Relativity | Astronomy Cast: Recent Episodes
Relativity | Astronomy Cast
Just a few years ago LIGO detected the first direct evidence of gravitational waves coming from colliding black holes. And there you have it. Boom! Black holes collide! But that wasn’t all we learned from gravitational waves, nor will we learn. Sure, the masses of merging black holes are nice to know, but what else can we learn from gravitational black holes?
Show Notes* Initial Discoveries and revisit to the groundbreaking detection of gravitational waves by LIGO * Beyond black hole mergers including Neutron Star Mergers * How gravitational wave detectors can observe other cosmic phenomena * Multi-Messenger Astronomy * Future Prospects including advanced detectors and space-based observatories * Technical challenges in gravitational wave detection
TranscriptFrasier Cain [00:00:50] Astronomycast episode 743, what else can we learn from gravitational waves? Welcome to Astronomycast, a weekly facts -based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I'm Fraser Cain. I'm the publisher of Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Sciences Institute and the director of CosmoQuest. Hey, Pamela, how are you doing?
Pamela Gay [00:01:11] I'm in the United States, so it's probably a question that you don't want me to answer. I'm also a soft money funded scientist, so that is definitely a question you don't want me to answer.
Speaker 4 [00:01:21] Okay.
Frasier Cain [00:01:21] Well, as a Canadian, now about to go into a trade war with you, my Canadian, sorry, my US friend, yeah, things are likewise bad.
Pamela Gay [00:01:34] Yeah, yeah. The thing that has me most concerned is the rule of law apparently no longer has any meaning in this country. There are unvetted people without security clearances that now have access to the Social Security numbers and payment history of every US taxpayer. Yeah, yeah, these are kids that are going in and installing these servers and getting access and it's just like, yeah.
Frasier Cain [00:02:15] Anyway, just a few years ago, LIGO detected the first direct evidence of gravitational waves coming from colliding black holes.
Speaker 3 [00:02:22] And there you have it.
Frasier Cain [00:02:23] Boom, black holes collide. But that wasn't all we learned from gravitational waves, nor will we learn. We'll get to it in a second, but it is time for a break.
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Frasier Cain [00:03:35] And we're back. So before we get into what else can we learn about gravitational waves, can you give us like the short version of what we did learn about gravitational waves from colliding black holes?
Pamela Gay [00:03:48] So what we learned was they politely do exactly what they were supposed to do. And it's really, really good when observation and theory match, especially when they match pretty much perfectly. So what we were eventually able to figure out, thanks to a suite of really long tunnels on the planet Earth that had mirrors and detectors and lasers that allowed us to consistently measure within a wavelength, the length of that tunnel, within a wavelength of optical light, the length of that tunnel, what we learned is as predicted when large gravitational waves pass over and through our planet, our planet will expand and squish, expand and squish and squish in a way that has a decaying frequency and amplitude that matches theory for what should happen as two masses come together and then collide.
Unidentified [00:04:55] Right.
Frasier Cain [00:04:57] So, you know, this was the prediction. I mean, it goes all the way back to Einstein when he did his theories of general relativity and said that masses moving through space time that experience, what is it, a quadrupole moment should generate gravitational waves and that, you know, you had the whole LIGO group come together to try to see whether they could actually demonstrate this. Now we knew that gravitational waves, like they had already been proven thanks to binary pulsars.
Speaker 4 [00:05:32] Right.
Pamela Gay [00:05:33] So, so what had earlier been figured out by Holst and Taylor was when you have pulsars orbiting around each other, these are two high mass objects that are not symmetric and as they go around, they are radiating energy. And because the radiating energy in the form of gravitational waves, their orbits are coming closer and closer together. The period of the orbit is changing. We can measure that extremely precisely thanks to changes in the pulsar timing as the objects move to and fro and the distances, those pulses have to travel change over the course of the orbit.
Speaker 3 [00:06:15] Right, right.
Frasier Cain [00:06:16] And they were actually able to measure that, that the pulsars as they're going around each other, they are bleeding off that rotational, the kinetic energy into gravitational waves, that's slowing down their, how quickly they're going around each other and you're actually measuring those pulsar timings and it all syncs up.
Pamela Gay [00:06:40] And they got a Nobel prize.
Frasier Cain [00:06:41] And they got a, yeah, Nobel prizes all around. But, but then, you know, when they were directly observed as opposed to indirectly just by the way that the orbits are changing, again, Nobel prizes all around. And so I guess what did we learn apart from, yes, gravitational waves are a thing, what did we learn from that original LIGO detection of, of gravitational waves?
Pamela Gay [00:07:07] Well, the one original detection was like, yay, merger. We did it. What we found from their, their population statistics is that group of intermediate mass black holes that we knew had to exist out there. And we hadn't been able to, until recently to directly detect was finally detectable through the gravitational waves that were produced when they merged with other objects. We have also been able to see a neutron star mergers. And back in 2017, we had that, we've, I think, dedicated an entire episode to it, that, that event that we detected through the neutrinos, through the gravitational waves and through the light. And now we know the majority of gold comes from neutron star mergers.
Frasier Cain [00:08:00] So, so I get, you know, I was going to take that to the, as the next part in this journey, but no problem. You're just going to speed run today's episode, which is perfectly fine by me. I can keep up. And that is that, yeah, we got this, this confirmation that the, that the black holes mergers are actually happening. And then that taught us that, that yes, indeed as predicted black holes get closer and closer to each other as they bleed off this kinetic energy through the gravitational waves. And that in fact, neutron stars can do the same. And this is detectable by LIGO when you've got these gravitational waves and then you've got, you know, we got a confirmation that a certain class of gamma ray bursts correspond to that merger of neutron stars that we see the wreckage of this collision. We see gold, we see other heavier elements that tells us that this is the way they probably formed and not necessarily with, with core collapse supernova. So then we get another finding just a couple of years ago with the nanograv facility about through pulsar timing arrays, we get another detection of gravitational waves.
Pamela Gay [00:09:12] And here I want to separate very carefully two separate ideas. Individual pulsars should be sources of continuous gravitational waves. We do not have the technological ability to detect those right now.
Frasier Cain [00:09:28] Only if they're unbalanced though.
Pamela Gay [00:09:31] Only if they're asymmetric at some, some level. And we believe that they are asymmetric at some level. It doesn't take a lot of asymmetry.
Frasier Cain [00:09:39] So it is wobble.
Speaker 4 [00:09:41] Yeah.
Pamela Gay [00:09:41] So pulsars are theorized to be a source of continuous gravitational waves. That's not what we're talking about right now. What you're talking about right now is we can measure the distance to pulsars through a variety of different means, and as long as that distance stays constant, the arrival of those pulses will stay constant. And this is more precise in timing than your standard atomic clock. What the nanograv facility has been doing is monitoring the pulsations of myriad different pulsars, looking for changes in arrival time that corresponds to a gravitational wave sweeping through our galaxy and changing the distance to these pulsars. And as you look out across space, we can see three -dimensionally all these different pulsars. We understand from all of the data we have so far that gravity appears to propagate at the speed of light. Gravitational waves appear to propagate at the speed of light. And we can, if we estimate the distance to this pulsar, we estimate to one back there, we look long enough and we haven't been able to do this yet. We'll be able to see pulsar delay here, pulsar delay there. That's a wave moving through space. What we're instead seeing is over here, there seems to be differences. We're seeing a myriad of different delays that statistically appear to There are gravitational waves regularly sweeping through our galaxy.
Frasier Cain [00:11:32] And a class of gravitational waves that we're not able to detect directly, which are the results of the merges of supermassive black holes.
Pamela Gay [00:11:40] And this is where it gets so cool to imagine all the different things we're going to be able to detect someday. And I don't know if you want to get to that right now, but nanograv is probing massive objects merging, LIGO is, is observing intermediate mass down to neutron star mass objects merging each different mass of black hole as it merges a neutron star as it merges produces a different frequency and amplitude of gravitational waves. And we can get at the distance of these events through the amplitude we observe and, and the frequency tells us what was happening.
Speaker 4 [00:12:29] Right.
Frasier Cain [00:12:31] So what's interesting as well is, is Meerkat, which is this incredible South African radio telescope array recently confirmed the existence of this background gravitational wave to the universe in a fraction of the time that the original nanograph was able to do. And so you've got this independent confirmation, you know, more telescopes better, more, they looked at more pulsars for a shorter period of time and got the confirmation. So, so I think that's where we stand today in, in what we have learned from gravitational waves so far. And so now we're going to move on in a second and talk about what we can learn, but it is time for another break.
