How Fast Is The Universe Expanding? Conflicting Estimates Continue To Fuel Debate

That the universe is expanding is a well known fact. What is less well known is the rate at which this expansion is taking place.

Initially, scientists had pegged the value of the Hubble Constant — a unit that is used to measure the rate of universe's expansion and is named after the astronomer Edwin Hubble — at roughly 43.7 miles per second per megaparsec (one megaparsec = 3.26 million light-years). However, a study published on the preprint website arXiv in April cast doubts over the figure.

The study, based on observations of thousands of Cepheid stars and hundreds of Type Ia supernovae, estimated, with an uncertainty of 2.4 percent, that the Hubble Constant is roughly 45.5 miles per second per megaparsec.

Just a month later, another set of measurements — made by the European Space Agency's Planck Satellite, which analyzed the pervasive cosmic microwave background radiation — put the value of Hubble Constant at approximately 41.6 miles per second per megaparsec — much lower than previous estimates.

Now, a new paper posted online on arXiv has also pegged the value of Hubble Constant at roughly 41.6 miles per second per megaparsec, with an error of 1.5 percent. The study is based on measurements by the Sloan Digital Sky Survey's Baryon Oscillation Spectroscopic Survey (BOSS), which studied patterns in the clustering of 1.2 million galaxies.

"At this point, I wouldn’t say that you would point at either one and say that there are really obvious things wrong," astronomer Wendy Freedman of the University of Chicago, who was not involved in any of the studies, told ScienceNews. "In order to ascertain if there’s a problem, you need to do a completely independent test."

An exact measurement of the rate of universe's expansion is crucial to further our understanding of "dark energy" — the mysterious force that is pulling galaxies apart. Depending on the value of the Hubble Constant, dark energy, which makes up roughly 68 percent of the universe, may turn out to be much stronger or weaker than previously estimated.

“You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right,” Adam Riess from the Space Telescope Science Institute and Johns Hopkins University, both in Baltimore, Maryland, who is the lead author of the supernova-based study, said in a statement in June. “But now the ends are not quite meeting in the middle and we want to know why.”

Whatever the reason behind the discrepancies, it gives rise to exciting possibilities — that Einstein’s theory of general relativity, which describes how gravity warps the fabric of space-time, may be incomplete, or that dark energy possessess some as-of-yet-unknown quality that is messing with the calculations.

"We know so little about dark energy, that would be my guess on where the solution most likely is," David Spergel, a theoretical astrophysicist at Princeton University, told ScienceNews.