ScienceWeek

COSMOLOGY: DARK MATTER AND THE EARLY UNIVERSE

The following points are made by J. Diemand et al (Nature 2005 433:389):

1) The Universe was nearly smooth and homogeneous before a redshift of z = 100, about 20 million years after the Big Bang[1]. After this epoch, the tiny fluctuations imprinted upon the matter distribution during the initial expansion began to collapse because of gravity. The properties of these fluctuations depend on the unknown nature of dark matter[2-4], the determination of which is one of the biggest challenges in present-day science[5].

2) The cosmological parameters of our Universe and initial conditions for structure formation have recently been measured via a combination of observations, including the cosmic microwave background (CMB), distant supernovae, and the large-scale distribution of galaxies. Cosmologists now face the outstanding problem of understanding the origin of structure in the Universe from its strange mix of particles and vacuum energy.

3) Most of the mass of the Universe must be made up of a kind of non-baryonic particle[1] that remains undetected in laboratory experiments. The leading candidate for this "dark matter" is the neutralino, the lightest supersymmetric particle, which is predicted to solve several key problems in the standard model for particle physics[5]. This cold dark matter (CDM) candidate is not completely collisionless. It can collide with baryons, thus revealing its presence in laboratory detectors, although the cross-section for this interaction is extremely small. In a cubic-meter detector containing 10^(30) baryon particles, only a few collisions per day are expected from the 10^(13) dark-matter particles that flow through the experiment as the Earth moves through the Galaxy.

4) The neutralino is its own anti-particle, and can self-annihilate, creating a shower of new particles including gamma-rays[5]. The annihilation rate increases as the density squared; the central regions of the Galaxy and its satellites will therefore give the strongest signal. However, the expected rate is very low -- the flux of photons on Earth is the same as we would receive from a single candle placed on Pluto. Numerous experiments using these effects are under way that may detect the neutralino within the next decade. Furthermore, in the next few years the Large Hadron Collider (LHC) at CERN will confirm or rule out the concepts of supersymmetry (SUSY).

5) The authors report supercomputer simulations of the concordance cosmological model, which assumes neutralino dark matter (at present the preferred candidate), and find that the first objects to form are numerous Earth-mass dark-matter haloes about as large as the Solar System. They are stable against gravitational disruption, even within the central regions of the Milky Way. The authors expect over 10^(15) to survive within the Galactic halo, with one passing through the Solar System every few thousand years. The nearest structures should be among the brightest sources of gamma-rays (from particle particle annihilation).

References (abridged):

1. Peebles, P. J. E. Large-scale background temperature and mass fluctuations due to scale-invariant primeval perturbations. Astrophys. J. 263, L1 L5 (1982)

2. Hofmann, S., Schwarz, D. J. & Stöcker, H. Damping scales of neutralino cold dark matter. Phys. Rev. D 64, 083507 (2001)

3. Berezinsky, V., Dokuchaev, V. & Eroshenko, Y. Small-scale clumps in the galactic halo and dark matter annihilation. Phys. Rev. D 68, 103003 (2003)

4. Green, A. M., Hofmann, S. & Schwarz, D. J. The power spectrum of SUSY-CDM on sub-galactic scales. Mon. Not. R. Astron. Soc. 353, L23 L27 (2004)

5. Jungman, G., Kamionkowski, M. & Griest, K. Supersymmetric dark matter. Phys. Rep. 267, 195 373 (1996)

Nature http://www.nature.com/nature

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ASTROPHYSICS: DARK MATTER AND DARK ENERGY

The following points are made by Sean Carroll (Nature 2004 429:27):

1) Humans seem to be extremely unimportant in the grand scheme of the Universe. This insight is often associated with Copernicus (1473-1543), who suggested (although not for the first time) that the Earth was not the center of the Solar System. A bigger step towards calibrating our insignificance was taken by Edwin Hubble (1889-1953), who determined that astrophysical nebulae are really separate galaxies in their own right. We now think there are about one hundred billion such galaxies in the observable Universe, with perhaps one hundred billion stars per galaxy.

2) But a metaphysically distinct blow to our importance came with the introduction of the idea of dark matter -- we are not even made of the same stuff that comprises most of the Universe. The need for dark matter, in the sense of "matter we cannot see", was noticed in 1933 by Fritz Zwicky (1898-1974), when studying the dynamics of the Coma cluster of galaxies. When galaxies are orbiting each other, their typical velocities will depend on the total mass involved, but when we observe clusters of galaxies, the velocities are consistently much higher than we would expect from the mass we actually see in stars and gas. Vera Rubin and others have driven the point home by examining individual galaxies. As we move away from the central galactic region, the velocity of orbiting gas becomes systematically higher than it should be. These observations imply the existence of an extended, massive halo of dark matter. Indeed, the picturesque galaxies we see in astronomical images are really just splashes of visible matter collected at the bottom of these more substantial, yet invisible, halos.

