Hubble Space Telescope refines Hubble’s constant

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 8 May, 2009

The rate at which the Universe is expanding, as described by the Hubble constant, has been refined to a precision where the error is smaller than five percent thanks to new data from the Hubble Space Telescope.

The Hubble constant, H0, is named after Edwin Hubble who first measured the expansion of the Universe nearly a century ago. He quoted the expansion as 500 kilometers per second per megaparsec (km/sec/Mpc), controversially equating to an expansion age of just two billion years when geologists had already shown that rocks on the Earth dated to three billion years. For almost a century this value has been continually refined, making it the least constant constant in mathematics. This week, new analysis from the Hubble Space Telescope has revised this value again, from an earlier measurement of 72 ± 8 km/sec/Mpc to a twice-as-precise 74.2 ± 3.6 km/sec/Mpc.

Hubble image of galaxy NGC 3021, one of the supernova hosts for the survey. The images in the insets were taken with NICMOS. Image: NASA, ESA, and A. Riess (STScI/JHU).

The new measurement resulted from the Supernova H0 for the Equation of State (SHOES) study led by Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University, which 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 that are reliable distance indicators for even farther measurements in the Universe.

The study is based on the concept of a billion light year long “cosmic distance ladder”, with cepheid variable stars forming the rungs. "Cepheids are the backbone of the distance ladder because their pulsation periods, which are easily observed, correlate directly with their luminosities,” says Lucas Macri, professor of physics and astronomy at Texas A&M, and a significant contributor to the results. “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."

The use of Hubble for a number of different measurements eliminated the systematic errors that can often be 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,” says Reiss. “You avoid compounding the little errors you make every time you move the yardstick. The higher the building, the greater the error."
The new value of H0 can now be used to test and constrain theories about the nature of dark energy, a form of energy that produces a repulsive force in space, which is that to be causing the expansion rate of the Universe to accelerate. The result is consistent with the simplest interpretation of dark energy: that it is mathematically equivalent to Albert Einstein's hypothesized cosmological constant, a concept created to push on the fabric of space and prevent the Universe from collapsing under the pull of gravity.

"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," says Riess. "That's progress, but we still have a long way to go to pin down the nature of dark energy."
Riess hopes to narrow down alternative explanations for dark energy to determine whether it is a static cosmological constant or a dynamical field — like the repulsive force that drove inflation after the big bang — by further refining measurements of the Universe's expansion history. Eventually he hopes that the Hubble constant will be refined to a value with an error of no more than one percent, to put even tighter constraints on solutions to dark energy.