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Fraser Cane: We are at the winter meeting of the American Astronomical Society in Long Beach, California. We are busy blogging and reporting news. Pamela is working on the International Year of Astronomy and we have found a quiet moment to do an episode of AstronomyCast because we thought it would be very cool to do this while we’re in the same location.
We’re going to stay on topic and have a real show for you. We did a popular three-part series about the center, size and shape of the Universe about a year ago, but every good trilogy needs a fourth episode. [Laughter]
This week we look at the age of the Universe. How old is the Universe and how do we know? And how has this number changed over time as astronomers have gotten better tools and techniques? Actually, let’s have this conversation backwards. How old do we know the Universe is right now?
Dr. Pamela Gay: Thirteen point 7 plus or minus .2 billion years.
Fraser: Thirteen point 7 plus or minus .2 billion years, right. Then how does that compare to the history of knowing how old the Universe is over time?
Pamela: One of the really cool things was except for the last few oscillations zooming in on the accurate number; the Universe just keeps getting older and older and older.
There’s the always what if it’s infinitely old? We’re going to ignore infinity. People who say less than infinity in general the age just keeps getting older and they seem to have finally settled on a nice old Universe to live with it.
Fraser: So how old then did people used to think the Universe was?
Pamela: It tended to settle out to a few thousand years based on a few thousand begats.
Fraser: Right then you would have written oral history in all cultures and as far back as people could remember as far back as people could describe their existence. That was the Universe.
They all had a mythological creation myth that described their beginning. I guess it all depends on the culture and what people would think was the beginning of the Universe.
Pamela: Everything from coming up out of the Earth to… Every society had its own way of getting the original humans on the planet from some sort of a holy beginning. Then you just count generations.
Fraser: Right so then when did science have a shot at it?
Pamela: People started trying to figure out how to think about this scientifically a few hundred years ago. There were a whole variety of techniques that were put into play. Everything from if the Sun is made of coal, how long could it have lasted to well we know how salt is getting into the ocean, we know roughly how it is coming out so based on the salinity of the ocean how old is the Earth?
Fraser: I guess that tells you how old the Earth is and I guess they thought the Earth and the Universe were formed at the same time.
Pamela: Right there was no reason to believe anything different. We’ve worked on all of these problems. One of the other things that also becomes problematic is we know that different things burn at different rates.
Wood burns at a different efficiency than coal which is very different from nuclear burning. By just starting from the base assumption that the Universe is the same age as the Sun, but then not understanding how the Sun burns, you end up with a far younger Universe.
By looking at the surface of the Earth and not taking into consideration things like plate techtonics – the fact that the surface of our Earth, the oceans of our Earth are constantly changing. They are being remodeled as the surface of our Earth radically changes. You also end up with a younger Earth. It’s only in starting to understand that well we have plate techtonics.
The surface of the Earth is constantly shifting, constantly resurfacing. The oceans we have today won’t be the oceans we had yesterday. Only by understanding, and these are all last century’s discoveries, it’s only by understanding that the Sun is fueled via nuclear burning not through some chemical process that we’re able to extend out the age of our own solar system to roughly 5 billion years.
Fraser: You have to admire the scientists in the last few hundred years for even taking a crack at it. Yes, the Sun isn’t made of coal, the Sun isn’t made of wood and so your calculations are worthless. [Laughter] At least they had a shot at it and I think that’s pretty great.
Pamela: They were coming up with ages older than the begats so they were going against current thought. Anytime a scientist is willing to say I did the calculations and it doesn’t match my philosophy but I’m going to trust the calculations. That’s a step forward.
Fraser: Then when did I guess scientists have a legitimate method of attempting to calculate the age of the Universe as opposed to something that in the end was fruitless?
Pamela: There are two things. First you have to get past the Earth wasn’t created with the Universe and then it gets a lot more complicated. But the first really legitimate scientific crack at getting a legitimate number was based on radioisotope dating.
You look at a pocket of material that you believe formed coincidental with the Earth. So, grab yourself a good chunk of rock from space – a meteorite – and measure the isotopic abundances of things that undergo radioactive decay.
Make an assumption about well these materials must have formed with the solar system and we know how much decay has taken place so if you assume you had a pocket of some pure radioisotope undergoing decay and you measure how many parent and child atoms are left, you can start to age the rock.
There’s a whole bunch of different radioisotopes that can get used so you can end up with many different things all giving you the same number so that you can trust your results. You can get error bars on your results. Through radioisotope dating we start getting to a planet that is 4.5 to 5.5 billion years old, which is our current understanding.
Fraser: Right and that we know fairly well. When that was first figured out did astronomers just think well that’s it, that’s the age of the Universe? They had a sense that planets and stars were forming at different rates at different times. Some galaxies had star formation…
Pamela: Yeah, well radioisotope dating started before we really understood there were other galaxies out there. That’s a very modern concept.
