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Posts Tagged ‘cosmology’

Expanding the cosmic search

Friday, March 20th, 2015

This article appeared in Fermilab Today on March 20, 2015.

The South Pole Telescope scans the skies during a South Pole winter. Photo: Jason Gallicchio, University of Chicago

The South Pole Telescope scans the skies during a South Pole winter. Photo: Jason Gallicchio, University of Chicago

Down at the South Pole, where temperatures drop below negative 100 degrees Fahrenheit and darkness blankets the land for six months at a time, the South Pole Telescope (SPT) searches the skies for answers to the mysteries of our universe.

This mighty scavenger is about to get a major upgrade — a new camera that will help scientists further understand neutrinos, the ghost-like particles without electric charge that rarely interact with matter.

The 10-meter SPT is the largest telescope ever to make its way to the South Pole. It stands atop a two-mile thick plateau of ice, mapping the cosmic microwave background (CMB), the light left over from the big bang. Astrophysicists use these observations to understand the composition and evolution of the universe, all the way back to the first fraction of a second after the big bang, when scientists believe the universe quickly expanded during a period called inflation.

One of the goals of the SPT is to determine the masses of the neutrinos, which were produced in great abundance soon after the big bang. Though nearly massless, because neutrinos exist in huge numbers, they contribute to the total mass of the universe and affect its expansion. By mapping out the mass density of the universe through measurements of CMB lensing, the bending of light caused by immense objects such as large galaxies, astrophysicists are trying to determine the masses of these elusive particles.

A wafer of detectors for the SPT-3G camera undergoes inspection at Fermilab. Photo: Bradford Benson, University of Chicago

A wafer of detectors for the SPT-3G camera undergoes inspection at Fermilab. Photo: Bradford Benson, University of Chicago

To conduct these extremely precise measurements, scientists are installing a bigger, more sensitive camera on the telescope. This new camera, SPT-3G, will be four times heavier and have a factor of about 10 more detectors than the current camera. Its higher level of sensitivity will allow researchers to make extremely precise measurements of the CMB that will hopefully make it possible to cosmologically detect neutrino mass.

This photo shows an up-close look at a single SPT-3G detector. Photo: Volodymyr Yefremenko, Argonne National Laboratory

This photo shows an up-close look at a single SPT-3G detector. Photo: Volodymyr Yefremenko, Argonne National Laboratory

“In the next several years, we should be able to get to the sensitivity level where we can measure the number of neutrinos and derive their mass, which will tell us how they contribute to the overall density of the universe,” explained Bradford Benson, the head of the CMB Group at Fermilab. “This measurement will also enable even more sensitive constraints on inflation and has the potential to measure the energy scale of the associated physics that caused it.”

SPT-3G is being completed by a collaboration of scientists spanning the DOE national laboratories, including Fermilab and Argonne, and universities including the University of Chicago and University of California, Berkeley. The national laboratories provide the resources needed for the bigger camera and larger detector array while the universities bring years of expertise in CMB research.

“The national labs are getting involved because we need to scale up our infrastructure to support the big experiments the field needs for the next generation of science goals,” Benson said. Fermilab’s main role is the initial construction and assembly of the camera, as well as its integration with the detectors. This upgrade is being supported mainly by the Department of Energy and the National Science Foundation, which also supports the operations of the experiment at the South Pole.

Once the camera is complete, scientists will bring it to the South Pole, where conditions are optimal for these experiments. The extreme cold prevents the air from holding much water vapor, which can absorb microwave signals, and the sun, another source of microwaves, does not rise between March and September.

The South Pole is accessible only for about three months during the year, starting in November. This fall, about 20 to 30 scientists will head down to the South Pole to assemble the camera on the telescope and make sure everything works before leaving in mid-February. Once installed, scientists will use it to observe the sky over four years.

“For every project I’ve worked on, it’s that beginning — when everyone is so excited not knowing what we’re going to find, then seeing things you’ve been dreaming about start to show up on the computer screen in front of you — that I find really exciting,” said University of Chicago’s John Carlstrom, the principal investigator for the SPT-3G project.

Diana Kwon


Where Do I Come From?

Wednesday, February 4th, 2015

It’s the oldest question in the world and it occurs to every child, sooner or later: where do I come from? Mum and Dad of course, but where did they come from? Genetics only takes us so far; our line of ancestors actually stretches back beyond our first single celled forebears. Chemistry proceeds biology, and before that the world was made only of protons, neutrons and electrons. Now this takes us pretty far back, to the first second of the universe. In many ways, our fate was decided in this instant. The protons and neutrons we are made of formed a millionth of a second after the proverbial lights went on, condensing out of quarks. But where did the quarks come from?

