Cosmological expansion and uncertainty

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Discussion Overview

The discussion revolves around the implications of cosmological expansion and its acceleration, particularly in relation to particle/antiparticle pairs and Hawking radiation. Participants explore theoretical scenarios and the nature of horizons in cosmology versus classical black holes.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant proposes that if cosmological expansion is accelerating, it could lead to particle/antiparticle pairs being separated at superluminal speeds, drawing an analogy to Hawking radiation.
  • Another participant counters this by explaining that the term 'acceleration' refers to the increasing rate of recession of distant objects, which is influenced by the Hubble Parameter that is declining over time.
  • A third participant questions whether Hawking radiation at the cosmological horizon is predicted due to the expansion, suggesting a connection to cosmological vacuum temperature.
  • One participant clarifies that while Hawking radiation is not observer-dependent for classical black holes, it is for the cosmological horizon, leading to disagreements among observers about its location.
  • A later reply acknowledges the previous point about observer dependence but suggests that the relative location of the cosmological horizon is what matters for vacuum temperature agreement among observers.

Areas of Agreement / Disagreement

Participants express differing views on the implications of cosmological expansion and the nature of horizons, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion involves complex concepts that rely on specific definitions and assumptions about cosmological models and observer perspectives, which remain unresolved.

T S Bailey
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Firstly, I assume that I'm correct in assuming that since expansion is accelerating it will increase to any arbitrarily large value at some point in the future. If this is true, there must be some point at which particle/antiparticle pairs (due to uncertainty) are carried away from one another at greater than the speed of light, analogous to the way in which hawking radiation is created at a BH's event horizon. Is there anything wrong with this thought experiment?
 
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That won't happen, because that's not what the 'acceleration' means. The rate at which particles move apart is proportional to the distance between them, and that constant of proportionality - the Hubble Parameter - is actually declining over time.

The use of the word 'acceleration' is to signify that a particular object that is currently receding at rate R m/s will in a billion years be receding at a greater rate (expressed in m/s), unless local gravitational irregularities interfere with that. It will be receding faster simply because it is farther away.

However if a pair were able to remain permanently separated, they would end up receding from one another superluminally once they were far enough separated.
 
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For a classical black hole, Hawking raditation is not observer dependent. This is untrue for the cosmological horizon. Every observer in the universe will disagree on the location of the cosmological horizon, whereas they will all agree on the location of the event horizon of a black hole
 
Chronos said:
For a classical black hole, Hawking raditation is not observer dependent. This is untrue for the cosmological horizon. Every observer in the universe will disagree on the location of the cosmological horizon, whereas they will all agree on the location of the event horizon of a black hole

Thanks, good catch!

Though I don't think that is essential for the derivation [but I didn't check]. Everyone at rest agrees on the vacuum temperature, so the relative CH location is what counts.
 

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