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I was reading this interesting article which talks about particle production in an expanding universe.

Usually this process is proposed to have occurred in the early universe, when the expansion was in the inflationary phase and it was so powerful that matter was created in particle production mechanisms.

However, can particles be produced in an accelerating expanding universe like our current one? Can particles be produced by the universe's expansion with the current conditions of our own one?

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    $\begingroup$ If the rate is absurdly small, is the right answer “yes”? $\endgroup$
    – Ghoster
    Sep 25 at 3:47
  • $\begingroup$ Matter production is assumed to have happened after inflation, not during it. When inflation ends, the inflaton field starts oscillating, and through these oscillations the matter particles can be produced (energy is transferred from inflaton to matter). $\endgroup$
    – Kosm
    Sep 25 at 13:24

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This was the hypothesis behind the steady state cosmology. Observations (particularly of the cosmic microwave background) do not agree with its predictions, so it is no longer accepted by most cosmologists.

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  • $\begingroup$ Please, se Anders Sandberg's answer to the question. Does his answer means that particle production associated with the universe's expansion can still be considered? @JohnDoty $\endgroup$
    – vengaq
    Sep 25 at 14:31
  • $\begingroup$ @vengaq It can be considered theoretically, but unless it has observable consequences it is beyond the scope of physics. $\endgroup$
    – John Doty
    Sep 25 at 14:34
  • $\begingroup$ @JohnDoty - last time I checked theoretical physics is counted as part of physics. $\endgroup$ Sep 26 at 9:09
  • $\begingroup$ @AndersSandberg When theories are testable, they are part of physics. When they aren't, they are mere mathematics. $\endgroup$
    – John Doty
    Sep 26 at 10:53
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You get an approach to a finite de Sitter temperature $T=\hbar H/2\pi k_B\approx 2.67\cdot 10^{-30}$ K in a universe expanding forever due to a cosmological constant, and this does produce a very meagre stream of particles as excitations of the fields. So this is something that likely happens in our universe in the standard $\Lambda $CDM cosmology.

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  • $\begingroup$ Thank you for your answer. Just one more question: Once reaching that temperature would this stream of particles (despite being meagre) be produced continually as long as the universe keeps expanding? Is this a hypothesis or is it based on mainstream/verified physics? @AndersSandberg $\endgroup$
    – vengaq
    Sep 25 at 16:11
  • $\begingroup$ @vengaq - Yes, those particles would be produced indefinitely. This is an unverified prediction of standard physics: the Unruh effect makes particle appear if you have an accelerating frame of reference, and the cosmological constant provides such an acceleration. $\endgroup$ Sep 25 at 22:46
  • $\begingroup$ They are produced indefinitely but also absorbed, so they don't accumulate. Their wavelength is on the order of the Hubble length, and (according to this answer by Luboš Motl) the total particle count is on the order of one. @vengaq has been asking various questions about extracting unlimited energy from cosmological expansion, and I'm pretty sure this is another method that won't work. $\endgroup$
    – benrg
    Sep 26 at 1:19

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