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Posts Tagged ‘dark energy’

Dark matter and dark energy feature prominently at the European Physics Society conference on particle physics in Vienna. Although physicists now understand pretty well the basic constituents of matter, all what one sees on Earth, in stars and galaxies, this huge amount of matter only accounts for 5% of the whole content of the Universe. Not surprising then that much efforts are deployed to elucidate the nature of dark matter (27% of the Universe), and dark energy (68%).

Since the Big Bang, the Universe is not only expanding, but this expansion is also accelerating. So which energy fuels this acceleration? We call it dark energy. This is still something absolutely unknown but the Dark Energy Survey (DES) team is determined to get some answers. To do so, they are searching a quarter of the southern sky, mapping the location, shape and distribution of various astronomical objects such as galactic clusters (large groups of galaxies) and supernovae (exploding stars). Their goal is to record information on 300 million galaxies and 2500 supernovae.

Galaxies formed thanks to gravity that allowed matter to cluster. But this happened against the dispersive effect of dark energy, since the expansion of the Universe scattered matter away. The DES scientists essentially study how large structures such as galactic clusters evolved in time by looking at objects at various distances, and whose light comes from different times in the past. With more data, they hope to better understand the dynamic of expansion.

Dark matter is just as unknown. So far, it has only manifested itself through gravitational effects. We can “feel” its presence but we cannot see it, since it emits no light, unlike regular matter found in stars and supernovae. As if the whole Universe was full of ghosts. A dozen detectors, using different techniques, are trying to find dark matter particles.

Not easy to catch such elusive particles when no one knows how and if these particles interact with matter. Moreover, these particles must interact very rarely with regular matter (otherwise, they would already have been found), the name of the game is to use massive detectors, in the hope one nucleus from one of the detector atoms will recoil when hit by a dark matter particle, inducing a small but detectable vibration in the detector. The experiments search for a range of possibilities, depending on the mass of the dark matter particles and how often they can interact with matter.

The plot below shows how often dark matter particles could interact with a nucleus (vertical axis) as a function of their mass (horizontal axis). This spans a wide region of possibilities one must test. The various curves indicate what has been achieved so far by different experiments. All possibilites above the curves are excluded. The left part of the plot is harder to probe since the lighter the dark matter particles is, the smaller the vibration induced.

CRESST-limitThe CRESST Collaboration uses small crystals operating at extremely low temperature. They are sensitive to the temperature rise that would occur if a dark matter particle deposited the smallest amount of energy. This allowed them to succeed where tens of previous experiments had failed: looking for very light particles. This is shown on the plot by the solid red curve in the upper left corner. All possibilities above are now excluded. So far, this area was only accessible to the Large Hadron Collider (LHC) experiments (results not shown here) but only when making various theoretical hypotheses. CRESST has just opened a new world of possibilities and they will sweep nearly the entire area in the coming years. Light dark matter particles better watch out.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline  or sign-up on this mailing list to receive an e-mail notification. You can also visit my website.


This article appeared in Fermilab Today on May 27, 2015.

The future Dark Energy Spectroscopic Instrument will be mounted on the Mayall 4-meter telescope. It will be used to create a 3-D map of the universe for studies of dark energy. Photo courtesy of NOAO

The future Dark Energy Spectroscopic Instrument will be mounted on the Mayall 4-meter telescope. It will be used to create a 3-D map of the universe for studies of dark energy. Photo courtesy of NOAO

Dark energy makes up about 70 percent of the universe and is causing its accelerating expansion. But what it is or how it works remains a mystery.

The Dark Energy Spectroscopic Instrument (DESI) will study the origins and effects of dark energy by creating the largest 3-D map of the universe to date. It will produce a map of the northern sky that will span 11 billion light-years and measure around 25 million galaxies and quasars, extending back to when the universe was a mere 3 billion years old.

Once construction is complete, DESI will sit atop the Mayall 4-Meter Telescope in Arizona and take data for five years.

DESI will work by collecting light using optical fibers that look through the instrument’s lenses and can be wiggled around to point precisely at galaxies. With 5,000 fibers, it can collect light from 5,000 galaxies at a time. These fibers will pass the galaxy light to a spectrograph, and researchers will use this information to precisely determine each galaxy’s three-dimensional position in the universe.

Lawrence Berkeley National Laboratory is managing the DESI experiment, and Fermilab is making four main contributions: building the instrument’s barrel, packaging and testing charge-coupled devices, or CCDs, developing an online database and building the software that will tell the fibers exactly where to point.

The barrel is a structure that will hold DESI’s six lenses. Once complete, it will be around 2.5 meters tall and a meter wide, about the size of a telephone booth. Fermilab is assembling both the barrel and the structures that will hold it on the telescope.

