# The temperture of the universe

1. Feb 16, 2010

### tiredryan

I am a new student to physics, and I am thinking about the average temperature of the universe. I have found that it is 2.725 +/- 0.002 K.

I am wondering what will the temperature be if we convert most or all of the mass of the universe into energy? If my understanding is correctly, mass in a system does not contribute to temperature whereas energy may. I am guessing in the very distance future we may be able to convert the entire masses of planets into usable energy, prolonging the possible eventual heat/cold death of the universe. Will the average temperature of the universe change when convert mass into energy?

Here are some back of the envelope calculations. Correct me if I am wrong since I am new to physics. The average mass of the universe is around 10^55 kg. Using E=mc^2, the energy of the universe, if converted to energy, is around 9x10^71 J.

References:
http://hypertextbook.com/facts/2006/KristineMcPherson.shtml

Last edited: Feb 16, 2010
2. Feb 17, 2010

### Chronos

2.73K is the CMB temperature. It is but one component of the brightness of the night sky. Think of it as the signal strength measured by a radio telescope pointed at the darkest part of the night sky. The CMB temperature is not affected by events occuring after the big bang, but, could fall below the lower detectabilty limit if every star in the universe decided to go supernova.

3. Feb 17, 2010

### tiredryan

Thanks for your response. If I understand this correctly, the value of 2.73 K is not affected by the events occurring after the big bang. If this is so, then the average current temperature may not be 2.73K if we included all the possible supernovas that occurred after the big bang. I am confused at the statement in wikipedia which says, "In a "heat death", the temperature of the entire universe would be very close to absolute zero." Is this statement correct? Wouldn't the ultimate temperature depend on amount of mass converted to energy at the time of heat death?

Reference:
http://en.wikipedia.org/wiki/Heat_death_of_the_universe

4. Feb 17, 2010

### mathman

Since the universe is continuing to expand, it will continue to cool down. Therefore the temperature will keep getting closer to absolute 0, especially since heat death is a long way off. Furthermore a lot of mass wouldn't be converted to energy - just a lot of dead stars, etc.

5. Feb 17, 2010

### tiredryan

Thanks for your response. If I understand this correctly, then temperature is inversely proportional to the size of the universe. As the universe expands, the temperature decreases.

I am a little confused though. As the universe expands, wouldn't only the vacuum/empty areas increase in size, and the massive objects like the earth remain the same size due to gravity being stronger than the expansion? I though temperature is dependent on the kinetic energy of molecules and on places like earth the volume/distance between molecules/kinetic energy won't change much. In empty space the volume will increase, but there would be little or no molecules whose kinetic energy will changes. It would seem that the expanding universe will have a minimal effect on the temperature of the universe or is my logic faulty?

Last edited: Feb 17, 2010
6. Feb 17, 2010

### Chronos

The CMB is a unique component of the temperature of the universe. Other events contribute to local temperature, but, are far weaker in the grand scheme of things compared to the big bang. In deep intergalactic space, CMB is the overwhelming contributor. This is expected given the BB was the most energetic event in the history of the universe.

7. Feb 18, 2010

### tiredryan

Thanks for your response. So if I understand this correctly the temperature of the universe is governed by the BB since it "the BB was the most energetic event in the history of the universe." So compared to "other events" such as the universe expansion mentioned by mathman, the BB is the major contributor to the current and ultimate temperature of the universe. Does this make sense?

Thanks.

Last edited: Feb 18, 2010
8. Feb 18, 2010

### mathman

big bang is responsible for most of the energy. Expansion is responsible for cooling down.

9. Feb 19, 2010

### tiredryan

Thanks mathman. Do you have an explanation for my prior post? I am having confusion on why physically it is cooling down. Is there something I am understanding wrongly? Here is my confusion again below.

