The Hubble Space Telescope has been used to measure the phenomenon first observed by the astronomer after which it was named: the expansion of the universe. It is the most precise measurement of the rate of expansion yet made, and it confirms something that’s been bothering astronomers for a while. The universe appears to be expanding faster than our measurements of the cosmic background radiation indicate it should be, suggesting either one of the measurements is wrong, or the expansion is being influenced by an unknown force.
As the universe expands, galaxies get further apart. Investigations into the way this is happening have been central to some of the most important discoveries in cosmology over the last century, but it seems we’re not done yet.
We can precisely measure the rate at which a galaxy is moving away from us from how much the wavelength of its light has stretched (known as red shift). Measuring the distance is harder, relying on the brightness of specific stars and supernovas known as “standard candles”. Comparing their brightnesses can indicate distance.
Using Hubble’s more exact measurements of certain standard candles, a paper to be published in The Astrophysical Journal reports the Hubble constant, a measure of the expansion of the universe, as 67 (42 miles) kilometers per second per megaparsec. This means that for every additional million parsecs (3.3 million light-years) away from us a galaxy is, its speed increases by 67 kilometers (42 miles) per second.
The problem is this figure is 9 percent larger than the value we calculate using observations of the situation shortly (378,000 years being “shortly” to cosmologists) after the Big Bang. This is based on the Planck satellite’s measurements of the cosmic background radiation created at that time, processed using models of how this should’ve changed as the universe evolved.
Previous estimates of the current Hubble constant had also been higher than that derived from the Planck data, but there was uncertainty in the measurements. Using new methods to calibrate the brightness of Cepheid variables, the standard candles used to measure relatively nearby galaxies, Hubble decreased its uncertainty range to 2.3 percent. Getting the Hubble constant accurate to within 10 percent was one of Hubble’s original goals, so this dramatically exceeds that. With this newfound precision, the chance of the current and early universe expansions aligning drops to one in 5,000, which seems a bit low to rely on.
“Both results have been tested multiple ways, so barring a series of unrelated mistakes, it is increasingly likely that this is not a bug but a feature of the universe,” said Professor Adam Riess of the Space Telescope Science Institute. Riess shared the 2011 Nobel Prize in physics for his discovery that the universe’s expansion is accelerating, not slowing down, leading to the acceptance of the idea of dark energy.
If both measurements are right, then our understanding of the laws of physics at large scales needs an overhaul. Several possible explanations have been proposed, but at this stage, we have little basis for choosing any of them.
One holds that dark energy, the force causing the universe’s expansion to accelerate, is itself accelerating. That is, the universe isn’t just growing faster, but growing faster at an increasing rate, like a skier going from a gentle slope to a steadily steeper downhill run. Given how little we know about dark energy, we can’t rule this out.
Another idea posits a currently undetected subatomic particle, affected by gravity but immune to the other three fundamental forces. Known as a sterile neutrino, such a particle would travel close to the speed of light, affecting how the universe evolves.
The observations could also be explained if astronomers have underestimated the interactions between dark matter, which we know of from the rotation rate of galaxies but don’t fully understand, and the directly observable parts of the universe.
Each of these theories, if correct, would interfere with the models we use to apply the data from the early universe to today’s situation. They’d also keep many theoretical physicists busy for years trying to explain the implications.