Big Bang for beginners-5: Some conceptual challenges

By Mano Singham

When we use the words 'Big Bang' it immediately conjure up certain images. We immediately think of familiar explosions, like bombs or firecrackers going off. We envisage a big noise and the exploding pieces hurtling away from the center of the explosion and spreading out into the surrounding space at great speed. This image captures correctly the idea of a hot compressed beginning with a fixed amount of matter spreading out through space and getting cooler and more dilute with time. But there are important ways in which the image is inaccurate.

One simple misconception is to think there was a loud noise at all. The very idea of sound at those huge densities is highly problematic and it is not helpful to think in those terms. But this is a minor misconception. The major misconception that people have is the idea that space always existed and extended all the way to infinity and that the Big Bang occurred in one small region of it and the matter that was created then spread out to fill increasing amounts of that pre-existing space.

What the theory actually says is that the only space that exists is the space occupied by the matter produced in the Big Bang and that, as the matter spread out, it did not fill already existing empty space, but instead it was space itself that was expanding, carrying the matter along with it.

To better understand this difficult idea, a good analogy is raisin bread baking in an oven. The raisins occupy more-or-less fixed positions in the dough. As the bread bakes, the dough expands, carrying the raisins along with it.

The wrong way to interpret this analogy to the Big Bang theory is to think of the dough and raisins as the matter expanding into the pre-existing space of the oven.

The correct way to view the analogy is to think of the bread dough as being space and the raisins as the matter. As the bread bakes, the dough (i.e., space) expands carrying the raisins (i.e., matter) along with it. The hard thing for people to grasp is that there is no space outside of the dough. There is no oven for the dough to expand into. So the 'explosion' we speak of is not of matter expanding into space but of space itself expanding.

In addition to the motion associated with the expansion of space, there is also what we call local motion caused by the forces between objects. So for example, Earth and the planets orbit our Sun under the influence of gravity, and our solar system rotates in the spiral arm of our galaxy the Milky Way, again under the influence of gravitational forces. Protons and neutrons in nuclei move under the influence of nuclear forces and electrons in atoms move under the influence of electromagnetic forces. All these motions are due to forces acting locally and not part of the motion caused by the expansion of space itself. Back to our raisin bread analogy, the raisins are not rigidly embedded in the dough. In addition to the raisins being dragged along by the expanding dough, they may also move around slightly in the dough due to (say) air pockets near them. But when we speak of the motion associated with the Big Bang, we are referring to the motion due to the expansion of space and not these local motions.

(It should be borne in mind that the well-known assertion that nothing can travel faster than the speed of light is popularly interpreted a little too broadly. That limit applies to the speed of particles and information flow. But there are things like the collapse of the wave function and the phase velocity of wave packets that occur at speeds greater than the speed of light. As I described yesterday, in the case of the early universe, space expanded at a much faster rate than the speed of light but that too is allowed by the theory of relativity. In other words, the dough can expand faster than the speed of light but the speed of the raisins relative to the dough has to be less than the speed of light.)

One consequence of the view that the Big Bang consists of space itself expanding is that it did not occur at a point 'in' space (like a blob of dough in an oven) but occurred everywhere in space simultaneously, and that it is space itself that was initially compressed. So it does not make sense to look for a point in the universe where the Big Bang occurred and treat it as the 'center' of the universe. There is also no 'edge' or boundary to the universe, so it does not make sense to ask what exists beyond the edge either.

I will come back to these last points later because they are undoubtedly hard to grasp, especially the idea about the absence of a boundary.

This series is archived here.

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)