In response to a flurry of emails prompted by my latest column, I decided to do a series of blog entries on common misconceptions about the expanding universe. Yesterday, I wrote about the expansion of time and how you can get rid of it using units.
Today I want to talk about the center of the universe, or really, one very important point:
The universe has no center, and the big bang didn’t occur at any particular place. It occurred everywhere.
Memorize that line as a mantra if you have to, because it’s important. But it might be more helpful to explain why people think that the universe should have a center, and why (as cosmologists understand it), it doesn’t. The fault lies (at least in part) in a too literal interpretation of the diagram at the top.
You’ve probably heard the explanation as follows: the universe is like an expanding balloon with ants or some two dimensional creatures walking around on the surface. As you inflate the balloon, all of the ants get further and further from one another without any given point being at the center. Everybody seems to be at the center, as seen from their own perspective.
But here’s the thing, actual balloons (like the kind you get on your birthday) really do have a center and markings drawn on an actual balloon really do get further away from that center as you inflate the balloon. If you take the analogy too far, it’s hard to get away from the idea that our universe really does have a center, perhaps in some unseen 4th dimension. You might be further inclined to think that the balloon is inflating into some higher dimensional room or box, one that has walls and a finite size.
This is not how general relativity understands the situation. I did a whole “Ask a Physicist” on the fact that the balloon model is kind of flawed. We go into a lot more detail behind the motivation in Chapter 6 of our book. The subheading is, naturally enough, “If the Universe is Expanding, what is it expanding into?”
The short answer is “nothing.”
You’ve heard, no doubt, that space-time gets warped by the presence of matter and energy. Well, this is a dynamic process. Rather than think of the universe literally expanding, simply think of shrinking all of your rulers. The universe stays “the same” (which is why “comoving distances” are useful — fixed galaxies stay a fixed comoving distance from one another as the universe expands), but the distances (the number of rulers you need to put end to end between the galaxies) increases over time.
This is, of course, a tough one to get your mind around, and it seems less natural than simply thinking of the universe as an inflating balloon, which is why most people (including us) use the analogy.
Even so, there’s a problem with this “shrinking ruler” analogy, and it’s not necessarily the one you think.*
The problem is that the expansion history of the universe looks something like this:
It’s all very well to talk about a ruler shrinking or growing when we’re talking about doubling or tripling it. But what happens when we literally start with a point? What then? Zero times anything is still zero. Start with a point, and you should end up with a point, and so naturally (the argument goes) you’ll have to expand out from a specific point and so the expansion must have started “somewhere.”
The answer is that we know that general relativity is flawed at the very instant of creation. Quantum mechanics must have come into play when the universe was small enough. Indeed, quantum mechanics doesn’t even really allow for infinitesimally small points at all. Everything is fuzzy in the quantum world.
It wasn’t until about after the big bang, the so-called “Planck time” that we can use plain old general relativity rather than an as-yet unknown theory of “quantum gravity” to describe the expansion.
What we typically call “the big bang model” doesn’t really start at t=0, it starts at the Planck time. The model describes the subsequent expansion, the generation of the light elements and the first atoms, and the decoupling of the various forces from one another. There are details (many of them) that we’re still not clear on. But the model itself doesn’t really say anything about what happened before the Planck time, which is why it is such a rich field of inquiry.
But, after that, as far as our current understanding goes, there is no center, there is no edge, and the “expansion” of the universe can just as easily be thought of as our cosmological rulers shrinking.
*Feel free to tell me all the way you think the shrinking ruler analogy is flawed in the comments. There are lots of them.