We might need new physics to explain the universe
“This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke”
Adam Riess
The universe is expanding faster than expected, suggesting that astronomers may have to incorporate some new physics into their theories of how the cosmos works, a new study reports.
The revised expansion rate is about ten per cent faster than that predicted by observations of the universe's trajectory shortly after the Big Bang, according to the new research. The study also significantly reduces the probability that this disparity is a coincidence, from 1 in 3,000 to just 1 in 100,000.
"This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke," study lead Adam Riess, a professor of physics and astronomy at The Johns Hopkins University in Baltimore, US, said in a statement.
"This is not what we expected," said Riess, who won the Nobel Prize for physics in 2011, along with Brian Schmidt and Saul Perlmutter, for showing, in the late 1990s, that the universe's expansion is accelerating. It's unclear what's driving this surprising acceleration, but many astronomers invoke a mysterious, repulsive force called dark energy.
In the new study, Riess and his colleagues used the Hubble Space Telescope to study 70 Cepheid variable stars in the Large Magellanic Cloud (LMC), one of the Milky Way's satellite galaxies. Cepheid variables dim and brighten at predictable rates and are therefore ‘standard candles’ that allow astronomers to calculate distances.
Riess and his team also incorporated observations made by the Araucaria Project, a collaboration involving researchers in the United States, Europe and Chile who studied various LMC binary star systems, noting the dimming that occurred when one star passed in front of its neighbour. This work provided additional distance measurements, helping the study team to improve their understanding of the Cepheids' intrinsic brightness.
The researchers used all of this information to calculate the universe's present-day expansion rate, a value known as the Hubble constant after American astronomer Edwin Hubble. The new number is about 74.03 kilometres (46 miles) per second per megaparsec – one megaparsec is roughly 3.26 million light years.
The ‘expected’ expansion rate, by contrast, is about 67.4 kilometres (41.9 miles) per second per megaparsec. This projected rate is based on observations that Europe's Planck satellite made of the cosmic microwave background – the light left over from the Big Bang that created the universe 13.82 billion years ago.
"One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding," Riess added. "If these values don’t agree, there becomes a very strong likelihood that we’re missing something in the cosmological model that connects the two eras."