Archive for August 5th, 2008

8/4/08: Launch Pad, day 5

Woke up early enough today for breakfast in the dining hall (which is only open 7-8am, what were they thinking?) and conversation with about half the gang. The downside is that I got less sleep than usual and am now tired and headachey, so this entry will be shorter than yesterday’s (if I know what’s good for me).

Mike Brotherton started off with a lecture about galaxies and cosmology. Almost everything we can see with the naked eye at night is in our own Milky Way galaxy (this is, apparently, its actual name — I expected it to have an official scientific name like Galaxy Number One or something, but no). One exception is the Andromeda galaxy, which is barely visible as a hazy star near Casseiopeia.

Herschel tried to determine the shape of the galaxy (1785) but didn’t do very well because so much of it is obscured by interstellar dust and gas. The galactic plane, in fact, is well above the bright line we can see, but obscured by dust. We can use wavelengths that are not obscured by dust (e.g. infrared) and “standard candles” such as Cepheid variable stars, whose absolute brightness can be determined from their periods, to determine the galaxy’s actual shape.

Stars in the galactic disk have nearly circular orbits, while “halo” stars outside the disk have highly elliptical orbits. The orbits of stars in our galaxy and others show that most of the mass of the galaxy is distributed smoothly throughout the galaxy rather than concentrated in the center. The speeds of the orbits tell us that there is a LOT more of this mass than we can account for through visible objects such as stars. But what is it?

Could this dark matter be ordinary dust and gas? No. We know the abundancies of baryons (protons and neutrons) in the universe from studying the Big Bang, and there aren’t nearly enough to account for the invisible mass.

Could it be WIMPs (Weakly Interacting Massive Particles) such as neutrinos? No. Although neutrinos don’t affect normal matter much, they do affect it, and we have performed experiments (using large quantities of dry cleaning fluid) that show there aren’t enough of them either. A theoretical WIMP called the “axion” has been proposed but never observed.

Could it be MaCHOs (Massive Compact Halo Objects) such as black holes and brown dwarfs? Maybe, but probably not. We can detect these objects through “gravitational lensing” (a distant object changing its brightness or apparent position as a MACHO passing in front of it warps its light) and we don’t see enough such events to account for the missing mass.

Could it be that we are simply wrong about gravity? No. MoND (Modified Newtonian Dynamics) seemed plausible until 2006. The Bullet Cluster consists of two clusters of galaxies that have recently passed through each other. We can see the hot gas of these two clusters (which is normal matter) using X-ray telescopes, but we can also find their centers of mass using gravitational lensing of the galaxies behind the cluster. The two centers of mass are farther apart than the visible gas. This tells us that the majority of the mass in the two clusters does not interact with itself or with the matter of the clusters in the same way as normal matter. (This Scientific American article includes a very helpful video simulation.)

I must say that I was both confused and skeptical about the very weird stuff called “dark matter.” (Note: not to be confused with “dark energy,” which we talked about later.) I didn’t understand why this strange non-interacting stuff had to be invoked when it could just be, well, matter that was just dark. But the Bullet Cluster was for me, you should pardon the expression, the smoking gun.

Many galaxies have spiral arms. If you look at a picture of a spiral galaxy it looks just like water going down the drain, or a hurricane, and you think you can tell which way it is rotating. The actual rotational direction is the other way. Spiral arms are, in fact, standing waves in the interstellar medium. At the leading edge of these waves (the inside edge of each sickle-shaped arm), new stars are born as the interstellar medium impacts the shockwave. The bigger, hotter stars burn out first, so the leading edge is brightest, fading away to blackness as most of the newborn stars burn out or fade away.

We can measure the distance to other galaxies by using Cepheid variables and type Ia supernovas (these are white dwarfs in binary systems that collapse when they accrete too much matter from their companion — we know exactly how bright they are because they explode immediately when their mass rises to a certain value). These “standard candles” tell us that distant galaxies are moving away from us with a speed proportional to their distance.

The galaxies aren’t moving through space, as any fule kno… it’s space that’s expanding. This expansion is happening everywhere, but it’s only visible in intergalactic space because at smaller scales the force of gravity is greater than the expansive force. This is why the galaxies are getting farther apart instead of just bigger.

By studying the three degree Kelvin background radiation that is the echo of the Big Bang, we can determine the initial conditions of the universe and determine that the total mass of the universe is almost exactly what is needed to make the universe “flat”, meaning that it will neither expand forever nor contract in a Big Crunch: the expansion will slow down and stop at some point. But there isn’t enough matter, even including dark matter, to account for this flatness, and when we measured the rate of deceleration, we got a surprise: it wasn’t slowing down at all, it was speeding up!

Turns out there’s a “cosmological constant” in Einstein’s equations, which was thought to be zero, but if we set it to a negative value it explains both the accelerating expansion of the universe and the missing mass. The missing mass is the mass equivalent of this weird anti-gravitic energy. We don’t know what this “dark energy” is — it has never been observed directly — but it makes the equations balance.

It may be that the cosmological constant itself is increasing. If it stays the same, the universe expands so fast that all other galaxies will eventually fade from view. If it is increasing, it will eventually get big enough to overcome atomic forces and everything in the universe will be torn apart: the “big rip.” For now, though, it’s less powerful than gravity and other forces, meaning its effect is only visible at the very largest scales.

After that cheery reassurance we went to the computer imaging lab where we got a talk by Chip Kobulnicky on imaging in astronomy. Raw images from the Wide-Field Planetary Camera on the Hubble space telescope look awful. They consist of four rectangles (three large, one small) with big visible seams between them, speckles of noise, and cosmic ray streaks. Scientists and technicians have to do a lot of processing to make them look all pretty and colorful. We also got some hands-on experience using a program called ds9 to combine the R, G, and B images of the Ring Nebula that were taken at WIRO on our field trip the other day into a single color image. Here’s the result:

(It’s kind of grainy because the exposure was short.)

The day ended with a talk on SETI by scientist/philosopher Jeffrey Lockwood. This talk was a bit of a surprise as we spent the whole time talking and writing about what messages we, as writers, would send to aliens, ignoring questions of transmission mechanism and language. It was an interesting writing exercise, and thought-provoking, but was so different from the hard science focus of the rest of the week that some of us felt kind of whiplashed.

One of the things I wrote during this session was a message to express the importance of “pattern” to humans while simultaneously encoding the Fibonacci sequence:

Another instance.
It happens again.
Why does it happen again?
Can we predict what the next instance is?
By observing phenomena, we learn about the universe and learn to predict events.
We find patterns and recurrences in all kinds of physical phenomena, from molecules to stars, simple to complex, insert and alive.
Once we have discovered a pattern, we can build devices, craft new experiments, build more knowledge on top of what we have already learned, and even begin to make changes and improve our environment.

We finished the evening on the roof of the physics building, looking at binary stars, globular clusters, the planet Jupiter, and various satellites (including the International Space Station) with night-vision goggles, binoculars, and two very nice amateur telescopes.

Apparently I do not know what’s good for me. Night, all!