Big Bang

The big bang theory was developed to explain the origin of the expanding universe, uniting cosmology with general relativity and elementary particle physics. About 13 to 14 billion years ago, an explosion called the big bang created energy and matter, space and time. Ever since, space has been expanding with time, carrying matter and electromagnetic radiation with it. As space has expanded, its contents have evolved.

Overview

Sir Isaac Newton’s law of universal gravitation led him to suggest that a static universe with a finite distribution of stars would collapse, but that an infinite universe could be stable. The possibility of an expanding universe, however, is contained within Albert Einstein’s general theory of relativity, which he published in 1915. In 1917, Einstein himself found that his general theory in its original form would not permit a static universe. Because the scientific consensus then was that the universe on the large scale is static and unchanging, Einstein added an arbitrary constant (later called the cosmological constant) to his field equations to allow static solutions. Physically, the cosmological constant represents a long-distance repulsion that would balance gravitational attraction on a cosmic scale and thus permit a static universe.

Just five years later, in 1922, the Russian mathematical physicist Alexander Alexandrovich Friedmann found two solutions to the original general relativistic field equations (without the cosmological constant) in which the universe initially expands with time. In one (called “open”), the universe continues to expand forever. In the other (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 field equations of general relativity that Friedmann had obtained earlier. However, Lemaître went further, speculating about the origin of the expansion. Extrapolating backward in time, he realized that everything in the universe would come together at the same time in the distant past, thus pointing to a unique beginning of the universe. He envisioned all matter and space compressed into a “primeval atom” that split into all the atoms of all the elements present in the universe. An enormous explosion initiated the expansion of space and its fragmented matter. As he described the aftermath,

The evolution of the world could be compared to a display of fireworks just ended—some few red wisps, ashes, and smoke. Standing on a well-cooled cinder,…we try to recall the vanished brilliance of the origin of the worlds.

Today, we know that the chemical elements could not have been created the way Lemaître proposed. However, Lemaître’s basic idea was prophetic. Many years later, the explosive origin of the universe was dubbed the “big bang.” Just before his death in 1966, Lemaître learned of the discovery of the cosmic microwave background, which is greatly redshifted radiation emitted just a few hundred thousand years after the big bang—the “vanished brilliance of the origin of the worlds” about which he had speculated so many years earlier.

Observational confirmation that the universe actually is expanding came in 1929, when Edwin Hubble, assisted by Milton Humason, showed there is a correlation between galaxy distances and the redshifts of their spectra; the farther away a galaxy is, the more its is redshifted. The cause of this redshift, termed cosmological, is the expansion of the universe. As the universe expands, wavelengths of electromagnetic radiation are stretched by the expansion, so visible light is shifted toward longer, redder wavelengths. (The term “redshift” has come to be applied to a shift to longer wavelengths of any part of the electromagnetic spectrum.)

Starting in 1935, Friedmann’s student George Gamow began work on more rigorously developing Lemaître’s hypothesis of an explosive origin. Gamow proposed that the very dense initial state would have been very hot, and the universe cooled as it expanded. In 1946, he suggested that the primordial substance, which he called “ylem,” had consisted of neutrons at a temperature of about 10 billion degrees, some of which decayed during the early stages of expansion to form protons and electrons. Successive interactions of the neutrons and protons then led to the formation of all the chemical elements by nuclear fusion reactions while the early universe still was very hot and dense. Gamow worked out the details of this nucleosynthesis of all the chemical elements with his colleague Ralph A. Alpher at George Washington University. Before they published their results in 1948, Gamow persuaded Hans Albrecht Bethe, the physicist who first described nuclear fusion reactions in stars, to allow them to add his name to their paper to make the list of authors “Alpher, Bethe, Gamow,” a wordplay on the first three letters of the Greek alphabet. This came to be referred to as the alpha-beta-gamma theory of the origin of the universe and its chemical elements. (Today we know that the early universe cooled too quickly for most of the chemical elements to have formed then; almost all the atoms heavier than helium were formed later by nuclear fusion reactions in stars.)

