Hubble’s Constant Redefined by Hubble Space Telescope
Credit NASA, ESA and A Reiss (STScl/JHU)
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 five percent. The new value for the expansion rate, known as the Hubble constant, or H0 (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). Earlier measurements made by Hubble gave the result as 72 ± 8 km/sec/megaparsec. The new measurement is now twice as precise.
A number of refinements were used to strengthen and streamline the construction of a cosmic “distance ladder” a billion light years in length that astronomers use to establish the expansion of the universe.
Pulsating stars called Cephid vairables have been observed by the Hubble Space Telescope to provide the cosmic mile marker. By using Hubble it removed the errors that are introduced by comparing measurements from different telescopes.
Dark energy is thought to be responsible for causing the expansion rate of the universe to accelerate. Wit the new value for the Hubble constant the properties of dark energy can be tested and constrained.
The 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.
Although the cosmological constant was dreamt up many years 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 characterise dark energy.
Before the Hubble Space Telescope 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 ten 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 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. 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 would eventually like to see the Hubble constant refined to a value with an error of no more than one percent, to put even tighter constraints on solutions to dark energy.
Credit: NASA, ESA and A Feild (STScl)
