Preposterous Universe

Tuesday, May 11, 2004

I notice that I'm actually writing much less about politics in this blog than I originally expected to. Part of that is because there are few things I have to say that aren't already being said more eloquently in some other blog. Another part is that the situation right now is so depressing and outrageous on multiple levels that there's an overwhelming temptation to forget about light and just throw heat.

So instead you will get some warmed-over cosmology. In particular, I was asked a while back to come up with a list of "most prevalent misconceptions about cosmology." I'm not sure among whom they should be prevalent; some of my colleagues have way-out ideas, but I'm not going to go about setting them all straight. Anyway, here was my suggested list of misconceptions. Comments welcome as to more misconceptions, although they have to be arguably prevalent -- not just your own personal misconceptions, we could be here a long time.
  • The universe is unchanging and infinitely old, or very young (thousands of years).
    These are basic misconceptions, but I think that most people actually don't have them. At least, not the people who might be reading this list. The universe is about 14 billion years old. At least, that's the time between the Big Bang and today.

  • The Big Bang model is controversial, or inflation is an alternative.
    The BB model is completely accepted by the community. Inflation and other ideas extend the model, but the basic BB picture is secure. Three pillars of the model -- the expansion of the universe, the cosmic microwave background, and primordial nucleosynthesis -- make it hard to imagine any credible alternative. The idea of inflation in the early universe is an add-on to the Big Bang, not a replacement for it.

  • The Big Bang is an explosion at a point in a pre-existing space.
    It's not; all of space comes into existence at the BB.

  • The universe has a center, or an edge, or something it is expanding into.
    The universe isn't expanding into anything else, and as far as we know it's quite homogeneous. Of course there is a point past which we can't directly see, so we can't say what goes on beyond there.

  • The universe is expanding faster than the speed of light, or perhaps it used to be, and this seems to conflict with relativity.
    The expansion of the universe is not a "speed", so this doesn't even make sense. We associate a speed to distant galaxies, but that's only an informal idea which works if they're not too far away. The apparent recession velocity of very distant galaxies can be greater than the speed of light, but that doesn't violate relativity, which only puts an upper limit on the relative velocity of two objects passing by each other.

  • Cosmologists used to believe in dark matter, and now in dark energy, and how do they know there isn't even more stuff out there?
    We believe in both dark matter and dark energy; the former seems to be made of particles that collect in galaxies and clusters, while the latter is evenly spread throughout space. The curvature of space puts limits on the total amount of energy, so we probably won't discover important new components.

  • Dark matter is just ordinary matter that we haven't found yet.
    Evidence from both the cosmic microwave background and primordial nucleosynthesis gives tight bounds on how much ordinary matter there is. We compare this to the total amount inferred from gravitational effects, and come up well short. The dark matter must be some new kind of particle, not yet discovered in the lab.

  • Scientists keep inventing new phenomena like dark matter and dark energy because they are desperate, or philosophically hidebound; these are just like epicycles or the aether, and will eventually go away.
    Well, maybe. But it's important to emphasize that we have been forced into these ideas by the need to explain observational data, they're not just cool ideas we've fallen for. The more data we to get, the more secure these ideas seem to become. Of course it's possible we're missing something big, but if so everyone would love to come up with the compelling alternative; there is no establishmentarian conspiracy to suppress other ideas.
Remember, these are misconceptions. I hope nobody reads the list without the preamble and thinks these are the "greatest discoveries of modern physics" or some such thing.

Ideas on culture, science, politics.
Sean Carroll

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