Our series on Universe weirdness continues, this time we learn how astronomers are struggling to make sense of the age of the Universe.
In this episode we mentioned donations. Click to learn more!
- How old is the universe anyway? (Popular Mechanics)
- Age of the universe (Wikipedia)
How Old is the Universe? (Space.com)
Cosmic microwave background (Wikipedia)
- Lambda Cold Dark matter models (Wikipedia)
The Formation and Evolution of Galaxies (Cosmotography)
Using Gravitational Lensing to Measure Age and Size of Universe (Universe Today)
The universe might be acting weird. Cosmic ‘lenses’ may help reveal why. (National Geographic)
Transcriptions provided by GMR Transcription Services
Hi, everyone. Producer Susie here. We greatly appreciate your support listening to Astronomy Cast, and we’d like to ask another small favor you can do to support our work. Just go over and subscribe for free to our CosmoQuest channel at www.youtube.com/c/cosmoquest. And while you’re there, subscribe to Fraser Cain’s channel, too, so you can check out his guide to space. Thank you.
Astronomy Cast, Episode 542
Weird Issues: The Age of the Universe
Fraser: Welcome to Astronomy Cast, our weekly fact-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 and with me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela. How you doing?
Pamela: I’m doing well. How are you doing?
Fraser: I’m doing great. Again, this is like a two-part episode. We’re actually recording them at the same time you are at the Planetary Science Institute working for the Planetary Science Institute near – all of the planetary science gets done in Tucson, Arizona. How are you enjoying your time there?
Pamela: It is remarkably hot.
Fraser: In October?
Pamela: It is. And there are little lizards everywhere. So, if you wish to be in the land of heat and little tiny lizards, I recommend Tucson, Arizona. But it’s been great getting to collaborate with everyone, see everyone face-to-face. I am not here long enough. There is so much interesting work to do, but we’re at least getting the face-to-face few moments to start to plan out what will happen in the coming months.
Fraser: Yeah, it’s beautiful landscape, and those desert skies if you can get a chance to get out and see them. Our series on universe weirdness continues, and this time we learn how astronomers are struggling to make sense of the age of the universe. All right, Pamela. Now, we have absolutely covered this topic several times in the past, I’m sure – how old is the universe – and I feel like last time we talked about this, we were a lot more definitive than we are today.
Pamela: This is why we are revisiting the subject.
Fraser: Exactly. Turns out, things are more complicated than we thought.
Pamela: And this has always been true with trying to figure out the age of the universe, whether you look back in history at the people who tried to figure it out from generations of begats to the scientists who tried to figure it out by looking at the total mass of the sun and figuring out how many years it could burn for given the current energy output to – well, once we figured out nuclear reactions, we started figuring out stellar evolution and figuring out well, how long should stars be able to live and what are the oldest stars that we see. And then finally, with the expansion rate, you can run that sucker backwards and figure out when our massive universe would have completely collapsed in on itself and get an age from that.
Fraser: And astronomers did the best they could using that, and so have they actually – did they get a number?
Pamela: Well, we’ve gotten lots of numbers.
Fraser: Right. But that was like one of the first ones, right? Which was like oh, stars should collapse – how long is that gonna take? And the answer is like a couple, a million, couple of million?
Pamela: Well, so what’s been lovely to watch is it started out with at about the same time that we figured our plate tectonics might need to be a thing, they also figured out that the sun needed nuclear reactions because without them, it would only live for like under 10,000 years, which is not enough time. Early stellar models looked at globular clusters and ended up with globular clusters being 15, 16 billion years old. We had quite a number of difficulties for a while because that tells us the universe has to be more than 15 or 16 billion years old because stars shouldn’t be able to exist in a universe that is younger than they are. That’s just a fundamental truth we hope.
And then with the expansion rate, when we didn’t know the expansion rate, we were able to get a universe that was quite short-lived, in the billions instead of the tens of billions of years if you just crank that Hubble constant up into the hundreds, which is where it was temporarily.
Fraser: So, I wanted to just return to those ideas for a second there. So, with the globular clusters, right? We know that all the stars in a globular cluster formed around the same time, and so astronomers literally just look for the oldest stars they can find, and that tells you – and so, if you find 13 billion year-old stars or 14 billion year-old stars, therefore the universe has got to be older than that age because all of the young ones have all died.
Pamela: And we try and make it even simpler than that. Stars go through a variety of different phases in their evolution, and the first and longest phase for the majority of stars is a hydrogen burning phase deep in their core. This is the main sequence part that stars like our own sun and most other stars go through. Some of the bigger ones have a slightly different process. We’re gonna ignore them. When we look at a globular cluster, we can look to see what are the most massive stars that are still undergoing hydrogen burning in their core?
