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The Dark Energy Survey (DES) is an astronomical survey designed to constrain the properties of dark energy. It uses images taken in the near-ultraviolet, visible, and near-infrared to measure the expansion of the universe using Type Ia supernovae, baryon acoustic oscillations, the number of galaxy clusters, and weak gravitational lensing.  The collaboration is composed of research institutions and universities from the United States,  Australia, Brazil,  the United Kingdom, Germany, Spain, and Switzerland. The collaboration is divided into several scientific working groups. The director of DES is Josh Frieman. 
The DES began by developing and building Dark Energy Camera (DECam), an instrument designed specifically for the survey.  This camera has a wide field of view and high sensitivity, particularly in the red part of the visible spectrum and in the near infrared.  Observations were performed with DECam mounted on the 4-meter Víctor M. Blanco Telescope, located at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.  Observing sessions ran from 2013 to 2019; as of 2021 [update] the DES collaboration has published results from the first three years of the survey. 
DECam, short for the Dark Energy Camera, is a large camera built to replace the previous prime focus camera on the Victor M. Blanco Telescope. The camera consists of three major components: mechanics, optics, and CCDs.
The mechanics of the camera consists of a filter changer with an 8-filter capacity and shutter. There is also an optical barrel that supports 5 corrector lenses, the largest of which is 98 cm in diameter. These components are attached to the CCD focal plane which is cooled to 173 K (−148 °F; −100 °C) with liquid nitrogen in order to reduce thermal noise in the CCDs. The focal plane is also kept in an extremely low vacuum of 0.00013 pascals (1.3×10−9 atm) to prevent the formation of condensation on the sensors. The entire camera with lenses, filters, and CCDs weighs approximately 4 tons. When mounted at the prime focus it was supported with a hexapod system allowing for real time focal adjustment. 
The camera is outfitted with u, g, r, i, z, and Y filters spanning roughly from 340–1070 nm,  similar to those used in the Sloan Digital Sky Survey (SDSS). This allows DES to obtain photometric redshift measurements to z≈1. DECam also contains five lenses acting as corrector optics to extend the telescope's field of view to a diameter of 2.2°, one of the widest fields of view available for ground-based optical and infrared imaging.  One significant difference between previous charge-coupled devices (CCD) at the Victor M. Blanco Telescope and DECam is the improved quantum efficiency in the red and near-infrared wavelengths.  
The scientific sensor array on DECam is an array of 62 2048×4096 pixel back-illuminated CCDs totaling 520 megapixels; an additional 12 2048×2048 pixel CCDs (50 Mpx) are used for guiding the telescope, monitoring focus, and alignment. The full DECam focal plane contains 570 megapixels. The CCDs for DECam use high resistivity silicon manufactured by Dalsa and LBNL with 15×15 micron pixels. By comparison, the OmniVision Technologies back-illuminated CCD that was used in the iPhone 4 has a 1.75×1.75 micron pixel with 5 megapixels. The larger pixels allow DECam to collect more light per pixel, improving low light sensitivity which is desirable for an astronomical instrument. DECam's CCDs also have a 250-micron crystal depth; this is significantly larger than most consumer CCDs. The additional crystal depth increases the path length travelled by entering photons. This, in turn, increases the probability of interaction and allows the CCDs to have an increased sensitivity to lower energy photons, extending the wavelength range to 1050 nm. Scientifically this is important because it allows one to look for objects at a higher redshift, increasing statistical power in the studies mentioned above. When placed in the telescope's focal plane each pixel has a width of 0.263″ on the sky, resulting in a total field of view of 3 square degrees.
DES imaged 5,000 square degrees of the southern sky in a footprint that overlaps with the South Pole Telescope and Stripe 82 (in large part avoiding the Milky Way). The survey took 758 observing nights spread over six annual sessions between August and February to complete, covering the survey footprint ten times in five photometric bands (g, r, i, z, and Y).  The survey reached a depth of 24th magnitude in the i band over the entire survey area. Longer exposure times and faster observing cadence were made in five smaller patches totaling 30 square degrees to search for supernovae. 
First light was achieved on 12 September 2012;  after a verification and testing period, scientific survey observations started in August 2013.  The last observing session was completed on 9 January 2019. 
Each year from August through February, observers will stay in dormitories on the mountain. During a weeklong period of work, observers sleep during the day and use the telescope and camera at night. There will be some DES members working at the telescope console to monitor operations while others are monitoring camera operations and data process.
