Several independent astronomical observations in different wavelength bands reveal the existence of much larger quantities of matter than what we would deduce from assuming a solar mass to light ratio. They are very high velocities of individual galaxies within clusters of galaxies, higher than expected rotation rates of stars in the outer regions of galaxies, 21 cm line studies indicative of increasing mass to light ratios with radius in the halos of spiral galaxies, hot gaseous X-ray emitting halos around many elliptical galaxies, and clusters of galaxies requiring a much larger component of unseen mass for the hot gas to be bound. The level of gravitational attraction needed for the spatial distribution of galaxies to evolve from the small perturbations implied by the very slightly anisotropic cosmic microwave background radiation to its current web-like configuration requires much more mass than is observed across the entire electromagnetic spectrum. Distorted shapes of galaxies and other features created by gravitational lensing in the images of many astronomical objects require an amount of dark matter consistent with other estimates. The unambiguous detection of dark matter and more recently evidence for dark energy has positioned astronomy at the frontier of fundamental physics as it was in the 17th century.
Astronomy and physics have had a mutually beneficial partnership. The late 16th century astronomical observations of planetary positions as a function of time by the Danish astronomer Tycho Brahe were analyzed and interpreted by Johannes Kepler. Three laws relating planetary motion and distance from the Sun were abstracted from Kepler’s analysis. Galileo’s telescopic observations of the Sun and planets in the early 17th century established the connection between Kepler’s analysis and the validity of the Copernican view of the solar system. With that knowledge in late 17th century Isaac Newton founded modern theoretical physics by explaining Kepler’s findings in the context of Galileo’s physical picture with his three laws of classical mechanics and the law of universal gravitational attraction.
Aside from the synergy between solar/stellar and laboratory atomic spectroscopy, from that time until the turn of the 20th century, astronomy and fundamental physics did not forge any more connections with such significance. Classifying and cataloging stars, noting their positions, color, and spectra, observing the occurrence of eclipses and the comings and goings of comets, all in accordance with Newton’s laws, were the preoccupations of astronomers. In fact, Simon Newcomb the most distinguished American astronomer of his era, founding member and first president of the American Astronomical Society said in 1888,
Beginning with the 20th century, profound changes in both astronomy and laboratory physics, plus the development of Einstein’s special and general theories of relativity, would bring the two disciplines together again at the frontiers of fundamental physics. Astronomy underwent a great expansion in scope thanks to the construction of new tools such as very large optical telescopes, solid state image sensors, large radio telescopes, and with access to space above the absorption of the atmosphere, far infrared, ultraviolet, X-ray and gamma-ray telescopes. This capability led to the discovery of neutron stars and black holes, objects whose existence could be explained by theoretical physicists following their discovery. Astronomers have also discovered evidence for dark matter, which seems to be primarily nonbaryonic and an even more enigmatic entity known as dark energy. At the current time the total mass-energy content of the known universes described by the so-called Lamda-Cold Dark Matter model is believed to be 5% ordinary matter, 23% dark nonbaryonic matter, and 72% “dark energy.”
Dark matter has been discussed by many authors. Trimble reviews our understanding of dark matter from an elegant historical perspective describing events up to 1987. She includes early indications of the existence of dark matter whose significance was not widely recognized at the time [
A variety of independent lines of evidence indicate that galaxies and clusters of galaxies contain much more gravitating matter than the total amount that has been detected across the entire electromagnetic spectrum. The excess is called dark matter (DM). The evidence includes galactic rotation curves, large oxygen-rich halos of star-forming galaxies, the velocity dispersions of stars in elliptical galaxies and of galaxies in clusters of galaxies, gravitational lensing, the confinement of hot gas in galaxy clusters, the pattern of acoustic oscillations in the power spectrum of the cosmic microwave background (CMB), and the imprint of these oscillations on the relative strength, and shape of the galaxy-distribution power spectrum at large wave numbers. The collective evidence suggests that dark matter comprises some twenty-three percent of the energy density of the universe today, (
Some baryons are tied up in dark forms, such as extremely low mass stars or black holes. The Hubble Space Telescope detected large (150 kiloparsec) halos of ionized oxygen surrounding star-forming galaxies. They contain a substantial amount of heavy elements and gas, perhaps exceeding the reservoirs of gas within the galaxies [
On a large scale, the spatial distribution of relatively nearby galaxies (Figure
The web-like spatial distribution of galaxies reported by Geller and Huchra [
The spatial distribution of hot baryons in the model of Cen and Ostriker [
The simulations show that the temperature of the baryonic portion of the clouds increases with time. This implies that clouds closer to the observer and further from a distant background quasar are warmer. The higher temperature hydrogen in nearby clouds is ionized and not capable of causing Lyman-alpha absorption. The clouds also contain heavier elements, for example, carbon and oxygen with abundance ~10−3 hydrogen. Warm oxygen in a cloud along the line of sight absorbs ~0.6 keV (2 nm wavelength) X-rays in the spectrum of a quasar or other type active galaxy. Because we do not know the distance and temperature of the X-ray absorbing cloud and the degree of ionization of the oxygen atoms, the exact wavelengths of the absorption lines are not known a priori.
