Ep. 639: Big Questions Update: Dark Matter, Dark Energy, Hubble Constant

Knowledge moves forward, and so, we must move with it. Today we’ll give you an update on some of the most fascinating, and fast-changing topics in astronomy, astrophysics and cosmology.

Download MP3 | Show Notes | Transcript

Show Notes

PODCAST: Ep. 4: The Search for Dark Matter (Astronomy Cast)

PODCAST: Ep 487: Dark Matter: 2018 Edition (Astronomy Cast)

Planck (ESA)

Max Planck (The Nobel Prize)

Mystery of the Universe’s Expansion Rate Widens With New Hubble Data (NASA)

Hubble’s Exciting Universe: Measuring the Universe’s Expansion Rate (NASA)

Cosmologists Debate How Fast the Universe Is Expanding (Quanta Magazine)

Planck and the cosmic microwave background (ESA)

The “Crisis in Cosmology” Might not be a Crisis After all (Universe Today)

How a Dispute over a Single Number Became a Cosmological Crisis (Scientific American)

In a Numerical Coincidence, Some See Evidence for String Theory (Quanta Magazine)

PODCAST: Ep. 598: How You Could Overturn Cosmology (Astronomy Cast)

WEBINAR: “The Hubble Constant Controversy” with Adam Riess (Johns Hopkins University, USA) (International Space Science Institute)

Supernova Cosmology Project (Lawrence Berkeley National Lab)

What are Cepheid Variables? (Universe Today)

Gaia (ESA)

Hubble Space Telescope

Type Ia Supernova (Swinburne University)

Quasar (Swinburne University)

Timing a sextuple quasar (Phys.org)

What Is Gravitational Lensing? (Hubblesite)

Large-scale Structure (Swinburne University)

What is the Inflation Theory? (NASA)

Dark Energy (Swinburne University)

Nancy Grace Roman Space Telescope (NASA Goddard)

Rubin Observatory


Sloan Digital Sky Survey (SDSS)

The universe is expanding faster than it should be (National Geographic)

What is “early dark energy” and can it save the expanding Universe? (Big Think)

No Release for the Hubble Tension (Sky & Telescope)

Back to Top


Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast Episode 639: Updates to Dark Matter, Dark Energy, and the Hubble Constant. 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. My name is Fraser Cain. I’m the publisher of Universe Today. I’ve been a space and astronomy journalist for over 20 years. With me is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How are you doing?

Dr. Gay:                      I am doing well. And I do need to congratulate you on the anniversary of Universe Today, which is old enough to drink in the United States, I believe.

Fraser:                         Oh, it’s been doing that for two years. It’s 23 years – is Universe Today now. So, yeah.

Dr. Gay:                      Excellent. Excellent. So, congratulations.

Fraser:                         Yeah, it’s been –

Dr. Gay:                      It’s awesome.

Fraser:                         Apparently, you just keep showing up every day and keep doing the same job and eventually you’ve done it for a long time. And I’ve found my life’s purpose, so I have no plans to change. I’ll do this for another 23 years.

Dr. Gay:                      You know, I am happy to keep doing this with you for another 23 years.

Fraser:                         There you go.

Dr. Gay:                      We are going to be dodder and gray-haired old people. So, one of my favorite things to do early in my career was to ask the oldest astronomers I encountered, “What was the biggest discovery of your life?” And I was shocked when I realized they consistently said galaxies. What do you think we’ll be saying?

Fraser:                         Oh, interesting. Photos from the surface of Titan.

Dr. Gay:                      Really?

Fraser:                         Ah, I don’t know. That’s gonna be cool, though, having a helicopter flying around Titan.

Dr. Gay:                      I was gonna go with dark energy, which is kinda related to the theme of our show today.

Fraser:                         Well, speaking of, let’s get into it. Knowledge moves forward and so we must move with it. Today, we’ll give you an update on some of the most fascinating and fast-changing topics in astronomy, astrophysics, and cosmology. So, this is gonna be just like a catchall of some interesting news that is pushing our knowledge of these topics forward. So, where do you wanna start?

Dr. Gay:                      Let’s start with why we made an episode like this.

Fraser:                         Okay, right. Wait, are you saying like the early episode that we did that says, “Dark Matter” and just call it a day? That’s not enough?

