Editing String theory (section)

== Phenomenology ==
{{main|String phenomenology}}

In addition to being an idea of considerable theoretical interest, string theory provides a framework for constructing models of real-world physics that combine general relativity and particle physics. [[Phenomenology (particle physics)|Phenomenology]] is the branch of theoretical physics in which physicists construct realistic models of nature from more abstract theoretical ideas. [[String phenomenology]] is the part of string theory that attempts to construct realistic or semi-realistic models based on string theory.

Partly because of theoretical and mathematical difficulties and partly because of the extremely high energies needed to test these theories experimentally, there is so far no experimental evidence that would unambiguously point to any of these models being a correct fundamental description of nature. This has led some in the community to criticize these approaches to unification and question the value of continued research on these problems.<ref name="Woit 2006"/>

=== Particle physics ===

The currently accepted theory describing elementary particles and their interactions is known as the [[standard model of particle physics]]. This theory provides a unified description of three of the fundamental forces of nature: electromagnetism and the strong and weak nuclear forces. Despite its remarkable success in explaining a wide range of physical phenomena, the standard model cannot be a complete description of reality. This is because the standard model fails to incorporate the force of gravity and because of problems such as the [[hierarchy problem]] and the inability to explain the structure of fermion masses or dark matter.

String theory has been used to construct a variety of models of particle physics going beyond the standard model. Typically, such models are based on the idea of compactification. Starting with the ten- or eleven-dimensional spacetime of string or M-theory, physicists postulate a shape for the extra dimensions. By choosing this shape appropriately, they can construct models roughly similar to the standard model of particle physics, together with additional undiscovered particles.<ref name=Candelas1985/> One popular way of deriving realistic physics from string theory is to start with the heterotic theory in ten dimensions and assume that the six extra dimensions of spacetime are shaped like a six-dimensional Calabi–Yau manifold. Such compactifications offer many ways of extracting realistic physics from string theory.<ref>{{cite arXiv|last1=Cvetic|first1=M|authorlink1=Mirjam Cvetič|last2=Halverson|first2=J.|authorlink2=|last3=Shiu|first3=G.|authorlink3=Gary Shiu|last4=Taylor|first4=W.|authorlink4=|date=2022|title=Snowmass White Paper: String Theory and Particle Physics|pages=|arxiv=2204.01742}}</ref> Other similar methods can be used to construct realistic or semi-realistic models of our four-dimensional world based on M-theory.<ref>[[#Yau|Yau and Nadis]], pp. 147–150</ref>

=== Cosmology ===
{{main|String cosmology}}

[[File:WMAP 2012.png|thumb|upright=1.4|A map of the [[cosmic microwave background]] produced by the [[Wilkinson Microwave Anisotropy Probe]]]]

The Big Bang theory is the prevailing [[physical cosmology|cosmological]] model for the universe from the earliest known periods through its subsequent large-scale evolution. Despite its success in explaining many observed features of the universe including galactic [[redshift]]s, the relative abundance of light elements such as [[hydrogen]] and [[helium]], and the existence of a [[cosmic microwave background]], there are several questions that remain unanswered. For example, the standard Big Bang model does not explain why the universe appears to be the same in all directions, why it appears flat on very large distance scales, or why certain hypothesized particles such as [[magnetic monopoles]] are not observed in experiments.<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 530–531</ref>

Currently, the leading candidate for a theory going beyond the Big Bang is the theory of cosmic inflation. Developed by [[Alan Guth]] and others in the 1980s, inflation postulates a period of extremely rapid accelerated expansion of the universe prior to the expansion described by the standard Big Bang theory. The theory of cosmic inflation preserves the successes of the Big Bang while providing a natural explanation for some of the mysterious features of the universe.<ref>[[#Becker|Becker, Becker and Schwarz]], p. 531</ref> The theory has also received striking support from observations of the cosmic microwave background, the radiation that has filled the sky since around 380,000 years after the Big Bang.<ref>[[#Becker|Becker, Becker and Schwarz]], p. 538</ref>

In the theory of inflation, the rapid initial expansion of the universe is caused by a hypothetical particle called the [[inflaton]]. The exact properties of this particle are not fixed by the theory but should ultimately be derived from a more fundamental theory such as string theory.<ref>[[#Becker|Becker, Becker and Schwarz]], p. 533</ref> Indeed, there have been a number of attempts to identify an inflaton within the spectrum of particles described by string theory and to study inflation using string theory. While these approaches might eventually find support in observational data such as measurements of the cosmic microwave background, the application of string theory to cosmology is still in its early stages.<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 539–543</ref>