By Paul Rincon

Science reporter, BBC News, Seattle



Hubble's new 3D map shows the "clumpy" nature of dark matter

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Dark matter does not reflect or emit detectable light, yet it accounts for most of the mass in the Universe.

The study, published in Nature journal, provides the best evidence yet that the distribution of galaxies follows the distribution of dark matter.

This is because dark matter attracts "ordinary" matter through its gravitational pull.

Scientists presented details of their research during a news conference here at the 209th meeting of the American Astronomical Society (AAS) in Seattle, Washington.

It involved nearly 1,000 hours of observations with the Hubble Space Telescope.

For the first time, we can see what's really out there

Prof Carlos Frenk, University of Durham

Ordinary matter - gas, stars, planets and galaxies - makes up just one-sixth of all matter in the Universe. The remainder is unseen.

While previous studies of dark matter relied on simulations, this one details its large-scale distribution in 3D.

For astronomers, the challenge of mapping the Universe has been described as similar to mapping a city from night-time aerial snapshots showing only street lights.

Dark matter is invisible, so only the luminous galaxies can be seen directly. The new images are equivalent to seeing a city, its suburbs and country roads in daylight for the first time.

Yet puzzling discrepancies remain.

Light bending

The map of mass distribution is based on measurements of about half a million distant galaxies.

Lead author Richard Massey and his colleagues used a technique called weak gravitational lensing to detect the dark matter.

To reach us, the light from galaxies has to pass through intervening dark matter.

The study traced subtle distortions in galactic light

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"We understand statistically what those galaxies are supposed to look like," said Dr Massey, from the California Institute of Technology (Caltech) in Pasadena, US.

"If you place some dark matter in the way, this dark matter - through its gravity - bends the path of light.

"As the light gets deflected, it distorts the shape of the background galaxies. So we end up seeing them in a distorted way, as if through lots of little lenses - and each of those lenses is a bit of dark matter."

To add 3D distance information, the Hubble observations were combined with multi-colour data from powerful ground-based telescopes.

The map of dark matter distribution confirms that galaxy clusters are located within clumps of this invisible material.

These clumps are connected via bridges of dark matter called filaments. The clumps and filaments form a loose network - like a web.

Cold and dark

Dr Eric Linder, from the Lawrence Berkeley National Laboratory, US, said the study was a "big step forward" in understanding the influence of dark matter on our Universe.

"It's still a small fraction of the sky we're talking about - something like two square degrees out of 40,000 square degrees in total; but they are definitely the clearest pictures we have to date," he said.

The study involved nearly 1,000 hours of Hubble observations

"In the next decade, I think most studies of the Universe's large-scale structure will be dark matter structure studies. In this sense, I think galaxies will be relegated to a secondary role."

He added: "For the first time, we can see what's really out there."

The University of Durham astronomer said that, overall, the results were a "beautiful confirmation" of the cold dark matter theory on which he works.

This theory is a leading model to explain how structures in the Universe evolved over cosmic time.

Soon after the Big Bang, cold dark matter formed the first large structures in the Universe, which then collapsed under their own weight to form vast halos.

The gravitational pull of these halos sucked in ordinary matter, providing a focus for the formation of galaxies.

'Naked' clumps

But astronomers will have to resolve discrepancies in the otherwise tight connection between ordinary matter and the dark "stuff".

Concentrations of ordinary matter almost always overlap with concentrations of dark matter - but not absolutely always.

Conversely, the researchers saw that dark matter concentrations sometimes seemed to have no corresponding ordinary matter.

"It's not forbidden, but you get a little uncomfortable because you would think the two should go together," said Dr Linder.

Carlos Frenk commented: "Finding what I would call 'naked' clumps of dark matter where there are no galaxies for me is very strange. All dark matter clumps of sufficient size should have galaxies - if our understanding is correct."

For the moment, no-one is talking about needing to revise cosmological models; but Professor Frenk said everything hinged on the size of these anomalies.

"What would be an enormous puzzle would be to find big, luminous galaxies sitting out there in the middle of nowhere with no dark matter around them. That really would be shocking."

The discrepancies could turn out simply to be artefacts, caused by noise in the data. But then again, said Carlos Frenk, they could be real.

Dr Massey said the anomalies were "tantalising" and that his team was eager to investigate them more closely.

But, he told BBC News, "the discrepancies are not yet at a level of significance where I am definitively convinced they are something other than noise or isolated defects in our analysis."

The findings come from the Cosmic Evolution Survey (Cosmos) project - the largest ever undertaken with Hubble Space Telescope.

The map stretches halfway back to the beginning of the Universe

Paul.Rincon-INTERNET@bbc.co.uk