# Expanding universe and absolute zero

1. Apr 20, 2004

### maxpowers_00

hi every one.

i was on my way out of my astronomy class today and i hit my self with an odd question. The universe is expanding and as a result it is cooling, now i was wondering if the universe expands infinitly then would it be possible for it to cool infinitly as well. eventualy reaching absolute zero?

2. Apr 21, 2004

### Nommos Prime (Dogon)

Yes

If you consider the Universe to be "open" and not "closed", then Yes, it will reach absolute zero.

3. Apr 21, 2004

### marcus

maxpowers00, I dont think it is possible for anything to reach absolute zero.
if the U keeps expanding the temp will keep falling and getting closer and closer to zero, but never actually getting there

so when the U has expanded 1000-fold, which it probably will some day, then the microwave background temp will be around 2.7 millikelvin

instead of the 2.7 kelvin it is today

but although that 2.7 millikelvin is nice and cold it is still not absolute zero

likewise when it has expanded a million-fold from what it is now
(any idea how long it might take to expand that much? I cant say offhand
but could probably find a calculator on the web that would)

4. Apr 21, 2004

### Mike2

Isn't there some ground state vibration going on between particles that would represent a minimum temperature?

5. Apr 21, 2004

### kurious

It would be great if we knew a minimum value for the temperature because then we would know the biggest size the universe can get.

6. Apr 21, 2004

### Nereid

Staff Emeritus
Let's not confuse the temperature of a piece of carbon (say) and the temperature of the CMBR. It may be, in the former, that there is some 'minimum temperature'; however, that wouldn't necessarily have any relationship to the vastly redshifted blackbody radiation that the CMBR would become if the universe (kept on, as Dogon points out) expanding for long enough. Why couldn't the peak of this future 'CMBR' have a wavelength of 10 trillion light years? Certainly not because atoms can have no lower temperature than (say; I'm making this up) 12 picodegrees K!

7. Apr 21, 2004

### Mike2

So what you are saying is that any zero point vibration between atoms in a material would itself not radiate. For if it did, then wouldn't it eventually be scattered to such an extent that it would ultimately contribute to background noise, CMBR?

8. Apr 21, 2004

### Nereid

Staff Emeritus
Even if it did, I can't immediately see how that would imply a minimum 'CMBR' temperature (as long as the universe continues to expand, of course!). After all, there is no net increase in the number of baryons in the universe (tho' what do we know about dark matter, right?), so even if there were to be some 'zero point EM', the effective radiation temperature of the universe would still continue to decrease ...

9. Apr 23, 2004

### bettysfetish

"Theoreticly", in a few billion trillion years, won't virtually "all" patricles disintigrate? Shouldn't it then be "0"-k degrees? Don't "all" systems seek their lowest state of energy if left un-hindered to do so? And then, would we not be left with a cold dark eternity waiting for anouther quantum fluxuation?
"Just a thought."
L8R----------bettysfetish.

10. Apr 23, 2004

Staff Emeritus
Your prediction about the particles is accurate according to current physics, but we won't get to 0 K becuse that would be a sharp value of 0 energy, which violates the uncertainty principle. The vacuum will still jiggle a bit, that's where the next quantum fluctuation will come from.

11. Apr 23, 2004

### Nereid

Staff Emeritus
Isn't this dependent on whether the proton is stable? If it isn't, then after some 1038 years, or more (which incidently is a few 'trillion trillion' times as many years as your 'billion trillion years'), the enormous universe would be just an incredibly dilute electron + positron plasma ... plus neutrinons and anti-neutrinos (and dark matter).

However, if the proton is stable ...

12. Apr 23, 2004

### LURCH

Also, if energy is quantized, and the temperature universally drops below the critical measurable value, isn't it 0oK, technically?

13. Apr 24, 2004

### meteor

Imagine that the temperature never reaches zero, but has a very little value, then surprisingly, random fluctuations can create new complex things, even whole galaxies. This is the proposal that appears in this Baez's article:
http://math.ucr.edu/home/baez/end.html

"However, Leonard Susskind has recently pointed out that in thermal equilibrium at any nonzero temperature, any system exhibits random fluctuations. The lower the temperature they smaller these are, but they are always there. These fluctuations randomly explore the space of all possible states of your system. So eventually, if you wait long enough, these random fluctuations will carry the system to whatever state you like. Well, that's a bit of an exaggeration: these fluctuations can't violate conservation laws. But conservation of energy doesn't count here, since at a nonzero temperature, a system is really in a state of all possible energies. So it's possible, for example, that a ice cube at the freezing point of water will melt or even boil due to random fluctuations. The reason we never see this happen is that such big fluctuations are incredibly rare.

Carrying this thought to a ridiculous extreme, what this means is that even if the universe consists of more or less empty space at a temperature of 10-30 kelvin, random fluctuations will occaisionally create atoms, molecules... and even solar systems and galaxies! The bigger the fluctuation, the more rarely it happens - but eternity is a long time. So eventually there will arise, sheerly by chance, a person just like you, with memories just like yours, reading a webpage just like this.

In short: maybe the universe has already ended! "

Nice if these things can happen. The idea of an universe consisting forever of cold radiation is certainly depressing

Last edited: Apr 24, 2004
14. Apr 27, 2004

### bettysfetish

Thanks SelfAdloint. I suppose your right, "Absolute" zero may not be obtainable: doesn't some theory state that you can never reach a point of zero by continually reducing the value by half? We will never reach a point where space will be as emtey as it was before the creation of matter, so that absolute value can never be obtained unless virtually all matter is gone from exsistance.
Nereid; The proton may not be stable but if it's lifetime exceeds the capability of any instrument we can devise then is it infinit "by defalt?" I stand corrected on the billion/trillion thing. These numbers "are" quite staggering.
Can anyone tell me a site where I can read more on "Massive Rotating Particle Theory?"
Selfadjoint, do you put any creedance in rotating electron/positron pairs containing mass energys that go off the charts? This seems an appealling theory to me. Would the pair attract because there opposits in the sense that there particle/anti-particle yet repluse equally because there the same, meaning both are electrons?
L8R------------bettysfetish

15. Apr 27, 2004