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Frasier Cain [00:14:15] And we're back.
Speaker 3 [00:14:17] All right.
Frasier Cain [00:14:17] So I think we've got a good sense of what we've learned so far about gravitational waves. And so now let's look into the future, which is, I guess, what are the kinds of questions that we think that gravitational waves can give us some kind of answer and then how can we detect them?
Pamela Gay [00:14:37] It's not always what kind of questions can be answered so much as what parts of the universe can be probed. And one of the things you and I have talked about since day zero of, of doing astronomy cast is we cannot observe with light earlier than the release of the cosmic microwave background, but gravity doesn't have that same issue. And in general, it's not like we can go out and probe the gravity field of before the cosmic microwave background, the way we can probe the gravity field of a world and map out its sides. There's no fly by of the big bang that any NASA probe's ever going to do. But what we can do instead is look for the gravitational waves that are radiating from all directions from that early universe. And this could be part of this background of stochastic gravitational waves that we believe is out there. Now, the problem is there are a whole lot of different things that can produce gravitational waves. All it takes is an asymmetric object rotating and you're going to start to get gravitational waves. A planet like Mars with a big old volcano on it is going to have gravitational waves, just not ones we can detect.
Frasier Cain [00:16:06] When you drive down the road, you are generating gravitational waves.
Speaker 4 [00:16:10] Yeah.
Pamela Gay [00:16:10] And, and so all these different things add up. And, and so we talk about there being the individual gravitational waves that we get from merger events that we get from supernovae explosions, unless there is somehow this miraculously, perfectly symmetric supernovae. And, okay, I don't know how that happens, but we'll go with that. Anything rotating that isn't perfectly symmetric is going to radiate gravitational waves continuously. So you have things that explode and merge do a burst of gravitational waves that we see. You have things that are rotating and are asymmetric that are giving off continuous gravitational waves. And there's this random distribution we believe of gravitational waves that we may not ever be able to figure out what is. This is the stochastic gravitational waves in the background. And some of those are probably going to come from pre CMB, pre cosmic microwave background formation physics. Now, well, there's going to be stuff we can never figure out. There's going to also be stuff we do figure out. And this may be the one and only way we can ever get information from before the cosmic microwave background, other than by happening to see things that are fossilized in the cosmic microwave background. And we're only going to get so far with that as well. So it's, it's cool to think we still have this one pathway to understanding the early universe.
Frasier Cain [00:17:56] And what will be the sources of that gravitational wave? I mean, the term is primordial gravitational waves and, and as opposed to the background gravitational waves, they're coming from the colliding supermassive black holes and us driving our cars down the road and so on, but there's going to be this class of gravitational waves that will be visible, that would have been generated within that first 380 ,000 years after the big bang and in theory, right from the very beginning, right from, you know, if inflation happened, hopefully there'll be evidence of, of that, those gravitational waves in, in, you know, coming from that inflation event, but even if there are, I mean, would there be like large masses merging and colliding early on in the universe? Like what would be that source of those first gravitational waves?
Pamela Gay [00:18:43] So there were the very own acoustic waves traveling through the early material that made up our universe, that was creating a variety of overdensities and under densities in this essentially fluid that was the early universe. And so you didn't so much have discrete objects that were merging in, in the early universe, but you did have changes in the mass distribution over time. And there's other things that people worry about as well. Echoes essentially from colliding black holes and neutron stars that, that are out there today could be hiding stuff that I have to admit, I don't fully understand a lot of the papers. I do know that we both chased the, the, uh, there was, what was it?
Speaker 4 [00:19:44] 2014.
Pamela Gay [00:19:45] Oh, the bicep two, the bicep two, where they thought they were able to detect the, the effects of gravitational waves in the data they were looking at, and they didn't. And so we should be able to see in the cosmic microwave background, depending on what's going on, a, a essentially bunching up of material changes and how the light is being radiated. And so far we haven't been able to find that. So that leaves the next question of, can we find these ripples from how the material was clumped up and not clumped up in the early universe? Can we find the gravitational waves from that directly? And, and that's the next thing that we're hoping for.
Frasier Cain [00:20:38] And I think that, you know, people are aware of the upcoming European space agencies, Lisa mission, the laser interferometer space antenna, and that's going to be three spacecraft flying in formation, firing lasers back
Speaker 3 [00:20:50] and forth.
Frasier Cain [00:20:50] And then as gravitational waves sweep past, they will change the length of the arms and they're like tens of thousands of kilometers long. And so it will change those and they will get direct evidence of those supermassive black holes merging. That's the hope, but people have proposed versions of Lisa that have like maybe 12 spacecraft that maybe have longer arms and this is called the big bang explorer. And in theory, that that's what gets you to those, those first gravitational waves, the ones, the echoes of the big bang itself. And hopefully that is, you know, is something that we will eventually see maybe in our, in our lifetimes. All right, we're going to continue on this conversation, but it's time for another break.
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Frasier Cain [00:22:39] All right. So we've talked about the, the potential for primordial gravitational waves to give us a look into the time before the microwave background radiation. What else can we learn from gravitational waves?
Pamela Gay [00:22:55] Oh man. So this is a conversation that Paul Matt Sutter is really the one. I'm just going to say any of you have the chance to ever talk to Paul Matt Sutter. He is the expert on this.
Frasier Cain [00:23:06] This is watch his videos.
Speaker 3 [00:23:07] Yeah.
Speaker 4 [00:23:07] Or watch his podcast.
Pamela Gay [00:23:08] Yeah. The dance videos, maybe not so much, but the podcasts for sure. Um, one of the things he talks about is how the early universe could have actually in essence fractured and, and these changes in the mass that look like fractures, if you were to try and draw them out, artistically could generate gravitational waves. He talks about how as the different forces split off from one another where first there was gravity and then there was the strong and the electromagnetic and weak. So strong went off and then the electroweak and the electromagnetic split apart. And as each of these things happened, the reality of our universe, and this is all happening in fractions of the first second, all of this could have potentially left a pattern through gravitational waves on the early universe. I don't know how we detect that.
Speaker 4 [00:24:12] I, I,
Frasier Cain [00:24:13] but in theory it's going to be giving off gravitational waves.
Pamela Gay [00:24:15] Yeah, yeah. And so we're at this point where things that I was always like, cool theory, dude, love it. We'll teach it. Can't observe it. I'm good with it though. We might actually be able to observe because there are people smarter than I am and more creative than I am. And I'm very grateful. Those people exist.
Speaker 3 [00:24:37] Yeah.
Frasier Cain [00:24:39] Um, okay.
Speaker 3 [00:24:41] What else?
Frasier Cain [00:24:42] I'll give you, this was your title. So what else can we learn from gravitational waves?
Pamela Gay [00:24:47] I, I got this idea from you.
Speaker 3 [00:24:50] Um, okay. Wait, what?
Frasier Cain [00:24:52] That was the title that couldn't have been the title.
Speaker 3 [00:24:54] I keep you. All right.
Frasier Cain [00:24:55] Well, I'll give you a couple more then.
Speaker 3 [00:24:56] Fine.
Frasier Cain [00:24:56] Uh, so one of the other ideas is that you had, um, cosmic strings that, that, you know, if they're, you know, one of the theories about the sort of underlying nature of matter is that, that it's made of these wiggly, jiggly strings.
Pamela Gay [00:25:11] And that theory, I'm so much of a proponent.
Frasier Cain [00:25:15] But if, and, and you can't direct them directly, but if, uh, that theory is correct, then, then those, what would have been tiny strings at the beginning of the universe would have sort of accreted more material, grown larger and could be potentially light years across and these giant cosmic strings moving through the universe, colliding, um, and causing gravitational waves. And so one of the possible things that you could detect with gravitational wave observatories is, is colliding.
Pamela Gay [00:25:48] Um, and over time we're getting more and more evidence that those suckers don't exist.
