When matter first formed after the big bang, it was very hot. I don’t know how hot, but you could probably cook an egg on it.
Now, the average temperature is something like 2.4K.
Where did the heat go?
I’m no phycisist, so I’ll offer a somewhat karmic WAG. The amount of energy created/expended at that moment is a finite thing. However, nobody knows the area into which that energy was released. The true size of the cosmos, so to speak.
You whip up three eggs, and drop them in an 8 in frypan, they’ll be about 1/16 of an inch deep. You drop them into a frypan the size of Toledo, and…well. You are left with eggs that are a thinness that is analagous to…2.6 degrees Kelvin.
Cartooniverse
The universe is expanding. The energy of the universe is being spread out over a greater volume. Less interaction/motion of atoms = less temperature.
OK, I’m a total zero when it comes to these matters, but they fascinate me beyond belief. Phobos, I can see your reasoning, however: if the universe is supposed to be of infinite size (it is understood to be, right?), how come there’s ANY residual heat (i.e. anything more then 0 Kelvin) at all? Is that 2.4 K caused by the stars heating things up still?
I’m sure I show my ignorance by this question, but I’m just trying to make sense of it all. Great question, BTW, howardsims. This is the reason why I love this place. People come up with questions that are actually NEW to me 
I believe you’re off on the assumed ‘infinite universe’. I don’t know offhand how big it’s supposed to be, but I’m sure someone will.
“an infinite universe means infinite possibilities”, meaning somewhere in the universe there’s a muppetsoup who knows the answer. Unortunately, it ain’t this one.
Rather than get into a debate over whether the Univserse is infinite or not, perhaps it’s better to think not of a finite amount of energy in the Universe, but rather a finite energy density. In any given 1 meter x 1 meter x 1 meter cube in the Universe there’s a finite amount of energy. Now, go back a few billion years, and that 1mx1mx1m cube was actually smaller–but with the same amount of energy, so it was hotter. (Sure, starlight and what have you comes into the box–but on average it’s going to just keep going and leave the box.)
To be honest, Coldfire, the fact that 99% of the universe is 2.7K (as I remember the figure to be) points to the fact that the universe is actually of finite size. If it were infinite, the background radiation would be a lot closer to, if not at, absolute zero because it would continually be dissipating into an area it hadn’t filled yet. Stars and galaxies just aren’t close enough together to fill the intervening void with the heat they produce.
Back to the OP. The heat given off by the Big Bang is still with us, essentially as the background radiation of the Universe. I think Phobos and Cartooniverse have got the amount of energy per unit of area thing right but Phobos mistakes the source of the energy.
Back in the first few seconds of the universe there was so much energy in so small a space that, if I understand it correctly, it was impossible for even the particle/wave dual existence to occur. It was all waves, baby 
As the universe expanded and the heat and light energy radiated outward without being reabsorbed by something else, it became possible for particles to exist, if only to bounce off each other because they still had too much energy to do anything else.
Further expansion allowed for further cooling of particles so they could finally clump together to form matter - quarks, atomic particles, atoms, molecules. From this state of things we get the localized clumping of matter - galaxies, stars, and planets - but an even spreading of the energy that matter had dissipated during the phases of its previous existence. So the energy originally didn’t come from the interaction of atoms and molecules but from their coming into existence in the first place.
Olentzero, perhaps I misunderstand your argument for a finite Universe.
You say “[the background radiation] would continually be dissipating into an area it hadn’t filled yet.” I don’t know what you mean by “it” in “an area it hadn’t filled”. I doubt you mean the Universe because any student of the Big Bang knows that the expansion of the Universe is an expansion of all space, not the expansion of stuff into an empty space. If you mean the background radiation, this is still erroneous. The background radiation occupies all space, regardless of whether space is infinite or finite. See my argument from energy density above.
Coldfire:
Considering that the universe has been in existence a finite amount of time (6-11 billion years, depending on who you listen to) and has been expanding at a finite speed (which has varied), it couldn’t possibly be infinite.
As I understand it, no finite thing can ever suddenly become infinite. Infinities are intrinsic; they must be there from the beginning.
Guys, you make my head hurt
Podkayne makes the most sense to me. Please tell me his version is right, so I can sleep peacefully at night? I’m sure the rest of you are equally smart if not smarter, but his explanation is the easiest to grasp for me.
He even used metric, so he has to be right
While others are covering the actual question about whether or not the Universe is finite, I’ll add something that might help visualize it. Steven Hawking, in A Brief History of Time describes the universe as “finite but unbounded”. The example he gives to help visualize this is to imagine the surface of a sphere, in particular the Earth. The surface of the Earth (or any sphere) is finite, since every part of it can be measured. However, there are no boundaries to the surface, given a starting point on the surface, an motion along that surface directly away from the initial point will eventually bring you back to that point.
I’ve read Hawking, and also came across that explanation. A wormhole is supposed to be a “shortcut” through the sphere, right? Just to make sure we’re talking about the same thing here.
While that sphere shape makes sense for the wormhole theory, it somehow doesn’t click in my head. Would that mean that if I jump in my hypothetical space shuttle and floor it, going straight upwards, I would eventually (after an impossibly long journey, of course) end up nose first into the earth, but on the other side? Somehow, I can’t make that sort of logic work. Am I missing something?
