Inflationary Theory Of The Universe

Type of physical science: Inflationary Theory of the Universe, Universe, expansion of the, Astronomy and astrophysics

Field of study: Cosmology

The inflationary theory of the universe modifies the big bang theory by postulating a brief and rapid period of exponential expansion of the universe shortly after its initial creation. This inflationary interval resolves a number of problems that were inherent in the original big bang theory.

Overview

The inflationary theory of the universe is an attempt to resolve several problems raised by the big bang theory of creation by postulating that a brief period of rapid expansion occurred shortly after the beginning of the universe. The inflationary theory was introduced in 1980 by Alan Guth, who was then a postdoctoral fellow at the Stanford Linear Accelerator Center. He was studying the connections between elementary particles and cosmology in the extremely high-energy conditions of the early universe, shortly after the big bang. In addition to explaining a number of arbitrary assumptions in the big bang theory, inflation also provided a plausible mechanism for the origin of matter and energy in the universe. Guth's original theory has since been modified into the new inflationary theory, also called slow-roll inflation, to address inconsistencies in the original theory.

After the brief interval of exponential inflation, the inflationary theory coincides with the standard big bang theory, which has been very successful in explaining the development of the universe over the last fourteen billion years. Among the most important successes of the big bang theory are its explanation of such phenomena as the redshifting of light from distant galaxies, the existence of cosmic background radiation, and the cosmic abundances of the light elements formed at high temperatures during the first seconds after the big bang.

Even before the formation of these light elements, temperatures were so high that the fundamental forces of nature began to converge. The electroweak theory, confirmed by high-energy accelerators in 1973 and 1983, argues that the electromagnetic force and the weak force (which acts on light particles called leptons, such as electrons and neutrinos) were originally combined into a single force, the electroweak force, but separated from one another at approximately one trillionth of a second (10^-12 second) after the big bang, when temperatures were about a million billion (10¹⁵) kelvins. In this theory, spontaneous symmetry breaking is produced by a Higgs field, as first suggested by Peter Higgs at Edinburgh University in 1964. Although various versions of grand unified theories (GUTs) have not yet been completed or confirmed, they agree that the electroweak force and the strong force (which acts on particles called quarks, from which protons and neutrons are made) separated from a single unified force at about 10^-36 second after the big bang, marking the end of the grand unification epoch and the beginning of the electroweak epoch.

In trying to apply grand unified theories to cosmology, Guth struck on the idea of a sudden exponential expansion of the universe, which he called "inflation" after the runaway monetary inflation of the late 1970s. He saw that cosmological inflation could create the initial conditions of the big bang during the grand unification epoch and account for the origin of matter. In the normal stages of big bang expansion, the energy density (sum of matter and radiation per unit volume) of the universe decreases as its volume increases. Today, the vacuum of space has a near-zero energy density, primarily from the weak electromagnetic fields of the background radiation. In the grand unification epoch, however, space was charged with the vast energy density of a zero-value Higgs field, creating what is called a false vacuum, which the theory requires to break the symmetry of the unified strong-electroweak force.

When Guth combined the idea of the false vacuum with Albert Einstein's equations of general relativity, he found a dramatic result in the form of a strong gravitational repulsion rather than the usual attraction that slows cosmic expansion. This repulsion has the same form as the cosmological constant that Einstein had introduced, but it operates for a limited time. It would have produced a rapid expansion, providing virtually all the previously unexplained momentum of the big bang. The universe would double in size every 10^-34 second for as long as it remained in the false vacuum state, doubling more than a hundred times and inflating by a stupendous factor of at least 10⁵⁰. After about 10^-32 second, a phase transition occurs that breaks the symmetry of the unified force, like the phase transition that changes steam to water or water to ice. This releases the energy of the false vacuum, the same way latent heat is released when steam condenses or water freezes, producing a vast number of particles and reheating the universe back to about 10²⁷ kelvins. From this point, standard big bang theory applies.

According to inflationary theory, most of the matter and energy in the universe were produced by the inflationary process without violating the principle of energy conservation. During inflation, the energy density of the false vacuum remains constant at the lowest possible value for which the grand unified symmetry is unbroken. As any region of the false vacuum expands with its constant energy density, its total energy content increases along with the volume by taking energy from its surroundings. This creates a negative pressure that forces the expansion. According to general relativity, normal big bang expansion is slowed by the gravitational effects of energy density and its pressure, the latter being a small relativistic correction. In a false vacuum, however, the pressure term is negative and overwhelms the energy density in its repulsive effect. This process stores a vast amount of energy in the false vacuum, supplied by the force of the expansion.

