Cosmology

Cosmology is the study of the structure and evolution of the universe, including the eventual development of our solar system. It combines many branches of astronomy and physics, including general relativity and high-energy particle physics.

Overview

Originally, cosmology was a branch of philosophy devoted to understanding the nature of reality and the origin and structure of everything that exists. With the growth of astrophysics during the nineteenth and twentieth centuries, cosmology rapidly became a significant area of research in astronomy and physics, and its focus narrowed to the origin and evolution of energy and matter in the universe as a whole. Modern cosmology is concerned with the universe's large-scale structure, including the distribution of billions of galaxies and galaxy clusters throughout space and time. Cosmologists use physical laws to derive mathematical models of the early universe. They also extrapolate physical processes into the distant future to predict the universe's future and its contents. Nevertheless, modern cosmology still retains many philosophical qualities.

Modern cosmology has its basis in Albert Einstein’s general theory of relativity, which he published in 1915. In 1922, the Russian mathematical physicist Alexander Alexandrovich Friedmann derived two types of solutions to the field equations of general relativity in which the universe initially expands with time. The universe continues to expand forever in one type (called "open"). In the other type (called "closed"), the universe expands to some maximum size, after which it contracts. In 1927, the Belgian priest and cosmologist Abbé Georges Lemaître independently derived the same two solutions to the general relativistic field equations and went on to speculate about the cause of the expansion; he suggested the universe began from a compact, dense initial state—the "primeval atom"—which disintegrated and dispersed into all the atoms in the universe.

Astronomer Edwin P. Hubble presented observational evidence that the universe is expanding (or at least appears to be) when, in 1929, he published his research that showed a linear correlation between a galaxy’s distance from Earth and the redshift of its spectrum. The cosmological explanation for the redshift is that, as the universe expands, the wavelengths of all electromagnetic radiation are stretched. The color red has the longest wavelength of all components of visible light. When wavelengths of visible light are stretched and shifted toward the red, they are said to be redshifted. The term "redshift" has also been extended to refer to a shift toward longer wavelengths in any region of the electromagnetic spectrum.

The generally accepted cosmological theory describing the origin and expansion of the universe is the Big Bang theory, which can be traced back to the proposal by Lemaître of a compact, dense initial state that somehow exploded. The physicist George Gamow expanded on Lemaître’s idea in the 1930s and 1940s, making it physically more rigorous and incorporating new work in nuclear physics. The name Big Bang was coined by Fred Hoyle, one of the developers of the rival steady state, continuous creation theory, as a derogatory term for an explosive origin, but it rapidly caught on and was adopted by proponents and opponents alike. By the mid-to-late 1960s, observational evidence had eliminated competing theories, and the Big Bang became generally accepted. The Big Bang has been further modified and refined throughout the remainder of the twentieth century to incorporate the latest work in high-energy particle physics.

The Big Bang theory maintains that space, time, energy, and matter were all created from a primordial explosion about thirteen to fourteen billion years ago. Space expanded isotropically, and as it grew, the universe cooled. Modern physical theories can trace the universe's development back to 10^-43 seconds after the Big Bang, when the temperature was 10³² kelvins, but before that time, the theories break down. At the even higher temperatures before that time, many scientists think all four fundamental forces of nature—gravity, strong nuclear, weak nuclear, and electromagnetism—were all unified as one force, indistinguishable from one another. As the universe expanded and cooled, the forces gradually separated from each other one by one—first gravity, then strong nuclear, and finally weak nuclear from electromagnetism at about 10^-10 seconds and 10¹⁵ kelvins.

The early universe is a hot, dense "soup" of interacting high-energy photons and subatomic particles, with energy and mass being transformed back and forth. When two photons with enough energy collided, their energy could be converted to mass, producing a matter-antimatter pair of particles in a process called pair production. When a particle and its antiparticle collided, they mutually annihilated each other, converting their mass into two high-energy gamma-ray photons.

As the universe expanded and cooled, eventually, photons did not have enough energy to produce more particle-antiparticle pairs. After that, the particles and their antiparticles collided and mutually annihilated. Since equal numbers of particles and antiparticles had been created, they all should have annihilated each other, and the modern universe would be devoid of matter and antimatter. Alternatively, some segregation process might have separated the matter and antimatter into distinct regions, and the universe would consist of equal but separate concentrations of matter and antimatter. However, the universe appears to be composed almost entirely of matter. An asymmetry in the weak nuclear force provides a way for antimatter but not matter to decay, so there was a slight excess of matter particles by about one in a billion. By the end of a few seconds, the temperature had dropped to several billion kelvins, and particle creation and annihilation had ceased. The small excess of matter that survived is the matter of the universe.

