A Cosmic Murder Mystery
Image courtesy of the Space Telescope Science Institute.
“…would you believe it? … I stepped right into the middle of a baffling murder mystery and they put me to work…”
–Nick Charles (William Powell), The Thin Man
They say there are a million ways to die in the big city. Sometimes a body goes out quietly. Sometimes, it goes out with a bang.
Either way, it’s our jurisdiction, and we’re on the case.
I can tell you about one we just had. Actually, it’s more like a serial. About once a day, Earth-bound telescopes catch a glimpse of bright flashes coming from distant regions of the universe. Sometimes, if they got enough in them, we can even see the flash with our bare eyes. The Hubble Space Telescope caught one of these things going off (top image).
This bright flash – one last cry before a star shoves off this coil – tells us how it lived.
In this artist’s rendering, jets of high-energy radiation shoot out from a Gamma-ray burst, signaling the death of a massive star. (Image courtesy of NASA/Swift/Mary Pat Hrybyk-Keith and John Jones, via Wikipedia)
Before they die, stars teeter on a delicate balance between gravity and nuclear fusion. Gravity pulls the matter of the star inward – close enough for the atoms to fuse together. The fusion produces the light we see, and it pushes outward on the matter of the star.
But this detente can only last so long. Once a star runs out of nuclear “fuel,” gravity takes over, and fusion slows down. Then, in a few short cosmic moments – as long as you can hold your breath – the dying star gives up more energy than our sun will produce in all its 10 billion years of burning. One more blast, a final scream to let us know it once lived.
These momentary flares are Gamma-Ray Bursts (GRBs), the death throes of some massive stars. At the top of this case file, the series of images shows the afterglow of the GRB (lower middle) next to its host galaxy (center).
Why do these things depart so violently?
Well, it ain’t the colonel with the candlestick.
The biggest clue is the blast. Since we see GRBs from all the way across the universe, the energy released by a GRB must be enormous, more powerful than a supernova. We deduce that much of the star’s mass and energy are converted to light and other particles, like neutrinos, during its final moments. A sketch artist’s rendering shows these jets of energy shooting out from the center of the burst. The only way this could happen is if a whole star’s worth of matter gets pulled close to a black hole in just a few seconds.
Now we know the perp (a black hole), and that its weapon is gravity.
We’ve seen these types before. Now, we’re ready, we’re watching for it.
Our detectives developed the DESAlert system as a way to put out an APB to fellow astronomers. When a GRB is detected – for example, by the Swift satellite – DESAlert uses data automatically generated by Swift to find the GRB’s location on the sky. DESAlert then looks within Dark Energy Survey data for observations in the same region of the sky.
Massive stars that end with a burst rarely die alone: usually, they’re near or inside a galaxy. Our fellow detectives use information about its location to look for a GRB’s galactic accomplice amongst the line-up of nearby galaxies.
Adriano Poci (left) and Raymond Lam (right), students at Macquarie University in Sydney, Australia, were among the first Dark Energy Detectives to work on the case of the dying stars. They are two of the creators of the computer program that controls the DESAlert “galactic APB” system.
For the suspect galaxies in DES data, we have entire astrophysical profiles – shape, size, brightness, distance from Earth – ready for comparison: we share this data with other GRB detectives, who continue the search, trying to catch the bursters.
Dark Energy Detectives and their astronomer colleagues can learn even more about how stars form, how they gather together into galaxies, and how they change during their lifetime leading up to their spectacular fiery death.
They say there are a million ways to die in the big city. How many could there be in the dark reaches of the cosmos?
–Det. Kyler Kuehn @kwkuehn [Australian Astronomical Observatory]
Memories from the edge of the universe
A week ago this morning, the last of the Dark Energy Survey observing teams bid farewell to the Cerro Tololo Inter-American Observatory (CTIO). Our third, six-month long, season of observations is over, and we won’t return until the Fall.
In the past, an astronomer would leave the summit with a suitcase full of data tapes and hand-written logbooks. However, in this digital age, our 57 DES shifters (members of the observing teams) leave only with their memories and photographs. Some of the shifters have generously shared these with us for this special Dark Energy Detectives case log.
Their memories include the expected (sunsets, weather, cute animals, food, and the beauty of the night sky) and the unexpected (meteors, friendships, setting observing records, and girl power).
Our favorite of all was the accidental observation of Comet Lovejoy. We have featured this observation in this case file (image at left). It reminds us that before we can look out beyond our Galaxy to the far reaches of the Universe, we need to watch out for celestial objects that are much closer to home!
The memories range from short sentences to long paragraphs. We have ordered them by increasing length to help those of you with only a few minutes to spare.
The avocados at the CTIO canteen. Delicious!
Setting four out of a possible five “best seeing” records for DES one night.
The shock of seeing an accidental observation of Comet Lovejoy pop up on the display in the control room!
Seeing my first fox on the mountain!
Finding a scorpion in my shoe one morning.
Spending Christmas night 2014 in the Blanco control room.
My first glimpse of a momma viscacha and her baby watching the sunset from the edge of the cliff. Adorable!
The DES atmospheric monitoring camera is amazing. I remember thinking “I wish I’d built this!” the first time I saw it.
The seemingly never ending variety of cookies that are supplied with the packed night lunch: it is a delicious mystery every time!
Getting an extra night of observing for DES because we finished the Liquid Nitrogen pump replacement a day early.
Some of my favorite things about observing at CTIO include: watching the sunsets,; getting to know the other observers; thermoses full of hot tea; galletas de coco; and being alone with my thoughts.
Looking up at the stars, with the silence only broken by the movement of the telescope dome. I’ll never look at the sky on a cloudy day the same way again, knowing what’s right there, just behind them.
I feel lucky to have become good friends with many of the CTIO staff in La Serena and on the mountain: the taxi driver, the cooks, the telescope operators…. Every time I go back, there are so many high-fives and smiles.
I’ve visited CTIO many times, but every time I return I feel like I am entering a new world: The dryness of the air, the brightness of the sun, the darkness of the night. But once I’ve caught my first glimpse of Milky Way plastered across the sky like a snow globe, at feel at peace, I feel like I’m home.
After dinner one evening, we headed up toward the summit to begin observing. It was clear when we set out, but by the time we got there, clouds had risen rapidly from the valley below. The clouds enveloped the summit and blocked out the sunset. We couldn’t even see the telescope dome 50 feet in front of us.
In the 0.9m control room: from the left: Claudia Belardi, Marcelle, Chihway, Catherine Kaleida, Pia Amigo, Sanzia Alves, Pamela Soto, Brittany Howard
We often have all-women observing crews observing for DES, but during my recent visit, there were all-women crews at all Tololo telescopes at once. Looks like the “old boys club” is truly becoming a thing of the past!
The very first time that I stepped out of the observatory to look at the night sky, I saw the bright flash of a meteor breaking up – it was huge and actually made a crackling sound as the embers fell from the sky. I thought the Third World War had started. My immediate reaction was to run back to the dome to protect the telescope!
One of the pleasures of being at the mountain is meeting other astronomers from around the world. I ate several times with a group of Koreans that had been working on a big new camera for several weeks. Some nights I wouldn’t see them at dinner, and I wondered why. When they invited me to have dinner with them at their guest house, I found out why: they had brought enough food from Korea to last for months!
For two days in a row, before our observing shift began, tried without success to catch sight of Iridium communication satellites (their positions can be found from this webpage www.heavens-above.com). One the third night, after some moments of silent expectation, we saw a spot brighter than Venus come into the twilight sky for a few seconds in the direction we were expecting it to be. I remember that I jumped in the air and yelled ‘wooho!’. I was so excited to know the prediction was true. It made me think of historical examples, such as the prediction of the existence and position of Neptune using only mathematics. I can’t imagine the excitement astronomers felt when they actually saw the planet in the place he predicted!
