“This is entering a new era of cosmology from galaxy surveys,” Frieman said. With DES’s first-year data, “galaxy surveys have now caught up to the cosmic microwave background in terms of probing cosmology. That’s really exciting because we’ve got four more years where we’re going to go deeper and cover a larger area of the sky, so we know our error bars are going to shrink.”
For cosmologists, the key question was whether DES’s new cosmic pie chart based on galaxy surveys would differ from estimates of dark energy and dark matter inferred from Planck’s map of the cosmic microwave background. Comparing the two would reveal whether cosmologists correctly understand how the universe evolved from its early state to its present one. “Planck measures how much dark energy there should be” at present by extrapolating from its state at 380,000 years old, Dodelson said. “We measure how much there is.”
The DES scientists spent six months processing their data without looking at the results along the way—a safeguard against bias—then “unblinded” the results during a July 7 video conference. After team leaders went through a final checklist, a member of the team ran a computer script to generate the long-awaited plot: DES’s measurement of the fraction of the universe that’s matter (dark and visible combined), displayed together with the older estimate from Planck. “We were all watching his computer screen at the same time; we all saw the answer at the same time. That’s about as dramatic as it gets,” said Gary Bernstein, an astrophysicist at the University of Pennsylvania and co-chair of the DES science committee.
Planck pegged matter at 33 percent of the cosmos today, plus or minus two or three percentage points. When DES’s plots appeared, applause broke out as the bull’s-eye of the new matter measurement centered on 26 percent, with error bars that were similar to, but barely overlapped with, Planck’s range.
“We saw they didn’t quite overlap,” Bernstein said. “But everybody was just excited to see that we got an answer, first, that wasn’t insane, and which was an accurate answer compared to before.”
Statistically speaking, there’s only a slight tension between the two results: Considering their uncertainties, the 26 and 33 percent appraisals are between 1 and 1.5 standard deviations or “sigma” apart, whereas in modern physics you need a five-sigma discrepancy to claim a discovery. The mismatch stands out to the eye, but for now, Frieman and his team consider their galaxy results to be consistent with expectations based on the cosmic microwave background. Whether the hint of a discrepancy strengthens or vanishes as more data accumulate will be worth watching as the DES team embarks on its next analysis, expected to cover its first three years of data.
If the possible discrepancy between the cosmic-microwave and galaxy measurements turns out to be real, it could create enough of a tension to lead to the downfall of the “Lambda-CDM model” of cosmology, the standard theory of the universe’s evolution. Lambda-CDM is in many ways a simple model that starts with Albert Einstein’s general theory of relativity, then bolts on dark energy and dark matter. A replacement for Lambda-CDM might help researchers uncover the quantum theory of gravity that presumably underlies everything else.