For the wide-area footprint observations, DES takes roughly every two minutes for each new image: The exposures are typically 90 seconds long, with another 30 seconds for readout of the camera data and slewing to point the telescope at its next target. Despite the restrictions on each exposure, the team also need to consider different sky conditions for the observations, such as moonlight and cloud cover.
In order to get better images, DES team use a computer algorithm called the "Observing Tactician" (ObsTac) to help with sequencing observations. It optimizes among different factors, such as the date and time, weather conditions, and the position of the moon. ObsTac automatically points the telescope in the best direction, and selects the exposure, using the best light filter. It also decides whether to take a wide-area or time-domain survey image, depending on whether or not the exposure will also be used for supernova searches. 
Dark Energy Group published several papers presenting their results for cosmology. Most of these cosmology results coming from its first-year data and the third-year data. Their results for cosmology were concluded with a Multi-Probe Methodology, which mainly combine the data from Galaxy-Galaxy Lensing, different shape of weak lensing, cosmic shear, galaxy clustering and photometric data set.
For the first-year data collected by DES, Dark Energy Survey Group showed the Cosmological Constraints results from Galaxy Clustering and Weak Lensing results and cosmic shear measurement. With Galaxy Clustering and Weak Lensing results, and for ΛCDM, , and at 68% confidence limits for ωCMD.  Combine the most significant measurements of cosmic shear in a galaxy survey, Dark Energy Survey Group showed that at 68% confidence limits and for ΛCDM with .  Other cosmological analyses from first year data showed a derivation and validation of redshift distribution estimates and their uncertainties for the galaxies used as weak lensing sources.  The DES team also published a paper summarize all the Photometric Data Set for Cosmology for their first-year data. 
For the third-year data collected by DES, they updated the Cosmological Constraints to for the ΛCDM model with the new cosmic shear measurements.  From third-year data of Galaxy Clustering and Weak Lensing results, DES updated the Cosmological Constraints to and in ΛCDM at 68% confidence limits, , and in ωCDM at 68% confidence limits.  Similarly, the DES team published their third-year observations for photometric data set for cosmology comprising nearly 5000 deg2 of grizY imaging in the south Galactic cap, including nearly 390 million objects, with depth reaching S/N ∼ 10 for extended objects up to ∼ 23.0, and top-of-the-atmosphere photometric uniformity < 3mmag. 
Weak lensing was measured statistically by measuring the shear-shear correlation function, a two-point function, or its Fourier Transform, the shear power spectrum.  In April 2015, the Dark Energy Survey released mass maps using cosmic shear measurements of about 2 million galaxies from the science verification data between August 2012 and February 2013.  In 2021 weak lensing was used to map the dark matter in a region of the southern hemisphere sky,   in 2022 together with galaxy clustering data to give new cosmological constrains.   and in 2023 with data from the Planck telescope and South Pole telescope to give once new improved constraints.    
Another big part of weak lensing result is to calibrate the redshift of the source galaxies. In December 2020 and June 2021, DES team published two papers showing their results about using weak lensing to calibrate the redshift of the source galaxies in order to mapping the matter density field with gravitational lensing.  
After LIGO detected the first gravitational wave signal from GW170817,  DES made follow-up observations of GW170817 using DECam. With DECam independent discovery of the optical source, DES team establish its association with GW170817 by showing that none of the 1500 other sources found within the event localization region could plausibly be associated with the event. DES team monitored the source for over two weeks and provide the light curve data as a machine-readable file. From the observation data set, DES concluded that the optical counterpart they have identified near NGC 4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources. 
In March 2015, two teams released their discoveries of several new potential dwarf galaxy candidates found in Year 1 DES data.  In August 2015, the Dark Energy Survey team announced the discovery of eight additional candidates in Year 2 DES data.  Later on, Dark Energy Survey team found more dwarf galaxies. With more Dwarf Galaxy results, the team was able to take a deep look about more properties of the detected Dwarf Galaxy such as the chemical abundance,  the structure of stellar population,  and Stellar Kinematics and Metallicities.  In Feb 2019, the team also discovered a sixth star cluster in the Fornax Dwarf Spheroidal Galaxy  and a tidally Disrupted Ultra-Faint Dwarf Galaxy. 