In the phase transition at the end of the inflationary process, the energy of the false vacuum is released in the form of particles that attract one another by their gravitational field. Since work is required to pull matter apart against its own gravity, the gravitational energy of a collection of particles is negative. When particles are added to a system, energy is released, and the gravitational energy decreases. The formation of heat and matter at the end of the inflationary period was exactly compensated by the negative gravitational energy of the newly created mass. No net energy appeared, and the total mass-energy of the system was conserved. Inflationary theory allows the entire observed universe to develop from a small amount of energy before inflation, equivalent to as little as ten kilograms of matter balanced by an equal amount of negative gravitational energy. Guth called this possibility of creation out of nothing "the ultimate free lunch."

Guth's original inflationary theory had a fatal flaw that was soon corrected. In the phase transition at the end of the grand unification epoch, symmetry broke down whenever the temperature fell below a level of about 10²⁷ kelvins. However, different regions of space would reach this temperature at different times. Each region would have to coalesce with all the others for the phase transition to be complete, like the merging of bubbles in boiling water to produce steam. Yet the "bubble" regions of asymmetric space are moving apart so fast as the universe expands that they can never merge, so the grand unification phase transition could never be completed.

In 1981, a solution was proposed by Andrei Linde of the Lebedev Institute in Moscow; the same solution was proposed independently in 1982 by Andreas Albrecht and Paul Steinhardt of the University of Pennsylvania. In this new inflationary theory, the entire observable universe originated inside a single bubble, where no merging was necessary to end the phase transition. This process would produce enough matter to be consistent with present observations. It also implies the existence of other, parallel universes totally cut off from our own. Between the bubbles would be the undifferentiated symmetric space filled with Higgs energy, where forces are still unified. Linde suggests that this model also explains why our universe has three dimensions. Bubble universes might have formed with a wide range of different dimensionalities, but only three-dimensional universes such as ours could have life as we know it.

Applications

Inflationary theory resolved several problems in the combined big bang and unified force theories. One problem that unified theories had already addressed was the antimatter problem—that is, why the formation of particles from energy at the end of inflation did not leave an equal number of antiparticles. Grand unified theories predict that during the very high energies of the grand unification epoch, hypothetical X and Y particles and antiparticles formed with masses of about a million billion protons. At the ultrahigh temperatures of this period, these particles would then decay into quarks and leptons and their antiparticles. Yet X and Y particles have a property, first predicted by Andrei Sakharov in 1967, that violates the symmetry of matter and antimatter when the temperature drops below 10²⁷ kelvins. At this point, there is not enough energy to re-create these particles, and they begin to decay faster than they can reappear. Yet they produce a slight excess of quarks and leptons over antiquarks and antileptons. When they begin to annihilate one another, about one particle remains from every billion annihilations, matching the observed excess of about a billion photons for every proton and electron in the universe and virtually no antimatter.

One of Guth's original concerns was the monopole problem, which came from the prediction of grand unified theories, as proved independently by Alexander Polyakov and Gerhard 't Hooft in 1975. They showed that a phase transition without inflation would have twisted the field, creating a great many magnetic monopoles, which are very massive particles (10¹⁶ proton masses) carrying a single north or south pole. If these particles existed, however, they would have dominated the mass of the universe, contrary to all the successful predictions of the big bang theory. At least one monopole is created in each horizon volume, corresponding to the distance light had traveled since the beginning of the universe (10^-34 light-second = 10^-26 meter). In the standard big bang model, about 10⁸⁰ horizon volumes are needed to create the matter in the present universe, which would also generate about 10⁸⁰ monopoles. Inflationary theory allows only a few monopoles, since just a single horizon volume (inflated by a factor of 10⁵⁰ to a size of 10²⁴ meters) produces the entire observable universe (about ten centimeters across after inflation).