Dr. Gay:                      No. No, sadly it’s not. It’s not. Back when we started this show, we thought, naïve summer children that we were, that with improved observations from Planck, from – continuing to look at supernovae, that we would see our understanding of the universe converge on a single value that explained the modern of expansion rate of the universe in the context of the creation of the cosmic microwave background radiation and everything that has happened in between.

And what we have instead seen is the two values had the error bars getting smaller and smaller and smaller. And they no longer overlap and the difference between them is statistically significant. And what we have actually learned is there is something fundamentally wrong, either with our understanding of the universe or our ability to observe things in many different ways.

Fraser:                         And this term – this is the crisis in cosmology, which gives it – it gives it this really negative sounding name. But you talk any astronomer about this and they’re gitty. They’re so excited that things are weird and they don’t understand and they don’t know why this is the way it is, and they can’t wait to figure this out. And it means a genuine puzzle that they can finally sink their teeth into because all it means is better measurements at different times to get – to understand is it a measuring problem or is it a physics as we don’t understand it problem? And they love it. So, I wish there was a better term than the crisis in cosmology. But, anyway, that’s the term we’re stuck with.

Dr. Gay:                      So, the thing is, for people who are even at the more senior levels and not retired and shouldn’t be retired in astronomy – we still have people in their 90s not retired. I’m excluding them from this conversation. For all of those of us who are below a standard retirement age, when we were learnings astronomy, all the really cool stuff was done before World War II. You had the mapping out of the atom, where it went from the plum pudding model to our model today with the nucleus and the cloud.

You had the resolution of quantum mechanics. You had all of this cool electromagnetism and quantum mechanics and gravity, and it didn’t really speak together real well. But other than gravity not behaving, all the big stuff we thought had been figured out. And let’s face it most of us are just like, “Okay, gravity, we have a problem.” And it was really hard when most of us didn’t want to be embroiled in because string theory is ugly. But, now, we have an approachable problem that even observers like me can get in on and have opinions and thoughts and debates and –

Fraser:                         Yeah, why don’t we try this. What if –

Dr. Gay:                      – it is awesome.

Fraser:                         – we just do that. Yeah.

Dr. Gay:                      Yeah.

Fraser:                         So, I guess, what’s the update because we’ve done an episode on the crisis in cosmology. So, what is the update?

Dr. Gay:                      So, the update is that the folks who are using supernovae, this is Adam Reiss’s team out of Johns Hopkins and the Space Telescope Science Institute, the more and more supernovae they look at, it is still converging at a higher value. And they have gone through and they have compared their data with cepheids and nearby galaxies with supernovae in nearby galaxies with cepheids in our galaxy, and Gaia data is allowing us to understand those cepheids as we have never understood them before.  

And Adam’s team has has figured out how to use the Hubble Telescope in ways that it really was designed for but works to make sure that they understand the photometry they’re taking like it’s never been understood before. And so, observationally, there’s nothing wrong with what he’s doing to think cepheids to supernovae – well, geometry to cepheids to supernovae and work our way out. All right. One day at a set.

Fraser:                         And so, just to clarify this, right, these are observations of type 1a supernovae, which in theory give off a set amount of radiation when they detonate. So, they act as standard candles. This is the most comprehensive survey of these supernovae to date. And it is –

Dr. Gay:                      And of the cepheids.

Fraser:                         And of the cepheids, right, at the same time, and is narrowing the error bars on both of them and they are still not overlapping.

Dr. Gay:                      No, no.

Fraser:                         They are more accurately disagreeing with each other.

Dr. Gay:                      Yes. So, then, we also have the folks that are looking at quasar timing and gravitationally lensed systems. So, if you have a large cluster of galaxies and then you have something behind that large cluster of galaxies, light that meant to go up here some where can get bent by the gravity to come down and reach us. And then light that meant to go down here can get bent up to reach us. And those paths that the light takes can cause the light to travel different distances, and it can also cause it to have multiple operations of the same distant object.

And, if that object happens to be a quasar or a quasi-stellar object with an active galactic nuclei, a black hole that is eating stuff, it can fluctuate in brightness over time. And we can use geometry to figure out the two path differences. And, then, we can use timing to see what’s going on. And, when you start using gravitational lensing of background quasars to figure out the expansion rate of the universe, it’s matching up with Reiss’s team on the supernovae. So, that data matches. Yay.

