Of all the enigmas in astronomy, the most perplexing is the invisible-mass mystery. Scientists now believe that more than 90% of the matter in the universe may be in a strange unseen form. It can't be detected directly, but its effects are plainly evident.
For example, spiral galaxies (systems of stars like our Milky Way), do not rotate as they should. Photographs show that the density in a spiral is highest near the center, dropping off toward the outer fringes. The gravitational pull on a given star, which results from all the other stars, should therefore be much less in the outer regions.
This means that stars in these regions should move slower than those closer to the center. But they don't. And the only explanation astronomers have come up with is that there is considerable mass that we can't see.
Studies of clusters (groups) of galaxies indicate a similar problem. They don't have enough mass to hold them together; they should be flying apart. But they aren't.
There obviously has to be a lot of invisible mass in them--in some cases 600 times as much as we see. Furthermore, the problem applies not only to galaxies and clusters, but also to the largest known structures in the cosmos--super-clusters (clusters of clusters).
Worse yet, there are difficulties with the universe itself: Without the missing mass, there does not appear to be enough matter to stop its expansion.
Where and what is this "dark matter," as it is sometimes called? The problem is so serious that it has been given top priority in cosmology. Thousands of astronomers around the world are working on some aspect of it, and numerous candidates for dark matter have been nominated.
The latest arrived only a few weeks ago from Harvard: electrically charged massive particles (CHAMPS) left over from the Big Bang that created the universe about 18 billion years ago.
The problem first came to the attention of astronomers in 1932 when Dutch astronomer Jan Oort decided to measure the mass of the Milky Way. He noticed that stars occasionally travel high above or below its disk but are eventually pulled back by the combined gravitational pull of all the stars in the disk.
By the time they get back to the center, however, they have considerable speed and pass right through.
Their overall motion, therefore, is a slow oscillation about the center of the disk. Oort measured the speeds of the stars as a function of their distance above or below the center. This allowed him to determine the net force acting on them, and therefore the mass of the region.
He then added the masses of the visible stars in the same region and was surprised when it was about 50% smaller than the overall mass. There was obviously a lot of matter that was not accounted for. Or, looking at it another way: for the amount of light visible, there was too much mass.
About a year later Fritz Zwicky of Mt. Wilson Observatory in California began a similar study of a group of galaxies called the Coma Cluster. After determining the total mass of the cluster by measuring the motions of the galaxies within it, he added up the masses of the individual galaxies and found that the overall mass was several hundred times larger than the sum of the individual masses. Something was obviously wrong.
Either there was a lot of unseen mass, or--since mass and energy are related through Einstein's famous formula E=mc 22, the mass was in the form of energy and the cluster was in a state of explosion. Since it didn't appear to be exploding, Zwicky assumed that some of the mass was in an invisible form--gas or dust or perhaps dim stars.
Further progress didn't come until 1974, when a group from Princeton University published a computer study of galaxies. Assuming that the stars were distributed according to the galaxy's brightness, they concluded that spiral galaxies could not be stable. But there was no evidence that they were breaking up or flying apart.
The only way galaxies could be stable, the researchers showed, was if they were surrounded by a massive halo providing sufficient gravitational pull to keep the stars in orbit. The halo's mass, in fact, had to be considerably greater than the mass of the visible material.
Was this possible? Strangely, observations had already indicated that it might be. In 1969, researchers at Carnegie Institution published the results of a study of a nearby galaxy in the constellation Andromeda.
Using an instrument called a spectrograph that splits the light from stars into its component colors, they were able to plot the speed of the stars as a function of their distance from the center of the galaxy, called a "rotation curve."
(Determining stars' velocity through their spectra is possible because of the Doppler effect: When a moving object is generating waves, they tend to bunch up when the object is approaching and to space out as it recedes. Thus a train whistle sounds higher-pitched as it get nearer, and lower as it moves away. The same is true of light waves: The faster a star is receding, the more its spectral lines move toward the long-wavelength, or red side of the spectrum. The phenomenon is called "red shift.")
The Carnegie scientists expected the stars to move in much the same way the planets in the solar system do: The farther from the sun, the weaker the gravitational field; thus, the outermost planets travel slower in their orbits.
This is called Keplerian motion, after the astronomer who discovered it. But when their results were calculated, they found that the stars' speeds were equal no matter how far they were from the galaxy's center.
But when the plot was made, they were surprised to find that they didn't. This was impossible unless there was considerable mass in the outer regions that couldn't be seen. They subsequently extended the study to many different types of galaxies, and in all cases the pattern was the same.
This leads to the question: Does the entire universe contain a large amount of invisible matter? Maybe and maybe not. For this reason we call it the "missing mass," although in fact it may not be missing.
Our universe is in a state of expansion. All galaxies are moving away from one another, and the farther away the faster they go. There are two possible outcomes: Expansion can continue, or the universe can stop and collapse back on itself. It all depends on the average density of matter. If it is greater than critical density (about one proton per cubic meter) the expansion rate of the universe will eventually decrease to zero, and a collapse will occur. If it is less, expansion will go on forever.
Astronomers now have a good estimate for the average density of the visible matter in the universe, and it is only about 1% of the critical density. Even making allowances for matter they suspect is there but can't see directly, they still only have a few percentage points more.
This suggests that the universe is "open" and will keep expanding. But a "closed" or collapsing universe is a more scientifically satisfying scenario to most astronomers. That, however, requires a lot of missing mass.
