The Big Bang was not an explosion from a single point, with a center and an edge. The picture below shows a 78 billion lightyear across square piece of a Universe ( Ho = 50 km/sec/Mpc, Omega = 1) at 6 different times. Each dot represents a "galaxy". Two of the galaxies have a ring of red photons moving out from them. Most of the photons are cosmic microwave background photons that were present since the Big Bang but which have been traveling freely only since 0.0003 Gyr after the Big Bang. These photons always travel at the speed of light relative to the galaxies near them. As the Universe gets older, the galaxies do not expand but the distance between the galaxies gets larger. The photons emitted by one galaxy do indeed spread out through the Universe, but photons from other galaxies can still be seen.
Note that at the current age of the Universe, 13 Gyr in this model, the
two "green galaxies" are just now becoming visible to each other.
Also note that they are now 39 billion light years apart.
But by comparing the photon positions relative to the galaxies that they
are near at 4.1 and 5.1 Gyr, or at 12 and 13 Gyr, you can see that the
photons are always traveling at the speed of light relative to nearby
The animation [left] shows the evolution of the galaxy density, positions and the photon positions within the 78 billion light year box. However, all parts of the Universe started with CMBR photons, not just the two green galaxies. The picture below shows the result of releasing a ring of 72 red photons from every dot on the picture. It makes a pretty quilt pattern, but except for this pattern imposed by the artificial regularity of my galaxy grid this pattern of photons is homogeneous and isotropic, as specified by the cosmological principle.
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© 1998 Edward L. Wright. Last modified 9-May-1998