Fraser:                         Right. So, when last we saw our heroes, the supernovae timing had been refined to the quasars had been measured and their timings had been refined close to and matching essentially what the supernovae are telling us. The cepheids had also been refined and they were completely disagreeing with the supernovae and the quasars. So, what does this mean then? Is this removing or really reducing the possibility that the measurements are wrong and that there’s something more fundamental going on here?

Dr. Gay:                      It is looking more and more like there is something fundamentally wrong with how we’re interpreting the Planck data of the cosmological background radiation.

Fraser:                         Right. And that’s the measurement done by Planck to – incredibly precise measurement that gives us a different number than the cepheid supernovae quasars. And you think about it, cepheids are close. Supernovae are in between. Gravitational lens quasars are really far. That gives you a giant chunk of time. That gives you what, 10-plus billion years. And that’s all on one side. And, then, you’ve got the cosmic microwave background radiation, which is the first few hundred thousand years after the big bang. And it’s those two numbers that disagree with each other.

Dr. Gay:                      So, this is where it gets interesting because we are directly measuring the expansion rate by looking at supernovae, by looking at quasars, by looking at cepheids that are distributed through space-time. With the understanding of the cosmic microwave background, what we’re doing is we’re looking at the point – I believe it’s .003 difference in temperature, which reflects density in the CMB and the size of those differences. And these differences arise from acoustic waves moving through the early universe.

And the way they interacted led to a what we call spectrum of spots, where you see a bunch of spots of one size, bunch of spots of another size, and so on through the spectrum. And the spectrum in our models and the spectrum we see beautifully match. Then you take this measurement of the hotspots, and you’re like, “Okay, that’s dense.” Those are going to become galaxies and clusters of different distributions. And you look at the coldspots and you say, “These are going to become emptiness.” And you project your model forward and you measure right now what is the distribution of the large-scale structure of the universe, in the walls and voids that we see.

You have two measurements. You assume from making other measurements what the geometry of space-time is. We believe the geometry is flat. And that’s backed up by multiple sets of evidence. And, when you run from cosmic microwave background at T=400,000ish to modern-day universe and you figure out how the expansion rate has evolved to get from that to this, you end up with just a slightly slower expansion rate.

But to do that there is a whole lot of assumptions in the physics going on. And we know that the physics is weird because within the first few minutes of the universe, there was a epic of massive inflation that blew our universe up from super tiny to solar system size. We know weird stuff happens. And there is this new idea that there was early dark energy. This is a terrible name, terrible, terrible name. There was early in the universe some factor that rushed the expansion and then stopped.

Fraser:                         But this is post-inflation.

Dr. Gay:                      Yeah.

Fraser:                         This is after the cosmic microwave background radiation was released but before the first quasars, the first really massive galaxies could form. There was this weird speed up and then slow down. Could it have been that just inflation carried on at an ever-decreasing rate until it stopped inflating the universe even after the cosmic microwave background?

Dr. Gay:                      So, saying slowdown is kind of – this is calculus.

Fraser:                         It’s less acceleration.

Dr. Gay:                      Yes. So, it’s a second derivative issue. So, it’s like you put your foot on the accelerator. You go from 30 to 50. You pull back on the accelerator, and, in the same amount of time, you go from 50 to 60. So, you’re still accelerating, but your acceleration is slowing down.

Fraser:                         Right. So, what I’m saying, though, is in the beginning of the universe, that expansion due to inflation is ludacris. The universe was something like the size of a volleyball, and then it was bigger by a factor of – I forget what it is.

Dr. Gay:                      Whatever the size of the solar system is.

Fraser:                         Yeah. Some enormous version of that. And that expansion happened in a fraction of a second. It happened in an incredibly short period of time. I’m gonna use all these very generic terms because I don’t have them in my head right now. But then it was still growing, but, now, it wasn’t growing as quickly –

Dr. Gay:                      Yes.

Fraser:                         – as it was in the beginning. Okay, right.

Dr. Gay:                      And, then, it was during this period that we can’t see because of the cosmic microwave background is opaque, that it’s thought that early dark energy played a role so that the universe wasn’t the size we thought it was when we’re looking at the cosmic microwave background.

Fraser:                         So, Pamela.

Dr. Gay:                      Yes.

Fraser:                         What does this mean?

