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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cosmic inflation | 5/9 | https://en.wikipedia.org/wiki/Cosmic_inflation | reference | science, encyclopedia | 2026-05-05T13:32:28.848172+00:00 | kb-cron |
Eventually, it was shown that new inflation does not produce a perfectly symmetric universe, but that quantum fluctuations in the inflaton are created. These fluctuations form the primordial seeds for all structure created in the later universe. These fluctuations were first calculated by Viatcheslav Mukhanov and G. V. Chibisov in analyzing Starobinsky's similar model. In the context of inflation, they were worked out independently of the work of Mukhanov and Chibisov at the three-week 1982 Nuffield Workshop on the Very Early Universe at Cambridge University. The fluctuations were calculated by four groups working separately over the course of the workshop: Stephen Hawking; Starobinsky; Alan Guth and So-Young Pi; and James Bardeen, Paul Steinhardt and Michael Turner.
=== Demise of topological defects rival === Once it became clear that the CMB would be a key experimental testing ground for cosmological models, possible tests for alternative approaches to inflation began to be considered. One rival theory that dates from the same time as Guth's work was the concept of cosmological topological defects. The idea was that as the very early universe cools from an initial hot, dense state it triggered a series of phase transitions much like what happens in condensed-matter systems such as vortices in liquid helium. Topological defects in cosmology are consequences degenerate vacuum states of the universe, called the vacuum manifold, after a symmetry-breaking phase transition. Magnetic monopoles were one example of a stable topological defect predicted by grand unified theories of the early universe. Development of these models throughout the 1980s and 1990s eventually resulted in predictions which could be tested by sensitive measurement of the CMB. Detailed measurements by the Wilkinson Microwave Anisotropy Probe provide strong evidence in favor of cosmic inflation instead. (Models which combine these concepts remain viable).
== Observational status == Inflation is a mechanism for realizing the cosmological principle, which is the basis of the standard model of physical cosmology: it accounts for the homogeneity and isotropy of the observable universe. In addition, it accounts for the observed flatness and absence of magnetic monopoles. Since Guth's early work, each of these observations has received further confirmation, most impressively by the detailed observations of the cosmic microwave background made by the Planck spacecraft. This analysis shows that the Universe is flat to within 1/2 percent, and that it is homogeneous and isotropic to one part in 100,000. Inflation predicts that the structures visible in the Universe today formed through the gravitational collapse of perturbations that were formed as quantum mechanical fluctuations in the inflationary epoch. The detailed form of the spectrum of perturbations, called a nearly-scale-invariant Gaussian random field is very specific and has only two free parameters. One is the amplitude of the spectrum and the spectral index, which measures the slight deviation from scale invariance predicted by inflation (perfect scale invariance corresponds to the idealized de Sitter universe). The other free parameter is the tensor to scalar ratio. The simplest inflation models, those without fine-tuning, predict a tensor to scalar ratio near 0.1 . Inflation predicts that the observed perturbations should be in thermal equilibrium with each other (these are called adiabatic or isentropic perturbations). This structure for the perturbations has been confirmed by the Planck spacecraft, WMAP spacecraft and other cosmic microwave background (CMB) experiments, and galaxy surveys, especially the ongoing Sloan Digital Sky Survey. These experiments have shown that the one part in 100,000 inhomogeneities observed have exactly the form predicted by theory. There is evidence for a slight deviation from scale invariance. The spectral index, ns is one for a scale-invariant Harrison–Zel'dovich spectrum. The simplest inflation models predict that ns is between 0.92 and 0.98 . This is the range that is possible without fine-tuning of the parameters related to energy. From Planck data it can be inferred that ns=0.968 ± 0.006, and a tensor to scalar ratio that is less than 0.11 . These are considered an important confirmation of the theory of inflation. Various inflation theories have been proposed that make radically different predictions, but they generally have much more fine-tuning than should be necessary. As a physical model, however, inflation is most valuable in that it robustly predicts the initial conditions of the Universe based on only two adjustable parameters: the spectral index (that can only change in a small range) and the amplitude of the perturbations. Except in contrived models, this is true regardless of how inflation is realized in particle physics. Occasionally, effects are observed that appear to contradict the simplest models of inflation. The first-year WMAP data suggested that the spectrum might not be nearly scale-invariant, but might instead have a slight curvature. However, the third-year data revealed that the effect was a statistical anomaly. Another effect remarked upon since the first cosmic microwave background satellite, the Cosmic Background Explorer is that the amplitude of the quadrupole moment of the CMB is unexpectedly low and the other low multipoles appear to be preferentially aligned with the ecliptic plane. Some have claimed that this is a signature of non-Gaussianity and thus contradicts the simplest models of inflation. Others have suggested that the effect may be due to quantum corrections or new physics, foreground contamination, or even publication bias.