Companion Dwarf Galaxy Almost Invisible

Segue 1 is 50 times dimmer than the star cluster pictured above but is 1000 times more massive, meaning most of its mass must be made up of dark matter. (Credit: Sloan Digital Sky Survey)

A team of astronomers has discovered the least luminous, most dark matter-filled galaxy known to exist. The Segue 1 galaxy is one of about two dozen small satellite galaxies orbiting our own Milky Way. This is a very faint galaxy, a billion times less bright than the Milky Way. But despite its small number of visible stars, Segue 1 is nearly a thousand times more massive than it appears, meaning most of its mass must come from dark matter. “Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass,” said Marla Geha, an assistant professor of astronomy at Yale and lead author on a paper about Segue 1.

Geha and her colleagues have observed about half of the dwarf satellite galaxies that orbit the Milky Way. These objects are so faint and contain so few stars that at first they were thought to be globular clusters – tightly bound star clusters that also orbit our host galaxy. But by analyzing the light coming from the objects using the Keck telescope in Hawaii, the researchers determined these objects are actually galaxies, but just very faint.

Looking only at the light emitted by these ultra-faint galaxies, Geha and her colleagues expected them to have correspondingly low masses. Instead, they discovered that they are between 100 and 1000 times more massive than they appear. Invisible dark matter, she said, must account for the difference.

Although dark matter doesn’t emit or absorb light, scientists can measure its gravitational effect on ordinary matter and believe it makes up about 85 percent of the total mass in the universe. Finding ultra-faint galaxies like Segue 1, which is so rife with dark matter, provides clues as to how galaxies form and evolve, especially at the smallest scales.

“These dwarf galaxies tell us a great deal about galaxy formation,” Geha said. “For example, different theories about how galaxies form predict different numbers of dwarf galaxies versus large galaxies. So just comparing numbers is significant.”

It’s only recently that astronomers have discovered just how prevalent these dwarf satellite galaxies are, thanks to projects like the Sloan Digital Sky Survey, which imaged large areas of the nighttime sky in greater detail than ever before. In the past two years alone, the number of known dwarf galaxies orbiting the Milky Way has doubled from the dozen or so brightest that were discovered during the first half of the twentieth century.

Geha predicts astronomers will find even more as they continue to sift through new data. “The galaxies I now consider bright used to be the least luminous ones we knew about,” she said. “It’s a totally new regime. This is a story that’s just unfolding.”

Source: Yale University

Dark Matter Halos? How About Disks, Too

A composite image shows a dark matter disk in red. From images in the Two Micron All Sky Survey. Credit: Credit: J. Read & O. Agertz.


Scientists are trying to understand the invisible and hypothetical ‘dark matter’ – the stuff that we know exists by inference of its gravitational influence on the matter we can see. The most common held notion of dark matter is that it exists in ‘halos’ or clumps that surround galaxies. But a new study predicts that galaxies like our own Milky Way, also contain a disk of dark matter. Using the results of a supercomputer simulation, scientists from the University of Zurich and the University of Central Lancashire say that if dark matter in fact resides as a disk within a galaxy, it could allow physicists to directly detect and identify the nature of dark matter for the first time.

Physicists believe dark matter makes up 22% of the mass of the Universe (compared with the 4% of normal matter and 74% comprising the mysterious ‘dark energy’). But, despite its pervasive influence, no-one is sure what dark matter consists of.

This ‘standard’ theory of dark matter is based on supercomputer simulations that model the gravitational influence of the dark matter alone. The new work includes the gravitational influence of the stars and gas that also make up our Galaxy.

Stars and gas are thought to have settled into disks very early on in the life of the Universe and this affected how smaller dark matter halos formed. The team’s results suggest that most lumps of dark matter in our locality merged to form a halo around the Milky Way. But the largest lumps were preferentially dragged towards the galactic disk and were then torn apart, creating a disk of dark matter within our Galaxy.

“The dark disk only has about half of the density of the dark matter halo, which is why no one has spotted it before,” said lead author Justin Read. “However, despite its low density, if the disk exists it has dramatic implications for the detection of dark matter here on Earth.”

