There’s a perplexing mystery going on in our universe. Measurements of the rate of cosmic expansion using different methods keep resulting in disagreeing results. The state of affairs has been called a “crisis.”
The crux of the problem is the Hubble constant. Named after the renowned American astronomer Edwin Hubble, this unit defines the rate of expansion of the universe at different distances from Earth. Using data from the European Space Agency’s (ESA) Planck satellite, scientists evaluated the rate to be 46,200 mph per million light-years i.e. 67.4 kilometers/second per megaparsec but calculations using pulsating stars known as Cepheids compute to 50,400 mph per million light-years i.e. 73.4 km/s/Mpc.
If the first calculation is right, it implies that for many decades now, scientists have been wrongly measuring distances to faraway objects in the universe. But if the second value is correct, then researchers might have to accept the existence of mysterious, new physics. Understandably, astronomers are pretty worried about this discrepancy.
To understand how important this discrepancy is, Live Science spoke to Barry Madore, an astronomer at the University of Chicago and a member of one of the teams responsible for measuring the Hubble constant.
The inconsistency has its roots back in 1929 when Edwin Hubble himself noticed that the more-distant galaxies were moving away from Earth much faster than their closer-in counterparts. He devised a linear relationship between the speed at which an object was receding away from our planet and its distance from us.
Madore told Live Science, “That means something spooky is going on. 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 understood that the universe was expanding, and it appeared to be doing so at a constant rate hence, the Hubble constant was computed. He measured the value to be approximately 342,000 miles per hour per million light-years i.e. 501 km/s/Mpc and nearly 10 times larger than the current measurement value. Over the years, researchers have revised that rate.
In the late 1990s, when two different teams of astronomers observed that distant supernovas were dimmer, and thus farther away than expected. This specified that not only was the universe expanding, but it was also accelerating in its expansion. Astronomers termed the cause of this mysterious phenomenon dark energy.
After accepting that the universe was doing something strange, cosmologists moved on to the next obvious task of measuring the acceleration as accurately as possible. They hoped that by doing this they may succeed in retracing the history and evolution of the cosmos from start to finish.
Madore compared this task to walking into a racetrack to catch a single glimpse of the horses running around the field. Was just a glance enough to deduce where all the horses started and which one of them would win?
Although it sounds impossible, scientists continue to try computing the expansion rate. For the last decade, the Planck satellite has been studying the Cosmic Microwave Background (CMB), a distant echo of the Big Bang, which offers a snapshot of the infant universe 13 billion years ago. With this observatory’s data, cosmologists could determine a number for the Hubble constant with an extremely small degree of uncertainty.
Madore said, “It’s beautiful. But it contradicts what people have been doing for the last 30 years”.
Over these three decades, astronomers have also been deploying telescopes to observe distant Cepheids to calculate the Hubble constant. These distant stars flicker at a constant rate dependent on their brightness, so researchers can pinpoint exactly how bright a Cepheid should be from its rate of pulsations. By studying how dim the stars actually are, astronomers can calculate the distance to them. Unfortunately, estimates of the Hubble constant using Cepheids don’t match the one obtained from Planck.
Although the discrepancy appears fairly small, each data point is quite precise with absolutely no overlap between their uncertainties. The differing sides have even pointed fingers at one another, claiming that their opponents have included errors that deviate their results, said Madore.
But he explained that each result also depends on large numbers of assumptions. Referring to the horse-race analogy mentioned earlier, Madore equated this situation to trying to work out the winner while having to deduce parameters like- which horse will get tired first, which will slip a bit on the wet patch of grass from yesterday’s rain, which will gain a sudden burst of energy at the end and many other difficult-to-determine variables.
The implication of the Cepheids teams being wrong is hat astronomers have been measuring distances in the universe incorrectly this whole time, Madore explained. But if Planck’s method is wrong, then it’s possible that new and exotic physics would need to be incorporated into cosmologists’ models of the universe, he added. These models comprise different dials like 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. In order to reconcile the Planck value for the Hubble constant with existing models, some of the dials may need to be tweaked, Madore clarified, but most physicists aren’t quite willing for that yet.
In a bid to provide another data point that could mediate between the two sides, Madore and his colleagues recently considered the light of red giant stars. These cosmic objects reach the same peak brightness at the end of their lives, so similar to the Cepheids, astronomers can gauge how dim they appear from Earth to work out a good estimate of their distance and, consequently, calculate the Hubble constant.
The results were released this July and presented a number squarely between the two prior measurements: 47,300 mph per million light-years (69.8 km/s/Mpc with the uncertainty containing enough overlap to potentially agree with Planck’s results.
But researchers aren’t popping their champagne corks yet, according to Madore.
He said, “We wanted to make a tie breaker. 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 also made attempts. A group named H0 Lenses in COSMOGRAIL’s Wellspring (H0LICOW) is concentrating on distant bright objects in the early universe known as quasars whose light is gravitationally lensed by massive objects located between us and them. After studying these quasars, the group recently calculated an estimated value closer to the astronomers’ side. Information from the Laser Interferometer Gravitational-Wave Observatory (LIGO which observes gravitational waves from crashing neutron stars, may provide another independent data point. But according to Madore, such calculations are still in their early stages and yet to reach full maturity.
For his part, Madore believes the middle number between Planck and the astronomers’ value will eventually prevail but he wouldn’t wager too much on that possibility at present.
He concluded, “A lot of froth has been put on top of this by people who insist they’re right. It’s sufficiently important that it needs to be resolved, but it’s going to take time.”