“It’s a big object that needs to be built very precisely,” said Gaston Gutierrez, a Fermilab scientist managing the barrel construction. “It’s very important to position the lenses very accurately, otherwise the image will be blurred.”

DESI’s spectrograph will use CCDs, sensors that work by converting light collected from distant galaxies into electrons, then to digital values for analysis. Fermilab is responsible for packaging and testing these CCDs before they can be assembled into the spectrograph.

Fermilab is also creating a database that will store information required to operate DESI’s online systems, which direct the position of the telescope, control and read the CCDs, and ensure proper functioning of the spectrograph.

Lastly, Fermilab is developing the software that will convert the known positions of interesting galaxies and quasars to coordinates for the fiber positioning system.

Fermilab completed these same tasks when it built the Dark Energy Camera (DECam), an instrument that currently sits on the Victor Blanco Telescope in Chile, imaging the universe. Many of these scientists and engineers are bringing this expertise to DESI.

“DESI is the next step. DECam is going to precisely measure the sky in 2-D, and getting to the third dimension is a natural progression,” said Fermilab’s Brenna Flaugher, project manager for DECam and one of the leading scientists on DESI.

These four contributions are set to be completed by 2018, and DESI is expected to see first light in 2019.

“This is a great opportunity for students to learn the technology and participate in a nice instrumentation project,” said Juan Estrada, a Fermilab scientist leading the DESI CCD effort.

DESI is funded largely by the Department of Energy with significant contributions from non-U.S. and private funding sources. It is currently undergoing the DOE CD-2 review and approval process.

“We’re really appreciative of the strong technical and scientific support from Fermilab,” said Berkeley Lab’s Michael Levi, DESI project director.

Diana Kwon


I don’t usually get to spill the beans on a big discovery like this, but this time, I DO!

CERN Had Dark Energy All Along!!

That’s right. That mysterious energy making up ~68% of the universe was being used all along at CERN! Being based at CERN now, I’ve had a first hand glimpse into the dark underside of Dark Energy. It all starts at the Crafted Refilling of Empty Mugs Area (CREMA), pictured below.

One CREMA station at CERN


Researchers and personnel seem to stumble up to these stations at almost all hours of the day, looking very dreary and dazed. They place a single cup below the spouts, and out comes a dark and eerie looking substance, which is then consumed. Some add a bit of milk for flavor, but all seem perkier and refreshed after consumption. Then they disappear from whence they came. These CREMA stations seem to be everywhere, from control rooms to offices, and are often found with groups of people huddled around them. In fact, they seem to exert a force on all who use them, keeping them in stable orbits about the stations.

In order to find out a little bit more about this mysterious substance and its dispersion, I asked a graduating student, who wished to remain unnamed, a little bit about their experiences:

Q. How much of this dark stuff do you consume on a daily basis?

A. At least one cup in the morning to fuel up, I don’t think I could manage to get to lunchtime without that one. Then multiple other cups distributed over the day, depending on the workload. It always feels like they help my thinking.

Q. Do you know where it comes from?

A. We have a machine in our office which takes capsules. I’m not 100% sure where those capsules are coming from, but they seem to restock automatically, so no one ever asked.

Q. Have you been hiding this from the world on purpose?

A. Well our stock is important to our group, if we would just share it with everyone around we could run out. And no one of us can make it through the day without. We tried alternatives, but none are so effective.

Q. Do you remember the first time you tried it?

A. Yes, they hooked me on it in university. From then on nothing worked without!

Q. Where does CERN get so much of it?

A. I never thought about this question. I think I’m just happy that there is enough for everyone here, and physicist need quite a lot of it to work.

In order to gauge just how much of this Dark Energy is being consumed, I studied the flux of people from the cafeteria as a function of time with cups of Dark Energy. I’ve compiled the results into the Dark Energy Consumption As Flux (DECAF) plot below.

Dark Energy Consumption as Flux plot. Taken March 31, 2015. Time is given in 24h time. Errors are statistical.


As the DECAF plot shows, there is a large spike in consumption, particularly after lunch. There is a clear peak at times after 12:20 and before 13:10. Whether there is an even larger peak hiding above 13:10 is not known, as the study stopped due to my advisor asking “shouldn’t you be doing actual work?”

There is an irreducible background of Light Energy in the cups used for Dark Energy, particularly of the herbal variety. Fortunately, there is often a dangly tag hanging off of the cup  to indicate to others that they are not using the precious Dark Energy supply, and provide a clear signal for this study to eliminate the background.

While illuminating, this study still does not uncover the exact nature of Dark Energy, though it is clear that it is fueling research here and beyond.


