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Alan Guth stated in this article

But let me start the story further back. Inflationary theory itself is a twist on the conventional Big Bang theory. The shortcoming that inflation is intended to overcome is the basic fact that, although the Big Bang theory is called the Big Bang it is in fact not really a theory of a bang at all; it never was. The conventional Big Bang theory, without inflation, was really only a theory of the aftermath of the Bang. It started with all of the matter in the universe already in place, already undergoing rapid expansion, already incredibly hot. There was no explanation of how it got that way. Inflation is an attempt to answer that question, to say what "banged," and what drove the universe into this period of enormous expansion. Inflation does that very wonderfully. It explains not only what caused the universe to expand, but also the origin of essentially all the matter in the universe at the same time.

Is it possible that time started with the universe inflating, and inflation was what caused the universe to start off expanding, then after inflation ended, the universe continued to expand from that kickoff ?

An anology would be : a spaceship that was driven by fuel, then after the fuel ran out, the spaceship continued to move from the inertia.

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  • $\begingroup$ I read that big bang is the inflation and the space itself can expand very very fast so no violation of relativity, the big question is what goes bang. $\endgroup$ – user6760 Jan 22 at 10:13
  • $\begingroup$ The Guth quote is just awful. The part in bold is totally inaccurate. Maybe he's just not very good at writing at the pop-sci level, or maybe he doesn't believe that it's important to be accurate when writing at that level. $\endgroup$ – Ben Crowell Jan 22 at 14:14
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I have to admit that the paragraph you cite does not look like an awfully great piece of scientific writing. (Unless it is followed by a much more understandable explanation of what Guth actually means.)

Let me try to describe the main issue the inflation theory resolved. Scientists observed the sky and measured the Cosmic microwave background in it, the light from the Big bang era. The Cosmic microwave background tells us about the distribution of temperature and density in the Early universe, and it turned out that it contained "too large" coordinated structures. What I mean by "too large" is the fact that when one used the usual Big Bang theory without inflation, the regions were coordinated over distances that could never come into contact.

Of course, you could imprint these coordinated structure in the initial conditions of the universe. The regions never came into contact, they were just created so as to be coordinated, Gottfried Wilhelm Leibniz, the Enlightenment philosopher and mathematician, might call this a "pre-established harmony".

However, inflation proposes a mechanism by which these large coordinated structures arise. One allows a primordial soup to interact with itself in a natural way in a pre-inflation era and then "claps", and inflates the universe by 26 orders of magnitude. Consequently, the resulting structures in the Early universe are exactly what we see in the cosmic microwave background, and they also fit well with the cosmic structures we see in the present era.

So this is what Alan Guth means when he says that inflation explains how "matter got that way" - he means that it provides a deeper explanation for the state of matter in the cosmos that we are able to observe directly (we are unable to observe any direct messenger from the inflation era).

This is of course not the whole story, the devil is in the details of how the mechanism of the inflation works, so this is what Guth means when he says "Inflation is an attempt to answer that question, to say what 'banged,' and what drove the universe into this period of enormous expansion." The answer that Guth proposes and which is accepted by the majority (but not an absolutely crushing majority) of cosmologists is a specific physical mechanism called spontaneous symmetry breaking.

I think that Guth is overselling a bit in the last sentence you cite: "[Inflation] explains not only what caused the universe to expand, but also the origin of essentially all the matter in the universe at the same time." Inflation definitely is a major actor in of how matter has formed in the Early universe, but I do not see a way how it could be considered its "explanation of origin".


As for your other questions about the "beginning of time", "universe getting started", and "what started it all", I recommend one of my older answers on the topic.

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  • $\begingroup$ To me this answer fails to address the main issue, which is that Guth's description is just plain wrong. He just seems to have been incredibly sloppy and inaccurate in a pop-sci description of his own theory. $\endgroup$ – Ben Crowell Jan 22 at 18:06
  • $\begingroup$ @BenCrowell: Well, the article is actually a transcript of a video that seems to be from the 90s or so - so what he is saying might have been an accurate representation of the state of the art. Inflation can create a lot of hot matter by inflaton decay during the final reheating period. If most matter-energy comes from this decay, this can be interpreted as the initial boost of the universe through the Friedmann equation. I have to admit I overlooked this aspect and have a hard time finding out how relevant it is given the CMB and other constraints. $\endgroup$ – Void Jan 22 at 18:43
  • $\begingroup$ All good info, but not al all what the OP asked: "I’m asking if it’s possible that the universe started with inflation, and that inflation was what started the expansion?" The answer simply is that it is not possible. Before you get to the spontaneous symmetry breakdown, you must already have the symmetry, the universe must already have expanded from the singularity before the inflation started. So the inflation cannot be the initial cause of the expansion. $\endgroup$ – safesphere Jan 23 at 5:55
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It is mathematically impossible for an inflating state to arise from a singularity, because the exponential function never reaches zero. If the universe started with inflation, then there was no primordial singularity, no arbitrary high energies, etc. However, if the Big Bang did originate from a singularity, then there must have been a brief period of expansion before the inflation began.

Fro more details see: Inflationary spacetimes are not past-complete

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  • $\begingroup$ Yes, I’m asking if it’s possible that the universe started with inflation, and that inflation was what started the expansion? $\endgroup$ – parker Jan 22 at 12:46
  • $\begingroup$ @parker: I think safesphere answered your question, and the answer was no. I agree with the answer. $\endgroup$ – Ben Crowell Jan 22 at 18:04

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