How can the initial universe (extremely dense) be hot when density and temperature are inversely proportional?

Recommended Posts

(Sorry if this seems obvious/stupid)

Correct me if I'm wrong. AFAIK when temperature is decreased, the molecules become closely packed,  thus increasing the density.
Then how do we say that the universe was extremely hot at the beginning when it was all packed into a few cubic centimeters?

Share on other sites

It's the other way round. When you reduce the volume (and increase the pressure) the temperature increases. Like when you pump up your bicycle tires: the pump can get very hot.

And when you release the pressure, it gets colder. If you use an aerosol paint spray, the can will get very cold after a few minutes.

This is how the heat pumps in refrigerators work.

Maybe you are thinking of cooling a gas so it condenses to a (denser) liquid and then freezes to a (denser) solid? But in that case, the heat has to leave the gas and the liquid in order for it to cool down. The universe is a closed system, so there is nowhere for the heat to go, so the temperature, volume and pressure are related by the ideal gas laws.

Share on other sites
2 hours ago, mo.sabith said:

(Sorry if this seems obvious/stupid)

Nothing is obvious in physics/cosmology. You don't come across as stupid at all. I think @Strange's answer just nailed what your confusion is about (+1.) I'll follow up if anything needs further clarification.

Cheers!

Share on other sites
4 minutes ago, joigus said:

Nothing is obvious in physics/cosmology.

Seconded.

Even some really simple ideas can take time to get your head around. It took years for me to really make sense of why we still see the cosmic microwave background; I mean, I understood the explanation but in a "yeah, I suppose so" sort of way. Eventually, a really good analogy(*) made me go "oh, of course".

(*) The "surface of last screaming"

Share on other sites
3 hours ago, mo.sabith said:

(Sorry if this seems obvious/stupid)

Correct me if I'm wrong. AFAIK when temperature is decreased, the molecules become closely packed,  thus increasing the density.
Then how do we say that the universe was extremely hot at the beginning when it was all packed into a few cubic centimeters?

The pressure was high.

Very high.

Share on other sites
9 minutes ago, Strange said:

a really good analogy(*)

9 minutes ago, Strange said:

(*) The "surface of last screaming"

If it's not too off-topic, I'd love to have a picture of the analogy. All analogies have limitations, but they're very useful tools.

Share on other sites
3 hours ago, mo.sabith said:

(Sorry if this seems obvious/stupid)

Correct me if I'm wrong. AFAIK when temperature is decreased, the molecules become closely packed,  thus increasing the density.
Then how do we say that the universe was extremely hot at the beginning when it was all packed into a few cubic centimeters?

You have cause and effect mixed up here, and also are making some implicit assumptions about what variables are fixed and what can vary

Yes, if you reduce temperature, if the volume decreases, density will increase. But if volume is held constant, that would not be the case. Pressure would drop (PV = nRT, in cases where you can assume an ideal gas)

But that's not what happened in the early universe, as others have explained

Share on other sites
1 hour ago, joigus said:

If it's not too off-topic, I'd love to have a picture of the analogy. All analogies have limitations, but they're very useful tools.

This analogy is good for just one thing, explaining the surface of last scattering, but it is very good at that: https://www.researchgate.net/figure/The-Surface-of-Last-Screaming-Consider-an-infinite-field-full-of-people-screaming-The_fig5_1931578

Share on other sites

"...temperature is the measurement of the average kinetic energy of the molecules and represents the motion of molecules."

"...Based on observations of distant objects and measurements of the cosmic background radiation, scientists have deduced the temperature at the Planck time, which is 10 million trillion trillion trillionths of a second. At that instant, the temperature was 100 million trillion trillion kelvins (180 million trillion trillion degrees Fahrenheit). The universe underwent a period of accelerated expansion that ended well before a second had elapsed. By this time, it had cooled to a temperature of 100 billion kelvins (180 billion degrees Fahrenheit)."

My question is how can the universe be so hot at the first Planck time  when the universe was so dense that the particles could not move?

BTW, at the center of a black hole are molecules in motion?  It seems to me that at near infinite density there is no space for motion.

Edited by Airbrush
Share on other sites
1 hour ago, Airbrush said:

"...temperature is the measurement of the average kinetic energy of the molecules and represents the motion of molecules."

"...Based on observations of distant objects and measurements of the cosmic background radiation, scientists have deduced the temperature at the Planck time, which is 10 million trillion trillion trillionths of a second. At that instant, the temperature was 100 million trillion trillion kelvins (180 million trillion trillion degrees Fahrenheit). The universe underwent a period of accelerated expansion that ended well before a second had elapsed. By this time, it had cooled to a temperature of 100 billion kelvins (180 billion degrees Fahrenheit)."

My question is how can the universe be so hot at the first Planck time  when the universe was so dense that the particles could not move?

BTW, at the center of a black hole are molecules in motion?  It seems to me that at near infinite density there is no space for motion.

I think average KE would stay the same as they only have each other to run into.

For the center of black holes the formulas fail to give meaningful values. Trying to find density, results in a division by zero situation, a mathematical singularity.