The Earth and Sun move at some 220 kilometres per second along a nearly circular orbit about the center of our Galaxy. Since the dark matter halo does not rotate, from an Earth-based perspective it feels as if we have a ‘wind’ of dark matter flowing towards us at great speed. By contrast, the ‘wind’ from the dark disk is much slower than from the halo because the disk co-rotates with the Earth.

“It’s like sitting in your car on the highway moving at a hundred kilometres an hour”, said team member Dr. Victor Debattista. “It feels like all of the other cars are stationary because they are moving at the same speed.”

This abundance of low-speed dark matter particles, the science team says, could be a real boon for researchers because they are more likely to excite a response in dark matter detectors than fast-moving particles. “Current detectors cannot distinguish these slow moving particles from other background ‘noise’,” said Prof. Laura Baudis, a collaborator at the University of Zurich and one of the lead investigators for the XENON direct detection experiment, which is located at the Gran Sasso Underground Laboratory in Italy. “But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”

If so, its possible that the dark disk could be directly detected in the very near future.

Sources: Monthly Notices paper, Royal Astronomical Society

Pushing the Polite Boundaries of Science About Dark Matter

Hubble and Chandra composite image showing possible dark matter. Credit: X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac)


Rumors are spinning faster than a neutron star about the possibility that a European satellite mission called PAMELA may have made a direct detection of dark matter, the mysterious particles thought to make up as much of 85% of all matter in the Universe. Word got out in August at a conference about dark matter in Stockholm, Sweden where the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) team presented their preliminary findings to a few selected physicists. What information has leaked out says the satellite has detected more positrons than can be explained by known physics and that this excess exactly matches what dark matter particles would produce if they were annihilating each other at the center of the galaxy. But the PAMELA team is not allowing any more information to be made public, until they re-analyze their data and allow other scientists to evaluate and verify the findings. This is good, if not wonderful, in all respects – making sure their findings are peer reviewed before publishing their work and going public. (Does anyone remember the cold fusion debacle?) But in what seems to cross the line of good science — as well pushing the boundaries of what is just plain polite, two other scientists have published an abstract based on what was revealed to them at the conference.

Ever since cosmologists “concocted” dark matter to explain the matter that was obviously missing from the universe’s equation, scientists have speculated, worked, created models and worked some more to determine exactly what dark matter is. Recent findings (see here and here)seem to be bringing us closer to finding this mysterious substance, providing clues to what this stuff might be. The PAMELA data seems to point towards positrons, or anti-electrons.

Marco Cirelli from the CEA near Paris in France and Alessandro Strumia from the Università di Pisa in Italy presented their own analysis of the PAMELA data in this abstract. They say the data agrees with their own model called Minimal Dark Matter in which the particle responsible is called the “Wino.” They do reference their own work but interestingly, many of their references are from talks given at the conference on August 18-22. At one point they note, “The preliminary data points for positron and antiproton fluxes plotted in our figures have been extracted from a photo of the slides taken during the talk, and can thereby slightly differ from the data that the PAMELA collaboration will officially publish.”

Is this just a desire to “publish” something first, or is this real science?

Sources: ArXiv, ArXiv blog, Nature

Minimum Mass for Galaxies Provides Insight on Dark Matter

Dwarf galaxies that are within 500,000 light-years from the Milky Way. Credit: UCI


More news on dark matter this week: By analyzing light from dwarf galaxies that orbit the Milky Way, scientists believe they have discovered the minimum mass for galaxies in the universe – 10 million times the mass of the sun. This mass could be the smallest known “building block” of the mysterious, invisible substance called dark matter. Stars that form within these building blocks clump together and turn into galaxies. Scientists know very little about the microscopic properties of dark matter, even though it accounts for approximately five-sixths of all matter in the universe. “By knowing this minimum galaxy mass, we can better understand how dark matter behaves, which is essential to one day learning how our universe and life as we know it came to be,” said Louis Strigari, lead author of this study from the University of California, Irvine.

Dark matter governs the growth of structure in the universe. Without it, galaxies like our own Milky Way would not exist. Scientists know how dark matter’s gravity attracts normal matter and causes galaxies to form. They also suspect that small galaxies merge over time to create larger galaxies such as our Milky Way.

