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Posts Tagged ‘cosmology’

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.

BigBang

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.

Galaxy-cluster-density-Subaru

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.

diagram

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

To be alerted of new postings, follow me on Twitter: @GagnonPauline
 or sign-up on this mailing list to receive and e-mail notification.

 

 

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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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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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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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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
 Lab.

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.

Anže

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