3) Of course, the air we breathe is invisible and transparent, just like dark matter. A sensible first guess might be that the extra mass we infer is ordinary matter, just in some form we cannot see. But we have independent ways to measure the amount of ordinary matter, through its influence on the early-Universe processes of primordial nucleosynthesis and the evolution of density perturbations. These constraints imply that ordinary matter falls far short of what is needed to explain galaxies and clusters (perhaps one-fifth of the total). Not only is dark matter "dark", it is a completely new kind of particle --something outside the standard model of particle physics, something not yet detected in any laboratory here on Earth.

4) And we have not even mentioned dark energy -- the mysterious form of energy that is smoothly distributed throughout space and (at least approximately) constant through time. Independent observations of high-redshift supernovae, the microwave background radiation, and the distribution of large-scale structure all require the existence of dark energy. The featureless, persistent nature of dark energy convinces us that it is not even a particle at all. About 70% of our current Universe is dark energy and 25% is dark matter. This leaves all the stuff we have directly observed at a paltry 5% of the whole Universe.

References:

1. Krauss, L. Quintessence: The Mystery of the Missing Mass (Basic Books, New York, 2001)

2. Peebles, P. J. E. From Precision Cosmology to Accurate Cosmology online at http://arxiv.org/abs/astro-ph/0208037

3. Rees, M. Our Cosmic Habitat (Princeton Univ. Press, Princeton, 2003)

Nature http://www.nature.com/nature

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ASTROPHYSICS: ON THE NATURE OF DARK MATTER

The following points are made by K. Zioutas et al (Science 2004 306:1485):

1) Astrophysical observations reveal that galaxies and clusters of galaxies are gravitationally held together by vast halos of dark (i.e., nonluminous) matter. Theoretical reasoning points to two leading candidates for the particles that may make up this mysterious form of matter: weakly interacting massive particles (WIMPs) and theoretical particles called "axions". Particle accelerators have not yet detected either of the two particles, but recent astrophysical observations provide hints that both particles may exist in the Universe, although definitive data are still lacking. Dark matter need not consist exclusively of only one of these two types of particles.

2) Precise measurements of the cosmic microwave background have shown that dark matter makes up about 25% of the energy budget of the Universe; visible matter in the form of stars, gas, and dust only contributes about 4%. However, the nature of dark matter remains a mystery. To explain it, we must go beyond the standard model of elementary particles and look toward more exotic types of particles.

3) One such particle is the neutralino, a WIMP that probably weighs as much as 1000 hydrogen atoms (henceforth, we refer to the neutralino as a generic WIMP). Neutralinos are postulated by supersymmetric models, which extend the standard model to higher energies. To date, no neutralinos have been created in particle accelerators, but in the future they may be produced in the world's most powerful particle accelerator, the Large Hadron Collider currently being built at CERN. A recent precise measurement of the magnetic dipole moment of the muon favors the existence of new particles such as neutralinos.

4) Another possibility for the direct detection of neutralinos is to seek evidence for the tiny nuclear recoils produced by interactions between neutralinos (created when the Universe was very young and very hot) and atomic nuclei. Because such interactions are rare and the effects small, they can only be detected in experiments that are conducted underground, where the high-energy cosmic radiation is suppressed by several orders of magnitude.

5) Astrophysical observations could provide indirect evidence for neutralinos. On astrophysical scales, collisions of neutralinos with ordinary matter are believed to slow them down. The scattered neutralinos, whose velocity is degraded after each collision, may then be gravitationally trapped by objects such as the Sun, Earth, and the black hole at the center of the Milky Way galaxy, where they can accumulate over cosmic time scales. Such dense agglomerates could therefore yield an enhanced signal for the postulated neutralinos of cosmic origin.(1-5)

References (abridged):

1. P. Jean et al., Astron. Astrophys. 407, L55 (2003)

2. F. Aharonian et al., Astron. Astrophys. 425, L13 (2004)

3. R. Irion, Science 305, 763 (2004)

4. R. D. Peccei, H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977)

5. R. D. Peccei, H. R. Quinn, Phys. Rev. D 16, 1791 (1977)

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