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I've read that astronomers have received a light (a photon) 13 billion years old. Here's my question:

If we start to turn the clock backwards all matter, energy, etc should start heading to the original "point" of the Big Bang. Then how come that the photon we have received and the stuff that makes planet Earth end up at the same location (the Big Bang), for the same amount of time (13 billion (+ a few millions but who cares)) by traveling with different speeds? (light travels with maximum speed, matter travels with less speed, etc)

EDIT: I know that the stuff that makes planet Earth was not even here, say 10 billion years ago: small particles form bigger ones, bigger ones gets shattered to smaller ones, etc. But imagine a hydrogen atom that some how managed to survive all the way up to here intact straight from the Big Bang (of a few million years after it). How come the atom gets here before the photon?

PAUSED: It looks like there are a lot of things I need to learn (or relearn) before asking the same question again.

yordan
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    Your hangup is assuming the big bang happened at a point and that like an explosion, everything zoomed away from that point. – Brandon Enright Oct 18 '14 at 02:37
  • Don't forget that the etc. includes the volume of space in which the matter and energy propagate. If you imagine the surface of a balloon deflating to essentially zero area, then can you see that the matter and energy propagating on that surface must be within that area? – Alfred Centauri Oct 18 '14 at 02:42
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    The cosmological solution was never "in one point". That's just a misunderstanding based on poor visualization in the media. If you could have "watched" the early eras of the universe, the only thing you would have noticed was a hot, dense and rapidly cooling particle "fog" surrounding you. There would have been a slight sound in addition. Only at the time of the surface of last scattering would the fog have lifted, to be replaced with a warm darkness. Some time later there would have been flashes of light popping up all around you... the first stars, but there was never a "center". – CuriousOne Oct 18 '14 at 02:52
  • to @Alfred Centauri: The concept that the volume of space is a physical entity is new to me. I thought the the volume of space is just an abstraction equal to all the matter and energy in the universe. Do you mean something like the "Luminiferous aether"? – yordan Oct 18 '14 at 02:54
  • to @CuriousOne and @"Brandon Enright": then why http://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-bang/ says: "in a single point in space"? – yordan Oct 18 '14 at 03:05
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    @yordan: NASA makes a lot of scientifically questionable statements in their materials for their layman audience. In some cases (like this one), they are dumbing the material down more than they should. On the other hand, most of their audience wouldn't be helped if they tried to explain it properly. There is no easy way to explain the current cosmological model, and this attempt tries to circumvent that problem with a statement that is plain wrong. – CuriousOne Oct 18 '14 at 04:01
  • The big bang and inflation do suppose that the extent of the universe (as measured by, say, the time it would have taken a photon to cross it) was very very small and then got vastly bigger. But the universe didn't expand "in space"; the amount of space itself expanded. – hobbs Oct 18 '14 at 04:03
  • http://en.wikipedia.org/wiki/Metric_expansion_of_space – hobbs Oct 18 '14 at 04:04
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    See Did the Big Bang happen at a point? – John Rennie Oct 18 '14 at 05:05

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When I was in high school I wondered the same thing. According to the Inflation models space time expanded exponentially between $10^{-36}$ and $10^{-32}$ seconds after the Big Bang. According to the theory a piece of space the size of a nucleus expanded to the size of a galaxy (rough order of magnitude, possibly even larger) in that very short time. Mathematically this can be modelled by a parameter in the space-time metric that increases, the metric usually assumed is that of deSitter space, a homogenous positively curved space. In this geometric picture the initially large curvature rapidly flattens out. This is well motivated as this kind of homogenous and isotropic structure with very little variation is seen at large distances by astronomers. In other words the current observed large scale structure is believed to be best explained by assuming that it is a gigantic magnification of an initially tiny region.

The crucial point is that that as space itself is expanding during inflation, even points that initially were close together become causally separated. In particular light reaching us from galaxies far away is from the time just after inflation. The Planck satellite and a number of experiments at the poles are looking for evidence of gravitational waves from the inflationary time, which would substantiate the inflation theory further.

orbifold
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