The Cosmos


The Dark Energy Survey: What Happens When You Can See Forever?

A new eye is now open to the cosmos. The Dark Energy Camera, which saw first light on September 12, 2012, is a spectacular new scientific facility with the grandest of goals: no less than understanding the evolution and fate of the entire universe.

For every telescope, “first light” is the moment when the optics and camera are assembled into a single instrument and turned to the night sky for the first time. But first light is just the beginning. While it often yields a spectacular photo or two, single photographs rarely lead to substantive results. Modern measurements require a subtle understanding of the equipment’s idiosyncrasies and the operators and scientists must spend a while familiarizing themselves with their instrument’s performance. After the facility has been put through its paces, real research begins. On January 9, Joshua Frieman, leader of the Dark Energy Survey (DES) collaboration, announced at the 221st meeting of the American Astronomical Society in Long Beach, California, that the team is wrapping up this getting-to-know-you phase, known as the commissioning period. They have already made interesting scientific observations, including discovering distant supernovae and clusters of galaxies.

The 570 megapixel Dark Energy Camera is hooked up to the venerable four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory, located in the Chilean Andes. Together, they will complete a study of the sky called the Dark Energy Survey, which may bring us closer to an answer to one of the deepest mysteries in cosmology: What is dark energy?

This question has been vexing astronomers since 1998, when astronomers discovered that, contrary to their expectations, the expansion of the universe wasn’t slowing down—it was speeding up! Cosmologists accounted for this surprising behavior by invoking a form of repulsive gravity first imagined by Einstein. But Einstein abandoned the idea when Hubble’s observation of the expanding universe made it seem unnecessary. Today, in the absence of a specific explanation, astronomers describe it with the generic term “dark energy.”

The Dark Energy Survey will help scientists probe the nature of dark energy. Over the course of 525 nights over five years, astronomers will survey a quarter of the southern sky to a depth of billions of light years, revealing the how the cosmic expansion rate has changed over nearly nine billions of years.

The Dark Energy Survey studies the universe in four distinct ways:

  1. It looks for 4,000 distant supernovae. By comparing their distance (determined by simultaneously observing their brightness and their redshift, the change in their color due to the expansion of the universe, and comparing these two numbers), astronomers will get a good handle on the cosmic expansion history.
  2. The camera will also study patterns in the spatial distribution of galaxies that are set by a phenomenon called baryonic acoustic oscillations. When the universe was smaller and hotter, the explosion of the Big Bang caused the universe to ring like a bell as the sound of the Big Bang rippled across the cosmos. About 370,000 years after the Big Bang, the universe cooled below a critical temperature, freezing these vibrations into patterns we can still see in microwave radiation and distribution of galaxies that are blazoned across the sky. This process is analogous to flash freezing the ripples on the surface of a pond. By comparing the apparent size of the ripples with their initial size, which can be calculated using information about the conditions that prevailed in the early cosmos, astronomers can provide crucial data on the shape of space itself: whether it is flat or curved and, if curved, exactly how.
  3. The camera will also have the capacity to study the size and makeup of vast clusters of galaxies. Because the properties of dark energy help determine how and when these clusters formed, by studying their history, we can gain new insight into dark energy.
  4. Finally, the Dark Energy Camera will see how light from distant clusters of galaxies is being bent by mass between those clusters and our telescopes. This information will tell us more about how dark energy has shaped the distribution of matter throughout the universe by studying the size and shape of clusters of galaxies over time. In total, the camera will be able to track three hundred million galaxies!

Through these four distinct strategies—each with different strengths and weaknesses—the survey will provide independent measurements of the dark energy of the universe.

The portion of the sky that the DES will study in detail is observable from Chile from September to February. Since first light, the collaboration has put their equipment through its paces. To get an early glimpse at a complete set of data, the DES collaboration will spend the rest of the 2012-2013 observation season studying a little under 5% of the region they will eventually explore. Using this strategy, they will have as good a measurement on a small portion of the sky after just a few nights of observation as they will over their entire target after five years. This will allow a relatively quick analysis of a subset of the sky and the caliber of this small study will already be world-class. The final shakedown is expected to be completed in February and in September of 2013, the survey will start in earnest, hopefully leading to new insights into the nature of dark energy.

