Just when the Universe was starting to make sense, the cosmos throws a curveball at us. Astronomers have been trying to accurately measure the expansion rate of the Universe as far back as Hubble. It’s been tough to nail down, and now astronomers are starting to figure out why.
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Accelerating expansion of the universe (Wikipedia)
Cosmologists Debate How Fast the Universe Is Expanding (Quantam Magazine)
- Dark Energy (Wikipedia)
- Type 1A Supernova (Wikipedia)
- Lambda Cold Dark Matter Model (Wikipedia)
- Hubble Constant (Space.com)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast episode 541. Weird issues: The expansion rate of the universe.
Welcome to Astronomy Cast for 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, publisher of Universe Today. With me as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hi Pamela, how you doing?
Pamela: I’m doing well. I’m actually at the Planetary Science Institute this week, and it has been an amazing adventure, but it also means the audio is going to sound a little bit more hollow today, because I got to borrow of friend’s corner office and it is awesome, but a little hollow sounding.
Fraser: Not as soundproofed as the home office that you normally record in.
Pamela: No. That’s true.
Fraser: Yep. Just when the universe was starting to make sense, the cosmos throws a curveball at us. Astronomers have been trying to accurately measure the expansion rate of the universe as far back as Hubble. It’s been tough to nail down, and now astronomers are starting to figure out why.
All right, Pamela, until this – I mean why is the expansion rate of the universe important to know?
Pamela: It’s one of those things that really defines what is the likelihood that different blobs of matter are going to have a chance to come together. It is the thing that defined what was the density of material when stars were ready to form. It is the thing that will eventually define does our universe tear itself apart or collapse back in on itself.
So by understanding the expansion rate along with other factors like the geometry of the universe, which is not a controversy, the mass density of the universe, which we think clearly handle on, the expansion rate is that thing that helps us to find the evolution of our universe.
Fraser: I mean you can kind of imagine – I love this idea, whether it was Hubble or some other people were working on this idea as well, where you observe these galaxies in all directions, you detect the red shift, and you realize that in every direction that you’re looking, galaxies are moving away from us.
And then you immediately realize like you’ve got to be like either everyone is afraid of us or the entire universe is expanding, and that means that it was once in a smaller location backwards in time and could have a definitive point.
So, was this like a – I don’t know, like a revelation? This must’ve been a big deal when it all came together.
Pamela: It was definitely a moment of what the – universe, you’re not supposed to act that way. It –
Fraser: Because it’s always just like if it’s always and forever, right?
Pamela: Yeah. So, back when these observations were first made by Vesto Slipher, he was the one that really got us into this mess, it was thought that the universe was a steady-state creation where the size of the universe was what it is and always had been and always would be, and if you look at it the galaxies, you should see random motions.
And it was off of this study-state idea that Albert Einstein stayed the universe in his equations so that it neither contracted nor expanded but instead just sat there going I’m a universe.
But when Vesto Slipher first looked out and started doing spectroscopy of galaxies and measuring their motions in the sky, we didn’t at that point know how to measure distances. But he expected to get a distribution of motions where some are coming towards us, some are going away from us with a variety of different speeds.
And what he found his yeah, there’s a few that are coming towards us, Andromeda, we’re looking at you, but the majority of that 20 some odd systems that he looked at were moving away, and this is not something that made any sense, and this was data that Hubble took and with the work of Milton Humason who did a lot of the observations for Hubble, they combined their red shift determinations. That measure of how fast things are moving away from us with Henrietta Lovette’s work that allows us to measure distances two things relatively using Cepheid pulsating variable stars.
And he was able to make that first measurement of this is how far away the galaxies are compared to one another. And this is there motion compared to one another. And the further they are away, the pastor they’re going.
Fraser: And so how do astronomers define this? Like if you say – if you ask an astronomer how fast is the universe expanding, what is the expansion rate of the universe? How do they describe this expansion rate?
