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Posts Tagged ‘LSST’

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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Lots of interesting particle physics news recently on the Cosmic Frontier front.

Science News reports that the National Research Council’s March 7 report for science in the coming decade recommends completion of the Large Synoptic Space Telescope.

…which will not only probe the nature of dark matter and dark energy but aid in tracking near-Earth asteroids.

LSST  is a huge public and private partnership, which includes many of the national labs, among them Fermilab, which hopes to build on its computing experience with the Sloan Digital Sky Survey to help manage the unprecedented flow of data expected from LSST. The February issue of symmetry magazine outlines the partnership needs the experiment will require.

…the LSST camera will produce 3.2-billion-pixel images and generate, on an average viewing night, about 15 terabytes of raw data, or 25,000 CDs worth. To display one of the LSST full-sky images on a television would require not just a high-definition screen, but 1500 of them.

While LSST is not expected to take data for quite sometime, its predecessor the Dark Energy Survey should start its first sky survey in October. The blog dark matter, dark energy, dark gravity explains how DES will be the first experiment to use four different methods at once to search for dark energy. Medill news services uses a great video to show physicists at Fermilab wrapping up tests on camera components before shipping the final parts to Chile for assembly on the 4-meter Blanco telescope. Sadly, the New York Times reports that the driving force behind making the telescope a bastion of U.S. science in Chile, Victor Blanco, passed away. 

Unlike DES and LSST, the holometer experiment aims not to record the sky as we see it but as Fermilab theorist Craig Hogan thinks it really is: a giant hologram.  The Little India newspaper explains Hogan’s theory and how it relates to black hole science.

Scientists have known for long time that information plays a key role in the creation of a system. Our computers and robots are just metals and wires if no information is exchanged in the form of bits. Our brain is inanimate if no information is carried by the neurons. Our genes are futile if no information is available from DNA that instructs how to function. In everything we know information is the key.

Similarly the entire information about our universe must be encoded elsewhere. Like a hologram on our credit cards, which contains the information in a thin film, and can generate 3D objects when viewed in proper light, the reality we tempt to believe is actually just one way of viewing information printed on a distant cosmic film. What we see and experience as reality are the shadows of the truth.

–Tona Kunz

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Hello again,

One of the commenters on our very first post wanted to hear more about the Large Synoptic Survey Telescope (LSST), one of the three cosmological projects that involve Brookhaven Lab. Set high on a mountaintop in Chile, LSST will be a very big and expensive ground-based telescope. Planning for the project started near the end of the 20th century and the experiment probably won’t start taking data in a scientific manner until 2020.

Artist rendering of LSST on Cerro Pachon, Chile. (Image Credit: Michael Mullen Design, LSST Corporation)

The story is that at a decadal survey 10 years ago, the person who first proposed that the word “synoptic” be used in the project’s name had a misunderstanding about what synoptic really means. Either way, the name has stuck. Synoptic, by the way, comes from Greek word “synopsis” and refers to looking at something from all possible aspects, which is precisely what LSST will do.

Astronomical survey instruments fall broadly under two categories: imaging instruments that take photos of the sky, and spectroscopic instruments that take spectra (that is, distribution of light across wavelengths) of a selected few objects in the sky. LSST falls into the first category — it will take many, many images of the sky in the five bands, which are a bit like colors, from ultra-violet light to infrared light.

(more…)

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