© Matt Strassler [March 16, 2014]
The Big Bang was an expansion of space, not like an explosion at all, despite what countless books, videos, articles and statements (even by scientists) often depict. Let’s look at the differences between an explosion of something into space versus an expansion of space.
[For an older article with a different perspective on the same subject, see here.]
In Figure 1 is shown a before and after of an explosion. Initially there is some space, with a seed — a bomb or a grenade or star or some other form of stored energy — sitting in it. The space is pre-existing, and so is the seed. Then something happens and the seed explodes. What was inside the seed undergoes some kind of transformation — for instance, a chemical reaction or a nuclear reaction — and energy is released. This creates tremendous heat and pressure inside the seed. The forces associated with the compressed heat and pressure cause the seed’s insides to expand outward as a hot ball of material. The energy comes out as the high speed and temperature of the seed’s insides, and the pressure and temperature gradually decrease as the interior of the seed expands outward into the pre-existing space in which it originally sat.
Notice that the cause of the explosion is a reaction that creates tremendous heat and pressure inside a tiny region. It is the imbalance between the huge pressure and heat inside the seed compared to the low pressure and temperature outside the seed that causes the seed to explode outward. And the things inside the speed move with high velocity, rushing apart from their initial location. Their speed relative to their starting point can’t be larger than light, so there’s a limit to how quickly they can recede from each other.
In Figure 2 is depicted the process (which may have already been going on before the moment of the left-hand figure) of an expansion of space. Between the left-hand picture and the right-hand picture, space has doubled in size, as you can see by the grid of lines. Things inside the space that are held together by powerful forces, such as chairs, tables, cats and people, do not expand — only the space in which they sit does. In short, space becomes more abundant, so there’s simply more room for the objects inside it.
Note that the objects do not intrinsically move! There’s no heat or pressure pushing them anywhere; they’ve not been kicked. It’s simply that the space between them and around them is growing, appearing out of nowhere, making the distances between them larger than before. And the increase (for uniform expansion) is uniform. In the right-hand picture, the distance between the cat and the table has doubled; so has the distance between the cat and the chair. That’s what happens when the universe doubles in size.
This kind of change in space itself is possible in Einstein’s theory of gravity but not in Newton’s older one. For Einstein, space is not just a place where things happen; it is a sort of thing itself, capable of growing, shrinking, deforming, wiggling, and changing shape. (Actually it’s space and time together which can do all of that!) Ripples in space-time are called “gravitational waves”.
Since it is space that is expanding, and it is not the objects that are moving, Einstein’s relativity puts no constraints on how fast the distance between the objects can grow — i.e., no constraints on how rapidly space between the objects can appear. It is possible for the distance between two objects to grow much, much faster than the speed of light. This is no contradiction with relativity.
People often say, with loose and imprecise words, that “relativity says that nothing can go faster than the speed of light”. But “nothing” and “go” are ambiguous, and in science we learn that imprecise words cause trouble. Einstein’s words (if you read them) are often ambiguous and easily misunderstood, though he tried to be precise. But Einstein’s equations are not ambiguous. The precise statement of relativity is that if two objects pass each other at the same point, then an observer who is moving with one of the objects will measure the speed of the other object to be less than or equal to the speed of light; and vice versa. But this is not in contradiction to the statement I’m making here: that the distance between two objects at different points can grow faster than this. And that will always happen, in a uniformly expanding universe, for two objects that are far enough apart.
Notice also, very importantly, that the cause of the expansion of the space need not have anything to do with heat or pressure… unlike an explosion. I’ve deliberately drawn normal objects like chairs and tables so that you can see that, in contrast to an explosion which will damage or destroy normal objects, an expansion can leave them untouched, just increasingly separated. Expansion can occur in a very hot universe — and early in the universe’s history, that did happen, during the Hot Big Bang. But expansion can also occur in a very cold universe. It is currently suspected that this, too, may have happened early in the universe, during the inflationary period. And of course our universe today is rather cold, yet not only is it expanding, the rate of expansion is increasing.
The Hot Big Bang, whose final stages we are living in, is an era that somehow began, at some moment in time, as a large region of space filled with a hot dense soup of particles, expanding and cooling very rapidly at first, then more and more slowly until just a few billion years ago. It did not begin as a point object that exploded into empty space. How the Hot Big Bang may have begun after inflation is explained at the end of this article on the Era of Inflation.