According to the Friedmann equations big bang happened everywhere, but if we account for inflation theory, big bang did not happened everywhere. How ? Anyone please explain. Please see the video for reference.

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    $\begingroup$ Could you explain what you mean by "_ if we account for inflation theory, big bang did not happened everywhere_", so that we don't have to watch a video? In the standard paradigm, inflation happened everywhere. $\endgroup$
    – pela
    Commented Jan 31, 2018 at 20:37
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    $\begingroup$ No, one of the main assumptions of the Friedmann equations is that the universe is homogeneous and isotropic. Thus is becomes one of the assumptions that if the BB happened anywhere, it happened anywhere. It is NOT a consequence of the FEQ. $\endgroup$ Commented Feb 1, 2018 at 0:45
  • $\begingroup$ The Big Bang did happen everywhere, and inflation then spread everything by exponentially expanding the space between things. No contradiction there. $\endgroup$ Commented Feb 1, 2018 at 5:24
  • $\begingroup$ @pela I am unable to grasp the logic given by Dr. Max tegmark so wouldn't be able to help on this. You can watch that video, it is of 2 minutes only. $\endgroup$
    – Gauti
    Commented Feb 1, 2018 at 6:26
  • $\begingroup$ The Big Bang happened everywhere in the inflation theory. $\endgroup$
    – peterh
    Commented Feb 1, 2018 at 11:02

2 Answers 2


Okay, I think I know what Max Tegmark is talking about in the video. He is referring to the fact that, when you observe the cosmic microwave background radiation (CMB) — i.e. the radiation that was "released" when the Universe had expanded and cooled sufficiently to allow protons and electron to combine into neutral hydrogen without immediately being ionized again — you find that it has almost (to within $1$ in $\sim10^5$) the same wavelength everywhere, no matter in which direction you look, meaning that the temperature of the Universe was almost the same when it was emitted.

In a universe like ours, but without inflation, this is puzzling. The reason is that, using the observed expansion rate and densities of the universe, you can calculate that at early times such a universe would simply expand so quickly that it would be impossible to reach thermodynamical equilibrium on such large scales as the Universe we observe today. In fact, when we look at the sky, any two points separated by more than roughly 1 degree would never have been in causal contact, i.e. been able to exchange information. Thus, there would be no way for one of these points to know e.g. what is the temperature of the other points, not to mention the rest of the universe.

This puzzle is called the horizon problem.

CMB Map of the CMB sky. The color coding shows the temperature of the radiation, and red is roughly $18\,\mu\mathrm{K}$ hotter than blue.

Credit: ESA/Planck Collaboration (plus my own annotations).

What Tegmark points at is that although we're told that "Big Bang happened everywhere", this doesn't make sense, because how can one part of this "everywhere" know about the other parts of "everywhere"?

One solution, obviously, is pure luck. After all, the regions on the sky are a little bit different. But this is a very unsatisfying solution, known as fine-tuning, which we loathe.

Enter inflation.

If you assume the our Universe was much, much denser in the beginning, but then went through a period of extreme (!) expansion, then in the very first tiny fraction of a fraction of a second, before this epoch which started when the Universe was $\sim10^{-36}$ seconds old, everything was so close to each other that a region much, much larger than the part of the Universe that we can see (the "observable Universe") could exchange information and reach thermodynamical equilibrium.

The reason that many people believe in this scenario, I think, is that it simultaneously solved two other puzzles, known as the flatness problem and the magnetic monopole problem (and that, so far, no better scenario has been proposed).

So, to recap, Tegmark is not saying that Big Bang didn't happen everywhere, but rather that, before Alan Guth proposed the inflation scenario in 1979, but after we first observed the CMB, this didn't make sense.

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    $\begingroup$ Great explanation! $\endgroup$
    – zephyr
    Commented Feb 2, 2018 at 16:33

You seem to be misunderstanding what inflation does. Lets say the entire universe is two stars and those two stars are gravitationally attracted to each other, but space is also expanding between them so the stars move apart from each other. Both stars happened inside the big bang, and as they move apart, the space between them that inflation creates happened inside the big bang as well.

Inflation doesn't happen beyond the big bang, it happens inside where the big bang happened. It could also be said that the big bang is inflation, at least, that's what it is as far back as we can tell what it is.

(I hope my layman's explanation isn't too far off). The Minute Physics Everywhere Stretch video explains it pretty well.

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    $\begingroup$ Your intention was to simplify things, but I think it confuses things. "Both stars happened inside the big bang" - but stars didn't form until at least 380 million years later; maybe "particles" would be better than "stars". And "the space between them that inflation creates happened inside the big bang" conflates two different processes: the metric expansion of space starting the instant after t=0 and continuing ever since, and a hypothesised inflationary epoch lasting for the briefest flash (t=10^-36 to t=10^-32 seconds). But well done on link to a good video :-) $\endgroup$ Commented May 4, 2018 at 0:45

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