Among the particles to survive were quarks. They combined to form protons and neutrons; a proton is two "up" quarks and one “down” quark, while a neutron is one "up" quark and two "down" quarks. Protons and neutrons are the particles that make up atomic nuclei. Single protons are hydrogen nuclei, and they combine with neutrons to form nuclei of other light elements such as deuterium (also called heavy hydrogen, with one proton and one neutron), helium (two protons and one or two neutrons), and small amounts of lithium (three protons and three or four neutrons) and beryllium (four protons and three neutrons). However, after about fifteen minutes, the temperature had dropped to a few hundred million kelvins, too cool for further nucleosynthesis. Heavier nuclei would be formed much later in nuclear fusion reactions in stars.

The early universe was dominated by electromagnetic radiation; that is, the spatial density of electromagnetic energy was greater than the spatial density of matter. Both densities decreased as the universe expanded, but the electromagnetic energy density decreased faster than the matter density. Several thousand years after the Big Bang, the electromagnetic energy density dropped below the matter density at a time referred to as the crossover time. After that, the universe was dominated by matter since its density was greater.

Around 300,000 to 500,000 years after the Big Bang, the temperature had dropped to about 3,000 kelvins. Electrons could then join with protons (bare hydrogen nuclei) to form electrically neutral hydrogen atoms. Free electrons are very effective at scattering photons, but electrons in atoms cannot do so. As a result, the universe changed from being very opaque to becoming transparent to electromagnetic radiation. Photons could then travel freely through the universe. This was the source of the cosmic microwave background (CMB) radiation observed at a temperature of about 3 kelvins because the wavelength of the electromagnetic radiation has been greatly stretched (redshifted) by the universe's expansion.

Within the first few hundred million years, matter clumped together by gravitational attraction to form protogalaxies or pregalactic fragments, and within them, further gravitational clumping created the first stars. These protogalaxies were relatively small, but through mergers, they developed into larger systems, the galaxies of the universe. Galaxies range in size from dwarfs, containing tens of millions of stars, to giants, with more than ten trillion stars. Galaxies are not distributed randomly through space but are grouped into galaxy clusters; poor clusters contain only a few tens of galaxies, while rich clusters have thousands of members. Galaxy clusters, in turn, are grouped into superclusters. Between them are large, nearly empty regions called voids.

Within galaxies, stars form from clouds of gas and dust called nebulae. Stars initially heat up and begin to shine by gravitational contraction, but this is a relatively brief stage in the life cycle of a star. During most of their energy-producing lives, stars generate energy by nuclear fusion reactions in which lighter atomic nuclei are fused into heavier nuclei with the release of energy. Stars with many times the Sun’s mass can synthesize nuclei as heavy as iron in their interiors. When they explode as Type II supernovae at the end of their energy-producing lives, the tremendous energy released synthesizes nuclei heavier than iron. The explosion disperses the elements the star formed during its life out into space to enrich the nebulae from which new generations of stars form.

It has been assumed that in the future, the expansion of the universe will slow down due to the gravitational attraction between galaxies. A prominent question, however, has been how rapidly the expansion is decelerating. If the deceleration were small, the universe would expand forever, at a gradually decreasing rate; if the deceleration were large enough, someday the universe would stop expanding and begin to contract at ever-increasing speed. Beginning in the 1990s, astronomers tried to determine how much the universe is slowing down by measuring the expansion rate at great distances (and hence at great times in the past) and comparing it to the expansion rate at smaller distances (and more recent times). Contrary to expectations, they found the expansion rate in the past was slower than it is now, indicating that the expansion of the universe is accelerating. The cause for this acceleration is unknown, but it is often attributed to dark energy, a controversial concept of which little is known. If the acceleration continues, the distances between galaxy clusters will grow at an ever-increasing rate. Eventually, all the matter in galaxies will be processed into stars, all the stars will use up their sources of energy and go out, and the universe will grow cold and dark.

