Measuring the universe
Posted: Monday, June 08, 2009 7:09 PM by Alan Boyle
Kochanek, Stanek, Prieto / Ohio State U.
The galaxy M81, seen here in an image from the Large Binocular
Telescope, is home to several ultra-long-period Cepheid variable stars that
could help astronomers fine-tune a new way to measure cosmic distances.
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How far away is that galaxy? The more precise your answer is, the more you can find out about mysterious dark energy. In the past, astronomers have used variable stars and a special kind of supernova to make their distance estimates - and now two new measuring sticks are being added to the toolbox.
One of the yardsticks is a particularly big and bright type of variable star, known as an ultra-long-period Cepheid variable, or ULP for short. The other yardstick makes measurements using the radio emissions from the supermassive black holes lying at the center of many galaxies, including our own.
If the yardsticks can be fine-tuned in the years ahead, they can give scientists a better read on the universe's expansion rate over time, which is tied to a key number known as the Hubble constant. Right now, the number carries an uncertainty factor of 5 percent either way. If you want to get technical about it, the latest value is 74.2 kilometers per second per megaparsec, plus or minus 3.6, derived by the SHOES Team. That estimate is based on a combination of distance scales, with short-period Cepheid variables used for relatively close measurements and Type 1A supernovae for farther measurements.
Cepheid variable stars are considered "standard candles" for measurement because their pattern of brightening and dimming correlates quite well with their intrinsic brightness. Thus, you can compare how bright the star looks with how bright the star really is, and come up with a pretty good distance estimate.
Type 1A supernovae reveal a similar linkage between brightness and distance. But you need to use a closer-in measurement method, like the Cepheids, to calibrate the farther-out supernova measurements. That's a classic source of uncertainty: For example, if you say that 1 meter is equivalent to 3 feet, then multiply that figure to estimate how many feet equal a kilometer, your estimate would be off by about the length of a football field.
A decade ago, the supernova measurements led astronomers to conclude that the universe's expansion rate was speeding up - apparently due to some repulsive energy in empty space, which has come to be known as dark energy. But what is dark energy? Is it a quality of the universe that varies over time, or has it been present throughout the universe's history? The Hubble Constant could hold the answer to that question, if we know the number accurately enough.
Right now, the accuracy is just about good enough for a decision: The evidence so far suggests that dark energy has been ever-present, as a constant push on the cosmic expansion rate. The new yardsticks could help clinch the case - or push a big "reset" button on the theory-making machine.
"There's going to be renewed interest in the Hubble Constant," because the uncertainty factor can be reduced with the new techniques, said Wendy Freedman, director of the Carnegie Observatories and one of the principal investigators for the Carnegie Supernova Project.
Freedman said she and her colleagues hope to get the uncertainty factor down to 2 percent, using hundreds of hours of time on NASA's Spitzer Space Telescope to observe farther-out Cepheid variables in infrared wavelengths. But she said the brand-new yardsticks offer "promising techniques" to weed out distance conversion errors.
The ultra-long perspective
Ohio State University's Jonathan Bird, one of the researchers behind the ultra-long-period Cepheid yardstick, agreed. He compared the measurement schemes to different rungs on an extension ladder.
"If we can extend all these rungs on the cosmic distance ladder, that's truly the way to beat down the systemic errors in the Hubble constant," he said Monday at a news conference during the American Astronomical Society's summer meeting in Pasadena, Calif.
The ULPs are something like classical Cepheid stars, only with a much longer cycle of brightening and dimming. They're also brighter, which makes them easier than the classical Cepheids to spot at greater distances. At distances beyond 100 million light-years, most Cepheids are too dim to detect - but the ultra-long-period stars can still be seen at distances of 325 million light-years or so.
The problem with the ULPs is that astronomers didn't think their periods correlated with their brightness, as is the case with the regular Cepheids. But Bird and his colleagues, led by Ohio State astronomer Krzysztof Stanek, pieced together data from previously published papers to come up with a system. Right now, the accuracy level is about 10 to 20 percent. With more observations, that accuracy should improve markedly, Bird said.
Seeing the swirl around a black hole
That's also the hope for the National Radio Astronomy Observatory's James Braatz and his colleagues with the Megamaser Cosmology Project. They are pioneering a method that relies on precise measurements of the linear and angular size of the disks of material that swirl around galactic black holes.
Radio emissions from the disk could be analyzed to determine how fast one edge of a disk was moving away, and how fast the other edge was coming closer - providing the necessary numbers for calculating distance. Normally, those emissions would be too faint to pick up, but water molecules inside the disk itself amplified the signals. In effect, the water functioned like tiny molecular-scale lasers, or "masers."
The maser technique was used to determine that a galaxy called UGC 3789 was 160 million light-years from Earth. Braatz told reporters that the observational feat was analogous to determining the size of a quarter sitting on his desk in Charlottesville, Va., as seen from the conference center in Pasadena, Calif.
"We measured a direct, geometric distance to the galaxy, independent of the complications and assumptions inherent in other techniques," he said in Monday's news release. "The measurement highlights a valuable method that can be used to determine the local expansion rate of the universe, which is essential in our quest to find the nature of dark energy."
The distance measurements for UGC 3789 were published in the April 10 issue of The Astrophysical Journal.
For more on the dark-energy distance quest, check out these archived reports:
Update/correction for 3:15 p.m. June 15: Astronomer Adam Riess, who was involved in the original observations of the cosmic speed-up as well as the latest observations relating to the Hubble constant, sent me an e-mail to correct the uncertainty factor I cited for the constant. I had a bit of conflicting information from Wendy Freedman and tried to reconcile what I was hearing with what I was reading, but Riess does a much better job of it. He's a professor at Johns Hopkins University as well as a researcher at the Baltimore-based Space Telescope Science Institute. Here's his e-mail:
"There are a few small problems in your piece on 'Measuring the Universe.' The old Hubble constant measure from the HST Key Project (Freedman et al., 2001) had an uncertainty of 10%. Our new one (which you quote without reference, its the SHOES Team, led by myself) has measured it to be 74.2 +/- 3.6. You have the new numbers right but note that this is now a 5% uncertainty (3.6/74.2=0.05), not 10%, which is now twice as good as the old one. Our measurement also already makes use of so-called ultra long period Cepheids. I hope you can correct some of these. Good work otherwise as usual."
I've made the corrections as suggested, and I've made sure to mention the SHOES team. Many thanks to Adam for setting all this straight, and thanks also to the commenters who wondered about the error percentages.
Correction for 2 a.m. ET June 9: I fixed a reference to light-year distances to add the word "million." That does make a difference. Sorry about the error, and thanks for setting me straight.
Stay tuned for further reports this week from the American Astronomical Society's summer meeting in Pasadena. Join the Cosmic Log corps by signing up as my Facebook friend or hooking up on Twitter. And if you really want to be friendly, ask me about my upcoming book, "The Case for Pluto."
