How scientists know 'dark energy' triggered expansion of Universe

Researchers have come up with a 3D map of the sky, which facilitates scientists to look into the time six billion years ago when dark energy became a dominant influence on the Universe?s expansion.

Using the new data, astronomers have been able to measure the exact distance to over a quarter of a million galaxies to gain new understanding of a key period in cosmic history.

?This is an incredibly exciting time to be working in cosmology, and we?re all privileged to be part of the Baryon Oscillation Spectroscopic Survey (BOSS),? the BBC quoted Prof Will Percival from the University of Portsmouth as saying at the UK National Astronomy Meeting in Manchester.

?What we?ve done is measure the 3D positions for just over 250,000 galaxies, covering the largest volume of the Universe ever surveyed. That gives us an amazing map that we can then analyse to try to understand how and why the Universe?s expansion is accelerating,? he said.

The discovery that everything in the cosmos is moving apart at a faster and faster rate was one of the major breakthroughs of the 20th Century, and the BOSS survey is a step to understand this phenomenon.

The quality of the BOSS map is big step forward on all previous such surveys.

It provides details on the position of galaxies out to some six billion light-years from Earth and gives those measurements to within 1.7% of their expected true value.

BOSS uses two techniques to understand the Universe?s acceleration, the first one of them being the so-called baryon acoustic oscillations.

These are pressure-driven waves that passed through the very early Universe and which were imprinted on the distribution of matter once conditions had cooled below a certain point.

Today, these ?wiggles? show themselves as a preferred scale in the separation of galaxies and can be used as a kind of standard ruler to measure the geometry of the cosmos.

?Because you can trace this pattern all the way through the Universe, it tells you a lot about its content,? said Portsmouth?s Dr Rita Tojeiro.

?If it had a different content - it had more matter, or it had less matter, or it had been expanding at a different rate - then you?d see a different map of the galaxies. So, the fundamental observation is this map,? she added.

The other technique involves ?redshift space distortions?. These describe the component in the velocity of galaxies that stems from the growth of structure in the Universe. The team can see if neighbouring galaxies are clustering in the way that would be expected from the action of gravity.

The BOSS project, which acquires all its data using the 2.5m Sloan telescope at Apache Point Observatory in New Mexico, is only a third complete and will continue to map the 3D positions of galaxies.

?We use these measurements to test a whole bunch of theories to try to understand dark energy a little bit better,? said Dr Tojeiro.

?The better your measurement, the more discriminant power you have between those competing theories. And if we can get an even greater volume of galaxies then we can start to perform new and different tests,? she added.

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