What Is the Nature of Dark Matter and Dark Energy?
AN EMERGING COSMIC RECIPE
What is the universe made of? The answer to this very simple question is not so simple. Baryons, the familiar neutrons and protons of which we, Earth, and the stars are made of, do not account for most of the mass in the universe. Instead, we appear to live in a universe composed primarily of new, unfamiliar, and unidentified forms of matter and energy. Three main pieces of evidence support this conclusion.
Big bang nucleosynthesis, the very successful theory of the nuclear origin of the lightest elements in the periodic table, together with recent measurements of the amount of deuterium (heavy hydrogen) in the universe, indicates that only about 4 percent of the mass and energy in the universe is in the form of ordinary matter (baryons), with the rest in an unknown form.
Since the pioneering work of Fritz Zwicky in the 1930s, astronomers have suspected that a dark component of matter—one that neither emits nor absorbs light—accounts for most of the mass of the universe. Over the last decade, the astronomy community reached a consensus that dark matter is ubiquitous in galaxies and accounts for most of the mass of clusters of galaxies and superclusters (larger systems composed of several clusters). Most of the mass of our universe is in dark matter. Further, because of the sheer quantity of dark matter, more than can be accounted for in the form of ordinary matter, it must be made of something exotic—with elementary particles produced in the early hot universe being the leading candidate. The leading candidate particles are axions and neutralinos (see Chapter 3).
Over the last few years, astronomers have made an even more remarkable and more puzzling discovery about the composition of the universe. Using high-redshift type Ia supernovae to probe the expansion history of the universe, they have found evidence that the expansion is
speeding up, and not slowing down as expected. This result implies the existence of large amounts of “dark energy” whose gravitational force is repulsive (see Box 5.1).
Perhaps the biggest puzzle of all is the odd mix that makes up our universe—ordinary matter, exotic dark matter, and dark energy, all in significant amounts. This odd arrangement may imply, as the Ptolemaic epicycles did, that we are lacking a deep enough understanding of the laws of physics underlying our universe. It is even possible that what we call dark matter and dark energy are the signatures of some unknown aspect of gravity or space-time itself.
The Universe Is Flat
According to Einstein’s theory of general relativity, the total density of matter and energy (mass or energy per unit volume) in the universe determines the spatial curvature of the universe (see Box 5.2). For one density— the so-called critical density—the universe is uncurved (“flat”) and the geometry is just that of Euclid. A supercritical (closed) universe curves back on itself (like the surface of a balloon, only in all three dimensions rather than two), and a subcritical (open) universe is curved away from itself, like a saddle. The contributions to the composition of the universe mentioned above sum to a value close to the critical density, indicating a flat universe.
The cosmic microwave background (CMB) can also be used to determine the shape of the universe and thereby provide an independent accounting of the total amount of matter and energy. The angular size of the hot and cold spots in the microwave background is directly related to the shape of the universe—in a closed universe the hot and cold spots appear larger than in a flat or open universe, because the overall curvature of space acts as a cosmic lens, magnifying or demagnifying the spots (see Figure 5.1). Researchers have recently made spectacular progress with the measurement of the angular scale of the hot and cold spots on the CMB. The BOOMERanG, MAXIMA, and DASI experiments have confirmed indications from earlier experiments that the universe is indeed flat, which implies a density deviating from the critical density by at most 6 percent.
These CMB experiments not only have determined the shape of the universe but also have provided an important cross-check on the accounting of the composition of the universe. Future CMB experiments, including the MAP and the Planck satellite missions, should reveal important clues about the nature of the dark matter and dark energy. However, more experiments will ultimately be needed to clarify the nature of both.
BOX 5.1 EINSTEIN’S GRAVITY CAN BE REPULSIVE
The most familiar feature of gravity is that it pulls, not pushes. According to Newton’s theory the gravitational force exerted by an object is always attractive and proportional to its mass. Not so, according to Einstein—in unusual circumstances gravity can be repulsive. Einstein’s theory, which extends our understanding of gravity into situations when gravity is very strong or particles move very fast, has other more familiar, amazing features, including black holes.
While the central idea of Einstein’s theory is the description of gravity as a property of curved space rather than a force, one can still discuss Einstein’s gravity in terms of forces. Because mass and energy are related through Einstein’s most famous equation, E = mc2, it is not surprising that energy replaces mass in Einstein’s version of a gravitational force. What is new is that pressure also generates gravity. (Recall that pressure can be thought of as momentum flowing in a particular direction.) When thinking of gravity as force in general relativity, it is the energy and the pressure (E + 3P) that determine the strength of the gravitational force exerted by an object.
The pressure of an ordinary gas is outward and positive. According to Einstein’s equations then, the gravitational pull of a ball of hot gas (e.g., our Sun) is greater than that of an equivalent mass of cold particles that have no pressure. For most situations the difference is too small to be of any importance. However, it is precisely this feature of Einstein’s theory that leads to the prediction of black holes, objects that cannot support themselves against the force of gravity and collapse to a singularity. For example, in any stationary object like the Sun or Earth the force of gravity must be balanced by an outward pressure if the object is to retain its shape and size. (In our Sun, the pressure arises from the hot gas of which it is made.) The extra gravitational force due to the pressure term is small. In more massive objects gravity is stronger and the corresponding resisting pressure must be stronger. But as the pressure increases, the correction to the pull of gravity also increases. For very massive objects, the extra gravitational forces due to the pressure itself can exceed the outward push of the pressure itself (remember the factor of 3 in front of pressure in the gravitational force equation) and is counterproductive. For very massive objects, in the end the pressure that initially supports the object against gravity only hastens the collapse to a black hole.