And the physics isn’t that terribly complicated conceptually. It’s always in the details that things get messy. It isn’t that terribly difficult to figure out okay, so you have hydrogen burning going on. We can figure out bits and pieces of how much mixing might be occurring, and how long can these stars possibly burn hydrogen before they have to evolve to that next step because they’re out of hydrogen in their cores.
Pamela: And even with that much simplified version of only having to figure out well, how long have things been around that have run out of hydrogen, we were still running into problems of globular clusters could be like 15 billion years old or more.
Fraser: Right. But look, the analogy is like you talk to a school and you find out what reunions are coming up for that school. And so, you’re going to have the 35th reunion, you’re gonna have the 25th reunion, you’re gonna have the 15th reunion. But if the school was only created 50 years ago, you’re not gonna have the 55th reunion, right?
Fraser: And so, you can tell – you can just sense how old that school is and therefore you can tell how old that globular cluster is. So, that’s the one way. Great. At least that gets you to within the – to the nearest 10 billion years. But the other method – and this one seems to make sense, right – is that we look out in all directions and we see these stars and these galaxies moving away from us. Run the clock backward, we should be able to calculate when they were all together smershed into a region of density that would tell us that was probably the beginning of the universe.
Pamela: And just to state something that we stated so many times before. They didn’t all smoosh down into the center of the universe. There is no such thing in the universe.
Fraser: Did you hear? I even was very careful. Did you hear I said that?
Pamela: I know. I know. You were so good.
Fraser: I know.
Pamela: And so here, what we’re looking at is if every one megaparsec of space, as we talked about in the last episode, is expanding by somewhere between 65-75 km/second, well you can take that one megaparsec of space and figure out when it was the size of an atom. And that is, quite simply, how we get at the age of the universe.
Fraser: Yeah. The way I sort of have come to describe this is that you imagine the universe today as this grid, like a three-dimensional grid that goes on in all directions forever – maybe forever. And so, over time, the squares, the cubes in that grid, are getting bigger. They’re getting less dense. And so, you could run that clock backwards all the way back to the beginning. Now, it might still have been a series of cubes – this grid that goes on forever. It’s just that it was way denser in the past than it is today.
But, it might have been infinite back in the beginning, but what we’re wondering is at what point were the gaps in that grid – from what are today, as you say, millions of lightyears apart – at what point were they all essentially really tightly packed together?
Pamela: How many times can you put the graph paper on the reduce setting on the Xerox machine?
Fraser: Yeah, exactly, exactly. And so, that feels like a relatively straightforward, newtonian physics question. You calculate the velocity, the time. That tells you – you know the distance, you know the velocity – that tells you the time.
Pamela: Right. And where it gets messy is if the expansion rate is a constant, you just math it. The age is equal to one over the expansion rate. But if that expansion rate isn’t constant, you now have to start figuring out how is it not constant? What factors are making it not constant? And this is where it gets really ugly because we having to take into consideration the changing mass density of the universe. We have to start taking into consideration also the geometry of the universe. That has always been a concern you have to figure out. Because if the geometry isn’t flat, everything goes slightly sideways.
And looking at all of this, we were able to get – even with the accelerating universe – a universe that appeared to be 13.8 billion years old, and stellar evolution was giving as globular clusters that were 11-12 billion years old – all good so far. We can also – another way we can judge the age of the universe is you look around for cold, white dwarf stars. And we know – we think, roughly – how long it takes a star to evolve all the way from being a main sequence star like our sun to becoming a red giant and doing the red giant thing all over the HR diagram to puffing out its outer atmosphere and leaving behind that hot core that we call a white dwarf star.
And over time, that hot core – because it’s not generating any of its own energy – it’s just gonna radiate heat into space and cool off. By looking for the coolest white dwarfs we can find, that also gives us a limit on the age of the universe. And we’re back to the point of – for a while, it looked like the stars were all older than the universe.
Fraser: Right. And then there’s this gift that the farthest thing we can see in all directions, the cosmic microwave background radiation, is red shifted, right? And you can essentially calculate how – when you know what color that light should have been when it was originally released, you look at the color that it is today – again, relatively straightforward math to calculate how long it has been expanding to help you – and that’s how we got the most accurate measurements of all, thanks to Plank.
Pamela: Right. And it was more complication than just looking at the temperature. But, all of these things should give us one age.
Fraser: Yes. Please.
Pamela: Or at least all of the stars should be younger than those Plank ages. And this is where things are currently veklempt for lack of a better word.
Fraser: It got weird.
Pamela: They went sideways. When we look at the expansion rate of the universe, we’re still getting like 13-point-something-ish close, rounds to 14. But, when we look at our lambda cold dark matter models, and we use the cosmic microwave background measurements from Plank, and also when we use models that we didn’t quite get to in the last episode where we’re looking at gravitationally lensed images of distant quasars as another way to get distances across the universe, this starts to get us to a younger universe that has a faster expansion rate of perhaps 72 km/second megaparsec.
