The past according to Planck: Cosmologists got a lot right

A new analysis of the universe’s first light has cosmologists simultaneously patting themselves on the back and scratching on their chalkboards. The results, obtained from the Planck satellite and posted online in February in a set of papers at arXiv.org, largely support the theoretical framework that cosmologists employ to describe the universe. But there are also some puzzling findings, hinted at in previous research, that could signal undiscovered physical phenomena.

“The old model of the universe is doing remarkably well,” says Shaun Hotchkiss, a cosmologist at the University of Sussex in Brighton, England. “But everything that was anomalous in the past is still anomalous.” The Planck results also have a lot to say about inflation, the theorized period just after the Big Bang in which the universe swelled rapidly.

Cosmologists can describe the universe reasonably well with a model based on six quantities, including the universe’s expansion rate and the density of ordinary matter. The Planck mission has chimed in with four years of measurements of the cosmic microwave background, or CMB, the universe’s first light that was emitted about 380,000 years after the Big Bang. The latest data support the standard cosmological model and pin down five of the six numbers with 1 percent precision, says Planck project scientist Charles Lawrence. Those numbers describe a universe that started with a brief episode of inflation and since has been guided by a combination of regular matter, an invisible substance called dark matter, and dark energy, which causes space to stretch apart at an ever-increasing rate.

Yet the results include discrepancies that also showed up in Planck’s 2013 data release and other microwave background observations, particularly the number of galaxy clusters. Planck detects subtle temperature variations in the CMB, which reflect quantum fluctuations in the early universe that got amplified by inflation into vast regions of varying density. Over time, mass in the high-density regions should have clumped together to form intricate galaxy clusters. But Planck data suggest there should be more clusters than scientists observe. The tension warrants further study but not any new theories just yet, Hotchkiss says.

Physicists are also pondering the expansion rate of the universe, known as the Hubble constant. Planck data indicate that objects located a megaparsec (about 3 million light-years) away from each other move apart at roughly 68 kilometers per second. That’s not much of a change from 2013, but it is about 6 kilometers per second slower than estimates obtained by measuring the distance and velocities of supernovas and stars called Cepheid variables that predictably fluctuate in brightness (SN: 4/5/14, p. 18). An improved Cepheid survey expected later this year will determine whether a discrepancy remains, says Daniel Scolnic from the Kavli Institute for Cosmological Physics at the University of Chicago.

Even if these issues are resolved, the standard cosmological model won’t be in the clear. Robert Kirshner, a cosmologist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., says the model relies on several unproven assumptions, including that the repulsive strength of dark energy per unit volume has remained constant. Kirshner and other scientists are performing surveys to track dark energy’s influence over time.

The model also fails to explain why and how inflation occurred. Planck’s data refuted last year’s claim by the BICEP2 team of detecting primordial gravitational waves, which would have helped physicists identify a mechanism and energy scale for that crucial instant in history (SN: 2/21/15, p. 13). Without a gravitational wave detection, “we do not have a definitive idea on how to connect our ideas about fundamental physics with inflation,” says David Spergel, an astrophysicist at Princeton University.

The Planck analysis also used BICEP2’s data to place more stringent limits on the value of r, which compares gravitational wave and matter density deviations in the CMB. The finding that r is probably less than 0.09 disfavors simple inflationary theories such as quadratic and natural inflation used in cosmology textbooks, Lawrence says. Those theories were already getting squeezed in 2013 when Planck suggested r was less than 0.11.

Theorists will hone their ideas to jibe with the latest data. Meanwhile, BICEP2 and a cadre of other experiments will continue probing the skies over the South Pole for the faint signals of gravitational waves.