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Dark Energy Survey bares clues to universe’s complexity

The decade-long effort puts forth findings in consonance with standard cosmological models, but open to more complex interpretations.

Way back in 1998, astronomers discovered the fact that the universe is expanding at an ever accelerating rate, owing to a mysterious force termed dark energy that makes up approx. 70% of the universe. The finding came as a surprise as it disputed the prevailing notion that the universe’s expansion should be slackening because of gravity.

Now, quarter century post the initial discovery, the Dark Energy Survey (DES) has released the outcomes of an analysis employing the same technique to further investigate the mysteries of dark energy and the phenomenon of expansion of the universe. In a presentation at the 243rd American Astronomical Society meeting on Jan. 8, and in a paper submitted to the Astrophysical Journal (ApJ), Masao Sako (Professor, University of Pennsylvania) and other DES astronomers reported results that are in consonance with the standard cosmological model of a universe expanding at an ever accelerating rate. Yet, the presenters conceded that their findings are not conclusive enough to rule out a far more complex model.

“It was exciting to ‘unblind’ our results and to look at the latest and greatest measurements on the amount of dark matter and dark energy, and see this effort come to an end,” states Sako. “It was definitely an emotional moment for me.”

Taking a unique approach to analysis

The DES is a global collaboration made up of over 400 scientists from about 25 institutions. Hosted by the Fermilab (U.S. Department of Energy’s Fermi National Accelerator Laboratory), DES mapped an area nearly one-eighth of the entire sky employing the Dark Energy Camera mounted on the Blanco 4-meter telescope operated by NOIRLab’s Cerro Tololo Inter-American Observatory (CTIO) in Chile. DES scientists acquired data for 758 nights over the course of six years.

To understand the nature of dark energy and determine the expansion rate of the universe, Dark Energy Survey scientists perform analyses using four different techniques, the supernova technique used in 1998 included.

Sako and his team started work on the DES in way back in 2010, when the camera was still under construction and their main contribution to the project was in designing the survey plan for the supernova technique. This included choosing which expanses of the sky to observe, to set the cadence for observations, and setting up simulations of the complete survey to exactly determine what to expect from their imaging data.

The researchers enlighten that this technique needs data from type 1a supernovae, which happens when an exceptionally dense dead star, termed as a white dwarf, attains a critical mass and explodes. Since the critical mass is the same for every white dwarf, all type 1a supernovae reach roughly the same levels of brightness. So, when astronomers match a type 1a supernova’s apparent brightness as seen from Earth to its actual brightness, they can determine how distant the supernova is from us.

 

There is a solid correlation between a supernova’s distance and its redshift (a measurement of how rapidly it is moving away from Earth). Combining these two values discloses how fast the universe was expanding when the supernova transpired. Astrophysicists can match that combined rate to the rate of expansion today to conclude whether the dark energy density has stayed constant or changed over time. “As the universe expands, the matter density goes down,” says Richard Kron (director of the DES). “But if the dark energy density is a constant, that means the total proportion of dark energy must be increasing as the volume increases.”

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