How many apparent horizons could the Universe have?

In summary, the paper by George Smoot assumes the holographic principle is true and suggests that our universe could be encoded on the "surface" of an apparent horizon as a weighted average of all possible histories. This raises questions about the existence of multiple apparent horizons and the potential for other universes to be encoded on them. It also considers the possibility of individual galaxies having their own cosmological horizons in the future. While this is a theoretical prediction, it has not yet been observed or tested experimentally.
  • #1
Suekdccia
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TL;DR Summary
Different universes in different apparent horizons?
I was reading a paper written by George Smoot [1], which assumes the holographic principle as true and conjectures that our universe would be encoded on the "surface" of an apparent horizon as the weighted average of all possible histories. In that way, there would be one world (or universe) that would be the average among all possible worlds.

However, could more than one apparent horizon exist? And if the answer is affirmative,then, wouldn't there be other universes "encoded" on those other apparent horizons if that was the case? And could some of these other universes be the result of the realization of one particular history (or of a particular set of histories) instead of the average of all possible histories?

[1]: https://arxiv.org/abs/1003.5952
 
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  • #2
Suekdccia said:
could more than one apparent horizon exist?
Yes and no.

In our universe, each observer has one cosmological horizon (which is what "apparent horizon" means in the context of this paper). But different observers at different spatial locations have different cosmological horizons.

Note that this means that the paper's use of the term "the apparent horizon of the universe" is not really correct. Our universe does not have one apparent horizon; it has an infinite number, one corresponding to each spatial location.

If what is actually mean is "the apparent horizon of our observable universe", then there is just one of those and the paper's analysis would apply to our observable universe. But our observable universe is not the same as the universe as a whole.
 
  • #3
PeterDonis said:
Yes and no.

In our universe, each observer has one cosmological horizon (which is what "apparent horizon" means in the context of this paper). But different observers at different spatial locations have different cosmological horizons.

Note that this means that the paper's use of the term "the apparent horizon of the universe" is not really correct. Our universe does not have one apparent horizon; it has an infinite number, one corresponding to each spatial location.

If what is actually mean is "the apparent horizon of our observable universe", then there is just one of those and the paper's analysis would apply to our observable universe. But our observable universe is not the same as the universe as a whole.
Okay. And then, another Hubble sphere (aka another observable universe) would have another apparent horizon, right?
 
  • #4
Suekdccia said:
another Hubble sphere (aka another observable universe) would have another apparent horizon, right?
Yes.
 
  • #5
PeterDonis said:
Yes.
Another question related to this:

As the universe keeps expanding we'll see fewer structures (like galaxies) until we'll reach a point where we wouldn't see any structures that would not be gravitationally bounded to us. Since this would happen also to every other galaxy (or groups of sufficiently near galaxies) then, would in the future each galaxy (instead of huge Hubble spheres) have its own cosmological horizon?
 
  • #6
Suekdccia said:
would in the future each galaxy (instead of huge Hubble spheres) have its own cosmological horizon?
You are thinking of this backwards. It isn't that Hubble spheres stop being "huge" and get small enough to only have one galaxy in each of them. What happens is that accelerated expansion, from the standpoint of any given galaxy (or more precisely gravitationally bound system), carries all other galaxies (other systems not gravitationally bound to yours) out beyond your cosmological horizon--and similarly, from their standpoint, you are carried out beyond their cosmological horizon. The proper distance to your cosmological horizon continues to increase (though it asymptotes to a finite distance, it does not increase without bound).
 
  • #7
Eventually a viewer from every galaxy (or bound system) would see nothing on the horizon? (Assuming enough time passes and each galaxy does not turn into a giant black hole). An observer would not see a 3K degree background radiation, it would eventually be close to, but never reach, zero Kelvin?
 
  • #8
KobiashiBooBoo said:
Eventually a viewer from every galaxy (or bound system) would see nothing on the horizon?
Since you are including background radiation, no, this is not correct.

KobiashiBooBoo said:
An observer would not see a 3K degree background radiation, it would eventually be close to, but never reach, zero Kelvin?
No. A cosmological horizon has a finite temperature. The background radiation you see will never drop below that temperature; it will just eventually be due to the cosmological horizon instead of the CMBR.
 
  • #9
PeterDonis said:
No. A cosmological horizon has a finite temperature. The background radiation you see will never drop below that temperature; it will just eventually be due to the cosmological horizon instead of the CMBR.
That is interesting. Has this been observed or tested experimentally or is it only a theoretical prediction?
 
  • #10
Suekdccia said:
Has this been observed or tested experimentally or is it only a theoretical prediction?
It's only a theoretical prediction at this point. The temperature of our cosmological horizon is far too low (about ##10^{-30}## K) for us to detect the corresponding radiation.
 
  • #11
PeterDonis said:
temperature of our cosmological horizon is far too low
And will be swallowed up by the ordinary CMBR. At all frequencies the CMBR will put out more eneegy.

Indeed, given the CMBR variations, I question whether it could be seen even in principle.
 
  • #12
Vanadium 50 said:
And will be swallowed up by the ordinary CMBR.
Now, yes. But at some point in the future the CMBR temperature will drop below the horizon temperature.
 
  • #13
OK, who wants to wait 1040 years for the answer? Hands please!
 
Last edited:

Related to How many apparent horizons could the Universe have?

1. How is an apparent horizon different from a black hole?

An apparent horizon is a theoretical boundary surrounding a black hole where the escape velocity is equal to the speed of light. It is the point of no return for any object or light trying to escape the gravitational pull of the black hole. In contrast, a black hole is a region of space where the gravitational pull is so strong that even light cannot escape.

2. Can multiple apparent horizons exist within a single black hole?

Yes, it is possible for multiple apparent horizons to exist within a single black hole. This can occur when the black hole is rotating rapidly, causing the shape of the event horizon to become distorted and creating multiple apparent horizons.

3. How many apparent horizons can the Universe have?

The number of apparent horizons in the Universe is not known for certain. It is possible that there could be an infinite number of apparent horizons, as the Universe is constantly expanding and new black holes are being formed. However, it is also possible that there is a finite number of apparent horizons, as the Universe has a finite amount of matter and energy.

4. Can apparent horizons be observed or measured?

No, apparent horizons cannot be directly observed or measured. They are theoretical boundaries and their existence is inferred from the behavior of light and matter around black holes.

5. Are apparent horizons the same as event horizons?

No, apparent horizons and event horizons are not the same. An event horizon is the boundary of a black hole where the escape velocity is equal to the speed of light. It is the point of no return for anything entering the black hole. An apparent horizon, on the other hand, is a theoretical boundary surrounding a black hole where the escape velocity is equal to the speed of light. It is not the same as the event horizon, but it is often used interchangeably in discussions about black holes.

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