Gamow and his associates tried to work out other physical processes that would have occurred in the intensely hot, compressed fireball from which the universe expanded. In the same year, 1948, Alpher and Robert C. Herman (another of Gamow’s colleagues) published a further analysis that predicted a left over as a kind of relic from the early hot, dense universe. Because of the expansion of the universe and the corresponding of this radiation, they predicted that it would have cooled from an initial high temperature to only about 5 kelvins at the present time. Since such radiation would be in the microwave part of the spectrum, they had no way of detecting it then, and their prediction was forgotten until the 1960s.

In the 1960s, a team of physicists at Princeton University—Robert H. Dicke, P. J. E. Peebles, P. G. Roll, and David T. Wilkinson—began planning to build an instrument to detect the cosmic background radiation predicted by Alpher and Herman. However, it was accidentally discovered first by Arno A. Penzias and Robert W. Wilson at Bell Telephone Laboratories in 1965. They were using a large radio horn antenna as part of a communication program when they detected microwave radiation coming uniformly from all directions and corresponding to a temperature of about 3 kelvins. Since the signal was so uniform, they thought it might be due to some instrumental noise. Pigeons roosted in the antenna, and Penzias described “a white sticky dielectric substance coating the inside of the antenna.” Chasing away the pigeons and cleaning out their droppings did not get rid of the signal. Eventually, it was identified by Dicke and his colleagues at Princeton University as the relic radiation from the primeval fireball predicted by Alpher and Herman. In 1978, Penzias and Wilson received the Nobel Prize in Physics for their serendipitous discovery that provided convincing confirmation of a big bang origin for the universe.

Measurements from Earth-orbiting such as the Cosmic Background Explorer (COBE), launched in 1989, and the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2003, as well as high-altitude balloons launched from Antarctica, including the Balloon Observations of Millimetric Extragalactic Radiation and Geomagnetics (BOOMERANG) project, have shown that the microwave background spectrum perfectly fits a thermal radiator at a temperature of 2.73 kelvins. The shape of the spectrum is exactly what would be expected for radiation from the early universe, when matter and radiation were in thermal equilibrium; the shape of the original blackbody spectrum has been preserved during the subsequent expansion and cooling. However, the background radiation is not precisely uniform in all directions. There are temperature variations of up to about 0.00001 over regions with an angular size of about one degree of arc. These variations are thought to represent slight differences in density in the early universe that ultimately produced the “lumpy” universe of clusters of galaxies that we observe today.

The equations that describe the expansion of the universe can be extrapolated back to very early times of incredibly high densities and temperatures, but only to about 10^-43 second (called the Planck time). Before that time, conditions were so extreme (for example, temperatures in excess of 1032 kelvins) that the current understanding of physics breaks down. Scientists believe that, at that time of high temperatures and correspondingly high energies, the four fundamental forces of nature—gravity, strong nuclear, weak nuclear, and electromagnetic—were indistinguishable from one another, or “unified.” However, physicists have no workable “theory of everything” (TOE) to describe this unification of forces. At about the Planck time, gravity would have separated (or “frozen out”) from the other three forces as temperature and energy decreased. After about 10^-35 second, when the temperature had decreased below about 10²⁸ kelvins, the separated from the other two. The energy released by this “freeze out” of the strong nuclear force may have initiated a brief period of cosmic inflation, during which the universe increased in size by a factor of 10⁵⁰ in 10^-32 second.

High-energy elementary particle physics has been employed to work out more details of the early development of the universe and its contents. The first particles and that would have materialized from energy according to ideas about mass-energy equivalence (as expressed by Einstein’s famous equation E = mc2) and particle-antiparticle pair creation are not well understood, although unified field theories are beginning to suggest their possible properties. At the very high temperatures of the early universe, particles moved so fast that they avoided any interaction, but as the universe cooled they could interact to produce new forms of matter, leading to an era dominated by quarks. The known laws of physics can account for the particles that would have existed after about 10^-12 second at a temperature of about 10¹⁶ kelvins. At that time, space was filled with photons, quarks, and leptons (electrons, neutrinos, and the like), along with their antiparticles.