One of the strangest moments I’ve ever had was Dorrit Hoffleit who unfortunately passed away a few years ago, was the oldest living female PhD and I had this conversation with her. She was almost a hundred years old and I asked her: what was the most amazing discovery during your lifetime?
I’m expecting like dark matter, expansion of the Universe, and she said that galaxies are separate island Universes. That really puts that there are people alive today who didn’t know that was the case.
Fraser: I might have mentioned this on an earlier show that my dad has an old astronomical text. It is an old planisphere actually, and it was so old that it had the Andromeda nebula listed on it.
Pamela: It’s fabulous finding these in old Celestial maps and things. So they were starting to understand that stars are stars about the time that they were getting radioisotope dating. In terms of we have temperature and color related to one another, we were starting to get a handle on the Hertzsprung-Russell diagram which basically tells us what sorts of nuclear generation is going on in the center of a star.
We have a whole episode describing different things that power stars of different masses. It’s a series we did. Once we started to get a handle on the evolution of stars and understanding that stars go from Hydrogen burning to all the way up through burning even Carbon in their cores. Once we started to understand the methods of how stars lived their lives, we could start to estimate how long it takes them to live these lives.
Looking around the Universe, we discovered there are some things that have been around a long time. We aren’t all of one generation. All because you know there are stars currently forming doesn’t mean the Earth wasn’t one of the first solar systems. Our solar system wasn’t one of the very first solar systems created. So we had to find things that we knew were older than our solar system that were associated with stars that had already lived lives far longer than our Sun’s life.
Fraser: How would you know that was the case?
Pamela: Well, the first thing you need to do is, we understand that larger mass stars evolve faster than lower mass stars. You look around and you start finding dead high mass objects. This is where white dwarfs come in handy.
Take a big star, but not too big, let it evolve. Eventually it is going to breathe off a planetary nebula and leave behind a white dwarf. Some of the old stars do this really fast and so we’d expect there to be white dwarfs even if all the stars formed at the same time that the Earth and the solar system formed.
The thing about white dwarfs is they form at extremely high temperatures and then cool off. We know the rate at which they cool so you look for cool white dwarfs. You can use these cool white dwarfs to place boundaries on age of the Universe.
Fraser: So a white dwarf of certain mass will tell you the size of the star that it was before it…
Pamela: Not so much.
Fraser: You won’t be able to tell it?
Pamela: Pretty much all white dwarfs are the same size.
Fraser: They’re all the same size but the temperature of the white dwarf will tell you when the star died.
Fraser: You can then look for the oldest white dwarf you can find.
Pamela: Exactly. The other neat thing that you can do is there is what is called a main sequence turn off fitting. Like I said, stars of different masses evolve at different rates. You look at a progression of well all of these high mass stars have died, the slightly lower mass section of stars have died and the even lower mass section of stars have died.
You start finding systems where all the stars the size of the Sun have died. With globular clusters, these are entire populations of stars orbiting galaxies like our Milky Way where all the higher mass stars have already died and so we look at them and go: Hmm, okay, so we know ten to fifteen billion years of evolution have gone on.
The problem is we don’t know the finest details of stellar evolution. We can’t exactly go in and explore the inner regions of a star.
Fraser: I’m thinking about for example I was watching the ceremonies for Remembrance Days in Canada and there was like ten World War I veterans at the ceremony in France. But then there was 300-400 World War II vets at that. Then there was a whole bunch of Korean War vets and so on.
It’s kind of like as the time goes on the older stars are the ones that are dying off. This is the same situation. The oldest globular clusters that you’re looking at it’s like the highest mass stars are gone, then the next mass stars are gone. If you can look at whatever is the highest possible mass star there then that will tell you ‘X’ amount of years have passed already.
Pamela: Then you have to start to figure out what is the nutrition of those humans, what is the ethnicity of those humans to know what they’re life expectancy is.
In the stars, we have similar complexities. We need to know the finer details of how do the different elements in the stars affect their total life. What are mass loss rates? There are a lot of fine details we’re trying go understand.
One of the great problems with astronomy as recently as 15 years ago was our understanding of stellar evolution gave us globular clusters that were 15-17 billion years old. Through other methods we kept coming up with a Universe that was 10 billion years old.
Fraser: What’s another method then? You’ve got one, boom, and plant your flag in the sand and say: “Aha! The Universe is 17 billion years old; the Universe is 20 billion years old thanks to these globular clusters.”
Pamela: Astronomers really like to have more than one method to get at every data point. We’re scientists. We like to collect matching facts. The other thing that we do is we look at the expansion of the Universe. Hubble back in the nineteen teens looked out and using initiallydata that showed that more galaxies are moving away from us than are moving toward us by significant numbers.
