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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.

 

 

 

 

 

 

 

 

 

 

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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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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!

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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.

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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.

 

 

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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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Lo mejor de lo mejor

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Teniente segundo, Jake Jenson. West Point. Graduado con honores. ¡Estamos aquí porque está buscando a los mejores entre los mejores de los mejores, señor! — Men in Black

 

Los mejores cielos proporcionan las mejores imágenes y nos dan las mejores pistas acerca de la expansión cósmica.

Si navegas entre los archivos de los detectives de la energía oscura, verás imágenes preciosas de galaxias, tomadas con la Cámara de la Energía Oscura (DECam). A pesar de su variedad en formas, colores  y tamaños, todas estas galaxias tienen algo en común: se alejan de nuestra Vía Láctea a toda velocidad, alcanzando decenas, cientos de millones de kilómetros por hora. El Universo se expande, algo que sabemos desde hace más de 90 años.

Si pudiéramos registrar las velocidades de estas galaxias a lo largo del tiempo, ¿qué encontraríamos? ¿Sería la misma, estaría aumentando o quizás disminuyendo? Dado que la gravedad de la Vía Láctea las atrae, Isaac Newton nos hubiera dicho que la expansión se iría ralentizando con el tiempo, al igual que si tiramos una manzana al aire se va frenando (y al final cae) debido a la atracción gravitatoria de la Tierra. Pero Isaac se hubiera equivocado: las galaxias se están acelerando, en lugar de frenarse. Esto es un hecho que sabemos desde hace tan sólo 17 años. Los trescientos detectives del Dark Energy Survey (DES) se han embarcado en una misión de cinco años para entender qué está pasando. Durante la misma, realizarán la mayor exploración del cosmos jamás realizada.

Estos objetivos son espectaculares y de gran calado, pero en el fondo lo que hace el proyecto DES es hacer fotos. Y muchas. En una noche típica, los detectives de la energía oscura toman 250 fotos del cielo. Tras cinco años, tendremos más de 80000 fotos en nuestro álbum. Por  cada instantánea, el obturador de la cámara se abre durante aproximadamente minuto y medio para acumular suficiente luz de las galaxias más lejanas. En cada imagen puede haber unas 80000 galaxias. Cuando las juntemos todas, y teniendo en cuenta que fotografiaremos cada zona del cielo unas cincuenta veces, tendremos unos 200 millones de galaxias.

Una de las maneras con las que podremos aprender más sobre la energía oscura, la misteriosa causa de la aceleración, es midiendo con gran precisión las formas de esos 200 millones de galaxias y comparándolas entre sí. Imagínate que hiciéramos fotos a 200 millones de personas para estudiar la diversidad de la especie humana (lo que sería una foto por cada 35 personas). Para tener la mejor información posible, nos gustaría tener a un fotógrafo profesional tomando las fotos en buenas condiciones  y lo más parecidas posibles entre sí: buena iluminación, enfoque, que no se muevan ni la cámara ni el sujeto fotografiado durante la toma, etc. Pero es inevitable que unas vayan a salir mejor que otras, simplemente por las circunstancias en las que se han tomado, que varíen de un momento a otro. En algunas, el sujeto estará algo borroso, en otras, quizás la iluminación sea excesiva o insuficiente.

En el Dark Energy Survey, intentamos conseguir las mejores imágenes posibles de estos 200 millones de galaxias. Como fotógrafos profesionales del cielo nocturno (es decir, astrónomos), usamos el mejor instrumental disponible: la Cámara de la Energía Oscura DECam, que construimos nosotros mismos. Es una cámara de 570 megapíxeles, con cinco lentes enormes. Consta de un sofisticado sistema de auto-enfoque, para conseguir las imágenes más nítidas posibles.

En cambio no nos hace falta flash, ya que las galaxias lucen con la luz de millones de soles.

