New discovery of universe's expansion speed puzzles astronomers

A team of astronomers has obtained what they believe to be the most precise measurement of how fast the universe is expanding, but the result seems to be puzzling, astrophysics news outlets reported Thursday.

Using data from the Hubble Space Telescope and the Keck I telescope in Hawaii, the astronomers report in an upcoming issue of the Astrophysical Journal that the current expansion rate of the universe is 73.24 (plus or minus 1.74) kilometers per second per megaparsec. A megaparsec equals 3.26 million light-years.

However, measurements by the European Space Agency's Planck Observatory a few years ago of fluctuations in the cosmic microwave background radiation, which documented the universe's early history, combined with the current Standard Model of physics, led to the prediction that the expansion rate is 66.53 (plus or minus 0.62) kilometers per second per megaparsec.

The discrepancy that the universe is expanding 9 percent faster than expected means either that measurements of the cosmic background radiation are wrong, or that some unknown physical phenomenon is speeding up the expansion.

"If you really believe our number -- and we have shed blood, sweat and tears to get our measurement right and to accurately understand the uncertainties -- then it leads to the conclusion that there is a problem with predictions based on measurements of the cosmic microwave background radiation, the leftover glow from the Big Bang," said Alex Filippenko, a professor of astronomy at University of California, Berkeley, and co-author of the new paper.

The cause, according to the astronomers, could be the existence of another unknown subatomic particle, perhaps an often-hypothesized fourth flavor of neutrino; or that the influence of dark energy, which pushes and accelerates the expansion of the universe, has increased over the 13.8 billion-year history of the universe. Or perhaps, Albert Einstein's general theory of relativity, the basis for the Standard Model, is slightly wrong.

Another possible explanation is that dark matter, as the backbone of the universe upon which galaxies built themselves into the large-scale structures, possesses some weird, unexpected characteristics.

"If we know the initial amounts of stuff in the universe, such as dark energy and dark matter, and we have the physics correct, then you can go from a measurement at the time shortly after the Big Bang and use that understanding to predict how fast the universe should be expanding today," said the leader of the study Adam Riess, of the Space Telescope Science Institute and Johns Hopkins University. "However, if this discrepancy holds up, it appears we may not have the right understanding, and it changes how big the Hubble constant should be today."

A former UC Berkeley post-doctoral fellow, Riess won the 2011 Nobel prize in physics for discovering the accelerating expansion of the universe in 1998.

About the new discovery, Riess believes that "this surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95 percent of everything and don't emit light, such as dark energy, dark matter and dark radiation."

"We've done the world's best job of decreasing the uncertainty in the measured rate of universal expansion," said Filippenko, "Yet we find that our measured rate of expansion is probably incompatible with the rate expected from observations of the young universe, suggesting that there's something important missing in our physical understanding of the universe."

"Maybe the universe is tricking us, or our understanding of the universe isn't complete," he said. Endi