The new studies run counter to calculations based on observations of the early universe. Is this bad news for the standard cosmological model?
The Hubble constant expresses the universe’s present-day rate of expansion. There’s only one current expansion rate of the universe, but different studies are coming up with different answers for what it is.
Calculations based on observations of the early universe — namely, the cosmic microwave background (CMB) that is a sort of afterglow of the Big Bang — produce one answer for the Hubble constant. Observations of the “late universe” instead compare the distances of astronomical objects, often standard candles with known distances, to the speed those objects are moving away from us. The two techniques provide different answers, a discrepancy that has become known as the Hubble-constant tension.
No one knows why the early and late methods give different answers. At first, people thought that more and better measurements would cause the numbers to converge. But instead, in study after study, the error bars have shrunk to the point where the difference has become statistically significant.
The first and most precise measurements of the current expansion rate were made using standard candles, sources with known luminosity. If we know how luminous a source is, then we can reckon its distance according to how bright it appears. The two most widely used standard candles are Cepheid variable stars and Type 1a supernovae, but there are many others.
Two independent groups using data from the Hubble Telescope have just published new studies measuring the Hubble constant in different ways, yet their results match. The results give further credence to the late-universe consensus of an expansion rate around ~73 km/s/Mpc. But it also serves to deepen the tension, as studies published in the last decade calculating the rate from the properties of the CMB give an answer around ~67 km/s/Mpc. This may seem like no big deal, but the difference is big enough that some astrophysicists are calling it a “crisis for cosmology.”
The first study, led by Adam Riess (Space Telescope Science Institute), uses Cepheid variables, which are typically nearby, as a stepping stone to Type Ia supernovae, which can be seen much farther away. Using Hubble to measure Cepheids in galaxies hosting Type Ia supernovae, they measure the Hubble constant more precisely than any late-universe method has done so far.
The second study, led by John Blakeslee (NSF's NOIRLab), uses surface brightness fluctuations (SBF) of 63 elliptical galaxies to calculate the Hubble constant. These fluctuations were first suggested as a tool for measuring intergalactic distances in 1988, but this is the first time they’ve been used in this way. The astronomers selected the galaxies from the MASSIVE galaxy survey, a study of the 100 biggest galaxies within 300 million light-years.
The SBF method looks for the differences in brightness between pixels within an image of a galaxy. For nearby galaxies, there are relatively few stars per pixel, and the statistical fluctuations in the number of stars per pixel is higher. As a result, nearby galaxies appear somewhat "bumpy" in their light distributions. For more distant galaxies, there are many more stars per pixel and the pixel-to-pixel variations consequently are lower, making the galaxies smoother in appearance. The relative smoothness of one galaxy compared to another, which may appear similar in total brightness, is a good indication that the smooth one is farther away.
“Surface brightness fluctuation is an alternative to methods such as Type 1a supernovae,” Blakeslee says. “It can be calibrated independently, and it occurs in different kinds of galaxies.”
Resolving the Tension
Some astronomers, like George Efstathiou (University of Cambridge, UK) think that all the late-universe measurements have systematic problems. But the more studies that are published from different groups, using different independent techniques, and looking at different parts of the sky, the less likely it seems that they would all give the same wrong answer.
It doesn’t seem likely that there is a problem with the CMB calculation, either. The measurement from the European Space Agency’s Planck mission is considered to be one of the most elegant and well-supported pieces of physics ever. But if all the late-universe calculations have been done correctly as well, then this could mean something is wrong with the standard cosmological model itself.
“Cosmic microwave background radiation doesn't give a direct measurement of the Hubble constant today,” Blakeslee says. “It needs to be combined with a cosmological model, which can then predict the expansion history of the universe.” And it’s possible that something vital has been “lost in translation.”
Hsin-Yu Chen (MIT), a member of the LIGO collaboration, and the author of a few studies pioneering the use of gravitational waves to measure the current expansion rate, says that everyone disagrees about the nature of the disagreement between early and late universe measurements.
“Perhaps the discrepancy is from systematic errors,” she says. “Or maybe the standard cosmological model needs to be fixed. Maybe it is wrong. Everyone has their own opinion.”
With the LIGO and VIRGO collaborations restarting more advanced operations next year, there will be a wealth of new gravitational-wave detections to analyze. Chen predicts that enough events will be observed in the next few years to give a Hubble constant measurement with only a 2% margin of error.
And when the James Webb Space Telescope launches this year, it will provide improved data for all the late-universe techniques. Whether the future is bleak or bright for the standard cosmological model, it seems as though we will have clarity as to the true nature of the disagreement before long.