Dr. Gay:                      It means that there are good folks running interesting models that are finding that cold, dark matter with dark energy doesn’t match the expansion rate of the universe as well as a model that adds early dark energy to the mix. And, as I wrote one of my friends this morning when she asked how I was, I think we know less about cosmology now than we thought when I did my dissertation, and I’m glad I graduated when I did.

Fraser:                         Wow, this is exciting then. This is good news.

Dr. Gay:                      It is. It is.

Fraser:                         It’s always better to know less. It’s awesome.

Dr. Gay:                      Yeah, yeah.

Fraser:                         There’s an analogy. When you work on a large project, you go with this naïve enthusiasm for the project. And you get into it. And there becomes this moment of clarity when you’re like, “Oh, now I know what I didn’t know before, which is how much I don’t know yet.” But you didn’t even know that in the beginning, and you get to the point where – and that – it feels to me like that’s the halfway point. And, then, the sort of the 90 percent point is when you now know all the stuff that’s left, and now you just have to do the work. But it still takes half of the time.  

So, the halfway project is when you finally understand what’s left to do. And we’re at the point now, where at least we know what we don’t know what we don’t know, which is exciting, as opposed to not knowing it. I’m sure none of that’s making any sense. But how will we get an answer to this then? How will we sort this out?

Dr. Gay:                      That is always the question. We need simultaneously both improvements in our understanding of the cosmic microwave background distribution, which we need for a variety of other reasons. If we can get high enough detailed observations, we can start to rule out certain geometries of space-time because we either will or won’t see the same features mirrored on opposite parts of the sky, which means light is going all the way around. We’re seeing in from the front and the back.

Fraser:                         But, if I understand this correctly, we pretty much can’t make a more accurate measurement of the cosmic microwave background. We’re literally – we can measure other factors in it like the polarization of the light, but you can’t just measure those temperature changes more precisely.  

We’re at Max – and Planck sort of closed the book on that portion of it, so now the polarization is gonna matter and all other – which will tell us essentially which way these blobs were turning in the early universe, which is very interesting and give us a sense of their interactions and give us a sense of whether there was gravitational waves and primordial gravitational waves and so on and so forth. But we’re gonna have to measure that in-between time, right? We really need that time from between 0 and about 10 billion years ago.

Dr. Gay:                      And this is where folks have hope for what was previously called the WFIRST and is now called the –

Fraser:                         The Nancy Grace Roman.

Dr. Gay:                      Thank you. I wanted to say Vera Rubin because I am in love with that telescope.

Fraser:                         Well, Vera Rubin will also help, so –

Dr. Gay:                      It will also help but not as much, not as much. The Nancy Grace Roman Telescope is really our next great hope for understanding dark energy. And, of course, there is the JWST always lingering in the background as Schrödinger’s telescope that either will or won’t solve all of our problems for us.

Fraser:                         Yeah, but it only does it – it’ll solve specific problems. James Webb is when you need to call – when you got something you need fixed, you bring in James Webb to take a good, close look at it. But Nancy Grace Roman is also in the infrared –

Dr. Gay:                      It’s a workhorse.

Fraser:                         – but it’s gonna be observing vast swaths of the sky, huge chunks. As with Vera Rubin, it is gonna be able to see big pieces of the sky. So, how will – and, ahh, I’m trapped in Pamela’s paradox now. We can’t talk about the Nancy Grace Roman Telescope because it hasn’t launched yet.

Dr. Gay:                      Exactly.

Fraser:                         And yet, in theory, it would be really interesting to know how it’s supposed to work. But how can we do that? Anyway –

Dr. Gay:                      Yeah, but there’s W bosons launch to discuss and we haven’t got there yet.

Fraser:                         Yeah. Well, that’s gonna have to be a whole other show because we’re running out of time for this one.

Dr. Gay:                      All right.

Fraser:                         Yeah. But the point being, all of these concepts, the Hubble constant, dark energy, dark matter, their interactions, their proportion of the universe, they’re all tied up together in this job of measuring the expansion rate of the universe at different times. And there is a giant gap that is about to get filled thanks to Nancy Grace Roman. But let’s imagine – okay, so fine – so, let’s imagine that you wanted to launch a Hubble-scale telescope. So, how does it, like – is it like the same size as the Hubble Space Telescope but it was all –

Dr. Gay:                      Like WFIRST.

