The raisin pudding analogy helps us to mentally picture how Hubble's Law relates to an expanding universe.
Consider a loaf of raisin pudding (or bread dough) baking in an oven, with raisins sprinkled evenly throughout.
As the pudding expands during cooking, all of the raisins are moved farther and farther
apart from each other. Seen from the viewpoint of any individual raisin, all the other raisins in the
pudding appear to be receding away with some velocity. The nearby raisins recede more slowly and distant
raisins recede more quickly.
It is important to remember that the raisins are NOT flying apart on their own due to some sort of explosion,
they are simply being carried
along by the expansion of the dough. In the same way, the galaxies are carried apart by the expansion of space itself.
The so-called big bang is not really perceived as a "bang." It is the growth of the fabric of space.
Running time backwards implies that the Universe used to be much smaller and extremely
dense. This scenario is sometimes referred to as "the big crunch." The implication of this crunch is that
the Universe had a beginning as a tiny and simple structure, and over time it has grown organically into
the large and complicated structure we see today.
The Wilkinson Microwave Anisotropy Probe (WMAP)
Temperature variations of the early Universe, about 380,000 years after the beginning.