The universe is expanding faster and faster. This accelerated expansion has been measured in a variety of ways, but there is a major problem. Depending on whether you’re looking at the very distant universe or not, you get two different numbers for the expansion rate of the universe.
This crisis in cosmology has been vexing astrophysicists for several years now. A new study has measured the expansion rate of the universe with a different method and found that the discrepancy in the number is definitely there.
In a paper accepted in The Astrophysical Journal, the team described how they measured the infrared properties of 63 elliptical galaxies within 330 million light-years (100 million parsecs) of Earth. The astronomers used the surface brightness fluctuation (SBF) technique, which works for elliptical galaxies because they are old and have a consistent population of old stars.
The researchers measured the Hubble constant – also known as H0 (H-naught or H-zero) – finding a value for it of about 73 kilometers per second per megaparsec. This means that if two galaxies are 1 million parsecs apart, they would appear to be moving away from each other at a speed of 73 kilometers (45 miles) per second,
This is consistent with other methods that looked at the expansion rate in the local universe, but it is larger than what was measured in the very early universe. Observation of the cosmic microwave background put H0 at around 68 kilometers (42 miles) per second per Megaparsec.
“For measuring distances to galaxies out to 100 megaparsecs, this is a fantastic method,” co-author Professor Chung-Pei Ma, from the University of California, Berkeley, said in a statement. “This is the first paper that assembles a large, homogeneous set of data, on 63 galaxies, for the goal of studying H-naught using the SBF method.”
The fact that several independent methods all find different values is truly a challenge. There could be something we have not taken into account in our theories – or maybe the uncertainty on the measurements is too optimistic.
“The jury is out,” Professor Ma said. “I think it really is in the error bars. But assuming everyone’s error bars are not underestimated, the tension is getting uncomfortable.”
Whatever the cause, this tension is here to stay. Cosmology is at an important crossroads. Humanity’s understanding of the universe is both the best it has ever been, and yet limited. New observatories, both on the ground and in space, will hopefully provide new ways to look at the universe and help solve this mystery.