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Memories from the edge of the universe

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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!

 

fox 

 Seeing my first fox on the mountain!    

       

 

 

 

 

 

 

 

 

 

scorpion

 

Finding a scorpion in my shoe one morning.

 

 

 

 

 

 

 

 

 

 

seeing

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!

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viscacha3

 

 

 

 

 

 

 

 

 

 

 

 

 

atmcam

 

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!

cookies1 cookies2

 

 

 

 

 

 

 

 

 

 

 

 

 

ValleyFog

 

 

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.

sunset3 sunset1sunset2

 

 

 

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.

 

 

controlroom

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)

 

 

 

 

 

 

 

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The best of the best

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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.

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巡天遥看一千河

地球相对于天空来说,约每24小时自转一周。地球上的我们也因此做到了“巡天”的旋转木马上。

每到傍晚,离我们最近的恒星,太阳,就会撤走遮天幕地的光芒。呈现给我们的将是一闪一闪宛如跳舞般的星光——那些古老遥远的星系和星星遍布了整个夜空。傍晚之后的几个小时给予我们机会细致观察笼罩着地球的点点星光,直到天色微曦,我们再次沐浴到阳光下。白天的时候,满天星斗依然存在,但强烈的阳光让我们几无可能观察到它们。一直到夕阳西下,那些跳舞般闪亮的星系和星星准时回到夜幕这张大舞台上,星空再次出现。

我们的整个太阳系都是银河星系的一部分。银河系呈盘状,其中的星星和气体物质又组成了银心和旋臂。我们的地球作为附属于太阳的一个淡蓝色小点,住在银河系的郊区,银盘的二分之一处。当地球日复一日的自转时,银河系也在缓慢的旋转,但银河系的旋转周期要比地球长的多。

在天空日复一日,夜复一夜的旋转中,银河系的平面也和其他星系、星星一起一遍遍经过我们上方的天空。当我们从伸手不见五指的帕穹山(Cerro Pachon)山顶向天空望去的时候,我们可以看到银河系的平面,看到银河系中心所在的方向。

在本页的视频中,相机经南由东向西移动,整晚每隔三十秒进行一次拍摄。地球的自转轴经过南极点,所以我们可以看到天空绕着向南的方向旋转:银河系的一边先落下,然后在这个十月凌晨一点的时候,另一边开始升起。

晚上好,欢迎和我们一起,“巡天”遥看一千河。

作者:暗能量侦探 B · 诺德 (B. Nord)

视频制作:暗能量侦探 · 诺德 (B. Nord)

翻译:暗能量侦探 张Y Y. Zhang

翻译编辑:暗能量侦探 李T T. Li

备注:

  1. 本博文引用的视频设制于帕穹山。其时,作者正在使用双子南座望远镜(坐落于帕穹山,Cerro Pachon)为暗能量巡天所发现的伴有强引力透镜现象的星系团进行后续观测,并非用布兰科望远镜(托洛洛山,Cerro Tololo)进行暗能量巡天项目的直接观测。 帕穹山和 托洛洛山的地理位置极其接近。
  2. 翻译的博文标题取自毛泽东《送瘟神》。
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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]

 

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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]

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Our dark, tangled web: Clues of dark energy

DES0006-4123_20141218_00_gri_20141219_000.edit1.0_950pxLurking 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.

Sloan Digital Sky Survey: Galaxy Map

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.”

Millennium simulation: http://www.mpa-garching.mpg.de/galform/virgo/millennium/seqB_063a_half.jpg

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

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Our dark, tangled web: Where’s Waldo?

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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

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