The way I've seen the expansion of the universe described, it seems to work a bit like an inside-out black hole: an event horizon beyond which there is no causal connection, light from beyond it can't reach us. The Big Rip is that event horizon getting ever closer (to all points at once rather than just "us", so the analogy is imperfect).

Answers to other questions here say that the equivalent of Hawking radiation in this context is Unruh radiation. If I understand correctly, Unruh radiation gets hotter as acceleration increases, which in this case also makes it like Hawking radiation, by increasing as the surface area of the event horizon shrinks.

I understand that there is some argument about whether or not Unruh radiation creates observable particles in the same way that Hawking radiation does.


If Unruh radiation does in fact create particles, would the positive gravitational pressure from these particles be expected to balance the negative pressure (whatever causes it) that is causing the accelerating expansion of the universe?

If the balance was perfect, I think this would prevent the acceleration from increasing, would my reasoning imply that current evidence says Unruh radiation doesn't create observable particles in the same way that Hawking radiation does?

(My physics level: enthusiastic amateur, I've tested myself with past exam papers as at least a good AS-level in the UK, and have formal qualifications of maths and further maths at A2 level, but my degree is software, not physics).

  • $\begingroup$ FWIW and people are welcome to disagree, but neither Unruh nor Hawking radiation exists. In relativity, some effects are relative and some are absolute. An event of detecting a particle is absolute. If it happens, it happens in all frames. If it doesn't happen in one frame, it doesn't happen in any frame. Quantum mechanics allows for superpositions, but they collapse upon the detection of a particle. So both types of radiation are just a wide spread misconception. $\endgroup$ – safesphere Jul 14 '18 at 3:42
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    $\begingroup$ In the context of cosmological horizons the term to use is Gibbons–Hawking radiation. $\endgroup$ – A.V.S. Jul 14 '18 at 5:56
  • $\begingroup$ @safesphere I understand that's the argument against Unruh radiation; but I thought, in the case of Hawking radiation, it reduces the mass of the black hole at a clearly defined rate? Doesn't that imply the Hawking radiation is absolute? $\endgroup$ – BenRW Jul 14 '18 at 13:48
  • $\begingroup$ It is not absolute, because a free falling observer does not see it (at least not all of it). Per the equivalence principle, both types are equivalent (in the uniform gravity, so the larger the hole, the closer both types are). A reduction in mass of the hole is a logical result, not the cause. The sungularity is irrelevant. A remote observer may see a frozen star with no singularity and no horizon, but per Hawking, the radiation would still exist and reduce the mass of the matter frozen outside the apparent horizon. $\endgroup$ – safesphere Jul 14 '18 at 15:23
  • $\begingroup$ @A.V.S. I see your earlier answer on the other type of radiation. Very interesting indeed and insightful as always: physics.stackexchange.com/questions/21830/… $\endgroup$ – safesphere Jul 14 '18 at 15:47

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