The smallest known galaxies, called dwarf galaxies, vary greatly in brightness, from 1,000 times the luminosity of the sun to 10 million times the luminosity of the sun. At least 22 of these dwarf galaxies are known to orbit the Milky Way. UCI scientists studied 18 of them using data obtained with the Keck telescope in Hawaii and the Magellan telescope in Chile, with the goal of calculating their masses. By analyzing stars’ light in each galaxy, they determined how fast the stars were moving. Using those speeds, they calculated the mass of each galaxy.

The researchers expected the masses to vary, with the brightest galaxy weighing the most and the faintest galaxy weighing the least. But surprisingly all dwarf galaxies had the same mass – 10 million times the mass of the sun.

Manoj Kaplinghat, a study co-author and physics and astronomy assistant professor at UCI, explains this finding using an analogy in which humans play the role of dark matter.

“Suppose you are an alien flying over Earth and identifying urban areas from the concentration of lights in the night. From the brightness of the lights, you may surmise, for example, that more humans live in Los Angeles than in Mumbai, but this is not the case,” Kaplinghat said. “What we have discovered is more extreme and akin to saying that all metro areas, even those that are barely visible at night to the aliens, have a population of about 10 million.”

Since dwarf galaxies are mostly dark matter – the ratio of dark matter to normal matter is as large as 10,000 to one – the minimum-mass discovery reveals a fundamental property of dark matter.

“We are excited because these galaxies are virtually invisible, yet contain a tremendous amount of dark matter,” said James Bullock, a study co-author and director of UCI’s Center for Cosmology. “This helps us better understand the particle that makes up dark matter, and it teaches us something about how galaxies form in the universe.”

The scientists say clumps of dark matter may exist that contain no stars. The only dark matter clumps they can detect right now are those that are lit by stars.

Scientists hope to learn about dark matter’s microscopic properties when the Large Hadron Collider in Switzerland becomes operational later this year. The device will accelerate two beams of nuclei in a ring in opposite directions and then slam them together to recreate conditions just after the Big Bang. By doing this, scientists hope to create the dark matter particle in the lab for the first time.

Source: University of California, Irvine

Dark Matter is Missing From Cosmic Voids

Map of distribution of galaxies. Credit: M. Blanton and the SDSS.


Cosmic voids really are devoid of matter. Astronomers have found that even the pervasive ‘dark matter’ which accounts for about 80% of the mass of the universe is not present in these voids, which are areas of vast emptiness in space that can be tens of millions of light-years across. “Astronomers have wondered for a quarter-century whether these voids were ‘too big’ or ‘too empty’ to be explained by gravity alone,” said University of Chicago researcher Jeremy Tinker, who led the new study using data from the Sloan Digital Sky Survey II (SDSS-II). “Our analysis shows that the voids in these surveys are exactly as big and as empty as predicted by the ‘standard’ theory of the universe.”

The largest 3-dimensional maps of the universe show that galaxies lie in filamentary superclusters interlaced by cosmic voids that contain few or no bright galaxies. Researchers using SDSS-II and the
Two-Degree Field Galaxy Redshift Survey (2dFGRS) have concluded that these voids are also missing the “halos” of invisible dark matter that bright galaxies reside in.

A central element of the standard cosmological theory is cold dark matter, which exerts gravity but does not emit light. Dark matter is smoothly distributed in the early universe, but over time gravity pulls it into filaments and clumps and empties out the spaces between them. Galaxies form when hydrogen and helium gas falls into collapsed dark matter clumps, referred to as “halos,” where it can form luminous stars.

But astronomers were not sure if the areas that are devoid of galaxies were also devoid of dark matter, or if the dark matter was there, but for some reason stars just didn’t form in these voids.
The research team used bright galaxies to trace the structure of dark matter and compared it with computer simulations to predict the number and sizes of voids.
Princeton University graduate student Charlie Conroy measured the sizes of voids in the SDSS-II maps. “When we used galaxies brighter than the Milky Way to trace structure, the biggest empty voids we found were about 75 million light years across,” said Conroy. “And the predictions from the simulations were bang-on.”

The sizes of voids are ultimately set, Conroy explained, by the small variations in the primordial distribution of dark matter, and by the amount of time that gravity has had to grow these small variationsinto large structures.