Pamela: Well, like so many things in the universe, since we can’t see the whole darn universe, we don’t say the universe is expanding at and give the rate the entire universe is expanding at. We instead break it down into manageable pieces.
And in this case, the manageable pieces are still really large. And what we say is the universe for every megaparsec of space that you have, that megaparsec is getting bigger by the expansion rate every second.
And that expansion rate is where we get ourselves into trouble, but its value has generally range from 0 to 1000 kilometers per second, depending on who took the measurements. So, it’s a 60, 70 kilometers per second for every megaparsec of space.
Fraser: Okay. So, let me just sort of see if I understand this. So, you’ve got a megaparsec, and that’s a gigantic chunk of space, right? A megaparsec is hundreds of thousands of light years? Million –
Pamela: Mega is millions.
Fraser: Millions of light-years? Yeah. Like a parsec’s like three light years and so, you know, a lot. And so every second that – you know, if you’re one side of that – a box that defines a megaparsec, and then someone else is on the other side of that box that defines the megaparsec, one second later, you are now roughly 70 kilometers farther away from each other.
Fraser: And then one second later, you’re now 140 kilometers away from each other. A billion years later, things have really moved.
Pamela: And it adds up. So, if you have 3 megaparsecs between you and your friend, first of all you’re never speaking to them, second of all, that 3 mark megaparsecs, it’s expanding apart at 210 kilometers per second. If you have four megaparsecs, you’re now at 280 kilometers per second. And it adds up, so that when we are looking out at the further stages of our universe, we’re seeing things that are trying to accelerate away from us at the speed of light.
And it’s not that they’re moving faster than the speed of light, it’s that all these different individual megaparsecs are all getting bigger, and it’s pushing things out of the way as it does it.
Fraser: And I’m glad you made that clarification at the beginning where, you know, I said like how fast is universe really kinda getting bigger, and I think, you know, we imagine if someone is inflating a balloon or inflating a bubble, you can imagine that bubble just getting bigger and bigger and bigger, and you could measure, you know, the bubble had a diameter of this number the second, and then one second later, the bubble has a diameter at that second, and those numbers are bigger.
But if the universe is infinite, you can’t say that it is getting bigger, because it’s forever before, and it’s still forever.
Fraser: And even if the universe is finite, we don’t know how big the finite is. So, it just doesn’t make any sense to say here’s how big this ball that you’re imagining in your mind, and I know you all are, that you can’t rely on that. And so that’s why astronomers just go let’s just measure the things that we can measure, which is smaller chunks of the universe.
Pamela: And this is what we do. And because we can see things multiple megaparsecs away, we’re able to systematically say yes, we see this change.
Now where it gets trickstery is while we have the ability to see several megaparsecs away, we aren’t always so good at measuring exactly how many megaparsecs things our way, and this has caused no end of difficulty across the decades.
Fraser: Right. And so you go back to Hubble’s original measurement, the Hubble constant, which really defines this, and what’s this? We’re closing in on 100 years –
Fraser: – since Hubble made these measurements? And astronomers still argue over what the Hubble constant is today. And in fact, you know, part of the reason why this is on this – we’re in this weird issues place is because it is something really complicated to try to understand.
So, can you just explain the challenge and why it’s become so complicated?
Pamela: And it all comes down to trying to measure distances, and space isn’t a place where we can just casually go out with the ruler and measure from point A to point B. It’s not some place where we can consistently go out and use radar to measure distances.
We can get away with that with asteroids and other planets occasionally, but in order to measure the distance to other stars, we really have to use parallax measurements. This is where we wait for the earth to be on one side of the sun, we measure a star against background galaxies, we wait for the earth to come around to the other side of the sun, we measure the position of the star against background galaxies, and our motion from one side to the other will cause the star to appear to move relative to this background object.
So, this is the kind of thing that you can experience for yourself just by holding your thumb out at arms length and blinking between two eyes and seen how your thumb appears to move against background trees, mountains, whatever.