Pero la naturaleza no siempre coopera a la hora de conseguir buenas fotografías, al igual que a veces los sujetos fotografiados tampoco. La Cámara de la Energía Oscura está montada en el telescopio Blanco, localizado en Cerro Tololo en los Andes chilenos. Este emplazamiento proporciona generalmente noches muy claras, pero de vez en cuando las nubes hacen acto de presencia. Además, la turbulencia de la atmósfera distorsiona las imágenes de estrellas y galaxias (por eso vemos las estrellas parpadear), incluso con un buen enfoque. La cámara toma sus fotos con la luz reflejada en un enorme espejo de cuatro metros de diámetro y 15 toneladas. Si un frente de aire frío enfría la cúpula del telescopio, el espejo irradia calor hacia dicha cúpula, lo que perjudica la calidad de la imagen. Las mejores imágenes son las tomadas directamente hacia arriba, verticalmente. Si tenemos que inclinar un poco el telescopio, la luz de las galaxias debe atravesar una fracción de atmósfera mayor. Pero queremos observar buena parte del Universo, con lo que no nos podemos conformar con dejar el telescopio apuntando hacia arriba únicamente.  También el viento puede colarse y mover ligeramente la estructura del telescopio, una vez más enturbiando nuestras fotos. Y además, mientras tanto, la Tierra sigue girando, y el gigantesco telescopio debe compensar suavemente el movimiento del cielo que es consecuencia de esta rotación (o si no, lo adivinaste, se enturbiará la imagen).

Por esta y muchas otras razones, la calidad de las imágenes de DES varía. Algunas noches, una combinación de circunstancias confluyen en unas imágenes estupendas. Otras, en cambio, nos dan fotos algo más borrosas de lo que quisiéramos, haciendo más difícil medir la forma de las galaxias. Si la foto es demasiado mala, no la incluimos en nuestro álbum. Ya volveremos otra noche a ese punto del cielo a tomar otra mejor. Hasta ahora, un 80% de las fotos han tenido la calidad suficiente para engrosar nuestra colección.

Durante la mayor parte de nuestra temporada de observación, tenemos a tres detectives operando la cámara. Cada uno de ellos permanece allí durante una semana, y en el transcurso de una temporada, unos 50 detectives distintos hacen turnos en la montaña. Durante la noche del 27 de enero de 2015, me encontraba realizando mi turno en el telescopio, junto a mis compañeros detectivescos Yuanyuan Zhang de la Universidad de Michigan y Andrew Nadolski, de la Universidad de Illinois en Urbana-Champaign. Esa noche, Andrew estaba manejando la cámara, yo me encontraba verificando la calidad de las imágenes, mientras que Yuanyuan hacía de jefa de observaciones.

Las condiciones de esa noche eran extraordinarias. Aunque había algo de humedad, la atmósfera era extremadamente estable. Tomábamos imágenes usando filtros que dejaban pasar únicamente la luz roja o infrarroja. La razón de esto es porque, al estar la luna en el cielo esa noche y dado que su luz es muy “azul”, con estos filtros podemos bloquear su brillo, de manera que podemos observar las galaxias rojas contra el oscuro fondo nocturno. En la famosa foto “Monolith, the Face of the Half Dome”, tomada en el Parque Nacional de Yosemite, Ansel Adams utiliza un filtro rojo para oscurecer el cielo azul diurno, creando un efecto espectacular.

A las 00:28 hora local, tomamos la foto número 403841, usando un filtro en el infrarrojo cercano llamado “banda z”. Esta banda del espectro electromagnético es tan roja, que se encuentra más allá del rango perceptible por el ojo humano. Pero las cámaras digitales, y en particular nuestra DECam, son muy sensibles a esta luz infrarroja. Los ordenadores en el telescopio analizan cada imagen inmediatamente después de tomarla y muestran los resultados en un conjunto de monitores, de manera que podemos determinar si pueden pasar a formar parte de nuestro álbum cósmico. Cuando los monitores mostraron la foto 403841, la pantalla indicaba que estábamos ante una extraordinaria nitidez de imagen. Un análisis más detallado mostró que, de las 35000 fotos que habíamos tomado hasta entonces durante dos años, sin duda era la más nítida.

De hecho era tan nítida, que la luz de cada estrella se dispersaba tan sólo 0.6 segundos de arco (0.00017 grados). Para que os hagáis una idea, ése es el tamaño aparente de un cráter de 1 km de diámetro en la luna, visto desde la Tierra. O el tamaño de un pelo a 30 metros de distancia.

Arriba, mostramos una pequeña sección de esta foto, usando colores falso, con una gran galaxia espiral más un conjunto numeroso de galaxias más pequeñas y débiles, junto con unas pocas estrellas de nuestra propia Vía Láctea. La luz de la estrella rodeada con un círculo rojo se ve dispersada en tan sólo 0.6 segundos de arco. Aunque no es tan bonita como otras imágenes que habréis visto en estos archivos, lo que es cierto es que ésta se parece más a una imagen sin procesar, tal cual sale de la cámara. Estas imágenes “en bruto” se envían para su procesado al National Center for Supercomputing Applications en Urbana-Champaign en Estados Unidos, para ser procesadas y prepadas para el análisis científico por parte de nuestros detectives.

