Observational evidence suggests that the universe began at an instant of time approximately 14 billion years ago, and it has been expanding ever since. The theory that the universe came into existence at one moment in time and subsequently has undergone great expansion is called the Big Bang theory. To understand the Big Bang is a very difficult task because nothing in our experience can help us understand an origin event of this sort.
Understanding the Big Bang Theory
The first step to imagining the Big Bang is to think about Hubble’s discovery that all the distant galaxies are moving away from us. The farther away the object is, the faster the motion, and everything seems to be expanding out.
Imagine playing a movie of this situation backwards, so all the galaxies would appear to be coming back towards us. The more distant galaxies would be coming towards us faster, and everything would seem to be converging back on a point; so at some point, the universe would appear to have begun at one point.
So, what was this Big Bang like? The Big Bang was not an explosion in which matter expanded into an existing space. The Big Bang was an expansion of space itself. The universe did not exist, and then the universe came into existence and kept getting larger, but it wasn’t expanding into anything, so all our analogies fail in that sense.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Analogies that Fall Short but Help
The surface of an expanding balloon provides one helpful analogy if you can imagine yourself as a two-dimensional being, utterly unaware of a third dimension, and imagine yourself living on the surface of a balloon. From your two-dimensional point of view, the expansion of a balloon would be very analogous to the expansion of the universe to us in three dimensions.
Imagine a balloon with galaxies on it. As you blow up the balloon, all the galaxies appear to get farther apart. From each galaxy’s point of view, all the other galaxies are getting farther apart, and the more distant the galaxy is on the surface of the balloon, the faster it seems to be moving away.
If you were a two-dimensional creature on the surface of the balloon, it wouldn’t appear as if the surface was expanding into anything; it would just appear that everything was getting farther apart, that there was more surface on which to play around with. Of course, the problem with this analogy is that when we look at the universe, we see a balloon expanding into volume. We can’t imagine being a two-dimensional creature.
Another analogy is provided by a batch of raisin bread, with lots of raisins in expanding dough, and you have to think of this as an infinite mass. As that raisin dough expands, every raisin gets farther away from every other raisins.
Nearby raisins only move very slightly, but if you have a raisin that’s, say, a mile away from you, that raisin moves very fast away from you, just from the same kind of expansion of all the dough, uniformly.
That’s what we think is happening in our galaxy—space is expanding uniformly, so more distant objects are carried away faster because the intervening space is larger.
Learn more about the redshift.
Note that in these analogies, the Earth doesn’t occupy a special place. According to all these observations, the Earth is just one of many possible points in the universe. That’s a philosophical point. It’s an assumption.
Most astronomers believe in a homogeneous universe, in which every point, wherever you are, is similar. Of course, locally, you may have a galaxy or not, but if you take a larger view of your area of space, every part of the universe has the same laws of nature, the same force of gravity, the same charge on the electron—no place is special.
There’s no reason why the universe has to be that way; perhaps some places are closer to a center, whatever that means. Maybe the gravitational constant does differ with distance from this center, but there’s no way to test this. There’s no way to know; it’s entirely a philosophical point. So the assumption of homogeneity is philosophy, not science, at this point.
Learn more about the discovery of galaxies.
Astronomical Philosophy before Edwin Hubble’s Discovery
Hubble’s discovery of the close relationship between a galaxy’s distance and its velocity away from us immediately opened a new realm of scientific inquiry and gave us even greater cosmic questions. Before those observations, any discussion of the origin of the universe was purely philosophical speculation.
It’s fascinating to think about the philosophical aspects of origin questions. Some astronomers, even without data, were very quick to debate this question, and many astronomers believed in a Biblical account—this includes, for example, the influential cosmologist, the Catholic priest Georges Lemaitre. He was committed to an origin event; he wanted a moment of creation in his cosmology.
There were others, such as the avowed British astrophysicist and atheist Arthur Eddington. He preferred a steady-state universe, with no beginning and no end.
Hubble’s reproducible observations pointed to an origin scenario that was both simple and testable.
Common Questions about Understanding the Nature of the Big Bang Theory
The Big Bang theory is about the expansion of space itself. Because we always think of it expanding into space that pre-exists, it’s hard to find an analogy that captures such a concept.
One analogy for understanding the Big Bang theory that works relatively well is the raisin analogy. It involves imagining a batch of raisin bread, with lots of raisins in expanding dough. As that raisin dough expands, every raisin gets farther away from every other raisin, just like what happened during the Big Bang.
Hubble’s discovery of the close relationship between a galaxy’s distance and its velocity away from us opened a new realm of scientific inquiry. Before those observations, any discussion of the origin of the universe was purely philosophical speculation.