Hubble Trouble: A Crisis in Cosmology?

Unfortunately, no one knows where the discrepancies come from. Researchers affiliated with Planck have even re-analyzed data taken by the Supernova H0 for the Equation of State (SH0ES) collaboration without success. "There’s a lot of back and forth with people checking analyses and results and consistency," says Bradford Benson of the University of Chicago. In an April Meeting session titled "Crisis in Cosmology," researchers recapped the latest ideas for resolving the discrepancy.

It’s "tempting," says Stephen Feeney of the Flatiron Institute, to think that some part of the standard model of cosmology is wrong. The model describes how the universe evolved since the Big Bang. Given an expansion rate and a specified amount of matter and energy both dark and bright, it tells you how to calculate the Hubble constant using the temperature fluctuations in the CMB — the speckles of hot and cold on the map, which correspond to expansions and contractions of matter in the early universe. By changing the model, you can make the Hubble constant value match the supernova and star-derived values.

For example, adjusting the number of neutrinos in the λCDM model can help get rid of the discrepancy. "Even though neutrinos are light particles, since there are so many of them, they can affect the evolution of the universe," says Benson, who works on CMB observations at the South Pole Telescope. Perhaps, he says, more neutrino species exist than are now known. But according to Feeney, it’s hard to physically justify these changes.

Planck Collaboration

Analysis of the cosmic microwave background from the Planck satellite — one way to measure the Hubble constant

Estimates of the Hubble constant based on measurements of the cosmic microwave background (CMB) differ significantly from those based on a distance ladder built from "standard candles" like Type 1A supernovae (Local).

However, the problem could be much more mundane. "Though it’s not a very exciting hypothesis, one of the main ones is that there’s systematic uncertainty in one or more of the measurements," says David Jones of the University of California, Santa Cruz, who is part of SH0ES.

But no source of uncertainty really stands out. "It could be a problem with the way you’re taking the measurement; it could be a problem with the way you’re interpreting the data; it could be that you’re assuming too much about the things you’re looking at," says Feeney. "All of these things have been tested, and there’s not a convincing explanation."

One source of uncertainty comes from celestial distance measurements, which are required to calculate the Hubble constant when using stars or supernovae. To measure distance, cosmologists look for objects whose absolute brightness is known, so-called "standard candles" such as Cepheid stars, which pulsate at a frequency related to their brightness, or Type 1a supernovae, exploding stars of a consistent brightness. By comparing an object’s absolute brightness to its observed brightness, they can calculate how far away it is. They hope to reduce the distance uncertainty, in part, by using new data from the Gaia space observatory, which will measure distances to Cepheids using geometrical methods.

They also want to improve their distance measurements by developing new celestial yardsticks based on gravitational waves produced in neutron star collisions, the first of which the Laser Interferometer Gravitational-Wave Observatory observed last October. If they measure the gravitational wave along with an accompanying electromagnetic signal, they can calculate the distance from Earth to the neutron stars. These measurements can be used to confirm the accuracy of the standard candles. "They’ve got nothing to do with Cepheids, or supernovae, or the CMB," says Feeney. "They’re kind of an independent arbiter."

Researchers also need to understand the structure of the CMB in more detail. For example, many massive objects are positioned between Earth and the CMB, which create distortions — so-called gravitational lensing — on CMB maps. They don’t understand these distortions well, which could mean they have misinterpreted past measurements, according to Feeney.

Solving the Hubble puzzle would mean cosmologists could calculate one fundamental value from two independent phenomena that bookend the history of the universe. "In a time sense, you’re comparing one end of the universe to other," says Jones. The CMB, the oldest observable light in the universe, doesn’t have much in common with a single supernova or Cepheid star.

According to Benson, cosmologists deserve some credit already, despite the discrepancy. For a value calculated from such unrelated astrophysical phenomena, the numbers are astonishingly close. "We’re saying this is a ‘crisis in cosmology,’ but you can turn that on its head," he says. "The fact that the numbers agree at a five percent level is pretty remarkable."

The author is a freelance science writer in Tucson, Arizona.