The Dark Energy Survey is an international collaboration comprising more than 400 astrophysicists, astronomers and cosmologists from over 25 institutions led by members from the U.S. Department of Energy’s Fermi National Accelerator Laboratory. DES mapped an area almost one-eighth the entire sky using the Dark Energy Camera, a 570-megapixel digital camera built by Fermilab and funded by the DOE Office of Science. It was mounted on the Víctor M. Blanco Telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory, a Program of NSF’s NOIRLab, in 2012. DES scientists took data for 758 nights across six years. [1]
This technique requires data from type Ia supernovae, which occur when an extremely dense dead star, known as a white dwarf, reaches a critical mass and explodes. Since the critical mass is nearly the same for all white dwarfs, all type Ia supernovae have approximately the same actual brightness and any remaining variations can be calibrated out. So, when astrophysicists compare the apparent brightnesses of two type Ia supernovae as seen from Earth, they can determine their relative distances from us. [1]
The standard cosmological model is ΛCDM, or Lambda Cold Dark Matter, a model based on the dark energy density being constant over cosmic time. It tells us how the universe evolves, using just a few features, such as the density of matter, type of matter and behavior of dark energy. The supernova method constrains two of these features very well: matter density and a quantity called w, which indicates whether the dark energy density is constant or not. [1]
According to the standard cosmological model, the density of dark energy in the universe is constant, which means it doesn’t dilute as the universe expands. If this is true, the parameter represented by the letter w should equal –1. [1]
The results found w = –0.80 +/- 0.18 using supernovae alone. Combined with complementary data from the European Space Agency’s Planck telescope, w reaches –1 within the error bars.
“w is tantalizingly not exactly on –1, but close enough that it’s consistent with –1,” said Davis. “A more complex model might be needed. Dark energy may indeed vary with time.” [1]
The history of the expansion universe can be traced by comparing recessional velocities (redshifts) with distances determined for each supernova. The DES result shows that the expansion has been accelerating with cosmic time, the signature of dark energy. Image: DES collaboration[1]
For the 2018 analysis, DES scientists combined data about the spectrum of each supernova to determine their redshifts and to classify them as type Ia or not. They then used images taken with different filters to identify the flux at the peak of the light curve — a method called photometry.[1]
Redshift is the term used to describe the stretching of wavelengths of the light with the expansion of the universe; the greater the object’s distance, the greater the redshift. The detailed history of the expansion of the universe is determined with a precise relation between the distances to galaxies — or supernovae — and their redshifts. Image: DES collaboration[1]
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