Photo courtesy of NASA

Photo courtesy of NASA

Baryogenesis as a concept is not too difficult to follow. Every molecule you see around you is a survivor of a vast catastrophe that struck the early universe, when 30,000,000 of every 30,000,001 quarks in the universe were destroyed. The culprit of this disaster is antimatter – the bizarro version of matter. The crux of the matter is that matter and antimatter have a love-hate relationship; they annihilate each other, but also prefer to be created together. In the present day our universe is just too cold to create matter out of thin air (actually, through interactions with particles like photons), but this was not always so. When we go far enough back, at temperatures of about 10^13 degrees Celsius pair creation kicks off and the universe is filled with massive amounts of matter and antimatter. While this is lukewarm for a particle physicist there are more orders of magnitude between this temperature and the sun’s core than the sun’s core and you. From what I have said, the origin of matter doesn’t seem like much of a mystery; pair creation made matter. The problem is that it also made antimatter, and (according to the Standard Model) in equal amounts. When the universe cooled, matter could no longer be created, only destroyed, and so both matter and antimatter dwindled into nothing.

Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.

Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.

Clearly this is not the case – as any child can see, our universe is a populated and interesting one, filled with stars and planets and puppies. Above all, our universe is made of matter – no antimatter allowed. So there must have been a kind of discrimination against antimatter for some matter to survive this rampant destruction. Either this asymmetry between matter and antimatter existed from the start, as some sort of initial condition, or it somehow has dynamically evolved since then. Inflation dilutes any primordial asymmetry even more than a homeopathic remedy, so there must have been some matter creating process – baryogenesis. In any case, simply citing “initial conditions” is almost like saying “just because”, which never really seems to work with children.

When you need to explain something, it is usually best to start by what broad features your theory must have. For baryogenesis, Sahkarov did this back in 1967. For any theory (that doesn’t violate CPT) to create an asymmetry between matter and antimatter, three conditions must be satisfied:

  1. Baryon number must be violated. If you wish to count the number of protons and neutrons, it turns out that assigning them a quantity called “baryon number” is useful, a proton and neutron each have a baryon number of 1, and a quark has a baryon number of 1/3. Antimatter versions have a negative baryon number. The process that leads to the predominance of matter over antimatter, and so baryons over anti-baryons, is referred to as “baryogenesis”. It turns out that the total baryon number of the universe is conserved under perturbative effects in the Standard Model, what is known as an “accidental symmetry”. If we want more protons than antiprotons this number cannot be inviolate. There is a similar counting of electrons and neutrinos called lepton number, which is also believed to be broken. Unfortunately as neutrinos are extremely difficult to observe there is no direct evidence of a total lepton asymmetry.
  2. Matter and antimatter must be treated differently. This means that charge conjugation (where you swap particles with antiparticles) and charge-parity conjugation (swap particles with antiparticles and also reflect them like a mirror image) cannot leave the physics unchanged. More succinctly, C and CP must be broken. While C violation is trivial (the weak force violates C maximally), CP is almost entirely preserved in the Standard Model. This is both a major failing of the Standard Model and a fantastic prediction – we know that CP violation is absolutely fundamental to our universe, and that there must be more of it than we have seen so far. You have probably seen CP violation mentioned many times, both on this site and through news reports. The necessity for CP violation to explain our own existence is the real reason why CP violation deserves our attention.
  3. The universe must go out of thermal equilibrium. In thermal equilibrium any process creating a baryon asymmetry would be balanced by its reverse. Fortunately for us, the fact that the universe expands creates periods of thermal non-equilibrium, such as phase changes (like when the Higgs mechanism breaks the electroweak symmetry of the Standard Model).


While the Standard Model does technically satisfy all three of these, it does so in a trivial way. The amount of CP violation is far too low, and a universe in which the Standard Model is entirely correct never gets far enough out of equilibrium to allow a large difference in matter and antimatter to form even if it did violate CP more. The only really useful element that the Standard Model has is baryon number violation; a non-perturbative process called sphalerons occurs above the electroweak phase transitions which violates baryon and lepton number. More importantly, it preserves a linear combination of the two, so if you manage to make a baron asymmetry or a lepton asymmetry, you automatically get both. Theories like leptogenesis use this to turn a lepton asymmetry into a baryon asymmetry. While there are many possible scenarios that could have lead to the present day world (my own work is in one of these, asymmetric dark matter), the truth is that we simply don’t know which of these, if any, is correct.