Methods of Study

Cosmology is studied both theoretically and observationally, the two complement each other. New observations need to be interpreted by theories, and theories need to be confirmed by further observations. Cosmological observations are made over the electromagnetic spectrum, from high-energy, short-wavelength gamma rays through X-rays, ultraviolet, visible light, infrared, and microwaves to long-wavelength radio waves. Not all electromagnetic radiation penetrates Earth’s atmosphere, so parts of the electromagnetic spectrum must be observed from satellites above Earth's orbit, unobstructed by the atmosphere.

The speed at which all forms of electromagnetic radiation travel through a vacuum is exceedingly fast but finite, being very close to 300,000 kilometers per second. Therefore, looking out to greater distances means looking further back in time. When astronomers look at Earth’s Moon, at a distance of about 400,000 kilometers, they see it as it was about 1.3 seconds earlier. When one observes the Sun at a distance of about 150 million kilometers, it appears as if it was eight minutes and twenty seconds earlier. The nearest star system outside our solar system, the Alpha Centauri system, is at a distance of 4.3 light-years, meaning that we see that system as it was 4.3 years ago. Distant galaxies are billions of light-years away, so we see them as they were billions of years ago. In this manner, it is possible to observe the early universe and its contents by observing at larger and larger distances.

There are two primary observational anchors in modern cosmology. The first, known as Hubble's constant, is the relationship between galaxy redshifts and distances, discovered by Edwin Hubble. This provides basic observational evidence that the universe is expanding. Other explanations for the redshifts of galaxy spectra have been proposed. For example, the "tired light" hypothesis posits that photons lose energy and are shifted to lower frequencies and longer wavelengths as they travel immense distances. However, none of these alternative explanations fits the observed data, with a minimum of extra assumptions, as well as the expanding universe concept does—which explains the redshifts of distant objects due to the stretching of wavelengths of electromagnetic radiation as the universe expands.

The second observational anchor in cosmology is the cosmic microwave background (CMB) radiation, the firmest evidence supporting a Big Bang origin to the expanding universe. It was first detected accidentally by Arno A. Penzias and Robert W. Wilson in 1965 as part of their work on a communication satellite project at AT&T’s Bell Laboratories in Holmdel, New Jersey. Using a large radio horn antenna, they found a uniform microwave background signal coming from all directions. It was identified by Robert Dicke and his colleagues at Princeton University as greatly redshifted radiation from a few hundred thousand years after the Big Bang when the universe became transparent. Subsequent observations of it by Earth-orbiting spacecraft—the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP)—and high-altitude balloons launched in Antarctica, including Balloon Observations of Millimetric Extragalactic Radiation and Geomagnetic (BOOMERANG), showed that the radiation exactly fits a blackbody spectral curve for a temperature of 2.73 kelvins. However, it is not precisely uniform. There are temperature variations of up to about 0.00001 Kelvin over sky areas with an angular size of about 1° of arc (about twice the apparent angular size of the Sun and full Moon as seen in our sky). These temperature variations represent slight differences in density in the early universe that ultimately produced the "lumpy" universe of clusters of galaxies observed in the modern sky.

These observations have made the Big Bang theory, particularly the so-called Lambda-CDM (cold dark matter) model, generally accepted as the standard model of physical cosmology. Other observations that support this view include the detection of gravitational waves, first thought to have been observed in the mid 2010s, and ongoing developments in particle physics.

On the theoretical side, modern cosmology draws primarily upon general relativity and high-energy particle physics. Solutions to the simplest form of the field equations of general relativity predicted an expanding universe before it was confirmed observationally. High-energy particle physics provides insights into the processes that likely occurred in the high-temperature, high-energy environment of the early universe.

Grand unified theories (GUTs) seek to unify the strong nuclear, weak nuclear, and electromagnetic forces as manifestations of a single, more fundamental force, and theories of everything (TOEs) try to include gravity with the other three forces. It is thought that this unification, meaning the forces are indistinguishable from each other, occurs at extremely high temperatures and energies, the conditions that existed in the very early universe. Thus, the very early universe is a laboratory to test such theories.

Context

The value of cosmology lies in understanding the structure and organization of the universe, where it came from, and how it will develop. Cosmology allows perspective on Earth's place in the universe. At the same time, cosmology provides a way to test the laws of physics on a grand universal scale.

