going off on a tangent...
Some notes on the PHD Comic:
- Some notes on other topics in the comic:
- Dark matter is the term we use to describe the invisible, non-interacting matter that we know makes up most of the matter in the universe. When I say "invisible" I mean that we can't see it, because it's not emitting light and it doesn't absorb or scatter it. "Non-interacting" refers to the fact that it doesn't seem to interact with other matter (visible or dark) in any way other than through its gravity. As alluded to in the comic strip, one of the best indications we have of the existence of dark matter has to do with the motions of galaxies. Specifically, when we watch the orbits of stars in other galaxies, we see that the rotation speed doesn't seem to decrease with distance from the center the way you'd expect if the visible matter was everything. We can estimate the mass of all the stars and gas in the galaxy, and we see that the outermost stars are still moving so fast that they should be flying off into space. So there must be something more that we can't see holding all the stars in their orbits; we call this dark matter. For a really fantastic discussion of dark matter, see this PHD Comic video.
- Dark energy is, as far as we know, not related to dark matter -- we just call it dark because we can't see it and we don't know what it is. What we do know is that something is causing the universe to accelerate in its expansion. I can't emphasize enough how weird it is for this to be happening. You may have heard that a long time ago, the universe was smaller and denser and hotter than it is now (we call this the Big Bang theory). So at some point, something caused this hot, tiny, dense universe to explode out and expand in all directions. We're fine with that; we see the expansion, and that's all good. What we expected was that over time the expansion would slow, just like if you throw a ball up in the air, it'll slow down at some point and fall down again. Or, alternatively, you could throw it so fast it would escape the Earth's gravity, and then it'd just coast forever. But you really don't expect it to speed up. That wouldn't make sense at all. But that seems to be what's happening to the universe. And "dark energy" is what we blame for it. It might be just a fundamental property of how gravity works in the universe (a cosmological constant), or it might be an actual substance. We don't know yet, and seeing as dark energy makes up something like 70% of the "stuff" in the universe, we'd really like to find out. We're working on it.
- The graph in the lower left-hand corner of the comic strip is a plot of the intensity of the cosmic microwave background as a function of the wavelength of the radiation. This spectrum is how we know that the universe started out hot and dense. It's called a blackbody spectrum, and it's the universal shape of the spectrum of anything giving off radiation in the form of heat. If you heat up a metal poker in the fire until it glows, and plot the intensity of the light in the same way, you'll get just that shape. It turns out that everywhere we look in the sky, there's a faint signal at microwave frequencies whose spectrum is the same shape as that hot poker. It's telling us that a long time ago, the whole universe was at about the same temperature, and since then the radiation from that "primordial fireball" has been travelling through the universe, cooling down, and eventually reaching our receivers.
- I won't go into a discussion of the anthropic principle and the string theory landscape, but I'll provide a couple of links that'll give you an idea of the issue in case you're interested. Here's an interview (from American Scientist) of Leonard Susskind talking about his book on the subject. Here's a New Yorker article about the debate (and other things).