1

If the universe is expanding at an accelerated rate it would mean that the more we go in the past, the slower the speed of expansion must have been. Provided the acceleration was always non-zero and in the same direction, at some point in time in the past, it should have been stationary, and before that the speed should have been in the opposite direction, and the universe should have been contracting. Does this make any sense whatsoever? Assuming it does, and assuming that the point at which it was stationary was during the big bang, would this imply that there was a 'big crunch' before the big bang and possibly what actually initiated it?

Urb
  • 2,608
  • 1
    The acceleration initially slowed with time then speeded up again. See How does the Hubble parameter change with the age of the universe? for a summary of how the expansion changed with time. – John Rennie Jan 05 '21 at 18:14
  • 1
    See pages 5 and 6 in https://arxiv.org/abs/hep-ph/9906447v1 where plots of the universe's evolution are given for some combinations of density parameters. – Photon Jan 05 '21 at 19:14
  • Thanks for the replies, it seems like the rate of expansion was more or less similar through time, with small deviations in the speed. Although I wanted to clarify two things: are those equations reliable in making predictions or estimated extrapolations based on limited data? Secondly is it still plausible that the universe contracted before, creating a singularity which then caused the big bang? –  Jan 05 '21 at 19:37
  • “If the universe is expanding at an accelerated rate” - But it doesn’t. This “discovery” has been debunked time and again. – safesphere Jan 06 '21 at 15:51
  • 2
    @safesphere How do you mean that? The way you state it, it sounds wrong to me, do you have some source for the statement? – Photon Jan 07 '21 at 16:08
  • @Photon You can search the web to find numerous sources. Here is just one by Sabine Hossenfelder, a highly respected German theoretical physicist: http://backreaction.blogspot.com/2019/11/dark-energy-might-not-exist-after-all.html – safesphere Jan 09 '21 at 00:45
  • @safesphere Ah, that's what you mean. Well, I don't have the impression that the analysis made in this paper is already consensus in the scientific community. Basically, they reverted the correction for the kinematic dipole, worked with the original data and got an acceleration parameter which is consistent with 0 at 1.4sigma (which is not great). But I haven't read the details of their analysis (which are rather technical) and thus still don't understand how they dealt with the kinematic dipole eventually. I mean, we still want to save the statistical isotropy assumption right? – Photon Jan 09 '21 at 09:10

2 Answers2

1

FLRW models have two types of solutions. In one type, which requires a large value of the cosmological constant, you get a "big bounce." Such large values of the cosmological constant have been ruled out by observation.

In the other type of solution, which is the type that is consistent with observation, you get a singularity rather than a bounce, and there is never a stationary point or any contraction.

user284781
  • 11
  • I see, but instead of a big bounce can it be said that the singularity was created in the first place by a big crunch, which then caused the big bang? –  Jan 05 '21 at 19:23
0

Re "In one type, which requires a large value of the cosmological constant, you get a 'big bounce'". https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric#Interpretation This says that: "The cosmological constant, on the other hand, causes an acceleration in the expansion of the universe." Therefore this does not seem to allow for a bounce.

Buzz
  • 354
  • 2
  • 11