1. Mar 12, 2015

### akashpandey

If temperature of universe is cooling then why universe is expanding.

Last edited by a moderator: Mar 12, 2015
2. Mar 12, 2015

### ZapperZ

Staff Emeritus
Maybe you got this all backwards. The "temperature" of the universe is cooling BECAUSE the universe is expanding! Your ideal gas law properties are consistent with that observation.

Zz.

3. Mar 12, 2015

### Suraj M

Think of it in another way.. go backwards
the universe is expanding ergo temperature is dropping.

4. Mar 12, 2015

### akashpandey

But according to the theory univers is expanding and in space there is vacuum
So why it consist of idea gas law properties.

5. Mar 12, 2015

### Suraj M

Which theory?
Just consider it as a system loosing it's internal energy so that it can expand, loss of internal energy= drop in temp. as it is an isolated system

6. Mar 12, 2015

### Suraj M

I don't know what you're talking about!
See the total energy of the universe is a constant, but its expanding, using up internal energy(i presume).. The internal energy is a function of temperature(directly proprtional), so the temperature has to drop.

7. Mar 12, 2015

### akashpandey

According to you suraj if internal energy of univers is end then expanding of univers will stop.

Last edited by a moderator: Mar 12, 2015
8. Mar 12, 2015

### Suraj M

from the past 7.2 billion years the temperature has dropped by just about 3°C.
Anyway leave that, the rate of the expansion of the universe is something else, its energy density, it used to be radiation a long time ago, recently dark energy became the dominant reason for expansion.

9. Mar 12, 2015

### akashpandey

But these is new theory .
There is a discution on that theory in present among scientist.

10. Mar 12, 2015

### Suraj M

11. Mar 12, 2015

### slatts

The original question and the first reply are so interesting that I'm formulating my thoughts as I type, so bear with me. Together they seem to describe that reversibility of time which I've heard is characteristic of physics.

In his popularization (called "Once Before Time") of Loop Quantum Cosmology, Bojowald points out that cooling results in contraction, and that the contraction of whatever content the universe or multiverse had before its expansion implies, IF the universe can be described as lying within a container, a pull that equates to negative pressure. As Guth has stressed since the beginning of inflationary theory, negative pressure generates the repulsive version of gravity. (Many texts show the small piece of the Einstein equations implying this, and a 21st-Century book ["The Little Book of the Big Bang"], that was endorsed by the Astronomer Royal, agrees with Vilenkin in analogizing it to tension, like the tension in a cable stretched by a weight suspended from it.) In Guth's original "Old Inflation", supercooling (cooling, preceding a phase transition, below the temperature normal for that transition) played a major role, but I don't know whether it transferred to "New Inflation", which has different contours in the abstractions that describe the underlying fields.

Repulsive gravity, which expanded whatever Planck-scaled mass was in the "container" to the size of our observable region in much less than a second, shares the unusual quality of being "negative energy" with the familiar attractive version of gravity. The temperature in our "container" jumped rather drastically when quantum perturbations slowed that part of the expanding universe containing that region (and probably a good deal of space outside it) which is now our "local universe" (or bubble, or island, or pocket).

So the continuing expansion of the bubble (or pocket, or island, or "local universe") that contains our observable region is purely inertial, and is usually posited to last forever. I don't know whether thermodynamics would normally require cooling, in the case of an expansion whose rate is not increasing.

Bojowald feels that the cooling would not have sufficed for the observed inflation, but my own problem with all this is whether the initial universe can be described as having been inside a container. If it was, its boundary must have been the boundary between its externally attractive and internally repulsive forms of gravity, although both Guth and Vilenkin appear to feel it was, instead, the walls of a quantum tunnel that got it into our space. Any views on why would be appreciated.

12. Mar 12, 2015

### phinds

This is ridiculous since it is totally controverted by tons of solid evidence which shows that the observable universe is expanding at an amazing accelerated rate. Things at the edge of our observable universe are receding at about 3c which certainly is not even remotely possible inertially.

13. Mar 12, 2015

### slatts

It is an accelerated rate, but, in the opinions of such physicists as Guth and Vilenkin, the vast part of it that's due to inflation per se is no longer an accelerating one anywhere in our observable region. I know the analyses of variable stars by two competing teams concluded with the surprising fact of some recent (cosmologically speaking) acceleration, but a recent thread points out that (in spite of much media hype), it's pretty negligible so far and could be an illusion due to fields between us and those stars.

14. Mar 12, 2015

### phinds

Explain to me how any of that has anything to do with obtaining a recession velocity of 3c due to inertia.

15. Mar 12, 2015

### slatts

Sorry, I'm not understanding how velocity necessarily results from any ongoing acceleration. Doesn't movement include direction and speed (-the only two components of velocity I can think of) even if it's relative to an object moving inertially (like my car, if I haven't depressed the gas pedal for awhile)?

16. Mar 12, 2015

### phinds

The acceleration being discussed is the acceleration of the expansion of the universe, and actually "velocity" is not quite the right term for what's happening unless you make it the phrase "recession velocity" so as to avoid confusion with local velocity which cannot exceed c.

17. Mar 12, 2015

### Staff: Mentor

ZapperZ didn't mean that the universe is actually an ideal gas. He meant that, for purposes of understanding the relationship between temperature and expansion, you can model the matter and energy in the universe as working like an ideal gas which is expanding adiabatically (meaning, without exchanging heat with anything else). As it expands, it cools.

The actual mathematical relationship between expansion and temperature depends on the specific kind of matter or energy: the relationship for ordinary matter is different from the relationship for radiation, which is different from the relationship for dark energy.

18. Mar 12, 2015

### Staff: Mentor

Be careful. What you actually mean here is "inertially in flat spacetime"; but the spacetime of the universe is not flat. The "recession velocity" of 3c is between comoving objects, and comoving objects are in free fall, so in that sense they are moving "inertially". They are just doing so in a curved spacetime in which the tidal gravity between them is sufficient to make freely falling objects at that separation have a relative "recession velocity" of 3c in comoving coordinates.

19. Mar 12, 2015

### phinds

Interesting. Thanks.