Although there are good reasons to believe that energy cannot be negative, negative pressure is a feature of anything that is elastic (e.g., a rubber band or sheet of rubber). For a rubber sheet or a rubber band, the small decrease in the gravitational attraction is too small to measure. However, there are situations in which the pressure can be comparable in magnitude to the energy, but negative, so that gravity becomes repulsive. The most extreme case is the energy of the quantum vacuum, where the pressure is the exact opposite of the energy, with the result that the gravitational pull is not only twice what Newton would predict but also repulsive! In Einstein’s theory of gravity, repulsive gravity is possible, but in Newton’s theory it is not.
In constructing his original static model of our universe, Einstein tuned one feature of his theory: He balanced the attractive force of matter in the universe against the repulsive force of his cosmological constant, which is mathematically equivalent to vacuum energy (described above). If the expansion of the universe is indeed speeding up and not slowing down, as current observations indicate, one need not go beyond Einstein’s theory for an explanation. We are simply seeing a new feature of gravity.
Not Much Ordinary Matter
There is now much evidence that the kind of matter we are made of accounts for only a small amount (around 4 percent) of the total mass and energy budget of the universe. Three independent methods point to this conclusion (see Figure 5.2).
During the first 3 minutes after the big bang, protons and neutrons fused together to form the nuclei of the lightest elements in the periodic table, hydrogen, deuterium (heavy hydrogen), tritium, helium, and lithium. The relative abundance of these elements, particularly deuterium, is sensitive to the density of ordinary matter. The recent measurement of the primordial
BOX 5.2 UNDERSTANDING THE CURVATURE OF SPACE-TIME
The central idea of Einstein’s theory of relativity is the curvature of space-time. While it is difficult (if not impossible) to visualize curved three-dimensional space, the tools of modern mathematics can describe it readily. However, it is possible to visualize a lower dimensional curved space. Imagine a universe with only two spatial dimensions, rather than the three of our space time. The two-dimensional analogue of our universe can take on three different shapes: flat, like a sheet of ordinary paper; positively curved (closed), like the surface of a ball; or negatively curved (open), like a saddle (or a potato chip), as shown in Figure 5.2.1.
Viewed from the luxury of our three space dimensions, these two-dimensional universes are seen to be very different. However, there are also simple mathematical measurements that the hypothetical two-dimensional inhabitants of these universes could make to discover the shape of their universe. The simplest involves one of the most basic truths of Euclidean geometry: In flat (Euclidean) space, the angles of a triangle sum to 180 degrees. This is not true for the open or closed spaces: for the closed universe (surface of a ball), the angles in a triangle always sum to greater than 180 degrees, and for the open universe (saddle) the sum is always less than 180 degrees. Without escaping to three dimensions, the two-dimensional inhabitants of these curved universes can determine the shape of their universe.
We can do the same. The trick in all of this is using really big triangles. In a tiny triangle laid out on the surface of a ball, the amount by which the angles exceed 180 degrees is too small to measure. In our universe, the largest triangle we can lay out extends to the surface of last scattering for the CMB. Measuring the size of hot and cold spots on the microwave sky uses the triangle method to determine the shape of our universe. The physical size of these spots depends on simple physics and not on the shape of the universe. However, the angular size of the spots does depend on the shape, through the triangle effect just discussed. By measuring the size of these spots, the BOOMERanG, MAXIMA, and DASI experiments were in essence determining the sum of the angles in the largest triangle we can lay out.
deuterium abundance in primeval gas clouds along the line of sight to distant quasars has provided a precision measurement of the average baryon density, about 4 percent of the critical density.
Secondly, the statistical properties of the fluctuations in the cosmic microwave background are sensitive to the baryon density. For example, when one takes pairs of points on the sky, calculates the difference in the microwave temperature between those points, and then looks at many pairs and many angles between the pairs, one finds that for some angles the temperature differences are larger than for others. When plotted as a function of angle, the curve shows peaks and valleys in the temperature differences. The higher the baryon density, the larger the ratio between the
amplitude of the first and second peaks in this plot. The results from BOOMERanG, MAXIMA and DASI (see Figure 5.3) give a value of the baryon density consistent with the density determined from big bang nucleosynthesis, but with a slightly larger uncertainty.
The third method, like the first, involves the study of primeval gas in the universe by its absorption of light from distant quasars. In this case, the total amount of light absorbed can be used to estimate the total amount of ordinary matter that existed in the universe only a few billion years after the big bang. The results of these studies, which are not as precise, are also consistent with a baryon density of only about 4 percent of the critical density.
These three methods of baryon accounting measure the amount of ordinary matter when the universe was a few minutes old, about a half million years old, and a few billion years old, respectively. While these results are consistent, it would also be nice to have a similar accounting of ordinary matter today.
