Whatever dark energy is, explanations for it have less
wiggle room following a
Hubble Space Telescope observation that has refined the
measurement of the universe's present expansion rate to a
precision where the error is smaller than 5 percent. The
new value for the expansion rate, known as
the Hubble constant or Ho — after Edwin Hubble who
first measured the expansion of the universe
nearly a century ago — is 74.2 kilometers per second
per megaparsec (error margin of ±3.6). The
results agree closely with an earlier measurement gleaned
from Hubble but are now more than twice
as precise.
The Hubble measurement, conducted by the SHOES
(Supernova Ho for the Equation of State)
team and led by Adam Riess, a professor in the Henry A. Rowland
Department of Physics and
Astronomy at Johns Hopkins' Krieger School of Arts and
Sciences and at the Space
Telescope Science
Institute, uses a number of refinements to streamline
and strengthen the construction of a cosmic
"distance ladder," a billion light-years in length, that
astronomers use to determine the universe's
expansion rate.
Hubble observations of pulsating stars called Cepheid
variables in a nearby cosmic mile marker,
the galaxy NGC 4258, and in the host galaxies of recent
supernovae directly link these distance
indicators. The use of Hubble to bridge these rungs in the
ladder eliminated the systematic errors
that are almost unavoidably introduced by comparing
measurements from different telescopes.
Explaining the new technique, Riess said, "It's like
measuring a building with a long tape measure
instead of moving a yardstick end over end. You avoid
compounding the little errors you make every
time you move the yardstick. The higher the building, the
greater the error."
Lucas Macri, a professor of physics and astronomy at
Texas A&M and a significant contributor
to the results, said, "Cepheids are the backbone of the
distance ladder because their pulsation
periods, which are easily observed, correlate directly with
their luminosities. Another refinement of
our ladder is the fact that we have observed the Cepheids
in the near-infrared parts of the
electromagnetic spectrum, where these variable stars are
better distance indicators than at optical
wavelengths."
This new, more precise value of the Hubble constant
was used to test and constrain the
properties of dark energy — the form of energy that
produces a repulsive force in space — that is
causing the expansion rate of the universe to
accelerate.
By bracketing the expansion history of the universe
between today and when the universe was
only approximately 380,000 years old, the astronomers were
able to place limits on the nature of the
dark energy that is causing the expansion to speed up. (The
measurement for the far early universe is
derived from fluctuations in the cosmic microwave
background, as resolved by NASA's Wilkinson
Microwave Anisotropy Probe in 2003.)
Their result is consistent with the simplest
interpretation of dark energy: that it is
mathematically equivalent to Albert Einstein's hypothesized
cosmological constant, introduced a
century ago to push on the fabric of space and prevent the
universe from collapsing under the pull of
gravity. (Einstein, however, removed the constant once the
expansion of the universe was discovered
by Edwin Hubble.)
"If you put in a box all the ways that dark energy
might differ from the cosmological constant,
that box would now be three times smaller," Riess said.
"That's progress, but we still have a long way
to go to pin down the nature of dark energy."
Though the cosmological constant was conceived of long
ago, observational evidence for dark
energy didn't come along until 11 years ago, when two
studies, one led by Riess and Brian Schmidt of
Mount Stromlo Observatory, and the other by Saul Perlmutter
of Lawrence Berkeley National
Laboratory, discovered dark energy independently, in part
with Hubble observations. Since then,
astronomers have been pursuing observations to better
characterize dark energy.
Riess' approach to narrowing alternative explanations
for dark energy — whether it is a static
cosmological constant or a dynamical field (like the
repulsive force that drove inflation after the big
bang) — is to further refine measurements of the
universe's expansion history.
Before Hubble was launched in 1990, estimates of the
Hubble constant varied by a factor of
two. In the late 1990s the Hubble Space Telescope Key
Project on the Extragalactic Distance Scale
refined the value of the Hubble constant to an error of
only about 10 percent. This was accomplished
by observing Cepheid variables at optical wavelengths out
to greater distances than obtained
previously and comparing those to similar measurements from
ground-based telescopes.
The SHOES team used Hubble's Near Infrared Camera and
Multi-Object Spectrometer and the
Advanced Camera for Surveys to observe 240 Cepheid
variable stars across seven galaxies. One of
these galaxies was NGC 4258, whose distance was accurately
determined through observations with
radio telescopes. The other six galaxies recently hosted
Type Ia supernovae that are reliable distance
indicators for even farther measurements in the universe.
Type Ia supernovae all explode with nearly
the same amount of energy and therefore have almost the
same intrinsic brightness.
By observing Cepheids with very similar properties at
near-infrared wavelengths in all seven
galaxies, and using the same telescope and instrument, the
team was able to more precisely calibrate
the luminosity of supernovae. With Hubble's powerful
capabilities, the team was able to sidestep some
of the shakiest rungs along the previous distance ladder
involving uncertainties in the behavior of
Cepheids.
Riess says he would eventually like to see the Hubble
constant refined to a value with an error
of no more than 1 percent, to put even tighter constraints
on solutions to dark energy.
The Hubble Space Telescope is a project of
international cooperation between NASA and the
European Space Agency and is managed by NASA's Goddard
Space Flight Center. The Space Telescope
Science Institute, which conducts Hubble science
operations, is operated for NASA by the
Association of Universities for Research in Astronomy. The
Space Telescope Science Institute is an
International Year of Astronomy 2009 program partner.