And that starts to give us a universe that’s like 11 billion years old, and that is younger than the globular clusters, which is problematic, which is where we start going “oh, expletive, what is wrong with the stars?” And what’s amazing is stars are not a solved problem. So, we recently figured out a breakthrough with white dwarfs of “oh, wait – we need to take into consideration how their structure treats energy differently because they’re crystalline in nature.” And so, crystals are going to radiate heat differently, go through phase transitions. The phase transitions take up energy, and that changes the cooling models.
Fraser: So, in theory, you find the oldest possible white dwarf, you know how long it takes you to cool down, and so that tells you when that white dwarf probably formed, and you are going to be within a few tens of millions of years of when the universe itself formed. Except that figuring out how a star’s mass of crystalline carbon, the largest diamond ever, takes to cool down, and all of the different layers and all of the different stuff that’s going on there turned out to be a little more complicated than anyone was expecting. Or they were all expecting it to be incredibly complicated, and nature was happy to oblige.
Pamela: Yeah. And so, what we end up with is when you look at stars, when you look at globular clusters, when you look at white dwarf cooling models, when you look at all these different things, you end up with the universe must be older than 12.9 billion years. When you look at Plank data, you end up with the universe – and when you also look at – sorry, when you look at the gravitational lensing of galaxies, you end up with the universe is like 11-something billion years. And all these different ages mean that we’re doing something fundamentally wrong and we don’t know what.
Fraser: Right. Right. And so, then again, as it sort of relates to the question that I asked in the last episode, right? What could it probably be? Because like again, up until about two years ago, we said with confidence that the universe is 13.8 billion years old. And we said that with confidence thanks to really the Plank data and the most accurate possible measurements of the background temperature of the universe. And as I said, that tells you, you know, you calculate the red shift, and that tells you how long it’s been expanding for.
So, why has this been thrown into dispute now? Why are we no longer saying it’s this precise number? What other measurement of the age of the universe has become so accurate to make it now confusing?
Pamela: It’s really doing that mathematical working backwards of if the expansion rate of the universe is this, then the universe was size zero at this time. And so, we’re getting 13.8 from Plank. That hasn’t changed. When we look at the geometry of space using gravitational lensing, to see different paths of light through the universe from distant quasars, that is consistent with a much shorter length of time for the universe to have been around under 12 billion years old. And that’s also where we’re getting from the expansion rate that’s measured from supernovae. And it all comes down to this stupid expansion rate. Who do you believe?
Fraser: Right. And if you could – since measuring the ages of stars and globular clusters, and measuring white dwarfs is complicated, possibly too complicated for us to get a really accurate number that we can feel really confident about, we have to rely on these other methods. And the problem is that – I mean, literally the last episode that we did really covers this – that now the measurements of the expansion today and the dark matter distribution back at the beginning of time are both accurate enough to be certain that they’re the right answer, but they don’t match.
Pamela: And so, the question becomes where all multiple places do we have bugs in our model of the universe? And this is usually a matter of everything is more complicated than we thought. We aren’t always creative enough to figure out what the universe is doing. And so, we’re in a position of well, we only just figured out hints of the complexity of white dwarf cooling models. It’s not just thermodynamics as I learned to treat it when I was an undergrad first learning this stuff. The physics coming out of the Plank models where we’re looking at the land of cold dark matter models of the universe.
Well, if the temperature structure was fuzzy with different pockets of different temperatures, that could change everything. It’s the details.
Fraser: And I know you don’t wanna talk about spacecraft that haven’t launched yet.
Pamela: That’s all right.
Fraser: But there is a telescope, a super-space telescope, that has just been packed up nice and tight, and is being sent to its launch site in South America that could help us finally answer this question, right?
Pamela: So, are you talking about the “it shall not be named” telescope?
Fraser: I’m saying the word. I’m saying the James Webb Space Telescope, due for launch in March 2021. That’s the one I’m talking about.
Pamela: So, it will help.
Fraser: Yeah. But we mentioned there was some brand new cutting edge thinking about just like that dark matter could have been fuzzy and that could provide the reason.
Pamela: Yeah. And I think we’re gonna need more than just observational breakthroughs. You can look at things all you want. But the problem is, we can’t see inside them. So, we need breakthroughs in thinking and understanding stellar evolution. We need breakthroughs in thinking in understanding lambda cold dark matter, or maybe lambda modified cold dark matter. We need breakthroughs in thinking that allow us to either accept or deny that type 1A supernovae are consistent in luminosity across the age of the universe, and all of these different things are going to require new technological breakthroughs beyond what JWUST represents.
They require new models, new ways of looking at the physics. We have no idea what dark energy is. And JWUST isn’t gonna solve that, at least as far as we know.