Such an accounting is more difficult because stars have been born and have died, baryons have been stirred up by the process of structure forma-
tion, and the universe is a more complicated place than it was long ago. Today baryons exist in bright stars, hot and cold gas, dark stars (faint stars such as white dwarfs, neutron stars, and black holes), and perhaps in other forms. A census of bright stars and cold gas shows that they account for only one third of the density inferred from big bang nucleosynthesis. The rest of the baryons are “dark” (see Figure 5.4).
Where are the dark baryons, and in what form do they exist? The most likely form is hot gas filling the regions between galaxies, the so-called hot intergalactic medium. Hot gas in the intergalactic medium is difficult to observe directly. Astronomers do have hints from observations of diffuse x-ray emission and from measurements of light absorption that there are significant quantities of gas in the hot intergalactic medium, but they are unable to characterize the quantity more precisely. Within our own galaxy, there is evidence that some of the dark baryons exist in the form of old white dwarfs.
While there is no evidence that the amount of ordinary matter today exceeds the big bang nucleosynthesis estimate, a full accounting of ordinary matter today is lacking. Such an accounting is high on the list of what astronomers would like to accomplish during the current decade.
EXOTIC DARK MATTER
Baryons account only for about 4 percent of the critical density, and several lines of evidence point to a matter density that is 35 percent of the critical density. The large discrepancy between these two numbers is the linchpin in the case for exotic (nonbaryonic) dark matter. While the additional matter cannot be seen with telescopes, its gravitational effects—from holding galaxies and clusters together to playing a critical role in the formation of large-scale structure—are very visible.
Evidence for the Existence of Dark Matter
The existence of dark matter is now well established on a variety of scales. In large spiral galaxies it is possible to measure the rotation velocity of gas clouds out to large distances from their centers. The behavior of rotation velocities implies that there is substantial mass well beyond the distance at which no more stars are observed and that the bulk of the matter that holds spiral galaxies together exists in a dark, extended halo (see Figure 5.5). Similar
dynamical evidence for dark matter is found in elliptical galaxies. Simply put, there is more mass than meets the eye in galaxies of all types.
The effect of dark matter is even more pronounced in clusters of galaxies. Cluster galaxies move at high speeds, and the mass necessary to hold them together in the cluster far exceeds that contained in all the stars that make up the galaxies. Cluster dark matter also creates a cosmic mirage: Light emitted by distant galaxies that passes by a cluster is bent by the gravitational effect of the cluster dark matter, as predicted by Einstein’s theory. Near the center of the cluster, this effect can be strong enough to produce multiple images of the distant galaxies. Farther out, it distorts the shape of each and every distant galaxy (see Figure 5.6). Gravitational lensing, as this phenomenon is called, based on one of the first tests of Einstein’s theory, is now used routinely to map dark matter in clusters and individual galaxies. This method shows very directly that clusters have 100 times more mass than can be accounted for in stars.
On even larger scales, the effect of dark matter can be detected through the pull it exerts on individual galaxies. By carefully measuring the velocities and distances of thousands of galaxies, it has been shown that virtually all galaxies—including our own—move with velocities over and above the velocities expected due to the expansion of the universe. These additional “peculiar velocities,” which arise from the gravitational effect of dark matter, provide a means of mapping the distribution of dark matter on scales even larger than those of clusters. This technique has revealed large dark-matter accumulations, including the well-known “Great Attractor.” Again, the amounts of matter revealed far exceed those that can be accounted for in stars or even in baryons.
Finally, the gravity of the luminous matter alone is not sufficient to account for the formation of the abundance of structure seen in the universe today—from galaxies to the great walls of galaxies. As discussed in Chapter 4, all objects seen today evolved from the tiny lumpiness in the matter that existed early on and was revealed by the Cosmic Background Explorer (COBE) experiment. This process was driven by the relentless, attractive force of gravity operating over the past 13 billion years. With the level of lumpiness revealed by COBE as the starting point, the structures seen today could not have been produced by the gravitational effect of luminous matter alone.
There is little doubt that dark matter exists. Deciphering its nature remains one of the great challenges of cosmology. Because there is strong evidence that it is not made of ordinary matter, discovering dark matter’s nature will also have deep implications for physics.
Amount of Dark Matter
How much dark matter is there? The most direct route to arriving at a quantitative estimate makes use of clusters to carry out a cosmic inventory of all forms of matter. Clusters are large objects and should offer a fair sample of matter in the universe. Thus, a measurement of the ratio of the total amount of matter to the amount of ordinary matter in a cluster can be used to determine the total amount of matter in the universe.
Most of the ordinary matter in clusters exists as hot, x-ray-emitting intracluster gas and can be inventoried from the clusters’ x-ray emission or from the slight distorting effect that clusters produce on the cosmic microwave background, known as the Sunyaev-Zel’dovich effect. Both methods arrive at the same value for the amount of cluster gas. The total mass can be measured through gravitational lensing, as well as by using the galaxy motions (or temperature of the hot gas) to infer the amount of matter needed to hold the cluster baryons together; all three methods give consistent results. The ratio of the total amount of matter to the amount of ordinary matter determined for clusters (about 9 to 1) and the big bang nucleosynthesis value for the amount of ordinary matter in the universe (about 4 percent) imply that the total amount of dark matter is about 35 percent of critical density.
