Dark Matter, Part 1. Gravity and stuff.

Image courtesy of CFHTLenS

There is a growing interest in the concept of dark matter and dark energy in astrophysics. Initially, I was skeptical, as it sounded too…well, fantastic for a real concept in science. It sounded like they were just making shit up at this point instead of simply saying "we have no idea."

Of course, as the years have passed, scientists' understanding and communication regarding dark matterand energy have increased dramatically, and now I get it. Not the math, mind you—I'm an anthropologist, not an astrophysicist, and there are reasons for that. I get the broader picture, a sense of the physics involved.

Dark matter and dark energy (which, like matter and energy, are merely two states of the same thing, and which I will abbreviate here on out by just referring to dark matter, though I mean both) are not directly observable, hence their monikers. A quick explanation:

Stars are incredibly massive objects. Even our sun, which is fairly modest in size, is unfathomably huge, and the stuff it is made out of is remarkably dense. All the naturally-occurring elements we know of were created, through fusion, in the hearts of stars, and those elements, and probably a lot we've never encountered, are still inside those stars. This is why the planets, even huge, far-flung ones like the gas giants, stay in orbit rather than slinging off into interstellar space, never to be seen again— the planets' own mass and inertia keep attempting a straight line of movement, but the star is so massive that its gravity counteracts the planets' tendency to shoot off in a straight line, and keeps them perpetually falling in toward it. Think about it a minute: Earth is a gigantic chunk of iron that is 93,000,000 miles away from the sun, sailing along at 67,000 mph (108,000 kph), and the sun is still so massive that the earth keeps falling towards it, and has for billions of years. Just. Think. About. That.

Objects this massive have a measurable effect on other objects that are even many light-years away. Stars and clouds of loose interstellar dust and debris all tug and pull towards one another. Inertia, stellar winds, ejecta, and the expansion of space push them apart. Collections of objects this massive—from star nurseries to full-blown galaxies—have an even more pronounced effect through the aggregation of all this mass and energy in relatively close proximity.

All of this can be measured, particularly now, since Hubble and Kepler were launched into orbit, eliminating atmospheric interference. I don't know all of the details (again, anthropologist), but I know that some of it involves measuring subtle shifts in emissive energies (which, if I'm not mistaken, serve as something of a fingerprint for galaxies, if not individual stars, though maybe those too), and some of it involves watching how the light from a more distant star or galaxy is bent ("lensed") as it passes by nearer objects. The effects of gravity are also observable by the interactions between masses in space—Star Trek: Generations played heavily with this very idea.

Tomorrow, part 2, where I discuss time.