Fraser: Right. But JWUST is going to be able to look right out to the edge of the observable universe and watch as those first galaxies are coming together. And my understanding is that the way dark matter behaves will have an effect in the way – when it was dominant early on and it was affecting the way these galaxies were coming together – that might help provide one more insight into whether dark matter is cold, whether it’s warm, whether it’s fuzzy, whether – it will confirm various models of dark matter and throw other ones out the door.
Pamela: I think that will happen. I think it will also cause a great deal of confusion initially because we’re gonna need new creative theories. We’re already starting to find results that we can get from ground-based systems looking at the most distant forming clusters of galaxies. These are systems that formed just a few hundred million years after our universe came into being, and galaxies are already forming and going gangbusters in massive structures, which is something we hadn’t really understood could be a possibility until recently. So, we need those theoretical breakthroughs as well. And James Webb is gonna be out there going “nope, you were wrong on this. You were wrong on this. You were wrong on this.”
Fraser: But right now, there isn’t a gadget that can do that.
Pamela: Not as easily as JWUST can do it. I have to admit I am so impressed with the science that they are pushing to get from ground-based instrumentation. And the big difference between what JWUST is going to be able to do and what these ground-based systems are able to do is our stupid atmosphere blocks infrared light in many wavelengths. And so, while we can do some infrared work from the planet’s surface, we can’t do everything we’d like to do because the atmosphere gets in the way. And where JWUST is going to shine is catching these colors of light that are ultraviolet stars that have had their light redshifted all the way into the colors that we can’t just have between the surface of our planet.
So, right now, we do the best we can. And I really think we need to give props to the people who are figuring out how to do the science that JWUST was supposed to do supporting missions like TASS, and they’re doing now in this gap before we have JWUST. So, basically, I just wanna say hands off to people running the mega-earth-based telescopes. You guys keep surprising me and it’s awesome.
Fraser: Oh, yeah. It’s pretty incredible. And when the next generation come online, and it’s not gonna be long, we’ve got…
Pamela: No. LSST is under construction.
Fraser: Yeah, 2021. The extremely large telescope in 2026. The 30-meter telescope is sort of to be announced whether it ends up in Hawaii or whether it moves to the Canary Islands. And of course, the Magellan telescope comes first, 2024. So, we are – it is not long for a revolution of just astronomy in general, and they will be used.
So, I guess which observation or which experiment do you think is going to allow us to then say again, with some level of precision, how old the universe is?
Pamela: At this point, I don’t think I’m creative enough to make that kind of a prediction. I really think we need some fundamental change in how we understand what drove inflation, what is driving the expansion today, what is dark energy. If we can figure out what the heck dark energy is, I think a lot of other things are gonna fall into place. But I don’t know if that answer is going to come from particle physics, come from observational cosmology, and so it could be that it comes to us from CERN or it could be that it comes to us from a telescope. And I don’t know which.
Fraser: Yeah. So, someone’s gonna figure out how to really actually measure the age of cooling white dwarf stars. Someone’s gonna figure out how to really accurately measure the age of globular clusters. Somebody is going to figure out how to track the changing expansion rate of the universe, or somebody’s going to figure out how to really accurately measure or predict what the right form of dark matter should do to explain what we see in the cosmic microwave background radiation.
And one of those is gonna be the one that astronomers are gonna mostly agree on, and that one will give us the most accurate measurement of the age of the universe that we can hope for for now, until they disagree again.
Pamela: And until then, we have from the Plank collaboration and expansion rate of roughly 67 km/second/megaparsec, and a 13.8-billionish-year-old universe. And from everything else, we’re looking at well greater than 70 km/second expansion rate to the universe, and a universe younger than some of the stars.
Fraser: There we go. Enjoy. As always, right? We are in the middle of these discoveries. And if this feels frustrating to you, then you have no sense of adventure.
Pamela: It’s true.
Fraser: It’s fun to watch the discoveries pile up to see the conversations go on. And just enjoy the journey. Stop being so inpatient. This is fun.
Pamela: This is why we science because we don’t know everything.
Fraser: Because we don’t know, yeah. All right, Pamela. Have you got some names for this week?
Pamela: I do. I would like to thank Brett Peterman, Jason Graham, Bart Flaridy, Kenneth Ryan, Russell Peto, Martin Dawson, Paul Wheeler, Dan Letman, Glen McDavid Dean, Shannon Humbard, Dwayne Isaac, Kristin Brooks, Ryan James, Skiela Penflico, Eric Ferringer, Darkel Daniels, Rachel Frye, Gregory W. Joiner, Brandon Wolverton, Ron Thorson, and Mathias Hayes.
Fraser: Thank you, everyone. And Pamela, we’ll see you next week.
Pamela: Sounds great, Fraser.
Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @astronomycast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle, and the show was edited by Susie Murph.
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Duration: 30 minutes