Stay tuned. This is a very exciting time.


The center of the Milky Way galaxy lends its awesome beauty to the skyline above the telescope domes at Cerro Tololo. The Greater and Lesser Magellanic Clouds grace the upper left corner of the photo. (Photo credit: Reidar Hahn)

Go Deeper
Editor’s picks for further reading

Baryon Oscillation Spectroscopic Survey: Baryon acoustic oscillations and dark energy
On this web page, learn more about how baryon acoustic oscillations can be used to study the history of dark energy.

Minute Physics: 2011 Nobel Prize: Dark Energy
In this video explainer, guest narrator Sean Carroll explains dark energy and cosmic expansion in honor of the 2011 Nobel Prize in physics.

NOVA scienceNOW: Cosmic Perspective: Dark Matter
In this short video, astrophysicist Neil deGrasse Tyson muses on just how much we don’t know about the mysterious components of the universe, dark energy and dark matter.

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Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Anonymous

      Layman here…many years ago I read that, since space is curved, if you proceed from any starting point and continue in what you think is a straight line, you will eventually round back to your starting point.
      Analogous to walking on the surface of a sphere. Is this correct? If so, when looking so far away or so far back in time, isn’t it possible that we are seeing our own neck of the woods from long ago via light curved in a loop by the curved universe? And if this is the case, what implications does this have for our observations of an accelerating expansion?

      • Physicists currently think that Universe is “flat”, in other words, it’s not curved. This has implications, though, a “flat” Universe is potentially infinite. However, to address the other side of this, it might SEEM flat or not curved to us because the Universe could be so vast that to it just seems flat. It could possibly take many more times the size of the observable universe to actually observe the curve of space. Think of it like this, we on Earth, by standing on the surface of the Earth, by observation, could possibly assume that the Earth is flat, but if we travel further and further, we start to notice the curvature. Same thing with space, we might be too small in our Universe to really notice the curvature of space. Just as an ant will never come to observe the curvature of the Earth.
        I personally think it’s flat, but who knows. It isn’t something anyone can say with certainty right now.

        • moll4y

          Both of you have chosen to ignore the possibility of negative curvature, i.e. hyperbolic geometry.

          • Did I “choose” to ignore the possibility of “negative curvature”? Or is it maybe possible that I didn’t know about “negative curvature” and therefore couldn’t have possibly brought it up in my comment?

            • Wondering

              Nice response. It is presumptive replies like the above that prevent me from posting questions. Shame, but I guess a little knowledge can indeed be a dangerous, or at least rude, thing

    • A well-written piece. These are exciting times to be alive,

    • Bertram M. Brown

      It is wonderful technology. Hopefully it show that dark energy does not exist. Maybe it show that the expansion acceleration is caused by gravity from dark matter or universes beyond our universe.

    • Steve Hartwell

      If they were to ‘talk’ based on what they do know, all they could say about the Universe is “we don’t know much” – and nobody would pay them to say that 🙂 But even scarier is to realize that when they use words like dark energy; dark matter; strange – up – down – quarks; even reverse gravity; everybody responds with ‘awesome, yes, I see’ pretending we know what they mean, and approves them being paid to say those things

    • Ujjal G.

      Dark energy and Dark matter are for real. Just as we cannot “see” gravity and air does not mean they do not exist. One day scientists are going to “find” dark energy which will serve humanity for all our needs.

    • Tony Abad

      Dark energy? They are just fast moving invisible “marbles”. When two or more collide and are forced to reduced their speed, they become “matter”.

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    • Universe gazing, dark matter, count me in!! These telescopic structures are cute, admiring the heavenly constructs of a great Creator!

    • Jason

      Thanks! I appreciate the way you put it for us laymen…I think the new discovery at CERN will push physics and the universe as we see it to a whole different level.

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    • Anonymous

      We will make a new approach for an effect known as “Dark Energy” by an effect on gravitational field.

