The temperature of the Universe can vary a dramatic amount from the hot cores of stars to the vast cold emptiness of deep space. What’s the temperature of the Universe now, and what will it be in the future?
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast, Episode 415: The Temperature of the Universe.
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. And with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmoquest. Hi, Pamela; how you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Good. How was Columbia?
Dr. Pamela Gay: It was – so I was in the city of Medellin, and it was an absolutely fabulous trip. Medellin is a city that people are working very hard to be very proud of. It is a city that is up and coming; that they’re putting in new metro; they’re putting in new gondola service up the sides of the mountains; they are building fabulous museums. And it felt kind of like going to a country five years after the war ended, where everyone is proud to get to rebuild the nation into what they want it to be. And the Communicating
Astronomy with the Public meeting is an absolutely amazing meeting that we had; I want to say, 23 different nations, 25 different nations worth of people. And it was cool.
Fraser Cain: Awesome.
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Fraser Cain: The temperature of the universe can vary a dramatic amount from the hot cores of stars to the vast, cold emptiness of deep space. What’s the temperature of the universe now? And what will it be in the future? Alright Pamela, what’s the temperature in the universe right now?
Dr. Pamela Gay: It is 2.725 degrees above absolute zero.
Fraser Cain: Do you say that? Do you say “degrees above absolute zero”? Whenever I would write an article, and I would say, “It’s degrees Kelvin” the pedants would pop out of the woodwork and say, “You can’t say degrees Kelvin.”
Dr. Pamela Gay: So, you don’t say degrees Kelvin. But I said, “Degrees above absolute zero.” And because Celsius and Kelvin are the same size, I can get away with that. So more correctly would have been to say, “It’s 2.7ish degrees Celsius above absolute zero.” So, it’s like five degrees above average in Austin right now, or something. It’s nomenclature gobbledygook. It was –
Fraser Cain: I would love to have that philosophical conversation with these pedants, and kind of go, “Is that how we’re supposed to say it then? It’s 2.7 degrees centigrade above absolute zero Kelvin?” Anyway, we’re not gonna go down that road today. We’re going to the – just don’t even think about it.
Dr. Pamela Gay: What I love is, I say Celsius, you say centigrade, when we mean the exact same thing.
Fraser Cain: Oh, we’re gonna get so many emails, even on that!
Dr. Pamela Gay: But they all mean the same thing! So, it’s okay. Just everyone, be chill; calm.
Fraser Cain: Okay, alright. Okay, so what I guess – so that’s the temperature of the universe right now; but actually, the temperature of the universe can vary a tremendous amount depending on where you are, and how far away you are from any kind of source of –
Dr. Pamela Gay: And when you are.
Fraser Cain: Yeah, and when you are! Alright, so let’s sort of find the hottest place we can possibly imagine. What do you think is the hottest place in the entire universe?
Dr. Pamela Gay: Oh. The hottest thing – the inside of a newly formed neutron star. This is where they have just finished – all of the protons and electrons happily combined; they gave off a blast of extra light; radiation particle-y bits; and those electrons and protons combined into neutrons. This occurs when a white dwarf achieves a specific size; and it happens via supernova. It can also happen when a very large star collapses at the end of its life. So at this moment that the neutron star is formed, it is 99, 999,999,726 degrees Celsius.
Fraser Cain: That is – that is very, very hot. I’ve actually heard that the Large Hadron Collider and various fusion reactors can even get hotter.
Dr. Pamela Gay: Got hotter, yes. Yeah, they do.
Fraser Cain: So you can say that the hottest place on the universe is at CERN; but obviously, except for the alien super-mega particle accelerators, right?
Dr. Pamela Gay: So when they collide lead ions at CERN – you have to be specifically the high mass of the lead, the high speed of the collider – when they combine lead ions in CERN, they get to five trillion five hundred billion-ish degrees Celsius.
Fraser Cain: Okay. So that is the hottest temperature in the universe and of course, you know, until the aliens send us some kind of screenshot of their supercollider accelerating, we take the record for the hottest temperature in the entire universe.
