Astronomers appear to have a heavenly crisis on their hands, and it concerns
material they can’t even detect.
Professional star watchers thought for years that they understood the basic theory
of how structure–galaxies and galaxy clusters–arose in the universe. Now, some are
worried that they don’t have it quite right. The recent observations that roused
their concerns, however, also are providing data that are beginning to put a face
on the mystery material that underlies the problem.
That stuff is called dark matter. As researchers piece together a profile of this
so-far elusive substance, which they believe makes up most of the universe’s
matter, they are starting to find out how it links the smallest structures in the
universe to the largest. While physicists continue to search for dark matter
particles using accelerators and underground detectors (SN: 2/26/00, p. 135),
astronomers have now joined the hunt.
Researchers first proposed the existence of this ghostly material in the 1930s.
That’s when Fritz Zwicky noticed that galaxies in the Coma cluster were spinning
so rapidly that all the visible material wasn’t enough to keep them from flying
apart. Some unseen matter, it seemed, had to be supplying the extra gravitational
glue.
Over the years, incentive to believe in this mystery material has only grown. In
the late 1970s, for example, researchers measured the velocity of the outer parts
of several galaxies. In galaxy after galaxy, they found that the outer regions
rotated so fast that it was a wonder any galaxy was still intact. Once again, the
laws of physics seemed to dictate that some unseen matter resides there and
provides the missing gravity.
Further evidence for dark matter comes from measurements on a more cosmic scale in
the 1980s and 1990s. Using remote quasars as flashlights whose beams pass through
primordial hydrogen clouds on their way to Earth, astronomers have measured the
amount of deuterium–a heavy isotope of hydrogen–that formed when the universe was
very young.
This measurement is supremely important because from it, researchers can infer the
cosmic abundance of baryons, which include the protons and neutrons that make up
all atomic nuclei. That exercise has led astronomers to calculate that baryons
account for less than 5 percent of all the matter in the universe. The rest must
be some sort of exotic material that no telescope can see.
Indeed, astronomers have come to think of luminous galaxies as mere bright flecks
embedded in a halo of dark material. In the prevailing theory of dark matter, the
mystery material has a one-dimensional personality. This type of dark matter,
known as cold dark matter, would consist of sluggish particles that exert a
gravitational tug but exhibit no other distinguishing feature. These particles
would give off no light and would interact with each other only slightly, through
the weak nuclear force–the same force that governs, for example, the radioactive
decay of atoms.
Because these putative particles move slowly, they would have clumped together
earlier in cosmic history than did baryons. Therefore, dark matter would have
provided the gravitational scaffolding necessary for the first galaxies to
coalesce when the universe was less than a billion years old. In that respect, the
cold-dark-matter model has proved remarkably successful at generating the kinds of
large-scale structures seen in the universe today.
When cosmologists apply the model to the finer scales of galaxies and smaller
objects, however, the theory seems to run into trouble. Computer simulations of
cold dark matter create universes that are far lumpier on these smaller scales
than the real universe appears to be.
The model predicts, for example, that the cores of galaxies are much denser than
recent, high-resolution observations indicate. It also holds that dwarf galaxies,
like the satellite galaxies orbiting the Milky Way, should be 100 to 1,000 times
more numerous than astronomers have detected.
There are other conflicts. According to the standard cold-dark-matter model, the
smallest galaxies were the first to form, coalescing at a time when the expanding
universe was younger and denser than it was when gravity later pulled together the
more massive objects. It follows that dwarf galaxies should contain a higher
density of matter than the others. Yet in reality, many dwarfs are no denser than
other galaxies and much larger objects, such as galaxy clusters.
Furthermore, several observations hint that the distribution of dark matter in
galaxy clusters is spherical rather than football-shaped, as models of cold dark
matter suggest.
Some researchers, such as Christopher S. Kochanek of the Harvard-Smithsonian
Center for Astrophysics in Cambridge, Mass., argue that many of the apparent
points of conflict between theory and observation may vanish when cosmologists
develop more sophisticated models for the complex effects that baryons have on
galaxy formation. Unlike dark matter, baryons radiate light and exert pressure,
and most computer simulations of cosmic evolution don’t accurately incorporate
these properties.