Frasier Cain [00:25:52] Just, just to be clear, because you would see them with gravitational lensing as well. And these large scale surveys haven't turned that up. So, uh, the other thing that I like is that we could use them to find aliens flying through space in their warp drives. That's right. And so in theory, a spacecraft as it is, you know, as when the, when the enterprise goes from star to star, it's going to be using the warp drive. And that's going to be going to be shifting space time. It's going to be bending space time to its will to be able to make this spacecraft go. And then in theory, that's going to cause a wake, a gravitational wave wake, um, which is pretty cool. And so, but the sort of the coolest idea about this, and this was like a paper that just came out fairly recently was people were saying, Oh, we won't necessarily be able to detect the wakes. That's like, there's not enough going on there. Um, but what we will be able to detect is the detonation, the catastrophic failure of the warp drives in these, uh, spacecraft. And so you'll have the spacecraft is going, the warp drive collapses, destroys the spacecraft, sends out ripples of gravitational waves. And that might be just within reach of what we could do with, with gravitational waves, which I think is, uh, is fantastic. So, uh, you know, you're wondering, and then the sort of last thing that's on my sort of mental list right now is that we could potentially use gravitational waves as a communications tool. So, you know, this is beyond our capability today, obviously, but gravitational waves pass nicely through almost anything. They'll pass through through planets. They'll pass through stars.
Pamela Gay [00:27:40] What don't they pass through nicely?
Speaker 3 [00:27:42] Uh, black holes. Yeah.
Pamela Gay [00:27:44] I mean, they pass through them.
Speaker 3 [00:27:47] So they'll pass, they'll pass. Yeah.
Frasier Cain [00:27:49] They'll pass around them.
Pamela Gay [00:27:50] Yeah.
Frasier Cain [00:27:51] Um, right. That a, that like when a gravitational wave passes a black hole, it will, any part of the gravitational wave that directly falls within the event horizon of the black hole gets added to the black hole. You convert the mass energy of the gravitational wave and you end up with, uh, additional mass in the black hole, but, but anything that, you know, but otherwise they get distorted. They get twisted as they go near the black hole. But in theory, if you could move a mass in a certain way, you could generate gravitational waves. You could modulate the gravitational waves. And if you have a detection system that is good enough, you could theoretically detect it. And it might very well be that, that some future advanced civilization could use these gravitational waves as a way to communicate. And in fact, that might be the best way to communicate. And so the reason we don't see any evidence of aliens out there is because they're all using gravitational waves to communicate with each other in some way that we haven't figured out yet. So, um, so there's a lot of like cool science fiction ideas on what you could use gravitational ways for.
Pamela Gay [00:28:55] What I love about doing the show is I don't generally keep up to date on all of the theoretical technology research going on, which is not my thing. Totally your thing. And, and so over the years, the show has totally become a collaboration because of all the interviews you've done with folks with NIAC funding, folks who are thinking out of the box with the technology for communications and thrust and everything else. Cause I would never have come up with those in any of the research that I was doing. I sort of hit the, uh, here are some papers. I don't fully understand on primordial, uh, primordial gravitational waves.
Frasier Cain [00:29:39] So that's the other thing is searching, potentially finding primordial gravitational waves, sorry, primordial black holes. So that there is a minimum size of black hole that should be created naturally through the collapse of a massive star and that then if we detect the mergers of any black holes that are not mergers between neutron stars that are lower than the mass of that minimum mass level, then that immediately confirms the existence of these primordial black holes. Uh, one of the things that we haven't seen so far is mergers between white dwarfs and neutron stars or white dwarfs.
Pamela Gay [00:30:20] And the frequency is wrong.
Frasier Cain [00:30:21] Well, but Lisa isn't your tool. Lisa is the one that gets us the colliding supermass of black holes. There's an extension to LIGO call. So there's a couple of extensions to LIGO and a new thing called the Einstein telescope. And that will have, so right now LIGO has arms that are a few 10 kilometers, 15, I forget the length of the arms on like that.
Speaker 3 [00:30:46] Yeah.
Frasier Cain [00:30:46] But, but the Einstein telescope will be 40. And so it'll be like the largest feasible gravitational wave observatory that you can put on earth. And what's nice is that it just blends in with the rest of the existing, uh, community. So, um,
Pamela Gay [00:31:01] did they change Lisa? Cause Lisa was originally billed as the white dwarf merger detector.
Frasier Cain [00:31:08] I, I don't, I don't think so. I mean, maybe Lisa will also be able to do white dwarfs, but it's the, it's the longer, slower mergers that they're going to go after. It's these longer baseline ground observatories, but like, like the gravitational wave observatories are kind of like telescopes. You tune them to specific frequencies and then that's what you're
Speaker 3 [00:31:28] looking for.
Frasier Cain [00:31:29] But, but yeah. So in theory, we will get these confirmations that white dwarfs collide with, with black holes, that white dwarfs collide with
Pamela Gay [00:31:35] neutral.
Frasier Cain [00:31:35] Like obviously this is happening, but that'll tell us which of the kinds of explosions that we see in the universe are matched with these kinds of mergers. So, uh, so it's a lot of things, but as soon as you move mass, then you get to observe the gravitational ways of that
Speaker 3 [00:31:51] thing.
Frasier Cain [00:31:51] So, all right. We've reached the end of our show. Thank you, Pamela.
Pamela Gay [00:31:55] Thank you, Fraser. And thank you everyone who is watching this video and apologies. I do not know why my camera decided it needed to, uh, completely lose its mind for a moment, but that is what it did. I mean, I understand. I think I've completely lost my mind for a moment, a few times over the weekend. Um, this week we would really like to thank, uh, some of our $10 or not patrons, uh, this week we would like to thank Alex Rayne, Andrew, Palestra, uh, Antasor, Astro Bob, Astro Sets, Benjamin Carrier, Benjamin Davies, Bill Smith, Bob Krell, Boogie Net, Brenda, Brian Kilby, Bruce Amazines, Manski, Claudia Mastriani, Cody Rose, David, David Rosetta, uh, Diane Philippon, Don Mundus, Frodo Tanenbe, I think I said that time, uh, Jeff, uh, McDonald Gold, Hal McKinney, Janelle, Jeremy Kerwin, Jim McGeehan, Jimmy Drake, Jordan Turner, Justin Proctor, Katie and Ulyssa, uh, Christian Magersholt, uh, Mark Schneider, Michael Purcell, Michael Regan, Nate Detweiler, Papa Hotdog, Rando, Robert Hundle, Robert Palasma, Ryan Amory, the Air Major, Thomas Gazetta, Timelord Irowe, Will Hamilton, William Andrews. Thank you all so very much. You make this show possible.
Frasier Cain [00:33:29] Thanks everyone. And we will see you next week.
Speaker 4 [00:33:31] Bye bye.
Pamela Gay [00:33:38] Astronomycast is a joint product of Universe Today and the Planetary Science Institute. Astronomycast is released under a Creative Commons Attribution License. So love it, share it, and remix it. But please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today's show topic on our website, astronomycast .com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at patreon .com slash astronomycast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomycast.
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Live Recording
It’s been over 20 years since astronomers first discovered that the expansion of the Universe is accelerating thanks to dark energy. And in these decades, astronomers still don’t have much evidence for what could be causing the increased expansion rate. Maybe there’s something else going on to explain it.
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Show NotesEscape Velocity Space News (CosmoQuest)
PODCAST: Ep. 4: The Search for Dark Matter (Astronomy Cast)
PODCAST: Ep. 11: A Universe of Dark Energy (Astronomy Cast)
The Dark Energy Survey
The Nancy Grace Roman Space Telescope (NASA JPL)
South Pole Telescope (University of Chicago)
Physical Review Letters (APS)
Hot new early dark energy: Towards a unified dark sector of neutrinos, dark energy and dark matter (Physics Letters B)
Casimir Self-Interaction Energy Density of Quantum Electrodynamic Fields (Physical Review Letters)
Dark matter (CERN)
Dark Energy (Hubblesite)
Doppler Shift (Swinburne University)
Equation of state (cosmology) (Wikipedia)
Hubble’s Exciting Universe: Measuring the Universe’s Expansion Rate (Hubblesite)
Planck (ESA)
WMAP- Content of the Universe (NASA)
Supernovae Were Discovered in all These Galaxies (Universe Today)
What are Cepheid Variables? (Universe Today)
Carnegie Supernova Project II: The Slowest Rising Type Ia Supernova LSQ14fmg and Clues to the Origin of Super-Chandrasekhar/03fg-like Events (The Astrophysical Journal)
Research team discovers unique supernova explosion (Phys.org)
Planck and the cosmic microwave background (ESA)
Hydrogen Epoch of Reionization Array (HERA)
FOLLOW-UP: What is the ‘zero-point energy’ (or ‘vacuum energy’) in quantum physics? Is it really possible that we could harness this energy? (Scientific American)
First principle (Wikipedia)
WMAP Inflation Theory (NASA)
Could a Dark Energy Phase Change Relieve the Hubble Tension? (Universe Today)
Michael S. Turner (University of Chicago)
What is the Casimir effect? (Scientific American)
The Nobel Prize
The Higgs boson (CERN)
Particle Fever (2013) (IMdB)
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TranscriptTranscriptions provided by GMR Transcription Services
Fraser Cain: Astronomy Cast Episode 669, Challenges to Dark Energy. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know but how we know what we know. I’m Fraser Cain, the publisher of Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela, how you doing?