Fraser: Yeah. Or like when you’re driving in a car really quickly and the trees are going past you while the mountains aren’t?
Pamela: Exactly. And this parallax effect requires objects to either be fairly close so you can see this jump or requires us to have extremely high-resolution detectors.
Now Cepheid variable stars, while wonderful in many ways, do not live near the planet Earth, and so we haven’t been able to get high quality parallax measurements of them until fairly recently. And it’s the debates we have had in trying to figure out the distance to the nearest Cepheid that has led to a lot of the squabbling in our community.
Now we have Gaia. Gaia is doing marvelous work. New methods of using the Hubble Space Telescope have been devised, and people are finally starting to get results that we think are accurate within a couple of percent for this first indicator that we use for measuring distances to galaxies.
So, once we can measure accurately the distance to the nearest Cepheids, that gives us the well, 0 point of our ruler.
Fraser: Right. This is a ladder, right? We’re building a ruler where the first piece of it is this parallax measurement that allows us to measure the stars out to – to what? Like the nearest 2,000 light years away?
Fraser: And then it’s that amazing relationship with Cepheid variables, how they oscillate tells you their intrinsic brightness, and then you can use that to tell how far away they are, and then that led to ladder up to now – because you can use Cepheid variables in Andromeda, and you can see them in other galaxies far away.
Pamela: And up until our recent spacecraft measurements, we could always say what was closer and what was further based on apparent brightness, the same way you can look out in a field of lightning bugs and know which lightning bugs are nearby and which are far away based on how bright they appear. Now it’s cool to know –
Fraser: Because you know how bright a lightning bug is.
Pamela: Well, you don’t even have to know accurately how bright lightning bugs are, you just need to know all lightning bugs are giving off basically the same amount of light. And as long as all lightning bugs are giving off the same amount of light, if you see one that’s faint, it’s far, if you see one that’s bright, watch out.
And so we didn’t even have to know exactly how bright Cepheids are to know the relative distances to galaxies. And this is where Hubble was able to get his first distance concept of I know this one is closer, I know this one is farther, but then accurately figured out exactly how much closer, exactly how much farther, this has been the stuff of legendary battles in the field of astronomy.
Fraser: Right. But of course, in 1998 with the discovery of dark energy, it turns out the expansion rate of the universe is changing over time. So, it’s not just that you have to make the most accurate possible measurement; you literally have to then figure out a way to measure distances at different points in time.
Pamela: So, we went from multiple decades of is it 50 or is it 100, because basically two different individuals, Alan Sandage and Gerard de Vaucouleurs were steadfastly arguing one way or the other, and you had to side with one of them or you were an evildoer.
And if you sided with the wrong one, you are still an evildoer. It was very partisan politics as far as the Hubble constant was concerned, and luckily I had instructors that were like we don’t know. Just use 100. It makes the math easier.
And then with the Cepheid key project that took place in the ‘90s with Parkas data with increasing accuracy from our spacecraft, we began to really hone in in the early ‘90s to it looks like it’s somewhere between 65 and 75. And then like you were saying, in ’98, it was just like oh expletive. As you plot things out farther and farther, that will get you more accuracy if it is a constant, because you can fit the line better if you have a longer line that you’re trying to fit.
But it wasn’t a line. It turned out that it was a slight curve, and as we went out to greater and greater distances with supernovae that were calibrated using the Cepheids, we saw this turn that could only be explained by the universe accelerated in itself apart.
Fraser: So, how much of an impact is this? Like here we are, and of course our next episode is why it’s been so difficult to talk about the age of the universe, but here we are some number of low teens billions of years into the age of the universe. Is that dark energy having a big impact on the rate that the universe is expanding apart?
Pamela: It’s not a huge impact, but –
Fraser: Like in that velocity number?
