Gravitationally lensed supernova yields novel Hubble constant result

The universe is expanding. That much is certain to astronomers. What continues to be up for debate is how fast the expansion is happening. Over the decades, different methods of measuring the universe’s expansion rate, also known as the Hubble constant or H₀, have produced different values .

A novel analysis of a supernova provides an important new result for H₀. Using a method called time-delay cosmography with a burst named Supernova Refsdal, a team of astronomers has reported in Science an H₀ value of 66.6 + 4.1 – 3.3 km/s/Mpc. That compares with values of 70–74 km/s/Mpc attained through measurements of standard candles—objects such as type Ia supernovae and pulsing red giant stars called Cepheids that have known luminosities, which allow astronomers to determine their distance from Earth by measuring how bright the objects appear. Measurements of H₀ made using the cosmic microwave background (CMB) radiation from the early universe, on the other hand, yield a rate of about 67 km/s/Mpc.

Although the new time-delay measurement is closer to the CMB value, there is still enough wiggle room for an agreement with standard candle measurements. “Measuring the Hubble constant using many different techniques is the way we’re going to ultimately convince ourselves that an accurate measurement has been made,” says Wendy Freedman, an observational cosmologist at the University of Chicago who was not involved in the research.

Multiple supernova images

Time-delay cosmography was pioneered in the 1960s by the namesake for the newly analyzed supernova, Norwegian astrophysicist Sjur Refsdal. When in line with a distant object, such as a supernova, a massive foreground galaxy or galaxy cluster bends the object’s light via strong gravitational lensing. As a result, the light takes different paths around the galaxy, and the object appears to observers as multiple images. Refsdal realized that by measuring the differences in arrival time of the photons from the respective images, astronomers could find the absolute distance between the galaxy and the object it is lensing.

That distance, plus a measure of velocity via redshift, would enable the calculation of H₀ with great precision. Previously, the H0LiCOW (H₀ Lenses in COSMOGRAIL’s Wellspring) collaboration used gravitationally lensed quasars to measure H₀. That collaboration’s latest result is a value of about 73 km/s/Mpc, which is consistent with standard candle measurements .

A team of astronomers at the University of California, Berkeley, first spotted multiple images of Supernova Refsdal in 2014 during a survey for gravitationally lensed supernovae. Just over a year later, another image appeared of the supernova, which is located about 14 billion light-years from Earth.

Patrick Kelly, an astronomer at the University of Minnesota who was on the team that discovered the supernova while he was a postdoctoral researcher, and his colleagues used the time delay to calculate the angular distance between the supernova and the foreground galaxy cluster that acted as the gravitational lens. The researchers modeled the mass distribution of the galaxy cluster, which is composed mostly of dark matter, to help figure out the path taken by the lensed light and thus the distance to the supernova. Finally, after using redshift to determine the supernova’s velocity relative to Earth, the team was able to derive a value for H₀. “This is an independent measurement of the Hubble constant,” says Kelly.

Resolving the tension

The novelty of this latest Hubble constant measurement means it is possible that there are sources of uncertainty that have not yet been identified. The uncertainties associated with the lensing method are completely different from those inherent in standard candle measurements. Until more measurements are made with other lensed supernovae, the researchers can’t be sure which systematic uncertainties are at play.

The discrepancy in H₀ values should be taken seriously, says Roger Blandford, an astrophysicist at Stanford University who was not involved in the new study. The measurements were attained by highly skilled teams of scientists who put a lot of effort into their research, he says. “That doesn’t take away the need to understand what is happening in these other measurements,” he adds.

There is a chance that the discrepancy is not an error but the reality of the universe. The CMB data provide a map of the universe from about 400 000 years after the Big Bang. Meanwhile, standard candle measurements of H₀ are direct measurements of the expansion rate of the much more recent universe. Freedman says it’s possible that both values are correct and that the expansion rate of the universe has changed over time. “Either we’re going to ultimately find that we agree with the cosmic microwave background … or that, in fact, the difference is real,” Freedman says. “So either way, it’s an important result.”

Resolving the Hubble tension will take future observations, likely with more powerful instruments. Freedman says her group is using data from the James Webb Space Telescope to do standard candle measurements with Cepheids as well as with two other classes of celestial objects. “The resolution of the telescope is fantastic,” she says. “It’s almost night and day comparing the data that are possible to get with Hubble with what JWST will now bring to this problem.” The new telescope will also observe another supernova, aptly named H0pe, to enable another time-delay cosmography measurement of the Hubble constant.