Bad Astronomy: The Expanding Universe

I'm not going to join the chatter on the BICEP2 result as there's already a lot of commentary out there (although ABC News 24 seem to have missed it, so no chance to mess it up :)

So, a tiny post today. Here's a picture from the BBC story.

This is supposed to be the Big Bang, with galaxies rushing from a  central explosion.

I've written about this before, but this is not what the Big Bang was. It was not an explosion in preexisting space.

If we take the cosmological parameters as we currently know then (and assume a simple topology) then the Universe is now infinite in extent. And it was a billion years ago, and it was 13.5 billion years ago, and it was throughout the history of the Universe.

When the Universe was a 100 million billion degrees celsius, it was this temperature everywhere throughout the infinite Universe. As it cooled down, it cooled down everywhere in the infinite Universe. When it got to a few hundred thousand years after the Big Bang and the plasma neutralized, and the radiation which became the CMB could flow freely through the Universe, this happened everywhere in this infinite Universe.

It was not an explosion.

I know I bang on about this, but hearing the media stumbling over this, unable to understand the meaning of things like BICEP2 because they don't get the basics of the Big Bang, gets a little depressing.


  1. This comment has been removed by the author.

  2. Two points. First, we really can't yet say whether the universe is infinite or not (even assuming a simple topology and assuming that it is a Friedmann-Lemaitre model) since we don't know lambda and Omega well enough to know whether or not the sum is less greater than 1 (which would mean it is finite, though very large).

    Second, if the universe is infinite, then how can the initial conditions be the same throughout infinite space? Even at t=0 the extent is infinite. This is of course a classic problem in classical cosmology, the isotropy problem (sometimes known as the horizon problem, though I prefer to avoid that term since some people use it to mean something else)---why are distant regions of space similar even though they have not been in causal contact? Inflation can solve this for the observable universe, and for an even larger portion of the universe, but the standard argument doesn't apply for an infinite universe.

    Note that if the universe is very big, but finite, then inflation could explained the observed isotropy.

    Max Tegmark argues that inflation can in effect make a finite initial universe appear infinite to us via relativistic effects, but I admit that I have not yet fully understood this argument. Do you understand it? If so, could you explain it?

    1. Yes, sorry, I was assuming flatness. And yes, the initial conditions are an issue - but hey, that's where the bleeding edge is in all of this.

      Not sure about Tegmark's argument - got a reference?

    2. OK, assuming flatness, yes then infinite. However, both the observational and (at least the classical) theoretical arguments point to a very large radius of curvature, but possibly finite and of unknown sign. However, to me, the qualitative question "finite or not" is very interesting.

      I ran across this in Tegmark's new book. I also recall seeing it in at least one of his papers, but it is easier to cite the book (which I recommend; look for a review in The Observatory in June) than track down one of his papers!

  3. Hi Professor,

    just saw you on The Drum and was very impressed by both the content of your comments and the accent with which you made them. I thought it was a mixture of Welsh and Irish (being from Belfast myself via Cambridge to Sydney), but from Wikipedia it seems it was more a contribution from North America that mixed up the Welsh.

    As a Pure Mathematician, I also get extremely frustrated with the inability of the media to explain that the Big Bang is not an explosion, but then I also get frustrated with my inability to explain the fascination of Topology to the average person. Having adopted the phrase "Big Bang" I guess physicists have no one but themselves to blame, and the phrase doesn't really help with the concept of sound in space either.

  4. My path was Old South Wales -> London -> Cambridge -> Long Island -> Vancouver Island -> Sydney, and my accent was mashed along the way. I have no idea where I sound like I am from, but I clearly don't sound like an Old South Walean.

    I, for one, appreciate the loveliness of topology!

  5. No doubt it will be very useful for my future projects. Would like to see some other posts on the same subject!


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