Cosmic expansion rate confirmed

Galaxy cluster

As the universe expands, galaxies move further apart from one another. The rate at which the expansion is proceeding is determined by the Hubble constant, which has been newly measured with high precision.

  • Hubble constant used to gauge size and age of the universe
  • Previous measurements had a level of uncertainty
  • New measurement method confirms earlier results

A STUDENT WITH THE with the International Centre for Radio Astronomy Research (ICRAR) at the University of Western Australia, has calculated how fast the Universe is growing by measuring the Hubble constant.

“The Hubble constant is a key number in astronomy because it’s used to calculate the size and age of the Universe,” said PhD candidate Mr Florian Beutler.

As the Universe expands, it carries other galaxies away from ours. The Hubble constant links how fast the galaxies are moving with how far they are away from us.

By analysing light coming from a distant galaxy, the speed and direction of that galaxy can be easily measured. But determining the galaxy’s distance from Earth is much more difficult.

Until now, this has been done by measuring the brightness of individual objects (such as certain kinds of stars) within a galaxy and using what we know about those objects to calculate how far away the galaxy must be.

This approach is based on some well-established assumptions but is prone to systematic errors, leading Mr Beutler to tackle the problem using a completely different method.

Plot of 6df Galaxy Survey data

In this plot of 125,000 galaxies from 6df Galaxy Survey data, each dot is a galaxy and Earth is at the centre. (The dark slices are regions blocked from view.) The amount of galaxy clustering has been used (along with other data) to measure the expansion rate of the universe.

New method uses super survey

Published in the Monthly Notices of the Royal Astronomical Society, Mr Beutler’s work draws on data from a survey of more than 125,000 galaxies carried out with the UK Schmidt Telescope in eastern Australia.

Called the 6dF Galaxy Survey, this is the biggest survey of relatively nearby galaxies, covering almost half the sky.

Galaxies are not spread evenly through space, but are clustered. Using a measurement of the clustering of the galaxies surveyed, plus other information derived from observations of the early Universe, Mr Beutler has measured the Hubble constant with an uncertainty of less than 5%.

The new measurement is 67.0 (±3.2) kilometres per second per megaparsec. A megaparsec is 1 million parsecs, or 3.26 million light-years.

Good agreement

“This way of determining the Hubble constant is as direct and precise as other methods, and provides an independent verification of them,” says Professor Matthew Colless, Director of the Australian Astronomical Observatory and one of Mr Beutler’s co-authors.

“The new measurement agrees well with previous ones, and provides a strong check on previous work.”

The measurement can be refined even further by using data from larger galaxy surveys.

“Big surveys, like the one used for this work, generate numerous scientific outcomes for astronomers internationally,” says Professor Lister Staveley-Smith, ICRAR’s Deputy Director of Science.

Adapted from information issued by ICRAR / Images courtesy ICRAR / Chris Fluke, Centre for Astrophysics & Supercomputing, Swinburne University of Technology / NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI),G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA.

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