Dr. Pamela Gay: I am doing well. The third episode of our new TV show aired Saturday and we got it to the station 90 minutes before it was due, due to computer crashes, because computers know.
Fraser Cain: Yeah.
Dr. Pamela Gay: But if you haven’t checked it out yet, it’s Escape Velocity Space News. It airs on Now Media and I am gonna be putting together a podcast version and loading that up later today.
Fraser Cain: Congratulations, that’s amazing. It’s been over 20 years since astronomers first discovered that the expansion of the universe is accelerating thanks to dark energy. And in these decades, astronomers still don’t have much evidence for what could be causing the increased expansion rate. Maybe there’s something else going on to explain it.
If you go back into the archive of Astronomy Cast shows, one of the first episodes that we did was Dark Matter and then after that was Dark Energy, like within the teens anyway and we’ve brought it up a couple of times since them, but I had been expecting in the 15 years that we’ve been doing this show that we would have something more to say on the matter. But to be honest, not much has been figured out, apart from some really interesting new surveys, like the Dark Energy Survey Telescope, the development and eventual launch of the Nancy Grace Roman Telescope. We still have no idea what this is.
Dr. Pamela Gay: One of the things that actually caught me by surprise is I was expecting to spend today talking about the new results from the Dark Energy Survey and the South Pole Telescope, and there are actually two really cool papers that have both come out in Phys Rev Letters in the past couple of months that are both clearly the, hi, we’re theorists, we have predictions, there are only two of us on this paper, please give us the Nobel Prize when our predictions prove true.
Fraser Cain: Right.
Dr. Pamela Gay: And so, we may be getting to the point that theorists are starting to figure out how to get a handle on things, and they’re finding answers that may also help confine dark matter, which is kinda cool.
Fraser Cain: Well, that’s great. So, then, I guess, what is the evidence for dark energy?
Dr. Pamela Gay: Basically it comes down to when you measure the distance to a supernova and you measure the rate at which that galaxy is moving away from us using doppler shifts. We find that the universe is actually accelerating over time, which is not something that was in anyone’s predictions but it was in the math in the form of a constant to the equations of state for the universe. When Einstein originally came up with these equations there was an integration factor, when you integrate you have to add a constant, and he assumed that the constant would have a value that caused the universe to be static.
A few years later, Hubble came along, found the universe is expanding. Now we have a constant that makes sense for that. But it turns out that if you have a universe that is accelerating apart, that is a value for the equations of state, and so now what we’re finding is, in order to explain the geometry of our universe, which is flat, flat, flat, very flat, and an accelerating universe, you have to have 70% of the universe made of something that isn’t observable matter, that’s about 4-1/2 percent of the universe, that isn’t dark matter that gets observed through gravitational lensing, gets observed through rotation curves of galaxies, gets observed through the motions of galaxies and clusters, that’s 27-ish percent and instead you have dark energy.
Fraser Cain: So, this measurement really relies on how good the measurements to those type 1A supernovae are.
Dr. Pamela Gay: Yes. And it also comes down to, once we realized, okay, so we have from the Planck Observatory the flat geometry and then you combine it with everything else and you look at the mass density of the universe, the only way to explain the mass density of the universe is to have this extra stuff as well. So, you can get to dark energy from a couple of different ways but to get at the value that we’re seeing, yes, that came very clearly from the 1998 observations that the universe is accelerating with time and its expansion.
Fraser Cain: And we did some coverage on a new database that came out a couple of months ago, where astronomers had gone through and like recalibrated, normalized all the data for about 1,000 total type 1A supernova measurements and if anything, have gotten even more accurate.
Dr. Pamela Gay: Yeah.
Fraser Cain: You overlap the Cepheid variables with type 1A supernovae, the distance ladder is beautiful, the error bars are ever shrinking and the amount of dark energy in the universe is zeroing in on this. They’re really nailing this number.
Dr. Pamela Gay: And one of the wild things about this is we keep trying to find an excuse that maybe further back in the universe these kinds of supernovas, due to the change in the chemistry, would have different properties, and we do keep observationally finding random exceptions. There was a super cool type 1A supernova that went off while inside of another star, which is one way to blow up a white dwarf.
Fraser Cain: You can imagine that would pollute the results a little bit.
Dr. Pamela Gay: Right. But these one-offs that we’re finding are super cool but they are one-offs. The vast majority of the type 1A supernovae are just boring white dwarfs that ate more than they could hold without changing states.
Fraser Cain: Right. And so I guess one possibility is that the type 1A supernovae aren’t the standard candles that astronomers had always believed but the evidence is continuing to build that yes, indeed, they are.
Dr. Pamela Gay: Yeah.
Fraser Cain: Except for these ones where one star blows up from inside another star.
Dr. Pamela Gay: Yeah.
Fraser Cain: Right.
Dr. Pamela Gay: And so, I mean it’s just like saying most human adults are between five foot and six foot, yes, there are people who are only three foot and there are people who are only seven foot, but the vast majority of us are between five and six foot. So, yeah, it’s averages.
Fraser Cain: So, then, let’s talk about some of the largescale surveys that have been developed to try to get to the heart of dark energy. Not necessarily explain it, I guess, but at least to confirm it, map it, try and nail down its parameters.
Dr. Pamela Gay: So, the big one with the obvious name is the Dark Energy Survey, which was done from down in Chile where they observed with extreme sensitivity vast swaths of the sky with the goal of looking to see how the structure of the universe evolved with time. So, the idea here is, we know that the universe started out as pretty much smooth distribution of particles. It wasn’t even anything more fancy than particles initially, with slight over and under densities that were created by soundwaves moving through the early universe.
That mostly smooth distribution that we can measure in the cosmic microwave background, then had to collapse down into galaxies, stars at the smallest scales, but then clusters of galaxies, walls of galaxies, super clusters at the largest scale and it does that over time. And we have models that basically say okay, here is the CMB, here is the modern universe, let’s fill in in between and the Dark Energy Survey was designed to get at the more recent few billion years.
The next survey that is super exciting to look at is HERA, which is being done in the radio, looking at the redshifted 20-centimeter line of cold hydrogen, that the detectors for this, they’re looking at wavelengths of light that instead of being just 20 centimeters are instead many, many feet, so, longer than us. And in these longer wavelengths we are able to start seeing how cold hydrogen was distributed in the early universe, start piecing together how cold gas clumped and then got re-ionized in the era of re-ionization.
And between these different surveys, we’re working our way through measuring what was the structure over time so that we can better confine our models and say, okay, was the amount of dark energy constant over time. Was there some sort of a phase transition? Was there a kick somewhere? And these are the kinds of questions that folks are trying to answer, is what observables can dark energy give us that will help us confine our theories?
Fraser Cain: And so, in addition to the type 1A supernovae measurements, they’re able to now look at these galaxy clusters and then on top of that they’re looking at the concentrations of these regions of hydrogen gas. And so, you’ve got like three independent lines of observation that theoretically should allow you to map out the amount of dark energy.
Dr. Pamela Gay: Yes.
Fraser Cain: And is the assumption that they’re gonna find it? Like I know the Dark Energy Survey is still partway through its survey. You didn’t even mention the Nancy Grace Roman, it’s gonna be launching in 2025, and –.
Dr. Pamela Gay: Well, you don’t count your satellites until they’re orbiting.
Fraser Cain: Fine, fine, okay. Pamela, put your hands over your ears. Audience, the Nancy Grace Roman, one of its main jobs is going to be to characterize dark energy, doing kinda the same thing that the Dark Energy Survey is but from space. So, this is classic, right. Like if you don’t have an easy answer then you build more instruments, more observatories. You keep trying to characterize the nature of the problem, building hypotheses, testing them against your observations, removing bad ideas one after the other and hopefully trying to pin down the final thing. So, we may never know what’s causing dark energy but we’ll have it measured to Six Sigma accuracy.
