• A Cepheid is a star that varies in light intensity over regular, measurable periods. This still from an animation shows a Cepheid in the arm of a spiral galaxy. (NASA)Source: NASA
Astrophysicists have discovered that the cosmos is expanding at a faster rate than expected, and it may be due to mysterious dark radiation.
Kemal Atlay

3 Jun 2016 - 11:49 AM  UPDATED 3 Jun 2016 - 11:49 AM

We know that everything in our universe has been spreading out since the Big Bang, but a new study shows this expansion is happening faster than previously thought, and we don’t yet know why.

An international team of researchers, including astrophysicists from Australian National University, used the Hubble Space telescope to study star movements and discovered that the universe is expanding around five to nine per cent faster than early in its life. The study was published today in Astrophysical Journal.

“What the project was to do was to measure accurately what our universe is doing right now, and if our model of the universe works and agrees with everything we should get what we expect,” ANU researcher Dr Brad Tucker tells SBS Science.

“It was a way to validate previous other measurements and make sure everything is in agreement - but it’s not.”

Measurements could be off

The researchers, led by Nobel Laureate Dr Adam Reiss from the Space Telescope Science Institute and the John Hopkins University in the US, measured the movements of around 2,400 Cepheid stars and about 300 Type Ia supernovae over two and a half years. This method is known as the cosmic distance ladder.

They then used the parallax shift to calculate the Universe’s expansion rate, also known as the Hubble constant, and found it to be 73.2 kilometres per second per megaparsec (one megaparsec is equal to 3.26 million light years) with a very low uncertainty of just 2.4 per cent. This means that the distance between cosmic objects will double in another 9.8 billion years.

The original Hubble constant – 70.4 kilometres per second per megaparsec – had been calculated using a different method that relied on measurements of the afterglow from the Big Bang 13.8 billion years ago.

“A simple answer could be that our measurements are off, that’s always possible,” says Tucker. “But if our understanding of everything else in the universe is accurate and true, then that probably means there’s potentially something else in the universe.”

Dark radiation at play?

One of the theories proposed to explain this Hubble constant discrepancy is that an undetected force called dark radiation – a collection of subatomic particles known as neutrinos that travel close to the speed of light – could be behind the Universe’s expansion by pushing galaxies away from each other faster than previously measured.

Dr Alan Duffy, an astrophysicist from Swinburne University of Technology, is excited by the findings, but he shares the researchers’ caution that there may be something amiss in the measurements.

“It’s a beautiful example of precision astronomy, they made a difficult measurement and did it incredibly well,” says Duffy.

“But, as incredible as it is, they’re using very ‘messy’ objects in stars brightening or blowing up to try to get these distances… I would love this to be right, but my guess is there’s some gremlin hiding in the data.”

The next step in the research will be to verify the findings using different techniques, such as the TAIPAN survey being led out of Siding Spring Observatory, and bring the uncertainty value down even lower. 

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