The signature of baryon acoustic oscillations (BAO) can be observed in the distribution of tracers of the matter density field and used to measure the expansion history of the Universe. BAO can also be measured using purely photometric data, though at less significance.  DES team observation samples consists of 7 million galaxies distributed over a footprint of 4100 deg with 0.6< < 1.1 and a typical redshift uncertainty of 0.03(1+z).  From their statistics, they combine the likelihoods derived from angular correlations and spherical harmonics to constrain the ratio of comoving angular diameter distance at the effective redshift of our sample to the sound horizon scale at the drag epoch. 
In May 2019, Dark Energy Survey team published their first cosmology results using Type Ia supernovae. The supernova data was from DES-SN3YR. The Dark Energy Survey team found Ωm = 0.331 ± 0.038 with a flat ΛCDM model and Ωm = 0.321 ± 0.018, w = −0.978 ± 0.059 with a flat wCDM model.  Analyzing the same data from DES-SN3YR, they also found a new current Hubble constant, .  This result has an excellent agreement with the Hubble constant measurement from Planck Satellite Collaboration in 2018.  In June 2019, there a follow-up paper was published by DES team discussing the systematic uncertainties, and validation of using the supernovae to measure the cosmology results mentioned before.  The team also published their photometric pipeline and light curve data in another paper published in the same month. 
Several minor planets were discovered by DeCam in the course of The Dark Energy Survey, including high-inclination trans-Neptunian objects (TNOs). 
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|Discoveries are credited either to|
"DECam" or "Dark Energy Survey".
The MPC has assigned the IAU code W84 for DeCam's observations of small Solar System bodies. As of October 2019, the MPC inconsistently credits the discovery of nine numbered minor planets, all of them trans-Neptunian objects, to either "DeCam" or "Dark Energy Survey".  The list does not contain any unnumbered minor planets potentially discovered by DeCam, as discovery credits are only given upon a body's numbering, which in turn depends on a sufficiently secure orbit determination.
Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood.
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect. Various astrophysical observations – including gravitational effects which cannot be explained by currently accepted theories of gravity unless more matter is present than can be seen – imply dark matter's presence. For this reason, most experts think that dark matter is abundant in the universe and has had a strong influence on its structure and evolution.
A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced KWAY-zar, and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from an AGN is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation. The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.
Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered during 1998 by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, which both used distant type Ia supernovae to measure the acceleration. The idea was that as type Ia supernovae have almost the same intrinsic brightness, and since objects that are further away appear dimmer, we can use the observed brightness of these supernovae to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift, which measures how much the universe has expanded since the supernova occurred; the Hubble law established that the further an object is from us, the faster it is receding. The unexpected result was that objects in the universe are moving away from one another at an accelerated rate. Cosmologists at the time expected that recession velocity would always be decelerating, due to the gravitational attraction of the matter in the universe. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery. Confirmatory evidence has been found in baryon acoustic oscillations, and in analyses of the clustering of galaxies.
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum.
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There may be 2 trillion galaxies in the observable universe, although that number was reduced in 2021 to only several hundred billion based on data from New Horizons. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical region centered on the observer. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.
The Sunyaev–Zeldovich effect is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy electrons in galaxy clusters, in which the low-energy CMB photons receive an average energy boost during collision with the high-energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the disturbance of density in the universe. Using the Sunyaev–Zeldovich effect, dense clusters of galaxies have been observed.
In the fields of Big Bang theory and cosmology, reionization is the process that caused matter in the universe to reionize after the lapse of the "dark ages".
An Einstein ring, also known as an Einstein–Chwolson ring or Chwolson ring, is created when light from a galaxy or star passes by a massive object en route to the Earth. Due to gravitational lensing, the light is diverted, making it seem to come from different places. If source, lens, and observer are all in perfect alignment, the light appears as a ring.
In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe: a measurement based on direct observations of an early state of the universe, which indicate an age of 13.787±0.020 billion years as interpreted with the Lambda-CDM concordance model as of 2021; and a measurement based on the observations of the local, modern universe, which suggest a younger age. The uncertainty of the first kind of measurement has been narrowed down to 20 million years, based on a number of studies which all show similar figures for the age and which include studies of the microwave background radiation by the Planck spacecraft, the Wilkinson Microwave Anisotropy Probe and other space probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time. The range of the estimate is also within the range of the estimate for the oldest observed star in the universe.
