January 11, 2018


Expanding cosmos hints at new physics (Paul Rincon Science editor, BBC News)

To calculate the Hubble Constant, Prof Riess and others use the "cosmic ladder" approach, which relies on known quantities - so-called "standard candles" - such as the brightness of certain types of supernova to calibrate distances across space.

However, a different approach uses a combination of the afterglow of the Big Bang, known as the Cosmic Microwave Background (CMB), as measured by the Planck spacecraft and a cosmological model known as Lambda-CDM.

The Hubble Constant obtained using these data is 66.9 kilometres per second per megaparsec. (A megaparsec is 3.26 million light-years, so it follows that cosmic expansion increases by 66.9km/second for every 3.26 million light-years we look further out into space).

The gap between the two is now at a confidence level of about 3.4 sigma. The sigma level describes the probability that a particular finding is not down to chance. For example, three sigma is often described as the equivalent of repeatedly tossing a coin and getting nine heads in a row.

A level of five sigma is usually considered the threshold for claiming a discovery.

However, Prof Riess said that at the three sigma level "this starts to get pretty serious I would say".

"In fact, in both cases of measurements, these are very mature measurements... both projects have done their utmost to reduce systematic errors," he added.

Indeed, a recent measurement of time delays in quasars that is completely independent of the cosmic distance ladder data gets very similar results to Prof Riess's late Universe Hubble Constant. For the early Universe, a 2017 analysis using the density of baryonic (normal) matter in the cosmos yields a very similar value as the one obtained by the Planck team.

What this all suggested, he said, was that the Universe is now expanding 9% faster than expected based on the data - a result he described as "remarkable".

Posted by at January 11, 2018 6:21 AM