En DES, tenemos un listado de récords, que incluye las imágenes más nítidas que se han tomado en cada uno de los cinco filtros. Nuestra amiga 403841 aparece orgullosa en la lista: la mejor de las mejores. Pero lo mejor de los récords es que están ahí para ser batidos.

 

Det. Josh Frieman [Fermilab y Universidad de Chicago]

 

NB: echa un vistazo al Reddit AMA del viernes 30, donde discutimos las pruebas y evidencias para la materia oscura y la energía oscura (https://www.reddit.com/r/science/comments/2u6yxp/science_ama_series_im_dr_josh_frieman_director_of/).

 

 


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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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Bailando en la oscuridad

 

“Trabaja como si no te hiciera falta el dinero. Ama como si nunca te hubieran hecho daño. Baila como si nadie te observara” – Satchel Paige

Un surtido de cuerpos celestes, grandes y pequeños, bailan toda la noche al son de la silenciosa tonadilla de la gravedad. En la oscuridad que yace más allá de Neptuno, esta compañía de objetos del cinturón de Kuiper (KBOs, por sus siglas en inglés), ha estado bailando como si nadie les observara, hasta ahora.

Es un baile pausado, lento, puesto que los objetos del cinturón de Kuiper tardan siglos en completar una sola órbita. Estos KBOs, cada uno de ellos de unos pocos cientos de kilómetros de tamaño, han sido descubiertos por DES durante los últimos dos años y medio (uno de ellos, fue descrito aquí anteriormente). Imagina que no sabes nada acerca de la gravedad. ¿Qué deducirías de un patrón así? ¿Cómo lo explicarías? Las leyes físicas que producen estos intrincados patrones, sin duda tienen que ser muy complejas ¿no es así?

Nuestros antepasados registraban las andanzas de los planetas noche a noche, estación a estación. Se dieron cuenta de que viajaban por el cielo a ritmos completamente dispares. A veces, parecían pararse para volver sobre sus pasos contra el mosaico celeste de estrellas fijas que servía como referencia para, al poco tiempo, retomar su viaje. Se desarrollaron modelos muy ingeniosos para dar cuenta de esta complicada danza. Pero fueron haciéndose progresivamente más enrevesados, y lo que es peor, empezaron a fallar en su descripción de nuevas observaciones que se hacían con cada vez mayor precisión.

Hicieron falta dos revoluciones científicas, la primera con Copérnico y después con Newton, para demostrar que el movimiento planetario puede ser explicado con una sencilla ecuación, la ley de la gravedad. El patrón oculto de pronto se aclaró.

La grácil pirueta que describe un KBO proviene de la combinación de dos tipos de movimiento. Su órbita, de siglos de duración, produce una deriva lenta hacia el este que lo arrastra aproximadamente lo que abarca la anchura de la cámara DECam en el cielo, cada año. Pero al estar observando estos objetos desde una plataforma móvil (¡la Tierra!) en su movimiento alrededor del Sol, observamos el KBO desde distintos puntos de vista. A veces a 150 millones de kilómetros de un lado del Sol, seis meses después desde 150 millones de kilómetros desde el otro lado y otras veces desde un punto intermedio. Esto resulta en un movimiento de ida y vuelta respecto al fondo de estrellas sobre el que se superpone el movimiento orbital del KBO. Para verlo tu mismo, simplemente extiende un dedo de tu mano delante tuya y observa cómo cambia de posición respecto a los objetos del fondo cuando mueves la cabeza.

La física intenta “destilar” el orden a partir de la complejidad, para explicar la vasta diversidad de fenómenos naturales, con un pequeño número de leyes sencillas. Más adelante, los físicos se dieron cuenta que la ley de la gravedad de Newton se quedaba corta en ciertas situaciones y fue superada por la teoría de la relatividad general de Einstein.

Hoy en día, la gravedad presenta un nuevo misterio para nuestra generación: ¿por qué se expande el universo aceleradamente? Quizás una nueva ley explique el misterio de la energía oscura con tanta elegancia y simplicidad como la ley de Newton explica el baile de los planetas. Y esa esperanza es la que mantiene a los Detectives de la Energía Oscura en pie, pacientemente escudriñando el cielo.

 

Det Dave Gerdes [Universidad de Michigan]

 


Video

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]

 


Video

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]


Image

Our dark, tangled web: Where’s Waldo?

DES0428-33_6122_20141105_43

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