The ΛCDM or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda associated with dark energy; second, the postulated cold dark matter ; and third, ordinary matter. It is frequently referred to as the standard modelof Big Bang cosmology because it is the simplest model that provides a reasonably good account of the following properties of the cosmos:
Redshift quantization, also referred to as redshift periodicity, redshift discretization, preferred redshifts and redshift-magnitude bands, is the hypothesis that the redshifts of cosmologically distant objects tend to cluster around multiples of some particular value. In standard inflationary cosmological models, the redshift of cosmological bodies is ascribed to the expansion of the universe, with greater redshift indicating greater cosmic distance from the Earth. This is referred to as cosmological redshift. Ruling out errors in measurement or analysis, quantized redshift of cosmological objects would either indicate that they are physically arranged in a quantized pattern around the Earth, or that there is an unknown mechanism for redshift unrelated to cosmic expansion, referred to as "intrinsic redshift" or "non-cosmological redshift".
While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak lensing regime, in which the deflection is impossible to detect in a single background source. However, even in these cases, the presence of the foreground mass can be detected, by way of a systematic alignment of background sources around the lensing mass. Weak gravitational lensing is thus an intrinsically statistical measurement, but it provides a way to measure the masses of astronomical objects without requiring assumptions about their composition or dynamical state.
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the universe does not expand at a constant rate; rather, the universe's expansion is accelerating. Understanding the universe's evolution requires knowledge of its starting conditions and composition. Before these observations, scientists thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background (CMB) suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how scientists could measure an accelerating universe. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research to understand the fundamental nature of dark energy. Assuming that the lambda-CDM model of cosmology is correct, as of 2013, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. Dark energy's density is very low: 6×10−10 J/m3 (~7×10−30 g/cm3), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space.
Ofer Lahav is Perren Chair of Astronomy at University College London (UCL), Vice-Dean (International) of the UCL Faculty of Mathematical and Physical Sciences (MAPS) and Co-Director of the STFC Centre for Doctoral Training in Data Intensive Science. His research area is Observational Cosmology, in particular probing Dark Matter and Dark Energy. His work involves Machine Learning for Big Data.
In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter of the universe, caused by acoustic density waves in the primordial plasma of the early universe. In the same way that supernovae provide a "standard candle" for astronomical observations, BAO matter clustering provides a "standard ruler" for length scale in cosmology. The length of this standard ruler is given by the maximum distance the acoustic waves could travel in the primordial plasma before the plasma cooled to the point where it became neutral atoms, which stopped the expansion of the plasma density waves, "freezing" them into place. The length of this standard ruler can be measured by looking at the large scale structure of matter using astronomical surveys. BAO measurements help cosmologists understand more about the nature of dark energy by constraining cosmological parameters.
Cosmic voids are vast spaces between filaments, which contain very few or no galaxies. The cosmological evolution of the void regions differs drastically from the evolution of the Universe as a whole: there is a long stage when the curvature term dominates, which prevents the formation of galaxy clusters and massive galaxies. Hence, although even the emptiest regions of voids contain more than ~15% of the average matter density of the Universe, the voids look almost empty for an observer. Voids typically have a diameter of 10 to 100 megaparsecs ; particularly large voids, defined by the absence of rich superclusters, are sometimes called supervoids. They were first discovered in 1978 in a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory.
Peter Lawrence Capak is currently the Architect of Perception Systems at the Oculus division of Facebook. His current focus is developing machine perception technologies, sensors, displays, and compute architectures for the next generation of augmented (AR), mixed (MR) and virtual reality (VR) systems. His research has focused on using physical modeling and advanced statistical methods including artificial intelligence and machine learning to extract information from very large multi-wavelength (hyper-spectral) data sets. He has primarily used this to study structure formation in the universe, cosmology, and the nature of dark matter and dark energy.
Mustapha Ishak-Boushaki is a theoretical physicist, cosmologist and professor at the University of Texas at Dallas. He is known for his contributions to the studies of cosmic acceleration and dark energy, gravitational lensing, and testing alternatives to general relativity; as well as his authorship of Testing General Relativity in Cosmology, a review article published in Living Reviews in Relativity. He was elected in 2021 as Fellow of American Association for the Advancement of Science (AAAS) with the quote: "For distinguished contributions to the field of theoretical cosmology, particularly for testing modifications to general relativity at cosmological scales, and for sustained excellence in teaching and mentoring of students."