What stays with me the most, when I leave the beautiful mountain, is the memory of the starry blanket that slowly envelops me when I step outside the dome on a moonless night. At first, the blackness is nearly absolute. But as my eyes adapt to the darkness, the brilliant and strange stars of the southern sky come into view with an intensity unmatched at home. High on that remote mountaintop, with the Milky Way arching overhead from horizon to horizon and the Southern Cross shining brightly, the human world is reduced to a dim orange glow off in the distance. In this private moment, I forget the official role that brought me there—“Observing Shift Run Manager”—and take on the only role that seems appropriate for one small human being living his brief moment in this vast cosmos: “awestruck participant.”
So, there you have it folks. Another DES season is over and our collaboration must now turn its attention from observations to analysis. Of course we love the analysis part (that when the fun science gets done), but I suspect most of our Season 2 DES shifters still wish they could click their heels together and be instantly transported back at our beloved mountain (and that would be especially nice, since it usually takes at least 24 hours to get there!)
We’d like to end with a conversation that one the final shifters of the season had with one of the CTIO telescope operating engineers: the tel-ops staff stay with us observers night after night throughout the year (even on Christmas Day), to make sure everything runs smoothly:
Q: So, do you work with DES folks a lot?
A: Yes, I work with DES people all the time. Every week there is a new team. I remember everyone. They are all a little different. The computer lady, the one with the hat, she is the best.
Q: Did you know it is our last night until September?
A: The last night?! No… Seriously? But I am sure you guys will be back. You always come back
Quotes from the following detectives who were on shift:
Jim Annis, Aurelio Rosell, Ross Cawthon, Chihway Chang, Alex Drlica-Wagner, David Gerdes, Ravi Gupta, Manuel Hernandez, Steve Kent, Christina Krawiec, Bob Nichol, Brian Nord, Andres Plazas, Kathy Romer, Marcelle Soares-Santos, Douglas Tucker, Yuanyuan Zhang
Pictures from the following detectives:
Jim Annis, Ross Cawthon, Chihway Chang, Kathleen Grabowski, Ravi Gupta, Christina Krawiec, Jennifer Marshall, Andres Plazas, Kathy Romer, Marcelle Soares-Santos, Douglas Tucker
Post written by Det. Kathy Romer (U. Sussex)
Loveyjoy image credit: Det.’s Marty Murphy, Nikolay Kuropatkin, Huan Lin, Brian Yanny (Fermilab)
The best of the best
Second Lieutenant, Jake Jenson. West Point. Graduate with honors. We’re here because you are looking for the best of the best of the best, sir! —Men in Black
The clearest skies give the best images and provide the best clues to cosmic expansion
Scroll down through these Dark Energy Detectives case files, and you’ll see beautiful images of galaxies taken with the Dark Energy Camera. While they come in different shapes, sizes, and colors, these galaxies all have one thing in common: they’re all speeding away from our own Milky Way, at speeds of tens to hundreds of millions of miles per hour. The Universe is expanding, something we’ve known for nearly 90 years.
If we could track the speeds of each of these galaxies over time, what would we find: would they stay the same, speed up, or slow down? Since the Milky Way’s gravity tugs on them, Isaac Newton would have told us they would slow down over time, just as an apple thrown straight up in the air slows down (and eventually falls) due to the pull of Earth’s gravity. But Isaac would have been wrong, the galaxies are getting faster, not slower. The expansion of the Universe is speeding up, something we’ve known for only 17 years. The 300 detectives of the Dark Energy Survey (DES) are embarked on a five-year mission to understand why this is happening. In this quest, they’re carrying out the largest survey of the cosmos ever undertaken.
While these goals sound lofty and profound (and they are), at its core DES is really about taking pictures. Lots of them. On a typical night, DES detectives snap about 250 photos of the sky. After five years, we’ll have over 80,000 photos in our album. For each snapshot, the camera shutter is kept open for about a minute and a half to let in enough light from distant galaxies. On each image, you can count about 80,000 galaxies. When we put them all together, and accounting for the fact that we’ll snap each part of the sky about 50 times, that adds up to pictures of about 200 million galaxies, give or take.
One of the ways we’ll learn about dark energy—the putative stuff causing the universe to speed up—is by measuring the shapes of those 200 million galaxies very precisely and comparing them to each other. Imagine taking photos of 200 million people, roughly one out of every 35 people on Earth, to learn about the diversity of the human race. To gain the most information about our species, you will want all of your photos to be taken by a professional photographer under identical conditions conducive to getting the best image: good lighting, camera perfectly in focus, no jiggling of the camera or movement of your human subject during the exposure, etc. But inevitably, with 200 million photos, given the vagaries of people and circumstance, some photos will come out better than others. In some, the subject may be a bit blurred. In others, there may be too much or too little background light to see the person clearly.
In the Dark Energy Survey, we’re striving to get the best, clearest snapshots of these 200 million galaxies that we can. As professional photographers of the night sky (a.k.a. astronomers), we’re using the best equipment there is—the Dark Energy Camera, which we built ourselves—to do the job. The camera has 570 Megapixels and 5 large lenses. It has a sophisticated auto-focus mechanism to always give us the crispest images possible.
No need for a flash, since galaxies burn with the light of billions of suns.
But as with human photography, Nature doesn’t always cooperate. The Dark Energy Camera is mounted on the Blanco telescope, located at Cerro Tololo in the Chilean Andes. This site has mostly very clear nights, but occasionally, clouds roll by. Turbulence in the atmosphere, which makes stars twinkle, leads to a slight blurring of the images of stars and galaxies, even if the camera is in perfect focus. The camera works by taking pictures of all the light that reflects off the 4-meter-diameter mirror of the telescope. If a cold front moves through, making the air in the telescope dome cooler than the 15-ton mirror, plumes of hot air rising off the mirror lead to blurry images. The sharpest images are those taken straight overhead—the further away from straight up that we point the telescope, the more atmosphere the light has to pass through, again increasing the blurring; since our survey covers a large swath of the sky, we cannot always point straight up. Strong wind blowing in through the open slit of the dome can cause the telescope to sway slightly during an exposure, also blurring the picture. Since the Earth rotates around its axis, during an exposure the massive telescope must compensate by continuously, very smoothly moving to stay precisely locked on to its target; any deviation in its motion will—you guessed it—blur the image.
For all these reasons and others, the quality of the DES images varies. On some nights, conditions conspire to give us very crisp images. On others, the images are a bit more blurred than we’d like, making it harder to measure the shapes of those distant galaxies. If an image is too blurred, we don’t include it in the album: we’ll come back another night to take a photo of those particular galaxies. So far, about 80% of the photos we’ve taken have been good enough to keep.
Most nights during our observing season, we have three detectives operating the camera; each of us is there for about a week, and in the course of a season about 50 detectives rotate through, taking their “shifts.” On the night of January 27, 2015, I was in the middle of my week-long observing shift at the telescope with two fellow detectives, Yuanyuan Zhang from the University of Michigan and Andrew Nadolski from the University of Illinois at Urbana-Champaign. That night, Andrew was manning the camera, I was checking the quality of the images as they were taken, and Yuanyuan was our boss.