Despite this being a question of the most fundamental kind, baryogenesis does not get nearly the same kind of media attention as dark matter or dark energy. Partly this is because we have little chance of experimentally finding an answer – baryogenesis could have occurred at almost any energy scale, which includes a good many far out of the reach of our colliders. But it is still important to push for an answer. Nothing is a better mark of our progress in understanding our origins than seeing how the question we ask about our origin evolves.


The Theory of Everything

Thursday, January 15th, 2015

Last night I went to see The Theory of Everything, the biographical film about Stephen Hawking, adapted from the memoir of his ex-wife, Jane Wilde Hawking. News literally just in – it has been nominated for the Best Picture and Adapted Screenplay Oscars, and there are Best Actor and Best Actress nominations for Eddie Redmayne (Stephen) and Felicity Jones (Jane). Arguably today’s most famous scientist, Stephen Hawking is a theoretical physicist and cosmologist, now holding the position of Director of Research at Cambridge’s Centre for Theoretical Cosmology. He suffers from motor neurone disease; a degenerative disease that has left him unable to move most of the muscles in his body. He now communicates by selecting letters and words on a computer screen using one muscle in his cheek. His computerised voice is world famous and instantly recognisable. He is responsible for ground-breaking work on black holes and general relativity.


I thought the film was fantastically made and the acting incredible; Redmayne’s portrayal of Hawking’s physical condition was uncanny. I shed a few tears at the plight of this man surviving against all the odds whilst doing incredible theoretical physics, and his wife, ever patient and loving, taking care of him and bearing his children despite his health getting only worse.
There have been some complaints about the lack of focus in the film on Hawking’s scientific work; the film instead focuses mainly on his relationship with Jane and their struggle as his condition deteriorates. This should not be a surprise when the film was adapted from Jane’s own writing. If you want to know more about Hawking’s work in physics, then I strongly recommend his physics books. I first attempted to read A Brief History of Time, his most famous publication, age 11. This was obviously optimistic of me, and I gave up after the first couple of chapters. I tried again during my A-levels but never got round to finishing it, but having now studied cosmology and general relativity in much more detail I fully intend to give it another try! I have however read The Universe in a Nutshell, a more accessible book on the history of modern physics and cosmology, as well as discussions on that holy grail of physics, and the title of the film, a ‘theory of everything’.

But what is a theory of everything? Also known as a ‘final theory’, an ‘ultimate theory’, and a ‘master theory’, it sounds rather grand. A ToE would elegantly explain our universe, maybe even in just one equation, linking all the aspects that we can not currently reconcile with each other. It would allow a deep understanding of the universe we live in, as Hawking himself professed despite being an atheist:

If we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason — for then we would know the mind of God.

Sounds good, right? The ultimate triumph. Unfortunately, so far, attempts at developing a ToE have not delivered. Why not? First we need to understand a little more about the physics we know and understand.
Our universe has four forces governing everything that happens within it:

  • Electromagnetism – the interaction of photons and charged particles that we are familiar with in electricity, magnets, etc.
  • Weak force – the interaction responsible for radioactive decay.
  • Strong force – the interaction that binds together the protons and neutrons in a nucleus
  • Gravity – the attraction of bodies with mass to each other, the reason we don’t fly away from the Earth and why the Earth orbits the sun (and also why we know about dark matter!)

Why four? No one knows. It has been shown that at electromagnetism and the weak force can be combined into an ‘electroweak’ force at high energies. This means that in our everyday low energy universe (as opposed to the hot dense universe shortly after the big bang) that electromagnetism and the weak force are just two faces of the same force.

If electromagnetism and the weak force can be combined, can we do the same with the strong force and gravity? Combining the electroweak and the strong force results in a “GUT” – a Grand Unified Theory, (NB despite being grand, this does not yet include gravity). The energy required to see the joining of the strong and the electroweak would be beyond the levels we could reach with particle colliders. We do not currently have a generally accepted GUT, but there are lots of complicated theories in the works.
The final step to a ToE would be the joining of gravity with a GUT theory. This is the real sticking point. As Jane illustrates with a pea and a potato over dinner in the film, the unification of quantum field theory (the pea) on the tiny scales with general relativity (the potato) on large scales has so far proven undoable.
Quantum field theory is what we particle physicists deal with, the standard model of particle physics, tiny things like photons and quarks and electrons, all interacting via electromagnetism, the weak force and the strong force. General relativity is far in the other direction; stars, galaxies, galaxy clusters. Big things with lots of mass, causing curvatures in space-time that manifests as gravity. Both quantum field theory and general relativity have been tested to extreme precision – they both work perfectly on their relative scales. So where does the problem in joining them lie?

