The ekpyrotic universe or ekpyrotic scenario is a cosmological theory of the origin of the universe. The name comes from a Stoic term for "out of fire". The ekpyrotic model of the universe is an alternative to the standard cosmic inflation paradigm, both of which accept that the standard big bang Lambda-CDM model of our universe is an appropriate description up to very early times. The ekpyrotic model is a precursor to, and part of the cyclic model. Cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. ... The word theory has a number of distinct meanings in different fields of knowledge, depending on the context and their methodologies. ... The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field. ... Stoicism is a school of philosophy commonly associated with such Greek philosophers as Zeno of Citium, Cleanthes, or Chrysippus and with such later Romans as Cicero, Seneca, Marcus Aurelius, and Epictetus. ... Cosmic inflation is the idea, first proposed by Alan Guth in 1981, that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ... According to the Big Bang theory, the universe emerged from an extremely dense and hot state (bottom). ... A pie chart indicating the proportional composition of different energy-density components of the universe. ... The cyclic model is a brane cosmology model of the creation of the universe, derived from the earlier ekpyrotic model. ...

Introduction

Brane cosmology assumes that the visible universe lies on a three-dimensional brane which moves in higher dimensional space. Our brane may be one of innumerable others moving through these extra dimensions. The ekpyrotic scenario was proposed by Khoury, Ovrut, Steinhardt and Turok in 2001. It suggests that the visible universe was empty and contracting in the distant past. At some time, our brane collided with another, parallel "hidden" brane, which caused the contracting universe to reverse and begin expanding. Hot matter and radiation was created in the collision, which started the hot big bang from which the present-day universe originated. The brane collision, from the four-dimensional perspective of the visible brane, looks like a big crunch followed by a big bang. Brane cosmology is a protoscience motivated by, but not rigorously derived from, superstring theory and M-theory. ... Branes are objects in M-theory and its offshoot, brane cosmology. ... This page may meet Wikipedias criteria for speedy deletion. ... Paul J. Steinhardt is the Albert Einstein Professor of Science at Princeton University and a professor of theoretical physics. ... Neil Geoffrey Turok (1967) is a South African cosmologist. ... 2001: A Space Odyssey. ... According to the Big Bang theory, the universe emerged from an extremely dense and hot state (bottom). ... In cosmology, the Big Crunch is a hypothesized collapse of the universe upon itself after its expansion eventually stops — a counterpart to the Big Bang. ...

The scenario is appealing because it replaces cosmic inflation with a theory that achieves many of the same successes in a framework that seems compatible with string theory. An important distinction between the ekpyrotic scenario and cosmic inflation is that in the ekpyrotic scenario, the primordial nearly scale invariant spectrum of quantum vacuum fluctuations, which is the seed for all structure in the universe today, is generated in a contracting universe, before the big crunch. In cosmic inflation they are generated immediately after the big bang, in an expanding universe. Cosmic inflation is the idea, first proposed by Alan Guth in 1981, that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ... Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles... In physics, scale invariance is the feature of physical objects of laws that do not change if the space is magnified, i. ... In physics, a virtual particle is a particle-like abstraction used in some models of quantum field theory. ... Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ...

Outstanding problems

There are major outstanding problems with the ekpyrotic scenario. Foremost among them is that colliding branes are not understood by string theorists, and nobody knows if the scale invariant spectrum will be destroyed by the big crunch, or even what happens when two branes collide. Moreover, like cosmic inflation, while the general character of the forces (in the ekpyrotic scenario, a force between branes) required to create the vacuum fluctuations is known, there is no candidate from particle physics. Moreover, the scenario uses some essential ideas from string theory, principally extra dimensions, branes and orbifolds. String theory itself is a controversial idea in the physics community. Particles explode from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... Kaluza-Klein theory (or KK theory, for short) is a model which sought to unify classical gravity and electromagnetism. ... Branes are objects in M-theory and its offshoot, brane cosmology. ... In topology and group theory, an orbifold (for orbit-manifold) is a generalization of a manifold. ...

Detractors of the original ekpyrotic scenario point out that it requires fine-tuned, nearly supersymmetric initial conditions, and thus replaces the problems solved by cosmic inflation with a new set of problems. The original scenario has been supplanted by a new more flexible and successful set of ideas, embodied in the cyclic model. In theoretical physics, fine-tuning is a necessary procedure of fudging and very accurate adjusting of the values of the parameters of a theory in order for various physical quantities to be very small. ... This article or section is in need of attention from an expert on the subject. ...

Popular reception

Despite these disputes, the ekpyrotic scenario has received considerable attention in the astrophysical and particle physics communities. Regardless of whether it is a correct model of the origin of the universe, it is an excellent indication of the new possibilities opened up by the development of brane cosmology. Spiral Galaxy ESO 269-57 Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature and chemical composition) of celestial objects such as stars, galaxies, and the interstellar medium, as well as their interactions. ... Particles explode from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... Brane cosmology is a protoscience motivated by, but not rigorously derived from, superstring theory and M-theory. ...

The ekpyrotic model has received considerable coverage in the media [1], [2], [3], [4].

Compare with: cosmic inflation, cyclic model. Cosmic inflation is the idea, first proposed by Alan Guth in 1981, that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ... The cyclic model is a brane cosmology model of the creation of the universe, derived from the earlier ekpyrotic model. ...

External links

• Britt, Robert Roy, "'Brane-Storm' Challenges Part of Big Bang Theory".
• Greene, Brian, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory, Vintage (2000).
• NOVA/PBS, "The Elegant Universe", especially hour 3, chapter 6.
• Khoury, Justin,"A Briefing on the Ekpyrotic/Cyclic Universe". Columbia Univ. Mar 2004. Astrophysics, astro-ph/0401579.
• Khoury, Justin, Burt A. Ovrut, Paul J. Steinhardt, and Neil Turok, "Density Perturbations in the Ekpyrotic Scenario". High Energy Physics - Theory, hep-th/0109050. Mar 2002, Phys.Rev. (2002) 046005
• Linde, Andre (2002). Inflationary Theory versus Ekpyrotic/Cyclic Scenario. Presented at Stephen Hawking's 60th birthday conference. (online)
• Khoury, Justin, Ovrut, Burt A., Steinhardt, Paul J. and Turok. Neil, The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang (the original article)
• Kallosh, Renata, Kofman, Lev and Linde, Andrei, Pyrotechnic Universe (the first paper to point out problems with the theory)
• Steinhardt, Paul J., Department of Physics, Princeton University http://feynman.princeton.edu/~steinh/
• Whitehouse, David, "Before the Big Bang". BBC News. April 10, 2001.
• Ekpyrotic universe on arxiv.org