The agreement between the simulations and the measurements holds for both red (old) and blue (new) galaxies, said Tinker. “Halos of a given mass seem to form similar galaxies, both in numbers of stars and in the ages of those stars, regardless of where the halos live.”

Tinker presented his findings today at an international symposium in Chicago, titled “The Sloan Digital Sky Survey: Asteroids to Cosmology.” A paper detailing the analysis will appear in the September 1 edition of The Astrophysical Journal, with the title “Void Statistics in Large Galaxy Redshift Surveys: Does Halo Occupation of Field Galaxies Depend on Environment?”

News Source: SDSS and The Ohio State University

Large Hadron Collider Could Generate Dark Matter

A simulation of a LHC collision (CERN)

One of the biggest questions that occupy particle physicists and cosmologists alike is: what is dark matter? We know that a tiny fraction of the mass of the universe is the visible stuff we can see, but 23% of the Universe is made from stuff that we cannot see. The remaining mass is held in something called dark energy. But going back to the dark matter question, cosmologists believe their observations indicate the presence of darkmatter, and particle physicists believe the bulk of this matter could be held in quantum particles. This trail leads to the Large Hadron Collider (LHC) where the very small meets the very big, hopefully explaining what particles could be generated after harnessing the huge energies possible with the LHC…

The excitement is growing for the grand switch-on of the LHC later this summer. We’ve been following all the news releases, research possibilities and some of the more “out there” theories as to what the LHC is likely to discover, but my favourite bits of LHC news include the possibility of peering into other dimensions, creating wormholes, generating “unparticles” and micro-black holes. These articles are pretty extreme possibilities for the LHC, I suspect the daily running of the huge particle accelerator will be a little more mundane (although “mundane” in accelerator physics will still be pretty damn exciting!).

David Toback, professor at Texas A&M University in College Station, is very optimistic as to what discoveries the LHC will uncover. Toback and his team have written a model that uses data from the LHC to predict the quantity of dark matter left over after the Big Bang. After all, the collisions inside the LHC will momentarily recreate some of the conditions at the time of the birth of our Universe. If the Universe created dark matter over 14 billion years ago, then perhaps the LHC can do the same.

Should Toback’s team be correct in that the LHC can create dark matter, there will be valuable implications for both particle physics and cosmology. What’s more, quantum physicists will be a step closer to proving the validity of the supersymmetry model.

If our results are correct we now know much better where to look for this dark matter particle at the LHC. We’ve used precision data from astronomy to calculate what it would look like at the LHC, and how quickly we should be able to discover and measure it. If we get the same answer, that would give us enormous confidence that the supersymmetry model is correct. If nature shows this, it would be remarkable.” – David Toback

So the hunt is on for dark matter production in the LHC… but what will we be looking for? After all dark matter is predicted to be non-interacting and, well, dark. The supersymmetry model predicts a possible dark matter particle called the neutralino. It is supposed to be a heavy, stable particle and should there be a way of detecting it, there could be the opportunity for Toback’s group to probe the nature of the neutralino not only in the detection chamber of the LHC, but the nature of the neutralino in the Universe.

If this works out, we could do real, honest to goodness cosmology at the LHC. And we’d be able to use cosmology to make particle physics predictions.” – Toback


Dark Matter is Denser in the Solar System

Dark matter was theorized to exist relatively recently, and we’ve come a long way in understanding what makes up a whopping 23% of our Universe. Our own galaxy is surrounded by a halo of dark matter that adds to its mass. A recent paper on the dark matter closer to home – right here in our own Solar System – reveals that it is denser and more massive than in the galactic halo.

Dark matter is just plain weird stuff. It doesn’t give off light, has mass and reacts gravitationally with “normal” matter – the stuff that we and our planet and the stars are composed of. Just like normal matter, it “clumps” up, or accretes, because of this gravitational attraction; we find more dark matter near galaxies than in the vast expanses between them.

Dark matter isn’t just far off in the Milky Way or somewhere on the other side of the Universe, though: it’s right here at home in our Solar System. In a recent paper submitted to Physical Review D, Ethan Siegel and Xiaoying Xu of the University of Arizona analyzed the distribution of dark matter in our Solar System, and found that the mass of dark matter is 300 times more than that of the galactic halo average, and the density is 16,000 times higher than that of the background dark matter.