      In an accelerated rocket, the dimensions of space towards movement due to ‘Lorentz Contraction’ are on continuous reduction.

      Using the equivalence principle, we presume that in the gravitational field, the same thing would happen.

      In this implicates in ‘dark energy effect’. The calculi show that in a 7%-contraction for each billion years would explain our observation of galaxies in accelerated separation.

      Lorentz Contraction

      If we suppose that gravitational field contracts the space around it (including everything within), we can explain the accelerated separation from galaxy through this contraction without postulating ‘dark energy’.

      The contraction of space made by gravity would cause a kind of ‘illusion of optic’, seem like, as presented below, that galaxies depart fastly.

      The contraction of space would be equivalent to relativistic effect which occurs in a special nave in high-speed L.M.: With regard to an observer in an inertial referential stopped compared to a nave, the observer and everything is on it, including own nave, has its dimension contracted towards to movement of nave compared to a stopped observer (Lorentz Contraction).

      This means that the ‘rule’ (measuring instruments) within the nave is smaller than the observer outside of moving nave.

      The consequence is, with this ‘reduced rule’, this moving observer would measure things bigger than the observer would measure out of nave.

      An accelerated rocket and its continuous contraction

      In the same way, if we think of an accelerated increasing speed rocket, its length towards movement – compared to an inertial reference – will be smaller, and ‘rule’ within the nave will decrease continuously compared to this observer.

      We would think of ‘equivalence principle’ to justify that gravitational field would have the same effect on ‘rules’ (measuring instruments) as an accelerated rocket would do within the nave, but, now, towards all gravitational field and not, in the case of rocket, only at acceleration speed.

      I.e., the gravitational field would make that all rules within this field would be continuously smaller regarded to an observer outside of gravitational field and this would make, as we can see, these observers see things out of field be away fastly.

      Anyway, even if “equivalence principle” can’t be applied into a gravitational field to show that the space is contracting around it, we can take it as a new effect on gravitational fields and this would explain the ‘dark energy effect’.

      The “dark energy” through gravitational contraction:

      Let’s think what would happen if a light emitted by a star from a distant galaxy would arrive into our planet:

      Our galaxy, as well as distant galaxies, would be in continuous contraction, as seen before, due to gravity.

      A photon emitted by a star from this distant galaxy, after living its galaxy, would go through by an “empty” big space, without so much gravitational influence, until finally arrives into our galaxy and, lastly, to our planet.

      During this long coursed way (sometimes billion years), this photon would suffer few gravitational effect and its wavelength would be little affected.

      However, during this period, our system (our rules) would still decreasing due to gravitational field, and when this photon finally arrives here, we would measure its wavelength with a reduced ‘rule’ compared to what we had had at the moment when this photon was emitted from galaxy.

      So, in our measurement would verify if this photon had suffered Redshift because, with reduced rule, we would measure a wavelength longer than those was measured. The traditional explanation is “Shift for Red” happened due to Doppler Effect compared to galaxy separation speed!

      End of Dark Energy

      Farthest a galaxy is from viewpoint, more time this light will take to arrive us and more shrunken our ‘rule’ will be to measure this photon since it had been emitted; so it would be bigger than wavelength, which would induce us to think of faster galaxy separation speed.

      This acceleration (this new explanation, only visible) from distant galaxies took astronomers to postulate the existence of a “Dark Energy” would have a repulsive effect, seems like they are getting away faster.

      But if acceleration is due to our own scale reduction, this dark energy wouldn’t be necessary anymore, because what makes this separation accelerated is, actually, our own special contraction. This would be the end of dark energy.

    • David Brown

      ” … one of the deepest mysteries in cosmology: What is dark energy?”
      I now have 2 decisive empirical tests of my quantum theory of gravity. “Is the space roar an empirical proof that the inflaton field exists?”

      On 12/20/13 5:17 AM, David Brown wrote:
      Prof. Witten: Do you have an opinion
      concerning the comments posted for?
      — D. Brown

      On Fri, Dec 20, 2013 at 3:54 AM, Edward
      Witten wrote:
      I am generally sympathetic with these
      Edward Witten