After 10^-6 seconds, the universe had cooled enough so that no more quark-antiquark pairs could be created. From then on, and antiquarks mutually annihilated each other, producing a brilliant fireball of gamma-ray photons. Equal numbers of quarks and antiquarks had been produced, but some asymmetry resulted in a slight excess of quarks over antiquarks by about one part in a billion. It seems that the asymmetry may occur in the weak nuclear force, which provides a way for antiquarks to decay but no equivalent way for quarks. After the quark-antiquark annihilations were over, all the antiquarks and most of the quarks were gone, but about one quark in a billion had survived; they combined to form protons and neutrons, which went on to become the matter in the universe today.

Neutrons decayed into protons by emitting electrons and antineutrinos, and protons combined with electrons to form neutrons and neutrinos. They were kept nearly equal in number by thermal equilibrium as long as electrons were abundant. When the universe was a few seconds old and the temperature fell to about 6 billion kelvins, photons no longer had enough energy to produce electron-positron pairs. Soon, all positrons and all but one electron out of a billion had mutually annihilated in another burst of gamma-ray photons. With so few electrons remaining, no new neutrons were formed, and the number of neutrons declined as they decayed into protons.

Before all the neutrons decayed, some joined with protons to form nuclei of deuterium (also called heavy hydrogen). However, while the universe was hot enough, gamma-ray photons could break deuterium nuclei apart. After about three to four minutes, when the temperature had dropped below about 1 billion kelvins, photons no longer had enough energy to break up deuterium nuclei, so they could survive. In rapid succession, the deuterium nuclei then collided with protons and neutrons to form helium nuclei, and soon almost all the remaining neutrons combined to form helium. When this began, there was about one for every six protons. Using almost all the neutrons to form ordinary helium nuclei resulted in one helium (two protons and two neutrons) for every ten hydrogen nuclei (each just a single proton), and this very closely matches the cosmic abundance of helium and hydrogen observed today. A few nuclei and even fewer beryllium nuclei also formed, but the temperature dropped too quickly for there to be time to form heavier nuclei. After about fifteen minutes, the temperature had dropped below 400 million kelvins, and nucleosynthesis ended. (The heavier elements eventually formed at much later times through nuclear fusion reactions in stars.) Throughout the early universe, the radiation density exceeded the matter density, but radiation density decreased more rapidly than matter density. After several thousand years, the two densities were equal, and from that time on, matter has been dominant.

When nucleosynthesis ended, one electron remained for each free or bound proton in hydrogen and helium nuclei, but the universe was too hot for electrons to combine with nuclei to form neutral atoms. Free electrons are very effective at scattering photons, so the universe was opaque to electromagnetic radiation. The universe expanded for several hundred thousand years before it was cool enough (about 3,000 kelvins) for electrons to combine with nuclei to form neutral atoms. When this happened, the lack of free electrons made the universe transparent to electromagnetic radiation, and photons were free to travel through space. This decoupling of matter and radiation was the source of the cosmic microwave background radiation. As the universe continued to expand, it stretched the wavelengths and effectively cooled the primeval “relic” radiation until it reached the present temperature of 2.73 kelvins.

Applications

The cosmological interpretation of redshifts attributes these spectral shifts to the stretching of wavelengths of electromagnetic radiation as space expands. The greater the redshift, the more space has expanded since the electromagnetic radiation was emitted, and the farther back in time one can observe. The Hubble law expresses basically the same idea; the greater the redshift, the farther away the sources and the greater the travel time of electromagnetic radiation to reach Earth. This ability to look back in time aids our understanding of distant objects with large redshifts. Such objects are seen as they were billions of years ago during early stages in the evolution of the universe.

For example, quasars (quasi-stellar radio sources) are mostly unresolved sources with very small angular sizes having very large redshifts and often rapid and erratic changes in brightness. If their redshifts are cosmological (a few astronomers dispute this assertion), then they are very distant. This conclusion, coupled with their apparent brightness, implies that they are incredibly luminous. Their rapid changes in brightness mean they are relatively small in actual size. All of this taken together suggests that they likely are extremely energetic compact phenomena in young galaxies, possibly supermassive black holes forming at the centers of developing galaxies.