Fraser:                         But was it infrared? But WFIRST doesn’t exist because it hasn’t launched yet. But I’m sort of envisioning a hypothetical space telescope that has infrared capability. And let’s say you launch it in say about 2026 or so that could do these surveys. How would it help in this project?

Dr. Gay:                      It’s going to give us a larger perspective on all the little things that we’re looking for, the discovering the supernova, the measuring the gravitationally lensed quasars, the looking for – Wendy Freedman is also out observing massive red stars that flare. And it’s going to give us this systematic understanding of also the distribution of the large-scale structure. So, that thing that we’re building on from the Planck observations of the CMB to our modern understanding of the structure of the universe, we’re gonna be able to get a much better understanding of how that structure has evolved over time. We started to get there with SDSS. Nancy Grace Roman is going to take us further than the Sloan Digital Sky Survey, and its amazingness that Apache Point could ever do from the surface of our planet, so –

Fraser:                         And in infrared, which –

Dr. Gay:                      Yeah, puts us further back in time.

Fraser:                         – SDSS can’t do because it’s visible light. And so –

Dr. Gay:                      It’s on the Earth.

Fraser:                         Right. And so, Nancy Grace Roman, because it’s viewing infrared, like James Webb, we’ll be able to see the stuff that is farther and older, which is that missing point in time. And, then, if it turns up any really interesting objects, cool, early gravitationally lensed quasars, then James Webb can step in and give us a better look and measure the redshift to these specific objects with precision.

Dr. Gay:                      And just to be clear, it’s not seeking supernovae. It is surveying the sky. And, if there happen to be supernovae in there, cool, cool. It will have the capacity to uncover these duplicate quasars that can get followed up, but it’s out there measuring the large-scale structure of the universe. And that’s a pretty awesome job.

Fraser:                         Mm-hmm. Mm-hmm. Well, that was good. It’s hard to kind of wrap together what this was, what we just talked about. But there is this crisis in cosmology.

Dr. Gay:                      It is awesome.

Fraser:                         It’s awesome. As astronomers have gotten better at measuring, it has resolved itself. What were thought to be error bars that would resolve themselves have actually turned into a giant gap in our understanding of how the universe worked at the earliest times, but we don’t have an answer. But at least we finally have a question, and, I think, that is enough to keep us busy and excited for years to come. Thanks, Pamela.

Dr. Gay:                      Thank you Fraser. And, yeah, we’ve got a lot of work to do. And we can keep bringing it to you here on Astronomy Cast thanks to all of you out there on Patreon, who support us week after week and allow us to pay all of the people that keep us organized. I’m talking about Nancy, Rich, Ally. There are names I’m sure I’m forgetting – Beth. Why do I always forget Beth’s name? I am sorry Beth.

Anyways, this week I would like to thank Scott Bieber, Matthias Heyden, Nial Bruce, Disasterina, Daniel Loosli, The Lonely Sand Person, Justin Proctor, Jim Schooler, Gregory Singleton, Paul L Hayden, Jeff Wilson, Cooper, Eran Segev, Paul D Disney, Kenneth Ryan, Nate Detwiler, Allan Mohn, Steven Shewalter, Omar Del Rivero, Tim McMackin, Alex Raine, Benjamin Müller, Don Mundis, NinjaNick, Micheal Regan, Dean McDaniel, Scott Briggs, Matt Rucker, Janelle Duncan, J. AlexAnderson, Father Prax, Philip Grand, Benjamin Carryer, Mark Heather, Widick, Frode Tennebø, Anitusar, Dwight Illk, Mark Steven Rasnake, Abraham Cottrill, schercm, Bruce Amazeen, Gabriel Gauffin, Dustin A Ruoff, Brent Kreinop, Jim McGihon, Gfour184, Kimberly Rieck, Glenn McDavid, Steven Coffey, john öiseth.

And if you would like to join us and have the potential of me either pronouncing or not pronouncing your name correctly, join us over at patreon.com/astronomycast. And feel free to add pronunciation instructions to your name.

Fraser:                         Or don’t

Dr. Gay:                      I love all of you.

Fraser:                         Or don’t if you want to laugh. Thanks, Pamela. We’ll see you next week.

Dr. Gay:                      Thank you, Fraser.  

Voiceover:                  Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast 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/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 Astronomy Cast.

Back to Top