Dr. Pamela Gay: It’s true.
Fraser Cain: So to take that supernova –
Dr. Pamela Gay: Well, what’s kind of amazing is that we’re able to get a couple orders of magnitude hotter than the universe gets in its day to day activities.
Fraser Cain: Even like, shortly after the Big Bang.
Dr. Pamela Gay: Well, shortly after the Big Bang, there we’re looking at – I had to actually look up the words, because once you get past a certain number of zeros I have to start looking it up – it was roughly one octillion degrees Celsius, ten to the negative 35 seconds after the Big Bang occurred.
Fraser Cain: That is ridiculously short after the Big Bang. So for just a moment, then, the Big Bang had the hottest temperature in the universe. Then we’ve taken the –
Dr. Pamela Gay: But that’s not the hottest it can possibly be.
Fraser Cain: Whoa. There’s an upper limit on how hot things can be?
Dr. Pamela Gay: We think there’s actually an absolute hot above which normal physics as we know it is just like, “I give up; I forfeit. I can’t deal with this temperature anymore; this is as hot as it can be.” And we called that the Planck temperature. The Planck temperature is one decillion – I’m not sure how to pronounce this; we hit like, I-left-my-vocabulary-behind temperatures – 420 nonillion degrees Celsius.
Fraser Cain: What is the physical rationale for why that is the hottest temperature?
Dr. Pamela Gay: We actually talked about this back when we did our show on Planck lengths, Planck times; it’s at a certain point, you hit the critical – all the forces come together and it stops being even logical to start talking about things as being separate anymore.
Fraser Cain: Got it.
Dr. Pamela Gay: Like, physics just breaks down once you get – physics stops.
Fraser Cain: Right, okay. We talked a bit about the Planck length and I don’t want to go down this rabbit hole, which is that the Planck length is this really neat sort of crossover of mathematics, but it’s not necessarily a physical constraint; it’s just a place where two numbers cross. It’s not like the resolution of the universe. But it sounds like on the high end of high temperatures, probably you’re gonna get to a place where temperature no longer has meaning.
Okay, so that’s like the hottest temperature; the hottest temperature we’ve been able to produce here on Earth. But something I always find really interesting is when you see these images in places like the Chandra X-ray Observatory, it’s looking at these galaxy clusters, and it’s seeing clouds of gas coming together at millions of degrees. But if you were to like, fly through them, you wouldn’t feel like you were heated up intensely. So what’s going on in those situations?
Dr. Pamela Gay: So, how we define temperature is kind of crazy sometimes. Temperature is not how hot it necessarily would feel on your skin. Temperature is, if you look at all the energy tied up in the atoms, the individual particles, in terms of their energy that they can deliver to other things. That energy is what we refer to as temperature. But if particles are so diffuse that they’re not hitting you, you may not even notice the temperature that you’re at. So an individual gamma ray has an extraordinarily high temperature, because it’s a gamma ray photon.
But one gamma ray photon, if it hits your DNA, it’s gonna kind of ruin that cell’s day, and potentially ruin your future. But most of the time that single gamma ray is just gonna go right through your body and you’re never going to notice. It’s when you’re being bathed in photon after photon after photon, like when you’re outside sunbathing, that your body is like, “Oh, I can tell I’m getting hit by all of these photons.” So what really matters – the fancy word is the flux density – if the flux density or the collision rate between particles is too low, that’s another way we look at it, is how often are you getting hit by one of these atoms?
Gamma ray is a photon of light. You also have high-energy ions; these are cosmic rays that come flying out of the sky. Single cosmic ray; again, you’re not gonna notice it. May cause cancer down the line, which is bad; but you’re not gonna notice that one cosmic ray hitting you. But if you’re getting hit with a deluge of these high-energy particles that is going to start to exert a pressure; it’s going to start to be noticeable, and fry you. So, things to avoid.
Fraser Cain: So if you were to interrogate each individual particle, it would report a temperature, and you would be able to sort of figure that out to know what the temperature of that thing is. But if you were to look at, like, how does it feel? The interior of the sun feels hotter than passing through this cloud of hot gas. Even though it could very well be the same temperature. Alright, so we talked about things that are very hot. Let’s talk about the insides of stars. What kinds of temperatures are we looking at there?
