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Why did the universe expand from the Big Bang singularity? If gravity was a separate force and in its current form by 1 Plank time, would not have gravity prevented expansion, unless there was some other stronger repulsive force?

What drove the inflationary expansion of the universe? I understand that inflationary expansion explains observed effects such as the uniformity of the universe, and that it may have prevented the universe from collapsing into black hole, but what was the cause, as well as the cause of the timing? Why did it stop, or did it?

Why is the rate of the expansion of the universe increasing? I understand this observation is the source of the realization that Dark Energy exists, but why accelerate?

FritzS
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  • The YouTube series PBS Space Time has some nice layman-accessible talks on this, both explaining why dark energy is a candidate for this (and why it's misnamed), and other candidates. They seem to be correct (inasmuch as I'm just an informed layman when it comes to general relativity and cosmology). There are some Royal Institution talks by noted physicists there, too. Study up! – TimWescott Dec 04 '19 at 19:09
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    The correct way to phrase "why is the rate of the expansion ... increasing?" should probably be "why is the apparent rate of expansion..." because the first answer is "well...". Google "crisis in cosmology". The answer, at this point, is "we dunno, and we're looking forward to new physics". – TimWescott Dec 04 '19 at 19:11
  • Do I interpret that your answer to the title question is, "we dunno"? That would be a fine answer, but I wonder why it was accepted for ~70 years before Dark Energy realization that the universe was expanding. Was there no reason other than Hubble observation? – FritzS Dec 04 '19 at 19:17
  • Dark energy is not the cause of the expansion. It just makes the expansion faster than without it. – D. Halsey Dec 04 '19 at 19:33
  • My best understanding of the interpretation of the phrase "dark energy" for non-cosmologists is "we need some term to drop into the equations to make the math work, and we're already using the term 'dark matter' so let's call it 'dark energy'". But it's never been the only accepted answer, and no one's ever been able to describe it in specific terms: the best that the physics community has done so far is to say it must be there and it's energy-like. – TimWescott Dec 04 '19 at 20:52
  • @D. Halsey, what then is the cause of expansion? – FritzS Dec 04 '19 at 21:46
  • @FritzS That is the subject of several other questions on this site, including this one of mine. https://physics.stackexchange.com/questions/430419/why-isnt-an-infinite-flat-nonexpanding-universe-filled-with-a-uniform-matter – D. Halsey Dec 04 '19 at 22:00
  • @D. Halsey, Unfortunately, my level of understanding prohibits me from seeing any similarity between my question and your question 430419, nor the question comments. Additionally, I do not see how the answers and answer comments address my question. – FritzS Dec 04 '19 at 23:46
  • @TimWescott, Does dark energy cause any gravitational effects, or is it gravitationally neutral, because it has uniform distribution? – FritzS Dec 05 '19 at 18:37
  • @TimWescott, I can imagine one gravitational effect of the expansion is / would be the weakening of all gravitation interactions due to increase in distance between interacting bodies? – FritzS Dec 05 '19 at 19:07

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Why did the universe expand from the Big Bang singularity? ... unless there was some other stronger repulsive force?

Expansion from the Big Bang has nothing to do with repulsive force. It's just an initial condition. If you throw a stone up, it's going upward (expansion) rather than downward (contraction), albeit the stone only experiences the downward gravity of the earth after it departs from your hand. But who sets the initial condition of the cosmos? Ask the almighty God.

What drove the inflationary expansion of the universe? but what was the cause ... of the timing?

The common inflationary expansion hypotheses DOES require repulsive force (to get the desired e-fold), which is usually incarnated as a scalar field. But who dictates the timing and graceful phasing out of the inflationary saga? Ask the semi-almighty Alan Guth.

Why is the rate of the expansion of the universe increasing?

It's widely believed that positive Dark Energy (by virtue of its negative pressure, bear in mind that a negative value of Dark Energy/cosmological constant would set the cosmos on a course of deceleration as opposed to acceleration) is the the driver of the accelerated expansion of the universe. But are we sure about the measured values of expansion rate (Hubble constant $H_0$ and the recent brouhaha surrounding the $H_0$ tension) and the acceleration rate (deceleration parameter $q$) for the current universe? Ask the bemused Adam Riess.

MadMax
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  • I have a hard time understanding an initial condition involving velocity / acceleration before space-time exists. – FritzS Dec 04 '19 at 19:28
  • @FritzS, for any question regarding "before space-time exists", you have to pray to the Almighty. He/She might twitter back to you. – MadMax Dec 04 '19 at 19:34
  • If the cause of initial expansion is initial condition, then should not gravity immediately begin reducing the rate of expansion? – FritzS Dec 04 '19 at 19:39
  • @FritzS,that's correct, it's decelerated expansion right after the Big Bang, due to gravity, until Alan downloaded the inflationary app which sets acceleration, and deceleration again after the app expires, and acceleration once more after the Adam downloaded Dark Energy. Confused? Wondering why? Ask Alan and Adam. There is non-zero chance that they will twitter back to you. – MadMax Dec 04 '19 at 19:46
  • I do have Alan's book "The Inflationary Universe" sitting on table next to me. I should read it, or will this not provide any answers? – FritzS Dec 04 '19 at 19:51
  • I do appreciate humor, and I was glad to be correct about something, which is not usually the case at this site. – FritzS Dec 04 '19 at 19:53
  • I take from your last comment that the answer to the title question is that Dark Energy did not enter into the picture until after the initial and inflationary expansions? – FritzS Dec 04 '19 at 19:55
  • @FritzS, in the early stages of the universe, Dark Energy (cosmological constant $\Lambda$ in the $\Lambda CDM$) is too weak to be of any consequences, compared with the other elements such as radiation, (dark) matter, and inflaton. – MadMax Dec 04 '19 at 20:01
  • The expansion of the universe is a result of assuming the universe as being homogeneous and isotropic. If you want to gather evidence to refute this assumption, just point a telescope at the sky. As for the recent acceleration of the expansion in our epoch, it's a artifact of the time component of the FLRW metric not accounting for the time dilation of an accelerated observer. – Cinaed Simson Dec 04 '19 at 20:08
  • @MadMax, at the risk of getting more down votes, I interpret your last comment to mean that dark energy is a property of space-time, as it is "too weak to be of any consequence" in the early stages of the universe when it is much smaller than today, where it has become ~70% of the energy of the universe? – FritzS Dec 05 '19 at 17:44