We know that the universe is expanding, but a strange discrepancy in just how fast that expansion is occurring continues to confound physicists — and make them wonder whether there's some new, unexplained physics afoot.
Strong gravitational lensing exampleImage: ESA/Hubble & NASA derivative work: (Wikimedia Commons)
For every 3.3 million light years, or one megaparsec, the universe expands around another 70km/s faster. There are two discrepant measurements of this so-called "Hubble constant." The light from the most distant parts of the universe reveals an expansion of 68km/s per megaparsec, while a method taken from extrapolating data from nearby sources reveals a rate of 73km/s per megaparsec.
Scientists can't explain this discrepancy by chance alone, which means they're leaving something out, either in their experiments or in the laws of physics. A team of researchers have an idea for another measurement that could help close the gap between these numbers — by measuring how gravity affects the light from distant supernovae.
"If you want to tell the difference between new physics and unknown errors, you need another measurement," study author Thomas Collett from the University of Portsmouth in the UK told Gizmodo. "If you have measurements that have completely independent methods and they're all pointing in the same direction, you can robustly believe it's new physics, not that they're screwing up in the same way."
Collett and the remaining researchers, all from the University of California, Berkeley, plan to look at an effect called strong gravitational lensing on the behaviour of type Ia supernovae. These stellar explosions are thought to occur in pairs of stars, where one star sucks so much gas from the other that it blows up. But if there's something really heavy in the way of the radiation, its gravity will warp the shape of space and cause some of the light to take a different, longer path towards Earth. This creates another, often magnified image in the sky.
Since the light from each image travels different distances, it arrives at different times here on Earth. Comparing those times for a lot of type Ia supernova could act as a useful tool to measure the Hubble constant. The researchers were concerned about microlensing, or smaller-scale effects smearing out the data. However, these microlensing effects don't change the supernova's colour during the supernova's first three weeks, so comparing the colours of the two images over time could yield very precise time delay measurements.
In short, scientists have theorised that learning the specifics of how massive galaxies warp the light of supernovae behind them could help solve the mystery of why we can't agree on how quickly the universe is expanding.
Sherry Suyu, principle investigator of the H0LiCOW experiment which measures the Hubble constant, told Gizmodo that she thought that eventually this time delay in these type Ia supernovae will be a great cosmological probe. But she warned: "This paper is more of a forecast of what might come, but it's not yet based on real observations." Scientists have only observed a single case of these strongly-lensed type Ia supernova to date.
More data is to come, however. Plenty of scientists are excited about the The Large Synoptic Survey Telescope, or LSST, currently under construction in Chile. Researchers at the National Energy Research Scientific Computing Center at the Lawrence Berkeley National Lab think these the LSST could spot almost 1,000 of these objects, according to modelling in a paper published recently in The Astrophysical Journal.
There are other probes, too. The discovery of gravitational waves from colliding neutron stars has offered scientists yet another conduit to take this measurement, by comparing analysis of the gravitational waves to the accompanying flash of light.
Ultimately, physicists love a good mystery. Suyu said: "I think it's a pretty exciting time that there's this discrepancy."