Other researchers say that the apparent problems with the theory of cold dark
matter are signs of a real crisis.
“If we only had one problem to worry about, you might blame it on [modeling], but
when you have five problems, it’s not so easy to dismiss them,” says Paul J.
Steinhardt of Princeton University.
No quick resolution
Theorists have developed two main approaches to resolving the cold-dark-matter
conundrums. Each of these alternatives invokes a different version of dark matter.
Last month, astronomers working with NASA’s Chandra X-ray Observatory reported new
findings that could rule out one of these. The findings suggest, however, that the
dark matter crisis may not be resolved any time soon.
Astronomers are looking to the Chandra observations, along with a host of other
ongoing studies, to reveal what dark matter is–and what it isn’t. At stake, notes
Steinhardt, isn’t just a deeper understanding of cosmic structure. The identity of
dark matter must fit with scientific understanding of the fundamental forces of
nature: electromagnetism, gravity, and the strong and weak nuclear forces, he
says.
Supersymmetry, the leading theory to unify those forces, includes several
elementary particles that make good candidates for dark matter particles. These
particles would interact only through the weak force. That’s a plus for the cold-dark-matter theory but may be problematic for the alternatives.
In one of the alternative models, researchers including Craig J. Hogan and Julianne J. Dalcanton of the University of Washington in Seattle propose that dark
matter particles are neither cold and sluggish nor hot and speedy. Rather, they
are just warm enough to slightly resist the mutual gravitational attraction that
brings them together.
This resistance could have made the first clumps of matter that coalesced in the
universe slightly puffier than they would be in the cold-dark-matter model, says
Dalcanton. Since these clumps formed the seeds from which bigger structures arose,
the puffiness could explain why dwarf galaxies aren’t as dense as cold-dark-matter
theory says they should be, she adds.
Because of their higher temperature, particles of warm dark matter move faster
than particles of cold dark matter. That motion might enable these particles to
avoid congregating at the centers of galaxies. This would fit with the observed
low density of galaxy cores.
One possible strike against warm dark matter is described in a paper to appear in
the Astrophysical Journal. Rennan Barkana of the Canadian Institute for
Theoretical Physics in Toronto, Zoltan Haiman of Princeton University, and
Jeremiah P. Ostriker, now at the University of Cambridge in England, note that the
material’s resistance to clumping might delay the early epoch when the very first
quasars–and the supermassive black holes thought to power them–came into
existence.
In another version of the dark-matter theory, the mystery material, known as self-interacting dark matter, remains cold but is a lot more sociable. As proposed by
Steinhardt and his Princeton colleague David N. Spergel, the particles interact
strongly with each other, colliding and scattering like billiard balls. As with
baryons, the collisions would occur more frequently in crowded quarters, such as
the cores of galaxies, than in the sparse expanses of intergalactic space. In the
simplest model, all dark matter particles would have the same likelihood of
colliding, regardless of their speed.
The jostling of self-interacting dark matter particles would tend to spread out
the galactic cores, reducing the density there. Farther from these cores, in less
compact regions, the particles would rarely meet and so behave like particles in
the standard cold-dark-matter theory.
Self-interacting dark matter could also explain the relative dearth of dwarf
galaxies–or at least why so few are found buzzing around large galaxies–says
Steinhardt. If there were interparticle collisions, the halo of dark matter
surrounding a big galaxy would have a more pronounced tussle with the halos of
nearby dwarf galaxies. The interactions would strip the dwarfs of their gas and
stars more rapidly than in the standard cold-dark-matter theory. So, more of these
dwarf galaxies would boil away or fall apart.
Mapping dark matter
Observations with the Chandra X-ray Observatory, reported last month in
Washington, D.C., seem to have dealt a blow to the self-interacting model. To test
the model, researchers used Chandra’s sharp optics to measure the temperature and
intensity of the hot, X-ray-emitting gas in a cluster called EMSS 1358+6245, which
is some 4 billion light-years from Earth. Just as lights on a Christmas tree
outline its dark branches, the X-ray-emitting gas provides a map of the dark
matter in the cluster.