Measurements of the peculiar motions of galaxies and the amount of mass contained in the halos of spiral galaxies point to a total density of matter that is consistent with this estimate. Using large, supercomputer numerical simulations of the formation structure in the universe, cosmologists arrive at a similar estimate for total matter density, based upon the gravitational pull needed to form the structure seen today from the lumpiness that existed at early times.
To summarize, there is strong evidence that dark matter holds the universe together, from the smallest galaxies to the largest structures observed. Although systematic uncertainties still exist, a number of independent lines of reasoning point to a dark matter density of about 35 percent of the critical density.
Different from Ordinary Matter
Three strong lines of evidence suggest that dark matter is something other than ordinary matter. The amount of dark matter (about 35 percent of the critical density) is significantly greater than the amount of ordinary matter inferred from big bang nucleosynthesis and from the cosmic microwave background (3 percent to 5 percent of the critical density). The discrepancy is nearly a factor of 10, which is far greater than the uncertainty in
either number. The amount of dark matter needed to produce the structure observed today from the lumpiness that existed early on is much greater than the known amount of ordinary matter. In addition, the pattern of structure that exists today cannot be produced even if baryons accounted for 35 percent of the critical density and there was no exotic dark matter. Putting aside the CMB and big bang nucleosynthesis measurements of the amount of ordinary matter, the implausibility of hiding 99 percent of the baryons in a form that is not detectable is daunting. For example, putting 35 percent of the critical density in white dwarfs or neutron stars would require far more star formation than the evidence supports. It is equally difficult to hide this many baryons in hot gas: Putting 35 percent of critical density in hot gas would jumble the cosmic microwave background by creating far too many hot and cold spots on the microwave sky.
Particle Debris from the Big Bang
The early universe was a powerful accelerator that produced the full complement of nature’s fundamental particles. At the earliest times there was a kind of particle democracy, where all particles were present in numbers comparable to the number of cosmic background photons. Over the course of time, as the universe cooled, massive particles disappeared when they were annihilated by their antiparticles. Without special circumstances, only photons (and other massless particles) would be left today.
Ordinary matter survived because of the tiny excess of matter over antimatter (see Chapter 4). Other massive particles might have survived if their self-annihilation had not been complete. Incomplete annihilation can occur if particle interactions are weak, like those of neutrinos. Massive neutrinos survived in great numbers because their annihilations failed to destroy them quickly enough. If nature is supersymmetric, the lightest superparticle, known as the neutralino (see Chapter 3), would survive in sufficient numbers to make a major contribution to the dark matter. (There are other, more complicated ways in which a particle can survive from the early universe.)
Over the years, many particles have been discussed as candidates for dark matter. Three deserve special consideration because they solve important problems in particle physics and their efficacy as dark matter is a cosmological bonus: massive neutrinos, neutralinos, and axions.
Neutrinos. Because the relic abundance of neutrinos is well determined, their contribution to the mass density of the universe hinges on the
question of their masses. To account for the dark matter, one (or the combination of all three) neutrino species would need a mass of around 30 eV. As discussed in Chapter 3, there is good evidence that at least one neutrino species has mass; however, the mass indicated is much smaller than this. Further, as discussed below, the pattern of structure formation seen in the universe is not consistent with the idea that neutrinos constitute the bulk of the dark matter. In fact, the neutrino mass implied by experiment indicates that neutrinos contribute between 0.1 percent and 5 percent, about as much mass as do bright stars. Neutrinos are part of the cosmic mix. This fact gives some credence to the idea of particle dark matter.
Neutralinos. The neutralino is well motivated by particle theory, it would lead to a pattern of structure formation that is consistent with observations, and it might eventually be produced at a particle accelerator with other supersymmetric particles. While estimates of its mass are uncertain, they are 50 to 500 times the mass of the proton. The neutralino is therefore a prime candidate for dark matter and for detection.
Axions. The axion was postulated to cure a serious but subtle problem with the otherwise very successful theory of quantum chromodynamics. Of course, this theoretical role is no guarantee that the axion actually exists. If it does, then axions would have been produced copiously in the early universe and would have survived in sufficient numbers to account for the dark matter today, in spite of the fact that the mass predicted for the axion, between about 0.00001 and 0.001 eV/c2, is so tiny.
These are the currently favored candidates. However, recent progress in string theory has led to a number of new ideas that could explain the dark-matter puzzle (and possibly even the dark-energy puzzle, too). Some have speculated that we and the weak, electromagnetic, and strong interactions actually live on a 4-dimensional “brane” in an 11-dimensional space, and that gravity can propagate through all 11 dimensions. If there are other branes in the 11-dimensional space, their gravitational interactions with our brane could in fact be what is called dark matter. It is also possible that the dark matter is made up of particles that are very much heavier than any of the three candidates discussed and that were produced at the end of inflation. Such ideas are at an early stage and have little predictive power. However, given their potential importance, they merit further attention.
