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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cosmic inflation | 3/9 | https://en.wikipedia.org/wiki/Cosmic_inflation | reference | science, encyclopedia | 2026-05-05T13:32:28.848172+00:00 | kb-cron |
=== Few inhomogeneities remain === Because the accelerating expansion of space stretches out any initial variations in density or temperature to very large length scales, an essential feature of inflation is that it smooths out inhomogeneities and anisotropies, and reduces the curvature of space. This pushes the Universe into a very simple state in which it is completely dominated by the inflaton field and the only significant inhomogeneities are tiny quantum fluctuations. Inflation also dilutes exotic heavy particles, such as the magnetic monopoles predicted by many extensions to the Standard Model of particle physics. If the Universe was only hot enough to form such particles before a period of inflation, they would not be observed in nature, as they would be so rare that it is quite likely that there are none in the observable universe. Together, these effects are called the inflationary "no-hair theorem" by analogy with the no hair theorem for black holes. The "no-hair" theorem works because the cosmological horizon is no different than that of a black-hole except in that there'd not be testable disagreements about what would be on the other side. One interpretation of the no-hair theorem is that the Universe (observable and unobservable) expands by an enormous factor during inflation. In an expanding universe, energy densities generally fall, or get diluted, as the volume of the Universe increases. For example, the density of ordinary "cold" matter (dust) declines as the inverse of the volume: when linear dimensions double, the energy density declines by a factor of eight; the radiation energy density declines even more rapidly as the Universe expands since the wavelength of each photon is stretched (redshifted), in addition to the photons being dispersed by the expansion. When linear dimensions are doubled, the energy density in radiation falls by a factor of sixteen (see the solution of the energy density continuity equation for an ultra-relativistic fluid). During inflation, the energy density in the inflaton field is roughly constant. However, the energy density in everything else, including inhomogeneities, curvature, anisotropies, exotic particles, and standard-model particles is falling, and through sufficient inflation these all become negligible. This leaves the Universe flat and symmetric, and (apart from the homogeneous inflaton field) mostly empty, at the moment inflation ends and reheating begins.
=== Reheating ===
During inflation, any pre-existing thermal bath is diluted to negligible levels. Consequently the universe emerges out of inflation in a non-thermal state, with all the energy density found in the inflaton field. Reheating is the process of converting this energy density into a thermal bath of Standard Model particles, necessary to initiate the Hot Big Bang. Due to the absence of direct observational probes of the universe at that time, it is currently unknown how reheating occurred. Proposed mechanisms are primarily constrained by the consistency requirements that the thermal bath has to be primarily composed of Standard Model particles and that the final temperature be above 1 MeV, necessary for Big Bang nucleosynthesis. Many reheating scenarios first begin with a period of preheating, during which there is a parametric resonance driving an exponential conversion from the inflaton into some final state particles. This ends once the inflaton condensate fragments due to various back-reaction effects. The universe then transitions into a non-linear phase sometimes characterised by turbulence and the formation of non-linear field configurations. This is followed by thermalisation, during which the resulting gas of particles establishes a state in local thermal equilibrium. The energy density of this state is dominated by radiation gas, so prior to the end of reheating, the massive inflaton particles must have completely decayed.
=== Aftermath === The history of the universe after inflation but before a time of about 1 second is largely unknown. However, the universe is known to have been dominated by ultrarelativistic Standard Model particles, conventionally called radiation, by the time of neutrino decoupling at about 1 second. During radiation domination, cosmic expansion decelerated, with the scale factor growing proportionally with the square root of the time. In the eras that followed the universe has continued to expand.
== History ==
=== Precursors === In the early days of general relativity, Albert Einstein introduced the cosmological constant to allow a static solution, which was a three-dimensional sphere with a uniform density of matter. Later, Willem de Sitter found a highly symmetric inflating universe, which described a universe with a cosmological constant that is otherwise empty. It was discovered that Einstein's universe is unstable, and that small fluctuations cause it to collapse or turn into a de Sitter universe. Historically, proposed solutions included the Phoenix universe of Georges Lemaître, the related oscillatory universe of Richard Chase Tolman, and the Mixmaster universe of Charles Misner. Lemaître and Tolman proposed that a universe undergoing a number of cycles of contraction and expansion could come into thermal equilibrium. Their models failed, however, because of the buildup of entropy over several cycles. Misner made the (ultimately incorrect) conjecture that the Mixmaster mechanism, which made the Universe more chaotic, could lead to statistical homogeneity and isotropy. In 1965, Erast Gliner showed that the simplest energy-momentum tensor usable in Einstein's theory of general relativity describes a vacuum-like state of matter. Based on the conservation of energy, this vacuum energy density would create negative pressure. This vacuum-energy idea led to two concepts, dark energy and a model for inflation. In the case of inflation, a primordial vacuum-energy is considered with orders of magnitude higher density. This density creates immense negative pressure, an anti-gravity force that expands the distance between the matter that condenses out of that energy. In the early 1970s, Yakov Zeldovich noticed the flatness and horizon problems of Big Bang cosmology; before his work, cosmology was presumed to be symmetrical on purely philosophical grounds. In the Soviet Union, this and other considerations led Vladimir Belinski and Isaak Khalatnikov to analyze the chaotic BKL singularity in general relativity. Misner's Mixmaster universe attempted to use this chaotic behavior to solve the cosmological problems, with limited success.
==== False vacuum ====