# I Observable Universe Tempature and CMB

1. Dec 28, 2017

### Arman777

It may sound stupid but something bothers me and I want to ask

This question come to my mind due to another thread,
https://www.physicsforums.com/threa...is-my-logic-faulty.935533/page-2#post-5910474

In Friedmann Equation we are assuming that universe is homogeneous and isotropic in large scales.

I think its possible to assume that the universe may not be homogeneous larger then the observable universe scale, since we cannot observe it.

"Moreover, theory suggests that this may not be the end of the story. According to inﬂationary theory, the universe continues to be homogeneous and isotropic over distances larger than 3000 Mpc, but it becomes highly inhomogeneous when viewed on scales much much larger than the observable patch."

Mukhanov, V. (2005). Physical foundations of cosmology (p. 3).

Lets assume universe is not homogeneous and isotropic , in that case, Is it possible that observable universe can get heat from surroundings, (you can think as its like a multiverse theory ), so that the tempature is also spreading homogenous and we dont see it at CMB.

Or , that the universe may lose heat ?

It's a hypothetical question but I am just asking is it possible or not.

Thanks

Last edited: Dec 28, 2017
2. Dec 28, 2017

### Staff: Mentor

If these surroundings could interact with the observable universe (as we see it) they would be part of the observable universe.
If A can influence B and B can influence C then A can influence C.

3. Dec 28, 2017

### Arman777

I didnt quite understand it.

4. Dec 28, 2017

### Bandersnatch

Let's say you're looking towards the edge of the observable universe, and you're seeing some temperature.
You're asking (I think) if that temperature could be due to something farther away transferring heat into the observable bit, or vice versa.
But since this transfer must be happening at speeds slower than the speed of light, then its effects couldn't be observed because you're already seeing to the farthest possible distance (at your current epoch).

In other words, the hypothetical regions of different temperature are beyond our past lightcone, so they couldn't have affected what we're observing now.

In yet another words, imagine a non-expanding universe (for simplicity) of finite age $t$, so that the distance to the edge of the observable patch is just $ct$. If there was a region of different temperature at a distance $ct+d$, then it is obviously beyond the observable universe. It would have to affect the observable patch before the age of the universe or at speeds faster than the speed light, neither of which makes sense.

5. Dec 28, 2017

### Arman777

But it might have been affected it in the past, or might it affect in the future ?

In this case wouldnt be able to see the affect of it after a time ? Since universe is not expanding ? So yes the different tempature region is in the position of $ct+d$ and lets suppose It started to affect at $t_0$. So the energy transfer will take some time $t_1$ and then this affect will come to us as speed of light so after a $t_1$ and $t$ later (or $\frac {R} {c}$ where R is the radius of that universe) we will see the difference ?

Last edited: Dec 28, 2017
6. Dec 28, 2017

### Arman777

Well since the universe expanding and while it expands the light travels more distance due to expansion of the universe, we might not able to detect it.

7. Dec 28, 2017

### Bandersnatch

You could see it in the future, yes.
Notice, however, that since the heat transfer must, necessarily, be slower than the speed of light, by the time the transfer is completed and its results communicated in the form of radiation, you will also see the region from which the transfer had originated. So, by then you'd not be seeing the observable universe exchanging heat with some unobservable part, but heat being exchanged between parts of the observable universe.

I'm not sure it changes anything. Just the fact of expansion doesn't limit our ability to observe the rest of the universe. What does affect it is the accelerated expansion, due to which causal patches tend to separate.

8. Dec 28, 2017

### Arman777

I see now. From this result can we conclude that the outside of the observable universe should be homogeneous or at least, thermal equilibrium with the observable universe, since we didnt see any change in the CMB so far ?

9. Dec 28, 2017

### Bandersnatch

I don't think it follows from the previous considerations. In principle, tomorrow the observable universe could start encompassing some region(s) with vastly different temperature or composition.
The argument one could make is a statistical one, which is what you seem to have expressed in the second part of the quoted bit: that since after all this time the ever-larger observable universe still looks homogeneous, it's unlikely that it looks different just outside what we can see. But, again, it's a Bayesian argument, not a deterministic one.

10. Dec 28, 2017

### Arman777

I see thanks a lot

11. Dec 31, 2017

### bapowell

12. Dec 31, 2017