Dr. Pamela Gay: So, I still classify the insides of stars as very hot; so –
Fraser Cain: I said it was hot, you know –
Dr. Pamela Gay: Well, you said cooler things. So we’re still hot though. So, inside our own sun, we’re starting to look at 15 million degrees Celsius. We’re pretty hot; this is where the temperature starts to allow nuclear reactions to occur; it’s also because of the densities. This is where we get back to that you need the individual particles to have sufficient velocity. That’s another way of looking at temperature, is what is the average velocity of the particles? But you’ll have to have sufficient density in order for the nuclear reactions to occur. So in the center of a star, we have sufficient density and sufficient temperature that it allows nuclear reactions.
Fraser Cain: Got it. Okay, and you said, what was it? 15 million Celsius? What’s that in Kelvin, about 15 million, right? Give or take a few hundred –
Dr. Pamela Gay: Right, because you add a couple hundred degrees. Yeah.
Fraser Cain: Right, okay; so that’s hot. Now that’s like the temperature inside our own sun, but you can have a bigger star have a hotter temperature, right?
Dr. Pamela Gay: Bigger stars, yeah, bigger stars have hotter cores. They also will eventually get to the point that they’re doing things like – we say burning – having nuclear reactions with heavier and heavier elements that again, the physics has to be a little bit different for that to happen. So we start with hydrogen burning, and then work our way up the food chain until we end up with an iron core; in which case, poof. It all goes away.
Fraser Cain: Right. And hotter temperatures just permit more interesting fusion reactions. You can – hotter, higher pressure – you can fuse, as you said, up to iron. Okay, so that’s pretty hot. What about the inside of gas planets like Jupiter?
Dr. Pamela Gay: So, this is a tricky thing to get at, but as near as we can tell from working through all of the physics, Jupiter probably has a core that’s around 24,000 degrees Celsius. So we’re looking at very hot, kind of molten, crystalline, depending on the pressure. One thing they talk about is you end up with hydrogen that starts acting like a metal; so you talk about metallic hydrogen inside these gas giants. So yeah, 24,000 degrees inside the center of Jupiter.
Fraser Cain: I love that idea, people are just like, “You could land a spacecraft on Jupiter and then dive down through the atmosphere.” Could you land on the surface? Because there is probably dozens of times the mass of the Earth in rock and metal; it’s down there.
Dr. Pamela Gay: It’s just – well, I wouldn’t say rock, because that implies that the –
Fraser Cain: It’s rock, heated. Silicon and oxygen atoms, heated to enormous temperatures, and mixed with molten hydrogen that acts like a metal.
Dr. Pamela Gay: Yes. It’s someplace you would die. Let’s just put it that simple. You would die.
Fraser Cain: Yeah; yeah, it’s just a race to which would kill you first. But in fact, you don’t have to get very deep down into Jupiter’s atmosphere –
Dr. Pamela Gay: Before the pressures –
Fraser Cain: –where the temperatures or the pressures raise up above boiling and what have you. You just go down a few hundred or thousand kilometers, and the temperatures start to crank up. Okay, that’s inside Jupiter; what about inside our own planet?
Dr. Pamela Gay: So inside our own planet it’s a paltry 6,000 degrees Celsius. And what’s kind of interesting to think about is, nuclear explosions are about 10,000 degrees Celsius; so Jupiter’s core is about twice as hot as being at the center of a nuclear explosion – nuclear bomb. The temperature inside of a conventional chemical bomb – so TNT, plastique – is about 5,000 degrees Celsius. So you can actually start to say, okay, bomb explosion core of the Earth; nuclear explosion core of Jupiter – just if you wanna go there. Random facts; now you know.
