Due to the limit of the speed of light, the farthest possible distance we can see is approx. 46.6 billion light years away. This radius describes a spherical boundary beyond which we cannot see.
It has been stated that our universe is, in fact, “inside a black hole”, and the observable universe boundary is an event horizon.
Does that event horizon emit hawking radiation?
I am not an expert on cosmology, but it seems to me that this whole notion that the boundary of our observable universe being an event horizon in the same sense as the event horizon of “normal” sized black holes falls apart if you examine it based on the following premises. First premise - there are no privileged observers, second premise - the universe is infinite, and third premise - the universe is homogeneous on large enough scales.
Now imagine all of the hypothetical non-privileged observers located in a spherical shell centred on the milky way with a radius of 1.01 light years times the age of the universe (current estimates are from 9 to 14 billion years according the NASA) and a thickness of say 100 million light years. Each of these observers has an observation boundary just like we do, and we are just outside the edge of all of them. If all of these observation horizons define black holes, and each of them emits hawking radiation, we should be bombarded on all sides by this. Is there any evidence of this happening?
The temperature and power of Hawking radiation scales inversely to the mass of the black hole. For a black hole with the mass of the observable universe, the temperature and power would be too low to be detectable.
There’s the Penrose style black hole cosmology to consider. From our perspective, we would be in a white hole attached to a black hole in some other universe. In other words, in this hypothetical our universe would appear as a black hole in some hypothetical parent universe. To us, it seems like a white hole, with our universe expanding due to the energy from all the matter the black hole sucks up in the parent universe and converts to “pure energy” AKA what we call dark energy.
From what I understand this is sort of what Roger Penrose has hypothesized. It seems like a nice explanation. At least for some reason it appeals to me on a personal level. Obviously that doesn’t make the hypothesis correct, but there’s at least one prominent cosmologist who holds to something like that.
That used to be a plausible hypothesis, a quarter-century ago. It’s since been disproved.
How has it been disproved? Everything I’ve read on the subject amounts to "We can’t rule it out ".
I can’t find anything that would indicate it has been disproved.
While I’m not qualified to judge it’s merits, here’s a link to part 1 of a paper published in 2022 that argues for a black hole universe: Symmetry | Free Full-Text | The Black Hole Universe, Part I
So it seems that peer-reviewed journals are still accepting papers on the subject.
Prof. Stephen Hawking proposed the idea that a black hole sheds mass via the interplay of virtual particles with the event horizon.
If the cosmological event horizon…aka the Hubble Horizon
Cosmological horizon - Wikipedia
…is in fact an event horizon which radiates hawking radiation, would that cause the horizon to recede away from us, or approach towards us? (assuming the horizon bounds a singularity being shed mass)
More precisely, the simple models where the Universe is a black hole have been ruled out. It used to be that the Big Question is cosmology was just how large the deceleration rate of the Universe is: A deceleration greater than some critical value would lead to a closed universe (which is, fundamentally, the same thing as a black hole), while a deceleration less than that critical value leads to an open universe.
And then, in the late 90s, using the Hubble Space Telescope (but since confirmed by many other independent measurements, using a wide variety of techniques), we discovered that the Universe wasn’t even decelerating at all: It actually has a positive acceleration. And while we have (and already had, for a long time before that) mathematical frameworks that can describe that positive acceleration, none of them are consistent with anything that fits the conventional label of “black hole”.
Now, it may be that even though it doesn’t fit the conventional label, it might fit something that’s qualitatively similar to a black hole in some way. And so we might want to expand the definition of what counts as a “black hole”, and the Universe might be one of these more complicated black-hole-like objects. So there’s still plenty of work that can be done on the nature of the Universe. But the simple question is settled.
I’m not sure there’s anything about black hole cosmology that requires a contracting universe for those inside it. Most things I’ve heard on the subject recently are imagining that we are on the white hole side of an Einstein-Rosen bridge.
The universe doesn’t have to be contracting initially, but it has to contract eventually, because everything is going to eventually reach the singularity.
A white hole’s singularity is in the past, so everything has already reached there.
And after all, since time and space seem to switch places in a black hole, I’m not sure the fact that everything will eventually reach the singularity means the same thing it does on our side of the event horizon.