The horizon problem arises from the uniformity of the cosmic background radiation, which varies by only a hundredth of a degree in any direction. Such thermal equilibrium implies that all parts of the universe must have been in causal contact at some time in the past. Yet radiation reaching Earth now from opposite directions was released fourteen billion years ago (about one hundred thousand years after the big bang) from two regions of the universe that today are separated by twenty-eight billion light-years. Since information cannot travel faster than light, these regions could not have influenced each other. When the universe was only one hundred thousand years old, the part of the universe we now see was about ten million light-years across, one hundred times as far as information could have traveled by then. Inflationary theory avoids the horizon problem, since it proposes that the observable universe expanded from a region much smaller than the horizon distance and thus was able to reach thermal equilibrium. When this small region inflated, it maintained its uniformity, so that the background radiation arriving today from all directions was once in close contact and could reach a common temperature.

The flatness problem refers to the fact that all observational evidence suggests that the universe is virtually flat, which seems vanishingly unlikely. The current average density of the universe is extremely close to the critical density, suggesting that it was infinitesimally close to the critical value in the earliest stages of the standard big bang, as the slightest deviation from critical density then would have been vastly magnified over time. For the universe to last fourteen billion years, it must have started at exactly critical density to within one part in 10⁶⁰. The essence of the flatness problem is why the universe would have formed with an initial density so close to critical density when there is no apparent reason for it to have done so. Inflationary theory addresses this problem as well: according to the equations governing it, the density of the universe is strongly driven toward the critical density during inflation, a condition that corresponds to a geometrically flat or Euclidean space. Rapid inflation causes space to become flatter, like the surface of a balloon when it is inflated. The theory predicts a density today matching the critical density to one part in ten thousand, producing a universe with a flat geometry.

Context

The concept of the big bang as the origin of the universe was first suggested by the Belgian priest and astronomer Georges Lemaître. In 1927, he showed that Albert Einstein's equations of general relativity implied an expanding universe in which the galaxies would have all been together at the same time in the distant past, thus pointing to a unique beginning of the universe. Einstein tried to avoid this conclusion and maintain a static universe by introducing an arbitrary constant in his equations, the cosmological constant, which represented a cosmological force that would balance gravitational attraction and stabilize the universe. In 1931, Lemaître envisioned all the matter and space of the universe compressed into a "primeval atom," which then exploded to form the receding galaxies.

The idea of an expanding universe was established in 1929 by Edwin Hubble, who demonstrated that the galaxies are moving away at speeds that increase with their distance, as measured by the redshift of their light. The big bang theory was elaborated in 1948 by George Gamow and his colleagues to explain the origin of the elements by nucleosynthesis under the high-temperature conditions of the early universe. Their analysis led to the prediction of cosmic background radiation resulting from the big bang, which would have cooled by expansion to a few degrees above absolute zero at the present time. The 1964 discovery by Arno Penzias and Robert Wilson at Bell Labs of microwave radiation distributed uniformly through space led to renewed interest in the big bang theory; subsequent measurements showed that this radiation had a temperature of 2.74 kelvins, matching the spectrum of the cosmic background radiation predicted by the big bang theory. This was confirmed in 1989 by the Cosmic Background Explorer (COBE) satellite, which showed that the microwave background radiation was uniform in all directions within a hundredth of a degree.

Refinements of Hubble's measurements showed that the universe began to expand between ten billion and sixteen billion years ago, providing sufficient time for the background radiation to cool to the observed temperature of 2.74 kelvins. Extrapolation back in time of the equations that describe the expansion of the universe made it possible to calculate the high temperatures in the early universe and the cosmic abundances of light elements such as hydrogen, deuterium, helium, and lithium, which matched the observed values close enough to substantiate the big bang theory back to the first few seconds. However, the original theory had to make unusual assumptions about the initial conditions that would have led to the outcome of the standard big bang theory. The modifications proposed by the inflationary theory improved on the original theory and opened up new possibilities in the understanding of the creation of the universe.

Since the introduction of inflationary theory, numerous variations and refinements have been proposed. In 1986, Linde proposed a chaotic inflation model, which itself is a version of what is known as eternal inflation. Essentially, eternal inflation is a logical extension of new inflationary theory; it posits that inflation is constantly taking place, creating new parallel bubble universes in addition to our own. Chaotic inflation builds on this, suggesting that inflation can occur in any universe that begins in a chaotic state, even ones in which the laws of physics might be vastly different from what we know. Linde later proposed a new model called hybrid inflation, which deals with scalar fields known as inflatons. The original inflation theory posited the existence of a single inflaton, which initiates inflation; hybrid inflation adds to this a second inflaton, which causes its termination. More recent work in inflation theory has focused on reconciling it with string theory.

