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Anti-lensing: the bright side of voids



11 January 2013

University of Sydney Astronomer Dr Krzysztof Bolejko from the Sydney Institute for Astronomy (SIfA) and his colleagues have uncovered a new anti-lensing effect for objects near cosmic voids by making a careful analysis of gravitational lensing. The paper, by an international team led by Dr Bolejko, was published in the journal Physical Review Letters on 10 January 2013.

Astronomers measure gravitational lensing to obtain the mass of the lensing object, or to estimate the mass density along a particular line of sight; this also causes background galaxies to appear brighter or distorted. Light is also affected by cosmic voids or large regions of low mass density. These voids are tens to hundreds of millions of light-years across and together account for more than half the volume of the universe.

"Objects on the far side of these cosmic voids are generally thought to be slightly smaller and dimmer, but contrary to the usual expectation, we found that objects on the far side of a void are brighter than they would be otherwise," said Dr Bolejko.

This image shows the distribution of matter in the Universe called the cosmic web. Between the filaments are cosmic voids (dark regions) which occupy more than a half of the volume of our Universe. Image source: Millennium simulation
This image shows the distribution of matter in the Universe called the cosmic web. Between the filaments are cosmic voids (dark regions) which occupy more than a half of the volume of our Universe. Image source: Millennium simulation

In their full analysis, the team included a typically neglected relativistic wavelength-stretching effect that occurs because voids are expanding faster than the universe in general. This added "redshift" leads to an overestimate of the distance to objects sitting near the far side of a void, and thus they will appear brighter than expected.

"In addition, our results show that the magnitude of astronomical objects depends not only on the properties of the object, but also on the environment locally and along the line of sight, which causes de-magnification," said Dr Bolejko. The result may affect the interpretation of large scale astronomical surveys.

"If we can measure this change of brightness between objects in front of and behind the void, then we can easily estimate the depth of a particular cosmic void. This method could also be used to measure dark matter distribution inside cosmic voids," said Dr Bolejko.

Read the paper in Physical Review Letters at: http://prl.aps.org/abstract/PRL/v110/i2/e021302

Contact: Tom Gordon

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