Fraser Cain: Now you know. I believe that is half the battle. I’m not sure which half. So okay, so we’ve got inside the core of our earth, which sometimes of course we get volcanoes and it oozes up, and we get to see how hot that stuff is. And there is a great – man, over on the Nerdist channel, have you ever seen their science channel? Kyle Hill did a great thing about what would happen if you jumped into lava. And that – you know, because you’re landing on rock, it wouldn’t be like you were splashing into water, you’d more just kind of smack onto the top of the rock and then explode into fire. It would just be a really awful way to go. So don’t jump in lava. Once again, your imagination is a –
Dr. Pamela Gay: It’s kind of a non-Newtonian –
Fraser Cain: Yeah, your imagination is no help to you when you think about what it would be like to actually interact with lava. Okay. So that’s the interior of the planet. We can assume Mars is probably a little cooler inside. So let’s talk about the hottest atmosphere, now. Let’s talk about, oh, Venus.
Dr. Pamela Gay: So, Venus is kind of hot; it’s the Mariner 2 data – so what I love about the temperature of Venus is, if you go back and you read old science fiction books, they talk about Venus as this tropical resort planet; and this is based on plain old atmospheric models. It’s closer to the Sun than we are; it’s about the same size we are; didn’t think about greenhouse gas properties at all. Then when little Mariner 2 got there, it realized that it’s like 900 degrees Fahrenheit; 460 degrees Celsius. It is a world of death. So do not go to Venus; and unfortunately, so many tropical paradise dreams were broken by that little spacecraft.
Fraser Cain: Yeah but unless you go into the high atmosphere, you end up – there’s like a perfect altitude where you can have both the temperature and the pressure be Earth-like.
Dr. Pamela Gay: And sulfuric acid filled.
Fraser Cain: And you could only breathe carbon dioxide. But oxygen, breathable air, is a lifting gas; you could have a balloon, you could sit outside in your protective suit – as it rots away from the sulfuric acid – briefly, and enjoy the view from the clouds. People always ask, they’re like, “What’s the most Earth-like place in the solar system?” It is the cloud-tops of Venus. I would rather stand there than on the surface of Mars.
Dr. Pamela Gay: You know, I’m kind of for the deepest valley on Mars, because that’s where you have –
Fraser Cain: The thickest atmosphere.
Dr. Pamela Gay: It’s where you have the thickest atmosphere; and middle of summer, on Mars, is warmer than a Boston winter day. So I’m thinking, put me down in the bottom of one of those valleys, one of those craters, and I’ll call it good. You can keep your sulfuric acid.
Fraser Cain: So then of course we’re going to – we’ll very quickly pass through the temperature of our own atmosphere here on Earth; right now, it’s kind of cold here. It can range in temperature here on the west coast of Canada from like 33 Celsius down to -15 Celsius, but average temperature of the Earth is what?
Dr. Pamela Gay: The average temperature of the Earth I believe is in the 20s, Celsius.
Fraser Cain: That’s a nice – literally the only place we’ve found in the entire universe that’s even marginally hospitable to life. Could very well be it. So that’s sad. You know when people say that the universe is perfect for life; it’s so not, right? We’ve talked about all these horrible, hellish, hot temperatures. And now we’re about to talk about awful, cold temperatures; we got this little place.
Dr. Pamela Gay: Well, what’s crazy is, a lot of these hot temperatures are – like, the melting point of silver is 960 degrees. So Venus’ atmosphere is only twice the melting point of silver. It’s – the boiling point of mercury is 357 degrees Celsius. So we’re looking at temperatures where bad stuff happens. Yeah, it’s kind of crazy, but even here on the surface of our earth; if you had to guess, what is the temperature of the hottest part of a wax candle flame? Like your normal “go to a restaurant where they don’t have the LED candles yet”; how hot do you think the hottest bright part of that flame is?
Fraser Cain: Is this a test? I’m going to guess –
Dr. Pamela Gay: Yes. It’s my turn!
Fraser Cain: Okay, okay, okay, okay – I’m going to say it is 1,000 degrees.
Dr. Pamela Gay: Unit, please?
Fraser Cain: Oh, Celsius.
Dr. Pamela Gay: Okay. You got 2/3 of the way there. It’s 1,400 degrees Celsius.
Fraser Cain: Right.