Over the history of the Solar System, Xu and Siegel calculate that 1.07 X 10^20 kg of dark matter have been captured, or about 0.0018% the mass of the Earth. To get a handle on this number, the mass of Ceres – the largest object in the asteroid belt between Mars and Jupiter – is about 9 times this amount.

Siegel and Xu calculated how much dark matter the Solar System has swept up over it’s 4.5 billion-year lifespan by modeling the composition of the background dark matter halo in the orbit of the Solar System around the galaxy, and calculating just how much dark matter would be trapped by the Solar System as it moves through this halo. They ran this calculation for the Sun and each one of the eight planets separately, giving the distribution of the matter throughout the Solar System, as well as the total amount captured.

Much like when you drive your car through a light snowfall, dark matter “sticks” to the Solar System when it is gravitationally bound by the Sun and planets. Just as some of the snow melts on your windshield (hopefully), some doesn’t stick to the hood and most just flies right by, dark matter isn’t distributed evenly throughout our Solar System, either. Some planets have more dark matter surrounding them than others, depending on where they are. Shown below is the density distribution of the dark matter in the Solar System

The first spike is Mercury, and the next two spikes are Venus and Earth (Mars doesn’t show up). The next is Jupiter, followed by a small bump from Saturn and finally Uranus and Neptune combined create the last small bump.

How does the local dark matter effect interactions in the Solar System? Well,it doesn’t have a large effect on the orbits of the planets, nor does it slow down the Solar System in its orbit around the galactic center appreciably.

“Planetary orbits, if there were enough dark matter present, would have their perihelia precess faster than if there were no dark matter. The amount of dark matter allowed from these observations is considerably greater than the amount I predict. The errors on the measurements of perihelion precession are in units of hundredths of an arc second per century…Even if you assume the dark matter is at rest with respect to the galaxy that the Solar System moves through (which is the extreme example), the Sun is of order 10^30 kg; capturing a 10^20 kg clump of dark matter will slow you down by about 20 microns/second over the lifetime of the Solar System. So that would be small.” – Ethan Siegel in an email interview.

And, alas, the mystery of the Pioneer anomaly is not going to be solved by this revelation, as the mass of the captured dark matter is not enough to explain the odd motions of that spacecraft.

The discovery of a higher density and mass of dark matter in our neighborhood may aid in the study and detection of dark matter, though. Knowing the mass and density distribution of the local dark matter – and thus knowing how much and where to look for it – will provide astronomers looking into solving exactly what it’s made up of with more information .

“Our determination of the local dark matter density and velocity distribution are of great importance to direct detection experiments. The most recent calculations that have been carried out assume that the properties of dark matter at the Sun’s location are derived directly from the galactic halo. By comparison, we find that terrestrial experiments should also consider a component of dark matter with a density 16,000 times greater than the background halo density,” wrote Xu and Siegel.

Source: Arxiv, email interview with Ethan Siegel

Primordial Stars Frozen Indefinitely by Dark Matter

Dark, cold stars from the young Universe could still be here today (University of Utah)

It is thought that primordial or “Population III” stars were born in dense clouds of dark matter, 100 million years after the Big Bang. During the period between birth and dark matter depletion, these first stars were effectively but into a “deep freeze” where normal star development was prevented. After this period when all the dark matter fuel had been consumed, these stars were allowed to commence normal stellar evolution, dying out within a few hundred thousand years. But say if a Population III star was born in an exceptionally dense cloud of dark matter? How long could “normal stellar evolution” be frozen for? According to new research, dark matter could theoretically freeze the star indefinitely, over timescales longer than the age of the Universe…

This amazing theory comes from research carried out by Gianfranco Bertone and his team at the Paris Institute of Astrophysics in France. The thought that the first stars, born over 14 billion years ago, could possibly inhabit the Universe today is a very impressive idea. These primordial stars are thought to have been seeded inside dense clouds of dark matter, where gravity caused dark matter compression. As the matter became concentrated, non-baryonic particles may have begun annihilating, stopping natural hydrogen fusion (the mechanism commonly associated with star creation). “Normal” stellar evolution was therefore paused and the “dark star” phase began as dark matter annihilation heated the stellar cores.

