The
present standard model of cosmology tells us that the universe started with an
initial singularity that produced the Big Bang, then expanded and cooled off
enough to permit the formation of particles, atoms, molecules, stars, planets,
and galaxies, and that it will expand at an accelerating rate until matter
becomes so diffuse that all such structure will come to an end, leaving the
universe a dark, empty, boring place forever that will ultimately contain only a
thin veneer of photons and leptons.However,
this is not the only story that fits the observable facts, and two recent works
in the field of cosmology have explored some interesting alternative scenarios
of the past and future of the universe.

Alexander
Vilenkin's group at
Tufts
University
has considered three alternatives to the standard model for how the universe
began, all hypothesizing that the universe has been in existence forever.These are: (1) a universe undergoing eternal expansion, (2) a universe
undergoing infinite repeating cycles, and (3) an “emergent” universe that
had existed for an infinite time in the past as a small static clump before
expanding into the present universe.Let’s
take these one at a time.

The
standard model describes the present universe as a number of isolated regions,
each in thermal equilibrium, all embedded is a continually expanding background
of underlying space-time.The
boundaries of the isolated regions (one of which we call our universe) expand
into the background at nearly the speed of light, but the space-time expansion
itself is even faster, so that the regions never touch, collide, or merge.It is not possible to send a signal from one of the regions to another,
once they are isolated by the expansion.This
picture appears to apply today, and (barring a Big Rip, see below) into the
infinite future.This raises the
question of whether the eternal expansion can be extended backwards, into the
infinite past.

Vilenkin’s
group has examined this question and has made a few mild and very plausible
assumptions about it.From these
they have proved mathematically that extension to negative infinity is
“incomplete” and that somewhere in the past there must be a singularity that
starts the expansion.Thus, even in
this seemingly eternal scenario, the universe must have had a beginning.

One
version of the cyclic scenario, the “clapping branes” model of Steinhardt
and Turok, was the subject of my
column in the November 2002 issue of Analog.Basically,
the idea is that two parallel extra-dimensional “branes” from superstring
theory, one of which is our universe, periodically separate and then collide,
with each “clap” creating conditions that closely resemble the immediate
aftermath of the Big Bang.The
resulting universe would be consistent with all of our observational details
about the cosmos, from primordial nucleosynthesis, the cosmic microwave
background, galaxy formation, etc, with one important exception.The large amount of energy that the standard model predicts should
present in the form of primordial gravitational waves from the Big Bang would be
absent in the Steinhardt/Turok brane-clapping universe, and the details of the
polarization of the cosmic microwave background would also be slightly
different.However, since we have
not yet been able to observe either the fine structure of the CMB polarization
or any gravitational waves, the Steinhardt/Turok model cannot (yet) be ruled
out.

Vilenkin’s
group has studied the class of recycling universe scenarios, of which the
Steinhardt/Turok model is a leading example.They conclude that such models also cannot be extrapolated into the
infinite past.Again, somewhere in
the past there must be a singularity that starts the clapping brane process, and
that represents a beginning point in time.

The
third scenario, the emergent universe or “cosmic egg” model, depicts the
universe as existing in a small dormant state back into the infinite past, until
some random fluctuation “hatches” the cosmic egg and the Big Bang commences.Vilenkin’s group has shown the same mathematical logic applies to this
model, and that it cannot be extrapolated backwards into the infinite past.The “egg” would have to begin with some singularity that brought it
into existence.

The
bottom line from these considerations is that, even in scenarios that seem to
extend into the infinite past, there must be some starting point.The two-way time-symmetry of a creation scenario that extends into both
the infinite past and the infinite future has been ruled out.The universe must have a starting point.If that is the case, the standard model’s singularity that started the
Big Bang would seem to me to be as good a choice for a starting point as any
other.

So
if the universe is required to have a start, what about its finish?Can it go on for an infinite time, as the current standard model of
cosmology seems to imply?A recent
paper by Li, Wang, Huang, Zhang, and Li (here abbreviated LWHZL) working at five
academic institutions in China
examined this question by considering the action of dark energy.Recent observations of Type 1a supernovas and of the detailed structure
of the cosmic microwave background radiation are now conventionally interpreted
as showing that about 74% of the mass-energy in our universe is in the form of
dark energy.Dark energy is related
to Einstein’s cosmological constant as a form of energy intrinsic to space
itself, which increases when a volume of space increases due to cosmological
expansion.The dominance of dark
energy in the universe produces a repulsive pressure that drives the observed
accelerating expansion of the universe.