DECam’s nearby discoveries

Monday, February 2nd, 2015

This article appeared in symmetry on Jan. 22, 2015.

The Dark Energy Camera does more than its name would lead you to believe. Image courtesy of NOAO

The Dark Energy Camera does more than its name would lead you to believe. Image courtesy of NOAO

The Dark Energy Camera, or DECam, peers deep into space from its mount on the 4-meter Victor Blanco Telescope high in the Chilean Andes.

Thirty percent of the camera’s observing time—about 105 nights per year—go to the team that built it: scientists working on the Dark Energy Survey.

Another small percentage of the year is spent on maintenance and upgrades to the telescope. So who else gets to use DECam? Dozens of other projects share its remaining time.

Many of them study objects far across the cosmos, but five of them investigate ones closer to home.

Overall, these five groups take up just 20 percent of the available time, but they’ve already taught us some interesting things about our planetary neighborhood and promise to tell us more in the future.

Far-out asteroids

Stony Brook University’s Aren Heinze and the University of Western Ontario’s Stanimir Metchev used DECam for four nights in early 2014 to search for unknown members of our solar system’s main asteroid belt, which sits between Mars and Jupiter.

To detect such faint objects, one needs to take a long exposure. However, the paths of these asteroids lie close enough to Earth that taking an exposure longer than a few minutes results in blurred images. Heinze and Metchev’s fix was to stack more than 100 images taken in less than two minutes each.

With this method, the team expects to measure the positions, motions and brightnesses of hundreds of main belt asteroids not seen before. They plan to release their survey results in late 2015, and an early partial analysis indicates they’ve already found hundreds of asteroids in a region smaller than DECam’s field of view—about 20 times the area of the full moon.

Whole new worlds

Scott Sheppard of the Carnegie Institution for Science in Washington DC and Chad Trujillo of Gemini Observatory in Hilo, Hawaii, use DECam to look for distant denizens of our solar system. The scientists have imaged the sky for two five-night stretches every year since November 2012.

Every night, the DECam’s sensitive 570-megapixel eye captures images of an area of sky totaling about 200 to 250 times the area of the full moon, returning to each field of view three times. Sheppard and Trujillo run the images from each night through software that tags everything that moves.

“We have to verify everything by eye,” Sheppard says. So they look through about 60 images a night, or 300 total from a perfect five-night observing run, a process that gives them a few dozen objects to study at Carnegie’s Magellan Telescope.

The scientists want to find worlds beyond Pluto and its brethren—a region called the Kuiper Belt, which lies some 30 to 50 astronomical units from the sun (compared to the Earth’s 1). On their first observing run, they caught one.

This new world, with the catalog name of 2012 VP113, comes as close as 80 astronomical units from the sun and journeys as far as 450. Along with Sedna, a minor planet discovered a decade ago, it is one of just two objects found in what was once thought of as a complete no man’s land.

Sheppard and Trujillo also have discovered another dwarf planet that is one of the top 10 brightest objects beyond Neptune, a new comet, and an asteroid that occasionally sprouts an unexpected tail of dust.

Mythical creatures

Northern Arizona University’s David Trilling and colleagues used the DECam for three nights in 2014 to look for “centaurs”—so called because they have characteristics of both asteroids and comets. Astronomers believe centaurs could be lost Kuiper Belt objects that now lie between Jupiter and Neptune.

Trilling’s team expects to find about 50 centaurs in a wide range of sizes. Because centaurs are nearer to the sun than Kuiper Belt objects, they are brighter and thus easier to observe. The scientists hope to learn more about the size distribution of Kuiper Belt objects by studying the sizes of centaurs. The group recently completed its observations and plan to report them later in 2015.

Next-door neighbors

Lori Allen of the National Optical Astronomy Observatory outside Tucson, Arizona, and her colleagues are looking for objects closer than 1.3 astronomical units from the sun. These near-Earth objects have orbits that can cross Earth’s—creating the potential for collision.

Allen’s team specializes in some of the least-studied NEOs: ones smaller than 50 meters across.

Even small NEOs can be destructive, as demonstrated by the February 2013 NEO that exploded above Chelyabinsk, Russia. The space rock was just 20 meters wide, but the shockwave from its blast shattered windows, which caused injuries to more than 1000 people.

In 2014, Allen’s team used the DECam for 10 nights. They have 20 more nights to use in 2015 and 2016.

They have yet to release specific findings from the survey’s first year, but the researchers say they have a handle of the distribution of NEOs down to just 10 meters wide. They also expect to discover about 100 NEOs the size of the one that exploded above Chelyabinsk.