The conditions that night were outstanding. Although it was a bit humid, the atmosphere was extremely smooth and stable. We were mainly taking pictures using filters that let in only very red or near-infrared light. This was because the moon was up, and the moon is actually quite blue: red filters block most of the moonlight that scatters off the atmosphere from entering the camera, enabling us to see red galaxies against the dark night sky. In his famous photograph “Monolith, the Face of Half Dome” taken in Yosemite National Park, Ansel Adams used a red (but not infrared) filter to darken the blue daytime sky to dramatic effect.
At 12:28 am local time, we snapped exposure number 403841, using a near-infrared filter called the z-band. The z-band is so red that it’s beyond the visible spectrum that can be seen by the human eye, but digital cameras, and the Dark Energy Camera in particular, are very sensitive to near-infrared light. Computers at the telescope analyze each image right after it’s taken and display the results on a bank of monitors, so we can tell whether we’re taking data that passes muster for our cosmic album. When 403841 came out, the screen showed that it was an exceptionally sharp image. Further analysis convinced us that it was in fact the sharpest image of the roughly 35,000 snapshots that DES has taken so far, going back two years to the beginning of the survey.
The image was so sharp that the light from each star was spread out over only about 0.6 seconds of arc or about 0.00017 degrees. For comparison, that’s how big a crater a kilometer across on the surface of the moon looks from Earth. It’s also the angular size of a typical human hair seen at a distance of about 100 feet.
A small portion of the 403841 image is shown above in false color, showing a great spiral galaxy plus a number of smaller, fainter galaxies and a few bright stars in our own Milky Way. The star inside the red circle at the lower right of the image has its light spread out over only 0.6 arc seconds. While not as pretty as the color images of galaxies in other DED case files, this is closer to what a raw image directly from the camera looks like. The raw DES digital images are sent for processing to the National Center for Supercomputing Applications in Urbana-Champaign, Illinois (if you’re under 40, ask your parents if they remember sending film out for processing), to make them science-ready for our fellow DES detectives.
In DES, we keep a “bragging rights” web page of the sharpest images we have taken in each of the five filters we use. Our friend 403841 is now prominently displayed there—the best of the best. But the best thing about records is that they’re made to be broken.
–Det. Josh Frieman [Fermilab and the University of Chicago]
N.B.: we just completed a Reddit AMA on Friday, Jan 30, where we discussed the cases and evidence for dark energy and dark matter.
Dancing in the dark
“Work like you don’t need the money. Love like you’ve never been hurt. Dance like nobody’s watching.”–Satchel Paige
To the silent tune of gravity, congeries of celestial objects – big and small – dance each night away. In the darkness beyond Neptune, this troupe of Kuiper Belt objects (KBOs) had been dancing like no one was watching – until now.
Their dance is a slow one, for Kuiper Belt objects take centuries to complete one orbit. These KBOs, each a few hundred kilometers in size, have been discovered by DES over the last two and a half years. (One of them was described here earlier.) Suppose you knew nothing about gravity. What would you make of a pattern like this? How would you explain it? The laws that give rise to such intricate celestial swirls must be incredibly complicated, right?
Ancient people marked the wanderings of the planets from night to night and season to season. They noticed that they moved across the sky at wildly different rates: sometimes, they appeared to stop, turn around, and move backwards against the canopy of fixed stars, before turning again and resuming their course. Ingenious models were developed to explain this complicated dance. But they became increasingly unwieldy, and even worse, failed to describe new and more accurate observations.
It took two scientific revolutions—first from Copernicus and then from Newton—to show that planetary motion could be readily explained by a single simple equation, the law of gravitation. The hidden pattern suddenly became clear.
The graceful pirouette executed by a KBOs arises from a combination of two motions. Its centuries-long orbit produces a slow eastward drift that carries it about the width of one DECam field of view per year. But we observe these objects from a moving platform, planet Earth. As the earth makes its journey around the sun, we observe the KBO from different perspectives, sometimes from 150 million kilometers on one side of the sun, six months later from 150 million kilometers on the other, and at other times from somewhere in between. This results in an annual back-and-forth motion relative to the distant stars that’s superimposed on the KBO’s own orbital motion. Watch how your fingertip moves against background objects when you move your head from side to side and you’ll get the idea.
Physics aims to distill order from complexity, to explain the vast array of natural phenomena with a small number of simple laws. Eventually, physicists learned that Newton’s law of gravitation fell short in certain situations and needed to be superceded by Einstein’s theory of general relativity.
Today, gravity confronts our generation with a new puzzle on the grandest of scales: Why is the expansion of the universe accelerating? Perhaps some new law will explain the mystery of dark energy with the as much elegance and simplicity as the dance of the planets. That’s the hope that keeps our dark energy detectives patiently looking up.
Det. Dave Gerdes [University of Michigan]
Cosmic soup for the soul
Amidst the dark forces and energies at work across the cosmos, a fire brews, a soup simmers.
The expansion history of the Universe is dominated by dark matter and dark energy. However, it is the elements in the periodic table that allow us to study and understand that history. In this posting we give a flavor for how the cosmic soup of elements came into existence.
Almost all the elements came into existence within 30 minutes of the Big Bang. The resulting broth was rather dull: 9 hydrogen nuclei (one proton) to every helium nucleus (two protons) and almost nothing of anything else. Even if you sifted through a billion nuclei you’d still be lucky enough to find anything as tasty as lithium (three protons).
Fortunately, over the intervening 13.7 billion years, the cosmic soup has become a little more interesting. Nuclear fusion – so hard to reproduce on Earth – is common place in stars: we have fusion to thank for the carbon in our cells, to the iron in our blood.
The flavor, density and temperature of the element soup varies widely. Consider our own Solar system: from the extreme pressures and temperatures inside the Sun’s core, to the cold and empty space between the planets. These variations are replicated throughout the Milky Way and in all the other galaxies in the universe.
These three concepts – that most elements were formed just after the Big Bang; that a smattering of heavier elements have been added since then; and that the elements are distributed non-uniformly – are of great benefit to the Dark Energy Survey.
Take for example clusters of galaxies, like those in the slideshow above (described in more detail later). These structures are so enormous that they can be considered to be mini Universes in their own right. Clusters contain several dozen galaxies, and sometimes as may as several hundred. In between the galaxies is the continuous haze of tenuous gas.
Both the gas and the galaxies are trapped within the confines of the cluster by dark matter. The dark matter acts like the lid on a sauce pan, where the lid stops the pan boiling dry, the dark matter stops the galaxies – which are moving at more than a million miles per hour – from flying away. However, at the outer edges of the very largest clusters, dark energy competes with gravity and the galaxies are starting to be peeled away. It is this interplay of gravity and dark energy that make clusters such useful cosmological probes.
The particles in the gas are so hot that electrons (negatively charged) and nuclei (positively charged) are stripped apart – this form of gas is known as a plasma. The plasma shines brightly in the X-ray part of the electromagnetic spectrum and can be detected by satellites such as XMM-Newton and Chandra. The plasma also casts a shadow on the Cosmic Microwave Background (a pulse of light that was emitted throughout the Universe one hundred thousands years after the Big Bang), meaning it can also be detected with shortwave radio telescopes such as the South Pole Telescope.
By contrast, the elements trapped in the stars are cooler, and at much higher densities, and shine in visible light. Starlight allows the Dark Energy Survey to not only to detect hundreds of thousands of clusters, but also to measure their distances (via a technique known as photometric redshifts), and to make a first estimate of their masses. Those masses need to be refined before we can use the clusters for cosmology, and information of the plasma from X-ray and radio telescopes is essential for that.
In the slideshow above we show several examples of the hundreds of Dark Energy Survey clusters that have also been observed by the XMM-Newton Cluster Survey. The intensity of the X-ray emission coming from the hot plasma is indicated by the red contours. X-ray specialists are working with these two datasets to calibrate the masses of Dark Energy Survey clusters.