It has long been the assumption that the “dark star” phase occurred for a short period of time in the early Universe where vast halos of dark matter may have dominated. Once the dark matter fuel ebbed away, primordial stars were left to self-destruct in a flurry of accelerated evolution. Now Bertone and his colleagues believe a few primordial specimens might be alive today, hidden inside particularly dense clouds of dark matter, in galactic centres, keeping some of the Universe’s first stars in a state of suspended animation.

There could be conditions in the early universe where stars form in big enough reservoirs of dark matter to last until the present day.” – Gianfranco Bertone.

One of the most exciting implications to come from this research is the fact that these ancient relics may be observed, what’s more, we may have already seen some. “A frozen star would appear much bigger and colder than a normal star with the same mass and chemical composition,” says Marco Taoso, co-investigator in the French group. If stars matching the characteristics of these frozen stellar bodies are (or already have been) found, the discovery would have huge consequences for the quantum search for supersymmetry, indicating dark matter was indeed made up of massive “superpartners” to ordinary matter.

If dark matter influenced stars a few hundred thousand years after the Big Bang, can it still influence stellar evolution today? Researchers believe this could be the case. Present-day stars evolving in regions of dark matter clouds may be influenced by non-baryonic particles. White dwarfs are formed after the death of Sun-like stars and it is believed that should the dwarf star encounter a cloud of dark matter, it could be resurrected as a dark matter burner, shining like 30 Suns.

It will be interesting to see if there have already been any observations of these primordial stars, possibly providing more indirect evidence of dark matter in our Universe.

Source: New Scientist

Could Dark Matter be the Root Cause of Flyby Anomalies?

The Galileo mission above Earth - the subsequent flybys caused an unexpected boost in velocity (credit: NASA)

When space probes Galileo, Rosetta, NEAR and Cassini carried out Earth flyby manoeuvre, scientists measured a bizarre and unpredictable jumps in orbital acceleration. To this day, the phenomenon remains unexplained, but there are many ideas as to how this flyby anomaly may be caused. As previously reported on the Universe Today, some of the scientific explanations can be pretty exotic (the Unruh Effect, after all, isn’t that easy to understand), but this new theory is just as captivating. In a new study from the Institute for Advanced Study, Princeton, one researcher thinks dark matter might be messing around with our robotic explorers…

Dark matter is probably one of the most interesting, yet controversial, ideas in advanced cosmological studies. We have reported on many of the existing theories as to how we might be able to detect the Universe’s “missing matter” and it is thought that the bulk of universal mass may be held in a range of sub-atomic to massive stellar objects.

The flyby anomalies have been attributed to measurement error (spaceships using the Earth as a gravitational slingshot have their velocities measured by Doppler radar instruments on ground-based observatories), the Unruh effect, even variations in the speed of light, but so far, dark matter hasn’t really featured. So if there is dark matter out there in space, perhaps it will influence the spaceships we send out there. Now Stephen Adler at the Institute for Advanced Study in Princeton examines this possibility and imposes some limits that dark matter may influence flyby anomalies.

The biggest challenge facing any anomaly theory is that spacecraft have experienced increases and decreases in acceleration, what could be the chief suspect causing these sudden changes in acceleration? Alder points to the strange physics behind dark matter accumulating around the Earth, confined within a planetary ring, much like the visible rings around Saturn. What’s more, to explain flyby observations, the ring would have to contain at least two types of dark matter (non-baryonic particles). Interestingly, I recently wrote about the proposed LUX detector to be buried in a disused South Dakota goldmine. This detector will be the first of its kind to attempt to measure the elusive Weakly Interacting Massive Particles (WIMPS) that have been theorized to contain large quantities of matter, hence a large proportion of the dark matter in our universe. This leads to the possibility that the Earth may be passing through “clouds” of WIMPs, giving some credence to the idea that dark matter varieties may also be contained in the volume of space surrounding Earth. As spacecraft orbiting Earth passes through this dark matter ring, perhaps there will be some complex interaction causing this sudden change in acceleration.

