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The ekpyrotic universe () is a
cosmological model Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of f ...
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 univers ...
that explains the origin of the large-scale structure of the
cosmos The cosmos (, ) is another name for the Universe. Using the word ''cosmos'' implies viewing the universe as a complex and orderly system or entity. The cosmos, and understandings of the reasons for its existence and significance, are studied in ...
. 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 pe ...
word ekpyrosis (ἐκπύρωσις, "conflagration"), which refers to a Stoic 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 The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
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 In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...
,
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 and
Planck satellite ''Planck'' was a space observatory operated by the European Space Agency (ESA) from 2009 to 2013, which mapped the anisotropies of the cosmic microwave background (CMB) at microwave and infrared frequencies, with high sensitivity and small ang ...
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 The multiverse is a hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The dif ...
. This is important because when the effects of quantum fluctuations 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 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.


See also

*
Cosmic inflation In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from  seconds after the conjectured Big Bang singular ...
*
Cyclic model A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 t ...
*
Physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of f ...


Notes and references


Further reading

*
A Brief Introduction to the Ekpyrotic Universe
by Steinhardt, Paul J., Department of Physics, Princeton University. * Greene, Brian, ''The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory'', Vintage (2000). * (the first paper to point out problems with the theory). * Whitehouse, David,
Before the Big Bang
. BBC News. April 10, 2001. * Discover Magazine
Before the Big Bang
February 2004 issue.
Parallel Universes, (BBC Two, 9 February 14, 2002)


* Yi-Fu Cai, Damien A. Easson, Robert Brandenberger
Towards a Nonsingular Bouncing Cosmology
arXiv:1206.2382, (June 2012). {{DEFAULTSORT:Ekpyrotic Universe Astrophysics Physical cosmology