Dr. Pamela Gay: Well, and the other side of it is the particle physics side. So, we know that dark energy, whether it’s a force, an energy, a field theory, whatever it is, puts into every cubic meter of space basically a proton-ish worth of energy. And so, how do you explain that energy existing? And by better understanding the quantum mechanic side of the universe, the particle physics, the vacuum energy, are sterile neutrinosactually a thing or not, this is another way of coming at the fullness of the universe by looking at the smallest factors inside of it.
Fraser Cain: All right. So, I guess, you found a few papers that have been proposing some alternative explanations that would explain the observations but not necessarily be new energy that’s being injected into every cubic meter of the universe, which I guess sounds satisfying. Like the fact that energy is appearing out of nowhere, that’s unnerving, so what are they proposing?
Dr. Pamela Gay: So, the first paper in here, I have to look at my notes, the first paper came out from Martin Sloth and Florian Niedermann, where they’re looking at new early dark energy. And the idea here is our universe has undergone phased transitions in terms of the energy of the entire universe. So, the first massive phased change occurred in the first fractions of a second, where we essentially went from every basically molecule sized bit of the universe expanding out via inflation, which we also don’t know what is, to be about the size of the observable universe according to some ways of looking at it.
And this massive, fairly instantaneous epic of inflation may have only been one of two phased transitions or there could have been a later phased transition that, if there testable ideas are right, leads through first principles to having a cosmological constant of 72, which is within error bars of what we see for the modern universe, and fixes the discrepancy we see with the old universe.
Fraser Cain: And so, sorry, so like, the idea of inflation –.
Dr. Pamela Gay: Yes.
Fraser Cain: Happening in just the first fraction of the Big Bang was developed I think back in the ’70s –.
Dr. Pamela Gay: Yeah.
Fraser Cain: To help explain a lot of the problems with the Big Bang. Like the Big Bang beautifully explains the universe as we see it today, but there are these flaws in the theory. How can vastly separated parts of the universe be similar temperature, there’s a bunch of these ideas. And so, one of the theories is that, in fact, there was this period of rapid inflation that carried everything away from each other really quickly and then it settled down.
So, I think baked into modern cosmology is already this idea, as you say a phased changed, a dramatic change in the expansion rate of the universe. So, it doesn’t seem that surprising that there could then have been others later on.
Dr. Pamela Gay: Right.
Fraser Cain: So, when would have this other, they’re calling it, what, early, what are they calling it, early –?
Dr. Pamela Gay: They’re calling it new, early dark energy.
Fraser Cain: Dark energy. Right, okay.
Dr. Pamela Gay: So, NEDE is the abbreviation.
Fraser Cain: New early dark energy, right. And so when would this have occurred in the timeline of the universe?
Dr. Pamela Gay: This still would have occurred during what they refer to as the dark times prior to the release of the cosmic microwave background. And it fits in with the way – Michael Turner, who is a prominent cosmologist, now professor emeritus from the University of Chicago, he says there’s basically these three unknown pillars of cosmology, inflation, dark energy which he actually named, dark matter. And so, this looks at that early period, makes some solid predictions for temperature details that we should be able to see with enhanced continuing to look at the cosmic microwave background, which tells us everything apparently.
But it also makes some finite predictions for what kinds of neutrinos should be out there, and what kinds of specific particles we can expect to find, thus, also perhaps explaining dark matter. So, we have one coherent theory spelled out in a letter-sized research article making concrete predictions. It seems good and just needs tested now but it’s not the only one out there.
Fraser Cain: All right. Well, let’s talk about the other one, then.
Dr. Pamela Gay: So, the other one is by Alexander, and I’m going to mispronounce this and I am sorry, this appears to be a Ukrainian last name, Tkatchenko, and the other one is Dmitry Federov. And they look at the vacuum energy of the universe, and vacuum energy is something we know is real because of the Casimir effect, which is just one of the best named effects in particle physics. The dude’s name was Casimir but it just sounds cool to say.
And what the Casimir effect says is if you take two plates that are capable of conducting electrons and you put them extremely close together but you’re not actually running charge through them and they’re not being exposed to any fields, nothing should happen I a vacuum. But the reality is, if you put these two plates just a couple nanometers apart within a vacuum, they will either attract or repel due to the constant creation and destruction of virtual particles that are in their creation and destruction, creating a field. So, if you have a proton spring into existence as a virtual particle between these two plates, that proton has a field and it creates the effect.
Fraser Cain: Right.
Dr. Pamela Gay: And we see this.
Fraser Cain: Right. And if I understand, like the Casimir effect, because the gap is so small the virtual particles of only certain sizes can pop into existence outside or as a field.
Dr. Pamela Gay: Right.
Fraser Cain: Outside the plates and then they are pushing inward, there is essentially a field on the outside and not so much field on the inside and it’s pushing on these plates. And once you move the plates farther and farther apart, now there’s room for the fields to appear both in between the plates and outside the plates.
Dr. Pamela Gay: Yeah.
Fraser Cain: And then that force goes away. And it sort of shows you the presence of this vacuum energy that is everywhere. And it’s been beautifully measured, so no one argues with the existence of vacuum energy. The assumption, though, is that this energy cancels out, goes away very quickly and doesn’t provide any ongoing force to the universe.
Dr. Pamela Gay: And that’s one school of thought. Another school of thought is, no, it’s totally providing dark energy-like forces, but when they do all the maths, they have consistently come up with an amount that is too large by a factor of 10 to the 120 for the theories that I like the best and that are among the prominent theories. And the best case for getting it down has, until this paper as far as I know, been 10 to the 30th. And when you’re off by somewhere between 10 to the 30 and 10 to the 120, it really doesn’t matter.
Fraser Cain: That’s a lot. Yeah.
Dr. Pamela Gay: It’s a lot. And this is where these two researchers, Tkatchenko and Fedorov, they said, well, okay, what if the vacuum energy has a polarizability, that the particles can actually have alignments that affect what can and can’t come into existence and annihilate. And they make again solid predictions that should be testable if we go looking with new technology that we don’t currently have laying around.
And it’s just one of those nice, simple, elegant ideas of we know stuff can be polarized. We know particles can be aligned. What if just the universe as a whole has this polarizability characteristic to it that we just hadn’t been including? And again, with this research, it makes set predictions on what to go look for. It looks at the particles that are out there and says, okay, here is what to see in particular physics and I don’t know which team I’m rooting for more. I like the polarizability one.
Fraser Cain: Why choose?
Dr. Pamela Gay: Well, I mean that’s – it’s two theory papers making predictions. Both of them have only two authors. Both papers are clearly gearing up for a Nobel Prize. It’s always good to cheer for people but I guess I’ll cheer for both.
Fraser Cain: Yeah. When you think about the winning Nobel Prizes, many of them start this way.
Dr. Pamela Gay: Yes.
Fraser Cain: That a theorist puts together a paper and says there should be a particle called the Higgs boson, right, and then 30 years later experimenters are finally able to find it, and this is the same thing. So, I mean I guess if dark energy makes you uncomfortable already –.
Dr. Pamela Gay: This doesn’t help.
Fraser Cain: Buckle up. Yeah.
Dr. Pamela Gay: Yeah.
Fraser Cain: You still got another 30 to 50 years of searching and scanning and trying theories to try and narrow in on an answer to it.
Dr. Pamela Gay: And if you want to get a feel for what it’s like within the field of particle physics to watch people so clearly chasing Nobel Prizes, watch the moving Particle Fever, it is very delightful. It shows the good, the bad and the ugly of scientists being scientists, and just the joy that comes from people getting to see their dreams come true through instrumentation.
Fraser Cain: I will check that out.
Dr. Pamela Gay: It’s really cool.
Fraser Cain: Yeah, that’s awesome. All right. Well, thanks, Pamela. I hope within the lifetime of Astronomy Cast we will do the show we finally know what dark energy is. After we do the show where we say we finally know what dark matter is, we will do both those shows. This is my promise to all of you.
Dr. Pamela Gay: And this is where it’s just sort of like we can hope, and when that finally happens is that when we retire?
Fraser Cain: No, no.