Principal terms

ANTIMATTER: a form of matter with particles that have the same mass as normal particles but carry opposite electrical charge and magnetic moment; if they meet, a particle and its antiparticle annihilate into radiation

BIG BANG THEORY: the theory that the universe began as an infinitesimal point of pure energy about fourteen billion years ago and expanded to form all space, matter, and radiation

COSMIC BACKGROUND RADIATION: fossil radiation from the big bang that has filled the universe and cooled by expansion to microwave frequencies at a temperature of 2.74 kelvins

CRITICAL DENSITY: the average density of the universe that provides just enough gravitational attraction to prevent a reversal of its expansion

DARK MATTER: nonluminous matter of unknown form that is evident from its gravitational effects and, according to inflationary theory, is in great abundance throughout the universe

EXPANDING UNIVERSE: the expansion of space-time that began with the big bang and continually increases the distances between galaxies

FALSE VACUUM: a condition of space in which the vacuum has a positive energy density, creating negative pressure and thus causing the universe to expand

HORIZON DISTANCE: the distance light can have traveled at any time since the origin of the universe, setting the limits of the observable universe

INFLATION: a sudden and rapid interval of exponential expansion of the universe shortly after its beginning

MAGNETIC MONOPOLES: massive particles formed shortly after the big bang that carry a single isolated north or south magnetic pole

Bibliography

Barrow, John, and Joseph Silk. The Left Hand of Creation: The Origins and Evolution of the Expanding Universe. New York: Basic, 1983. Print. Has several sections relating to inflationary theory, including a good discussion of magnetic monopoles. Includes a good glossary of terms in astrophysics and cosmology and an index.

Davies, Paul. Superforce: The Search for a Grand Unified Theory of Nature. New York: Simon, 1984. Print. Gives a good account of elementary particle theory, grand unified theories applied to the expanding universe, and inflationary theory. Includes diagrams, a short bibliography, and an index.

Gefter, Amanda. "Bang Goes the Theory." New Scientist 30 June 2012: 32–37. Print.

Gorbunov, Dmitry S., and Valery A. Rubakov. Introduction to the Theory of the Early Universe: Cosmological Perturbations and Inflationary Theory. Singapore: World Scientific, 2011. Print.

Greenstein, George. Symbiotic Universe: Life and Mind in the Cosmos. New York: Morrow, 1988. Print. A well-written book with more than seventy figures. Includes the chapters "The Moment of Creation" and "Grand Unification and the Inflationary Universe." Has a short glossary and a good index.

Guth, Alan, and Paul Steinhardt. "The Inflationary Universe." Scientific American May 1984: 116–28. Print. An excellent article by two of the founders of inflationary theory. Includes seven color diagrams and authoritative but readable descriptions of difficult ideas.

Hetherington, Norriss S., ed. Cosmology: Historical, Literary, Philosophical, Religious, and Scientific Perspectives. New York: Garland, 1993. Print. A good anthology of essays on cosmology, with sections such as "The Expanding Universe" and "Particle Physics and Cosmology." Includes "The Inflationary Universe," a readable essay by Alan Guth. Several diagrams and an index.

Leslie, John, ed. Physical Cosmology and Philosophy. New York: Macmillan, 1990. Print. An anthology of essays on cosmology, including Paul Davies's "What Caused the Big Bang?" and Andrei Linde's "The Universe: Inflation out of Chaos." Has a good bibliography and index.

Lieu, Richard. "Has Inflation Really Solved the Problems of Flatness and Absence of Relics?" Monthly Notices of the Royal Astronomical Society 435.1 (2013): 575–83. Print.

Linde, Andrei. "Towards Inflation in String Theory." Journal of Physics: Conference Series 24.1 (2005): 151–60. Print.

Riordan, Michael, and David Schramm. The Shadows of Creation: Dark Matter and the Structure of the Universe. New York: Freeman, 1991. Print. An authoritative and readable book on cosmology, with such chapters as "The Big Bang Universe," "The Creation of Matter," and "A Burst of Inflation." Includes many good diagrams and photographs, a bibliography, and an index.

Trefil, James S. The Moment of Creation: Big Bang Physics from Before the First Millisecond to the Present Universe. New York: Scribner's, 1983. Print A well-written book on big bang theory with good chapters on grand unification and inflation. Includes more than fifty diagrams, a glossary, and an index.