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Posts Tagged ‘SDSS-III’

The last time I wrote a post was in February, ages ago in this fast-paced world. Since then, a lot of things have happened. I just flew back from the April Meeting of the American Physical Society in California, where I presented our most recent result from the Sloan Digital Sky Survey-III: the creation of the largest-ever 3D map of the distant universe. Thanks to the power sockets kindly provided by United on this particular flight, I can actually spend my time doing something useful while in this metal tube — attempt to write a marginally interesting post.

Anyway, my recent results were about the Lyman-alpha forest, a completely new technique that my colleagues and I got to work for the first time. It means a lot to me, mostly because there was a great deal of skepticism in the community on whether this technique would ever work, and given that I spent the past two years in the trenches trying to make it happen, I’m very happy. You can read more about it here. Fun fact: the media attention surrounding this announcement caused my name to appear on Fox News, a somewhat bizarre occurence for a European-style liberal like me.

What I want to tell you more about today is dark energy, which is a problem that is relevant both for my Lyman-alpha work as well as the Large Synoptic Survey Telescope (LSST) science that I discussed in this post. To put it mildly: dark energy is one big embarassement for modern physics. So, what is it?

A rendering of the Large Synoptic Survey Telescope, a proposed 8.4-meter ground-based telescope that will survey the entire visible sky deeply in multiple colors every week from a mountaintop in Chile. (Image credit: LSST Corporation/NOAO)

First, one important clarification: dark energy is not dark matter. Dark matter is a substance that is omnipresent in our universe and essentially behaves as a cold, invisible dust that collapses under its own gravity. The observational evidence for dark matter is overwhelming, but there are also many good theoretical ideas about what dark matter might be. Physicists have embarked on a long program to establish a “theory of everything,” or at least a “theory of many things.” We have unified electrodynamics and the weak interactions of particles — and the strong force can be self-consistently added — but how we combine these three forces with gravity is still an unsolved problem. There are many proposals on how to do it: supersymmetry, string theories, quantum loop gravity, etc.  The beauty of all these proposals is that that, in addition to having observational signatures at the Large Hadron Collider (LHC), they naturally explain dark matter. Most of these theories have at least one stable, weakly-interacting particle that could act as dark matter. The picture hasn’t quite clicked together yet, but this will indeed happen in the next couple of years, as more results come from the LHC.

Dark energy, on the other hand, doesn’t have such beautiful connections to fundamental physics. Nobody has the slightest idea of what if could be and how it could fit into the bigger picture. So what do we know?

In the late 1990s, observations of the dimming of distant supernovae showed that the universe is undergoing a phase of accelerated expansion. In other words, the universe is expanding faster and faster. This is very counterintuitive: if you throw a ball upwards, it keeps slowing down until it reaches its maximum height and then it falls down. The universe does something similar: After the initial kick, which we call the Big Bang, the expansion of the universe went slower and slower. But, some 7 billion years ago, the universe started to accelerate. It’s like throwing a ball in the air and watching it do what it’s supposed to do for a while before it suddenly changes its mind and zooms off to the skies!

A simulated 15-second LSST exposure from one of the charge-coupled devices in the focal plane. (Image Credit: LSST simulations team)

After the initial discovery of dark energy through distant supernovae, the evidence grew stronger and stronger and now we see it in many different measurements of the universe: the Lyman-alpha forest measurements that I mentioned earlier, as well as measurements from the Dark Energy Survey and LSST will all constrain the behavior of dark energy. The amazing thing is that we can describe this accelerated expansion of the universe by putting an extra term in the equations that describes the evolution of the universe — the so-called cosmological constant. At the moment, all observations are consistent with adding this one simple number to our equations. But this number has nothing to do with the physics that we know; it is of a wrong order of magnitude and shouldn’t be there to start with. So we all measure like wackos and hope that we will detect some small deviation away from this simple solution. This would indicate that dark energy is more complicated, somewhat dynamical, and thus, give us a handle on understanding it. But it might turn out that it is just that — a cosmological constant with no connection to anything else. In the latter case we are stuck for the foreseable future hoping that someone will eventually be lucky enough to make an observation or theoretical insight that will bring everything together.

-Anže

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The following news release from the Sloan Digital Sky Survey-III (SDSS-III) collaboration was first posted on Brookhaven’s website.

Scientists from the Sloan Digital Sky Survey III (SDSS-III) have created the largest ever three-dimensional map of the distant universe by using the light of the brightest objects in the cosmos to illuminate ghostly clouds of intergalactic hydrogen. The map provides an unprecedented view of what the universe looked like 11 billion years ago.