Share on other sites

All would be in a Bose condensate state ie thermal equilibrium but in that state they would have identical Compton wavelengths. They will still vibrate such as the harmonic oscillator. So they are never truly motionless regardless of how dense. Remember particles has no discernable volume. They aren't little corpuscular billiard balls of solid matter like. For example you can stack an infinite number of Bosons in the same space.

Edited by Mordred
Share on other sites

Thank you all for the answers. I'm glad I could clear this up for me!

Share on other sites

But then, why have a read several times that the temperature inside a Black Hole is freezing cold?

Share on other sites
26 minutes ago, michel123456 said:

But then, why have a read several times that the temperature inside a Black Hole is freezing cold?

The first article is about the temperature of the Hawking radiation from the event horizon, not the inside.

The second is probably about the same thing, but is really incoherent nonsense.

Share on other sites
22 hours ago, Strange said:

The first article is about the temperature of the Hawking radiation from the event horizon, not the inside.

The second is probably about the same thing, but is really incoherent nonsense.

What do you mean, that it is not freezing cold? (I posted my question because I have read this info-that it is cold-  in a french Science & Avenir).

Share on other sites
1 hour ago, michel123456 said:

What do you mean, that it is not freezing cold?

The temperature is very low. But it is not the temperature inside the black hole.

I don’t know if we can say anything meaningful about the inside.

Share on other sites
8 hours ago, Strange said:

The temperature is very low. But it is not the temperature inside the black hole.

I don’t know if we can say anything meaningful about the inside.

In the article I read it was loosely explained that there is no room for the particles to move at all, which leads to the very low temperature.

Share on other sites

Whoever wrote the article wouldn't know. However consider this the average surface temperature of a neutron star is roughly 600,000 K.

The BB intial temperature is roughly $10^36$ K.

Yet expansion causes cooling. So consider this how does a cold initial singularity lead to such an extreme high temperature due to the BB expansion ?

Obviously the temperature would initially be hotter.

Share on other sites
2 hours ago, michel123456 said:

In the article I read it was loosely explained that there is no room for the particles to move at all, which leads to the very low temperature.

That doesn't seem to be in the first article.

And the second article was just a random collection of words with no intelligence behind them. So I don't really care what it says. 🙂

Share on other sites

Black Holes don't have a 'temperature' as we would normally use the word.
And temperature is not one of the conserved quantities that survive stellar collapse to manifest in the mathematical construct of the Event Horizon.
So even if before collapse a star had a temperature of 10 million degrees K, that information, according to GR, is not conserved like mass, charge and angular momentum are ( Quantum Gravity theory will probably say different ).

In the middle 70s, J Bekenstein, working with S Hawking, suggested that BHs cannot violate thermodynamic laws, and therefore must have entropy, and that the entropy is proportional to the surface area of the Event Horizon divided by the Planck area. This has been further refined over the years with a statistical interpretation of microstates.

Of course, it follows that anything that has entropy, must have a temperature.
And that is where the 'temperature' of a black hole comes from.
I assure you Michel, no one has ever stuck a thermometer past the Event Horizon, to take a BH's temperature.

Share on other sites
22 hours ago, Mordred said:

....Yet expansion causes cooling. So consider this how does a cold initial singularity lead to such an extreme high temperature due to the BB expansion ?

Obviously the temperature would initially be hotter.

Are you saying that since at the first Planck time the temperature was already astronomically high, that would mean that BEFORE the first Planck time, the temperature would be even higher?  It would not make sense for the temperature to change from zero to trillions of trillions of trillions of degrees in such a short moment?

Edited by Airbrush
Share on other sites
1 hour ago, Airbrush said:

Are you saying that since at the first Planck time the temperature was already astronomically high, that would mean that BEFORE the first Planck time, the temperature would be even higher?  It would not make sense for the temperature to change from zero to trillions of trillions of trillions of degrees in such a short moment?

Temperature is average kinetic motion. I would think the kinetic motion, and therefore temperature, in a singularity or extremely compressed state would be next to zero. The moment of inflation would be the highest point of temperature once the constituents could move or vibrate, then it cooled with expansion.

Share on other sites
2 hours ago, Airbrush said:

Are you saying that since at the first Planck time the temperature was already astronomically high, that would mean that BEFORE the first Planck time, the temperature would be even higher?  It would not make sense for the temperature to change from zero to trillions of trillions of trillions of degrees in such a short moment?

Correct at the moment prior to Singularity the temperature is Planck temperature $10^{32}$ correction to previous temp I mentioned at Planck time $10^{-43}$ in a Planck length.

You will arrive at these values by extrapolating expansion backwards  prior to those you have a singularity condition which we cannot mathematically describe.

Note at this point the diameter is only 1 Planck length. What does this say for the argument particles have no room to move ? ( That argument stems from thinking particles are like billiard balls ) an elementary particle has no discernable volume. They can still vibrate as per the quantum harmonic oscillator.

The other detail is we do not know if the universe is infinite nor finite. That Planck length is only our observable universe portion at 10^-43 seconds.

The remainder of the universe at that time could very well be infinite or simply a much larger finite portion outside our shared causality described by the observable universe.

With a BH we don't even know if the singularity described is even feasible. We can only speculate

Edited by Mordred

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account