How can progress be made in choosing between these possibilities and deciphering the nature of the dark matter? Two complementary approaches are important: characterize as much as possible the clumping properties of
dark matter (a clue to its nature) and attempt to directly detect the dark matter particles in our halo and/or produce them at an accelerator.
Hot, Cold, or Something Else?
The different particle dark-matter candidates are characterized by how fast the particles are moving: dark matter whose individual particles move fast (i.e., at speeds close to that of light) is called hot, and dark matter whose individual particles move slowly (i.e., at speeds much less than that of light) is called cold. Neutrinos fall into the first category, hot dark matter, while axions and neutralinos are cold dark matter. Neutralinos move slowly because they are heavy, while axions do so because they were produced in a cold state by a quantum process akin to Bose-Einstein condensation. (Other candidates are intermediate in speed and are referred to as warm dark matter.)
The difference between hot and cold is crucial for structure formation: hot dark matter particles move fast enough to wash out lumpiness on galactic scales (particles from high-density regions spread out quickly to fill in lower-density regions). In turn, this means that galaxies must form by fragmentation of larger structures (superclusters). With cold dark matter, structure forms from the bottom up—galaxies form first, cluster together to form clusters of galaxies, and so on. In the 1980s this led to two competing theories of structure formation—the hot dark matter and cold dark matter scenarios.
The observational evidence is now very clear. Structure formed from the bottom up—galaxies came into existence before clusters of galaxies and superclusters. This fact all but rules out neutrinos (or other future hot dark matter candidates) as the dominant part of the dark matter. Moreover, the formation of structure with cold dark matter has been simulated on supercomputers, and the predictions agree well with a wide array of observations, including the masses and abundances of galaxies, clustering of galaxies and clusters of galaxies, the distribution of gas clouds at high redshift, and fluctuations in the cosmic microwave background radiation. This concordance, together with the failure of hot dark matter to account for the evidence, provides convincing indications that the dark matter particles are cold.
That being said, there are hints of problems with the cold dark matter model. Computer simulations of the evolution of cold dark matter predict more substructure within galactic halos than is seen and a higher concentration of dark matter at the centers of galaxies and clusters than is measured.
It could be that there are physical mechanisms for smoothing the dark matter distribution that are not included in the simulations or some problem in interpreting the observations. However, no such solution has yet been found. These results have given rise to speculation that the observations are revealing some new property of the dark matter.
Identifying the Dark Matter Particle
Based on clues from astrophysics, cosmology, and particle physics, progress is now being made in the search for the dark matter particles in the laboratory. After a decade of effort, attempts to look for the three main candidates are now reaching the level of sensitivity needed to test their candidacies directly.
In spite of the fact that neutrinos alone cannot form the large-scale structure observed in the universe, measurements of their masses are needed to clarify the role they have in cosmology. The measuring techniques include the study of beta-decay spectra, the study of neutrinoless double-beta decay, and searches for oscillations between the different neutrino species. Neutrino oscillation studies using neutrinos produced by cosmic rays in Earth’s atmosphere and by the Sun have already produced strong evidence for nonzero neutrino mass and indicate a minimum cosmic density of neutrinos comparable to the mass density of stars. At this minimum mass density, neutrinos might have a small, but detectable, influence on the formation of large-scale structure, which could permit a cosmological determination of their masses.
Experimentalists are pursuing two general approaches: (1) direct detection of elastic scattering of the neutralinos that presumably exist in the halo of our galaxy in very sensitive laboratory detectors and (2) detection of the annihilation products of neutralinos that accumulate within the Sun, Earth, or the halo of our galaxy. The direct-detection field is very active, especially in Europe, with experiments based on germanium of unprecedented radioactive purity, the operation of very large sodium iodide scintillators, and the development of totally new cryogenic sensors. The DAMA collaboration at the Gran Sasso laboratory in Italy says it has detected a signature of neutralinos via a yearly modulation of the signal of ±2 percent in 100 kg of sodium
iodide that would be caused by Earth’s annual movement through the local cloud of neutralinos. However, the U.S.-based Cryogenic Dark Matter Search (CDMS) experiment and other experiments at European laboratories appear to contradict this result. Even if the DAMA result is not confirmed, this is an exciting time for dark matter searches, because the current generation of experiments is now achieving the sensitivity levels needed to detect neutralinos. A number of second-generation experiments (CDMS II in the United States, and CRESST II and Genius in Europe) will be even more sensitive and will explore a significant amount of the theoretically favored parameter space. Neutralinos in the galactic center and trapped in the Sun or Earth can annihilate and produce high-energy neutrinos. Neutralino annihilations in our galactic center would produce gamma rays, antiprotons, and positrons; GLAST will have significant sensitivity to the gamma-ray signature of halo neutralinos. The current generation of high-energy neutrino detectors is about as sensitive to neutralinos as are the direct-detection experiments. Finally, it is possible that the neutralino will be produced and detected in an accelerator experiment (i.e., at the Fermilab Tevatron or at CERN’s Large Hadron Collider).