As
described in my column “The Big Rip at the End of Time” in the March-2005
issue of Analog, despite settling
on a standard model, there is not universal consensus among the theorists
working in general relativity and cosmology about just what the equation of
state of the universe is and how the density of dark energy is related to the
size and accelerated expansion of the universe.In particular, the equation of state parameter w, a dimensionless number
representing the pressure produced by dark energy divided by its density,
assumed by Einstein to have a constant value of -1, could
be time-dependent and/or could have other values that might lead to disastrous
and spectacular consequences for the fate of the universe.In particular, for values of w
more negative than -1,
the repulsion of dark energy will grow as the universe expands until its force
is large enough to rip apart the galaxy, unbind the Solar System, detach the
Moon, explode the Sun and the Earth, dissociate atoms, and rip apart nuclei.

For
estimating w there is relevant
information from astrophysical observations: Type 1a supernovas, the cosmic
microwave background, the Sloan Digital Sky Survey data, and the Hubble Space
Telescope estimates of the value of the Hubble constant.These can lead to estimates of the value of w
in the present era and in eras in the past.However, the information content of such data is capable of determining
no more than two independent parameters, so various two-parameter models have
been proposed by cosmologists.The
technique for using such models is to separate the available observational data
into “bins”, i.e., groups characteristic of a particular range of values for
the red-shift factor z,
and attempt to determine if w
is constant with z or is changing in a particular way.LWHZL criticizes several other two-parameter models for characterizing w
as containing unacceptable divergences, and they focus on the divergence-free
model given by the equation: w(z)
= w_{0} + w_{a}[ln(2+z)/(1+z) – ln(2)],
where ln is a natural logarithm and w_{0
}and w_{a}
are the two parameters to be determined from the data.LWHZL then fits the available data with this model, and they extract the
parameter values: w_{0}=
-1.0862+0.4689-0.2993
and w_{a}=
-0.0567+13.8399-2.2345.Here the plus and minus errors represent possible excursions in the
reported value that fall within one standard deviation of the central value.

The
quintessence model of Steinhardt is disfavored by this analysis.It indicates that while the parameter values consistent with Einstein’s
cosmological constant model (w_{0}=
-1,
w_{a}=0)
fall just within the 68.3% confidence limits of the analysis, the best fit to
the data is obtained around w_{0}=
-1.09
and w_{a}=
-0.06,
i.e., w is slightly more negative than -1 and its
negativity is increasing as the universe expands.These parameter values, if taken seriously, would imply that our universe
is doomed, and that in particular it is in for a Big Rip in times to come.

LWHZL
then calculates just when the arrival of the Big Rip of doomsday is likely to
occur.Fore Einstein’s values, the
universe would last forever, but for their best fit values, the Big Rip would
come about 103.5 billion years in the future.The soonest it might come, using the values at the “fast” edge of the
95.4% confidence limit, would be about 16.7 billion years.

Writers
of SF may be interested in just what occurs when this doomsday comes, as our
world and our universe are ripped apart by the repulsive effects of the growing
dark energy.About 33 million years
before the end in the final stages of the Big Rip, our Milky Way galaxy comes
apart.About 2 months before the
end, the Solar system becomes unbound and the planets detach from the Sun and go
their separate ways.About 5 days
before the end, the Moon leaves its orbit around the Earth.About 28 minutes before the end the Sun becomes gravitationally unbound
and explodes.At 16 minutes before
the end the Earth also explodes.At
3 x
10^{-17}
seconds before the end, all the electrons come loose from their atoms and
chemistry ends.Just a bit later
nuclei come apart into clouds of neutrons and protons, and a bit later still the
neutrons and protons come apart into quarks.

You
wouldn’t want to stay around for that.

So
there you have it.Recent work in
cosmology indicates that our universe must have had a definite starting point,
and may have a well-defined ending point.Nothing
is forever, not even universes.

SF
Novels by John Cramer:my two hard SF novels,TwistorandEinstein's Bridge, are newly released as eBooks and are
available at :http://bookviewcafe.com/bookstore/?s=Cramer
.

AV
Columns Online:
Electronic reprints of about 170 "The Alternate View" columns by
John G. Cramer, previously published in Analog
, are available online at: http://www.npl.washington.edu/av.

References:

The
Start of the Universe:

“Did the universe
have a beginning?”,Audrey Mithani and Alexander Vilenkin, 20 April, 2012,
arXiv 1204.4658v1 [hep-th].

The
Finish of the Universe:

“Dark
Energy and the Fate of the Universe”, X-D Li,
S. Wang
, Q-G Huang, X. Zhang, and M. Li, 24 April, 2012, arXiv 1202.4060v2 [astro-ph.CO].

SF
Novels by John Cramer:my two hard SF novels,TwistorandEinstein's Bridge, are newly released as eBooks and are
available at :http://bookviewcafe.com/bookstore/?s=Cramer
.

AV
Columns Online: Electronic
reprints of about 170 "The Alternate View" columns by John G. Cramer,
previously published in Analog
, are available online at: http://www.npl.washington.edu/av.