Pamela: Yeah. It’s not a huge impact on the velocity number, but it’s enough that it makes people uncomfortable and start to deeply question well, is dark energy real, because none of our particle physics models requires it or can explain it. Is this an error in our measurements? Do type 1a supernovae, which we use to calibrate dissent galaxies, are they actually all the same actual luminosity or do they come in a variety of changing luminosities or to the history of the history of the universe that causes us to measure a false acceleration? Or is there something else going on that we can’t even imagine?
So, when we see it, it raises all of these questions. And then, we’re now in an era where we can also start to measure the expansion rate of the universe kind of from first principles where we look out at the cosmic microwave background and we go okay, we know what was required to get to the cosmic microwave background. Check. We see it.
We now measure the lumps and bumps in it. The hotter places and the colder places that reflect the sound waves moving through the early universe, and we measure those, we get to the geometry of the universe.
And then we look at when we see various structures starting to appear, and we build what’s called a lambda cold dark matter model of the universe where lambda is our dark energy component. We assume a temperature gradient to dark matter that allowed galaxies to form when we see galaxies forming. It allowed clusters to form when we see clusters forming. We measure some other parameters of the universe. And all of this can be put together to get us out a Hubble value, and it doesn’t quite match.
It’s more like 72. And 72 and 68 are not the same number. But everything matched until recently within error bars. It’s only now that we are starting to get really good tiny error bars that we have hit this problem. You can go back to past episodes of the show, and I very firmly stated the age of the universe is 16.8 plus or minus 0.2 billion – and no. No. We don’t know. We don’t know the age of the universe right now.
Fraser: Yeah. It was like 13.77. It was getting really accurate. For a while there it was 13.7. We were rounding it up. And then it got a little more accurate thanks to I think Planck.
Fraser: Yeah. We upgraded it to 13.8, and now the error bars have widened again, and we’re having to go 14-ish.
Pamela: Well, and it’s not really the error bars in our measurements that are problematic, and that’s the frustrating part.
Pamela: It’s somewhere we don’t understand something fundamental, which is disturbing. As near as anyone can tell, all of our measurements of the expansion of the universe based on what we can see and measure the distance to, those are all fine.
So, we think we finally nailed the parallax measurements, got the distances to Cepheids, have confirmed with other kinds of objects, we’ve measure the distances to nearby galaxies with Cepheids, we’ve compared it using red variables, planetary nebula, all these other methods we’ve come up with to measure distances with a more or less error built in, all match nicely.
We’ve extended out using supernova, calibrating them wherever we can with all these other distance measurements. Everything seems to work. And then, we compare these measurements to early universe measurements based on cosmic microwave background and formation rates, and expansion of the universe – well, that’s what you’re calculating is the expansion of the universe, and that number doesn’t match.
And so, the question starts to become is the universe maybe a mix of temperatures early on. Is there a mix of cold and warm dark matter? Was it fuzzies the way it was put in a recent paper?
Fraser: Yeah. That’s brand-new. Isn’t it? Yeah.
Pamela: Yeah. And there’re questions about is dark matter changing? Is dark energy changing over time? More likely, it would be dark energy is the variable culprit, but we don’t know what the heck it is, so saying anything really you can’t do.
So, we’re at this point of we know there’s something fundamentally different about the universe than what we thought.
Fraser: Hm-mm. So, in the few minutes that we have left, let’s explore some of those possibilities to try to understand at least a little bit where this could be coming from. So, let’s start with I guess the expansion rate changing over time in a way that astronomers weren’t predicting
So, how would that be causing – you know, what could that look like?
Pamela: So, the expansion rate changing over time currently seems to be driven by every cubic meter of universe has a couple protons of energy within it. And as near as we can tell within recent times, that amount of energy per cubic meter has stayed constant, and so as the universe expands, you end up with more and more of this dark energy in the universe.
