> times and that all of history, including our own lives, repeats itself.
You have to understand that things are changing very fast in this
field these days. What we don't know far exceeds what we know.
The universe is now thought to evolve, but not just in the generic
sense of becoming more ordered over time. But in the ...Darwinian
sense. The universal constants evolve and adapt over time. Dark
energy and dark matter emerge at different times, guiding the cyclic
evolution of the universe.
To quote Steinhardt, one of the founders of the inflationary theory
and now the cyclic model.
"Most of the energy in the universe is not matter." For its first 300
years, physics has focused on the properties of matter and
radiation, including dark matter. Now we know that they
represent less than 30%% of the composition of the universe.
The rest consists of something we know virtually nothing about.
Most of the energy in the universe is not gravitationally attractive.
We are probably the last generation to have been taught that
"gravity always attracts," a notion which has been presented
as a basic fact of nature for hundreds of years. We are now
aware that gravity can repel, as well. We must rewrite the
textbooks to explain that the gravitationally self-attracting matter
with which we are familiar is the minority in the universe today and
for the indefinite future.
We live at a special moment in cosmic history, the transition between
a decelerating, matter-dominated universe and an accelerating, dark
energy dominated universe.
The recent proposal of a "cyclic" universe presents a whole new outlook on
cosmic history in which dark energy plays a central role (Steinhardt & Turok,
2002a, 2002b). in this model, the conventional cosmic history is turned
topsy-turvy. The big bang is not the beginning of time. Rather, it is a bridge
to a pre-existing contracting era. The Universe undergoes a 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 big bang" is moderated. 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).
Dark energy recurs as the dominant form of energy every cycle roughly
15 billion years after each bang. and it replaces two of the key roles of
inflation. Although it causes the universe to accelerate at an pace
100 orders of magnitude slower than inflation, by maintaining the
acceleration for a trillion years or so, the dark energy homogenizes and
flattens the universe. In particular, it is the dark energy of a
cycle ago that made the universe homogeneous and at prior
to our own big bang,
A second critical feature of the dark energy is that it is not stable.
It naturally decreases with time as the universe expands. As a result, the
acceleration ultimately stops and the universe begins to decelerate.
It eventually triggers a period of contraction, during which
there is the quantum generation of a nearly scale-invariant spectrum of
perturbations that accounts for the temperature fluctuations of the cosmic
microwave background and large scale structure.
Finally, the dark energy is responsible for insuring that the cyclic evolution
is an attractor solution to the evolution equations. If random fluctuations
kick the universe away from the ideal cyclic evolution, the period of
dark energy domination red shifts" away the transient behavior and
drives the universe back towards the regular cyclic solution.
http://wwwphy.princeton.edu/~steinh/steinhardt.pdf