You can tell I’m not exactly an expert here huh
Podkayne’s explanation is probably the simplest, dealing with densities rather than totals. As for the size of the Universe, we don’t know. We’re pretty sure that it’s unbounded, but don’t yet know if it’s finite (although current evidence seems to be suggesting that it’s infinite). Da Ace is correct in that if it’s finite, it was always finite, and vice versa, though.
Coldfire, you’re correct about the implications of a finite-but-unbounded Universe: It is theoretically possible to circumnavigate it and arrive back where you started.
As for the expansion speed, it’s a function of distance, so very distant objects are expanding away faster than nearby ones. The speed of light isn’t actually a limitation here, so it’s still possible for a finite-aged Universe to be infinite in size.
Einstein wasn’t sure about the Universe, and even today, neither is anyone else.
Could you direct me (us) to somewhere that talks about the ‘current evidence’. As far as I’ve seen, it’s still thought of as finite. But I haven’t been keeping up as closely as I should lately.
Not so. At some point 20 billion years ago, the background radiation definitely occupied a finite space. That space is now going through a rapid expansion and, though it’s been expanding at a tremendous rate for a very very long time, it is still finite. While the background radiation could occupy all space if the Universe was finite, it could not do so if the Universe was infinite. It’s expanding from a point, and the limit of the space it can occupy is determined by how far it can travel (I’m assuming at the speed of light here) over the given time.
Sorry, Olentzero, but your understanding of the Big Bang is flawed. Saying that the background radiation is expanding from a point is incorrect, except insofar as one could say that all points in the Universe were once vanishingly close to being “a” point. The background radition has always existed at all points. The background radiation is not expanding and filling an empty space; it is expanding (and cooling) along with space.
And, Coldfire, I’m a she, not a he. No offense taken, of course.
When people say they average temperature of the universe is that averaging by space or mass? What I’m trying to say is, is that the average for the temperature taken at every point in space? or the average for each unit of mass?. For example. The sun’s mass is 99% + of the mass in the solar system, and is thousands of degrees. So the average piece of mass in the solar system must be more 2.5 K. Most solar systems are going to be the same way, and the relatively small amount of extra-solar mass won’t effect the equation very much. Unless you start adding in the possibility of dark, cold matter, which will screw everything up tremendously.
So, unless I’m missing something, and the 2.5 K is the average temperature by mass, it must be the average temperature by location. So has anyone figured the average temperature of the universe by mass?
You’re right, wolfman. The 2.7 K figure we’ve been flinging around is the temperature of a blackbody fit to the cosmic microwave background radiation. If there was an object at 2.7 Kelvins, it would “glow” with the same spectrum of light as the CMBR, which we can observe with radio telescopes, in the same way that humans glow in the infrared or that a heate iron bar glows red or white-hot, depending on its temperature. The CMBR is made up of photons, not matter. (If you were to stick an object out into empty space, it would eventually radiate away its energy, but since it’s in that bath of 2.7 K photons, it could never get down below the temperature of the background radiation.)
When someone says the average temperature of the Universe is 2.7 K, they’re invoking the fact that the Universe is mostly empty space, with a few trivial clumps of matter (like, oh, say, galaxies and stars and people and what have you) sprinkled pretty sparsely throughout it. The volume-averaged tempeature of somewhere relatively dense, like the solar system, would probably be just a bit larger than 2.7 K, but the mass-averaged temperature in the solar system would be considerably larger. I don’t know of a mass-averaged temperature for the whole Universe, though I do recall that something like 99% of the Universe is plasma (as opposed to solid, liquid, or gas.)
I’m with Podkayne on this one, Olentzero. The energy/matter of the universe is not shooting out into space from a central point. Rather, every point of the visible universe used to be very close together. So, the energy of the visible universe was already everywhere to begin with (i.e., the Big Bang happened everywhere). As space expanded, this energy (or something like a quark-gluon plasma…this is still an unknown area of cosmology) was able to cool and essentially condensate into atoms of matter. The Cosmic Background Radiation (found everywhere) is what’s left over.
Also, as Chronos said, the finite-aged universe may in fact be infinite or unbounded.
A key phrase is the “visible universe”. There is more out there than we can see. Even when the universe was fresh from the Big Bang, it was still potentially infinite/unbounded. The only boundary we know about is the one at Time = 0.
But thinking about infinities & points can make one’s head spin.
Another quick point…recent evidence is that the universe is “flat”. A topology which further implies an infinite/unbounded extent.
OK, I can see this. Been a while since I read the 1980s version of “Cosmology for Dummies”.
I think that’s what I was trying to say in my first post. The more room the energy had to radiate around in, the more likely parts of the plasma/energy cloud/big ball o’ superheated Play-Doh would be able to cool down and condense into matter.
I like the ‘finite but unbounded’ theory myself, if only because of my materialist viewpoint and the thought that, if I had a powerful enough telescope, I could sit down, look straight up, and see my own butt.
Seriously, though, I remember reading in TIME magazine many years ago that several astronomers had pretty much done that - they’d located a quasar in a certain position, then checked the sky exactly 180º opposite and found a similar, if not identical, object just as far away. Did they ever fully confirm this finding or withdraw the assertion?