“A lot of anticipation”: cosmologists await Planck’s views of the universe’s first lightby Jeff Foust
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| “It’s roughly the difference from going from a one-megapixel camera to a six-megapixel camera,” said Partridge, comparing upcoming Planck data with existing WMAP data. |
“There’s a lot of anticipation in the community because on March 22nd they’re supposed to announce their big science results,” said Marc Kamionkowski, a professor of physics and astronomy at Johns Hopkins University, during a session about the cosmic microwave background at last month’s annual meeting of the American Association for the Advancement of Science (AAAS) in Boston. Kamionkowski will be among the participants in the NASA briefing on Thursday.
The key item of the Planck data release is expected to be a new map of the cosmic microwave background, or CMB. “After years of painstaking work, scientists have now been able to remove the bright foreground emissions that lie between us and the Universe’s first light,” the ESA media advisory stated. “Planck’s map of the cosmic microwave background over the whole sky is the best ever made.”
Creating that new-and-improved map of the CMB is the key purpose of Planck, launched in 2009 along with the Herschel infrared space telescope. It follows in the footsteps of two previous NASA missions, the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP), which previously provided maps of the cosmic microwave background, measuring its minute variations across the sky.
Planck, though, plans to offer more detail than those earlier missions. “Planck will provide even more precise measurements with an accuracy set by fundamental astrophysical limits,” ESA states on its Planck website. “In other words, it will be impossible to ever take better images of this radiation than those obtained from Planck.”
“It’s roughly the difference from going from a one-megapixel camera to a six-megapixel camera,” said Bruce Partridge, a professor of astronomy at Haverford College who is involved in the Planck mission, on the improvement in resolution of the Planck data versus WMAP. An increased range of wavelengths sampled by Planck will also improve the quality of the data, he added.
“Planck has worked great. The data are wonderful,” said James Bock of Caltech. The sensitivity of the spacecraft’s instruments turned out to be better than expected, and the mission lasted longer than planned. “We know the data are fantastic.”
| “For a future measurement of inflationary gravitational waves, we’re going to require another satellite,” Bock said, although such a mission likely won’t fly before the 2020s. |
The cosmic microwave background has been a key tool for cosmologists. Its discovery—by accident—in the 1960s by Arno Penzias and Robert Wilson provided evidence for the Big Bang model of the universe’s origin. Penzias and Wilson won the Nobel Prize in physics in 1978 for that discovery. The COBE map of the cosmic microwave background, which revealed its variations, or anisotropies, further confirmed the Big Bang model and won Nobel Prizes in 2006 for two of the mission’s scientists, John Mather and George Smoot.
What will Planck have to offer? Partridge, speaking at the AAAS meeting, couldn’t go into details as the findings of the mission remain under embargo, but he did offer some hints about what to look for on Thursday. One issue is the Hubble constant, the measure of how fast the universe is expanding. “There is some tension between the CMB and supernova measurements of the rate at which the universe is expanding,” he said, on the order of two to three kilometers per second. Could Planck resolve this tension? “Come back on the 22nd of March.”
Another issue is the number of types, of species, of neutrinos. Current theory calls for three species of neutrinos, but there is some evidence that there may be more. Measurements of the power spectra of anisotropies in the CMB by other telescopes have allowed astronomers to fit models where there are more than three neutrino species. “Basically, if you add a neutrino, you add more energy density, you have a faster expansion rate, and you’ll get a different diffusion,” said John Carlstrom of the University of Chicago at the AAAS meeting.
Carlstrom’s team has tried to calculate the number of neutrino species using CMB observations from the South Pole Telescope, where they see tantalizing hints of additional neutrino species—maybe. Their best fit for the number of neutrino species is 3.6, with an error bar of plus/minus 0.5. In other words, there could be four species of neutrinos, or only three. “Well, that’s interesting,” he said at the AAAS meeting, reminding the audience that theory predicts only three. “We’re on the fence.”
The Planck data might help push astrophysicists off the fence, in one direction or the other. Partridge discussed the potential conflict between theory and observation, but as to what Planck might do to resolve this, said only, “Come back on the 22nd of March.”
While ESA bills Planck as the ultimate mission to study the CMB, astronomers at the AAAS meeting said there’s plenty of additional observations to make, from groundbased and balloon-borne telescopes as well as from spacecraft. “If you ask cosmologists what we want to do next, we just want to put more where we have it,” said Mark Devlin, of the University of Pennsylvania, who uses the Atacama Cosmology Telescope in Chile to probe the CMB at smaller angular scales than WMAP.
“For many of us in this room, the future is coming frighteningly soon,” said Bock, referring to the impending release of the Planck results. There are efforts underway on various detector technologies to further improve sensitivities to enable better CMB observations, including polarization and searches for the signature of gravitational waves caused by inflation, the sudden, enormous expansion of the universe immediately after the Big Bang.
Some of that work will require space missions. “For a future measurement of inflationary gravitational waves, we’re going to require another satellite,” Bock said. There have been proposals for such a mission, called the Inflation Probe, but such a spacecraft is unlikely to fly before some time in the 2020s.
So the focus for now is what new insights Planck has to offer on the cosmic microwave background. “I am thrilled to see what is possible,” said Mather at the conclusion of the AAAS session, “and eager to see what will happen.”