Number 651, August 28, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
The Big Rip: A New Cosmic Doomsday
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). 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
(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)
Ultracold Molecular Bose Gases
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 (see PNU
#237), 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 and lab website)
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