Homing in on Dark Energy With Supernova Studies in Space
Supernovae studied with the Hubble Space Telescope sheds new
light on dark energy, according to the latest findings of
the Supernova Cosmology Project, recently posted at
http://www.arxiv.org/abs/astro-ph/0309368, and soon to appear in the Astrophysical Journal.
Light curves and spectra from the 11 distant supernovae
constitute “a strikingly beautiful data set, the largest
such set collected solely from space,” says Saul Perlmutter,
an astrophysicist at Lawrence Berkeley National Laboratory
and leader of the Supernova Cosmology Project (SCP). The SCP
is an international collaboration of researchers from the
United States, Sweden, France, the United Kingdom, Chile,
Japan, and Spain.
Type Ia supernovae are among astronomy’s best “standard
candles,” so similar that their brightness provides a
dependable gauge of their distance, and so bright they are
visible billions of light years away.
The new study reinforces the remarkable discovery, announced
by the Supernova Cosmology Project early in 1998, that the
expansion of the universe is accelerating due to a
mysterious energy that pervades all space. That finding was
based on data from over three dozen Type Ia supernovae, all
but one of them observed from the ground. A competing group,
the High-Z Supernova Search Team, independently announced
strikingly consistent results, based on an additional 14
supernovae, also predominantly observed from the ground.
Because the Hubble Space Telescope (HST) is unaffected by
the atmosphere, its images of supernovae are much sharper
and stronger and provide much better measurements of
brightness than are possible from the ground. Robert A.
Knop, assistant professor of physics and astronomy at
Vanderbilt University in Nashville, Tenn., led the Supernova
Cosmology Project’s data analysis of the 11 supernovae
studied with the HST and coauthored the Astrophysical
Journal report with the 47 other members of the SCP.
“The HST data also provide a strong test of host-galaxy
extinction,” Knop says, referring to concerns that
measurements of the true brightness of supernovae could be
thrown off by dust in distant galaxies, which might absorb
and scatter their light. But dust would also make a
supernova’s light redder, much as our sun looks redder at
sunset because of dust in the atmosphere. Because the data
from space show no anomalous reddening with distance, Knop
says, the supernovae “pass the test with flying colors.”
“Limiting such uncertainties is crucial for using supernovae
— or any other astronomical observations — to explore the
nature of the universe,” says Ariel Goobar, a member of SCP
and a professor of particle astrophysics at Stockholm
University in Sweden. The extinction test, says Goobar,
“eliminates any concern that ordinary host-galaxy dust could
be a source of bias for these cosmological results at
high-redshifts.” (See “What is Host-Galaxy Extinction?”
under additional information, below.)
The term for the mysterious “repulsive gravity” that drives
the universe to expand ever faster is dark energy. The new
data are able to provide much tighter estimates of the
relative density of matter and dark energy in the universe:
under straightforward assumptions, 25 percent of the
composition of the universe is matter of all types and 75
percent is dark energy. Moreover, the new data provides a
more precise measure of the “springiness” of the dark
energy, the pressure that it applies to the universe’s
expansion per unit of density.
Among the numerous attempts to explain the nature of dark
energy, some are allowed by these new measurements —
including the cosmological constant originally proposed by
Albert Einstein — but others are ruled out, including some
of the simplest models of the theories known as
quintessence. (See “What is Dark Energy?” under additional
information, below.)
High-redshift supernovae are the best single tool for
measuring the properties of dark energy — and eventually
determining what dark energy is. As supernova studies with
the HST demonstrate, the best place to study high-redshift
supernovae is with a telescope in space, unaffected by the
atmosphere.
Nevertheless, “to make the best use of a telescope in space,
it’s essential to make the best use of the finest telescopes
on the ground,” says SCP member Chris Lidman of the European
Southern Observatory.
For the supernovae in the present study, the SCP team
invented a strategy whereby the Hubble Space Telescope could
quickly respond to discoveries made from the ground, despite
the need to schedule HST time long in advance. Working
together, the SCP and the Space Telescope Science Institute
implemented the strategy to superb effect.
The current study, based on HST observations of 11
supernovae, points the way to the next generation of
supernova research: in the future, the
SuperNova/Acceleration Probe, or SNAP satellite, will
discover thousands of Type Ia supernovae and measure their
spectra and their light curves from the earliest moments,
through maximum brightness, until their light has died away.
SCP’s Perlmutter is now leading an international group of
collaborators based at Berkeley Lab who are developing SNAP
with the support of the U.S. Department of Energy’s Office
of Science. It may be that the best candidate for a correct
theory of dark energy will be identified soon after SNAP
begins operating. A world of new physics will open as a
result.
“New constraints on omega-m, omega-lambda, and w from an
independent set of eleven high-redshift supernovae observed
with the HST,” by Robert A. Knop and 47 others (the
Supernova Cosmology Project), will appear in the
Astrophysical Journal and is currently available online at
http://www.arxiv.org/abs/astro-ph/0309368.
For more about the Supernova Cosmology Project visit
http://supernova.lbl.gov/. For more about the Hubble Space
Telescope and the Space Telescope Science Institute visit
http://www.stsci.edu/resources/. For more about the SNAP
satellite visit http://snap.lbl.gov/.
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.
Additional information:
“What is Host-Galaxy Extinction?”
Type Ia supernovae are among the best standard candles known
to astronomy — objects whose distance can be determined
because their intrinsic brightness is known or can be
computed, just as the distance to a 100-watt bulb can be
calculated by comparing its apparent brightness with its
actual brightness.
Determining the expansion rate of the universe by comparing
the brightness and redshift of far-off Type Ia supernovae
therefore critically depends on accurate measurements of
both.