What form does this "dark matter" take? There are several possibilities. One is dim stars beyond the range of telescopes. Our nearest neighbor, Proxima Centaurus, for example, is so dim that despite its proximity it was not discovered until 1915.
It is of a type called "red dwarf"--stars ranging in mass from about one-half to one-tenth the mass of the sun. (Stars are classed by the wavelength they emit, which in turn reflect their masses and chemical makeup.
Lower-energy, "cool" stars tend toward the red end of the spectrum, high-intensity "hot" stars toward the blue. Dwarfs result from the collapse of larger stars, giants from expansion and cooling.)
Large numbers of red dwarfs have been observed, and most astronomers believe that there are many we can't see. Indeed, the general trend throughout the galaxy is known to be thus: the smaller the mass of the star, the greater the number. In the Milky Way region of space there are almost as many red dwarfs as there are all other types of stars put together. But does this pattern extend to extremely small stars of, say, one-tenth the solar mass? Research indicates that the number of dim stars falls off rapidly below about one-fifth solar mass.
The major problems with red dwarfs, however, is the enormous number it would take to account for the dark matter. Also, since red dwarfs occur along with brighter stars throughout the visible part of the galaxy, it is reasonable to assume that if they make up the halo of the galaxy, a few bright stars would also be present. So far, though, there is no evidence of them.
Another candidate is the brown dwarf. Such stars, if they exist at all, range from about one-tenth solar mass down to slightly more than that of Jupiter. Since there are a large number of red dwarfs, it seems likely that there are also many brown dwarfs.
There may even be more. They are not stars in the proper sense of the word, but gaseous clouds that weren't massive enough to become stars. They heated as they condensed, but their central temperature never rose high enough to trigger nuclear reactions. Nevertheless, the compression of the gas is sufficient to cause them to glow for a while.
Unfortunately, scientists have considerable difficulty detecting them, although there have been several possible sightings in recent years.
Other types of stars are also possibilities. When a star such as the sun runs out of fuel, it slowly collapses, eventually becoming a white dwarf--a small, extremely dense object only a little larger than Earth. They are so dense that if they existed in large numbers, they would be excellent candidates.
About 500 are visible within the range of our telescopes. But if this is typical of how they are distributed throughout our galaxy, there are only enough to account for about 10% of the dark matter. Then there are neutron stars--smaller, even denser objects produced in the collapse of massive stars.
But stars large enough to make neutron stars are rare. So are the even more massive stars that collapse to form another dark-matter candidate, black holes-entities so dense that even light cannot escape them.
In recent years, the search for dark matter has turned to various exotic subatomic particles--most of which exist only as mathematical concepts.
One of the few that we know exists is the elusive neutrino, which has no charge and travels at or near the speed of light. It was not taken seriously for many years because it was assumed that, like the photon (the particle of light), it had zero rest mass.
But in the mid-1970s Soviet scientists showed that it might have a small mass. Astronomers suddenly took another look at it. The neutrino is so abundant that it would make an excellent candidate. But so far, experiments haven't proved that it has mass.
Other exotics include magnetic monopoles (formed in the first infinitesimal sub-fraction of a second of the Big Bang, each would have a mass 10 times that of a proton); the so-called super-symmetric particles (hypothesized to be the massive "super-partners" of known particles); and axions, presumed to be a trillion times lighter than electrons and to cluster around galaxies. So far, however, no one has detected any of them.
In short, none of the candidates is satisfactory. And even in combination, they would account for only a small fraction of the missing matter.
Were this not baffling enough, the cosmos contains yet another vast anomaly. "Echoes" of the Big Bang still reverberate through the universe in the form of loose photon-particles of light or other electromagnetic radiation.
During the first half-million years of expansion, it is believed, photons were mixed in with everything else in the seething primordial plasma. As the exploding universe expanded and cooled, photons "decoupled" and flew off into space, leaving protons and electrons to form the elements we know today. These photons (the "background noise" of the universe) are easily detectable.
And that's the problem: Point an instrument in any direction, and this background radiation is the same, suggesting that the universe expanded in perfect symmetry. Thus it follows that the distribution of galaxies should also be perfectly uniform.
And so astronomers believed until the 1930s, when Fritz Zwicky showed that many galaxies were grouped in clusters. As the years passed, researchers found further asymmetries: clusters of clusters, and on a larger scale "bubbles" containing chains of clusters.
But if the universe was created by smoothly uniform expansion in all directions, why would there be clumps of matter in certain areas and voids in others? The answer, again, is dark matter, which we now know to be intimately tied to the large-scale structure of the universe.
It is impossible to say how this riddle will be solved. Perhaps we will need a new theory of gravity. Newton's laws of motion are perfectly valid for the everyday momentum of cars, billiard balls and rockets, but break down at the microcosmic level of atoms and particles.
Similarly, when we deal with huge macrocosmic structures such as galaxies--as far removed from the normal dimensions of human perception as are subatomic particles--it is possible that traditional laws break down there too.
Mordehai Milgrom of Israel's Weizmann Institute believes that Newton's rules of gravity are invalid when attraction becomes exceedingly weak, and that a new definition takes care of the dark-matter problem by eliminating it.
Or the breakthrough may be as radical as the revolution that shook physics when Niels Bohr's concept of the atom replaced Rutherford's earlier in this century.
Meanwhile, we have galaxies that do not rotate right, a universe that may be missing 99% of its mass and a stupendous amount to learn.