Dr. Pamela Gay: Okay.
Fraser Cain: Because there will be there will be a hundred new mysteries that are even –.
Dr. Pamela Gay: We keep going until we die.
Fraser Cain: Yeah, there will be a hundred new mysteries that are even more complicated and more troubling, so, no, that’s how this whole process works. All right, Pamela, thanks a lot.
Dr. Pamela Gay: Thank you. And thank you to all the patrons out there. I don’t actually have a thing of names, I just discovered, to pull out because it’s the very beginning of the month and the list hasn’t been generated yet but I do want to say thank you to all of you. We have been reading the names of just a subset of you on air, it’s one of the perks associated with the different levels but we love all of you, whether you’re a $1.00 a month contributor or a $100.00 a month contributor. You allow us, through our ongoing efforts, year after year to allow us to pay our humans and keep the show going, and occasionally replace cables from my camera when they die. So, thank you.
Fraser Cain: I will take this opportunity then to sort of mention a trend, a disturbing trend that you who are a fan of educational content should be aware of and that is artificial intelligence generated content.
Dr. Pamela Gay: Yeah.
Fraser Cain: That more and more websites are moving to this model of getting ChatGPT and other artificial intelligence to generate en masse the material and this trend is going to accelerate.
Dr. Pamela Gay: Yeah.
Fraser Cain: And it’s just another career that is gonna be –.
Dr. Pamela Gay: Go away.
Fraser Cain: Go away, sunsetted. The career of the science communicator and that’s because it’s way cheaper to let a large language model generate explainer content. And hopefully, Pamela and I have value and place in this society as more and more of this content shifts into being generated by enormous databases. I know it sounds shocking and surprising but this is where it’s all gonna go and it’s gonna happen startlingly fast. So.
Dr. Pamela Gay: We have had people write in and say, hey, why don’t you switch over to using ChatGPT to generate your scripts.
Fraser Cain: Right.
Dr. Pamela Gay: It’s already people are suggesting it.
Fraser Cain: Yeah, yeah.
Dr. Pamela Gay: And we won’t do that.
Fraser Cain: Yeah. I will get ChatGPT to recommend ideas but I won’t –. See, here’s the key, there are no scripts, that’s the trick.
Dr. Pamela Gay: Right, right.
Fraser Cain: So, how could we have a script if there are no scripts? Smart. Anyway. So, if the work that we’re doing, like if the loss of science communicators at the mainstream media was already troubling to you, now they’re coming for all science communicators. So, if supporting the work that we do is important to you to make sure that we can keep doing the show, paying the people on the team, keeping the servers running, all of that, join our Patreon.
Dr. Pamela Gay: Thank you.
Fraser Cain: Patreon.com/astronomycast. Thanks, everyone, and we’ll see you next week.
Dr. Pamela Gay: Bye-bye.
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Astronomers have made extremely accurate measurements of the expansion rate of the Universe and come up with different results. And the error bars for the observations don’t overlap, so there’s something strange going on. What’s the answer and how can the Crisis in Cosmology be resolved?
Download MP3 | Show Notes | Transcript
Show NotesSee Comet ZTF (C/2022 E3) Dash Between Big and Little Dippers (Sky & Telescope)
Comet Hale-Bopp (NASA JPL)
Comet Hyakutake (NASA JPL)
Comet NEOWISE, the best comet of 2020 (EarthSky)
Comet McNaught over the Pacific Ocean (ESO)
Hubble Tension Headache: Clashing Measurements Make the Universe’s Expansion a Lingering Mystery (Scientific American)
Hubble telescope refines universe expansion rate mystery (Space.com)
Megaparsec (Swinburne University)
Messier 87 (NASA)
Andromeda galaxy: All you need to know (EarthSky)
Ask Ethan: Is there a better way to measure cosmic time? (BigThink)
Type Ia Supernova (Swinburne University)
How do astronomers measure the brightness of something? (Astrobites)
Luminosity (Swinburne University)
Standard Candle (Swinburne University)
What are Cepheid Variables? (Universe Today)
Gravitational Lensing (Hubblesite)
Dr. Adam Riess (Space Telescope Science Institute)
Astronomical deep-sky photometry and spectroscopy (BBC Sky at Night)
Gaia (ESA)
Baryon Acoustic Oscillations (NASA)
COBE (NASA)
Planck (ESA)
LAMBDA – ΛCDM Model of Cosmology (NASA)
Astronomers Grapple with JWST’s Discovery of Early Galaxies (Scientific American)
How Did Inflation Happen — and Why Do We Care? (Space.com)
The Big Bang (NASA)
IMPROVING MEASUREMENTS OF THE COSMIC EXPANSION WITH GRAVITATIONAL WAVES (LIGO)
Cosmic Inflation Theory Faces Challenges (Scientific American)
Sloan Digital Sky Survey
The Dark Energy Survey
JWST (NASA)
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TranscriptTranscriptions provided by GMR Transcription Services
Fraser Cain: AstronomyCast, Episode 668, “The Crisis in Cosmology.” Welcome to AstronomyCast, your weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, the publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how you doing?
Dr. Pamela Gay: I am doing well. How are you doing?
Fraser Cain: Doing great. Yeah, nothing to report.
Dr. Pamela Gay: That, I think, in 2023, is really the best any of us could ever ask for.
Fraser Cain: Yeah, the weather is fine, the snow is gone, I’m getting out and doing a bunch of hikes and stuff in the nature, even though it’s wintertime, but still, garden’s coming along.
Dr. Pamela Gay: Dang.
Fraser Cain: I saw the comet last night. It sucks, but…
Dr. Pamela Gay: Oh well.
Fraser Cain: Yeah, it’s gotten quite diffuse at this point, so although it’s bigger and brighter, it’s also more of just a cloud.
Dr. Pamela Gay: It’s spread out, so the light from any given place – its surface brightness is really low.
Fraser Cain: That’s right, so you really don’t get that nice, little, tight nucleus with the tail, you just get this – what looks like a little cloud in the sky. But still, it’s easy to find. It’s so easy to find because it was right beside Ursa Minor, right beside the Little Dipper, and then, it’s moving towards Cassiopeia. So, if you have never seen a comet – you can’t see it with your eyes, but you can see it in a pair of binoculars or a small telescope. It’s easy to find, and that’s nice, as opposed to one where it’s in a fairly difficult constellation to discover. But unfortunately, completely inaccessible now to the folks in the Southern Hemisphere, so this one is just for the folks in the north. So, if you haven’t already –
Dr. Pamela Gay: Who do not have cloud like the Midwestern folks in the north.
Fraser Cain: Right, yeah. It’s gonna peak in just a couple of days from now, so now is your chance, and then it’s just gonna get – but it mostly sucks. I always compare comets to Hale-Bopp and Hyakutake, and people are even like, “Oh, didn’t you like Comet NEOWISE?” I’m like, no.
Dr. Pamela Gay: No.
Fraser Cain: No, it sucked. I could see it with my eyes. That does not a good comet… That is necessary – sufficient but not necessary? Anyway, just barely being able to see a comet with the unaided eye does not – you do not declare victory in the comet world. No, you want the one that is gigantic –
Dr. Pamela Gay: The tail.
Fraser Cain: The tail spans multiple handspans across the sky, that you can see it even in light-polluted skies. That’s a comet, and everything else the universe is sending our way right now is mediocre, and I reject them. So, no, NEOWISE sucked, McNaught sucked, this one sucks. We demand better. I will wait, but I’ve been patient for too long. Come on, comet!
Dr. Pamela Gay: I…I can’t argue with that. All of that is true. It’s all true.
Fraser Cain: Yeah. For people who are like, “Oh yeah, Comet NEOWISE was fine,” no, it wasn’t! It wasn’t, and you’re settling. You deserve better. I deserve better. We deserve better comets. The universe can provide it; it’s done it in the past. It’s time to put up or shut up. All right.
Dr. Pamela Gay: Yeah…
Fraser Cain: Astronomers have made extremely accurate measurements of the expansion rate of the universe and come up with different results, and the error bars of the observations don’t overlap, so there’s something strange going on. What’s the answer, and how can the crisis in cosmology be resolved? So, what is the crisis in cosmology?
