“Big Bang” : A terrible name for a great theory [from the archives]
[This was originally posted in June of 2006 on an earlier incarnation of this blog that's no longer available. It was collected in "The Open Laboratory: The Best Writing on Science Blogs 2006" I'm reposting it here so that it will again be available to the broader net.]
Perhaps, from a marketing point of view, “Big Bang” is a great name. It’s short, it’s punchy, it evokes memorable images, and it’s easy to remember. But, marketing is something I really don’t understand; as we all know, it’s easy to fear what we don’t understand (particularly when it has a huge amount of control over us and the world around us), and to hate what we fear, and as such I’m very convinced in my opinion that Marketing is Evil.
So why is the Big Bang a terrible name? Because the name itself evokes and supports many misconceptions about the theory. Read on for my musings on the matter.
The Big Bang was not an explosion.
The name suggests an explosion: some point that blew up, and everything outside it is now rushing away from it due to the force of the explosion. It’s an analogy that does provide some useful intuition (without Dark Energy, the Universe is expanding because of the left over expansion that started way back when and hasn’t been stopped by gravity), but there are a lot of problems with it. Most notably, galaxies are not in fact flying away from each other! A much more modern way to look at it, and a way that matches the mathematics that physicists and astronomers really use when they work with the model of the expanding Universe, is that galaxies (except for little local motions) are pretty much fixed, but space itself expands. Galaxies get farther apart from each other because, as space expands, there’s more space between them. (Note that the galaxies themselves are not expanding; they are held together by their own forces.) An analogy is rising raisin bread: the bread expands, and the raisins get farther apart, but the raisins aren’t moving through the bread. Another analogy would be pennies pasted on the surface of a balloon.
[Addendum added 2009-08-21: there is some debate in the astronomical community as to whether "galaxies flying apart" or "space expands" is a better qualitative description of the mathematics of the Big Bang. Obviously, I'm in the "space expands" camp. I'll write more about this in a later post.]
The Universe didn’t all start from one point.
Sometimes you will hear people say this. The classical Big Bang model (but see below) has an initial singularity; at that beginning, any two points which are separated by a finite distance now were separated by zero distance. That sure seems like things begin at an initial point. It is, however, much better to say that the initial singularity is a point where our models break down, and that all we can really say is that densities in the extremely early universe, just “after” the “bang”, were extremely high. The problem with thinking of it all starting from a point is that it suggests that there is a center that everything is rushing away from. In fact, however, the Big Bang happened everywhere. If you must think about it as that point, that point has itself expanded to the whole infinite Universe… and we, right here, are within that point, as is a distant galaxy a billion light-years away. In the Big Bang theory, the Universe has no center. Any point is as good as any other, and indeed, an observer looking from any point would see things around her as if she were at the center of the expansion! Which leads me to the T-shirt I want to have made one day. On the front: “Yes, in fact, I am at the center of the Universe!…” ; on the back: “…but so are you.”
We know little or nothing about the moment of “Bang” itself.
This is perhaps the most egregious. The Big Bang theory is tremendously successful, tremendously well supported by observations, yet… the thing that the theory is named after is something that the theory as it exists right now really can’t address. Sad, huh? What the Big Bang theory tells us is that the Universe evolved from a hot and dense state to what we have now. There are various different epochs we have been able to probe using different techniques, and the whole picture hangs together very well using these very different techniques. Right now, we have the expansion of the Universe. At a moment a few hundred thousand years after the “beginning”, we have the cosmic microwave background observed by the WMAP satellite and large numbers of other ground, balloon-borne, and space experiments. A few minutes after the Big Bang, we have the time when the Universe was so hot and dense that nuclear reactions routinely happened, at which the primordial elements were created… in proportions that match what we observe. And, finally, a tiny fraction of a second after the beginning, we have Inflation, which isn’t as well supported as those other bits, but which does explain a lot and even has stood up to one test from recent WMAP results. (Indeed, many people, myself included, are attracted to the notion of calling the end of Inflation “the beginning”, as subtracting that tiny fraction of a second from any later epoch won’t make any difference in the time we quote, and to talk about things before that you need Physics we don’t know how to do.)
All those different epochs, tied together by one expanding Universe theory that started from an extremely hot and dense state, all supported by a vast array of different observations. Pretty cool.
The classical Big Bang– what you get if you just look at the Universe with Einstein’s General Relativity, ignoring Quantum Mechanics– gives a well-defined “beginning”, that time we call t=0 and measure all the other times from. That beginning is the singularity, where things diverge, where densities go infinite. “What caused it” is something we don’t even know how to address (unless you’re a string theorist talking about branes, but that’s a whole nuther issue).
The problem is, the classical Big Bang ignores Quantum Mechanics, which is a bad idea: Quantum Mechanics is another extremely well-supported theory in Physics we know to be right. It’s the theory of the very small (speaking very roughly). At some time a tiny fraction after the moment of the classical Big Bang, the Universe got large enough that it became reasonable to do your Quantum Mechanics (e.g. for the nuclear reactions a few minutes after the beginning) without worrying about gravity, and to do General Relativity (which describes gravity) without worrying about Quantum Mechanics. However, before that moment, you have to worry about both at once. Here’s the rub: the two theories don’t work together. At the moment, we don’t have a working theory that can handle gravity and quantum mechanics at the same time, although the string theorists keep telling us that they’re working on it.
What the previous paragraph says is that our extremely successful physical theories are incapable of predicting what happened during the first tiny time interval after the moment of the classical big bang. Indeed, if you think about it in reverse– start now and go back in time– we reach an early period before which we can say almost nothing. We can go back through the present day expansion, through the epoch when galaxies first formed, through the epoch when the Universe was a plasma and the Cosmic Microwave Background was emitted, through the epoch when the elements formed, through the epoch when quarks and gluons were all mushed about in one big continuous mess, to Inflation… but we can’t go before that.
We don’t know what happened before that. Was there even a “t=0″ beginning? Dunno. Was there really a singularity? Dunno. What happened before Inflation? Dunno. What was the deal with the actual Big Bang itself? Dunno.
Kind of sad that this extremely successful, extremely well-tested theory, a theory so good that we can call it right and true, doesn’t really address the moment it seems to be named after.
That’s why, marketing reasons aside, Big Bang is a terrible name for that theory. But I sure as hell can’t come up with a better name that doesn’t violate all sorts of marketing, so I guess we’re stuck with it.