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Fermilab | Batavia, IL | USA

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Why you should care about SDSS’s giant dark matter map

The largest map of dark matter made with direct measurements, unveiled today by two teams of physicists at the U.S. Department of Energy’s Fermilab and Lawrence Berkeley National Laboratory (Berkeley Lab) removes a key hurdle for tracing the history of dark energy in the universe using ground-based telescopes.

This work done by members of the Sloan Digital Sky Survey collaboration points to greater successes for upcoming sky surveys, including the Dark Energy Survey, which will turn on the Dark Energy Camera on the Blanco Telescope later this year, and then the Large Synoptic Survey Telescope and the HyperSuprimeCam survey.

To find and map the invisible dark energy and dark matter that make up about 96 percent of the universe, physicists look at their effects on the matter and radiation we can see, namely galaxies.

Surveying galaxies from Earth-based telescopes is cheaper than satellite-based experiments but had traditionally had the drawback of having to make due with a less clear view of the sky. The same atmospheric distortions that make stars twinkle blurs attempts to track invisible dark matter in the universe made by measuring the distortion of background galaxy shapes, a process called weak lensing. DES and LSST will use this technique to create the largest galaxy surveys ever, covering more than one-eighth of the sky.

Layering photos of one area of sky taken at various time periods, a process called coaddition, can increase the sensitivity of the images six fold by removing errors and enhancing faint light signals. The image on the left show a single picture of galaxies from the SDSS Stripe 82 area of sky. The image on the right shows the same area with the layered effect, increasing the number of visible, distant galaxies. Credit: SDSS.

Layering photos of one area of sky taken at various time periods, a process called coaddition, can increase the sensitivity of the images six fold by removing errors and enhancing faint light signals. The image on the left show a single picture of galaxies from the SDSS Stripe 82 area of sky. The image on the right shows the same area with the layered effect, increasing the number of visible, distant galaxies. Credit: SDSS.Particle physicists and astronomers from Fermilab and Berkeley Lab have demonstrated a new technique for weak lensing that lessens the blurriness and allows researchers to see fainter galaxies, providing a younger picture of the universe. The two teams essentially layered snap shots of these distorted galaxies, in a process called coaddition, to remove errors caused by equipment or atmospheric effects and to enhance very faint light signals coming from deep in the universe.

Both teams depended upon extensive databases of cosmic images collected by the Sloan Digital Sky Survey, SDSS, which were compiled in large part with the help of Berkeley Lab and Fermilab.

“These results are very encouraging for future large sky surveys. The images produced lead to a picture of the galaxies in the universe that is about six times fainter, or further back in time, than is available from single images,” says Huan Lin, a Fermilab physicist and member of SDSS and DES.

Surveys of galaxies across large swaths of the sky track how clumps of dark matter have changed over time as dark energy exerts its repulsive push on them. Clumps of dark matter not only distort the images of galaxies behind them, but they determine how galaxies cluster around them. By combining this information with redshift data, the observed change in the color of light emitted by a star or other celestial object that is moving away from Earth, it’s possible to trace how the distribution of matter in the universe has evolved over time, offering insight into the growth of dark energy.

Researchers hope this new tool will help answer one of the largest questions for upcoming dark energy surveys and in cosmology: whether dark energy is what Einstein called a “cosmological constant”, a counterbalance to gravity’s pull on matter? Or is it something else such as gravity behaving differently at cosmic scales. The variation or lack of separation between clusters of galaxies and within the clusters across time will lead to new insight into this question.

To build one of the largest maps of dark matter and track its evolution across eras, the teams looked at two manifestations of gravitational lensing: those caused by large galaxy clusters and those caused by the overall distortion spread across the large scale structure of the universe. This second effect is called cosmic shear. Both of these distortions are caused by the gravitational fields of clumps of dark matter acting as lenses, bending the light from galaxies behind them. This distorts the shapes of these distant galaxies, making them look more elliptical. By measuring the ellipticities, or amount of distortion, physicists can infer properties of the dark matter, such as its abundance and how clumpy it is and the masses of the clusters.

“This image correction process should prove a valuable tool for the next generation of weak-lensing surveys,” Lin says.

– Tona Kunz

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