Full speed ahead
The universe is expanding faster than scientists thought possible.
Imagine you’re driving down a hill and the car’s brakes fail, sending you careening downward at an ever-increasing speed. That scenario is frightening enough, but now imagine that the car begins to accelerate even faster, seemingly defying the laws of gravity. In a similar way, scientists have discovered our universe is accelerating much faster than they thought possible.
A team of American and Australian astronomers found that distant galaxies are flying away from us about 8% more rapidly than they should be, given the starting speed of acceleration shortly after the Big Bang.
“A funny universe just got funnier,” said Brad Tucker, an Australian National University astronomer and co-author of the work, which can be found on Arxiv and will appear in The Astrophysical Journal. This faster-than-expected rate of expansion could mean the universe is gearing up to tear itself apart in what some are calling the “Big Rip”.
Astronomers have known since the 1930s that the universe is expanding. But in the 1990s scientists made a strange discovery: the universe wasn’t just drifting apart in the aftermath of the Big Bang, it was accelerating. Brian Schmidt, Adam Riess and Saul Perlmutter, who won the 2011 Nobel prize for that finding, named the driving force “dark energy”. Scientists still don’t know what it is, but they do know it represents the chief ingredient of the universe, comprising 70% of its matter-energy mix.
Now a team of astronomers, led by Riess at Johns Hopkins University, have found that not only is the universe expanding, it’s accelerating faster than expected. Using the Hubble Space Telescope, they clocked the speed of 18 retreating galaxies over a two-year period. They focused on specific beacons within each galaxy – 2400 Cepheids stars and 300 Type Ia supernovae. Since both types have a standard luminosity, it’s a bit like gauging the distance to a town from the brightness of its streetlights. The astronomers also measured the speed of the stars’ retreat using Doppler shift – the same principle police radar scanners use to pinpoint a passing car’s speed.
Riess and colleagues have calculated the most accurate figure so far for the expansion rate of the universe, known as the Hubble constant. The new calculation shows the universe is expanding at about 73 kilometres per second per megaparsec (3.26 million light-years) with an uncertainty of 2.4%. This means that a galaxy one million light years from us is rushing away at about 22.4 kilometres per second, while another galaxy two million light years distant is flying away twice as fast, at 44.8 kilometres per second.
The problem is, this speed of expansion disagrees with that predicted from measurements using the faint glow of radiation left over from the Big Bang, called the cosmic microwave background. Previous measurements had hinted that our universe was expanding faster than predicted, but the measurements were too inaccurate to be sure. The new set of precision measurements adds more weight to the view that there is something missing from our best theory of the universe.
“Maybe the universe is tricking us, or our understanding of the Universe isn’t complete,” says Alex Filippenko, an astronomer at University of California, Berkeley and co-author of the paper.
One possibility is that dark energy, already known to be accelerating the universe, may be growing stronger – in other words, that the acceleration of the universe is itself speeding up.
“The new results have an impact on the dark energy issue, although it’s too soon to say what,” says cosmologist Paul Davies at Arizona State University.
Alternatively, some physicists propose a yet-to-be-discovered particle may have been present during the early expansion of the universe and altered its acceleration.
One candidate for this particle is dark radiation, which is like the dark matter version of the photon.
But there is another possibility: that these distance measurements may not be as reliable as we think. So don’t throw out your physics textbook just yet.
MAYBE THE UNIVERSE IS TRICKING US, OR OUR UNDERSTANDING OF THE UNIVERSE ISN’T COMPLETE.