Dr. Pamela Gay: So that candle, if you could get your silver in the exact right part of the candle flame, you could melt your silver jewelry.
Fraser Cain: I want to do this. Alright; so we were talking hot. We passed through lukewarm, and room temperature, and comfort. And now let’s move into cold. So let’s go to the atmosphere of Mars. Minus hundreds of degrees. It can be, right? It can be anywhere from minus 100 Celsius, even colder at the poles at night –
Dr. Pamela Gay: So for Mars, we’re looking at someplace that it’s actually temperature we get to on Earth; but it’s a temperature you don’t want to experience on Earth, and it’s that temperature year-round. And that temperature, on average, is -55 degrees Celsius. For those of you who speak Fahrenheit, we’re now down to -67 degrees Fahrenheit. The two temperature systems cross at -40 which you experience if you go to Michigan state every year in February.
Fraser Cain: Ironically, right, the temperature in Canada, in Winnipeg, sometimes can be colder than the temperature on Mars. A couple years ago, the temperature of Winnipeg was colder than what the Curiosity rover was experiencing at the time.
Dr. Pamela Gay: I don’t remember if it was Spirit or Opportunity; one of the days that one of those two first landed, I was in Boston, and they were talking on Science Friday about the temperature on Mars. And I realized it was warmer on Mars, and I was kind of wishing I could be there with those little rovers, because it was cold in Boston.
Fraser Cain: Yeah, I’m sure Curiosity is like, “Ha ha, suckers! Come back to – come on over to Mars!” Alright, so that’s – Mars is cold, but it is definitely not the coldest place in the solar system. So as we move further and further out, to the outer edges of the solar system, how cold do we get?
Dr. Pamela Gay: Well, it’s one of these things where, first of all – how far out do you want to go? Because when you start getting out to the Oort Cloud, you’re starting to get down pretty close to the average temperature of the universe. But it’s a 1 over R-squared process. So every time you double your distance, the amount of light that you’re getting from the sun goes down by a factor of 4. So it gets worse and worse, the further out you go. And by the time you get out to Pluto, our friendly neighborhood hyper belt object, we’re now down to -218 degrees Celsius, which is -360 Fahrenheit.
Fraser Cain: And how much – that’s only a few dozen degrees above absolute zero at this point, right?
Dr. Pamela Gay: Yeah. It’s pretty darn close.
Fraser Cain: Pretty darn close. Okay, so that’s sort of within our solar system. So, now as we sort of get out into the interstellar gulfs, the regions between the stars, the temperature drops even more dramatically. The only thing that’s really heating us up at this point is the temperature from the stars.
Dr. Pamela Gay: Right. So if you imagine that you are in the intergalactic voids, these parts of the universe where you’re no longer in a galaxy, you’re nowhere near a star, you just see all this distant light. Well, you are getting photons from those galaxies, from those stars. The number of those photons is so low that it’s not really heating you up at all. And in fact, you’re sinking down toward the mean temperature of our entire universe, which is that cosmic microwave background radiation at about 2.7 degrees above absolute zero.
Fraser Cain: Which is really, really, really cold.
Dr. Pamela Gay: But it’s not the coldest place in the universe.
Fraser Cain: No. Once again, like the hottest place in the universe, the coldest place in the universe is really close to home.
Dr. Pamela Gay: And as far as we know, it’s the Boomerang Nebula, which is about 5,000 light years away. And it’s acting like a heat pump; just pumping all of the warmth out of it, and super-cooling the center, the same way your air conditioning unit does in the summer. It just doesn’t have your electricity bill.
Fraser Cain: So it’s colder than the background temperature of the universe. Wow, that’s amazing.
Dr. Pamela Gay: Heat pumps will do that.
Fraser Cain: Okay, so that’s naturally occurring coldest place in the universe; which is amazing. That is mind-bending. But we’ve done a whole show on Bose-Einstein Condensate; just this idea that you can cool things down. We did that whole show on absolute zero; you can get pretty close to absolute zero with lasers.