One worrisome possible source of error in measuring distant
supernovae has been host-galaxy extinction, the filtering
effect of dust peculiar to the galaxy in which the supernova
occurs. Dust occurs in our own galaxy too, but has been so
extensively studied that it is of less concern in supernova
distance measurements.
The concern is that distant supernovae appear dimmer not
because of the accelerating effects of dark energy but, more
prosaically, because of dust. There is a straightforward way
to distinguish these effects, however, since dust normally
reddens the light passing through it. Shorter, bluer
wavelengths are absorbed and scattered more readily than
longer, redder wavelengths.
“When you want to measure a supernova’s brightness you can
measure the light that was blue when it left, or the light
that was red,” says Greg Aldering, a member of the Supernova
Cosmology Project and leader of the Nearby Supernova Factory
program, which concentrates on studying the intrinsic
properties of Type Ia supernovae. “Both measurements are
valid, but what you want is to make sure you get the same
answer on both sides of the spectrum. If the blue is
fainter, you’ve got a dust problem.”
The extraordinarily high quality of photometric data from
the 11 distant supernovae studied by the Hubble Space
Telescope in this study allowed their intrinsic brightness
to be determined and compared in both bands.
The study determined that no anomalous effects of
host-galaxy extinction occur at great distance; distant
supernovae are comparable to nearby supernovae in this
respect, underlining their utility as standard candles.
“What is Dark Energy?”
When SCP researchers initially set out to measure the
expansion rate of the universe, they expected to find that
distant supernovae appeared brighter than their redshifts
would suggest, indicating a slowing rate of expansion.
Instead they found the opposite: at a given redshift,
distant supernovae were dimmer than expected. Expansion was
accelerating.
Not only did this discovery mean that the universe would
never come to an end, more fundamentally it implied that a
large part of the universe is made of something we know
nothing about — the mysterious whatever-it-is that goes by
the name “dark energy.”
Later, new measurements of cosmic microwave background (CMB)
radiation provided strong evidence that the universe is flat
(having an overall geometry of space like Euclid’s, in which
parallel lines never meet or diverge) — and because there
is not enough matter in the universe, whether visible or
dark, to produce flatness, the difference can be attributed
to dark energy, providing a strong confirmation of the
supernova measurements.
The first attempt to explain the nature of dark energy was
by invoking Albert Einstein’s notorious “cosmological
constant,” an extra term he introduced early in the the
equations of the theory of general relativity in the 20th
century under the mistaken impression, shared by astronomers
and cosmologists of the time, that the universe was static.
The cosmological constant, which Einstein signified by the
Greek letter lambda, made it so.
Einstein happily abandoned the cosmological constant when,
in 1929, Edwin Hubble found the universe was not static but
expanding. However, lambda came back strong — albeit 70
years later! — when supernova studies led to the discovery
that expansion was accelerating.
“For the cosmological constant, the vacuum — space itself
— possesses a certain springiness,” says Eric Linder, a
cosmologist at Berkeley Lab and director of the Center for
Cosmology and Spacetime Physics at Florida Atlantic
University. “As you stretch it, you don’t lose energy, you
store extra energy in it just like a rubber band.”
Such springiness, whether of matter, energy, or space
itself, is described mathematically by a term called the
equation-of-state parameter (w). For lambda, the value of
this parameter is minus one, corresponding to a component of
the universe that has “negative pressure” — unlike matter
or radiation, which have zero or positive pressure. True to
its name, the cosmological constant doesn’t change over
time: the energy stored by lambda scales uniformly,
increasing exactly as the volume of the universe increases.
The problem is that the most obvious source for lambda’s
stored energy is what quantum theory calls the energy of the
vacuum ?? so much more powerful (10 to the 120th power!)
than what’s been observed for lambda, Linder says, that if
this were the dark energy “it would overwhelm the expansion
of the universe. It would have brought the universe to a
swift end a miniscule fraction of a second after it was
created in the big bang.”
Other explanations of dark energy, called “quintessence,”
originate from theoretical high-energy physics. In addition
to baryons, photons, neutrinos, and cold dark matter,
quintessence posits a fifth kind of matter (hence the name),
a sort of universe-filling fluid that acts like it has
negative gravitational mass. The new constraints on
cosmological parameters imposed by the HST supernova data,
however, strongly discourage at least the simplest models of
quintessence.
Quite different “topological defect” models attribute dark
energy to defects created as the early universe cooled,
during the phase changes that precipitated different forces
and particles from a highly symmetrical early state.
Certain of these theoretical defects, known as domain walls,
could have partitioned space into distinct cells whose
boundaries would have repulsive gravity, thus filling the
role of dark energy. But the new HST supernovae study rules
out — with 99 percent certainty — domain walls as the
source of dark energy.
While the case for the cosmological constant looks strong by
comparison to these alternatives, many other exciting
possibilities remain. Some even propose a cosmos in which
our universe, having three dimensions of space, is afloat in
a higher-dimensional world, with gravity free to interact
among the dimensions.
Or there could be a time-varying form of dark energy that
only temporarily mimics lambda. If it becomes less
gravitationally repulsive in the future, it could bring
acceleration to a halt, perhaps even causing expansion to
reverse and triggering the collapse of the universe.
The opposite is also possible: superaccelerating dark
energy. These models have w, the equation-of-state
parameter, less than minus one — unlike lambda, stored
energy would not scale uniformly as the universe expands but
increase faster than the increase in volume.
“One of the goals of the SuperNova/Acceleration Probe
satellite is to determine whether w may be changing with
time,” says Saul Perlmutter, coprincipal investigator of the
SNAP satellite now under development. “This will help us
narrow the possibilities for the nature of dark energy.
That’s an exciting prospect for physicists, because
understanding dark energy will be crucial to finding a
final, unified picture of physics.”