Space waste

Most surveys looking for “space junk”—inactive satellites, parts of spacecraft and the like in orbit around the Earth—can see only pieces larger than about 20 centimeters. But there’s a lot more material out there.

How much is a question Patrick Seitzer of the University of Michigan and colleagues hope to answer. They used DECam to hunt for debris smaller than 10 centimeters, or the size of a smartphone, in geosynchronous orbit.

The astronomers need to capture at least four images of each piece of debris to determine its position, motion and brightness. This can tell them about the risk from small debris to satellites in geosynchronous orbit. Their results are scheduled for release in mid-2015.

Liz Kruesi


Costumes to make zombie Einstein proud

Wednesday, October 29th, 2014

This article appeared in symmetry on Oct. 21, 2014.

These physics-themed Halloween costume ideas are sure to entertain—and maybe even educate. Terrifying, we know. Image: Sandbox Studio, Chicago with Corinne Mucha

These physics-themed Halloween costume ideas are sure to entertain—and maybe even educate. Terrifying, we know. Image: Sandbox Studio, Chicago with Corinne Mucha


So you haven’t picked a Halloween costume, and the big night is fast approaching. If you’re looking for something a little funny, a little nerdy and sure to impress fellow physics fans, look no further. We’ve got you covered.

1. Dark energy

This is an active costume, perfect for the party-goer who plans to consume a large quantity of sugar. Suit up in all black or camouflage, then spend your evening squeezing between people and pushing them apart.

Congratulations! You’re dark energy: a mysterious force causing the accelerating expansion of the universe, intriguing in the lab and perplexing on the dance floor.

2. Cosmic inflation

Theory says that a fraction of a second after the big bang, the universe grew exponentially, expanding so that tiny fluctuations were stretched into the seeds of entire galaxies.

But good luck getting that costume through the door.

Instead, take a simple yellow life vest and draw the cosmos on it: stars, planets, asteroids, whatever you fancy. When friends pull on the emergency tab, the universe will grow.

3. Heisenberg Uncertainty Principle

Here’s a great excuse to repurpose your topical Breaking Bad costume from last year.

Walter White—aka “Heisenberg”—may have been a chemistry teacher, but the Heisenberg Uncertainty Principle is straight out of physics. Named after Werner Heisenberg, a German physicist credited with the creation of quantum mechanics, the Heisenberg Uncertainty Principle states that the more accurately you know the position of a particle, the less information you know about its momentum.

Put on Walter White’s signature hat and shades (or his yellow suit and respirator), but then add some uncertainty by pasting Riddler-esque question marks to your outfit.

4. Bad neutrino

A warning upfront: Only the ambitious and downright extroverted should attempt this costume.

Neutrinos are ghostly particles that pass through most matter undetected. In fact, trillions of neutrinos pass through your body every second without your knowledge.

But you aren’t going to go as any old neutrino. Oh no. You’re a bad neutrino—possibly the worst one in the universe—so you run into everything: lampposts, trees, haunted houses and yes, people. Don a simple white sheet and spend the evening interacting with everyone and everything.

5. Your favorite physics experiment

You physics junkies know that there are a lot of experiments with odd acronyms and names that are ripe for Halloween costumes. You can go as ATLAS (experiment at the Large Hadron Collider / character from Greek mythology), DarkSide (dark matter experiment at Gran Sasso National Laboratory / good reason to repurpose your Darth Vader costume), PICASSO (dark matter experiment at SNOLAB / creator of Cubism), MINERvA (Fermilab neutrino experiment / Roman goddess of wisdom), or the Dark Energy Survey (dark energy camera located at the Blanco Telescope in Chile / good opportunity for a pun).

Physics-loving parents can go as explorer Daniel Boone, while the kids go as neutrino experiments MicroBooNE and MiniBooNE. The kids can wear mini fur hats of their own or dress as detector tanks to be filled with candy.

6. Feynman diagram

You might know that a Feynman diagram is a drawing that uses lines and squiggles to represent a particle interaction. But have you ever noticed that they sometimes look like people? Try out this new take on the black outfit/white paint skeleton costume. Bonus points for going as a penguin diagram.

7. Antimatter

Break out the bell-bottoms and poster board. In bold letters, scrawl the words of your choosing: “I hate things!,” “Stuff is awful!,” and “Down with quarks!” will all do nicely. Protest from house to house and declare with pride that you are antimatter. It’s a fair critique: Physicists still aren’t sure why matter dominates the universe when equal amounts of matter and antimatter should have been created in the big bang.

Fortunately, you don’t have to solve this particular puzzle on your quest for candy. Just don’t high five anyone; you might annihilate.