With these data, John S. Arabadjis and Mark W. Bautz of the Massachusetts
Institute of Technology, along with Gordon P. Garmire of Pennsylvania State
University in State College, found that the density of the dark matter is greater
the closer it is to the center of the cluster. Chandra could probe no closer than
130,000 light-years from the center, a distance much greater than the radius of an
individual galaxy’s core. Nevertheless, the findings still rule out the simplest
model of self-interacting dark matter, Arabdjis’ team says.
“What we’re seeing is the farther we go, the denser [the dark matter] gets,” says
Bautz. That’s in contradiction to the self-interacting dark matter model, in which
the particles bump into each other and keep the density from rising by puffing up
the core. “So, our data completely support the standard picture [of cold dark
matter],” says Bautz.
Ostriker notes that having data from a single cluster isn’t enough to knock down
the self-interacting theory, but he says that further observations “can
potentially provide a clue about what the dark matter is.”
Bautz agrees. “We’re not saying that we now understand something about dark matter
that we didn’t before, but we’re undoubtedly going to know more when all the
Chandra data are in,” he says.
In some models of self-interacting dark matter, adds Ostriker, the force between
the particles declines dramatically with speed. That’s a crucial feature because
the greater gravity in a galaxy cluster makes particles there move faster than
they do in an individual galaxy. Consequently, self-interacting dark matter
particles may have substantial collisions in a galaxy but act in a galaxy cluster
just as inertly as do cold-dark-matter particles. The Chandra observations can’t
rule out that possibility, notes Bautz.
Nor do they rule out warm dark matter. At the cores of galaxies, the faster-moving
particles of this version of dark matter could offer some resistance to gravity,
preventing the dark matter from congregating there. However, warm-dark-matter
particles would be no match for the gravity of galaxy clusters. Warm dark matter
would therefore behave no differently than cold dark matter in such a weighty
environment.
Prying into secrets
The Chandra observations have extended the search for dark matter–once limited to
particle accelerators and underground detectors–into the realm of astrophysical
observations, says Steinhardt.
With longer-term observations, Chandra will be able to peer even more closely into
the centers of galaxy clusters and place new limits on models for dark matter,
adds Bautz.
Already, the Chandra observations are prying into dark matter’s secrets. By
placing limits on the strength of the interaction between dark matter particles,
the results suggest that if the particles do collide, they do so relatively
weakly.
Several other astrophysical studies may also illuminate the dark matter mystery,
says Steinhardt. For instance, astronomers can measure the density of small
galaxies or the cores of larger galaxies by determining how well they act as
gravitational lenses. Any dense object serves as a lens. It bends the light
passing by it from a background body, such as another galaxy, into multiple images
or arcs. The higher the density of dark matter, the greater the distortion.
Since some dwarf galaxies may be essentially starless, and so all but invisible,
the only way to detect them is through their distortion of the images of
background objects (SN: 9/29/01, p. 203: Gravity’s lens: Finding a dim cluster and Gravity’s lens: Finding a sextet of images). Gravitational lensing thus provides an
accurate count of dwarf galaxies in a given patch of sky, a critical number for
testing the predictions made by different dark matter models.
Closer to home, increasingly detailed maps will provide an accurate estimate of
the abundance of dwarf galaxies near the Milky Way, Steinhardt says. Their
distribution provides another hint about the nature of dark matter.
The cold-dark-matter theory predicts that dwarfs would be randomly distributed
throughout the universe, but the self-interacting model suggests that relatively
few should lie near big galaxies like the Milky Way. In contrast, the models
indicate that warm dark matter would reside in sheets.
More information about the nature of dark matter may come from the abundance of
tidal tails, the streams of stars and gas that are gravitationally torn by the
Milky Way from small galaxies, such as the nearby Large Magellanic Cloud. Self-
interacting dark matter would hasten the stripping of these satellites, increasing
the number of tails. It would also shrink the size of these satellite galaxies.
“The exciting thing about this is that the realm of local astronomy is a new
window on the nature of dark matter,” says Steinhardt. “We’re not talking about
measuring distant galaxies but rather measuring satellites in our neighborhood and
the neighborhood of the [nearby] Andromeda galaxy.”
With these studies, he says, the dark matter crisis may ultimately be resolved.