The most promising method for detecting the hot part of the WHIM is through absorption of far-ultraviolet (FUV) and X-radiation from a background source. Over the last few years, detections have been reported by a number of authors, but these have been biased toward the strongest systems, or trace extreme galaxy overdensity regions, and therefore may not be representative. Chandra grating spectromers, combined with earlier XMM-Newton and Chandra observations, gave a 4σ detection of the OVII K
Papers by Zwicky in the 1930s [
The Coma Cluster of galaxies. An optical image (a) shows two giant galaxies near the center plus a number of smaller galaxies. The raw X-ray image (b) was taken by the Chandra X-Ray Observatory. The size of the optical and X-ray fields shown is about 600 kpc (2 million light years). (c) is an overlay of the optical image with a high contrast X-ray image that displays the regions with high X-ray surface brightness around two giant elliptical galaxies that dominate the cluster.
Applying the virial theorem to the observed kinetic energy indicates that the potential energy and consequently the mass of the Coma Cluster is much larger than the mass obtained by assuming a solar-like mass to light ratio. The popular term for this disparity became known as the “missing mass.” Although there were other indications of a discrepancy between the solar mass to light ratio and the mass of some other objects, it took thirty years for other types of measurements to confirm the existence of dark matter as convincingly as had Zwicky’s measurements of the Coma Cluster.
Zwicky and his contemporaries did not know that the Coma Cluster contains a halo of hot, ~108 K gas whose mass is ~5 times larger than the mass of the stars in the galaxies. That became known some forty years later after X-ray telescopes were launched into space. Although the baryonic mass becomes much larger after taking the hot gas into account, it is still a factor ~6 too small to explain the high velocities of the galaxies. Also the existence and presumably the stability of the hot gas halo is also an indication that a larger quantity of mass is present. Determining the mass of an object by measuring the parameters of a hot gas halo is discussed in more detail in Section
A 1959 theoretical paper by Kahn and Woltjer showed that the Local Group of galaxies can be dynamically stable only if contains an appreciable amount of intergalactic matter that is not seen [
As was first discovered a hundred years ago, the stars and clouds of a spiral galaxy are generally rotating about the galaxy’s center. A measurement of the orbital velocities of stars as a function of distance from the center of rotation in spiral galaxies yields the mass interior to the stars’ positions. Rubin and her colleagues studied the rotation of several spiral galaxies including the Milky Way and the Andromeda galaxy, M31 [
(a) shows the rotational velocity of stars as a function of distance from the center for three galaxies, the Milky Way, M31, and NGC 4258. (b) is a pictorial representation [
These optical and radio observers did not claim that the dark matter was nonbaryonic. However there were others who claimed that indeed the halo mass contained a nonbaryonic component [
The same general analysis applied to elliptical galaxies, with the dispersion of the stellar velocities replacing the circular velocities, also provides evidence for the existence of dark matter. Dwarf spheroidals (dSphs) are small galaxies, with Mtot ~107 solar masses, and relatively large dispersion velocities. In some extreme cases the deduced amount of dark matter is an order of magnitude or higher than that deduced for spiral galaxies, implying that dark matter constitutes more than 95% of the matter in these galaxies [
A modification of Newtonian dynamics (MOND), according to which the acceleration term in Newton’s second law become nonlinear for very small accelerations, corresponding to large distances, has been offered as an alternative explanation for the flattening of the rotation curves of galaxies [
M87 is a giant elliptical galaxy with an extended corona of X-ray emitting hot gas and two opposing jets projecting out from a giant black hole at the center. The mass of M87’s giant central black hole is 3 × 109 solar, where the solar mass is 2 × 1033 grams [