8. Entangled particles

Einstein described quantum entanglement as “spooky action at a distance”—the perfect costume for Halloween. Entangled particles are extremely strange. Measuring one automatically determines the state of the other, instantaneously.

Find someone you are extremely in tune with and dress in opposite colors, like black and white. When no one is observing you, you can relax. But when interacting with people, be sure to coordinate movements. They spin to the left, you spin to the right. They wave with the right hand? You wave with the left. You get the drill.

You can also just wrap yourselves together in a net. No one said quantum entanglement has to be hard.

9. Holographic you(niverse)

The universe may be like a hologram, according to a theory currently being tested at Fermilab’s Holometer experiment. If so, information about spacetime is chunked into 2-D bits that only appear three-dimensional from our perspective.

Help others imagine this bizarre concept by printing out a photo of yourself and taping it to your front. You’ll still technically be 3-D, but that two-dimensional picture of your face will still start some interesting discussions. Perhaps best not to wear this if you have a busy schedule or no desire to discuss the nature of time and space while eating a Snickers.

10. Your favorite particle

There are many ways to dress up as a fundamental particle. Bring a lamp along to trick-or-treat to go as the photon, carrier of light. Hand out cookies to go as the Higgs boson, giver of mass. Spend the evening attaching things to people to go as a gluon.

To branch out beyond the Standard Model of particle physics, go as a supersymmetric particle, or sparticle: Wear a gladiator costume and shout, “I am Sparticle!” whenever someone asks about your costume.

Or grab a partner to become a meson, a particle made of a quark and antiquark. Mesons are typically unstable, so whenever you unlink arms, be sure to decay in a shower of electrons and neutrinos—or candy corn.

Lauren Biron


This Fermilab press release came out on Aug. 18, 2014.

This image of the NGC 1398 galaxy was taken with the Dark Energy Camera. This galaxy lives in the Fornax cluster, roughly 65 million light-years from Earth. It is 135,000 light-years in diameter, just slightly larger than our own Milky Way galaxy, and contains more than 100 billion stars. Credit: Dark Energy Survey

This image of the NGC 1398 galaxy was taken with the Dark Energy Camera. This galaxy lives in the Fornax cluster, roughly 65 million light-years from Earth. It is 135,000 light-years in diameter, just slightly larger than our own Milky Way galaxy, and contains more than 100 billion stars. Credit: Dark Energy Survey

On Aug. 15, with its successful first season behind it, the Dark Energy Survey (DES) collaboration began its second year of mapping the southern sky in unprecedented detail. Using the Dark Energy Camera, a 570-megapixel imaging device built by the collaboration and mounted on the Victor M. Blanco Telescope in Chile, the survey’s five-year mission is to unravel the fundamental mystery of dark energy and its impact on our universe.

Along the way, the survey will take some of the most breathtaking pictures of the cosmos ever captured. The survey team has announced two ways the public can see the images from the first year.

Today, the Dark Energy Survey relaunched Dark Energy Detectives, its successful photo blog. Once every two weeks during the survey’s second season, a new image or video will be posted to www.darkenergydetectives.org, with an explanation provided by a scientist. During its first year, Dark Energy Detectives drew thousands of readers and followers, including more than 46,000 followers on its Tumblr site.

Starting on Sept. 1, the one-year anniversary of the start of the survey, the data collected by DES in its first season will become freely available to researchers worldwide. The data will be hosted by the National Optical Astronomy Observatory. The Blanco Telescope is hosted at the National Science Foundation’s Cerro Tololo Inter-American Observatory, the southern branch of NOAO.

In addition, the hundreds of thousands of individual images of the sky taken during the first season are being analyzed by thousands of computers at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, Fermi National Accelerator Laboratory (Fermilab), and Lawrence Berkeley National Laboratory. The processed data will also be released in coming months.

Scientists on the survey will use these images to unravel the secrets of dark energy, the mysterious substance that makes up 70 percent of the mass and energy of the universe. Scientists have theorized that dark energy works in opposition to gravity and is responsible for the accelerating expansion of the universe.

“The first season was a resounding success, and we’ve already captured reams of data that will improve our understanding of the cosmos,” said DES Director Josh Frieman of the U.S. Department of Energy’s Fermi National Accelerator Laboratory and the University of Chicago. “We’re very excited to get the second season under way and continue to probe the mystery of dark energy.”

While results on the survey’s probe of dark energy are still more than a year away, a number of scientific results have already been published based on data collected with the Dark Energy Camera.

The first scientific paper based on Dark Energy Survey data was published in May by a team led by Ohio State University’s Peter Melchior. Using data that the survey team acquired while putting the Dark Energy Camera through its paces, they used a technique called gravitational lensing to determine the masses of clusters of galaxies.