The most promising method for detecting axions is through their interaction within the halo of our galaxy with a very strong magnetic field. In the presence of the magnetic field, axions would produce a faint microwave radiation detectable in a tunable microwave cavity. The U.S. axion experiment has reached the sensitivity needed to begin testing the axion dark matter hypothesis. The recently approved upgrade incorporates novel SQUID amplifiers that will enable it to search for axions over a mass range spanning one order of magnitude (out of the three that are still allowed by current theories). A Japanese collaboration is developing very sensitive photon detectors using atoms in highly excited states that will cover the same mass range.
Is Expansion of the Universe Speeding Up Rather Than Slowing Down?
Type Ia supernovae, the thermonuclear explosions of white dwarf stars slightly more massive than the Sun, have remarkably uniform peak luminosities (when corrected for the rate of decline of brightness). Since
they are very bright, they can be seen and studied throughout the observable universe and can be used as cosmic mileposts to study the expansion rate of the universe at earlier times. For 70 years, astronomers have been trying to measure the slowing effect of gravity on the expansion of the universe to determine the total amount of matter in the universe. In a surprising and exciting turn of events, two teams independently studying supernovae at high redshift (corresponding to great distance) (see Figure 5.7) recently found that the expansion of the universe is speeding up, not
slowing down! The acceleration requires something new, as matter in any amount should cause the universe to decelerate. The nature of this new component, called dark energy in the spirit of Zwicky’s naming of dark matter, is not understood.
Since the supernova observations yield such a surprising result, astronomers are carefully searching for sources of systematic error in their measurements or interpretations. There are a number of possible sources of error. For example, type Ia supernovae might not remain “standard” at earlier epochs. It is possible that large dust particles (so-called “gray dust”) pervade intergalactic space, absorbing some of the light from distant supernovae, mimicking the effect of an accelerating universe. Thus far, no systematic error has been found that would invalidate the result.
Independent Confirmation of an Accelerating Universe
While the supernovae results themselves may be subject to unknown errors, there is an independent confirmation of the extraordinary discovery that the expansion of the universe is speeding up, not slowing down. It comes from the CMB measurements, which give a top-down accounting of matter and energy in the universe. The measurements indicate that the universe is flat; this, in turn, implies that summed together, matter and energy must equal the critical density. Since independent accounts of the amount of matter indicate that it is about 35 percent of the critical density, the CMB measurements point to another component that does not clump with matter and contributes about 65 percent of the critical density. This missing mass/energy revealed by the CMB is consistent with the dark energy component indicated by supernovae measurements.
Why Is the Expansion of the Universe Speeding Up?
According to Einstein’s theory, in unusual circumstances gravity can be repulsive (see Box 5.1). What is needed is a form of energy that is elastic (negative pressure). There is a form of energy known to have this property— the energy of the quantum vacuum (originally referred to as the cosmological constant by Einstein). However, this fact does not wrap up the story of dark energy. All efforts to calculate the energy of the quantum vacuum using our current understanding of physics give a value that is at least 55 orders of magnitude (1055 times) too large. Perhaps advances in understanding of the quantum vacuum will ultimately solve the discrepancy, but many theorists believe that such an advance will lead to an explanation of why
the vacuum energy should be precisely zero rather than the tiny but finite value needed to explain the accelerating universe.
Other explanations for the dark energy have been proposed. There is the possibility that a particle field, poetically named “Quintessence,” is at work. If this were the case, we would be at the beginning of a (hopefully mild) inflationary episode. Topological defects arising from a phase transition in the early universe could get tangled and exert a negative pressure, exactly as a bunch of tangled rubber bands when one tries to separate them. Or we could be experiencing the effects of additional space-time dimensions.
The ultimate fate of the universe depends on the nature of the dark energy. If the dark energy is truly the energy of the quantum vacuum, then the fate of the universe is continued acceleration of its expansion. As a result, 150 billion years from now only a thousand or so galaxies will still be visible to even the most powerful telescope because the vast majority of galaxies in the universe will be moving away too fast to be seen. On the other hand, if the universe is only experiencing a mild and temporary spurt of inflation, then it will once again begin to slow down and the sky will remain filled with galaxies until all their stars burn out.
Finally, there is the “why now?” puzzle. We appear to live at a special time when the expansion of the universe is transitioning from deceleration to acceleration. Is this fact just a coincidence, or does it have a deeper explanation?
TWO MAJOR CHALLENGES: DECIPHERING DARK MATTER AND DARK ENERGY
Occasionally a science reaches a precipice—a juncture where all paths seem to lead to confusion. These crises often precede major conceptual breakthroughs. By any measure, cosmologists and physicists now find themselves in such a (wonderful) quandary. Their picture of the universe now requires a strange kind of dark energy with negative pressure, along with dark matter that is made of something other than the baryons that make up the stars and us.
Deciphering the nature of the dark matter and dark energy is one of the most important goals in the physics of the universe. Resolving both puzzles is key to advancing current understanding of both cosmology and particle physics. The solutions to these problems will cast light not only on the fate of the universe but also on the very nature of matter, space, and time. Through a combination of new approaches and increasingly pow-
erful instrumentation, scientists are poised to make great progress in the coming years.
