The ekpyrotic universe () is a cosmological model of the early

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...

that explains the origin of the large-scale structure of the

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. The model has also been incorporated in the cyclic universe theory (or ekpyrotic cyclic universe theory), which proposes a complete cosmological history, both the past and future.

Origins

The original ekpyrotic model was introduced by Justin Khoury, Burt Ovrut,

Paul Steinhardt Paul Joseph Steinhardt (born December 25, 1952) is an American theoretical physicist whose principal research is in cosmology and condensed matter physics. He is currently the Albert Einstein Professor in Science at Princeton University, where ...

and

Neil Turok Neil Geoffrey Turok (born 16 November 1958) is a South African physicist. He holds the Higgs Chair of Theoretical Physics at the University of Edinburgh since 2020, and has been director emeritus of the Perimeter Institute for Theoretical Physi ...

in 2001. Steinhardt created the name based on the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

word ekpyrosis (ἐκπύρωσις, "conflagration"), which refers to a

Stoic Stoic may refer to: * An adherent of Stoicism; one whose moral quality is associated with that school of philosophy * STOIC, a programming language * ''Stoic'' (film), a 2009 film by Uwe Boll * ''Stoic'' (mixtape), a 2012 mixtape by rapper T-Pain * ...

cosmological model in which the universe is caught in an eternal cycle of fiery birth, cooling and rebirth. The theory addresses the fundamental question that remains unanswered by the Big Bang inflationary model, "What happened before the Big Bang?" The explanation, according to the ekpyrotic theory, is that the Big Bang was actually a big bounce, a transition from a previous epoch of contraction to the present epoch of expansion. The key events that shaped our universe occurred before the bounce, and, in a cyclic version, the universe bounces at regular intervals.

Applications of the theory

The original ekpyrotic models relied on string theory,

branes In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. Branes are dynamical objects which can propagate through spacetime accordin ...

and

extra dimension In physics, extra dimensions are proposed additional space or time dimensions beyond the (3 + 1) typical of observed spacetime, such as the first attempts based on the Kaluza–Klein theory. Among theories proposing extra dimensions are: ...

s, but most contemporary ekpyrotic and cyclic models use the same physical ingredients as inflationary models (quantum fields evolving in ordinary space-time). Like Big Bang cosmology, the ekpyrotic theory has accurately described essential features of our universe. It predicts a

uniform A uniform is a variety of clothing worn by members of an organization while participating in that organization's activity. Modern uniforms are most often worn by armed forces and paramilitary organizations such as police, emergency services, ...

flat universe The shape of the universe, in physical cosmology, is the local and global geometry of the universe. The local features of the geometry of the universe are primarily described by its curvature, whereas the topology of the universe describes gen ...

with patterns of hot spots and cold spots, in agreement with observations of the

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

(CMB), observations confirmed to higher precision by the

WMAP The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...

and Planck satellite experiments. Observation of a CMB has long been considered evidence of the Big Bang, but proponents of the ekpyrotic and cyclic theories contend that the CMB is also consistent with a Big Bounce as posited in those models. Other researchers argue that data from the Planck observations of the CMB "significantly limit the viable parameter space of the ekpyrotic/cyclic scenarios." Primordial gravitational waves, if ever observed, may help scientists distinguish between various theories about the origin of the universe.

Implications for cosmology

An advantage of ekpyrotic and cyclic models is that they do not produce a multiverse. This is important because when the effects of

quantum fluctuations In quantum physics, a quantum fluctuation (also known as a vacuum state fluctuation or vacuum fluctuation) is the temporary random change in the amount of energy in a point in space, as prescribed by Werner Heisenberg's uncertainty principle. ...

are properly included in the Big Bang inflationary model, they prevent the universe from achieving the uniformity and flatness that the cosmologists are trying to explain. Instead, inflated quantum fluctuations cause the universe to break up into patches with every conceivable combination of physical properties. Instead of making clear predictions, the Big Bang inflationary theory allows any outcome, so that the properties we observe may be viewed as random chance, resulting from the particular patch of the multiverse in which the Earth resides. Most regions of the multiverse would have very different properties. Nobel laureate Steven Weinberg has suggested that if the multiverse is true, “the hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our Big Bang is doomed, for their values would be an accident of the particular part of the multiverse in which we live.” The idea that the properties of our universe are an accident and come from a theory that allows a multiverse of other possibilities is hard to reconcile with the fact that the universe is extraordinarily simple (uniform and flat) on large scales and that elementary

particles In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

appear to be described by simple symmetries and interactions. Also, the accidental concept cannot be falsified by an experiment since any future experiments can be viewed as yet other accidental aspects. In ekpyrotic and cyclic models, smoothing and flattening occurs during a period of slow contraction, so quantum fluctuations are not inflated and cannot produce a multiverse. As a result, the ekpyrotic and cyclic models predict simple physical properties that are consistent with current experimental evidence without producing a multiverse.

Origins

Applications of the theory

Implications for cosmology

See also

Notes and references

Further reading