In June, Dark Energy Survey researchers from the University of Portsmouth and their colleagues discovered a rare superluminous supernova in a galaxy 7.8 billion light years away. A group of students from the University of Michigan discovered five new objects in the Kuiper Belt, a region in the outer reaches of our solar system, including one that takes over a thousand years to orbit the Sun.

In February, Dark Energy Survey scientists used the camera to track a potentially hazardous asteroid that approached Earth. The data was used to show that the newly discovered Apollo-class asteroid 2014 BE63 would pose no risk.

Several more results are expected in the coming months, said Gary Bernstein of the University of Pennsylvania, project scientist for the Dark Energy Survey.

The Dark Energy Camera was built and tested at Fermilab. The camera can see light from more than 100,000 galaxies up to 8 billion light-years away in each crystal-clear digital snapshot.

“The Dark Energy Camera has proven to be a tremendous tool, not only for the Dark Energy Survey, but also for other important observations conducted year-round,” said Tom Diehl of Fermilab, operations scientist for the Dark Energy Survey. “The data collected during the survey’s first year — and its next four — will greatly improve our understanding of the way our universe works.”

The Dark Energy Survey Collaboration comprises more than 300 researchers from 25 institutions in six countries. For more information, visit http://www.darkenergysurvey.org.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @FermilabToday.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

The National Optical Astronomy Observatory (NOAO) is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation.


This article appeared in symmetry on May 1, 2014.

Scientists stay inspired in their sometimes tedious task of inspecting photographs taken in the Dark Energy Survey’s ambitious cataloging of one-eighth of the sky. Image courtesy of Dark Energy Survey

Scientists stay inspired in their sometimes tedious task of inspecting photographs taken in the Dark Energy Survey’s ambitious cataloging of one-eighth of the sky. Image courtesy of Dark Energy Survey

Physicists working on the Dark Energy Survey can expect to pull many an all-nighter. The international collaboration of more than 120 scientists aims to take about 100,000 photographs peering deep into the night sky. Scientists must personally review many of these photos to make sure the experiment is working well, and they’ve come up with ways to stay motivated while doing so.

DES scientists collected almost 14,000 photographs from August 2013 to February 2014, in the first of five seasons they plan to operate their sophisticated Dark Energy Camera. Even for those of us who aren’t trying to take the most detailed survey of the universe, it might not come as a surprise that complications can occur during operation. For example, the telescope may not always sync up with the natural movement of the night sky, and passing airplanes can create trails in the images. Software bugs can also cause issues.

Two of the DES researchers, Erin Sheldon of Brookhaven National Laboratory and Peter Melchior of The Ohio State University, created the DES Exposure Checker, an online gallery of images from the telescope. Team members use the photo repository as a way to spot imperfections and other issues with the images so they can fix problems as quickly as possible.

“These problems are easier for an actual person to see rather than some automated program,” Sheldon says. “And then we can create an inventory to help diagnose troubles that may occur with future images.”

When reviewing photos, DES scientists flag the ones that show symptoms of different problems, such as long streaks from satellites; unwanted reflections, called ghosts; or marks left by cosmic rays. But the process can get overwhelming with thousands of photos to look over. So the DES researchers decided to add a positive classification to the mix—an “Awesome!” category. When someone sees an incredible photo, they can mark it as such in the database.

Sheldon points out one of his favorite images, one that captured a passing comet. “It was just so serendipitous. We couldn’t find that if we pointed the telescope in the same place at any other time,” he says.

Steve Kent, Fermilab scientist and head of the experimental astrophysics group, says one of his favorite images from the survey shows a dying star. In the color photo, a bright blue oxygen haze surrounds the hot remnant of what was formerly a giant red star.

A second way to encourage team members classifying images is the leader board posted on the DES Exposure Checker website, honoring individuals who have categorized the most photos. Researchers compete to see their names at the top.

But more than friendly competition drives the DES team to categorize images. They’re also seeking answers to questions about the past and future of our universe such as: Has the density of dark energy changed over time? Why is the expansion of the universe speeding up?

“For me, it’s a mystery,” Sheldon says. “I have this question, and I have to find out the answer.”

Amanda Solliday


The Dark Energy Survey begins

Wednesday, September 4th, 2013

This article appeared in symmetry on Sept. 3, 2013.

Over the next five years, scientists will capture some of the grandest images of the cosmos ever seen and use them to probe the mystery of dark energy. Image courtesy of the Dark Energy Survey Collaboration

Over the next five years, scientists will capture some of the grandest images of the cosmos ever seen and use them to probe the mystery of dark energy. Image courtesy of the Dark Energy Survey Collaboration

Space: the final frontier. These are the voyages of the Dark Energy Survey. Its five-year mission: to map a portion of the southern sky in unprecedented detail. To use the world’s most powerful digital camera to probe the mystery of dark energy. To boldly photograph where no astrophysicist has photographed before.

