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 fu ...

of the early

universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...

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

cosmos The cosmos (, ; ) is an alternative name for the universe or its nature or order. Usage of the word ''cosmos'' implies viewing the universe as a complex and orderly system or entity. The cosmos is studied in cosmologya broad discipline covering ...

. 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 Professorship in Science, Albert Einstein Professor in ...

and Neil Turok 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 classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

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 is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

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 and

extra dimension In physics, extra dimensions or extra-dimensional spaces are proposed as additional space or time dimensions beyond the (3 + 1) typical of observed spacetime — meaning 5-dimensional or higher. such as the first attempts based on the Ka ...

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 costume worn by members of an organization while usually participating in that organization's activity. Modern uniforms are most often worn by armed forces and paramilitary organizations such as police, emergency serv ...

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

cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

(CMB), observations confirmed to higher precision by the WMAP 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 The multiverse is the hypothetical set of all universes. Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describ ...

. 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 In the physical sciences, a particle (or corpuscle 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