Hubble started figuring out what’s the distance to these galaxies. He realized that more distant galaxies are receding away from us faster than nearby ones. He was able to build up a whole picture of the Universe expanding – we did an entire episode on this.
Fraser: Right and so I can see another analogy. We’ve used this one a few times. You imagine your car and you know where it is now and you just measure backward and that tells you where it came from and you can calculate the time. It’s physics 10 [Laughter] experiment math, right?
Pamela: The only problem is that we generally start with the assumption – let’s just for the sake of making the math easy – assume that the expansion rate today matches the expansion rate in the past.
Fraser: Which we know is a bit of a mistake.
Pamela: That’s kind of a huge leap of faith. We look and say okay, Universe currently expanding at this rate. Let’s run it backward until the entire Universe collapses in on itself. That was giving us a number of roughly 10 billion years.
The thing is we didn’t know the expansion rate real accurately. When I was an undergraduate, one of the most frequently used lines was: “Well it’s somewhere between 50 and 100, use 100 it makes the math easier.” It turned out the number IS between 50 and 100. It’s 72 as near as we can tell – which is a nice convenient number.
It’s 72 kilometers per second from mega parsec of space. So grab yourself a mega parsec of the Universe, wait a second and it’s going to be 72 kilometers bigger in diameter. Without knowing accurately what the expansion rate is when you try and work backwards to how long did it take to get where it is, you can’t get an accurate solution.
As we made our estimates, we’re ending up with the Universe being the wrong size. We also sort of missed out on the whole fact that while the expansion rate is changing…
Fraser: But I think that if you’ve got one line that tells you it is 10 billion and you’ve got the stars that tell you it is 17 billion, that’s within an order of magnitude.
That’s good, right? Does it go as high at a 100 billion? [Laughter] It is a trillion? Is it a quadrillion? You’ve got something.
Pamela: And for the most part astronomers go like “we’re on the right track!” We were good with it. We knew we had issues to solve. We knew that we didn’t fully understand stellar evolution. We knew it was all complex physics.
We knew that eventually we’d be able to measure the expansion rate of the Universe as we’re able to probe larger and larger distances in those larger and larger look-back times in the Universe. We knew we’d get there.
The problem is you’re trying to explain this to the public. It’s really confusing to say to the public, well the stars are older than the Universe. They don’t really buy that and so you kind of get egg on your face when you have to explain we don’t really know how stars form in detail. We don’t know how they evolve in detail.
There are a lot of people who ask: “If you don’t know every single detail, you don’t know anything.” So they don’t want to hear any of the story. In one of those rare instances of putting all of our eggs in one basket, the astronomy community built “The Little Probe That Could”, the WMAP Cosmic Microwave Background Explorer.
In general in astronomy we invest tons of money in instruments that serve multiple purposes. Hubble Space Telescope was launched to help us identify in detail the expansion rate of the Universe. The Large Synoptic Survey Telescope that’s getting built is being justified as something that’s going to find the asteroid that could possibly destroy the Earth and map out all the asteroids. These are really multipurpose instruments. They can solve lots of different scientific problems.
The Wilkinson Microwave Anisotropy Probe had one mission. That was to in great detail map out the little hot and cold wiggles in the color in the temperature of the cosmic microwave background. The distribution in the sizes of those little changes in temperature helps us get at what exactly was the size of the Universe at the moment the cosmic microwave background was formed.
By looking at those distributions in size, we’re able to start to figure out what is the geometry of the Universe. What is the size at the moment that those parts were let loose?
We’re able to by considering the density of mass in the Universe will look back time to that moment the radiation is released. We’re able to get an extremely accurate measurement of many of the cosmological parameters that describe how our Universe is expanding, how it’s evolving, how it’s changing and how it’s eventually going to die.
We were able to ask all these fundamental parameter problems using one wall of light – the cosmic microwave background; using one little probe – the Wilkinson Microwave Anisotropy Probe. That’s what gave us the final 13.7 plus or minus point 2(!) billion year old Universe.
Fraser: In the listeners’ minds right now, they’re going to say and I’m asking: How by looking at the variations in the temperature of the cosmic microwave background radiation do you know the Universe is 13.7 billion years old? What is the method that tells you that?
Pamela: One of the things that gets that is how quickly is the Universe evolving initially? As the Universe was first formed it was very hot dense plasma. It was nothing more than light and matter constantly in contact with one another constantly scattering. Electrons and protons were de-coupled. Light couldn’t really get anywhere without hitting an electron or proton and being absorbed and re-emitted.
It was basically a lot like a hotter denser version of what’s inside of a fluorescent light bulb. Within this dense plasma waves were able to form. We call these acoustical waves. You see them in the Sun as well. Any hot dense medium, in fact any medium that behaves in a lot of ways like a liquid.
Fraser: Like boiling water on the stove.