Two of the major unsolved questions in cosmology involve the nature of dark matter and dark energy. There are numerous situations in astronomy in which a study of the dynamics of a system implies gravitational forces that far exceed what the observed mass can account for. The deficit in observed mass was originally called "missing mass." However, the dynamical mass calculations seem reliable, so astronomers generally use the term dark matter since the mass is not missing. It is just not observable in any part of the electromagnetic spectrum. Dark matter probably includes nonluminous ordinary matter that has not been observed yet, such as small conglomerates of non-radiating matter, black dwarfs, and black holes. However, indications are that most dark matter is much more exotic—completely unknown forms of matter that do not interact with ordinary matter except gravitationally. Possible candidates include a class of particles called WIMPs (weakly interacting massive particles) and cosmic strings (long, thin, massive lines of unbroken symmetry left over from the early universe in which the strong, weak, and electromagnetic forces remain unified).

Dark energy, which drives the acceleration of the expansion of the universe, is even more enigmatic. Mathematically, it may take the form of Einstein’s cosmological constant in the equations derived from general relativity that describe the expanding universe. Physically, its nature is entirely unknown.

Various observations indicate that the geometry of the universe is almost precisely flat. That means the average density of matter and energy throughout the universe must almost exactly equal a value called the critical density, about 10^-26 kilograms per cubic meter. Observed luminous matter accounts for about 1 percent of this. Allowing for probable nonluminous but ordinary dark matter gives about 3 percent more. There is approximately twenty-six times more dark matter (both ordinary and exotic) than luminous matter, so exotic dark matter accounts for 23 percent of the critical density. The total for all forms of matter comes to about 27 percent; thus, dark energy contributes about 73 percent of the average density of the universe. That means approximately 96 percent of the universe consists of dark matter and dark energy, about which we know virtually nothing. Only about 4 percent consists of ordinary matter, both luminous and nonluminous.

Another cosmological puzzle is that the universe seems "fine-tuned" for life. If the physical laws and constants of the universe were much different from what they are, life as we know it would be impossible. Stars and planets would not form or not last long enough for life, especially intelligent life, to develop. One explanation for this fine-tuning is called the anthropic principle: the idea that the universe has to be the way it is because otherwise, we would not exist to ask about such things. However, some scientists find the odds overwhelmingly against the universe being the way it is solely by chance, proposing instead that the universe, in some way, may have been deliberately designed for life. To avoid the theological implications of deliberate design, other scientists suggest that the universe is just one of many alternate parallel universes, each with its own unique set of laws and constants; human beings occupy the one that allows the existence of life.

Workable GUTs and TOEs are needed to describe the very early universe. One theory for unifying all four forces requires eleven dimensions—the familiar three dimensions of space and one of time, plus seven more dimensions. The extra dimensions would be rolled up into structures too small to detect. In some versions of this theory, particles, such as quarks and electrons, are multidimensional membranes wrapped around the extra dimensions. Multidimensional membranes are also called M-branes or just branes. It has even been suggested that collisions between branes lead to big bangs, creating a new universe.

Through the 1980s, most of the parameters that characterize the universe and its expansion were very poorly known, often with more than a factor of two uncertainty. Since then, new observations have dramatically narrowed the range of uncertainty. For example, modern determinations of the Hubble constant—the slope of the redshift-distance relation, which is the rate at which the universe is expanding—are generally between sixty-five to seventy-five kilometers per second per megaparsec. (A megaparsec is a million parsecs or 3,260,000 light-years.) These values mean that the average speed with which other galaxies recede from us increases by sixty-five to seventy-five kilometers per second for every million parsecs (or 3,260,000 light-years) of distance from us. This small range of values for the Hubble constant, together with the matter and energy density percentages (27 percent matter, 73 percent energy), yields a time back to the Big Bang of thirteen to fourteen billion years ago.

Cosmology started as a branch of philosophy but has become an integral part of astronomy and physics. It has shifted from being primarily a speculative, qualitative endeavor to becoming a precise, quantitative science. Nevertheless, many aspects of cosmology remain philosophical in scope and, in some cases, verge on the spiritual. Cosmologists speculate whether our universe is the only universe, whether there are additional dimensions to the known universe—even a multiverse—that have yet to be discovered, and how the four principal forces of nature might have been unified in the very early universe right after the Big Bang. Since humans are part of the universe, it can be said that "we are the universe contemplating itself."

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