By Robert Hazen, Ph.D., George Mason University
Measurements of the redshift of galaxies reveal surprises that point to new mysteries in science. In the 1970s, Carnegie Institution astronomer Vera Rubin was engaged in a methodical study of these galactic motions. Rubin’s work was conducted at Kitt Peak Observatory in Arizona, and it was a tedious task. After a few nights, a strange and curious pattern emerged.
Rubin’s Confusing Discovery
Rubin expected the rotation speed of the stars to drop off as you got farther away from the center—the stars should be rotating more slowly or orbiting more slowly around the galactic center, because there’s less mass, and the distance is greater. But she found that there was no drop-off in velocity whatsoever.
Even the most distant stars from the center were rotating much too fast, perhaps three times faster than you’d expect from a normal galactic distribution, just based on the visible stars; and that’s the only way we can estimate the distribution of mass in the galaxy.
Newton’s laws provide a very straightforward solution to orbital dynamics, and Newton’s laws said that those galaxies were rotating too fast, based on the stars.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Missing Mass of the Universe
The only solution was that there had to be mass that you couldn’t see, missing mass. The problem was compounded when people started doing radio-telescope observations. Radio telescopes can pick up emissions from hydrogen gas, not coalesced into stars, but just huge clouds of gas that extend beyond the visible galaxy in what’s called a halo.
Our Milky Way galaxy, for example, may extend 100,000 light-years beyond the visible stars in a halo of hydrogen, and the hydrogen was rotating much too fast too. There’s a vast quantity of mass that’s missing.
This is the problem of the missing mass. Perhaps ten to 100 times more mass is contained in a galaxy than what you can see in the stars. This missing mass or dark-matter problem confounds astrophysicists and particle physicists today.
What’s Dark Matter?
First, we can be pretty sure what it isn’t. It doesn’t seem to be ordinary dust or gas because that material would block out light from the galaxy. Also, it would radiate out infrared radiation or heat.
Other obvious possibilities include things called MACHOs, Massive Compact Halo Objects. These are things like planets, brown dwarf stars, comets, asteroids, the ordinary things we see in our solar system. However, all these objects also should emit infrared radiation, and we don’t see that radiation. So we can’t make up 90 percent of a galaxy’s mass with ordinary matter.
Maybe it’s massive black holes, but black holes should act as gravitational lenses. They should make more-distant stars twinkle, that is get brighter and dimmer as the black hole passes in front, but a systematic study of stars doesn’t show enough twinkling going on to account for all that missing mass.
There are other possibilities: exotic particles, neutrinos with mass, for example, certainly could make up part of the dark-matter budget. Other astronomers invoke things called WIMPs. These are Weakly Interacting Massive Particles, exotic particles for which, right now, there’s no direct evidence whatsoever. The fact is, we just don’t know what the missing mass is; it’s one of the great mysteries of science.
Learn more about the life cycle of stars.
Future of the Universe
We know how the universe began, and we know its present state. What about its future? Predicting the future requires playing the tape forward, and we really can’t be sure; there are three possible fates.
One of them is that the universe does not have enough mass, that it’s going to continue to expand forever, becoming more and more spread out and growing gradually colder and colder. Ultimately, when all the stars have died, the universe would then cool to a uniform temperature.
This is a phenomenon that’s been called the heat death of the universe. If that happens, it would be hundreds of billions of years in the future.
This open universe, the ever-expanding universe, seems aesthetically unappealing to most physicists, although some astronomers just don’t care.
Learn more about the evolution of life.
Other Possible Fates of the Universe
Many people feel uneasy about a universe that just keeps getting larger and dissipates in kind of a whimper.
The universal expansion might also be flat—that’s, if there’s exactly the right amount of mass to compensate for the expansion. Gravity eventually slows things down, but it never totally stops.
This flat universe is quite aesthetically appealing because it seems to signify a kind of symmetry in nature between the total amount of mass and the expansion.
Finally, the universe might have more than the necessary amount of mass. Eventually, gravitation causes the expansion to stop, coming back into a series of crunches, maybe another bang, and another crunch. This is incredibly appealing to those people who want a steady-state universe because bangs and crunches would then give a steady-state—albeit one interspersed with violent events.
As of now, the open model seems like the most likely one. It appears that the universe is expanding far too fast, based on our current data, ever to slow down and stop—but keep watching science news. That’s one hot story where the final answer is not yet in.
Common Questions about Two Great Mysteries: Dark Matter and Future of the Universe
While studying the motions of galaxies, Vera Rubin expected the rotation speed of the stars to drop off as they got farther away from the center. However, she found that there was no drop-off in velocity whatsoever.
There is perhaps ten to 100 times more mass contained in a galaxy than what you can see in the stars. This is the missing mass or dark matter problem, and it confounds astrophysicists and particle physicists even today.
One theory regarding the future of the universe is that the universe does not have enough mass and that it’s going to continue to expand forever, becoming more and more spread out and growing gradually colder and colder. Ultimately, when all the stars have died, the universe would then cool to a uniform temperature. This phenomenon has been called the heat death of the universe.