Hawking’s greatest work is on black holes; the infinitely small and dense aftermath of the collapse of an enormous star. Once a star greater than about 23 solar masses runs out of fuel to produce energy, its core collapses under its own weight, expelling its outer layers in an explosion called a supernova that outshines its own galaxy. If the core is big enough, it will continue collapsing until it becomes a ‘space-time singularity’ – a point in space infinitely small and dense, where not even light can escape.
When we try to understand the physics inside that point, we start encountering problems. We need both quantum field theory and general relativity – we have a tiny tiny space but a huge mass, and infinities start popping up all over the place. The maths just doesn’t work.

The evolution of stars, showing how a sufficiently large star can end its life as a black hole

The evolution of stars, showing how a sufficiently large star can end its life as a black hole

Stephen Hawking, with the computerised speech system that has allowed him to communicate after losing his ability to speak

Stephen Hawking, with the computerised speech system that has allowed him to communicate and continue his physics work after losing his ability to speak

Hawking has dedicated much of his life to trying to unify these two pillars of modern physics, so far with no luck. This begs the question, if his incredible mind cannot do it, what hope do we have? Currently, a popular approach is string theory – the theory that everything is made of tiny strings, vibrating in many (up to 26!) dimensions. This may sound silly, but it’s actually quite elegant – each different particle is made of a string vibrating in a different mode. An issue with string theory currently is it offers no easily testable predictions. Some of the best minds of today are working on this, so there is still hope!

Stephen Hawking is clearly an incredible man. He has a level of intelligence and a talent in mathematics and physics most of us physicists can only dream of. However, I believe Jane also deserves a huge amount of credit. The diagnosis of motor neurone disease came only shortly after they began dating, but she embarked on a life with him, marrying him and having his children, taking on the mammoth task of caring for him mostly alone, despite his prognosis of only 2 years to live.

Of course, Hawking has far exceeded those two years. He is now 73, reaching what is basically a normal life expectancy despite having a disease that has an average survival from onset of only 3-4 years. He was diagnosed aged only 21. Diseases such as his are tragic, leaving a person’s mind totally intact but trapped inside a failing body. Many would just give up, but Hawking’s love for both Jane and physics drove him to persevere and become the esteemed professor he is today.

I strongly recommend watching The Theory of Everything, even to those uninterested in cosmology. It’s a beautiful, romantic drama set in picturesque Cambridge, emotionally powerful and moving, and certainly does not require you to understand the physics!


Cosmology and dark matter

Monday, July 8th, 2013

Third part in a series of four on Dark Matter

I have already reviewed how it reveals its presence through gravitational effects and the lack of direct evidence of interaction with regular matter. Let’s now look how cosmological evidence also supports the existence of dark matter.

Galaxy seeds

It is now widely accepted that all matter (dark and visible) started out being uniformly distributed just after the Big Bang. To make a long story short, a rapid expansion followed where the Universe cooled down and particles slowed down enough to form nuclei three minutes after the Big Bang. The first atoms appeared 300 000 years later while galaxies formed between a hundred and a thousand million years later.


How did the Universe change from being a gigantic cloud of uniformly distributed matter to containing large structures?  Dark matter is probably the one to be blamed.

Dark matter is heavier than regular matter and slowed down earlier. Small quantum fluctuations eventually turned into small lumps of dark matter. These lumps attracted more dark matter under the effect of the gravitational attraction, in a very slow snowball effect. Since dark matter also interacts very weakly, these planted seeds survived well through the stormy moments of the early Universe.

Once matter cooled off as the Universe expanded, it started accumulating on the lumps of dark matter. Hence, dark matter planted the seeds for galaxies. “All this could have happened without dark matter, although it would have taken much more time,” explains Alexandre Arbey, theorist at CERN.

Simulating the formation of the Universe

Not convinced? Nowadays, scientists can reproduce this process using computer simulations. As a starting point, they inject into their models how much matter and dark matter there was right after the Big Bang. The observations of the cosmic microwave background provide these estimates. Then they let it evolve under the attractive effect of gravity and the repulsive effect of the Universe expansion.