The cosmic microwave background radiation is extremely uniform in all directions, but it does have a small asymmetry, being slightly warmer systematically in one half of the sky compared to the other half by a maximum of about 0.007 kelvin. This is interpreted as due to the motion of our through the background radiation field; the temperature difference of 0.007 kelvin in thermal radiation corresponds to a spectral Doppler shift produced by a speed of 380 kilometers per second. Presumably this speed is a combination of the movements of our solar system in the galaxy, the Milky Way galaxy in the Local Group of galaxies, the Local Group relative to the Virgo of galaxies, and maybe other motions as well.

The measured temperature of the makes it possible to calculate the expected cosmic abundances of light elements: about 74 percent (by mass) hydrogen, 26 percent helium, a thousandth of a percent deuterium, and a millionth of a percent lithium. All these match the measured abundances within the observational uncertainties. Since no other source for deuterium production is known, these measurements provide additional confirmation for the standard big bang model.

The uniformity of the cosmic background radiation implies thermal equilibrium throughout the universe, even in regions so far apart that electromagnetic radiation has not had time to travel from one to the other. In the early 1980s, Alan Guth proposed “inflation” as a solution to this “horizon problem.” He suggested a very early period of rapid expansion at an exponential rate, when the universe increased in size by a factor of 10⁵⁰ in 10^-32 second. Before inflation, the universe would have been small enough for electromagnetic radiation to travel between all parts of it.

Uniformity also raises a “galaxy problem.” How could galaxies and stars form in such a uniform universe? Fine-scale sky maps of the cosmic background radiation made with data from sensitive detectors on the COBE and WMAP spacecraft and BOOMERANG balloons show small temperature (and hence density) variations in the early universe that could have grown by gravity to develop into clusters and superclusters of galaxies. The density variations probably are due to small random quantum fluctuations in the very early universe.

The discovery of the W and Z particles in 1983 by Carlo Rubbia and Simon van der Meer provided support for the electroweak theory, which predicts the electromagnetic force and the weak nuclear force become “unified” or indistinguishable from each other at temperatures above about 10¹⁵ kelvins. The quark theory predicts a weakening of the strong nuclear force at even higher temperatures; grand unified theories (GUTs) propose that the strong nuclear force is unified with the electromagnetic and weak nuclear forces at temperatures above about 10²⁷ kelvins. Theories of everything (TOEs) go still further and suggest the unification of gravity with the other three forces at temperatures above about 10³² kelvins.

The high temperatures required to unify the forces occurred shortly after the big bang, making the very early universe a high-temperature laboratory in which it may be possible to test these theories. Gravity would have decoupled at the Planck time, 10^-43 second. The decoupling of the strong nuclear force at about 10²⁷ kelvins would have occurred after about 10^-35 second and may have released the energy that drove the sudden inflationary expansion of the universe. The last decoupling of the weak nuclear and electromagnetic forces would have occurred at 10^-11 second, when the temperature had cooled to 10¹⁵ kelvins.

Context

Several competing theories have attempted to avoid the creation implications of the big bang theory, but they have not been able to sustain successful alternatives. One of the first was Einstein’s early attempt to obtain a solution for a static universe and his introduction of an arbitrary cosmological constant to balance gravitational attraction. When it was later shown that his field equations of general relativity without the cosmological constant were compatible with the observed expansion of the universe, Einstein is reported by Gamow to have remarked that the cosmological constant was the greatest blunder of his life. It is ironic that the early era of cosmic inflation and the recently discovered of the expansion both involve repulsive forces similar to Einstein’s original cosmological constant.

The most serious attempt to defeat the big bang theory was the steady state theory of the universe, introduced in 1948. About the same time that Gamow and his colleagues were working out the details of a Lemaître-type explosive origin, the British cosmologists Hermann Bondi, Thomas Gold, and Fred Hoyle were developing an alternative—the steady state continuous creation model. It was Hoyle who coined the term “big bang” as a derogatory name for the Lemaître-type primordial explosion. However, the name is short and catchy, and it was quickly adopted by most astronomers and physicists no matter which side (if either) they supported.

The s did not invoke a moment of creation for the entire universe but assumed instead the continuous creation of new matter throughout space at a rate that keeps the mean density of the universe constant for all times as the universe expands. Continuous creation would occur so gradually that it could not be observed until enough matter had been created to form stars and galaxies. Such a steady state universe would be infinite and eternal. Ironically, it required the religious idea of creation ex nihilo (from nothing) to avoid another religious idea of a unique creation event in the remote past (the big bang).