A slice through the three-dimensional map of the universe. SDSS-III scientists are looking out from the Milky Way, at the bottom tip of the wedge. Distances are labeled on the right in billions of light-years. The black dots going out to about 7 billion light years are nearby galaxies. The red cross-hatched region could not be observed with the SDSS telescope, but the future BigBOSS survey, the proposed successor to BOSS, could observe it. The colored region shows the map of intergalactic hydrogen gas in the distant universe. Red areas have more gas; blue areas have less gas.

The new findings were presented on May 1 at a meeting of the American Physical Society by Anže Slosar, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory, and described in an article posted online on the arXiv astrophysics preprint server.

The new technique used by Slosar and his colleagues turns the standard approach of astronomy on its head. “Usually we make our maps of the universe by looking at galaxies, which emit light,” Slosar explained. “But here, we are looking at intergalactic hydrogen gas, which blocks light. It’s like looking at the moon through clouds — you can see the shapes of the clouds by the moonlight that they block.”

Instead of the moon, the SDSS team observed quasars, brilliantly luminous beacons powered by giant black holes. Quasars are bright enough to be seen billions of light years from Earth, but at these distances they look like tiny, faint points of light. As light from a quasar travels on its long journey to Earth, it passes through clouds of intergalactic hydrogen gas that absorb light at specific wavelengths, which depend on the distances to the clouds. This patchy absorption imprints an irregular pattern on the quasar light known as the “Lyman-alpha forest.”

An observation of a single quasar gives a map of the hydrogen in the direction of the quasar, Slosar explained. The key to making a full, three-dimensional map is numbers. “When we use moonlight to look at clouds in the atmosphere, we only have one moon. But if we had 14,000 moons all over the sky, we could look at the light blocked by clouds in front of all of them, much like what we can see during the day. You don’t just get many small pictures — you get the big picture.”

The big picture shown in Slosar’s map contains important clues to the history of the universe. The map shows a time 11 billion years ago, when the first galaxies were just starting to come together under the force of gravity to form the first large clusters. As the galaxies moved, the intergalacitc hydrogen moved with them. Andreu Font-Ribera, a graduate student at the Institute of Space Sciences in Barcelona, created computer models of how the gas likely moved as those clusters formed. The results of his computer models matched well with the map. “That tells us that we really do understand what we’re measuring,” Font-Ribera said. “With that information, we can compare the universe then to the universe now, and learn how things have changed.”

A zoomed-in view of the map slice shown in the previous image. Red areas have more gas; blue areas have less gas. The black scalebar in the bottom right measures one billion light years. Image credit: A. Slosar and the SDSS-III collaboration.

The quasar observations come from the Baryon Oscillation Spectroscopic Survey (BOSS), the largest of the four surveys that make up SDSS-III. Eric Aubourg, from the University of Paris, led a team of French astronomers who visually inspected every one of the 14,000 quasars individually. “The final analysis is done by computers,” Aubourg said, “but when it comes to spotting problems and finding surprises, there are still things a human can do that a computer can’t.”

“BOSS is the first time anyone has used the Lyman-alpha forest to measure the three-dimensional structure of the universe,” said David Schlegel, a physicist at Lawrence Berkeley National Laboratory in California and the principal investigator of BOSS. “With any new technique, people are nervous about whether you can really pull it off, but now we’ve shown that we can.” In addition to BOSS, Schlegel noted, the new mapping technique can be applied to future, still more ambitious surveys, like its proposed successor BigBOSS.

When BOSS observations are completed in 2014, astronomers can make a map ten times larger than the one being released today, according to Patrick McDonald of Lawrence Berkeley National Laboratory and Brookhaven National Laboratory, who pioneered techniques for measuring the universe with the Lyman-alpha forest and helped design the BOSS quasar survey. BOSS’s ultimate goal is to use subtle features in maps like Slosar’s to study how the expansion of the universe has changed during its history. “By the time BOSS ends, we will be able to measure how fast the universe was expanding 11 billion years ago with an accuracy of a couple of percent. Considering that no one has ever measured the cosmic expansion rate so far back in time, that’s a pretty astonishing prospect.”

Quasar expert Patrick Petitjean of the Institut d’Astrophysique de Paris, a key member of Aubourg’s quasar-inspecting team, is looking forward to the continuing flood of BOSS data. “Fourteen thousand quasars down, one hundred and forty thousand to go,” he said. “If BOSS finds them, we’ll be happy to look at them all, one by one. With that much data, we’re bound to find things that we never expected.”

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Top left image shows SDSS-III's view of a small part of the sky, centered on the galaxy Messier 33. The middle top picture is a zoomed-in image on M33, showing the spiral arms of this galaxy, including the blue knots of intense star formation. The top right-hand image shows a further zoomed-in image of M33 highlighting one of the largest areas of intense star formation in that galaxy. Credit: SDSS

The world’s largest, digital, color image of the night sky became public this month. It provides a stunning image and research fodder for scientists and science enthusiasts, thanks to the Sloan Digital Sky Survey, which has a long connection to Fermilab.

