Where in the universe did the Big Bang happen?

The Big Bang did not happen inside the 3-dimensional space of our universe, at least that's what our best understanding of physics seems to be telling us during the last 70 years! That means it did not happen 'over there' a billion light years beyond Antares.

The only guide we have for answering this question is Big Bang cosmology and EInstein's general theory of relativity. These make very specific predictions for what happened to space and time during the Big Bang.

The figure above shows the expansion of the universe predicted by general relativity. Note that the latitude and longitude positions of the 'star' galaxies remains the same but the distance between them is dilating as the radius of the sphere increases. This is a representation of how our universe is expanding. From this geometric analogy, if the surface of the sphere represented the 3-d volume of our universe at a specific time since the big bang, you see that the volume of space is increasing but space isnt being added in from some where else! This is a sphere in 4-dimensions with a 3-d surface, in geometric analog to a 3-d basket ball with a 2-d surface. Also, as you shrink the sphere's radius, the volume of 3-d space decreases steadily until it approaches the condition where the radius is zero. You can do this with a mathematical 'balloon' but not a real one. At zero radius we also have the condition where the 3-d volume of space also vanishes.

Now suppose that this spherical surface was filled with atoms. As you shrink the volume of space, the density of this matter increases steadily. When you get close to the time where the radius is zero, the average density of matter in the 3-d space of this sphere has grown enormously. When the radius becomes zero, the density becomes infinite and we have what physicists call a singularity.

So, the best, non-mathematical description that any cosmologist can create for describing the Big Bang is that it occurred in every cubic centimeter of space in the universe with no unique starting point. In fact, it was an event which our mathematics indicate, actually brought space and time into existence. It did not occur IN space at a particular location, because it created space ( and time itself) as it went along. There may have existed some state 'prior' to the Big Bang, but it is a state not described by its location in time or space. This state preceded the existence of our time and space.

Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.