The SDSS telescope at Apache Point, N.M.

Applying cutting-edge computer science to a wealth of new astronomical data, Pitt researchers and their colleagues in the Sloan Digital Sky Survey (SDSS) reported April 26 the first robust detection of cosmic magnification on a large scale, confirming a prediction of Einstein’s General Theory of Relativity applied to the distribution of galaxies, dark matter, and distant quasars.

These findings, accepted for publication in the Astrophysical Journal, detail the subtle distortions that light undergoes as it travels from distant quasars through the web of dark matter and galaxies before reaching the Earth.

The SDSS discovery ends a two-decade-old disagreement between earlier magnification measurements and other cosmological tests of the relationship between galaxies, dark matter, and the overall geometry of the universe.

“The distortion of the shapes of background galaxies due to gravitational lensing was first observed more than a decade ago, but no one had been able to reliably detect the magnification part of the lensing signal,” said lead researcher Ryan Scranton, a postdoctoral fellow in Pitt’s Department of Physics and Astronomy.

As light makes its 10-billion-year journey from a distant quasar, it is deflected and focused by the gravitational pull of dark matter and galaxies, an effect known as gravitational lensing. The SDSS researchers definitively measured the slight brightening, or “magnification,” of quasars and connected the effect to the density of galaxies and dark matter along the path of the quasar light. The SDSS team has detected this magnification in the brightness of 200,000 quasars.

Astronomers have been trying to measure this aspect of gravitational lensing for two decades. However, the magnification signal is a very small effect—as small as increases of a few percent in the light coming from each quasar. Detecting such a small change required a very large sample of quasars with precise measurements of their brightness.

The top panel shows a grid of points representing background quasars. The bottom panel is the same grid after being “gravitationally lensed” by the galaxy shown in the center of the panel. (The magnitude of the effect is exaggerated to make it apparent by eye.) As predicted by Einstein’s Theory of General Relativity, the light from the background images is bent and distorted as it passes by the gravitational potential of the galaxy and the surrounding dark matter on its way to observers on Earth. The images nearest the foreground galaxy have a larger area than their counterparts in the left panel, making them brighter. This effect is known as cosmic magnification.

The breakthrough came earlier this year, using a precisely calibrated sample of 13 million galaxies and 200,000 quasars from the SDSS catalog. The data available from the SDSS solved many of the technical problems that had plagued earlier attempts to measure the magnification. However, the key to the new measurement was the development of a novel way to find quasars in the SDSS data. By using new statistical techniques, SDSS scientists were able to extract a sample of quasars 10 times larger than conventional methods, allowing for the extraordinary precision required to find the magnification signal.

Recent observations of the large-scale distribution of galaxies, the Cosmic Microwave Background, and distant supernovae have led astronomers to develop a standard model of cosmology. In this model, visible galaxies represent only a small fraction of all the mass of the universe, the remainder being made of dark matter.

But to reconcile previous measurements of the cosmic magnification signal with this model required making implausible assumptions about how galaxies are distributed relative to the dominant dark matter. This led some scientists to conclude that the basic cosmological picture was incorrect or at least inconsistent. However, the more precise SDSS results indicate that previous data sets were likely not up to the challenge of the measurement.

“With the quality data from the SDSS and our much better method of selecting quasars, we have put this problem to rest,” Scranton said. “Our measurement is in agreement with the rest of what the universe is telling us and the nagging disagreement is resolved.”

Andrew Connolly, associate professor in Pitt’s Department of Physics and Astronomy and a coauthor of the paper, said, “Now that we’ve demonstrated that we can make a reliable measurement of cosmic magnification, the next step will be to use it as a tool to study the interaction between galaxies, dark matter, and light in much greater detail.”

The SDSS will map in detail one-quarter of the entire sky, determining the positions and absolute brightness of several hundred million celestial objects. It also will measure the distances to more than a million galaxies and quasars. The site of the SDSS telescopes, the Apache Point Observatory, is operated by the Astrophysical Research Consortium (ARC).

The SDSS is managed by the ARC for the participating institutions. A complete list of authors and SDSS participating institutions can be found at www.sdss.org. Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the participating institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.