There are several astronomical approaches to determining the properties of dark matter and dark energy. They rely on information of two different types. The expansion rate of the universe depends on its composition. By measuring the distances to objects as a function of redshift, astronomers can determine the expansion history and thereby infer the relative abundances of dark matter and dark energy, as well as probe the nature of the dark energy. The rate at which structure in the universe forms is sensitive to the properties of both the dark matter and the dark energy. On large scales, the growth rate of structure is sensitive primarily to the amount of dark energy and its properties. Once the matter collapses to form galaxies and clusters, the properties of the galaxies and clusters are sensitive to the detailed properties of the dark matter.
There are several powerful techniques for getting at the expansion rate of the universe. Observations of the angular size of hot and cold spots on the microwave sky can determine the distance to the surface where the radiation originated; that distance depends on the expansion history of the universe from 400,000 years ago until today. The MAP and Planck satellite missions will accurately make this measurement in the coming years.
Observations of distant supernovae can probe the detailed expansion history directly back to redshifts of around 2, corresponding to times from a few billion years after the big bang until the present. The current data have already provided the first strong evidence for the existence of dark energy. Large-field-of-view telescopes are needed to find larger and more uniform samples of supernovae. These telescopes will vastly increase the samples of supernovae (by a factor 10 or more). Large samples will enable not only greater statistical accuracy but also better control of systematic errors. For example, a large sample of supernovae can be divided into smaller subsamples to search for systematic trends and test the validity of the results.
The probability of the gravitational lensing of distant objects is sensitive to the distance to the lens and the background source; these distances depend on the expansion history of the universe. Observations of clusters containing multiple-lensed sources at several different redshifts can measure the relative distances to the lenses and sources. These observations will require detailed maps of the mass of the cluster based on gravitational lensing observations.
Like the distance to high-redshift objects in the universe, the volume associated with a given redshift is sensitive to the expansion history. Counting objects of known (or calculable) instrinsic number density can be used to infer the volume as a function of redshift, and from it the expansion history. Galaxies and clusters of fixed mass both show promise for this use.
There are several powerful techniques for probing the growth rate of structure in the universe. The emergence of the first stars and galaxies (a few hundred million years after the big bang) is sensitive to the nature of the dark matter. For example, hot dark matter (neutrinos) completely inhibits early galaxy formation, and the study of the formation of the large gas clouds seen by their absorption of light from distant quasars constrains the fraction of dark matter in the form of neutrinos to less than about 10 percent. Observations of “weak” gravitational lensing (i.e., the small distortions of the shapes of galaxies) can be used to measure the large-scale clustering properties of matter during the last 10 billion years. The amplitude of this clustering as a function of scale and redshift depends on the amount of dark energy and its properties. Observations of “strong” gravitational lensing measure the distribution of mass within galaxies and clusters. The density profile at the core of galaxies and clusters and the number of dwarf systems around large galaxies are sensitive tests of the properties of the dark matter. The absence of central condensation of dark matter could be indicative of new dark-matter physics—or even of an inconsistency in the paradigm of particle dark matter. Measurement of the peculiar velocities of galaxies and clusters as a function of redshift is a powerful probe of the growth of structure in the dark matter. In the next few years, supernovae can be used systematically to extend current peculiar-velocity surveys by a factor of 2. Progress can also be expected in the use of galaxy surface-brightness fluctuations to provide very accurate distance measurements (±3 percent). Using this method on huge optical telescopes with adaptive optics should allow the measurement of peculiar velocities on scales 10 times as large as is currently possible. Peculiar velocities of clusters can also be measured by using the distorting effect of the hot gas in the cluster on the microwave background (Sunyaev-Zel’dovich effect). Counts of galaxies and clusters are also sensitive to the growth rate of structure in the universe. Both the growth rate and the expansion rate affect the counts.
No one of these methods will suffice by itself. A combination of methods is needed to definitively determine the nature of dark matter and dark energy. These methods are often complementary—that is, they provide different information—but at other times, they provide important crosschecks on one another. No method is immune from the possibility of subtle
or unknown systematic error, so such cross-checks are critical. The importance of complementary measurements and crosschecks is illustrated by the current data in Figure 5.2.
For the study of dark energy and dark matter, a new type of telescope may be called for. Wide field-of-view (more than 1 degree) telescopes with gigapixel charge-coupled device cameras enable the search of large regions of the sky for supernovae (type Ia supernovae occur at a rate of about 1 per second over the universe but are spread over the 40,000 square degrees of the sky) and the mapping of the distribution of dark matter on large scales.
Finally, theory and large-scale computing will play a critical role. To gain the full benefit of measurements of the development of structure in the universe, large-scale numerical simulations of the predictions of the different cold dark matter models are crucial. The need for more dynamic range and better input physics challenges existing computing resources. Likewise, a better theoretical understanding of type Ia supernovae is key to exploiting them as cosmological mileposts. Achieving this understanding will require advances in large-scale scientific computing. Currently, computing resources are often the time-limiting factor in analyzing cosmic microwave background data, and the situation will become more critical with MAP, Planck, and other experiments that give high-resolution and large-sky coverage.
