The Dark Energy Survey The Big Questions The Discovery of Dark Energy The telescope The camera The science Expected Results

The Big Questions What is the universe made of? Where in the universe are we? When in the universe are we? How does Fermilab help with these questions?

What is the Universe Made Of?

Where are we in the Universe? Our galaxy, the Milky Way, is a spiral galaxy similar to this one (called M81). M81 is 12 Million light years away. Arrow shows where our sun would be if this were the Milky Way

How Far We’ve Come Since 1920’s Hubble Space Telescope Hubble Middle School In 1920, state of knowledge: our galaxy = the universe, but contained many “nebulae” (fuzzy, unresolved clouds) By 1929, Edwin Hubble had established that nebulae were galaxies like ours, very far away AND that they were almost all receding, at faster speeds if they were farther away Edwin P. Hubble (1884-1953) (Grew up in Wheaton, IL U of C graduate, also played basketball)

Where are we in the Universe? 160 billion over entire sky 10,000 here

When are we in the Universe?

How Fermilab is helping answer these questions Matter at the level of quarks and leptons CDF and DØ, ATLAS and CMS Neutrino experiments (MINOS, MiniBooNe, Noνa) Dark Matter searches CDMS, COUPP, DAMIC CDF/DØ, ATLAS/CMS Dark Energy measurements Sloan Digital Sky Survey (completed) Dark Energy Survey (being constructed now)

How Dark Energy Was Discovered Image courtesy of David A Hardy Supernovae: “Standard Candles” Allow us to measure the expansion of the Universe as we look back in time

The Dark Energy Survey

What is the Dark Energy Survey? Will survey 5000 square degrees of the Southern Hemisphere sky, repeatedly using different filters to capture different wavelengths of light Will take images of more than 300,000,000 galaxies, reaching back to a redshift of 1.3 (~7.7 billion years for the light to reach us from the farthest galaxies we will see)

Cerro Tololo Inter-American Observatory Altitude 2215 m Climate dry and mild – among the best sites for astronomical observation

The Blanco Telescope

The Blanco Telescope The mirror is 4 m in diameter, made of a single piece of Cervit glass Here the mirror is being removed for re-aluminization

Popular Science, September 1969

The Dark Energy Camera Imager

DECam Imager A 520M pixel digital camera DECam Focal Plane A 520M pixel digital camera Has a 3-square-degree field of view Contains 74 CCDs which need to be operated in a vacuum and at a temperature of -100 degrees C Uses custom built very-low-noise readout electronics Each image taken by the camera is 1 GB in size Prototype Imager

DECam CCDs Each CCD: 4K x 2K pixels Converts light to charge Charge from each pixel is moved along the chip and read out sequentially Very efficient for red/ infrared light (design by LBNL)

The Dark Energy Camera Optical Corrector

DECam Optical Corrector C2 & C3 C4 C5 Filter-changer and shutter chamber (two filters shown) C1 cover Internal stray light baffle Electrical isolator

DECam Optical Corrector Contains 5 fused silica lenses – largest is about 1 m in diameter The lenses are carefully designed to produce the best focused image at the location of the plane of CCDs They have taken about two years to manufacture

Focusing the Camera Hexapod positions camera to 1 micron Camera weighs ~8000 lbs

Testing DECam We will place the camera in a specially built stand to tilt and rotate it Allows us to verify operation in all orientations before shipping it to Chile

The Testing Stand for DECam

Fixtures for Mounting Imager on the Telescope

How does the survey work? Take data for about 100 nights each year for 5 years (the survey area is visible from Oct-Feb) Obtain about 300 images per night Send data to NCSA at Univ of Illinois for processing Raw dataset: ~150,000 images Processed dataset: > 1 petabyte of data

Science with DECam We expect to identify 300,000,000 galaxies, 30000 galaxy clusters and 2000 new supernovae Identify distance for each galaxy and supernova by comparing its light intensity in the 5 different DECam filters (For some fraction of objects, get more precise distance measurement using other telescopes)

Science with DECam The supernovae will give us an independent measure of expansion with a new dataset, similar to what other surveys have done but out to a larger distance How the density of galaxies varies with distance will also tell us how the metric of the universe has changed over time, and thus whether the expansion we see has been constant over that time

Science with DECam cont’d Look for ‘bending’ of light from distant sources around massive galaxy clusters which are closer to us

Systematic Errors Worry about effects of dust causing supernova to be less standard Worry about correctly identifying and measuring the distance for very faint galaxies Worry about correcting for the effects of changes in the instrument over time Worry about effects from the atmosphere

Results: A measurement of Dark Energy which is ~ 5 times better than current measurements

Further results A very capable instrument on a fine telescope at an excellent site A library of 300,000,000 astronomical objects available for other scientific studies Can be correlated with results from other instruments