Finally… why “for the soul”? Well “soul’’ happens to be a synonym for “quintessence”, and Quintessence has been widely adopted by cosmologists as a catch all term to describe theories that allow for a time variation in the properties of Dark Energy.
Det. Kathy Romer [University of Sussex]
Image Credit: Det.’s Phil Rooney [University of Sussex] and Chris Miller [University of Michigan]
Our dark, tangled web: Clues of dark energy
Lurking beneath a sea of light, an intricate pattern rustles and changes ever so slowly. It is built from dark, and nearly invisible, cosmic forces. Amidst the clumps and knots of galaxies lay empty, usually fallow spaces. While each galaxy, with its billions of stars, has a unique story of birth and evolution, we don’t miss the forest for the trees. Taken as a whole, the pattern of clusters and voids in our galaxy maps can tell us about the dark forces that shape our universe.
Mapping of galaxies by the Sloan Digital Sky Survey out to 2 billion light-years away. Red and green points indicate positions of galaxies, with red points having a larger density of galaxies. The fully black areas on the sides are parts of the sky inaccessible to the survey. (See also the SDSS fly-through.)
Looking at the image from the Dark Energy Camera (above), we can see a plethora of celestial objects, including many blue, red and yellow smudges, many of which are distant galaxies. It may appear that these galaxies are randomly strewn about the cosmos. However, astronomers charting the locations of these galaxies across large distances have found that galaxies are organized into structures, into cosmic patterns that can span swaths of space and time much larger than what is seen in this image. The figure on the right, from the Sloan Digital Sky Survey, shows a map of millions of galaxies. These galaxies appear to cluster into knots and filaments (areas with many galaxies), and leave behind voids (areas with few or no galaxies). Some filamentary structures stretch across a billion light-years – 60 trillion times the distance from the Earth to the Sun!
Like any good detective, we cannot ignore a pattern. How do galaxies, separated by up to billions of light-years, eventually coalesce into the great cosmic structures we see today? It turns out the ‘mastermind’ of this cosmic operation is a familiar friend (and foe) to us on Earth: the force of gravity.
Using computer simulations, astronomers have investigated how gravity acts among so many galaxies over such very large distances. The Millennium Simulation, and others like it, show that a mostly random distribution of matter will naturally cluster into filaments and voids through the force of gravity. When we statistically compare the simulation results to our data (observations of many galaxies), the patterns are the same: gravity’s influence throughout the visible universe has fostered this grand filamentary structure, which has been dubbed, “The Cosmic Web.”
The Millennium Simulation: brighter areas are where more matter and galaxies have concentrated. (See more of this simulation in this fly-through video).
What does this mean for the detectives working on the Dark Energy Survey? It turns out that gravity has a nemesis in its goal for creating web-like order across the universe: dark energy, the invisible force causing the accelerated expansion of space throughout the universe. The faster space grows and accelerates, the greater the distances galaxies must travel to form filaments and clusters. If there is more dark energy, gravity needs more time to pull galaxies together, and web-like structure develops slowly. If there is no dark energy, the web gets built quickly. By studying how quickly or slowly the cosmic web was built across time, we learn how strong dark energy has been and if it is growing stronger or weaker.
The battle between gravity and dark energy, manifested in the evolving structure of the cosmic web, is a key way to study dark energy. In fact, the cosmic web is particularly important for answering one specific question: is there even dark energy at all?!
Most astronomers agree that there is overwhelming evidence for the accelerated expansion of the universe. For many reasons, the most plausible source of this acceleration is some new force or otherwise unseen, “dark” energy. The leading alternative theory though is a change in the laws of gravity (specifically, in Einstein’s laws of general relativity). Since physicists and astronomers have tested Einstein’s laws numerous times on Earth, the Solar System, and within galaxies, the change would only manifest itself at much larger distance scales. It could be causing the appearance of cosmic acceleration, such that there might be no dark energy.
This second hypothesis would re-write our case file on the cosmic web. Perhaps instead of fighting against dark energy, gravity is just not carrying quite the influence across billions of light years that we’ve predicted. Measurements of the cosmic web, in conjunction with other measures of cosmic acceleration, will be key in telling us whether our universe is a battleground for dark energy and gravity, or if gravity is just different than previously thought. Either conclusion (or perhaps an even stranger one!) would signify a fundamental revision in how we think about the workings of our universe.
As the Dark Energy Survey collects more beautiful images of hundreds of millions of galaxies over a five-year span, our detectives will be carefully logging their positions, charting out the cosmic web, hoping to identify what forces are at work in the dark.
Detective Ross Cawthon (University of Chicago)
Image: Det.’s Marty Murphy and Reidar Hahn
Our dark, tangled web: Where’s Waldo?
Cosmic structures woven together during the tug of war between gravity and dark energy present a multi-faceted challenge for scientists, as we seek to untangle each galaxy from the luminous cacophony of filaments and clusters across large swaths of space and time.
We love staring at the beautiful images taken by the Dark Energy Camera (DECam) at the Blanco telescope. The image above shows a cluster of galaxies laid on a backdrop of even more distant galaxies. To investigate the mysteries of the accelerating expansion, Dark Energy Survey (DES) scientists need to do a bit more – we need to develop a comprehensive census of the content across the universe: how many stars and galaxies are there in a given swatch of space-time fabric?
A critical step comes in creating a high-fidelity and detailed list of the observed celestial objects: these are called “catalogs” by astrophysicists and astronomers. The most common pieces of information are the position and brightness: this is the minimum information necessary to know where a galaxy resides in spacetime.
With our hard-working scientists in the data management team and the powerful computers at National Center for Supercomputing Applications (NCSA), DES has developed new algorithms and pipelines for efficiently sifting the objects out of our images. We start with raw images straight from DECam, and then we refine them to remove artifacts, like satellite trails, cosmic rays and faulty pixels. From these “reduced” images, we must then find and characterize discrete objects, like galaxies and stars – cut the wheat from the chaff.
However, there is a limit to what we can do. For example, a very far-away object may appear extremely small and faint – so faint that it will look like a piece of the sky and get missed during the cataloging procedure. In some cases, it is not possible to tell the difference between a faint object and a noisy patch of sky. In addition, not every astronomical object is “willing” to be cataloged: it can be disguised as a part of another object. For example, near the center of today’s image, there is a very large, bright galaxy with many smaller neighbors. Discerning all the objects here is similar to the difficulty one might have in noticing a flea in a picture of an elephant.
Objects also tend to hide from the computers when a piece of the sky is full of them: spotting a small object becomes as difficult as finding Waldo (Wally) on a crowded beach!
DES takes more detailed images than previous projects, like the Sloan Digital Sky Survey (SDSS). Thus, we are more pestered by the “hiding” objects problem. We see a more tangled web. As one solution, a group of DES scientists have employed an image restoration algorithm, derived from work by computer vision scientists. This algorithm successfully eliminates the impact of close neighbors when cataloging the “hiding” objects. Upon application to DES images, they have been able to find many “Waldos,” so we can add them to DES catalogs.
For more detailed description of the method, you can find a preprint of the paper here: http://arxiv.org/abs/1409.2885.
Det.’s Yuanyuan Zhang and B. Nord
Image: Det.’s Marty Murphy and Reidar Hahn
In the air tonight
Sometimes, you can feel it coming in the air of the night.
Weather is fickle, but when a night of observing begins, we usually know how it will go. The first part of this season was often rainy and gray. The last several weeks, however, have allowed for new records in precision the precision of DECam data.
