Universe is expanding faster than physics can explain


Astronomers have calculated the most precise measurement of how fast the Universe is expanding and perhaps only dark physics can explain it. Cathal O'Connell reports.


Cepheid variable stars in this spiral galaxy, known as UGC 9391, and a Type Ia supernova (not visible in this image) were used by a team of astronomers to calculate the expansion rate of the modern Universe.
NASA / ESA / L. FRATTARE (STSCI)

The Universe is expanding much faster than it should be according to known laws of physics, a collaboration of American and Australian astronomers has found.

Using the Hubble Space Telescope, the team performed the most precise measurement ever of how fast distant galaxies are flying away from us, and found it’s about 8% faster than our best predictions dictate.

The discrepancy could mean that dark energy, the mysterious force that’s accelerating the expansion of the Universe, is increasing in strength. Or, it could indicate the existence of an exotic new particle that gave the Universe a kick in its early days.

“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.

So is it time to gather your physics books into a small pile and strike a match?

The idea that the Universe is expanding isn’t a new one. Edwin Hubble uncovered that fact in the 1930s.

But in the 1990s, things got even weirder. Physicists discovered that that expansion was accelerating – and whatever’s driving that acceleration, so-called dark energy, is the major constituent of the Universe.

Now, scientists have found that even that acceleration is faster than expected, and it could mean that the Universe is gearing up to tear itself apart in the so-called Big Rip.

In the new work, researchers led by Adam Riess, who shared the 2011 Nobel Prize for discovering dark energy, used the light of two kinds of star to clock the speed of 18 galaxies.

For two and a half years the team used the Hubble Space Telescope to monitor the brightness of 2,400 Cepheid stars and about 300 Type Ia supernovae within these galaxies.

Astronomers use these astronomical objects as distance markers in the cosmos because they each have well-known luminosity (or inherent brightness). That brightness decreases predictably the further away they are from Earth – a bit like gauging the distance to a town from the brightness of its streetlights.

The astronomers also measure the speed using Doppler shift. Wavelengths of light can be stretched and compressed depending on relative motion – police radar scanners use the Doppler effect to pinpoint a passing car’s speed.

Reiss and colleagues calculated the expansion rate of the Universe (known as the Hubble constant) by looking at how the speed of galaxies and their distance from us are related.

Pulsating Cepheid variable stars and Type Ia supernovae were used to calculate how fast the Universe expands.
NASA / ESA / A. FEILD (STSCI) / A. RIESS (STSCI / JHU)

The new calculation shows the universe is expanding at about 73 kilometres per second per megaparsec (a megaparsec is 3.26 million light-years) with an uncertainty of 2.4%.

This means that a galaxy a 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 from us 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.

Astrophysicists make these predictions by looking at a picture of the early Universe, and running it forward in time while incorporating all known physics (including values for dark matter and dark energy, and assuming dark energy density stays constant).

Previous measurements had hinted at a discrepancy, but the uncertainty was always too great to know for sure. The new measurement puts daylight between the measurement and our best theory of the Universe – and this means something has to give.

“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 – pushing galaxies away from each other with ever greater force. In other words, that the acceleration of the Universe is itself accelerating.

Alternately, the Universe’s expansion rate may have been given a kick in the past through the action of a previously undocumented subatomic particle whizzing through the early cosmos at near the speed of light. One candidate for this particle is dark radiation, which is like the dark matter version of the photon.

There is, of course, a third possibility: that these distance measurements may not be as reliable as we think.

So don’t throw out your physics textbook just yet.

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Cathal O'Connell is a science writer based in Melbourne.