# If the universe is full of dark matter, why is it only 2.73 K cold?

people!

I am just a physics layman, but I recently watched a documentary about the universe and it was told that

1. the universe is full of dark matter and energy and
2. the universe is empty, so that the average temperature measured by the cosmic microwave background is about 2.73 Kelvin.

They also said, that temperature is (also) depending on gravity or mass respectively. That's why the sun is so hot, since there is huge mass concentrated, and the empty space is cold, since there is no mass. Or is it?

So now my question(s): IF there is almost only dark matter and energy in the universe with HUGE mass effects which explains interstellar movement etc., why is the universe only 2.73 K cold? Shouldn't it be warmer? Isn't the 2.73 Kelvin the temperature you get, if you calculate the temperature without dark matter and energy?

The temperature 2.73 K is not calculated, it is measured. The cosmic microwave background (CMB) has the properties of a blackbody radiation at the temperature 2.73 K. It is based just on the measurement of CMB, no calculation of dark matter or dark energy is involved.

Temperature does not simply depend on mass or gravity. Temperature is a quantity which in a lot of situations is difficult to define. For example a black hole can be extremely massive and have huge gravity, but the black body radiation from such a black hole is extremely cold. If there is nothing falling into the black hole, there won't be any other radiation.

The dark energy particles (if dark energy is made up of particles) do not interact electromagnetically or strongly (and maybe not even weakly), so they play virtually no role in our discussions about what we call "temperature of the universe".

We know basically nothing about dark energy, so it also is not included in our definition of temperature.

• So basically dark matter ONLY exists to describe something, but it doesn't generate no heat, no light or any other measurable particle/wave/effect except for gravitation. Dec 6, 2013 at 15:18
• I know that this does not directly relate to the question, but can you please elaborate a little upon " If there is nothing falling into the black hole, there won't be any other radiation."? Dec 6, 2013 at 15:18
• Is it possible that there is no dark matter or energy and something is wrong with the mathematical methods we use? I have no idea about them, it's just: it seems strange that dark energy and dark matter was calculated or "measured" just in the moment the movement of galaxies could not be described... Dec 6, 2013 at 15:21
• @JanHirschner: Since it is hard to hit the black hole exactly, most infalling matter will end up orbiting the hole for some time until it loses enough energy from inelastic collisions to finally fall in. This lost energy eventually becomes heat and is radiated away in addition to the Hawking radiation from the hole itself. (This glowing matter is how we actually observe most objects considered to be black holes). (And even a directly infalling object would be heated a bit by tidal stresses before it reaches the event horizon, I suppose). Dec 6, 2013 at 15:49

Mass in and of itself will not generate heat. Heat comes about when two objects interact with each other i.e. when they change from one state with a given energy to another. More specifically, if an object has more energy after an event than it did before, the net gain in heat would be negative for the surroundings (the environment becomes colder), and vice versa.

Note I use the word "object" here to mean anything that can partake of a physical interaction via a known force (gravitational, electromagnetic, weak or strong).

Now we know dark matter is called "dark" because it is very hard to detect i.e. it does not partake of physical interactions readily. We cannot "see" it in the sense that it doesn't emit light (so they do not interact electromagnetically). The question of whether dark matter is baryonic (partakes of strong interactions) or not is still open to debate. So if dark matter particles only exert a gravitational force on their surroundings (the weakest of the known forces), then the amount of "heat" generated would be very little.

You can also think of it like a balloon. When a balloon expands under constant internal pressure, it's inside cools down. Similarly, the Universe' expansion cools it down. The fact that it is 2.7K despite all the stuff inside it is down to how much empty space there actually is.

Light itself has temperature.

If you let it fall upon a cold black body long enough, the body would warm up to that temperature and emit light of that temperature itself.

What's the temperature of sunlight on the earth? About 5000K. It's the temperature that the surface of the sun had about 8 minutes ago. The sun could have extinguished 8 minutes ago, but for the next 8 minutes the light would have that temperature.

The big bang was a flash that filled the then-universe. Due to expansion of the universe that light has been getting colder (i.e. longer wavelength), so now it's about 2.73K, and it's all around us. It does not come from the matter in the universe.