Don’t ask. We don’t know. We can’t explain. But this was what seems to be observed. Now there are hints that may or may not prove real, that at different times in the universe that energy per cubic meter may have changed in which case you would’ve ended up with driving factors that affected the expansion at different rates. And we know the expansion rate couldn’t have always been constant, because there was in epic of inflation that got us from nothing to solar-system sized in the first few moments after the Big Bang.
Inflation is a thing that appears to be necessary to understanding the universe. So, it isn’t unheard of to imagine that the universe has a variable expansion rate, it’s just not something we can explain.
Fraser: So, I’m just sort of imagining, before we imagined that the universe started out from the Big Bang and then it expanded, everything got farther away from each other at some set rate that was slowing down thanks to the gravity of all these things pulling them together, and it should be a fairly straightforward although very complicated measurement to make.
In 1998, that throws in dark energy. So, okay. So now, you’ve got this accelerating process. So, the universe has got its foot on the accelerator, and so things aren’t slowing down. In fact, they’re speeding up, but do some calculus, you should be able to measure that acceleration rate.
But it could be that it’s a variable, that in fact there’s something early on with dark energy that maybe made it expand more quickly and then it slowed down, and then it sped up later on.
And so, if you’ve got that one measurement early on, and you’ve got this other measurement now, and they’re both accurate, and they don’t agree, then the universe did something weird in between the times that you were able to make those measurements from the beginning of the universe to today.
Pamela: So basically, dark energy could really screw us up. And the other thing that could really screw us up is dark matter, because all –
Fraser: Right. And that was the other thing that you said, right? The temperature, the – so, can you just explain? What is that? Like cold dark matter, one dark matter, why does that matter matter?
Pamela: It’s a matter of the temperature determines how rapidly particles are moving. And if particles are moving too quickly, they are not going to settle down into politely collapsing blobs that forms stars and galaxies.
So, if you have a universe that starts out filled with hot, dark matter, it’s not going to form these beautiful halos that are necessary to collect in the material that formed the first stars, galaxies, and clusters.
Now if instead, you have extraordinarily too cold dark matter, it will collapse too fast. So, you have to have dark matter that’s just the right temperature distribution, which we still judge as cold.
And based on that cold dark matter, we’re able to get the controlled early collapse of material into halos and structures that can explain the universe as we see it.
Fraser: Now we’ve got a second episode that we are going to be coming shortly I guess for the people who are listening to this is as a podcast, it comes in a week. And it is going – it’s very connected, and is very related, and so if it feels like we haven’t fully looked at this problem from every degree, that’s okay. We’re going to spend another episode going over it from a different perspective as well. So, don’t worry. Next week, the story continues.
Pamela, before we go, do you have some names to read out?
Pamela: I do. I would like to thank our patrons on. Patreon.com/AstronomyCast. Without you, we couldn’t do any of the things we do. Thank you so much. We know that basically one in ten of you out there donate to this show, and –
Fraser: 1 in 1,000.
Pamela: Oh. You’re right. You’re right.
Pamela: Dang it. I can’t math.
Pamela: Yeah. 1 in 1,000 of you donate.
Fraser: Which is cool. I mean that allows for every 1 in 1,000 of you who donate to the show, you allow that other 1,000 people to get an education in astronomy from a PhD astrophysicist for free. So, thank you.
Pamela: And the least we can do is say thank you. And I wish we could do more. But for now, I’m going to say thank you by name.
So, thank you to Jessica Felts, Arthur Latz-Hall, Tyrone Thong, Brandon Wolverton, Jay Alex Anderson, Joshua Pearson, Omar Del Riviera, William Lauer, Jack, Jeremy Kerwin, Brian Kilby, Brent Krenop, Jill Wilkinson, Mark Stephen Resnak, Dustin A. Ralph, Kevin Nitka, Chad Collopy, Claudia Mastroianni, and Nuder Dude.
Thank you so much for everything you do, and I wish could say it to your face someday.
Fraser: All right. Thanks, Pamela. We’ll see you next week.
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Duration: 29 minutes