PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 651 August 28, 2003   by Phillip F. Schewe, Ben Stein, and
James Riordon

THE BIG RIP: A NEW COSMIC DOOMSDAY scenario takes the present
acceleration of the expansion of the universe to new extremes.
Dartmouth physicist Robert Caldwell and his colleagues Marc
Kamionkowski and Nevin Weinberg at Caltech have determined that if
the supposed dark energy responsible for the acceleration is potent
enough not only will the space between galaxies continue to increase
but that the galaxies themselves will fly apart as will, at
successive times stars, planets, and even atoms and nuclei.  Since
the acceleration idea became established with astronomers a few
years ago in the wake of observations of distant supernovae, it has
been conventional to apportion the supposed energy inventory of the
universe as follows:  5% in the form of conventional baryon matter
(out of which atoms are made), 25% in the form of dark matter, and
the biggest part, 70%, in the form of dark energy.  Not a lot is
known about dark matter, and even less about dark energy.
Cosmologists have taken to discussing the enigmatic properties of
the dark energy with the use of a new parameter, w, which is the
ratio of its average pressure to energy density.  The degree of this
runaway expansion impulse is expressed by w.  What is the nature of
dark energy and how does it overcome the attractive pull of
gravitation in order to speed up the cosmic expansion, and what is
the proper value of w?  In the best known model, the "cosmological
constant" in Einstein's famous equations of general relativity
corresponds to energy and pressure of the universal quantum vacuum,
and is constant in space and time.  Here the value of w is -1.  In a
second popular model, the "quintessence"model, the dark energy is
associated with a universal quantum field relaxing towards some
final state.  Here the energy density and pressure of the dark
energy are slowly decreasing with time, and the value of w is
somewhere between -1/3 and -1 (w must  be smaller than -1/3 in order
for cosmic acceleration to occur).
In Caldwell's "phantom energy" model, there is no stable vacuum
quantum state and the energy density and the expansionary pressure
exerted on the universe seems to increase even as the spacetime
itself expands (with ordinary gases, pressure falls with
expansion).  In this scenario  w is less than -1. The implications
of this new type of cosmology are that bound systems should in the
course of time be ripped up (see figure at
http://www.aip.org/mgr/png/2003/200.htm ).   For example, at a w
value of -1.5 the universe would last for 35 billion  years before
being ripped apart. About 60 million years before the end, the Milky
Way would be torn apart.  About 3 months before the end the solar
system would become undone.  About 30 minutes before that the Earth
would explode.    And about 10^-19 seconds before the ultimate
moment of doom, atoms would be pulled apart.  Caldwell
(robert.r.caldwell{at}dartmouth.edu, 603-646-2742) suggests that
deciding between this model and the others might be possible in
coming years with much better data coming from microwave background,
supernovae, and galaxy measurements.  (Caldwell et al., Physical
Review Letters, 15 August 2003; text at www.aip.org/physnews/select
)

ULTRACOLD MOLECULAR BOSE GASES, where the gas consists of diatomic
molecules of fermionic atoms (atoms with an overall half-integral
spin value), provide two important opportunities---the chance to do
high-precision spectroscopy of molecules and the chance to study the
process by which fermions (normally unable to form into coherent
quantum condensates) amalgamate into pairs.  The pairs are bosons
(entities with a whole-number valued spin) and can form
condensates.  Randy Hulet and his colleagues at Rice University, the
first to engineer a Bose Einstein condensation (BEC) in lithium-7
atoms (http://www.aip.org/enews/physnews/1995/split/pnu237-1.htm ),
have gotten a batch of Li-6 atoms to pair up (at least 50% of them
at a time) at micro-kelvin temperatures by manipulating an external
magnetic field.  Although the group does not yet have evidence that
the pairs, or molecules, have taken the final plunge by forming a
BEC, the atoms have held together (in an optical trap) in their
paired state for as long as 1 second, compared to millisecond times
for previous experiments of this type.  Hulet hopes that as the
molecular gas hangs together long enough, it will cool off
sufficiently through the evaporative process to form a BEC.   Having
a true BEC of molecules would give researchers the chance to study
the Cooper pairing mechanism at work in superconductivity and in
superfluidity of liquid helium-3.  In ordinary molecules (joined by
chemical forces) the constituents (atoms) are very close together.
In the Cooper pairs characterizing superconductivity, the
constituents (electrons) are only weakly coupled and are far apart
from each other.  Hulet and his group hope to dissociate the
molecular condensate in order to produce Cooper pairs that fall in
between these two cases, both as to the size and in the strength of
the force holding the pairs together.  One might even be able to
simulate high-temperature superconductivity by loading ultracold
fermion gases into an "optical lattice" consisting of crossed laser
beams.  (Strecker et al., Physical Review Letters, 22 August 2003;
see figure at http://www.aip.org/mgr/png/2003/199.htm and lab
website at http://atomcool.rice.edu; text at
www.aip.org/physnews/select )

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