Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the universe, part 10: Enter the dark forces of the universe!

In 1985, Very Rubin presented evidence that there is something weird about how galaxies rotate.



Vera Rubin in 1970, studying galactic spectra.

Galaxies were supposed to rotate in much the same manner that our own solar system is rotating. After all, a galaxy can be regarded as a monstrously blown-up version of a solar system. In the middle of both these systems you have a large concentrations of mass - in our solar system it is the Sun, and in a galaxy it is the supermassive black hole that is found in the center of most galaxies. Further out you have all the matter orbiting the center of mass. In our solar system it is all the planets, asteroids and comets that orbit the Sun. In a galaxy it is all the stars, planets and gas clouds that orbit the central black hole.



The planets orbit the Sun.



In a galaxy, all stars, planets, gas clouds and other matter orbit the central black hole.

In our solar system, planets orbit faster the closer they are to the Sun. Mercury, the innermost planet, orbit the fastest. Neptune, the outermost planet, ponderously moves around the Sun at the slowest speed. (Pluto doesn't count here since it is no longer considered a planet, just a dwarf planet.)



Pluto, a planet no more.

Anyway. In our solar system, objects move faster the closer they are to the Sun. They move more slowly the farther they are from the Sun. There is a reason for that. Neptune (and Pluto!) feels the Sun's gravity much less than Mercury does, and if Neptune (and Pluto) moved as fast as Mercury does, then their own speed "along the tangent of their orbits" would easily overcome the Sun's attempts to "rein them in" with its own gravity. Conclusion? If Neptune and Pluto moved as fast as Mercury does, then their own velocity along the tangents of their orbits would flung them clear out of the solar system until they were lost in space!

Lost in space, but for real!

It was expected to be the same way with galaxies as it is with our solar system, namely, that objects orbiting far from the center of the galaxy would have to orbit much more slowly than objects orbiting deeper inside. The farther away you got from the center, the more slowly the stars, gas and other matter was expected to rotate.

It turned out that it was not so, as Very Rubin found out.



This is a "rotation curve" of stars at different distances from the center of our smallish neighbouring galaxy, M33.

It is not too hard to measure how stars rotate in galaxies, as long as the galaxies are somewhat tilted to our line of sight, so that the stars move either toward us or away from us during some parts of their orbits. When they move toward us or away from us their light will be either blueshifted or redshifted, and by measuring how much the spectral lines have moved from their "proper place" you can rather easily deduce how fast they move. (See part 4 of my series for an explanation of redshift and blueshift.

It was, as I said, Very Rubin who was the one who started systematically measuring the rotation curve of galaxies. What she found was that stars farther out from the galaxies' centers don't move more slowly than stars closer to the galactic center. This was completely unexpected.



A typical galactic rotation curve. But how can it be explained?

In 1983, Mordehai Milgrom proposed a new theory of gravity called MOND, Modified Newtonian Dynamics.



According to MOND, Newtonian gravity as we know it doesn't work for objects where the gravitational field is low. What MOND means for galactic rotational curves is that stars close to the center of a galaxy move according to classical Newtonian laws of gravity, but stars further out from the center move according Modified Newtonian Dynamics. Interestingly, MOND has proved itself quite good at predicting how how stars in galaxies and galaxies in galactic clusters will move. Even so, MOND isn't very popular with astronomers, at least not so far. So we need to take a look at the only other explanation that astronomers have come up with to explain the strange rotation curves of galaxies, and that is dark matter.

Ann