The universe is expanding at an accelerating rate. This is a generally accepted scientific understanding, the discovery of which yielded a Nobel prize in 2011. But what exactly is the speed of that expansion?
According to new research, soon to be published in the Astrophysical Journal, the rate of expansion, or the Hubble constant, is about 73.24 kilometers per second per megaparsec (a unit equivalent to 3.26 million light-years): somewhere between five and nine percent faster than we thought.
A multitude of reasons has been put forth for this discrepancy, including the possibility that these new calculations are inaccurate. But if they continue to withstand scrutiny, they could challenge some of the basic precepts surrounding the functioning of our universe.
"If you really believe our number – and we have shed blood, sweat and tears to get our measurement right and to accurately understand the uncertainties – then it leads to the conclusion that there is a problem with predictions based on measurements of the cosmic microwave background radiation, the leftover glow from the Big Bang," co-author Alex Filippenko, an astronomy professor at the University of California, Berkeley, said in a press release.
"Maybe the universe is tricking us, or our understanding of the universe isn't complete," he added.
If the science is correct, then one possible explanation has to do with the mere 95 percent of the universe constituted of mysterious elements that do not emit light, such as dark matter, dark energy, and dark radiation.
Dark energy, thought to be responsible for the acceleration of our expanding universe, could be pushing galaxies apart with even greater vigor than scientists believed. Or dark matter, the "backbone of the universe," in NASA's words, could have some as-yet undiscovered oddities.
And then there's dark radiation: subatomic particles that travel at velocities approaching the speed of light. Maybe one such particle exists, or existed, that is unknown to science, perhaps the much-hypothesized "fourth flavor of neutrino."
Aside from all that darkness, there is another possible explanation: Einstein's theory of relativity could be inaccurate.
"You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right," study leader and Nobel Laureate Adam Riess, an astrophysicist at Johns Hopkins University and the Space Telescope Science Institute, said in a NASA press release. "But now the ends are not quite meeting in the middle and we want to know why."
The new numbers are based upon the team's efforts to acquire a more precise understanding of the universe's current expansion rate. Using data from the Hubble Space Telescope and Hawaii's Keck I telescope, they refined that to an uncertainty of only 2.4 percent.
The new data were gleaned by analyzing two types of star: Cepheids and Type la supernovae. Cepheid stars allow astronomers to determine how far they are from us, and thereby how distant their galaxy is, by analyzing two characteristics: their true brightness, gleaned from the rate at which they pulsate, versus their apparent brightness as seen from Earth. Type la supernovae, a kind of exploding star, can be detected even further away.
The researchers measured about 2,400 Cepheid stars occupying 19 nearby galaxies, compared the apparent brightness of both types of star, and were able to determine the true brightness of the Type la supernovae.
Armed with this information, they could calculate the distance to some 300 Type la supernovae in distant galaxies.
"We've done the world's best job of decreasing the uncertainty in the measured rate of universal expansion and of accurately assessing the size of this uncertainty," Dr. Filippenko said in the Berkeley release, "yet we find that our measured rate of expansion is probably incompatible with the rate expected from observations of the young universe, suggesting that there's something important missing in our physical understanding of the universe."
Some scientists uninvolved with this study urge caution, including NASA astrophysicist John Mather, Princeton astrophysicist David Spergel, and California Institute of Technology physicist Sean Carroll, who said that the most likely explanation is a mistake in the most recent calculations.
"It's far too early to jump up and down to say the universe is messing with us," Dr. Carroll told The Washington Post, but he conceded that the research was carried out by scientists both solid and careful.