Evolution of the Universe

About thirteen to fourteen billion years ago, the Big Bang occurred, creating the space, time, matter, and energy of our universe. Since then, space has expanded, galaxies of stars have formed in that space, and the stars in the galaxies have evolved. The evidence indicates that the universe will continue to expand forever. Indeed, the expansion appears to be accelerating due to some unknown cause referred to as “dark energy.”

Overview

The evidence we have from observing radiation in all ranges of the electromagnetic spectrum—particularly the infrared—has led astrophysicists and cosmologists to conclude that, approximately thirteen to fourteen billion years ago, an explosive event dubbed the Big Bang created space and time, matter, and energy from an unimaginably hot, dense state whose origin is unknown. Ever since the Big Bang, the universe has been expanding and evolving. Events following the Big Bang have been reconstructed in great detail using known physical laws to predict the behavior of matter and energy as the universe expands.

As the universe expanded, its temperature and the density of both matter and electromagnetic radiation all decreased. In the early, very hot, very dense universe, cosmologists think that all four fundamental forces of nature—gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force—were unified as one force, indistinguishable because as yet unseparated from each other. Models that would successfully combine these four forces are called theories of everything (or TOEs), but there is no workable TOE.

All TOEs, however, assume that the four forces were unified only at the extremely high energies, corresponding to extremely high temperatures, that characterized the first few seconds of the universe. Sometime between ten and forty-three seconds after the Big Bang (called the Planck time), the temperature dropped to about 10³² kelvins, and gravity separated (“froze out”) from the other forces. After about 10^-35 seconds, the temperature dropped to about 10²⁸ kelvins, and the strong nuclear force separated from the remaining two forces. The “freeze-out” of the strong nuclear force may have been what initiated a period of rapid inflation, in which the universe expanded exponentially, increasing in size by a factor of about 10⁵⁰ in the next 10^-32 seconds. This rapid inflation explains why distant regions of the present universe appear so similar and why space on the large scale is so nearly flat. Then, the universe resumed the slower expansion it had been undergoing before inflation occurred. After about 10^-10 seconds, the temperature dropped to 10¹⁵ kelvins, and finally, the weak nuclear and electromagnetic forces assumed their separate characteristics.

The early universe was very hot and was filled with high-energy gamma-ray photons. When two gamma rays with sufficient energy collide, their energy can be converted to mass, as described by Einstein’s famous equation, E = mc², which says that energy, E, and mass, m, are equivalent and related by the speed of light, c, squared. The mass appeared as a particle-antiparticle pair in a process called pair production. When a particle and its antiparticle encountered each other, they mutually destroyed each other in a process called annihilation, in which their mass was converted into energy as a pair of gamma-ray photons. The rates of pair production and annihilation were equal, and matter and radiation were in a state of thermal equilibrium. However, as the universe continued to expand, the temperature dropped to the point that the photons no longer had enough energy to produce specific particle-antiparticle pairs. Once the temperature had dropped below the threshold temperature for a particular type of pair production, no more of those particle-antiparticle pairs were formed, and those that had formed previously quickly annihilated each other.

The threshold temperature for proton-antiproton pair production is about 10¹³ kelvins; this temperature was reached when the universe was about 10^-4 seconds old, after which no more proton-antiproton pairs were produced, and those protons and antiprotons that had been produced previously annihilated each other. The threshold temperature for electron-positron (another name for an antielectron) pair production is about 6 10⁹ kelvins; that temperature was reached after a few seconds, after which no more electron-positron pairs were produced, and those electrons and positrons that had been produced previously annihilated each other. If exactly equal numbers of particles and antiparticles had been created, they all would have annihilated each other, and there would be no matter or antimatter in the modern universe. However, the modern universe is composed predominantly of matter. Consequently, a slight excess of particles over antiparticles must have been created by about one part in a billion; they survived and constitute the matter in the present universe.

Electrons could combine with protons to form neutrons, and protons and neutrons could combine to form nuclei of deuterium (also called heavy hydrogen), each deuterium nucleus consisting of one proton and one neutron held together by the strong nuclear force. High-energy gamma rays could break deuterium nuclei back into protons and neutrons as fast as they had formed, but by about three minutes after the Big Bang, the temperature had dropped to about one billion kelvins, and photons no longer had enough energy to break up deuterium nuclei. This began the time of nucleogenesis when deuterium nuclei could fuse into helium nuclei and even form some lithium and beryllium nuclei. However, after about fifteen minutes, the temperature dropped to a few hundred million kelvins, and the nuclei no longer were moving fast enough to overcome their electrical repulsion. The nucleosynthesis of heavier elements by fusion would have to wait till much later when it would occur in stars. This established the overall composition of the modern universe—about one atom of helium for every ten atoms of hydrogen, with only very small amounts of all the other chemical elements.