A Chandra X-Ray Observatory image of the giant elliptical galaxy M87 in the constellation Virgo. (provided by W. Forman of CfA). There is a large X-ray halo as well as features created by episodic outbursts emanating from a giant black hole at the center [
From the equation of hydrostatic equilibrium shown below, the total mass,
Within a radius of 87 kpc, or 275 million light years, (20 arc minutes) the optical luminosity is 6.6 × 1010 solar (solar luminosity = 2 × 1033 ergs/sec), the mass of the gas
These results are confirmed, within the uncertainties, by observing the velocities of 161 globular clusters orbiting M87 [
The X-ray measuring technique has been widely applied, especially to data from the Chandra X-Ray Observatory, to measure the baryonic and dark matter content of many hydrodynamically relaxed galaxy clusters, with the general result that the mass of dark matter in galaxy clusters is ~5 to 10 times that of the baryonic matter [
Gravitational lensing is a technique that uses the distorted images of distant galaxies as a tracer of dark matter in a foreground object. The patterns of the distortions reflect the density of matter along the line of sight. The process is essentially the same as that which Arthur Eddington used for measuring the change in positions of stars when their positions are close to the solar disk, as observed during a solar eclipse. After applying some corrections, Eddington reported that the changes in stellar positions were in agreement with the predictions of General Relativity. On a cosmic scale the same process can be used to map the distribution of matter in a foreground object by observing the distorted and often multiple images of a distant point source. The geometry is far more complex because unlike the Sun the foreground lens is not a simple sphere but a cluster of galaxies. Figure
Gravitational arc from a distant galaxy behind a foreground galaxy cluster [
One of the most significant examples of utilizing gravitational lensing to trace the location of mass is shown in Figure
A composite X-ray/optical image of the galaxy cluster 1E 0657-56, also known as the “Bullet Cluster.” This cluster was formed following the collision of two large clusters of galaxies. The X-ray image, pink, is from the Chandra X-Ray Observatory. The optical image is from the Magellan telescope and the Hubble Space Telescope; the galaxies are orange and white. The distribution of mass as determined by gravitational lens analysis is blue.
Since the discovery of the Bullet Cluster, a half dozen or so other examples have been found, establishing cluster mergers as important cosmic laboratories for the study of the constraints on the cross-sectionfor dark matter self-interactions. At present the best constraints are still from the Bullet Cluster.
The 2.73 K thermal CMB is the strongest evidence that our universe was indeed created in a Big Bang. The general picture is that quantum fluctuations caused the newborn post-Big Bang ionized matter to be slightly nonuniform. These fluctuations were frozen-in by a sudden inflation or expansion in volume, which has been described as a change in state [
The current structure of the microwave background radiation represents the structure of the universe as it was 4 × 105 years after the Big Bang. The amount of mass needed to provide the level of gravitational attraction required for the structure of the universe as it was 4 × 105 years ago to evolve into the web-like structure of the galaxies, clusters of galaxies, and the voids we observe today is much larger than what we can detect over the entire electromagnetic spectrum, and it is consistent with other measurements of the amount of dark matter. Although this conclusion is based entirely upon theoretical considerations it is one of the strongest pieces of evidence for the existence of dark matter. However, it provides no direct indication of whether the matter is baryonic or nonbaryonic. Light element abundances and models of Big Bang nuclear synthesis point to the latter (Section
Three spacecraft with increasing resolution and sensitivity were launched to map the cosmic microwave background. The first was NASA’s Cosmic Background Explorer (COBE) launched in 1989 It was followed by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) in 2001 and ESA’s Planck in 2009. A collection of papers on the results of Planck are available online [
Planck all-sky map of the intensity of the cosmic microwave background radiation is shown in (a). (b) is an analysis of its angular structure and multipole components.
