Mar 16, 2021
One of the longest standing and most controversial questions in astronomy is — how fast is the universe expanding today? New work, including measurements made by Las Cumbres Observatory, has applied new techniques to the problem and found a surprising answer.
Astronomers call the local expansion rate of the universe the Hubble constant, H0, (pronounced H-naught). Measurements have gotten extremely precise in recent years — some claim to have measured it to better than a few percent. Different groups have come up with results that vary by more than 10% — far larger than the claimed uncertainty. Complicating matters, the measurements seem to cluster high or low depending on where they are made in the universe. The Hubble constant measured from nearby supernovae tends to be high, while measurements built up from the afterglow of the Big Bang — the Cosmic Microwave Background — give a low value. Some have argued that this is a crisis for the field, one requiring “new physics.” Perhaps an unknown property of Dark Energy is causing the local expansion rate of the universe to be highly sensitive to the distance at which it is measured. Others argue that there must be some kind of mistake in building the “distance ladder” — in using one set of distance indicators to calibrate another.
The new study, released March 12 in the journal Astronomy & Astrophysics, involves an international team of scientists led by Nandita Khetan, a PhD student at the Gran Sasso Science Institute in Italy, and an associate researcher at the Istituto Nazionale di Fisica Nucleare. It used the Surface Brightness Fluctuations of galaxies to calibrate the distances to nature’s best distance indicators — Type Ia supernovae. Type Ia supernovae are used as “standard candles” to map out distances in the universe. They were used to determine that the universe was accelerating in its expansion, leading to the discovery of Dark Energy that resulted in the 2011 Nobel Prize in Physics.
The standard candle method relies on measuring the apparent brightness of a distant known light, say a 100W light bulb, and using the difference between the apparent and intrinsic brightness to work out how far away the light is. This requires knowing the intrinsic power output —the wattage — of the “standard candle,” something that is unknown for Type Ia supernovae. Astronomers have to calibrate their brightness using a handful of nearby supernovae in galaxies with distances determined by other means. Traditionally this has been done with galaxies whose distances are known from observations of Cepheid variable stars. The new paper research swaps out the Cepheids for a different fundamental calibrator, Surface Brightness Fluctuations. This measures the resolution of individual stars in different galaxies, since stars tend to blur together the farther away a galaxy is. It is similar to how a street will appear rough when photographed up close, but smooth when seen from farther away.
The new study found an answer that is in between the two discordant values of the expansion rate of the universe. This argues that perhaps new physics isn’t needed after all. It may be that previous researchers overestimated the precision of their studies.
Andy Howell, a staff scientist at Las Cumbres Observatory, and adjunct faculty at the University of California Santa Barbara, is the Principal Investigator of the Global Supernova Project, a worldwide collaboration that provided some of the observations of supernovae used in the study. He explains, “At a recent conference about this Hubble Constant crisis, after each speaker walked through their methodology, I couldn’t find any problems with what they were doing. I started to question whether we do need new physics to explain the different Hubble constants. But now we, like several studies before ours, found an answer in the middle. Maybe there’s some weirdness to some of the other measurements that we don’t fully understand. That’s more comforting, because you don’t want to upend our understanding of physics unless you have to.”
The new work does not undermine the discovery or characterization of Dark Energy, since that relies on only relative, not absolute, measurements of supernovae and has been verified by other means.
The new supernova observations were obtained with Las Cumbres Observatory’s worldwide network of robotic telescopes, specifically designed to study time-variable phenomena like supernovae. Howell adds, “Supernovae are hard to observe, because you need just a little bit of telescope time per night, over months. But a robotic telescope network is perfect for this — nobody has to travel — the telescopes can make the observations wherever and whenever they are needed. This is what we built Las Cumbres Observatory for and I’m delighted to see it being used to refine our understanding of the universe.”
The study “A new measurement of the Hubble constant using Type Ia supernovae calibrated with surface brightness fluctuations” involves an international team of scientists with expertise in supernova observations, Surface Brightness Fluctuations, and theory working, at the Gran Sasso Science Institute, INAF, INFN, DARK-Niels Bohr Institute, University of Copenhagen, Centre for Astrophysics and Supercomputing, Swinburne University, Las Cumbres Observatory, UC Santa Barbara, and UC Davis.