On Nov 11 and Nov 18, 2014, the Dark Energy Survey took exquisite data of all of our supernova fields – the regions of sky selected specifically to look for exploding stars. It was clearer than anything we’d seen previously. The video above is from a night early in this season, when the weather was also extremely good (but only for a few days). It is a view, from inside the dome, of DECam and the Blanco Telescope scanning the sky over the course of one night in August, 2014.
After a few nights of clouds or rain, it usually takes another night or two for the atmospheric turbulence to die down. This turbulence deflects light as it comes through the layers of Earth’s atmosphere, effectively blurring an image. But when this turmoil is no longer there, the conditions can be pristine.
Sometimes, you can feel it coming in the air of the night. It’s the final moment for so much starlight.
We are here to see what it did, see it with DECam’s 570 million eyes. DECam’s been waiting for this moment all of its life. Now we know where you’ve been, traversing the dark night skies.
The light of distant galaxies and stars has been waiting for this moment all that time.
Now forever, we remember where the light has been, how could we forget. When our detectors capture it, it’s the first time, the last time, we’ve ever met. We know the reason you kept your silence up. When it was cloudy, how could we know. When it’s clear, the signal still grows, the universe no longer a stranger to you and me.
Sometimes, you can feel it in the air of the night.
Det. B. Nord
[Hat tip to Phil Collins.]
As the sky turns: the fall and rise of the Milky Way
We are swept up in a cosmic merry-go-round.
Earth spins relative to the sky – about one revolution every 24 hours.
After twilight, our nearest star, the Sun removes its warm blanket of light, revealing the dancing lights overhead: collections of aeons-old galaxies and constellations of distant stars fill the night sky. For some precious hours, we have exquisite access to these pinpricks and smudges of light that have always swirled overhead – until we bask again in the Sun’s rays. During the day, all blinking tapestry is still above us, but the Sun washes out any hope of seeing it. Again, after dusk, familiar patterns fill the sky, as the dancers return like clockwork to their positions on the celestial stage.
Our entire solar system resides in a galaxy, the Milky Way. The Galaxy’s structure includes spiral arms and a disk of stars and gas: our pale blue dot, tethered to the Sun, is nestled in the suburbs, halfway to the edge of the Galactic disk. As we turn from day to night to day, the Galaxy itself also spins (over much longer periods than Earth’s day).
During the course of our daily/nightly sweep of the heavens, just as the stars and galaxies move across our sky, so does the disk of Milky Way. When we look up from the dark mountain tops of Cerro Pachon, we look into the plane of the Milky Way, into the heart of our Galaxy.
In the video above, the camera rotates from East to West through South – taking a picture every 30 seconds over the course of the night. Earth’s axis goes through the South pole, so we see the sky spin about that point: one side of the Milky Way sets, and by 1am on this October night, another side begins to rise.
Good night, and keep looking up,
Det. B. Nord
Across the world and up all night
For the last week, detectives from the Dark Energy Survey have been coordinating across four continents to bring to light more evidence of how the fabric of spacetime is stretching and evolving.
In Sussex, England, over 100 detectives met to discuss the current state and the future of the Survey that is conducted at the Blanco telescope, located at Cerro Tololo in Chile. At this semi-annual collaboration meeting (with a new venue each time), we continued to strategize analyses for the many probes of spacetime evolution and dark energy: as I write, several early results are being prepared for publication.
At Cerro Tololo, a team of observers operated the Dark Energy Camera (DECam) on the Blanco telescope, as we make our way through the second season of observing for the Survey. Each season goes August through February, during the Chilean summer.
The Anglo-Australian Telescope at Siding Spring Observatory in Australia is home to the OzDES Survey – long-term project for obtaining highly precise distance measurements of objects discovered by DES, such as supernovae and galaxy clusters. These “follow-up” measurements will be very important evidence in pinning down the culprit for dark energy.
At Cerro Pachon, just east of Cerro Tololo, another team of two agents began to search for evidence of highly warped space in the distant cosmos, using the Gemini (South) Telescope (@GeminiObs). We spent six nights working to measure highly accurate distances of strong gravitational lensing systems. These systems are galaxies or groups of galaxies that are massive enough to significantly distort the fabric of space-time. Space and time are so warped that the light rays from celestial objects – like galaxies and quasars – behind these massive galaxies become bent. The resulting images in DECam become stretched or even multiplied – just like an optical lens. In future case reports, we’ll expand on this phenomenon in more detail.
All the while, supercomputers the National Center for Supercomputing Applications (NCSA) are processing the data from DECam each night, turning raw images into refined data – ready for analysis by the science teams.
The image above doesn’t display any obvious strong lenses, but it is an example of the exquisite lines of evidence that DES continues to accumulate each night.
Here are positions of some of the galaxies above. What information can you find about them? There are several electronic forensic tools to assist your investigation (for example, http://ned.ipac.caltech.edu/forms/nearposn.html; take care to enter the positions with the correct formatting, as they are below). Tweet your findings to our agents at @darkenergdetec, and we can compare case notes.
RA: 304.3226d, Dec: -52.7966d
RA: 304.2665d, Dec: -52.6728d
RA: 304.0723d, Dec: -52.7044d
Good night, and keep looking up,
Det. B. Nord
Det. M. Murphy [image processing]
Tributaries of Time: Autumn Leaves
Across North America, as the transition toward winter begins, we see symptoms in the changing colors of tree leaves. The lively green hue of summer gives way to yellows, oranges, reds and purples. Living cells inside the leaves have instructions for how to react to cooler and cooler environments: this reaction reduces the production of the green pigment, chlorophyll, allowing other colors (caused by the pigments of the carotenoids and anthocyanins) to dominate. When spring returns, so do leaves, newly filled with oxygen-producing chlorophyll.
Year after year, we watch the cycle of death and rebirth in the life-giving foliage around us.
But what if we were insects? What if, like the mayfly, we lived for only a day or two? Would we have any way of understanding the immense tapestry evolving around us? Imagine for one day on Earth, looking at leaves all over the globe – in different environments and in various states of health and age. With just this one day to create a coherent picture, could we piece together the clues of color, environment and the internal workings?
This is the challenge we face in understanding the life-cycle of galaxies, the leaves on our cosmic tree of matter and light. To these celestial objects, we are indeed the mayfly, living for only a blink of an eye in cosmic time.
Consider the cornucopia of dusty swirls in the image above, their colors spanning the entire visible rainbow and beyond. Each puff of light contains billions of stars. Through our telescopes, images and spectrographs, we learn about the kinds of chemicals, of matter that reside within galaxies. Through an understanding of physics, we link this information to the possible physical processes, from gravity to quantum mechanics.
Similar to that of tree leaves, the colors of galaxies are the result of the chemical constituents, and they reflect their ages. Blue galaxies, still young, are cold enough to be forming stars, because young stars and the gas enshrouding them release bluer light to the cosmos. Red galaxies have had their star-formation extinguished: their gases are too warm for the force gravity to collapse them into energy-generating balls of fusion. These ‘red and dead’ galaxies represent the end of the galactic life-cycle.
While we have ways of peering inside galaxies to reveal some of their guts, we still have no way to watch an entire galaxy come into being, much less live out a full life. Each galaxy represents its own tributary of time, its own puzzle piece in the delta of the cosmic web.