Analysis of the Planck data is ongoing following a new release in May, 2013. Methods of accounting for terrestrial, solar system, or galactic foreground radiation are being refined, but these corrections are unlikely to change the basic conclusions on the amount of dark matter in the Universe [
Simulation by computation of complex hydrodynamic and magnetodynamic processes that cannot be studied in the laboratory or observed in the cosmos with any type of telescope has become an important tool in astrophysics, indeed virtually a separate branch of astrophysics on a par with theory and observation. It is being applied to studying the evolution of structure in the universe under the influence of gravity. For example, starting with the structure of the microwave background, which represents the universe as it was 4 × 105 years after the Big Bang determining what is required for it to evolve to the web-like structure it has today with the correct number of spiral and elliptical galaxies and the observed quantities of dark matter and baryonic matter. This effort has been occurring over a period of two decades at various institutions. The most recent and the most detailed simulation so far is the work of the
Some models of the decay of DM predict that high energy positrons will be produced [
Another investigator team presents evidence of a 110–140 GeV gamma-ray emission feature from the inner galaxy where dark matter is expected to be enhanced, in the Fermi data [
We consider the possible identities of dark matter only in general terms. If an element of dark matter is a particle we do not provide a specific description of its properties or attempt to find a place for it within the Standard Model of subatomic particles. It would not be one of those already detected in the laboratory. Other articles in this issue are likely to address this, as has a recent paper by Bergstrom [
As discussed in Section
It has been long thought that dark matter could be explained by an as yet undiscovered massive, weakly interacting elementary particle (WIMP),that is, a thermal relic of the Big Bang. Initially the early Universe was dense and hot, and all particles were in thermal equilibrium. The Universe then cooled to temperatures below the pair creation of dark matter particle’s with mass
Observations of large-scale cosmological structures imply that dark matter must be stable, or at least metastable, on Gyr time scales. This rules out all unstable Standard Model particles. Furthermore, the observation that galaxies formed at redshifts
Another line of investigation into the nature of dark matter is to look for its effects in the X-ray spectra of cosmic sources. One class of dark matter candidates is called axion-like particles (ALPs). The presence of a magnetic field is predicted to induce conversions of photons into ALPs. The absence of anomalous irregularities in the Chandra X-ray spectrum of the Hydra A cluster produces the most stringent constraint to date in the range of very low mass ALPs, with mass <10−11 eV [
One possibility is that there is no such thing as dark matter. The effects described in Section
The limits on the baryonic mass-energy density rule out black holes created through the process of star formation and evolution. However, it is possible that primordial black holes were created in the very early universe and thereby evades Big Bang nucleosynthesis and CMB constraints. Primordial black holes with masses much less than ~1015 g (= 2
As in the 16th and 17th centuries, the interests of astronomers, now more appropriately called astrophysicists, and laboratory physicists are converging upon the same issue at the frontier of fundamental physics, identifying the source and nature of the elusive dark matter. Astronomers are developing a new generation of tools including ground optical based telescopes with 28 to 40 m segmented apertures provided with adaptive optics to correct for atmospheric jitter. Large area optics will be stationed in both the northern and southern hemispheres. The Large Synoptic Survey Telescope (LSST) is likely to find numerous examples of gravitational lenses that will shed light upon dark matter. There will be a second generation space telescope, the James Webb Space telescope (JWST), with extended infrared sensitivity to view objects at very large redshift, which are closer in time to the first generation of star formation and perhaps unveiling the condition of dark matter at that epoch. Radio telescopes can reach even further back in time to study structure at earlier times by mapping the redshifted ubiquitous 21 cm hydrogen line. Radio astronomers are constructing the low frequency array (LOFAR), an international partnership led by The Netherlands. LOFAR will be able to map redshifted 21 cm lines that reveal structure at earlier epochs. Radio astronomers are also planning the Square Kilometer Array. The European Space Agency has approved “Athena,” an X-ray telescope system that will have a much larger aperture and much better spectroscopic resolution than the currently orbiting Chandra X-Ray Observatory and XMM-Newton. It will be capable of detecting and measuring the mass of the WHIM by observing oxygen and other elemental absorption lines by foreground clouds in the spectra of quasars with orders of magnitude more sensitivity and better resolution than Chandra and XMM-Newton.
These astronomical facilities will no doubt improve our knowledge about the behavior of dark matter, but a definitive resolution of what the dark matter particles actually are and where they fit in the Standard Model of fundamental particles, if they do indeed fit within, or upset the Standard Model, will ultimately require a positive detection of individual particles either in a debris of particles created by the Large Hadron Collider or by a ground, subterranean or deep oceanic based large area detector array.
The authors declare that there is no conflict of interests regarding the publication of this paper.