The star RS Puppis varies in brightness by almost a factor of five every 40 or so days
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe CollaborationAcknowledgment: H. Bond (STScI and Penn State University)
Why is the universe expanding 9 per cent faster than we thought?
By Joshua Sokol
The expansion of the universe is getting out of hand. According to recent measurements by a Nobel prizewinning team, space is stretching 9 per cent faster than we think it should be – yanking distant galaxies away from us at a rate that defies easy explanation.
This accelerating expansion of the universe is usually explained by invoking a mysterious substance called dark energy. But either our observations are wrong, or dark energy isn’t enough to explain the situation. Maybe something even stranger is lurking in the cosmos.
“This is really an end-to-end test of our understanding of the universe,” says Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland.
A paper detailing the conflict, reported on the pre-print server Arxiv in April by Riess and his colleagues, has now been accepted by The Astrophysical Journal. It reveals the disagreement between the two best methods we have of measuring the expansion of the universe.
One method looks at dimples in the cosmic microwave background (CMB), a glow left behind by the hot, soupy universe just a few hundred thousand years after the big bang. Space-based observatories like WMAP and Planck have measured small fluctuations in temperature in the CMB. Assuming we understand the physics at play, the size of these fluctuations should let us calculate how quickly the universe was expanding when the universe began, some 13.7 billion years ago.
The other method, practised by Riess and his colleagues, measures how distant galaxies appear to recede from us as the universe expands, using stars and supernovae of known brightness to gauge the distance to those galaxies. These measurements led to the discovery of dark energy, and earned Riess and two other physicists a Nobel prize in 2011.
The trouble comes when we compare the two estimates. “They don’t agree,” Riess says.
Hints of this problem have been brewing since 2011, and the most recent results have only made the problem worse. Still, we could do away with the issue if one of the two benchmark measurements is just a little bit wrong, says David Spergel of Princeton University in New Jersey.
“Right now I think the discrepancy is at the level of, ‘This is interesting, it’s worth thinking about’,” he says. “But it’s not yet, ‘Let’s panic, we know that our current model must be wrong.’”
Storm in a teacup?
The measurements both of the ancient universe and the present day will get more accurate, which could expose a systematic effect that is tricking us into seeing a conflict where there isn’t one.
For example, Riess’s modern measurements rest on accurately knowing the distance to nearby pulsing stars. The European Space Agency’s Gaia mission, which is working to measure the distance to 1 billion Milky Way stars, should make those benchmark measures more precise.
The rate of expansion soon after the big bang might also be a little off if we aren’t correctly measuring the sizes of fluctuations in the cosmic microwave background. By looking at the microwave background in polarised light, observatories like the South Pole Telescope in Antarctica and the upcoming Simons Observatory in Chile might affect this.
“The results are pretty clear right now,” Spergel says. “In a year or two they will be better. Either the discrepancy will start to go away as the data improves, or it will turn out to be the signature of new physics.”
The situation has many physicists, including Riess, excited. It could mean that dark energy is growing denser over time, and the universe will eventually end in a catastrophic “big rip“. It could mean Einstein was wrong, requiring changes to his well-tested theory of relativity. Or it could invoke a shady new particle secretly pulling the universe’s strings.
Stay tuned for updates from us.
Journal reference: Astrophysical Journal, http://hubblesite.org/pubinfo/pdf/2016/17/pdf.pdf
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