For more technical information, have a read of the arXiv publication: “Can the flyby anomaly be attributed to earth-bound dark matter?” by Stephen L. Adler.

Source: arXiv blog

Digging for Dark Matter: The Large Underground Xenon (LUX) Detector

The Hubble Space Telescope distribution of dark matter - indirect observations (HST)

How do you catch a WIMP? No, I’m not talking about bullying the weakest kid in class, I’m talking about Weakly Interacting Massive Particles (those WIMPs). Well, it isn’t easy. Although they are “massive” by definition, they do not interact with the electromagnetic force (via photons) so they cannot be “seen” and they do not interact with the strong nuclear force, so they cannot be “felt” by atomic nuclei. If we cannot detect WIMPs via these two forces, how can we possibly ever hope to detect them? After all, WIMPs are theorized to be flying through the Earth without hitting anything, they are that weakly interacting. But sometimes, they might collide with atomic nuclei but only if they collide head-on. This is a very rare occurrence, but the Large Underground Xenon (LUX) detector will be buried 4,800 feet (1,463 meters, or nearly a mile) underground in an old South Dakota goldmine and scientists are hopeful that when an unlucky WIMP bumps into a xenon atom, a flash of light will be captured, signifying the first ever experimental evidence of dark matter

Galaxies observed from Earth have some strange qualities. The biggest problem for cosmologists has been to explain why galaxies (including the Milky Way) appear to have more mass than can be observed by counting stars and accounting for interstellar dust alone. In fact, 96% of the Universe’s mass cannot be observed. 22% of this missing mass is thought to be held in “dark matter” (74% is held as “dark energy”). Dark matter is theorized to take on many forms. Massive Astronomical Compact Halo Objects (astronomical bodies containing ordinary baryonic material that cannot be observed; like neutron stars or orphaned planets), neutrinos and WIMPS all are thought to contribute toward this missing mass. Many experiments are in progress to detect each contributor. Black holes can be indirectly detected by observing the interactions in the centre of galaxies (or gravitational lensing effects), neutrinos can be detected in huge tanks of fluid buried deep underground, but how can WIMPs be detected? It seems a WIMP detector needs to take a leaf out of the neutrino detector’s books – it needs to start digging.

Super-Kamiokande, a neutrino detector in Japan, holds 50,000 tons of ultra pure water surrounded by light tubes (Super-Kamiokande)

To avoid interference from radiation such as cosmic rays, low energy detectors such as neutrino “telescopes” are buried well below the Earth’s surface. Old mine shafts make ideal candidates as the hole is already there for the instrumentation to be set up. Neutrino detectors are huge containers of water (or some other agent) with highly sensitive detectors positioned around the outside. One such example is the Super Kamiokande neutrino detector in Japan which contains a vast amount of ultra-purified water, weighing in at 50,000 tons (pictured left). As a weakly interacting neutrino hits a water molecule in the tank, a flash of Cherenkov radiation is emitted and a neutrino is detected. This is the basic principal behind the new Large Underground Xenon (LUX) detector that will use 600 pounds (272 kg) of liquid xenon suspended in a 25 foot high tank of pure water. If WIMPs exist beyond the realms of theory, it is hoped that these weakly interacting massive particles will collide head-on with a xenon atom, and like their light-weight cousins, emit a flash of light.

Robert Svoboda and Mani Tripathi, UC Davis professors, have secured $1.2 million in National Science Foundation (NSF) and U.S. Department of Energy funding for the project (this is 50% of the total required). When compared with the Large Hadron Collider (LHC) costing billions of Euros to build, LUX is a highly economic project considering the scope of what it might discover. Should there be experimental evidence of a WIMP interaction, the consequences will be enormous. We will be able to begin to understand the origins of WIMPs and their distribution as the Earth sweeps through the possible dark matter halo that is indirectly observed to exist in the Milky Way.

Detecting dark matter “would be the biggest deal since finding antimatter in the 1930s.” – Professor Mani Tripathi, LUX co-investigator, UC Davis.

The gold mine in South Dakota was closed in 2000 and in 2004 work began to develop the site into an underground laboratory. LUX will be the first large experiment to be housed there. It is hoped that the installation will start late summer, after water has been pumped out of the mine.

Original source: UC Davis News