Refined Hubble Constant Narrows Possible Explanations for Dark Energy

Whatever dark energy is, explanations for it have less wiggle room following a landmark 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 five percent.

The new value for the expansion rate, known as the Hubble constant or H_o (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, accepted for publication in the Astrophysical Journal, agree closely with an earlier measurement gleaned from Hubble of 72 +/- 8 km/sec/megaparsec but are now more than twice as precise.

This latest measurement of the Hubble constant was conducted by the SHOES (Supernova H_o for the Equation of State) Team, which is led by Adam Riess of Space Telescope Science Institute and the Johns Hopkins University and includes Lucas Macri, an assistant professor of physics and astronomy at Texas A&M University and a significant contributor to the results. The team used a number of refinements to streamline the construction of a cosmic distance ladder — a billion light-years in length — that astronomers use to determine the Universe’s expansion rate.

Hubble Space Telescope observations of pulsating stars called Cepheid variables around 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 systematic errors introduced by comparing measurements from different telescopes.

“It’s like measuring a building with a long tape measure instead of moving a yard stick end over end,” Riess explains. “You avoid compounding the little errors you make every time you move the yardstick. The higher the building, the greater the error.”

Macri adds, “Cepheids are the backbone of the distance ladder because their pulsation period correlates with their luminosity. Another refinement of our ladder is the fact that we have observed the Cepheids in the infrared where these variables 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, which 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 put limits on the nature of the dark energy which 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, WMAP, 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 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 says. “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 studies from two teams independently announced the discovery of dark energy, in part with Hubble observations. One paper was led by Riess and included Nicholas Suntzeff, director of Texas A&M’s astronomy program and holder of the Mitchell-Heep-Munnerlyn Endowed Chair in Observational Astronomy in the Texas A&M Department of Physics, as a significant contributor to the results. The other paper was led by Saul Perlmutter of Lawrence Berkeley National Laboratory. Since then, astronomers have been pursuing observations to better characterize dark energy.

The approach of the SHOES Team 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, the 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 a team that included Macri which observed Cepheid variables at optical wavelengths out to greater distances than previously obtained and compared those to similar measurements from ground-based telescopes.

The SHOES Team used Hubble’s Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Advanced Camera for Surveys (ACS) to observe 240 Cepheid variable stars across seven galaxies. One of these galaxies was NGC 4258, whose distance was very accurately determined through observations with radio telescopes and whose Cepheids were discovered by Macri in 2006. 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 7 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.

The SHOES Team is scheduled to make additional observations of Cepheids in NGC 4258 and other hosts of Type Ia supernovae once the upcoming Shuttle servicing mission to the Hubble Space Telescope is carried out successfully. These additional observations will continue to narrow the uncertainty in the Hubble constant and could eventually lead to a refined value with an error of no more than 1 percent, to put even tighter constraints on solutions to dark energy.

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Contacts: Lucas Macri, (979) 862-2763 or lmacri@physics.tamu.edu