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.

Introduction

Brane cosmology assumes that the visible universe lies on a four-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.

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 before the big bang, in an expanding universe.

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.

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.

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.

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

Compare against: cosmic inflation, cyclic 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/