Published by the Students of Johns Hopkins since 1896
October 28, 2021

Dark matter used to assess universe expansion

By Vivek Sinanan | October 4, 2012

A group of international scientists have captured their first images of distant galaxies using the world’s most powerful digital camera, a crucial first step towards understanding the reason for the accelerated expansion of the universe.

In 1998, a collaboration of astrophysicists from Chile, Australia, Europe and the United States, using data gathered through observations of distant supernovae, discovered that the universe was expanding at an accelerated rate. Their work won them the 2011 Nobel Prize in Physics, an award shared among three scientists, including Adam Riess, a professor in the Department of Physics and Astronomy at Hopkins.

Their discovery was crucial to the field of astronomy since, according to Albert Einstein’s Theory of General Relativity, the rate of expansion of the universe should slow down under the influence of gravity. Research in the field of so-called “dark energy” has flourished in an attempt to reconcile these observations with Einstein’s theory, and it is now believed that dark energy accounts for approximately 74 percent of the total mass-energy in the universe.

The Dark Energy Survey, a collaboration of more than 120 scientists, astronomers and engineers from 23 institutions in Brazil, Germany, Switzerland, Spain, the United Kingdom and the United States, is designed to probe the accelerating universe in order to more fully understand the nature of dark energy.

Their work over the past eight years has been centered on the building and designing of a 570-Megapixel digital camera (the Dark Energy Camera or DECam), that will capture data from galaxies so distant, that light from these galaxies would have left them when the universe was less than half as old than it is today.

The camera is mounted on the Blanco 4m Telescope, which is located at the Cerro Tololo Inter-American Observatory in the Chilean Andes, 7,200 feet above sea level.

The DECam has a 2.2 degree field of view, one so wide that it can record data from an area of the sky 20 times the size of the Moon as seen from Earth in a single image. It has also been designed to be extra-sensitive to the red-shifted light of galaxies, which center on one extreme of the spectrum of visible light.

The Survey will use the DECam to measure four modes of observing dark matter. The first, Type Ia supernovae, was what was used by Dr. Reiss and his collaborators in the 1990s when they first discovered the accelerating universe. These supernovae occur upon the death of a star and achieve brief brightness on the scale of entire galaxies composed of billions of stars.

They will also look at Baryon Acoustic Oscillations, which act as a cosmic “ruler” in order to measure and compare the distances that they will get from observing the supernovae. Additionally, by looking at galaxy clusters and the rate at which they expand, they will be able to tell how much dark energy contributed to the increase in volume of the clusters.

Finally, they will look at gravitational lensing, a phenomenon that results from the effect of gravity on light itself, causing it to bend to such a degree that multiple images of a galaxy are formed. All of these will come together and be used to understand the interplay of gravity and dark energy on the acceleration of the universe’s expansion.

Their work came to fruition earlier this month, when the DECam achieved first light, and took photos of the Fornam cluster of galaxies, 60 million light years from Earth.

Over the next five years, the camera will survey a 500 square degree swath of sky over a period of 525 nights in order to capture light from over 4,000 supernovae, 100,000 galaxy clusters and 300 million galaxies, some so far away that the Dark Energy Survey has described them as being 1 million times fainter than the dimmest star that can be seen with the naked eye.

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