Ekpyrotic model

The ekpyrotic universe (/ˌɛkpɪˈrɑːtɪk/, AHD: /ĕk′pī-rŏt′ĭk/) is a cosmological model of the early universe that explains the origin of the large-scale structure of the cosmos. 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.

The original ekpyrotic model was introduced by Justin Khoury, Burt Ovrut, Paul Steinhardt and Neil Turok in 2001. Steinhardt created the name based on the early word ekpyrosis (Ancient Greek: ἐκπύρωσις, ekpurōsis, "conflagration"); it refers to an ancient Stoic cosmological model in which the universe is caught in an eternal cycle of fiery birth, cooling and rebirth.

The name is well-suited to the theory, which 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.

The original ekpyrotic models relied on string theory, branes and extra dimensions, but most contemporary ekpyrotic and cyclic models use the same physical ingredients as inflationary models (quantum fields evolving in ordinary space-time). The theory has shown impressive success in accurately describing what we know so far about our universe. It predicts a uniform, flat universe with patterns of hot spots and cold spots now visible in the cosmic microwave background (CMB), and has been confirmed by the WMAP and Planck satellite experiments. Discovery of the CMB was originally considered a landmark test of the big bang, but proponents of the ekpyrotic and cyclic theories have shown that the CMB is also consistent with a big bounce, as posited by the ekpyrotic and cyclic theories. The search for primordial gravitational waves in the CMB (which produce patterns of polarized light known as B-modes) may eventually help scientists distinguish between the rival theories, since the ekpyrotic and cyclic models predict that no B-mode patterns should be observed.

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