Shedding Light on the Nature
of Dark Energy
June 7, 2001
A team of scientists has suggested tests that should be able to shed
light on the mysterious dark energy that is thought to be causing the
expansion of the universe to accelerate. The tests should determine
whether the present epoch is a special time in the history of the
universe, or whether extra dimensions of space or modifications of the
theory of gravity are required.
As presented today by Kevork Abazajian of the University of
California, San Diego (UCSD) at the summer meeting of the American
Astronomical Society in Pasadena, CA, observations of distant
supernovas, quasars and clusters of galaxies can be used to determine
whether the simplest
model for dark energy is correct, or radical departures are needed.
of these observations are possible with today's technology, whereas
others may require specially designed space observatories.
Observations of distant supernovas, the cosmic microwave background,
and the distribution of galaxies all point to the existence of
amounts of dark energy in our universe. This dark energy contributes a
majority of the mass-energy, and causes an acceleration of the
expansion of the universe in recent times.
The mystery, in addition to the unknown nature of this dark energy,
is why the dark energy is just starting to be effective in the present
epoch. If the theoretical amount of dark energy is tuned to fit the
observations, then dark energy was negligible for essentially all of
the previous history of the universe, but for the indefinite future all
other forms of energy will become negligible and the universe will
undergo an ever more rapid expansion.
"The good news is that simple models with dark energy can provide an
excellent fit to many observations", said Abazajian. "The bad news
is that these models give no explanation of the implied cosmic
coincidence that we are living in a very special time in the history of
"It is possible to get around the coincidence problem by modifying the
theory of gravity, or invoking extra dimensions", explained Neal Dalal,
a coauthor who is also from UCSD. "The tests we have proposed can
give us insight into whether these far-out ideas are worth pursuing."
In particular, Abazajian, Dalal and their colleagues show that effects
of dark energy on the evolution of the universe can be assessed
through observations of the dimming of supernovas with distance, the
apparent angular separation of intergalactic clouds in front of quasars,
(extremely bright cores of galaxies that can be seen at great distances),
and the rate of formation of giant clusters of galaxies with cosmic time.
Present supernova surveys, together with data being accumulated by
the Sloan Digital Sky Survey, and deep X-ray surveys by NASA's Chandra
X-ray Observatory and the European XMM-Newton X-ray observatory may
provide enough information to confirm or deny the simplest dark energy
The proposed Supernova Acceleration Probe, (SNAP), an optical infrared
space telescope, would make an even more definitive test.
Other members of the UCSD team are Elizabeth Jenkins and Aneesh
Manohar, also of UCSD. This research was supported by the U.S.
Department of Energy, NASA, and the ARCS foundation.
Kevork Abazajian (858) 534-5687
Neal Dalal (858) 534-5687
To read more about cosmic coincidences and dark energy
see the Physical Review Focus article:
"A Cosmic Yardstick"