

Galactic lensing shows cosmology is on right track

KEITH COOPER

ASTRONOMY NOW

Posted: 25 March



A subtle form of gravitational lensing acting on hordes of distant galaxies has provided a triple whammy of independent evidence for not only the strength of dark energy, but also the distribution of dark matter throughout the Universe and the sanctity of Einstein’s General Theory of Relativity.

A false-colour reconstruction of the distribution of dark matter within the COSMOS survey field. This matter is sufficient to cause more distant galaxies to be very weakly lensed. Areas coloured white, blue and green are closer to use than those in orange and red. Image: NASA/ESA/P Simon (University of Bonn)/T Schrabback (Leiden University). Using observations of 446,000 galaxies taken by the Hubble Space Telescope for the Cosmic Evolution Survey (COSMOS), which is the most intense galactic survey ever accomplished, a multinational team of astronomers led by Dr Tim Schrabback of Leiden University in the Netherlands searched for slight distortions that indicate that the light from the galaxies is being gravitationally lensed by foreground objects. Known as weak lensing, the effect is much more subtle than the dramatic smeared arcs and multiple lensed images found around strong magnifiers like massive galaxy clusters. In the case of weak lensing, the effect is caused mainly by smaller galaxy groups whose gravity is not as strong. The amount of weak lensing gives information on the distribution of matter throughout space that is doing the lensing, particularly the invisible dark matter that makes up 23 percent of the Universe. Furthermore, dark energy – the mysterious force that is accelerating the expansion of the Universe – has affected large clumps of matter in the Universe by counteracting their gravitational pull, causing galaxy clusters to grow more slowly, limiting them in size and affecting the degree to which galaxies are weakly lensed. Dark energy also means that the lensed galaxies in the background are more distant than they would otherwise be, and because gravitational lensing is strongly dependent on geometry, this makes the lensing effect more efficient and noticeable. And because the degree to which the galaxies are lensed is a direct result of Einstein’s theory of gravity described in General Relativity, the observations are more proof that Einstein’s theory holds true.

Weak lensing helps map the distribution of matter in the Universe, including dark matter. With the added inclusion of redshift data, astronomers were able to measure how distant specific clumps of matter are. Because of the finite speed of light, the farther away we look, the farther back in time we go, so we can measure how the distribution of matter has changed over billions of years. Image: NASA/ESA/P Simon (University of Bonn)/T Schrabback (Leiden University). “The most important aspect is that our measurements and basically all other major cosmological probes are in agreement,” says Schrabback. “They can all be described by a relatively simple cosmological model based on Einstein’s General Theory of Relativity.” To provide more accurate analysis, redshift data for 194,000 of the 446,000 galaxies was provided by the ground-based Subaru Telescope in Mauna Kea, Hawaii, to help create a three-dimensional map of the galaxies all the way up to a redshift of 5, which denotes a light-travel time of about 12 billion light years. Comparisons were then made with the Millennium Survey – one of our best supercomputer models of the distribution of mass in the Universe – to double check the statistical uncertainty inherent in the measurements, for weak lensing is measured by averaging over the shapes of all the galaxies. As a result, Schrabback’s team have provided the most accurate measurements of galaxy shape as influenced by weak lensing ever made, and is one of the few times that such data has been combined with accurate redshift information. In the coming years, deep range galaxy surveys such as the Dark Energy Survey, or those conducted by the Visible and Infrared Survey Telescope for Astronomy (VISTA) in conjunction with the VLT Survey Telescope at the European Southern Observatory in Chile, will also be used to measure weak lensing, as will the upcoming Euclid space mission that is being considered by the European Space Agency for a launch window in either 2017 or 2018. More about COSMOS

The Cosmic Evolution Survey was initiated by the California Institute of Technology’s Dr Nick Scoville and 70 astronomers stationed around the world. It was a mega-attempt to map a patch of sky two-degrees square across a broad range of wavelengths with the world’s biggest telescopes: Hubble, the Spitzer Space Telescope, the Chandra X-ray Observatory, XMM-Newton, NASA’s Galaxy Evolution Explorer (GALEX), the Subaru Telescope, the Very Large Array, the Very Large Telescope, the United Kingdom Infrared Telescope (UKIRT), the Canada–France–Hawaii Telescope and more. In total over two million galaxies were detected. Hubble’s contribution was made by 579 pointings of its Advanced Camera for Surveys (ACS) during 640 orbits of Earth, covering 1.64 square degrees (77 arcminutes square) – the largest ever area imaged by Hubble. The main aim of the COSMOS survey was to study how the large scale distribution of matter affected how, when and where galaxies formed, and in 2007 data from the survey was used to assemble the first ever three-dimensional map of the distribution of dark matter in the Universe, showing how dark matter has grown increasingly clumpy over the history of the Universe.