Pamela: Well, boiling water is actually different physics but you can set up oscillations. Seismic waves propagating through the Earth is another type of acoustical wave.
Fraser: Okay, I see now.
Pamela: The wave length of the acoustic waves was a function of the density and the size of the Universe.
Fraser: So they by measuring the temperature they could calculate that how dense and how big the Universe was at that point.
Pamela: Right, using the distribution of those acoustic waves. There is a lot of really scary complicated math involved. But it is math that we know how to do.
Fraser: I’ve got a pen.
Pamela: I’m not going to do it right now, sorry. The theorists have gone through and made this wonderful prediction of: these are the distributions of sizes of the hot and cold spots that we should see. And the actual data – the error bars in the data are smaller than the typical line-width of the expected theoretical distribution. It’s just amazingly dead-on data.
One of the things that we get at with the geometry is: you expect the average size to be roughly a degree for one part of the distribution. If it is smaller it tells you the Universe is one geometry because it’s like looking at something on a saddle – you see the angles of the triangle are less than 60 degrees on an equilateral triangle if it’s on a saddle.
Whereas if you put that equilateral triangle on a circle instead, the angles on the corners of the triangle expand out greater than 60 degrees; but if it’s on a flat surface you have nice 60 degree corners. We expect the sizes to vary with the geometry of the Universe. Just by looking at the sizes we’re able to get at what is the geometry.
By looking at the distribution we’re able to start learning properties about the density. Lots of different things can be learned and we’ve put all the pieces together, plugged through all the math. Figuring out the density of the Universe is one of the coolest things that we’ve done in our lifetime in some ways.
Fraser: What impact did dark energy have on those calculations? If you knew the size of the Universe at a certain point, and then you know the size of the Universe now, or you think you know the amount of expansion that’s happened then you calculated it back. In 1998 the discovery of dark energy has kind of thrown that whole thing, right?
Pamela: Right and luckily WMAP came out after dark energy.
Fraser: So we probably would have had an incorrect [Laughter] number if we hadn’t accounted for the additional acceleration coming from dark energy WMAP that math would have then been wrong.
Pamela: It wouldn’t have been as huge an error as you might worry because in the early moments of the Universe we were dominated by matter. It’s only as we expand more and more and more that dark energy is starting to dominate how our Universe’s size is changing over time.
So, yeah, it would have introduced error but it wouldn’t have been a devastating result. Luckily we figured out dark energy first.
Fraser: Right and then could account for that as well knowing what kind of a change you need in the velocity of the expansion and how that would have changed over time.
Pamela: And how that affects our densities and everything else. It’s all the different cosmological parameters. There’s a really good book – you’re the one who pointed it out to me – Six Basic Numbers?
We’ll link to it in the show notes. There’s a really good book that we’re both blanking on because we’re face-to-face which…
Fraser: We don’t have our internets. [Laughter] No research! No Wikipedia! Six Numbers – yeah.
Pamela: We’ll put a link to it in show notes. It goes through and documents all these different numbers that help us understand the way our Universe is evolving. It’s really kind of amazing to see how far we’ve come from 6,000 years based on a bunch of begats to order of 62 million years based on well we know how much salt is going into the ocean, we know how much salt is coming out of the ocean.
We know the current salinity of the ocean so we come up with the 62 million years to well we’re not quite sure but certainly not a billion years based on what the Sun is burning to realize oh, nuclear burning. It’s good. Then we get on to radioisotope dating of the planet getting us to 4ish billion years to radioisotope dating for meteorites getting us out to 5ish billion years. With aging of stars we have our stellar evolution models that can get us out to unfortunately15 billion years. We’re still working on those.
Fraser: Stars are still older than the Universe. We don’t worry about that.
Pamela: Yeah, we’re pretty sure we know where the problems are. It’s called mass loss that’s an issue. I use white dwarf cooling. We use evolution models too. Now we’re using the parameters that define the shape of space.
The Universe has finally stopped getting older except for the rate at which it’s aging. So, now we’re roughly 27 minutes older than we were when we began this show. [Laughter]
Fraser: It’s amazing that scientists can put such a definitive number down because it’s only been 4 years now I think that that number was known.
So if we’d had this conversation 4 or 5 years ago we would had said it’s somewhere between 10 and 20 billion years old. We don’t know exactly. And now we can give you a very precise number.
Pamela: I do miss the days of: “children, use 100 it makes the math easier.”
Fraser: Now it is children, use 72. I know it makes the math hard but suffer.
Pamela: You still tend to use 70 a lot.
Fraser: Alright, thanks Pamela and as you said, we’re going to try and record some more episodes while were here but hopefully this one reflects that we’re in the same room together, seeing each other as we have the conversation. I don’t know when we’re going to talk next.
Pamela: Probably over dinner. [Laughter]