Dr. Pamela Gay: So, some people call it the Crisis, some people call it the Hubble Tension. A lot of us just put “WTF?” and call it a day. So, what’s happening is before the supernova teams did such an amazing job of measuring the present expansion rate of the universe, we were like, “Nah, the universe is expanding somewhere between 50 kilometers per second per megaparsec to 100 kilometers per second per megaparsec,” and I had so many profs that were like, “Just use 100. It makes the numbers easier.” It was pleasing.
Fraser Cain: Right. Just so people understand this idea, that you take a megaparsec of space, which is about 33 million lightyears of space, and when you think about that, that is the distance between us and faraway galaxies, like Andromeda’s really close. We’re talking about galaxies that are 30 million lightyears away, like M87 is kind of in that – when you think about the supernova image. Every second that goes by, those objects are now 100 kilometers farther apart, or 50 kilometers farther apart.
Dr. Pamela Gay: And it is a function of how far something is away from you, so the further something is away, the faster it appears to be moving away from you. A lot of people use a raisin bread analogy on this because the raisins stay the same size as the bread dough expands, so two raisins that start really close together will end up a little further apart, two raisins that are really far apart initially will end up seriously far apart by the time that bread is done rising.
Fraser Cain: And why is knowing the expansion rate of the universe important?
Dr. Pamela Gay: It’s one of those things that allows us to put together all the rest of our cosmological ideas of how you go from our universe being a single point to expanding out, to forming hydrogen and helium, trace amounts of lithium and beryllium, to – the whole story is tied up, and it slowed down or it sped up, and understanding what rate we’re going now, since there’s no accelerator pedal that we know about, what we see has to be defined by the physics of our universe, and we can start to define all that physics of the universe if we know this one number that refuses to be measured.
Fraser Cain: I apologize, I was off by a factor of 10 there, so, sorry, and you should have caught me – it’s surprising you didn’t – but a megaparsec is 3.3 million lightyears, not 33, so Andromeda, roughly, is in that ballpark range. So, apologies.
Dr. Pamela Gay: Yes. The way I think about it is someone in Andromeda looking back at us would be seeing Neanderthals.
Fraser Cain: Right, but astronomers don’t think in lightyears, they think in megaparsecs, so if I get off by a factor of 10, that’s fine by you because you don’t even think about it, so apologies in –
Dr. Pamela Gay: It’s true.
Fraser Cain: Yeah, the general public thinks in lightyears while astronomers only think in parsecs and megaparsecs, but yeah, so, apologize, let’s continue. So, why – you were discussing why knowing the expansion rate of the universe is important.
Dr. Pamela Gay: It basically just gives us this reference point that we can work all the other maths back from.
Fraser Cain: So, how long the universe has been around for?
Dr. Pamela Gay: How long the universe has been around for, basically –
Fraser Cain: What will happen in the future…?
Dr. Pamela Gay: The one that gets me is by understanding the current expansion rate, we can actually figure how fast the universe went from being a mostly smooth distribution of gases to forming galaxies, to forming galaxy clusters. The rate at which we formed large-scale structure, at a certain level, hinges on how fast our universe is expanding. It’s everything.
Fraser Cain: Right. And so, in the olden days, we used to get “How old is the universe?” and people would say, “Well, it’s kind of somewhere between 10 and 20 billion years old,” and that’s that range of measurement. If you get 50 kilometers per second per megaparsec, you get one age of the universe because you just measure how long the universe is expanding, but if you get 100, you get a different one, and they are very different, and knowing that is important. So, how do astronomers measure the expansion rate of the universe at the close and at the far?
Dr. Pamela Gay: So, there are two totally different suites of mechanisms. The “local time” way of doing it is we look for supernovae, which give off a set amount of light if they’re Type 1A supernovae – explode a white dwarf star, and you get essentially the same explosion over and over and over again, with errors that we’ve discussed in other episodes.
Measure how bright that explosion appears, measure how fast the galaxy the supernova is in is moving, and this tells you the distance using measured brightness and known luminosity, and it tells you the expansion rate by looking at the Doppler shifting. So, we’re literally measuring how much the colors of the different bands of atomic lines have been shifted by the galaxy’s motion, and that gets us a velocity.
Fraser Cain: Right. And so, we have all of these standard candles, from the Cepheid variables, to the supernova, to – I saw a list. There must have been 30 potential standard candles overlapping, going from – some of which are very well known, others of which are poorly known, but you go from local measurements using parallax that overlaps with Cepheid variables that overlaps with Type 1A supernova, and you just get this really beautiful, smooth measurement, and what number did we get from the local methods of measuring the expansion rate of the universe?
Dr. Pamela Gay: So, we’re getting around 70 kilometers per second per megaparsec, and this is using not just supernovae, but as you point out, there’s a bunch of other methods. So, folks are looking at red giants, they’re looking at planetary nebulae, they are looking even at the distant gravitationally lensed galaxies that we’re able to see multiple versions of using crazy geometry when we can see the galaxies’ lenses do the same thing at different times. All these methods are giving us definitely over 70, and narrowing in on 74, so it seems pretty constant.
Fraser Cain: Right, and the error bars are really tightening up. The quality of the observations is exquisite. I talk to a lot of astronomers, and they talk about how good of a job they’ve done with those observations, and they just gush.
Dr. Pamela Gay: The SHOES survey by Adam Reiss – they’re quoting an error of 1.3%, and they are basically going from the nearby Cepheids that they have taken some of the most precise photometry of that anyone has ever taken, then using Gaia parallax data, and then working all the way out. How often does anything in astronomy get done with that level of accuracy? We know this. The local value is basically 74 kilometers per second per megaparsec.
Fraser Cain: Right. And so, let’s go the other end of the range because there is another group of measurements that are taken not locally.
Dr. Pamela Gay: Right, and this is where things are squirrely. In the cosmic microwave background, we see these baryonic oscillations, these soundwaves that move through the early universe, causing slight over- and underdensities, and we can map so beautifully this distribution with our theoretical models, and by combining our understanding of, okay, the universe had this much regular matter, this much dark matter, this much – putting all of these base understandings that we come at from the theory, combining our average temperature information that we got from COBE and other missions, putting it all together, it gets us in the 60s, 68, generally.
Fraser Cain: But the most accurate version of this was the Planck satellite from the European Space Agency.
Dr. Pamela Gay: Right, and this is where it’s important to note the Planck data was used to get at that distribution of baryonic oscillations, and that was used in combination with a mean temperature that they were, in a lot of the papers, referring back to COBE data. So, Planck got us, very specifically, deviations about the mean, and we just fed all the data together in the context of what’s called lambda cold dark matter.
This is a theoretical framework that says that our universe is not just expanding, but it’s accelerating as it expands, that the dark matter, the stuff that we’re not really sure what it is, that may be related to neutrinos in some way – whatever it is, it wasn’t moving extremely fast early in the universe, so that’s where the “cold” part comes in. So, we have lambda, the dark energy, and cold dark matter, two things we have very poor understanding of. When you combine those with the data, it gets you, again, roughly 60 kilometers per second per megaparsec with error bars that don’t overlap.
Fraser Cain: Right, and this is the key. So, you look at the local neighborhood and you get a measurement that’s in the low 70s with very tight error bars, you look at the early universe, you get 68 with very tight error bars. Both are exquisite observations, both are trying to tell you the same thing, and they disagree with one another, and this, at the heart, is the crisis in cosmology.
Dr. Pamela Gay: Correct.
Fraser Cain: So, this is the crisis in cosmology, so what’s the answer?
Dr. Pamela Gay: Well, this is where I personally am a bit excited, and I don’t know how many people are with me on this one because I wasn’t at the meeting, but at the American Astronomical Society meeting, there was a lot of discussion about how JWST images of gravitationally lensed early galaxies appear to be showing from two different studies that have both made it through peer review that there were already well-formed galaxies 350 million years after the Big Bang, and that’s early.
There is other work that is being done that is still going through peer review that is showing there may have already been galaxies – massive ones – at 200 million years after the Big Bang. So, with galaxies forming this early in the history of the universe, it tells us that that model we have, lambda cold dark matter, is off somewhere because while we thought there would be a couple, a few massive galaxies early on, those baryonic oscillations didn’t lead us to believe there would be as many as we are now finding.
And so, we have to figure out a new way to get from mostly smooth universe with the cosmic microwave background to galaxies forming in bigger and probably larger numbers than we anticipated to our present structure, and folks are putting out ideas like maybe there was a bit of leftover inflation, maybe the value of dark energy hasn’t been constant, and all of these different ideas – I don’t think we can really throw anything out yet, and I am the first person to want to throw out ideas.