In the early universe, the density of electromagnetic radiation was greater than the density of matter, but as the universe expanded, the radiation density decreased more quickly than the matter density. Several thousand years after the Big Bang, at a time called the crossover time, the density of radiation and matter were equal. From that time on, the universe has been dominated by matter. At the high temperatures of the early universe, the matter was ionized, meaning it consisted of free electrons and bare atomic nuclei. Free electrons are very effective at scattering photons of electromagnetic radiation, so the early universe was opaque; photons could not travel far before encountering free electrons and being scattered in new directions. About 300,000 to 400,000 years after the Big Bang, the temperature dropped to about 3,000 kelvins, and the electrons could combine with nuclei to form neutral atoms. Neutral atoms are able to absorb only certain specific photon energies, so the universe became transparent to most photons, and matter and radiation decoupled. The photons could travel freely through the universe, and we observe them as the greatly redshifted cosmic microwave background radiation.

After matter and radiation decoupled, small fluctuations in the distribution of matter started to grow; slightly denser regions gravitationally attracted matter from surrounding areas and became denser still. Within the first billion years, they developed into small protogalaxies or pregalactic fragments in which the first stars formed by gravitational contraction of clouds of gas. Through mergers, the protogalaxies formed larger systems of stars, the galaxies that make up the modern universe. Within the galaxies, stars continue to form by the gravitational contraction of gas clouds, and at least some stars develop families of planets as a by-product of their own formation. It is stars that synthesize the heavier chemical elements. Some of the elements are formed by nuclear fusion processes during the active, energy-producing lives of the stars. Others are formed when stars explode as supernovae. Whether stars end their energy-producing lives violently as supernovae or more quietly, they expel some or most of their mass, and this disperses the heavier elements into the interstellar material, enriching the clouds of gas and dust from which new stars form.

The Milky Way galaxy formed more than twelve billion years ago. The Sun and its solar system formed from a cloud of gas and dust about 4.5 billion years ago. Without the nucleosynthesis of heavier elements by earlier generations of stars, there would be no carbon, oxygen, silicon, iron, or any of the many other elements needed to form a rocky planet like Earth and the life it supports.

Observations of distant galaxies indicate that the expansion of the universe is accelerating. The cause is unknown, but it has been given the name “dark energy.” If the expansion continues to accelerate, the distances between galaxies will grow ever greater. Eventually, all the matter in the galaxies will be processed into stars, the stars will all run out of energy and die, and the universe will grow dark and cold.

Methods of Study

An expanding universe is predicted by Albert Einstein’s general theory of relativity. This conclusion was arrived at independently by Alexander Friedmann in 1922 and Georges Lemaître in 1927 from solutions they found to the field equations of general relativity applied to the structure of the universe. In 1929, Edwin Powell Hubble showed that the universe actually is (or at least appears to be) expanding when he discovered that the distances of thirty-one galaxies were correlated with the redshifts of their spectra. The cosmological explanation for this relationship is that, from the perspective of observers on Earth, the expansion of space stretches the wavelengths of electromagnetic radiation as it travels through space from the source to the observer. The greater the distance between the source and observer, the longer it takes electromagnetic radiation to travel the distance, the longer the universe has been expanding, and the more the wavelengths are stretched. The long-wavelength end of the visible light spectrum is the red end, so the expansion of space shifts visible light to longer, redder wavelengths. The term “redshift” refers to a shift toward longer wavelengths of a photon of light in any portion of the electromagnetic spectrum, whether visible light or not, hence a shift toward the lower energy of that photon.

The cosmic scale factor R(t) is defined to be a measure of how the universe changes in size as a function of time. Changes in the cosmic scale factor can be determined directly from the spectral redshifts. The amount the wavelengths are lengthened tells cosmologists how much the universe has expanded since the light was emitted; for example, if the features of some spectrum all have double their expected wavelengths, then the size of the universe and the cosmic scale factor have both doubled since the light was emitted; to put it another way, the universe, and cosmic scale factor then both were half as big as they are now.