Although the steady state theory provided the main competition for the big bang theory during the 1950s and early 1960s, it did not stand the test of time. Since stars and galaxies would form throughout space from the continuous creation of new matter, young and old galaxies should exist side by side. This is contrary to the evidence that galaxies all formed at about the same time, and the galaxies that seem to be much younger (such as quasars) are observed only at great distances and hence at great times in the past. The steady state theory was virtually abandoned after the 1965 discovery of the cosmic background radiation, the relic radiation predicted from the big bang fireball. Even Hoyle, chief spokesman for the steady state theory, helped work out some details of the standard model of the big bang in 1967.

One other attempt to avoid a finite age for the universe was the idea of an oscillating universe. If the density of matter in the universe were large enough eventually to reverse its expansion by gravitational attraction, the universe would collapse toward a “big crunch.” The o theory proposed that another big bang might follow each big crunch, giving rise to a series of oscillations between successive big bangs, extending indefinitely into the past and future. However, such speculation was laid to rest by the discovery in the 1990s that the expansion of the universe is accelerating, so no contraction seems possible.

While the religious theory of the Creation, which posits that life and the universe originated through divine acts, has conflicted with scientific concepts such as evolution and the big bang theory for some time, in 2014, Pope Francis delivered an address during which he asserted that the two theories can coexist. He argued that the theory of the Creation and the big bang theory actually rely upon one another, making both theories valid and not necessarily contradictory. In 2017, Pope Francis invited leading scientists to the Vatican to attend a conference in honor of Lemaître in which they would explain concepts such as gravitational waves and black holes.

Meanwhile, scientists continued to refine their understanding of the big bang theory, which was challenged by new ideas introduced through more advanced technologies. The James Webb Space Telescope, launched in late 2021, provided powerful images of the early universe, revealing mature galaxies that challenged conventional notions about the big bang. Such information led some scientists to posit that the universe has not been expanding from a single explosive event, but rather that the redshift observed in distant galaxies might be due to light losing energy as it travels through space. This theory, called the Tired Light theory, was actually proposed in 1929 but largely fell out of favor among astronomers as the big bang theory gained consensus.

Bibliography

Barrow, John D., and Joseph Silk. The Left Hand of Creation. Basic Books, 1983.

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed., Addison-Wesley, 2008.

Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages to the Stars and Galaxies. Brooks/Cole-Thomson Learning, 2006.

Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed., W. H. Freeman, 2008.

Hawking, Stephen W. A Brief History of Time: From the Big Bang to Black Holes. Bantam Books, 1988.

Jastrow, Robert. God and the Astronomers. Warner Books, 1978.

Lang, Kenneth R., and Owen Gingerich, eds. A Source Book in Astronomy and Astrophysics, 1900–1975. Harvard UP, 1979.

McMillan, Tim. "Time to Rethink the Big Bang? New Research Suggests Universal Expansion May Not Be What It Seems." The Debrief, 12 Sept. 2024, thedebrief.org/time-to-rethink-the-big-bang-new-research-suggests-universal-expansion-may-not-be-what-it-seems/. Accessed 6 Dec. 2024.

Schneider, Stephen E., and Thomas T. Arny. Pathways to Astronomy. 2nd ed., McGraw-Hill, 2008.

Silk, Joseph. The Big Bang. Rev. ed., W. H. Freeman, 1989.

Tharoor, Ishaan. "Pope Francis Says Evolution Is Real and God Is No Wizard." The Washington Post, 28 Oct. 2014, www.washingtonpost.com/news/worldviews/wp/2014/10/28/pope-francis-backs-theory-of-evolution-says-god-is-no-wizard/?utm‗term=.4cd5be42a21e. Accessed 24 Oct. 2017.

Trefil, James S. The Moment of Creation: Big Bang Physics from Before the First Millisecond to the Present Universe. Charles Scribner’s Sons, 1983.

Weinberg, Steven. The First Three Minutes. Bantam Books, 1977.