# Average temp of universe is 3K. So what is colder than 3K?

The average temperature of the universe is 3K. Now we know of many stars which are millions of Kelvin hot and even Pluto is like 50K - so which universe bodies are colder than 3K?

Do you have a reference for the average temperature of the universe?

If we knew how the average was estimated, we might understand what is estimated to be less than 3K.

Are you talking about the 2.73 Kelvin background temperature of the universe? Because that’s not an average,

From Wikipedia ( look for links to published paper)

“The average temperature of the universe today is approximately 2.73 kelvins (−270.42 °C; −454.76 °F), based on measurements of cosmic microwave background radiation.”

https://en.m.wikipedia.org/wiki/Absolute_zero

Yes. Wikipedia calls it average and many physics books call it average too.

I believe the only places colder than 2.7K are on Earth or other inhabited planets with the technology to cool below the temperature of the cosmic background radiation (CBR).

If you put a rock at an arbitrary point in the universe, it is highly likely to be far removed from any radiation source that would heat it significantly. It would radiate away it’s own thermal energy until it cooled to 2.7K and came into equilibrium with the CBR.

It is conceivable that some naturally occurring phenomenon could act as a refrigerator, but I haven’t heard of such a thing.

It’s an “average” in the sense of a ‘typical’ number, not as a mean. Think of it as the median, if you like.

Though, I wouldn’t be surprised if the mean was pretty darned close to it, too, at least up to a few decimal places.

The 2.725K figure is the average temperature of the CMBR. In a map of it such as this one, the difference between the darkest blue (coldest spots) and the reddest red (warmest spots) is 0.0005K. This is where the average comes from.

My WAG: The entire universe is acting as the evaporator end of a refrigerator, because it’s expanding. It started off unimaginably hot, and has been cooling ever since.

Now it’s not quite that simple because space is actually being created out between the galaxies, and because space contains intrinsic vacuum energy. But on balance, it seems that the thermal energy of the universe is being dissipated, so it’s cooling. And to the extent that expansion hasn’t been uniform, some parts of the universe may be colder than the present CBR.

Wolfpup, you know a lot about MP3 players don’t you? Can I get your comments on Pono?

Black holes are considerably colder. By which I mean the hole itself, not the miscellaneous junk in its vicinity, which is probably quite hot. But a stellar mass black hole will have a temperature of about a millionth of a kelvin, and the larger ones are proportionately colder (thus, a billion-solar-mass black hole will have a temperature of about 10^-15 K).

Photosynthesis, which seems to occur naturally on a planet very nearby, might be said to be a naturally occurring refrigerator. This endothermic process absorbs energy (ambient sunlight), and uses that energy to build hydrocarbons and carbohydrates which sequester some of that energy in chemical bonds. That heat energy is removed from the universe outside of the plant itself. That seems to be, roughly speaking, what a refrigerator does.

Black dwarfs maybe? Can they even exist yet?

I have some real trouble with someone reporting an average temperature of the universe, considering that the overwhelming volume of it has a density of less than one hydrogen atom per cubic meter. Can you even say that has a temperature? It mostly just isn’t there. What is the temperature of interstellar space?

Beetlejuice, not to be confused with Betelgeuse, which is a red supergiant.

Plus, is this an average temperature of an area, or the average temperature of a given unit of mass? I’d think that all things considered in the thread, if we are averaging the temperature of an area (for most reference frames), then it would be pretty darn close to the CMB. If instead we are asking the average temperature of a kilogram of mass, then if this theory of the temperature of intergalactic filament density and temperature is correct, then the average mass temperature is much higher, since the temperature from the warm intergalactic medium would overwhelm even the coldness of black holes and possible cold dark matter. Assuming only 5% of the universe is traditional mass, and the rest is at 0K, and thus 2.5% of the universe is in warm intergalactic medium with a low-end temp of 10^5, that still means the average temperature of mass is at least 2500K!

Right, is it a mass-weighted average or a volume weighted average? I’m guessing volume, which isn’t the way to weight an average temperature.

It might make more sense if you think of it as “energy per unit volume of space” rather than the intuitive concept of temperature. The alleged “temperature” is just the blackbody equivalent of the wavelength of photon emissions that began with photon decoupling shortly after matter first came into being after the Big Bang, and which wavelength has been steadily increasing and thus the blackbody temperature dropping ever since.

As a side note, a black hole is really and truly cold, but only in the peculiar sense that matter and energy go in and nothing ever comes out except Hawking radiation, so we would measure extremely low temperature at the event horizon. In reality, as mentioned, the accretion disk outside would be incredibly hot, and if you happened to fall through the EH, depending on the rate of matter influx, I imagine it might be hotter still inside. So yes, a large black hole is very cold indeed, but in a practical sense fairly useless for cooling a case of beer!

Formerly-white black dwarfs do exist in the Universe, but formerly-red ones do not (at least, according to our models). They don’t fit in this discussion, though, because while they’re a lot colder than live stars, they’re still not going to be any colder than the CMB at an absolute lower bound, and probably a lot warmer than that.