Ever wonder whether the Big Bang isn't a trendy or paradigmatic
cosmology? After all, many physicists in the first half of the 20th
century were occupied with building big bangs of a different sort.

Is it possible that we have multiple big bangs, at perhaps the galactic
level but not generally beyond this (never involving all available
matter/energy), and no singular beginning or end to all matter/energy,
but just a bunch of localized recycling?


Jonathan wrote:

The universe as an attractor solution.

s





The Endless Universe:
A Brief Introduction to the Cyclic Universe

Paul J. Steinhardt
Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544, USA

http://wwwphy.princeton.edu/~steinh/cyclintro/

http://wwwphy.princeton.edu/~steinh/


Over the last century, cosmologists have converged on a highly successful theory of
the evolution of the Universe - the big bang/inflationary picture.[1] According to
this picture, space and time sprung into being 15 billion years ago in a `big bang.'
When the Universe emerged, it was filled with particles and radiation of nearly
infinite temperature and density. Instants later, the Universe underwent a period of
extraordinarily rapid, superluminal expansion (`inflation') which made the Universe
homogeneous and flat and which created fluctuations that seeded the formation of
galaxies and large-scale structure.

In the last decades, cosmological observations have supported the predictions of the
big bang and inflationary theory in exquisite detail.[1,5] They have also provided one
major surprise. It appears that, billions of years after the big bang, following the
formation of galaxies, the Universe was overtaken by some form of dark energy that is
causing the expansion rate to accelerate. Although dark energy was unanticipated and
has no particular role in the big bang/inflationary picture, the general view has been
that it can simply be added by fiat to the initial make-up of the Universe. There is
no compelling reason for a new theoretical approach. Quite the contrary, many
cosmologists regard the basic cosmic story as being settled.

In this context, a new paradigm has been recently proposed by Paul Steinhardt
(Princeton) and Neil Turok (Cambridge) - the cyclic universe - that turns the
conventional picture topsy-turvy. (Perhaps the model should be called an old paradigm
since it reinvigorates ancient cosmic mythologies and philosophies, albeit using the
tools of 21st century physics.) In this picture, space and time exist forever. The big
bang is not the beginning of time. Rather, it is a bridge to a pre-existing
contracting era. The Universe undergoes an endless sequence of cycles in which it
contracts in a big crunch and re-emerges in an expanding big bang, with trillions of
years of evolution in between. The temperature and density of the universe do not
become infinite at any point in the cycle; indeed, they never exceed a finite bound
(about a trillion trillion degrees). No inflation has taken place since the big bang.
The current homogeneity and flatness were created by events that occurred before the
most recent big bang. The seeds for galaxy formation were created by instabilities
arising as the Universe was collapsing towards a big crunch, prior to our big bang.

The prospects for an alternative cosmology that is so different from the
well-established convention would seem extremely dim. Yet, the cyclic model recoups
all of the successful predictions of the big bang/inflationary theory and has
sufficient additional predictive power to address many questions which the big
bang/inflationary model does not address at all: What occurred at the initial
singularity? What is the ultimate fate of the Universe? What is the role of dark
energy and the recently observed cosmic acceleration? Does time, and the arrow of
time, exist before the big bang? or after the big crunch?

In the new paradigm, each cycle proceeds through a period of radiation and matter
domination consistent with standard cosmology, producing the observed primordial
abundance of elements, the cosmic microwave background, the expansion of galaxies,
etc. For the next trillion years or more, the Universe undergoes a period of slow
cosmic acceleration (as detected in recent observations[1]) which ultimately empties
the Universe of all of the entropy and black holes produced in the preceding cycle and
triggers the events that lead to contraction and a big crunch. Note that dark energy
is not simply added on - it plays an essential role. The transition from big crunch to
big bang automatically replenishes the Universe by creating new matter and radiation.
Gravity and the transition from big crunch to big bang keep the cycles going forever.
In fact, as will be discussed, the cyclic behavior is a strong attractor. That is,
even if the Universe were disrupted from its periodic behavior, it would rapidly
reconverge to the cyclic solution.

The linchpin to the new paradigm is the transition from big crunch to big bang. The
transition was thought to be an impossible passage in which the laws of physics blow
up. However, recent developments in superstring theory suggest that the cosmic
singularity is otherwise, as the two authors have argued in a recent paper with Justin
Khoury (Princeton), Burt Ovrut (Penn) and Nathan Seiberg (IAS). Superstring theory
relies on the idea that the Universe contains nine or ten spatial dimensions,
depending on the formulation, all but three of which are curled up in a compact
manifold of microscopic size. In this framework, the big bang and big crunch may be an
illusion. Expressed in the usual variables of general relativity, it may appear that
our usual space and time are disappearing. However, viewed with the proper variables,
our usual space dimensions actually remain infinite and time runs continuously. The
transition from big crunch to big bang is due, instead, to the collapse, bounce and
re-expansion of one of the extra dimensions. For example, in a variant known as M
theory, the Universe consists of two branes (surfaces) bounding an extra dimension,
and the singularity corresponds to a collision and bounce of the two branes. The
temperature and density of ordinary radiation and matter remain finite at the bounce,
and particles move continuously in a natural and intuitive way. By dispelling the myth
that the big bang is a beginning of space and time, superstring theory opens up new
possibilities for the cosmological history of the Universe. Six months ago, the
``ekpyrotic model"[4] was proposed by Khoury, Ovrut, Steinhardt and Turok as one new
possibility based on the idea of making a universe from a single collapse of the extra
dimension. The cyclic model builds on lessons learned from the ekpyrotic example to
produce a picture with remarkable predictive and explanatory power.