Fraser Cain: Right. So, the challenge here is that you’ve got – the most obvious possibility is that one or both measurements is incorrect –
Dr. Pamela Gay: Yes.
Fraser Cain: – which is what would be everybody’s first instinct, that someone’s wrong, that one of these measurements is incorrect. But, because of this dichotomy, both measurements have been scrutinized and scrutinized, and teams have gone back, and all they’re doing is narrowing the error bars. They’re not finding a large discrepancy. So, that’s the one that is most likely, and yet, that seems to be less and less of the case, and so, you’re left with the universe –
Dr. Pamela Gay: New physics.
Fraser Cain: New physics, right, that our understanding of Einstein, our understanding of what those acoustic oscillations should be in the cosmic microwave background, is wrong.
Dr. Pamela Gay: And this is where I’m not gonna lay blame on anybody. Relativity seems to work so far, it doesn’t seem to be the problem, but our understanding of the distribution of kinds of matter, the way different forces interplayed, whatever the heck inflation might be – we have no idea what inflation might be – it is somewhere in this physics of how we get from the Big Bang to now that we are missing something, and it’s kind of awesome.
We don’t get a whole lot of surprises anymore, it feels like, some days. We’ve found all the particles in a standard model. We didn’t find any of the particles from supersymmetry. This gives us something new to chase, and the fact that even the folks using gravitational waves to measure distances – they’re still getting the same local universe numbers.
Fraser Cain: And that’s harder to get wrong.
Dr. Pamela Gay: Yeah!
Fraser Cain: I know I’ve talked to some people working in gravitational wave observatories, and I’ve had this conversation, and effectively, the more precise, the more powerful these gravitational wave observatories get, the farther they’re able to see out into the universe. You can rely on their measurements, so it’s another layer of observation, but one that’s very trustworthy, and it only goes so far. It doesn’t take you all the way up to the end of the universe, but maybe some future observation will.
So then, new physics – so, maybe we don’t understand how the early cosmic microwave background worked, maybe we don’t understand how Cepheid variables work, maybe we don’t understand all these different pieces, but there have been a few hints. Maybe Type 1A supernovae aren’t the standard candles that we thought they were.
Dr. Pamela Gay: And this is where there are so many different things that are getting us that local 73-ish that, yeah, I’m happy saying there are discrepancies from one Type 1A supernova to another in weirdo special cases. We’ve got to talk about some of those. If the white dwarf ends up inside of another star, that explosion’s gonna be a bit different, and that happens, but it really seems like there is something about how you get from there – cosmic microwave background – to here – gravitational waves, Cepheids, planetary nebulae, supernovae, all these other mechanisms. There’s something in the science that we have yet to uncover.
Fraser Cain: And then, another possibility is if the rate of expansion of the universe changed. So, perhaps there was a – the model that I’ve heard is this idea of late inflation.
Dr. Pamela Gay: Yeah, that’s the one I was looking at as well, where whatever it was that caused us to initially blow up, there’s a little bit of that left over that caused another kick, but we don’t understand what’s going on there.
Fraser Cain: Right, right. And so, if you had an expansion rate of 74 early on – or, sorry, 68 early on, and then it slowed down, you could get the one that’s today, and that’s even taking into account dark energy. I’m sure people are like, “What about dark energy?” That’s layered on top of this. That’s accounted for. And so, you would have this almost – instead of the universe smoothly applying the accelerator on the gas, it was like putting the accelerator a little harder, and then pulling the foot off the pedal a little bit, and then putting it on harder again, and who knows what kind of shenanigans it got up to in the intervening period? So, what is the way forward at this point? What is the way out of the crisis in cosmology?
Dr. Pamela Gay: We need to basically do a survey of just what was the distribution of galaxies and galaxy clusters in the early universe. We have done a beautiful job, first with the Sloan Digital Sky Survey, doing a volume around our galaxy. Then, with the Dark Energy Survey, we have pushed out even farther in some areas of the sky. With JWST, we’re going to be able to continue pushing the survey of structure size further and further out, watching how the universe goes from being Swiss cheese with giant holes in it to being Swiss cheese with smaller and smaller holes in it –
Fraser Cain: Right.
Dr. Pamela Gay: – and by measuring how that large-scale structure changes over time, that will start to put a different form of constraint on our models. We need to be out there, counting early galaxies.
Fraser Cain: Right, and so, you’ve got this structure of the cosmic microwave background radiation, and the hot spots and cold spots should map to equivalent clusters and distributions of galaxies, and so, you’ll know that this transition from the farthest that you can see to more recent is smooth –
Dr. Pamela Gay: Yes.
Fraser Cain: – and then, that will tell you, and you can keep moving forward at that point, but that’s a much harder observation. Weird as it sounds, the galaxies are much dimmer and harder to spot and map out than the cosmic microwave background radiation, which is everywhere in all directions.
Dr. Pamela Gay: Yeah. More telescopes.
Fraser Cain: Yeah. And then, the other side of that is going farther with gravitational waves, and hopefully, you’ll get to this point where the two overlap, where the gravitational waves reach the cosmic microwave background, or shortly after.
Dr. Pamela Gay: That is technology that someone maybe someday will fund, and we’re at that frustrating point where the next big discovery beyond what we can do with the new, massive radio telescopes that have started to catch star formation at earlier periods, and what we can do with JWST – it’s gonna take multiple nations getting together to build these 30-, 80-, however-many-meter telescopes that are being discussed to be able to look back.
Fraser Cain: All right, place your bets. It’s the close observations are wrong, the CMB observations are wrong, or there’s new physics.
Dr. Pamela Gay: New physics.
Fraser Cain: Really? That’s the most exciting outcome possibility, is new physics, so if that’s true, that would be wonderful.
Dr. Pamela Gay: Or at least a new understanding of that cold dark matter temperature.
Fraser Cain: It would be huge thing, like there was a revision to relativity that nobody saw coming.
Dr. Pamela Gay: So, again, I’m not sure saying a revision to relativity is the right way to say it because I think that what we’re looking at is something coming out of the realm of particle physics, and particle physics and relatively do not talk to one another, and I really think it’s going to be something about how particles interact in different regimes, and whatever the heck this dark energy is that’s gonna be what gets us to the solution to this discrepancy.
Fraser Cain: I think this “crisis” makes it sound like a bad thing, but you talk to astronomers, and they couldn’t be more excited. They’re so happy to not understand something, what was considered to be this bedrock idea, because the problem is bedrock is you get this ossification. Suddenly, you have this space that has opened up, where the solution is in there somewhere, and a lot of interesting ideas and theories, and a lot of brainstorming, and a lot of intellectual power gets to be put onto this problem, and they love it. They love it. So, I feel said that the term “crisis” – because you get a lot of pseudoscientists sort of rolling their eyes at scientists at this thing.
Dr. Pamela Gay: I like “the Hubble Tension.”
Fraser Cain: “Hubble Tension” – yeah, but “Crisis in Cosmology” is a better name, so I’d rather reel them in with “the Crisis in Cosmology,” and then help people understand that, in fact, astronomers couldn’t be more excited and happy to have this opportunity. All right, Pamela, thank you so much.
Dr. Pamela Gay: Thank you, Fraser, and thank you to everyone out there who makes this show possible through your patronage at Patreon.com/AstronomyCast. This week, I would like to thank Camy Raissian, Gabriel Gauffin, Benjamin Davies, Steven Coffey, john öiseth, Arcticfox, Dean, Corinne Dmitruk, Bart Flaherty, The Lonely Sand Person, John Drake, Nate Detwiler, Lew Zealand, Brian Kilby, Naila, The Air Major, Ron Thorrsen, Arthur Latz-Hall, Leigh Harborne, Jason Kardokus, Robert Hundl, Kim Barron, Paul Esposito, Ruben McCarthy, Bob Zatzke, Jordan Turner, Timelord Iroh, Daniel Donaldson, Frank Stuart, Ian Abdilla, and Geoff MacDonald. Thank you all so much for making everything we do possible.
Fraser Cain: Thanks, everyone, we’ll see you next week.
Dr. Pamela Gay: Bye-bye.
Voiceover: AstronomyCast is a joint product of Universe Today and the Planetary Science Institute. AstronomyCast is released under a Creative Commons attribution license, so love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, AstronomyCast.com.
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