Det. B. Nord
Image: Dark Energy Camera [Edited and logged by Det. M. Murphy]
Unsung Hero Cold Cases – The Slipher File
As the Milky Way sets, light from nearby villages and mining towns turns the stream of clouds overhead into a rippling river of fool’s gold. On this night in October of 2013, during the first season of observations of the Dark Energy Survey, we pumped caffeine into our bodies to stay awake, to keep ready for when the conditions would change. Every field we can observe, every galaxy we can capture will make a contribution to the greater measurement of their vast patterns – patterns distorted (or created) by a dark energy.
One hundred years ago, an American astronomer by the name of Vesto Slipher became the first to measure streams of galaxies in our local neighborhood. Slipher used the 24-inch telescope at Lowell Observatory to measure velocities of spiral nebulae (i.e., galaxies), through a method known as “spectroscopy.” Most of the galaxies that Slipher measured are receding from the Milky Way, rather than moving toward it – the first indication of cosmic expansion.
This result laid the groundwork for the definitive discovery of the expanding universe. Unfortunately, Edwin Hubble of Mount Wilson is most often accredited with this finding. Hubble measured distances via Cepheid Variables to distant nebulae and then correlated them with Slipher’s velocity (redshift) data to create the famous distance-velocity plot for his 1929 paper.
Hubble provided no citation of Slipher’s work.
Slipher is the first to measure Doppler Shifts (velocities) of galaxies, to show that spiral galaxies rotate, and to detect that collections of stars and dust are actually nebulae outside our own Milky Way.
Let us remember Vesto Slipher – among modern cosmology’s most influential unsung heroes.
Det. B. Nord
Distant Wanderer
After a great journey, a long-hidden member of our solar system has returned. Not since the 9th century, when Charlemagne ruled as Emperor of the Holy Roman Empire and Chinese culture flourished under the Tang Dynasty, has this small icy world re-entered the realm of the outer planets.
This distant wanderer is among first of its kind discovered with data from the Dark Energy Survey (DES). Now officially known as 2013 TV158, it first came into view on October 14, 2013, and has been observed several dozen more times over the following 10 months as it slowly traces the cosmic path laid out for it by Newton’s law of gravitation. We see this small object move in the animation to the left, comprised of a pair of images taken two hours apart in August, 2014.
It takes almost 1200 years for 2013 TV158 to orbit the sun, and it is probably a few hundred kilometers across – about the length of the Grand Canyon.
In eight more years, it will make its closest approach to the sun – still a billion kilometers beyond Neptune. At this distance, the sun would shine with less than a tenth of a percent of its brightness here on earth, and would appear no larger than a dime seen from a hundred feet away.
That’s what high noon looks like on 2013 TV158.
Then it will begin its six-century outbound journey, slowly fading from the view of even the most powerful telescopes, eventually reaching a distance of nearly 30 billion kilometers before pirouetting toward home again sometime in the 27th century.
This object is just one of countless tiny worlds that inhabit the frozen outer region of the solar system called the Kuiper Belt, an expanse 20 times as wide and many times more massive than the asteroid belt between Mars and Jupiter. The dwarf planet Pluto also calls the Kuiper Belt its home. The orbits of Jupiter, Pluto and 2013 TV158 around the sun can be seen in the image to the lower right.
Scientists believe that these Kuiper Belt Objects, or KBOs, are relics from the formation of the solar system, cosmic leftovers that never merged into one of the larger planets. By studying them, we can gain a better understanding of the processes that gave birth to the solar system 4.5 billion years ago.
Because they are so distant and faint, KBOs are extremely difficult to detect. The first KBO, Pluto, was discovered in 1930. Sixty-two years would pass before astronomers found the next one. Astronomers have identified well over half a million objects in the main asteroid belt between Mars and Jupiter. To date, we know of only about 1500 KBOs.
DES is designed to peer far beyond our galaxy, to find millions of galaxies and thousands of supernovae, but it can also do much more. DES records images of ten specific patches of the sky each week between August and February. These images are a perfect hunting ground for KBOs, which move slowly enough that they can stay in the same field of view for weeks or even months. This allows us to look for objects that appear in different places on different nights, and eventually track the orbit over many nights of observations.
So far we’ve searched less than one percent of the DES survey area for new KBOs. Who knows what other distant new worlds will wander into view?
Det. D. Gerdes
Revolution
When is the last time you watched the sky revolve around us?
Earth rotates on its axis at 1,000 miles per hour (1600 kilometers per hour). At the same time, it flies around the sun at 67,000 m/h (110,000 km/h). And the Sun, with all its planets and rocks and dust in tow, makes its way around the center of the Galaxy, our Milky Way, at 520,000 m/h (830,000 km/h). And then, the Milky Way itself is hurtling toward the nearby Andromeda galaxy at 250,000 m/h (400,000 km/h).
The fastest space craft (and fastest man-made object in history), Juno, will slingshot around Earth on its way to Jupiter, eventually reaching a speed of 165,000 m/h. The NASA space shuttle reaches speeds of 17,000 m/h (27,000 km/h).
The average human walking speed is 3.1 m/h (5.0 km/h).
Though we sit in this coordinated maelstrom, we can still understand all of space and time on the largest scales. But, to do so, we must consider it statistically, on the whole, at great breadth and as a collection – not merely the sum of disconnected parts or separate events.
All across the universe, there are supernovae – exploding stars that blink in a cataclysmic, cosmically infinitesimal moment. Quasars are small regions that surround the supermassive black holes at the centers of galaxies that flash on and off on the timescales of hours to months. Each galaxy in the universe is creating some dimple in space-time due to its mass. Imagine a vast expanse of sand dunes: all light passing by these galaxies must traverse through it, resulting in distorted images by the time they get to us.
These are just some of the events that go on constantly around us, without regard for our existence, as we spin round and round, imagining a static quilt of stars turning about us. And they are just some of the celestial targets that will tell us more about how fast the universe is expanding.
To better understand these events, and the acceleration of spacetime, we wait for the targets to be at a place in the sky when we can see them – when the sun is down and this part of Earth is pointed in their direction. Our targets come from a large swath of sky, one-eighth of the celestial sphere. And across this expanse, we will obtain a uniform sample of targets. The uniformity – homogeneity or constancy – is crucial: we must observe all galaxies brighter than a certain amount, and within a certain distance to have a clean, uniform sample. Otherwise, variations in that information could be misconstrued, or at best they could muddy our measurement of dark energy.
Building the collection starts with amassing a set of deep images of the sky: these are but snapshots of long-gone eons, and they are the first step in our process of discovery. From the images, we distill vast catalogs of celestial bodies – galaxies, stars, motes and seas of hot gas and dust – an accounting of what the universe has so far created. This catalog can be further distilled when studied as a whole. The final concentrate is a small set of numbers that summarizes the fate of our universe: a measurement of the strength of dark energy.
Our spaceship Earth is a pebble in the swirling cosmic sea around us. We watch it as if we are separate, sometimes forgetting we come from it. As we look up from within our snowglobe on a mountaintop in the Chilean Andes, it becomes easier to remember that we are a conduit between the finite and the infinite.
Good night, and keep looking up.
Det. B. Nord
Light-years Away, Right at Home
As the Galaxy sets behind the Blanco telescope, our home away from home, we are reminded of where we really are. Earth resides in a mere village of planets, one of many in a city of stars – our Milky Way galaxy – which, as these detectives see it, is our true home.
But it is the distant stars and galaxies, just like those we call home, that betray patterns in our cosmos.
We operate in the dark of night to find as many as we can, as carefully as we can. We track locations, movements, interactions, explosions and lifetimes of millions of individuals. Only these clues in aggregate (for the most part), will lead us down a starlit path to an understanding of our universe’s greatest tug of war: that which is between the pull of gravity and the accelerating expansion of dark energy.
