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If I understand it right, the universe will at some point in time expand so fast, that from any particular point in space, an observer will see an event horizon around himself.

This event horizon will radiate Hawking Radiation relative to the size of its surface.

As the universe expands faster, the event horizon becomes smaller.

The smaller a region of space is, the bigger its surface is relative to the space it contains.

So the smaller the event horizon gets, the less hawking radiation is emitted, but also the more “dense” the hawking radiation inside the event horizon gets.

The Hawking radiation bends space-time so that the expansion is locally slowed.

So, the faster space expands, the more it is slowed down by Hawking radiation?

Is there a point where the Hawking radiation would be so “dense”, that a black hole would form, that itself would give off Hawking radiation?

Would this create a region of space, that has an outer and an inner event horizon, the outer from its own expansion, the inner from a black hole inside?

Could the pressure from Hawking radiation press matter together?

Is there a speed of expansion, where the hawking radiation gets so “dense”, that it can locally slow down the expansion enough for matter from the Hawking radiation to accumulate?

Would the region slowed down by Hawking radiation be limited enough, that the surrounding space would still expand fast enough to create an event horizon?

Could the accumulation of matter happen fast enough for the mass of a universe to accumulate, before the event horizon would “break down” from the slowed down expansion?

Would such a slowed down region of space with it's newly created matter expand and behave in a way, that is compatible with what has been observed for our universe?

In this description, I might not always use the right terms and there are a few sub-questions, that come from parts of the question, that I am aware are unclear to me and that I did not want to formulate as statements. An answer to my question can take the sub-questions into account by using answers to them to answer my question, or by showing, that they are not relevant.

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I suspect you have heard a slightly garbled account of a de Sitter horizon.

Dark energy makes the expansion of the universe accelerate, and since the amount of matter (both ordinary matter and dark matter) in the universe is approximately constant the expansion dilutes the matter and reduces its density. By contrast dark energy is not diluted and the result is that the universe becomes dominated by dark energy. Under these conditions an event horizon called a de Sitter horizon forms as a sphere around the observer. The de Sitter horizon produces Hawking radiation that shines inwards towards the observer at the centre of the sphere. This is discussed in the answers to de Sitter cosmological limit though the discussion there is rather technical.

You can kind of think of this as the opposite of a black hole. With a black hole the observer is outside the horizon and sees the Hawking radiation shining out. With a de Sitter horizon the observer (i.e. us) is inside the horizon and sees the Hawking radiation shining inwards. Though don't take this too literally as when we look at the maths the two horizons have important differences.

Anyhow, the intensity of the Hawking radiation depends on the radius of the de Sitter horizon and the smaller the radius the higher the intensity (again much like a black hole).

But as far as we know the dark energy concentration is constant and doesn't change as the universe expands, and this means the de Sitter horizon will have a fixed radius of somewhere between 15 and 20 billion light years. A horizon this big produces a vanishingly small amount of Hawking radiation and that isn't going to have any effect on the expansion. In this case the scenario you suggest is impossible.

The de Sitter horizon can only shrink if the density of the dark energy increases as the universe expands. If the density of dark energy increases with the expansion then the increased density makes the expansion accelerate even faster, and that increases the dark energy density even more, and a vicious cycle develops. This leads to a catastrophic conclusion called the Big Rip. If I understand your question correctly you're asking what happens as a Big Rip approaches.

And the simple answer is that we have absolutely no idea because the Big Rip is a singularity and that makes it impossible to calculate what happens at the rip or afterwards. In principle a theory of quantum gravity would allow us to calculate what happens at the rip, but since we currently have no theory of quantum gravity we are unable to perform that calculation.

But while the idea of a Big Rip sounds exciting I should emphasise that there is absolutely no evidence for it at the moment. As far as we can tell the dark energy density is constant and that means the universe will settle down to a steady state with a constant de Sitter radius.

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  • $\begingroup$ I think what he may be thinking of is not a "Big Rip", but inflation. If the Universe expanded far faster at an early point, could the horizons become small enough that matter is thus produced by their incandescence? $\endgroup$ – The_Sympathizer Jul 6 at 23:24
  • $\begingroup$ @The_Sympathizer In most theories of inflation (and there are many, many theories of inflation) the energy density during inflation is far too high for matter as we see it today to exist. The matter we see today was created at the end of inflation when the inflaton field decayed and transferred its energy to the other quantum fields. This is not related to the Hawking radiation produced by the de Sitter horizons. However the Hawking radiation was important as it created the primordial density fluctuations that we see in the cosmic microwave background. $\endgroup$ – John Rennie Jul 7 at 11:25
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So far as we know, nothing ever leaves the universe, so it cannot emit Hawking radiation though black holes within it can. The black hole has to form first, the Hawking radiation comes after. Current thinking is that the universe, mysteriously speeded up by dark energy, will go on expanding for ever, but not everyone agrees. The newly created matter you refer to was created more than 13 billion years ago, very shortly after the Big Bang. This should have created an equal quantity of antimatter at the same time, but nobody knows what happened to it. New matter is not created on a large scale today. Hawking radiation does not slow down the expansion of the universe, but we need to find something which does if we support the Big Bang-Big Crunch-big Bounce theory of an eternally recycled universe. Gravity was doing the job until dark energy came along and threw a spanner in the works. We await further developments.

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