I found these press releases interesting
Office of Communications Stanhope Hall, Princeton, New Jersey
08544-5264 Telephone 609-258-3601; Fax 609-258-1301
Contact: Justin Harmon, (609) 258-3601
Date: May 27, 1999
Scientists Conclude Anti-Gravity Force Is Accelerating Expansion of the Universe
PRINCETON, N.J. -- After reviewing recent astronomical observations, Princeton
scientists have concluded that the evidence strongly supports the existence of a
mysterious anti-gravity force that is causing the expansion of the universe to accelerate.
They presented their argument in a review article that will be published in the May 28
edition of the journal Science. The researchers are Neta Bahcall and Jeremiah Ostriker of
the Department of Astrophysics and Paul Steinhardt of the Department of Physics, in
collaboration with Saul Perlmutter of Berkeley National Laboratory.
Scientists have known since the 1920s that the universe is expanding, and they
discovered in the last year that the expansion is likely to go on forever. In recent
months, however, evidence has emerged to suggest that not only will the expansion
continue, it will accelerate. The only way to account for such acceleration is the
existence of a force to counteract the gravitational forces that would stabilize or shrink
The Princeton scientists have now bolstered that idea by reconciling three independent
sets of data and showing that the data have a surprising degree of agreement. The data,
some of which was generated at Princeton, have been used to answer three questions: How
much matter is in the universe? Is the expansion rate slowing down or speeding up? And, is
the universe flat? The Princeton scientists used a framework they call the "Cosmic
Triangle," to relate the three questions and show for the first time how they merge
into a unified picture of a universe that is flat, lightweight and expanding at an
"It's a very exciting time because we are starting to reveal the status of the
universe and it tells us something very unexpected," says Bahcall. It is the
acceleration idea that is most surprising, she says. Bahcall cautions, however, that these
conjectures must be confirmed by further improvements in the data, which are expected to
come from a variety of sources over the next few years.
The expansion of the universe can be described in terms of a car coasting along a road
as a result of a big push (the Big Bang). The mass of the universe, with the gravitational
pull it exerts, is analogous to the friction and wind resistance that slow the car. In
this analogy, there is so little resistance (gravitational tug) that the car never stops.
The only way it could accelerate is if it were rolling downhill or if someone were
depressing the gas pedal. The new force in the universe is like the downhill tug or an
engine pushing the car.
"The evidence is now getting stronger that there really is a force in the universe
that competes with gravity and causes repulsion instead of attraction," says
To account for this force, referred to as cosmic dark energy, scientists recently have
revived a concept called the cosmological constant. In their paper, the Princeton
scientists describe this cosmic dark energy as "a vacuum energy assigned to empty
space itself, a form of energy with negative pressure." Einstein first introduced the
cosmological constant in 1917, but later withdrew it, calling it the worst mistake of his
life. Understanding the source and nature of this force poses deep new problems for
physicists. "It's of very profound physical significance," says Ostriker.
The work to explain the source of this force already has begun. Steinhardt, a
co-author, recently introduced a possible new force called quintessence, which may account
for the dark energy.
Another implication of this new understanding of the universe is that it gives
scientists a radically new picture of the future of the universe. It appears that the dark
energy could eventually overwhelm the gravitational forces of matter. The density of
matter in the universe would then become insignificant, so that the universe would
approach an essentially uniform force field of dark energy. The researchers conclude that
understanding dark energy, and hence the future of the universe, will be "one of the
grand challenges of the millennium to come."
Lawrence Berkeley National Laboratory
Contacts: Saul Perlmutter, (510) 486-5203, firstname.lastname@example.org Paul Preuss, (510)
May 25, 1999
Dark Energy Fills The Cosmos
BERKELEY, CA -- In an article titled "The Cosmic Triangle: Revealing the State of
the Universe," which appears in the May 28, 1999, issue of the journal Science, a
group of cosmologists and physicists from Princeton University and the Department of
Energy's Lawrence Berkeley National Laboratory survey the wide range of evidence which,
they write, "is forcing us to consider the possibility that some cosmic dark energy
exists that opposes the self-attraction of matter and causes the expansion of the universe
Dark energy is hardly science fiction, although no less intriguing and full of mystery
for being real science.
"The universe is made mostly of dark matter and dark energy," says Saul
Perlmutter, leader of the Supernova Cosmology Project headquartered at Berkeley Lab,
"and we don't know what either of them is." He credits University of Chicago
cosmologist Michael Turner with coining the phrase "dark energy" in an article
they wrote together with Martin White of the University of Illinois for Physical Review
In the May 28 Science article, Perlmutter and Neta Bahcall, Jeremiah Ostriker, and Paul
Steinhardt of Princeton use the concept of dark energy in discussing their graphic
approach to understanding the past, present, and future status of the universe. The Cosmic
Triangle is the authors' way of presenting the major questions cosmology must answer:
"How much matter is in the universe? Is the expansion rate slowing down or speeding
up? And, is the universe flat?"
The possible answers are values for three terms in an equation that describes the
evolution of the universe according to the general theory of relativity. By plotting the
best experimental observations and estimates within the triangle, scientists can make
preliminary choices among competing models.
The mass density of the universe is estimated by deriving the ratio of visible light to
mass in large systems such as clusters of galaxies, and in several other ways. For several
decades the evidence has been building that mass density is low and that most of the mass
in the universe is dark.
Changes in expansion rate are estimated by comparing the redshifts of distant galaxies
with the apparent brightness of Type 1a supernovae found in them. These measurements
suggest that the expansion of the universe is accelerating.
Curvature is estimated from measurements of the anisotropy (temperature fluctuation) of
the cosmic microwave background radiation (CMB), a remnant of the Big Bang. Although
uncertainty is large, current results suggest a flat universe.
The Cosmic Triangle eliminates some popular models, such as a high- density universe
that is slowing down and will eventually recollapse, as well as a nearly empty universe
with no dark energy and low mass. While the evidence from galactic clusters shows that
mass density is low, supernova evidence for acceleration shows that dark energy must be
"These two legs of the Cosmic Triangle agree with the evidence from the CMB that
the universe is flat," Perlmutter says, adding that "this is a remarkable
agreement for these early days of empirical cosmology."
Thus the Cosmic Triangle suggests that the standard inflationary scenario is on the
right track: one of its key predictions is a flat universe.
Various types of dark energy have been proposed, including a cosmic field associated
with inflation; a different, low-energy field dubbed "quintessence"; and the
cosmological constant, or vacuum energy of empty space. Unlike Einstein's famous fudge
factor, the cosmological constant in its present incarnation doesn't delicately (and
artificially) balance gravity in order to maintain a static universe; instead, it has
"negative pressure" that causes expansion to accelerate.
"The term Cosmic Triangle sounds kind of New Agey," says Perlmutter,
"but it's a good way to portray the quantities in these comparisons, and it's fun for
people who like to plot the possibilities" -- an evolving task that, among other
choices, will require finding an answer to "the most provocative and profound"
issue of all, the nature of cosmic dark energy.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research and is managed by the
University of California. Visit our website at http://www.lbl.gov .