The detectives have gone back to work for Season 2 of observing and combing the logs of photons as they stream into the trap we’ve set, the Dark Energy Camera (DECam). In the coming months, we’re turning these streams into nuggets of knowledge, the first puzzle pieces to be revealed by the Dark Energy Survey (@theDESurvey).
And ultimately this knowledge brings us back home, to understanding our place in the cosmos.
I’m here now at the Blanco, writing this as we prepare for our third night of observations and tracking in Season 2 of DES, with bags under our eyes, coffee mugs in hand, watching the fires in the sky.
Det. B. Nord (@briandnord)
If you run into us where the electrons roam (FB, Twitter, Reddit, etc.), don’t be afraid of the dark – get in touch. We’ll report every two weeks (and occasionally more), and we’ll have more detectives and more ways to tell the stories.
Today, there’s an announcement about the beginning of Season 2, along with a spate of videos and images about the team of detectives, the location and the machine we’ve built.
New Beginnings: Our Darkness (Re)Lit
What clues early in humanity’s search of the sky told the Universe’s story? Emerging from the darkness long ago, what diffuse beacons in the fabric of spacetime offered a glimpse into our place in the cosmos?
When Galileo first pointed his telescope at Jupiter and saw its moons he inevitably would have looked at other parts of the sky. He would have noticed fuzzy patches of light in the sky. Early astronomers could only guess what those fuzzy patches of light were. Collectively, they were referred to as “nebulae”, due to their nebulous forms. Intentional or not, this was the beginning of modern astronomy.
Until the early 1900s, scientists believed the entirety of the universe was contained in what we now call our Milky Way galaxy. They believed that the fuzzy nebulae were much closer to Earth than they actually are. It wasn’t until Edwin Hubble’s observations of Cepheid variable stars in the Andromeda and Triangulum nebulae in the early 1920s that astronomers began to appreciate the size and scope of the universe.
Hubble discovered that those little fuzzy patches of light were entire collections of stars much farther away from us than the rest of the stars in the night sky. In short order, these collections of stars would be referred to as galaxies. That name was natural: the Greeks had already been referring to the fuzzy disc of the Milky Way as a galaxy (the Greeks referred to it as galaxias kyklos which means milky circle; the Latin word galaxias literally means milky way).
Our Milky Way was now one of many, many galaxies.
After changing our notion of what a galaxy is and our place in the universe, Hubble set out to categorize the different kinds of galaxies. Pictured above are many types of galaxies captured by the Dark Energy Camera (DECam). There are at least five or six easy-to-spot galaxies – the edge-on spiral on the right side, the pair of colliding spirals at the bottom center, a big spiral in the top-left, and an elliptical on the far left.
Hubble’s discovery rocked astronomy, and as the fields of astronomy and physics inevitably came together, many new questions emerged. Is the universe static? How did the universe come into being? How old is the universe?
Our current understanding is that the universe is not static. Nobel prize-worthy research conducted in the late 1990s used exploding stars (supernovae) to reveal that the cosmos is expanding at an increasing rate. Some new form of energy (dark energy) is overwhelming the force of gravity between all the massive objects in the universe. The fate of the cosmos is once more brought to light.
So what is driving that expansion? What is causing galaxies to move away from one another, overcoming gravity’s pull? The answer appears to be dark energy. Very little is known about dark energy, but we believe it makes up about 2/3 of the energy in the universe.
And so we are at the beginning again. Our answers lead us to new questions. There are many more questions to answer, and many more measurements to make.
If you’re interested in seeing these galaxies for yourself, point your telescope toward RA 05:00:34 Dec -62deg 4’.
Written by Det. Marty Murphy [FNAL]
Image by Det. Marty Murphy
DECam Tracks Near-Earth Asteroid
In the early evening of February 3rd, 2014, the DES team received an urgent request for optical imaging of a Near Earth Object (NEO) on a “potentially hazardous orbit.” This asteroid had first been spotted by the NEOWISE (NEO Wide-field Infrared Survey Explorer) team. However, they had been unable to pin down its orbit. Additionally, poor weather in Hawaii and Arizona had stymied all other attempts to image this object. To make matters even worse, the asteroid was rapidly moving towards lower solar elongations which would bring it in line with the Sun and make later observations impossible.
Luckily, the Dark Energy Survey (DES) was on the scene as humanity’s best, last, and only line of defense. Cerro Tololo was enjoying some of the finest weather Chile has to offer, and DECam’s large field of view makes it an excellent instrument for tracking down errant asteroids. Soon after sunset, the Blanco 4m telescope swung towards the best guess for the asteroid’s position and DECam took five images, dithering slightly to make sure the asteroid couldn’t slip through the gaps between CCDs, DECam’s digital imaging chips.
After rapid processing, the DECam images revealed a new Apollo-class asteroid, 2014 BE63. The NEOWISE team confirmed that 2014 BE63 will cross the Earth’s orbit; however, the closest approach to Earth itself will be at a safe distance of 18 million miles.
We dark energy detectives can rest easy knowing that, in the words of Steve Kent [FNAL], “2014 BE63 poses no threat to DES observations (and no threat to Earth).
Written by Detective Alex Drlica-Wagner [DES, FNAL]
Video by Alex Drlica-Wagner
Beyond the Veil, but not Beyond Reach
When I awake each afternoon during an observing mission at the Cerro Tolo Inter-American Observatory (CTIO), I have one priority. Before I eat, before I check e-mail, before I even stretch, I step out the door and look to the west: are our skies clear? Clouds can cast a shroud over a night’s observing program for the Dark Energy Survey (DES), which is now in full swing, each night gathering a terabyte of clues to dark energy. If our view is blocked by clouds, if we’re not taking data and peering into the deep black, we’re missing precious opportunities to observe space-time’s expansion.
To mitigate this, the Dark Energy Survey has developed another tool to pierce the veil of Earth’s atmosphere: the Radiometric All Sky Infrared Camera, or RASICAM.
The video above shows RASICAM closed during the day and then open after sun-down. RASICAM sees the entire sky in the infrared wavelengths, where our eyes are blind, but the clouds show up clearly. DES scientists and engineers at the Stanford Linear Accelerator Center (SLAC) National Laboratory designed and constructed this all-seeing eye on the infrared sky, and it’s been operational at CTIO since 2011 (http://today.slac.stanford.edu/feature/2010/rasicam.asp). In future posts, we’ll look at the sky from RASICAM’s point of view.
RASICAM is critical to DES operations. We use this camera to help inform us about how many clouds are in the sky, as well as where they are. We can then adjust our observing strategy and better analyze the image data. The instrument is brought to us by Rafe Schindler, Peter Lewis and Howard Rogers. Data analyses and maintenance are performed regularly by Kevin Reil, Dave Burke, Peter Lewis and Zhang Zhang.
Occasionally, clouds may appear or rain may fall, but dark energy cannot hide from us.
Det. B. Nord
One Star Sets, Others Rise
The first season of the Dark Energy Survey is now drawing to a close. For another few weeks, we will continue to watch the sky from the summery Southern Hemisphere. After that, others in the astronomy community will take the reins of the Dark Energy Camera (DECam) until September.
Early in the season, the clouds (and occasionally rain) interrupted this work. For example, in October of 2013, late-evening skies of plum-golden hue gave us the sunsets you see in today’s picture. Even though there were cloudy nights early in the season, this was anticipated. We’re using basic climate and weather models to plan our survey, so we can still observe fruitfully when visibility isn’t the best. Moreover, we can use the data from this past year to improve our survey strategy for the coming four years.