"I am a little confused though. As the universe expands, wouldn't only the vacuum/empty areas increase in size, and the massive objects like the earth remain the same size due to gravity being stronger than the expansion? I though temperature is dependent on the kinetic energy of molecules and on places like earth the volume/distance between molecules/kinetic energy won't change much. In empty space the volume will increase, but there would be little or no molecules whose kinetic energy will changes. It would seem that the expanding universe will have a minimal effect on the temperature of the universe or is my logic faulty?"

Last edited: Feb 19, 2010
10. Feb 19, 2010

### bapowell

Hi tiredryan. As mentioned previously, the temperature that you quote in the beginning of the post is that of the CMB, the radiation "left over" from the big bang. The temperature of the universe is proportional to the energy density. The energy density of intergalactic molecules is exceedingly low, as you correctly reasoned in your post -- and so the CMB is single handedly responsible for most of this energy density.

The CMB is made up of light. Light behaves a bit differently than matter, in that it is wavelike. As the universe expands, so too does the wavelength of the light. Wavelength is inversely proportional to energy density (recall from highschool physics that the energy of a photon is $$E = hf = h/\lambda$$, where $$\lambda$$ is the wavelength). So, as the universe expands, the photons of the CMB redshift: their wavelengths increase along with this expansion, and they lose energy in the process. Hence, in the far future, the CMB will be in the neighborhood of absolute zero, and the universe will be a cold place indeed.

11. Feb 19, 2010

### tiredryan

Thanks. It makes more sense now. I never thought the energy of unabsorbed background photons/light/em radiation is related to temperature. I thought that a dense volume of photons in a vacuum will have the same temperature as a vacuum with no photons. I was trying to imagine the temperature as a function the kinetic energy of molecules, specifically the KE = (3/2)kT. Does that mean that KE = (3/2)kT is not entirely correctly and it should really include a photonic energy density in that equation? This would mean that T = (Proportionality Constant)(Kinetic Energy of Molecules) + (Proportionality Constant)(Energy Density of Photons not Absorbed by Molecules). Thanks.

Reference:
http://hyperphysics.phy-astr.gsu.edu/Hbase/kinetic/kintem.html

Last edited: Feb 19, 2010
12. Feb 19, 2010

### bapowell

In principle, that's correct. If you were to stick a thermometer out in intergalactic space, there would be a contribution to the temperature of both matter particles and photons. However, the contribution from matter is exceedingly low.

BTW, it's a common habit of cosmologists to think of energy and temperature as more or less synonymous quantities (just set c = h = k =1). So, yes, anything with energy (like radiation) also has a temperature.

13. Feb 20, 2010

### tiredryan

Thanks so much for your help. It completely makes sense now. I'll change the title so it says its solved.

Edit: Nevermind. I guess I can't edit posts that are too old. But thanks again bapowell, mathman, and Chronos for your help.

14. Feb 20, 2010

### Chronos

What ATM view are you championing, tiredryan? Your posts have troll written all over them.

15. Feb 21, 2010

### tiredryan

What does ATM mean? I'm just a physics student trying to get a better physical understanding beyond the equations. You can look at my other posts. For example, I still haven't gotten a response for a color question I had. Do you have insights on it? Here's what I posted a several days ago.

"Does anyone have any insights on why solid gold is yellow and colloidal gold is red? What is the physical basis of the red shift in colloidal gold? Here is a link to a picture of colloidal gold.

http://en.wikipedia.org/wiki/File:ColloidalGold_aq.png" [Broken]

PS:
The closest thing I could find for ATM is atmosphere (pressure), but atmosphere (pressure) views does make sense. Also ATM means "at the moment" from wikipedia. Is it what are my views now? CMB was easy to figure out since there is no disambiguitity.

References:
http://en.wikipedia.org/wiki/ATM
http://en.wikipedia.org/wiki/CMB

PS 2: Also I just wanted an introductory physics explanation for a beginning physics student so I posted this under "general physics" but somehow it is now in "astrophysics."

Last edited by a moderator: May 4, 2017