Laboratory-based experiments are a complementary approach to identifying the particle dark matter. All three dark-matter candidates—axions, neutralinos, and neutrinos—can be sought out in the laboratory. This search will involve accelerators, specialized dark-matter detectors, and large underground detectors.
Accelerators will help provide important constraints on the properties of some particle dark-matter candidates or perhaps even discover the sought-after particle (e.g., the neutralino). For example, the combination of long-baseline oscillation experiments using neutrinos from existing accelerators and future muon colliders with atmospheric and solar neutrino studies will enable unraveling the mass and mixing structure of the neutrino sector. This will make it possible to describe more precisely the role of neutrinos in structure formation to be made. The search for neutrinoless double-beta decay provides additional constraints on the properties of neutrinos. Similarly, the searches for supersymmetry at the LHC and for neutralinos in the halo of our galaxy will be complementary and will reach very similar sensitivities.
In the direct search for neutralinos, it is important to begin the preparation for third-generation detection experiments, with two possible scenarios in mind. If the second-generation experiments discover a signal, the emphasis will probably shift to obtaining directional information to link the signal to the galaxy and provide information on the halo and the distribution of dark matter within it. If the second-generation experiments fail to find a signal, the emphasis will be on techniques to reduce background signals. This second route would become compelling if supersymmetry were discovered at the Tevatron or LHC. The indirect searches for neutralinos will benefit from new instruments such as the Gamma Ray Large Area Space Telescope (GLAST), which will have the energy resolution to look for gamma rays from neutralino annihilations at the galactic center. The large-area neutrino detectors being planned should complement direct-detection experiments for high-neutralino masses. Although the primary motivations for these instruments are different (see Chapter 7), they should be designed to permit such exploration.
The challenge of searches for axions will be to extend the explored mass range to the higher end of the mass range discussed in Chapter 6, subsection “Cosmic Accelerators and High-Energy Physics.” Unless broad-band methods can be devised, the current approach will have to be extended to higher frequencies using, for example, tunable “photon bandgap” cavities.
Last but not least, the tests of gravity either at small scale with Cavendish-type experiments or at large scale with equivalence-principle tests (see Chapter 3) may shed some light on the dark matter and dark energy problems. These puzzles may be a sign that scientists do not understand gravity, and some models of dark energy predict the existence of new, weaker-than-gravity forces that could be discovered with more sensitive tests of the equivalence principle.
As the new century begins, scientists have a first, tentative accounting of the universe: one-third matter and two-thirds dark energy, adding up to the critical density and a flat universe. This accounting raises a new set of deeper questions whose answers will have profound implications for both cosmology and particle physics and whose answering will involve both astronomers and physicists. Scientists are poised to make progress in addressing two key questions about the makeup of our universe and the very nature of space, time, and matter.
What Is Dark Matter?
Dark matter—stuff that neither emits nor absorbs light—holds the universe together. Its nature is a mystery. There is strong evidence that the bulk of it is not the ordinary matter of which we and the stars are made. The working hypothesis is that it is composed of elementary particles left over from the earliest moments of creation. The leading candidates for dark matter are new particles whose existence is predicted by theories that attempt to unify the forces and particles of nature. Showing that one (or several) of these particles compose the dark matter not only would answer a key question in cosmology but also would shed new light on the fundamental forces and particles of nature.
Clues to the nature of the dark matter will come from astronomical observations of its distribution in the universe as well as from study of the evolution of structure in the universe. There are also important opportunities to detect the dark matter particles holding our galaxy together, either directly or indirectly, by detecting the particles into which they annihilate. Accelerators may also be able to produce the dark matter particle.
What Is the Nature of Dark Energy?
Two independent lines of evidence indicate the presence of a new form of energy pervading the universe that accounts for two-thirds of the critical density and is causing the expansion of the universe to speed up rather than slow down. This is an extraordinary result, so extraordinary that the first order of business is establishing further evidence for accelerated expansion and the existence of dark energy. Assuming that it exists, this mysterious dark energy exhibits a hitherto unseen feature of gravity (it can sometimes be repulsive), and understanding its nature could lead to progress in our understanding of space, time, and matter. Explanations put forth for the dark energy range from the energy of the quantum vacuum to the influence of unseen space-time dimensions. Science magazine was not exaggerating when in 1998 it chose as the scientific breakthrough of the year the discovery that the expansion of the universe was accelerating.
Dark energy by its very nature is extremely diffuse, and its effect can be felt only on the largest scales, where it influences the expansion of the universe and the growth of structure within it. Getting at the nature of dark energy will necessarily involve telescopes rather than accelerators, and key opportunities exist to use supernovae, galaxies, and clusters of galaxies to probe both the expansion and the formation of structure.
In studying dark energy and dark matter a new kind of special-purpose telescope may prove useful. The mapping of dark matter with gravitational lensing and the search for distant supernovae both require the search of large swaths of sky. These searches could be achieved with a wide-field-of-view (1 degree or more) telescope and a commensurately large CCD camera (probably gigapixel or larger). The Sloan Digital Sky Survey (a project to digitize the sky and map large-scale structure, depicted in Figure 4.3) and the MACHO project (a search for dark stars in the halo of our galaxy through microlensing) have shown the value of such special-purpose telescopes.