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One Number Shows Something Is Fundamentally Wrong with Our Conception of the Universe

an image of the large magellanic cloud showing cosmic expansion rate discrepancy
An image of the Large Magellanic Cloud taken with a ground-based telescope. The inset image was captured by the Hubble Space Telescope, and shows a galaxy cluster teeming with variable Cepheids, a class of stars that flicker regularly. Using this pulsation rate, scientists have calculated the universe's expansion rate, but that number doesn't match with values derived from other cosmic phenomena, such as the echo of the Big Bang known as the cosmic microwave background radiation. (Image credit: NASA, ESA, A. Riess (STScI/JHU) and Palomar Digitized Sky Survey)

There's a puzzling mystery going on in the universe. Measurements of the rate of cosmic expansion using different methods keep turning up disagreeing results. The situation has been called a "crisis."

The problem centers on what's known as the Hubble constant. Named for American astronomer Edwin Hubble, this unit describes how fast the universe is expanding at different distances from Earth. Using data from the European Space Agency's (ESA) Planck satellite, scientists estimate the rate to be 46,200 mph per million light-years (or, using cosmologists' units, 67.4 kilometers/second per megaparsec). But calculations using pulsating stars called Cepheids suggest it is 50,400 mph per million light-years (73.4 km/s/Mpc). 

Related: The Biggest Unsolved Mysteries in Physics

If the first number is right, it means scientists have been measuring distances to faraway objects in the universe wrong for many decades. But if the second is correct, then researchers might have to accept the existence of exotic, new physics. Astronomers, understandably, are pretty worked up about this discrepancy.

What is a layperson supposed to make of this situation? And just how important is this difference, which to outsiders looks minor? In order to get to the bottom of the clash, Live Science called in Barry Madore, an astronomer at the University of Chicago and a member of one of the teams undertaking measurements of the Hubble constant.

The trouble starts with Edwin Hubble himself. Back in 1929, he noticed that more-distant galaxies were moving away from Earth faster than their closer-in counterparts. He found a linear relationship between the distance an object was from our planet and the speed at which it was receding. 

"That means something spooky is going on," Madore told Live Science. "Why would we be the center of the universe? The answer, which is not intuitive, is that [distant objects are] not moving. There's more and more space being created between everything." 

Hubble realized that the universe was expanding, and it seemed to be doing so at a constant rate — hence, the Hubble constant. He measured the value to be about 342,000 miles per hour per million light years (501 km/s/Mpc) — almost 10 times larger than what is currently measured. Over the years, researchers have refined that rate.

Things got weirder in the late 1990s, when two teams of astronomers noticed that distant supernovas were dimmer, and therefore farther away, than expected, said Madore. This indicated that not only was the universe expanding, but it was also accelerating in its expansion. Astronomers named the cause of this mysterious phenomenon dark energy

Having accepted that the universe was doing something strange, cosmologists turned to the next obvious task: measuring the acceleration as accurately as possible. By doing this, they hoped to retrace the history and evolution of the cosmos from start to finish.

Madore likened this task to walking into a racetrack and getting a single glimpse of the horses running around the field. From just that bit of information, could somebody deduce where all the horses started and which one of them would win?

That kind of question may sound impossible to answer, but that hasn't stopped scientists from trying. For the last 10 years, the Planck satellite has been measuring the cosmic microwave background, a distant echo of the Big Bang, which provides a snapshot of the infant universe 13 billion years ago. Using the observatory's data, cosmologists could ascertain a number for the Hubble constant with an extraordinarily small degree of uncertainty. 

"It's beautiful," Madore said. But, "it contradicts what people have been doing for the last 30 years," said Madore. 

Over those three decades, astronomers have also been using telescopes to look at distant Cepheids and calculate the Hubble constant. These stars flicker at a constant rate depending on their brightness, so researchers can tell exactly how bright a Cepheid should be based on its pulsations. By looking at how dim the stars actually are, astronomers can calculate a distance to them. But estimates of the Hubble constant using Cepheids don't match the one from Planck.

The discrepancy might look fairly small, but each data point is quite precise and there is no overlap between their uncertainties. The differing sides have pointed fingers at one another, saying that their opponents have included errors throwing off their results, said Madore. 

But, he added, each result also depends on large numbers of assumptions. Going back to the horse-race analogy, Madore likened it to trying to figure out the winner while having to infer which horse will get tired first, which will gain a sudden burst of energy at the end, which will slip a bit on the wet patch of grass from yesterday's rain and many other difficult-to-determine variables. 

If the Cepheids teams are wrong, that means astronomers have been measuring distances in the universe incorrectly this whole time, Madore said. But if Planck is wrong, then it's possible that new and exotic physics would have to be introduced into cosmologists' models of the universe, he added. These models include different dials, such as the number of types of subatomic particles known as neutrinos in existence, and they are used to interpret the satellite's data of the cosmic microwave background. To reconcile the Planck value for the Hubble constant with existing models, some of the dials would have to be tweaked, Madore said, but most physicists aren’t quite willing to do so yet. 

Hoping to provide another data point that could mediate between the two sides, Madore and his colleagues recently looked at the light of red giant stars. These objects reach the same peak brightness at the end of their lives, meaning that, like with the Cepheids, astronomers can look at how dim they appear from Earth to get a good estimate of their distance and, therefore, calculate the Hubble constant.

The results, released in July, provided a number squarely between the two prior measurements: 47,300 mph per million light-years (69.8 km/s/Mpc). And the uncertainty contained enough overlap to potentially agree with Planck's results. 

But researchers aren't popping their champagne corks yet, said Madore. "We wanted to make a tie breaker," he said. "But it didn't say this side or that side is right. It said there was a lot more slop than everybody thought before."

Other teams have weighed in. A group called H0 Lenses in COSMOGRAIL's Wellspring (H0LICOW) is looking at distant bright objects in the early universe called quasars whose light has been gravitationally lensed by massive objects in between us and them. By studying these quasars, the group recently came up with an estimate closer to the astronomers' side. Information from the Laser Interferometer Gravitational-Wave Observatory (LIGO), which looks at gravitational waves from crashing neutron stars, could provide another independent data point. But such calculations are still in their early stages, said Madore, and have yet to reach full maturity. 

For his part, Madore said he thinks the middle number between Planck and the astronomers' value will eventually prevail, though he wouldn't wager too much on that possibility at the moment. But until some conclusion is found, he would like to see researchers' attitudes toned down a bit. 

"A lot of froth has been put on top of this by people who insist they're right," he said. "It's sufficiently important that it needs to be resolved, but it's going to take time." 

Originally published on Live Science.

Adam Mann
Adam Mann is a journalist specializing in astronomy and physics stories. His work has appeared in the Wall Street Journal, Wired, Nature, Science, New Scientist, and many other places. He lives in Oakland, California, where he enjoys riding his bike. Follow him on Twitter @adamspacemann.