Cosmic Question Mark: The Planck Mission's Data Put a Kink in Precision Cosmology

By Siegfried, Tom | Science News, April 5, 2014 | Go to article overview

Cosmic Question Mark: The Planck Mission's Data Put a Kink in Precision Cosmology


Siegfried, Tom, Science News


For as long as humans have wondered about it, the universe has concealed its vital statistics--its age, its weight, its size, its composition. By the opening of the 21st century, though, experts began trumpeting a new era of precision cosmology. No longer do cosmologists argue about whether the universe is 10 billion or 20 billion years old--it was born 13.8 billion years ago. Pie charts now depict a precise recipe for the different relative amounts of matter and energy in the cosmos. And astronomers recently reached agreement over just how fast the universe is growing, settling a controversy born back in 1929 when Edwin Hubble discovered that expansion.

Except now the smooth path to a precisely described cosmos has hit a bit of a snag. A new measurement of the speed of the universe's expansion from the European Space Agency's Planck satellite doesn't match the best data from previous methods (SN: 4/20/13, p. 5). Just when all the pieces of the cosmic puzzle had appeared to fall into place, one piece suddenly doesn't fit so perfectly anymore.

"Something doesn't look quite right," says astrophysicist David Spergel of Princeton University. "We can no longer so confidently go around making statements like all our datasets seem consistent."

In other words, different ways of measuring the universe's expansion rate--a number called the Hubble constant--no longer converge on one value. That calls into question the whole set of numbers describing the properties of the cosmos, known as the standard cosmological model. Accepting the new Hubble constant value means revising the recipe of ingredients that make up the universe, such as the dark matter hiding in space and the dark energy that accelerates the cosmic expansion.

Over the years, the Hubble constant's value has been as elusive as it is important. Hubble himself badly overestimated the expansion speed, which depends on distance--the farther away two objects are, the faster space's expansion pushes them apart. Hubble calculated that objects separated by a million parsecs (roughly 3 million light-years) would fly apart at 500 kilometers per second. At that rate, the universe would be, paradoxically, younger than the Earth.

Refined measurements gradually reduced the estimate to a more realistic realm. By the 1970s, experts argued over whether the Hubble constant is closer to 100 or to 50. By the late 1990s, Hubble Space Telescope observations of supernovas and other data placed the expansion rate value in the 70s, eventually settling in at around 73 km/s/megaparsec.

Confidence in that value was enhanced by measurements of the radiation glow left over from the Big Bang, primarily by a satellite probe known as WMAP. Its value for the Hubble constant was about 70, close enough to 73 that the margins of error for the two values overlapped (SN: 3/15/08, p. 163).

But last year, the Planck satellite reported even more precise measurements of that glow--known as the cosmic microwave background radiation--implying a Hubble constant around 67. That was about 10 percent lower than the Hubble telescope value, a difference that most physicists found too big to ignore.

"We seem to be having some disagreement," says Wendy Freedman of the Carnegie Observatories in Pasadena, Calif., and leader of the team that measured the expansion rate using the Hubble telescope.

An inconvenient discrepancy

Freedman, Spergel and other experts expect that further refinements of the measurements will eventually resolve the conflict with no major repercussions. Nevertheless, the discrepancy was a constant topic of discussion in December at the Texas Symposium on Relativistic Astrophysics, held in Dallas. Krzysztof Gorski of the Planck team acknowledged the disagreement during his talk at the symposium, but he noted that much of the Planck data has not yet been analyzed. "I think we should just stay calm and carry on," he said. …

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