Oh, yeah, and the image is  free.

The image, which would require 500,000 high-definition TVs to view in its full resolution, is comprised of data collected since the start of the survey in 1998.

“This image provides opportunities for many new scientific  discoveries in the years to come,” said Bob Nichol, SDSS-III scientific spokesperson and professor at University of Portsmouth.

Fermilab oversaw all image processing and distribution of data to researchers and the public from 1998 through 2008, for the first seven batches of data. These batches make up a large chunk of the ground-breaking more than a trillion-pixel image. The eighth batch of raw, reduced data, which was released along with the image at the 17th annual meeting of the American Astronomical Society in Seattle was processed by Lawrence Berkley National Laboratory. LBNL, New York University and Johns Hopkins University distributed that data. Fermilab’s SDSS collaboration members now focus solely on analysis.

“This is one of the biggest bounties in the history of science,” said Mike Blanton, professor from New York University and leader of the data archive work in SDSS-III, the third phase of SDSS.  “This data will be a legacy for the ages, as previous ambitious sky surveys like the Palomar Sky Survey of the 1950s are still being used today. We expect the SDSS data to have that sort of shelf life.”

The release expands the sky coverage of SDSS to include a  sizable view of the south galactic pole. Previously, SDSS only imaged small, spread out slivers of the southern sky. Increasing coverage of the southern sky will aid the Dark Energy Survey and the Large Synoptic Survey Telescope both southern sky surveys that Fermilab participates in.

Comparing the two portions of the sky also will help astrophysicists pinpoint any asymmetries in the type or number of large structures, such as galaxies. Cosmic-scale solutions to Albert Einstein’s equations of general
relativity assume that the universe is spherically symmetric, meaning that on a large enough scale, the universe would look the same in every direction.

Finding asymmetry would mean the current understanding of the universe is wrong and turn the study of cosmology on its head, much as the discovery of particles not included in the Standard Model would do for collider physics.

“We would have to rethink our understanding of cosmology,” said Brian Yanny, Fermilab’s lead scientists on SDSS-III. So far the universe seems symmetric.

Whether the SDSS data reveals asymmetry or not it undoubtedly will continue to provide valuable insight into our universe and fascinate amateur astronomers and researchers.

Every year since the start of the survey, at least one paper about the SDSS has made it in the list of the top 10 astronomy papers of the year. More than 200,000 people have classified galaxies from their home computers using SDSS data and projects including Galaxy Zoo and Galaxy Zoo 2.

In the three months leading up to the image’s release a record number of queries, akin to click counts on a Web page,  occurred on the seventh batch of data. During that time, 90 terabytes of pictures and sky catalogues were down loaded by  scientists and the public. That equates to about 150,000 one-hour long CDs.

Scientists will continue to use the old data and produce papers from it for years to come. Early data also works as a check on the new data to make sure camera or processing flaws didn’t produce data anomalies.

“We still see, for instance, data release six gets considerable hits and papers still come out on that in 100s per year,” Yanny said.

So far, SDSS data has been used to discover nearly half a billion astronomical objects, including asteroids, stars, galaxies and distant quasars. This new  eighth batch of data promises even more discoveries.

Fermilab passed the job of data processing and distribution on to others in 2008. The eight batch of data was processed by Lawrence Berkley National Laboratory and distributed by LBNL, New York University and Johns Hopkins University.

Fermilab’s four remaining SDSS collaboration members now focuses solely

illustration of the concept of baryon acoustic oscillations, which are imprinted in the early universe and can still be seen today in galaxy surveys like BOSS. Credit: Chris Blake and Sam Moorfield and SDSS.

on analysis. They are expected to produce a couple dozen papers during the next few years. The group touches on all of SDSS-III’s four sky surveys but focus mainly on the Baryon Oscillation Spectroscopic Survey, or BOSS, which will map the 3-D distribution of 1.5 million luminous red galaxies.

“BOSS is closest to our scientists’ interests because its science goals are to understand dark energy and dark matter and the evolution of the universe,” Yanny said.

For more information see the following:

* Larger images of the SDSS maps in the northern and southern galactic hemispheres are available here and here.

*Sloan’s YouTube channel provides a 3-D visualization of the universe.

*Technical journal papers describing DR8
and the SDSS-III project can be found on the arXiv e-Print server.

*EarthSky has a good explanation of what the colors in the images represent and how SDSS part of an on-going tradition of sky surveys.

*The Guardian newspaper has a nice article explaining all the detail that can be seen in the image.

— Tona Kunz

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