The Dark Energy Survey officially began on Saturday, Aug. 31. Using the Dark Energy Camera, a 570-megapixel imaging device built at Fermilab, scientists plan to take clear, dazzling pictures of the largest number of galaxies ever studied in such a survey. The camera is mounted on a telescope at the Cerro Tololo Inter-American Observatory in Chile, which offers a mountaintop vista perfect for obtaining crystal-clear, high-resolution images.

“With the start of the survey, the work of more than 200 collaborators is coming to fruition,” says Fermilab physicist Josh Frieman, director of the Dark Energy Survey, in a press release. “It’s an exciting time in cosmology, when we can use observations of the distant universe to tell us about the fundamental nature of matter, energy, space and time.”

Over five years, scientists will capture full-color photographs of 300 million galaxies, 100,000 galaxy clusters and 4000 new supernovae. The camera is powerful enough to see light from more than 8 billion light years away. The Dark Energy Camera’s 62 charged coupled devices will provide a previously unheard-of level of sensitivity to red light. This will help determine the distances to galaxies—those that appear red are generally farther away, while those that appear blue are nearer by.

But the survey is not just about collecting pretty pictures. Scientists are searching for the answer to a fascinating mystery: Why is the expansion of the universe accelerating? The Dark Energy Survey will use four methods to probe dark energy, the phenomenon believed to be pushing the universe apart:

1. Counting galaxy clusters. While gravity pulls mass together to form galaxies and clusters of galaxies, dark energy pushes it apart. The Dark Energy Camera will see light from galaxy clusters billions of light years away, and counting those clusters at different points in time will offer insight into the cosmic competition between gravity and dark energy.

2. Measuring supernovae. A supernova is a star that explodes, becoming as bright as an entire galaxy of billions of stars. By measuring how bright it appears on Earth, scientists can tell how far away a supernova is and then use that information to determine how fast the universe has been expanding since the star’s explosion.

3. Studying the bending of light. When light from distant galaxies encounters dark matter in space, it bends around it, causing those galaxies to appear distorted in telescope images. The survey will measure the shapes of 200 million galaxies, exploring how gravity and dark energy mold the lumps of dark matter throughout space.

4. Using sound waves to map the universe’s expansion. Sound waves created hundreds of thousands of years after the big bang left an imprint on the way galaxies are distributed across the universe. The survey will measure the positions in space of 300 million galaxies to find this imprint and use it to infer the history of cosmic expansion.

The Dark Energy Camera achieved first light in September of last year. During the subsequent commissioning phase, the camera was put through its paces. Along the way, it captured dozens of sharp, clear images. You can see some of them at the survey’s official photo blog, Dark Energy Detectives, and even more in an interactive mosaic of 62 DECam images.

With the survey now officially underway, scientists will collect data between September and February each year through 2018. This “season” was chosen because the portion of the sky scientists wish to observe will be overhead during those months, and the weather in Chile will be the most cooperative.

Andre Salles


This article originally appeared in Fermilab Today on April 26, 2013.

Images from the Dark Energy Camera before (left) and after (right) a supernova explosion in a galaxy about 2 billion light-years away.

Images from the Dark Energy Camera before (left) and after (right) a supernova explosion in a galaxy about 2 billion light-years away.

The Dark Energy Survey (DES) collaboration has captured images of 176 star explosions, called supernovae, including 16 that occurred farther than 7 billion light-years away and when the universe was only about half as old as it is today. A new type of CCD detector contained in the Dark Energy Camera enabled identification of the distant supernovae, making DECam about 10 times more sensitive than other optical cameras to the long-wavelength (red and near-infrared) light coming from these very distant explosions. This improved sensitivity will allow the DES collaboration to find more supernovae from this period in the history of the universe than any other project.

Our current understanding is that the universe is made up of about 70 percent dark energy and that this dark energy is causing the universe to expand at an accelerating rate. Measuring Type 1a supernovae is a way to study dark energy. The fainter the observed explosion, the further away it is, similar to the difference in brightness between nearby and distant candles. As the light of the explosion travels to us, it is stretched by the expansion of the universe and becomes redder. By combining the measured brightness and information about how much the light is stretched, cosmologists can calculate the expansion rate of the universe.

The Dark Energy Survey collaboration includes scientists, postdocs and graduate students from around the world, who worked together to build the camera, collect the images and identify the supernovae described in this result.