All these guesses must converge to reproduce the amount of dark matter leftover today, a quantity called the “relic abundance”. If all is properly tuned, scientists can recreate the whole evolution of the Universe in fast motion from the moment of the Big Bang until today.computer-simulation

The results are striking as can be seen on the three pictures above. These computer-generated images show the distribution of dark matter 470 million years after the Big Bang, then 2.1 and 13.4 billion years later (today). Dark matter first formed small lumps, then long filaments and finally large-scale structures appeared.

Scientists from the French National Centre for Scientific Research (CNRS) just released an amazing video showing how they are now using these mega simulations in the hope to discriminate against different dark matter and dark energy models by comparing these images with current observations.

Cold dark matter

Another way to figure out which theory of dark matter best fits the reality was provided last month by a group of scientists working with the Subaru telescope. They studied the distribution of dark matter in fifty galaxy clusters. Averaging all the data, they found that the dark matter density gradually decreases from the centre of the clusters to their diffuse outskirts.

This new evidence conforms to the predictions of cold dark matter theory (CDM), which states that dark matter is made of slow moving particles. Hot dark matter candidates like neutrinos would be made of particles moving close to the speed of light.


Cold dark matter theory predicts that central regions of galaxy clusters have a lower dark matter density while individual galaxies have a higher concentration parameter.

Unexplained signals from outer space

Astronomers are not just providing clues to the mystery of dark matter but also raising questions.  For example, a decade ago, the INTEGRAL-SPI experiment found an intense gamma ray source at 511 keV coming from the galactic centre, exactly where dark matter is most concentrated. This value of 511 keV is precisely the energy corresponding to the electron or positron mass.


This smelled incredibly like dark matter particles annihilating or decaying into pairs of electron and positron, which in turn can annihilate into gamma rays as depicted on the diagrams above. Unfortunately, nowadays the excitement has somewhat wound down since theorists have a hard time reconciling its characteristics with numerous other observations.

Several satellite experiments (HEAT, PAMELA and FERMI) have observed an excess of positrons in cosmic rays. A positron is the antimatter counterpart of the electron. Given matter prevails over antimatter in the Universe (otherwise, we and the galaxies would not be there), astrophysicists have to figure out where these positrons come from.

Many theorists have attempted to explain this in terms of astronomical phenomena but the jury is still out. Could this be the first concrete sign of dark matter? The AMS experiment on-board the International Space Station has already shown that they have high quality data and could provide a definitive answer very soon.

Dark matter remains a mystery but this field is fast evolving. In my next blog, I will look at what the Large Hadron Collider (LHC) at CERN could do after restart in 2015.

First part in a Dark Matter series:       How do we know Dark Matter exists?

Second part in a Dark Matter series:  Getting our hands on dark matter

Third part in a Dark Matter series:     Cosmology and dark matter

Fourth part in a Dark Matter series:  Can the LHC solve the Dark Matter mystery?

Pauline Gagnon

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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.”


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.



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


Looking Up: Cosmology and Me

Friday, January 7th, 2011

Welcome to Quantum Diaries. I’m Anže Slosar, a scientist living in New York and working at Brookhaven National Laboratory. I’ll also be occasionally blogging on this site. Let me tell you a little about myself.

I am originally from Slovenia, a small country with population of 2 
million, which was created in 1991 after the disintegration of
 Yugoslavia. I studied physics as an undergrad at the University of Cambridge, UK, and, in a typical 
Oxbridge fashion, continued my graduate studies at Cambridge. My Ph.D. 
thesis was on the Very Small Array, an interferometric radio telescope 
designed to measure fluctuations in the cosmic microwave 
background. After receiving my Ph.D., I spent six years doing postdocs at the University of Ljubljana (in the
 capital of Slovenia), Oxford University, and Lawrence Berkeley National Laboratory before moving to Brookhaven

Here, I work on a mixed salad of projects, both theoretical and
 experimental, but inevitably connected to the universe we live 
in. These days, my scientific focus is on the Lyman-alpha forest, part 
of a dark energy project called BOSS (Baryonic Oscillation Spectroscopic Survey), a topic
 that takes more than a healthy amount of my time and nerves but holds
 promise of being truly revolutionary. I will no doubt write more about 
it in the coming posts.

In my spare time, I do a lot of things at a very low level: real ales,
 real food, yoga, classical guitar. And then I do things New Yorkers do, 
namely following the real estate and thinking about cunning ways 
of getting rich while burning cash on restaurants, theaters, clubs,
 galleries, and a myriad other places.

More to come.