However, the rest of the season has been great, with many nights of very little air turbulence in the atmosphere, meaning we captured very clear images. Astronomers talk about this using the term, “seeing,” which is measured in “arcseconds.” The lower the seeing, the clearer and crisper the images. At the Cerro Tololo Inter-american Observatory, typical values are near one arc-second.
Right now, our squads are sifting through and preparing these images for science, and preparing them to share with you.
In less than a month, the sun will rise on our first season, but the long nights of work will continue.
Det. B. Nord
CSI: Early Universe
For this installment of Cosmic Scene Investigation, we travel to one of the earliest collisions of large-scale structures in the known universe.
A splatter of red (denoting galaxies) lies at the center of this image, and extends toward the lower left. These are the remnants of a cosmic collision. Aeons ago, one group plunged through another at millions of miles per hour, leaving in its wake a wreckage. The galaxy cluster ‘El Gordo‘ is all that remains of this raucous event, which took place less than a billion years after the universe started.
From the deserts of Chile, the Atacama Cosmology Telescope was the first to detect this prodigious system. NASA’s Chandra X-ray Observatory, the European Southern Observatory’s Very Large Telescope, and NASA’s Spitzer Telescope have also collected forensic evidence across the energy spectrum, from the infrared to the X-ray. All put together, we see a system similar to the infamous Bullet Cluster: a pair of clumps converted to a churning, violent amalgam of hot gas, dust and light.
An extremophile in the truest sense, El Gordo is the earliest-occurring cluster of its caliber. Its hot gas is burning at 360 million degrees Fahrenheit (200 million degrees Celsius), and it weighs in at a million billion times the mass of Earth’s sun. Compare this to the Virgo cluster of galaxies, the celestial city that holds our Milky Way and its neighbors. El Gordo’s mass is about the same, but it is over a hundred times hotter.
Dark energy is the name given to that substance, that energy, that is making spacetime spread out faster and faster. In the early universe, the small chunks that make up El Gordo were able to overcome dark energy (if it even existed then) and move toward each other to produce this cosmic crash scene. How many more like it are out there? The case remains open.
To read more about the curious case of the big old cluster, see the peer-reviewed paper and the press release from Chandra X-ray Observatory.
Written by: Det. B. Nord [FNAL]
Image by: Det.’s Nikolay Kuropatkin and Martin Murphy [FNAL]
River of Dreams
The Dark Energy Survey, in its search for distant cosmic secrets, needs many nights of clear sky. Unfortunately, on this night, a river of clouds flowed overhead. We often look so far away to the faintest objects, that a wisping of cottony clouds moving over the Blanco telescope requires extra attention. In these cases, we might change our observing strategy to look at parts of the sky that require less detail.
As we sift through the tons of sediment and the terabytes of data, occasionally the clouds get so heavy and consuming that we must close the telescope dome to preserve the instrument. In these cases, we still use the time wisely: our Dark Energy Calibrations (DECal) team has developed a method to precisely measure and characterize the flow of light through the telescope at every relevant wavelength, and to monitor for any changes in that flow over the years of the survey.
As we fish for light in the sky, we cast a broad net. In the river of dreams, we sift through sediment for celestial gold.
Lights Burning and Churning Through Time
The light is going the distance.
Some celestial (light-producing) objects are farther away than others. The three larger galaxies in today’s image are nearby, located in the Fornax galaxy cluster. As you can see, light comes from other galaxies much farther away: the smaller points are galaxies, too. We measure the patterns in the galaxies that are far away from us and compare that to the patterns of the nearby galaxies. Differences in this pattern give us indispensable clues about how dark energy has affected space-time over the last several billions of years.
And some of the bright points (galaxies) are behind the large galaxy near the center (NGC1374), yet we can see them. What does that tell us about the brightness in different parts of galaxies? If you look closely, we can see background galaxies (those behind) everywhere—except a small patch near the center. This is due to the fact that the centers of galaxies are brighter than their outskirts.
Light from distant regions of space can travel up to billions of light-years, to accidentally land here on Earth — to land on a small spot on the side of a mountain in Chile, where the Dark Energy Camera is waiting to capture it. To measure the expansion of the universe, the Dark Energy Survey needs to measure light from hundreds of millions of galaxies, all differing in their colors, brightnesses and locations. These properties will betray their patterns, which we must decipher.
Light travels the distance, and we’re hunting it all the way.
Written by: Det. B. Nord [FNAL]
Image by: Det.’s Nikolay Kuropatkin [FNAL], Martin Murphy [FNAL]
The Cosmos: A Digital Frontier
Ever since we started looking up, we tried to picture where the lights in the night were coming from as they moved through the universe. What were these objects – fiery stones, gods?
We kept dreaming about and measuring a world that was hard and too dark to see. And then one day, we could see into its depths.
At first, we only looked out with our eyes, and then we captured light on photographic plates of glass, and then we learned to convert the photons of light into electrical signals that we can store and analyze in computers.
When you sit down at the console, the eight-panel control module first looks like a gamer’s dream. And then you look to your left and you see another multi-panel module. These two consoles alone control the hundreds-ton telescope and $50M Dark Energy Camera (DECam) that captures the light from distant galaxies and exploding stars. These two consoles initiate the process of turning the ancient light into the recorded history of the cosmos. But if you want to know what goes on inside the machines, how we cyber crunch the sky, you have to look behind the scenes at the Data Management system that turns photons into bytes.
The Dark Energy Survey (DES) acquires hundreds of images – nearly a Terabyte of data – every night, and will continue to do so for the next five years. After the light is collected and stored, it must be transferred and cleaned before we can mine it for the faint clues of dark energy. As the images are taken, they are transferred via a high-speed pipeline from the mountaintop of Cerro Tololo in Chile to the corn fields of Illinois, where computer clusters at the National Center for Supercomputing Applications (NCSA; image, upper left) perform calibrations and tests to ensure data are free of error and contamination, to prepare them for cosmological analyses.
While we can observe and measure the sky to great depths, a key partner in this endeavor is our theoretical understanding of cosmic evolution. For this, we must create fake universes, with a variety of different parameters, inside these super-computers. DES will simulate several universes, in which we will test the hundreds of thousands, millions of lines of code in preparation for working with actual data. What’s more, we can compare the simulated universe to our own, telling us which aspects we’ve simulated correctly and which parts of our theories are solid, which need revision. Super-computers, like those at NCSA shown in today’s image (top right and bottom), play a critical role in constructing these universes.
It’s amazing how productive watching the sky can be. But, to look out, we had to bring the universe in.
(Hat tip to Tron: Legacy.)
By: B. Nord [FNAL]
Image: NCSA
Hotel Tololo
The dormitory rooms at the Cerro Tololo Inter-American Observatory (CTIO) in Chile cast a stout shadow over the desert fauna in the light of the early risen moon last February. This night’s moon is so bright, it prevents the Dark Energy Camera from looking at that part of the sky. On bright nights like these, we aim the telescope elsewhere, but still looking, still searching for supernovae and distant galaxies.
We start the night’s work early with an inter-continental tele-conference before dinner. After dinner, we prepare the software and telescope until sunset, when the hunt begins. Working through the night (and through a few pots of coffee and bags of cookies), we emerge a few hundred images closer to understanding dark energy and its effects on the celestial objects deep in the night sky. Just after sunrise, we hit the hay, but our minds often keep crunching numbers or sifting puzzles that arose during our observations, as the work from our night bleeds into our dreamscape.
Welcome to Hotel Tololo. We’ll turn the light off for you.
Written by: Det. B. Nord [FNAL]
Image by: Det. B. Nord
Dark Energy Detectives 