Dr. Pamela Gay: It’s true. You can pretty much stop atoms – almost – from moving. And they’re still; they have some temperature, because you can’t stop them from moving altogether. But you can lock them into states where they’re pretty darn close to absolute zero. But I have to say, we’re about to skip over my absolute favorite random science fact.
Fraser Cain: Oh, well, please tell me what it is then.
Dr. Pamela Gay: So that -272 degrees Celsius that is the temperature that you get at for the Boomerang Nebula – the extremophile, the water bear, tardigrade, these cute little critters that kind of look like they came straight out of the first Airbender; we know that those little critters can actually live to -273 degrees Celsius and be resurrected from that experience. So it turns out that this little extremophile that seems to be pretty much impossible to kill can survive the coldest naturally occurring places in our universe.
Fraser Cain: That’s why they really make ideal candidates for these upcoming Alpha Centauri probes that we talked about, right? They’re small, they don’t mind extreme temperatures either way; they’re ready to go. They’re ready to go to Alpha Centauri for us. They will colonize the universe. So we passed through; we talked about the coldest place in the lab; people use lasers, they cool things down to just a teeny-tiny fraction above absolute zero. So let’s, in order to wrap this up, let’s talk just a second about time. Which is that, right now, here we are: 13.8 billion years after the Big Bang, the temperature, that 2.7 Kelvin is how far the universe has cooled to by now.
Dr. Pamela Gay: Yes.
Fraser Cain: But it’s going to get cooler and cooler over time, right?
Dr. Pamela Gay: It is. And over the fullness of time – and here we’re looking at trillions of degrees – it’s going to very slowly work its way down toward, without actually achieving, absolute zero. So in terms of putting it into Celsius, which is where all of our brains seem to be working today, absolute zero is at -273.15 degrees. The coldest we’ve gotten in a laboratory so far is -273.144 degrees. So we’ve gotten really close to absolute zero; but it took a whole lot of energy and whole lot of work to do that, because we had to essentially counteract all of the different internal momentums in that atom to get it to stop jiggling, moving, all of that, absolutely as much as possible.
It was actually a copper vessel that they managed to supercool in India. The universe – as far as we know – doesn’t have a bunch of external lasers ready to start slapping around the atoms and say, “Stop vibrating.” So our own ability to get that cool, we’re probably not going to get that cool in the fullness of time. But we’re going to slowly slow down so that all you have left is the random vibrations of isolated atoms.
Fraser Cain: And the universe in the far, far future will have reached the coldest possible temperature, but I guess, when it hits infinite time.
Dr. Pamela Gay: Yes.
Fraser Cain: So at infinite time, we will have the coldest possible temperature, which will be, essentially, absolute zero.
Dr. Pamela Gay: Part of the reason I’m being kind of vague and squirrelly on this is there’s a lot we don’t know about how the universe will end. We know it’s – when we first started filming Astronomy Cast, it was, we didn’t know if it was by fire or by ice. Then we eventually figured out we do live in a forever expanding, actually accelerating apart universe; well, we knew that when we started recording. But there were still some open questions. We now know we’re certainly going to die by heat death of the universe, which means everything freezes.
But what we don’t know is, do protons – one of the fundamental building blocks of the universe – do they actually decay or not? If they decay, that means that eventually all the atoms in the universe end up just poofing off into energy. Now, every year that goes by without detecting the decay of a proton is another year that says, no, protons aren’t going to decay, they’re forever stable. But, without knowing that, it’s hard to say is the universe going to someday be a bunch of cold isolated atoms, or is it going to be nothing more than a background of low-level radiation? And it’s amazing what we don’t know. And this is why we keep doing science.
Fraser Cain: Awesome. Alright, well thanks, Pamela; we’ll see you next week.
Dr. Pamela Gay: My pleasure, Fraser. I’ll talk to you later.
Male Speaker: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at AstronomyCast.com. You can email us at firstname.lastname@example.org. Tweet us @AstronomyCast. Like us on Facebook, or circle us on Google+. We record our show live on Google+, every Monday at 12 p.m. Pacific, 3 p.m. Eastern, or 2000 Greenwich Mean Time.
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Duration: 34 minutes