# Why do they say universe will become cold with expansion?

The Universe is expanding and they say it will eventually become cold, and new star formation will stop.

But galaxies are only moving away from each other, and each galaxy itself stays intact and the star formation etc activities happens inside galaxies.

So why would expansion of universe affect this when galaxies themselves are not expanding?

While the universe is expanding, it is true that this expansion does not affect the relative position of astronomical bodies within galaxies.

But when and how galaxies become quenched$$^1$$, is a complex process and the exact mechanism is the subject of many astronomical studies.

It seems unlikely that universal expansion on it's own is the reason why galaxies become quenched.

$$^1$$ Galaxies that have stopped star formation are referred to as being “quenched.”

My layman understanding is that the galaxies expend their energy and get cold and with nothing new coming in due to expansion they stay that way.

• What do you mean by expending their energy? Total energy is conserved. I take it you mean kinetic thermal energy. How do they use it up? Jul 8 '21 at 4:05
• Planets cool, stars exhaust their fuel and burn out, etc. The energy is used up and ultimately converted into low-grade heat which itself spreads out and once the distribution of energy reaches equilibrium no more energy can flow back and forth to do work. Jul 8 '21 at 4:06
• Where does the energy go? You are saying that it is converted to heat over time, which goes against the idea of cooling down Jul 8 '21 at 4:09
• For heat to do work there needs to be a gradient across which it flows. In a closed system, if the temperature of everything inside it is the same, no heat flow occurs because it is already at equilbriium. Agreed? At that point the heat can do no work because it cannot flow. If the heat from all the galaxies spreads out over expanding space, everything eventually reaches equilibrium (even if the universe wasn't expanding, as long as it isn't shrinking). That's why car engines, which are heat engines, run much hotter than ambient. Jul 8 '21 at 4:11
• The question however is not about a galaxy, or the ability to produce work, but about the temperature of the universe. The temperature is the average kinetic energy per particle. It is an intensive quantity. For a given amount of thermal energy and a given system, the average temperature is the same. I don't agree with the idea that spreading the energy means a lower average temperature (distribution varies but average is the same). Lowering the temperature implies a transfer of thermal energy to something else. Jul 8 '21 at 4:17

When space expands, the overall temperature goes down, because there will be more empty & cold space. Even if the galaxies themselves aren't cooling down, for the vast majority of the universe it'll be cold.

In the very far future even galaxies will be cold, since there won't be enough material left to form stars.

• Expanding a gas doesn't mean cooling it. It very much depends how you do it, but you can have isothermal expansions (they are notably part of a carnot cycle). Ideal gas law doesn't linearly relate temperature and volume, unless you are barostating a closed system . Even if it did, modelling the universe as an ideal gas seems like a bold idea to me Jul 8 '21 at 3:57
• When a gas expands, it cools down (see ideal gas law).” The ideal gas cools down when expanding only if it’s in contact with something cooler or if it does work. In your model, what is the universe doing work against or being cooled by through contact? Jul 8 '21 at 4:02
• Ok, let's remove the comparison to ideal gases (although that is how I remember the relation between the scale factor and the temperature of the universe). Jul 8 '21 at 4:22