The Dark Energy Survey collaboration includes scientists, postdocs and graduate students from around the world, who worked together to build the camera, collect the images and identify the supernovae described in this result.

The amount and wavelength of a supernova’s light determines its age and type. Researchers use filters that divide optical light into four separate parts, with each filter allowing only certain wavelengths to pass through. We know these 16 supernovae are about 7 billion light-years away because most of the light was observed with the filter that allowed only the reddest light to pass through and be measured by the special red-sensitive detectors in the camera. Less sensitive cameras require time-consuming follow-up observations to determine the supernova age.

To search for supernovae, the DES observers take images of the same patch of sky every four to seven days. Then they subtract the images from each other and search for differences. Computers and teams of people looked at thousands of sets of DECam images to find the 176 candidate supernovae. So far five of the candidates have been followed up, and all five were confirmed to be type 1a supernovae.

The Dark Energy Survey will measure more than 3,000 type-1a supernovae in the next five years and provide new information about the mysterious nature of dark energy. For more information, see the Dark Energy Survey website.

Brenna Flaugher


The great vacuum in the sky

Thursday, March 29th, 2012

This is the zone rockets traverse in Thomas Pynchon’s novel Gravity’s Rainbow. I got e-mail from a reader who didn’t understand the concept of the vacuum. The writer didn’t think it possible, and is in good company. Neither Plato, nor Aristotle, nor even Descartes believed that a pure vacuum could exist.

A ‘vacuum’ in the most common sense is simply the absence of matter in some volume. Early experiments by physicists Torricelli and Boyle with vacuum pumps demonstrated that at least a partial vacuum was possible and could be created on earth. A standard measure of the purity of a vacuum is often expressed in the unit of pressure called a “Torr”, after Torricelli. The pressure at the surface of the earth is 760 Torr. The creation of vacuums of increasingly rarefaction has been possible with more and more powerful pumps. First, there is a mechanical pump, much like a piston engine in a car, which can achieve a pressure of about 10E-5 Torr. Then, there is a turbomolecular pump that uses a high-speed turbine to rid a chamber of gas. Beyond this, there are ion pumps, which trap atoms in a chamber by bombarding them with ionized atoms. At very low temperatures, physicists can take advantage of cryopumping where molecules can be made to stick to cold surfaces.

Why are vacuums important to the LHC? As you might be aware, we have to cool the magnets to a degree or so above absolute zero. In order to do this, we effectively have to create a giant thermos bottle to help keep the magnets cold. This uses a vacuum as the first stage of insulation from the outside world, which prevents the transmission of heat across the barrier of the vacuum.

The beam pipes of the LHC must have a very clean vacuum in order to keep protons circulating in the accelerator tubes without colliding with errant gas molecules. To do this, the pipes the protons travel through are typically maintained to a vacuum of 10E-9 Torr. At the interaction points, where the collisions take place in the middle of the detectors, extra care has to be taken to reduce the number of gas molecules even further, so more cryopumping is used to get the vacuum down to a level of 10E-11 Torr.

To give you some idea of what 10E-11 Torr is like, it’s akin to the pressure in interplanetary space. Present estimates of the vacuum of space far between galaxies is more than 1000 times lower than that, with 6 hydrogen atoms per cubic meter.

In a sense, these are all ‘partial vacuums’ – meaning that there are still atoms floating around. But, if we were able to make a perfect vacuum pump, would this mean that there’s absolutely nothing but space in such a creation?

The answer is ‘no’ and somewhat bizarre. In quantum field theory, there is a concept of ‘virtual’ particles, which are always being created and destroyed in empty space. For example, an electron and an anti-electron (called a positron) can be created momentarily in free space and can then fall back together again. If we introduced a free charge to this perfect vacuum, these electron-positron pairs would polarize and tend to screen the charge of the particle.

Beyond these virtual pairs of particles, there is something even stranger, that we sometimes associate with the Higgs boson, called a ‘vacuum expectation value’. This is to say, in a perfect vacuum we expect that there is some non-zero amount of the Higgs field floating around. Now, one may be quick to dismiss this as just some figment of a theorist’s imagination that has no consequence. Measurements of the rate of expansion of the universe, however, indicate a strange ‘dark energy’ that permeates free space and is forcing the universe to accelerate its expansion. This dark energy appears to be an energy that will inhabit space devoid of any matter whatsoever and is akin to the ‘vacuum expectation value’ in many ways. No one knows why this dark energy exists, but it is permitted by Einstein’s equations describing the large-scale structure of the universe. We just didn’t expect to see it, and it seems to lurk everywhere.

So, perhaps the ancient philosophers were right: there may not be a pure vacuum in nature after all.