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Expansion of space measurement improved

A team of astronomers, led by Wendy Freedman, director of the Carnegie Observatories, have used NASA's Spitzer Space Telescope to make the most accurate and precise measurement yet of the Hubble constant, a fundamental quantity that measures the current rate at which our universe is expanding. These results will be published in the Astrophysical Journal and are available online.

The Hubble constant is named after 20th Century Carnegie astronomer Edwin P.Hubble, who astonished the world by discovering that our universe is expanding now and has been growing continuously since its inception. Astronomers now know that the universe exploded into being in a Big Bang about about 13.7 billion years ago. Determining Hubble's constant, a direct measurement of the rate of this continuing expansion, is critical for gauging the age and size of our universe.

Spitzer's new measurement, which took advantage of long-wavelength infrared instead of visible light, improves upon a similar, seminal study from NASA's Hubble Space Telescope by a factor of three, bringing the uncertainty down to only three percent, a giant leap in accuracy for a cosmological measurement. The newly refined value, in astronomer-speak, is: 74.3 ± 2.1 kilometers per second per megaparsec (a megaparsec is roughly 3 million light-years).

"Spitzer is yet again doing science it wasn't designed to do," said Michael Werner, the mission's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who has worked on the mission since its early concept phase more than 30 years ago. "First, it surprised us with its pioneering ability to study exoplanet atmospheres, and now, in the mission's later years, it's become a valuable cosmology tool."

In addition, the findings were combined with published data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) to obtain an independent measurement of dark energy, one of the greatest mysteries of our cosmos. In the late 1990s, astronomers were shocked to learn that the expansion of our universe is speeding up over time, or accelerating. Dubbed dark energy, this force or energy is thought to be winning a battle against gravity, pulling the fabric of the universe apart. Research documenting this acceleration garnered the 2011 Nobel Prize in physics.

"This is a huge puzzle," said lead author Freedman. "It's exciting that we were able to use Spitzer to tackle fundamental problems in cosmology: the precise rate at which the universe is expanding at the current time, as well as measuring the amount of dark energy in the universe from another angle."

Spitzer was able to improve upon past measurements of Hubble's constant due to its infrared vision, which sees through dust to provide better views of variable stars called Cepheids. These pulsating stars are vital "rungs" in what astronomers called the cosmic distant ladder: a set of objects with known distances that, when combined with the speeds at which the objects are moving away from us, reveal the expansion rate of the universe.

Cepheids are crucial to these calculations because their distances from Earth can be readily measured. In 1908, Henrietta Leavitt discovered that these stars pulse at a rate that is directly related to their intrinsic brightness. To visualize why this is important, imagine somebody walking away from you while carrying a candle. The candle would dim the farther it traveled, and its apparent brightness would reveal just how far.

The same principle applies to Cepheids, standard candles in our cosmos. By measuring how bright they appear on the sky, and comparing this to their known brightness as if they were close up, astronomers can calculate their distance from Earth.

Spitzer observed ten Cepheids in our own Milky Way galaxy and 80 in a nearby neighboring galaxy called the Large Magellanic Cloud. Without the cosmic dust blocking their view at the infrared wavelengths, the research team was able to obtain more precise measurements of the stars' apparent brightness, and thus their distances, than previous studies had done. With these data, the researchers could then tighten up the rungs on the cosmic distant ladder, opening the way for a new and improved estimate of our universe's expansion rate.

"Just over a decade ago, using the words 'precision' and 'cosmology' in the same sentence was not possible, and the size and age of the universe was not known to better than a factor of two," Freedman said. "Now we are talking about accuracies of a few percent. It is quite extraordinary"

The research team included former and current Carnegie scientists Barry Madore, Vicky Scowcroft, Andrew Monson, Chris Burns, Mark Seibert, Eric Persson, and Jane Rigby.

The Carnegie Institution for Science is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

Wendy Freedman | EurekAlert!
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