Lemaître, in 1927, was the first to propose that the expansion of the universe began from a compact, dense initial state—the “primeval atom,” as he called it—which “fissioned” into all atoms in the universe. Although wrong in the details, Lemaître’s basic idea of expansion and evolution from a compact initial state has been developed into the Big Bang model of modern cosmology. Events following the Big Bang have been reconstructed in great detail by using known physical laws to predict the behavior of matter and energy as the universe expands.

Space is filled with electromagnetic radiation that has been traveling freely since the universe became transparent a few hundred thousand years after the Big Bang. Its serendipitous discovery by Arno Penzias and Robert Wilson in 1965 was a significant confirmation of a hot, dense Big Bang origin for the universe. This is thermal blackbody radiation, so the wavelength at which it is “brightest” is inversely proportional to the temperature. As the universe has expanded, the wavelength at which it is brightest has increased linearly with the cosmic scale factor R. Thus, the temperature T decreases as the inverse of the cosmic scale factor R. Since the temperature of the cosmic background radiation is about three kelvins, and the temperature at which the universe became transparent was about 3,000 kelvins, the universe has expanded by a factor of about 1,000 since then.

The expansion of the universe causes the density of both matter and radiation to decrease with time. Since volume increases as R³, the density of matter decreases as the inverse of R³. However, the density of the energy of electromagnetic radiation decreases as the inverse of R⁴. This is because the number of photons of electromagnetic radiation per volume of space decreases as the inverse of R³, and the energy of each photon decreases as the inverse of R; the wavelength associated with each photon increases linearly with R, and wavelength and photon energy are inversely related. Thus, the density of electromagnetic energy decreases as the inverse of R³ times the inverse of R, which equals the inverse of R⁴. This means that the density of radiation energy decreases more rapidly than matter density as the universe expands. The density of matter is several thousand times the density of electromagnetic energy, but at an earlier time, when the universe and the cosmic scale factor were several thousand times smaller than they are now, the density of matter and electromagnetic energy were equal. The time when the densities of matter and electromagnetic radiation were equal is called the crossover time; before that, the density of electromagnetic radiation was greater than the density of matter. Estimates of the crossover time place it several thousand years after the Big Bang.

The discovery that the expansion of the universe is accelerating was completely unexpected; in fact, it was discovered during an attempt to measure how much the expansion was decelerating. The models of the universe derived from the simplest form of general relativity all predict the expansion should be slowing. If it were slowing only a little, then the universe would be open, and the expansion would continue forever. If it were slowing enough, then eventually, the expansion would stop, and the universe would begin to contract back on itself. It was expected that measuring the redshifts of very distant galaxies would show how much faster the expansion was in the long-ago past when the light we receive left those galaxies. However, the observational evidence indicates the expansion was slower in the past, meaning the expansion has been accelerating.

Context

Throughout history, humankind has wondered about the origin and fate of the Earth, its life, and the universe. This desire to understand our origins has led nearly every culture to form some kind of creation myth. In Western culture, many religious and philosophical beliefs about the origin of the universe can be traced back thousands of years to the creation myths of the Middle East. Although science cannot explain the origin of what Lemaître called the primeval atom, evidence from observations of phenomena billions of light-years old has provided more definitive answers to many questions that humans have pondered for thousands of years.

Perhaps the most crucial of these questions is how the universe formed. Physics, coupled with astronomical observations, has helped us work out the events and processes that likely occurred in the aftermath of the Big Bang. Our models of the evolution of the universe have profound implications for understanding life on Earth. The universe seems fine-tuned for the existence of life as we know it. If the physical laws and constants were changed slightly, the universe would be a very different place, and life as we know it could not exist. Some scientists explain this by invoking what is called the anthropic principle: the universe has to be the way it is because we are here; if conditions did not permit the development of life, we would not be here to speculate about it. Others argue that the odds are too great against it just being chance that the universe is the way it is and suggest that it may have been deliberately designed that way. Still, others speculate that our universe is just one of many universes, each with its own physical laws and constants; we live in the one in which life as we know it is possible.

Another aspect of the evolution of the universe with profound implications is its future and ultimate fate. If the expansion continues to accelerate, eventually, some billions of years from now, all matter will be processed into stars, all stars will run out of energy, and the universe will grow cold and dark.

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