A Cosmic Crisis?

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.

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