Cosmologically speaking, diamonds may actually be forever (Forwarded)

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Vanderbilt University
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4-25-2007

Cosmologically speaking, diamonds may actually be forever

If you've ever wondered about the ultimate fate of the universe, Lawrence
Krauss and Robert Scherrer have some good news ... sort of.

Writing in the journal Physical Review D, the two physicists show that
matter as we know it will remain as the universe expands at an
ever-increasing clip. That is, the current status quo between matter and
its alter ego, radiation, will continue as the newly discovered force of
dark energy pushes the universe apart.

"Diamonds may actually be forever," quips Krauss, professor of physics and
astronomy at Case Western Reserve University (CWRU) who is spending the
year at Vanderbilt. "One of the only positive things that has arisen from
the dark-energy dominated universe is that matter gets to beat radiation
forever!"

Although this may not sound surprising, it actually runs contrary to
conventional wisdom among cosmologists. Today, there is more matter than
radiation in the universe. But there were periods during the early
universe that were dominated by radiation due to particle decays. The
generally accepted view of the distant future has been that ordinary
matter particles -- protons and neutrons in particular -- will gradually
decay into radiation over trillions upon trillions of years, leaving a
universe in which radiation once again dominates over matter; a universe
lacking the material structures that are necessary for life.

It is only in the last decade that the existence of dark energy has been
recognized. Before that Krauss and collaborators argued for its existence
based on indirect evidence, but the first direct evidence came in 1998
when a major survey of exploding stars, called supernovae, revealed that
the universe is apparently expanding at an increasing rate. Dark energy
acts as a kind of anti-gravity that drives the expansion of the universe
at large scales. Because it is associated with space itself, it is also
called "vacuum energy." A number of follow-up observations have supported
the conclusion that dark energy accounts for about 70 percent of all the
energy in the universe.

"The discovery of dark energy has changed everything, but it has changed
the view of the future more than the past. It is among the worst of all
possible futures for life," says Krauss, who has spent the last few years
exploring its implications. In an eternally expanding universe there is at
least a chance that life could endure forever, but not in a universe
dominated by vacuum energy, Krauss and CWRU collaborator Glenn Starkman
have concluded.

As the universe expands, the most distant objects recede at the highest
velocity. The faster that objects recede, the more that the light coming
from them is "red-shifted" to longer wavelengths. When their recessional
velocity reaches light speed, they disappear because they are traveling
away faster than the light that they emit. According to Krauss and
Starkman, the process of disappearance has already begun: There are
objects that were visible when the universe was half its present age that
are invisible now. However, the process won't become really noticeable
until the universe is about 100 billion years old. By ten trillion years,
nothing but our local cluster of galaxies will be visible.

From the perspective of future civilizations, this process puts a finite
limit on the amount of information and energy that will be available to
maintain life. Assuming that consciousness is a physical phenomenon, this
implies that life itself cannot be eternal, Krauss and Starkman argue.

"Our current study doesn't change the process, but it does make it a
little friendlier for matter and less friendly for radiation," says
Scherrer, professor of physics at Vanderbilt.

In their paper, Krauss and Scherrer analyzed all the ways that ordinary
matter and dark matter could decay into radiation. (Dark matter is
different from dark energy. It is an unknown form of matter that
astronomers have only been able to detect by its gravitational effect on
the ordinary matter in nearby galaxies. At this point, the physicists have
no idea whether it is stable or will ultimately decay like ordinary
matter.) Given known constraints on these various decay processes, the two
show that none of them can produce radiation densities that exceed the
density of the remaining matter. This is counter-intuitive because, when
matter turns into energy, it does so according to Einstein's equation,
E=mc2, and produces copious amounts of energy.

"The surprising thing is that radiation disappears as fast as it is
created in a universe with dark energy," says Krauss.

The reason for radiation's vanishing act involves the expansion of space.
Expanding space diminishes the density of radiant energy in two ways. The
first is by increasing the separation between individual photons. The
second is by reducing the amount of energy carried by individual photons.
A photon's energy is contained entirely in its electromagnetic field. The
shorter its wavelength and the higher its frequency, the more energy it
contains. As space itself expands, the wavelengths of all the photons
within it lengthen and their frequency drops. This means that the amount
energy that individual photons contain also decreases. Taken together,
these two effects dramatically reduce the energy density of radiation.

Protons and neutrons, by contrast, only suffer from the separation effect.
Most of the energy that they carry is bound up in their mass and is not
affected by spatial expansion. In an accelerating universe, that is enough
of an advantage to maintain matter's dominance ... forever.

NOTE: A multimedia version of this story is available on Exploration,
Vanderbilt's online research magazine, at
http://www.vanderbilt.edu/exploratio...matterdom.html