Cosmology

V

Cosmological Evidence

Cosmologists use telescopes, astronomical satellites, and other instruments to study the universe. The data that these instruments provide allow scientists to evaluate current theories and to come up with ideas to better explain the universe. Modern cosmologists are continuously calculating the age, density, and rate of expansion of the universe.

The universe’s density, expansion rate, and age are all related. The density of the universe’s matter determines how much the gravitational force will slow the expansion rate. The rate of expansion depends on the age and density of the universe. If cosmologists measure the rate of expansion by examining galactic redshifts and estimate the density of the universe, they can calculate an estimate of the universe’s age. Cosmologists calculate the expansion rate of the universe by finding the relationship between the distance of an object from Earth and the rate at which it is moving away from Earth. This relationship is represented by Hubble’s constant (H) in the formula v = H × d, where v is velocity (or the speed of the object) and d is the distance between the object and Earth. If Hubble's constant is relatively large, the universe is expanding relatively rapidly. A measure of the distance scale in a universe that is rapidly expanding would be larger than a measure of the distance scale in a universe of the same age with a smaller value of Hubble's constant.

For a universe with very low density, the age of the universe would be directly related to its expansion rate. This universe would expand forever; this eternal expansion defines an open universe. If, on the other hand, the density of a universe is sufficiently high, the expansion rate is changing—slowing down as the universe ages. This universe would eventually stop expanding and begin contracting, which defines it as a closed universe. Astronomers and cosmologists have been able to estimate the density of the universe, but until the Wilkinson Microwave Anisotropy Probe (WMAP) results were released the density estimates covered a wide range of values. Some estimates of density fell in the range for an open universe, others in the range for a closed universe, and still others near the boundary between the two. Age calculations for the higher densities are about two-thirds of those for the lower densities.

Estimates of the age, density, and expansion rate of the universe include many possible sources of uncertainty. For example, many galaxies orbit each other as members of clusters of galaxies. The velocity of any one galaxy in the cluster as seen from Earth varies over time as it circles the cluster, moving toward Earth through part of its orbit and away through the remainder. Cosmologists, therefore, must find the average expansion velocity of the entire cluster. Recent studies drawing on data collected by the Hubble Key Project, the Hipparcos satellite, and WMAP have helped reduce the uncertainty of estimates for age, density, and expansion rate.

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A

Hubble Key Project

Several groups of astronomers conducted observational projects to determine Hubble's constant, the most important cosmological parameter, during the late 1990s. Notably, the Hubble Key Project, carried out by American astronomers Wendy Freedman, Robert Kennicutt, and Barry Madore, used the Hubble Space Telescope to observe Cepheid variable stars in distant galaxies, following the Leavitt-Shapley method. The Hubble Space Telescope can distinguish and follow such stars in galaxies much farther away from Earth than ground-based telescopes can. Their final value was a Hubble constant of 72 kilometers per second per megaparsec (45 miles per second per megaparsec). A parsec is about 3.26 light-years (a light-year is the distance that light could travel in a year—9.5 × 10¹² km, or 5.9 × 10¹² mi). The units of the Hubble constant mean that for each million parsecs (megaparsec) of distance between two objects, the space between them expands by 72 kilometers every second. Their result was accurate to within about 10 percent. It corresponds to an age of the universe of 12 billion to 14 billion years, depending on the rate of deceleration.

B

Hipparcos

The European Space Agency’s (ESA) Hipparcos satellite made accurate measurements of the distance between Earth and 100,000 different stars, and moderately accurate measurements of the distance between Earth and 1 million other stars, from 1989 to 1993. The ESA released the data to the scientific community in 1997, and the measurements soon began affecting cosmological theories. For example, the measurements changed the accepted distances to some globular clusters (clusters of stars outside the main disk of the Milky Way Galaxy) and led to revisions of calculations of the ages of these clusters. Before the Hipparcos data, some of these clusters appeared to be older than the universe (as predicted by Hubble’s constant), but the revised distance measurements give the clusters an age within cosmologists’ estimates of the age of the universe.

C

Wilkinson Microwave Anisotropy Probe

In 2003 astronomers released results from the Wilkinson Microwave Anisotropy Probe (WMAP) that thoroughly confirmed existing ideas of cosmology and also produced several revelations about the nature of the universe. The probe studied the distribution of the ripples in the cosmic background radiation. A major conclusion from WMAP data linked with other observations is that the universe follows Euclidean geometry—that is, given any line in the universe, one and only one parallel line may be drawn through any point not on the original line. Such a universe is known as 'flat,' although it extends infinitely in all directions. If the universe is flat, it must be at the critical density that marks the boundary between an open and closed universe.

WMAP results also confirmed that the density of baryons—the elementary particles that make up regular matter—account for only 4 percent of the critical density. The probe further showed that another 23 percent of the universe consists of dark matter, a mysterious substance that does not shine in any part of the spectrum. The gravity of dark matter, however, is detectable. It binds clusters of galaxies together and causes the outer portions of galaxies to rotate faster than they would otherwise. Astronomers do not know the composition of dark matter, but they can theorize what it might be like. A slowly moving, cold dark matter, for example, could consist of not-yet-discovered particles that have names such as axions and weakly interacting massive particles (WIMPs). A rapidly moving, hot dark matter could be made up of particles called neutrinos, but measurements of neutrino mass indicate that they are too lightweight to account for much of the dark matter.

Since normal matter and dark matter account for only 27 percent of the material necessary for the universe to be at the critical density, the remaining 73 percent of the universe must be composed of a still more mysterious substance that astronomers have named 'dark energy.' The composition of dark energy is not known, but its effect on the universe is detectable. Dark energy exerts a negative pressure that acts as antigravity, accelerating the universe's expansion. The effect of dark energy was smaller in the past, allowing gravity to slow the universe's expansion, but on the largest scale the repulsive force of dark energy now overwhelms the attractive force of gravity.

WMAP results also showed that the universe is 13.7 billion years old, with an uncertainty of only 0.2 billion years, and that the cosmic background radiation was set free 389,000 years after the big bang, a value uncertain by only 8,000 years. WMAP estimated a value for the Hubble constant of 71 kilometers per second per megaparsec (44 miles per second per magaparsec), in near agreement with the value predicted by the Hubble Key Project. WMAP’s wide-ranging results will be refined as the spacecraft makes additional observations. Observations made by the European Space Agency's Planck spacecraft, scheduled for launch in 2007, will be even more precise.