Tuesday, 28 October 2014


( —The same phenomenon that causes a bumpy airplane ride, turbulence, may be the solution to a long-standing mystery about stars' birth, or the absence of it, according to a new study using data from NASA's Chandra X-ray Observatory.
Galaxy clusters are the largest objects in the universe, held together by gravity. These behemoths contain hundreds or thousands of individual galaxies that are immersed in gas with temperatures of millions of degrees.
This hot gas, which is the heftiest component of the galaxy clusters aside from unseen dark matter, glows brightly in X-ray light detected by Chandra. Over time, the gas in the centers of these clusters should cool enough that stars form at prodigious rates. However, this is not what astronomers have observed in many galaxy clusters.
"We knew that somehow the gas in clusters is being heated to prevent it cooling and forming stars. The question was exactly how," said Irina Zhuravleva of Stanford University in Palo Alto, California, who led the study that appears in the latest online issue of the journal Nature. "We think we may have found evidence that the heat is channeled from turbulent motions, which we identify from signatures recorded in X-ray images."
Prior studies show supermassive black holes, centered in large galaxies in the middle of galaxy clusters, pump vast quantities of energy around them in powerful jets of energetic particles that create cavities in the hot gas. Chandra, and other X-ray telescopes, have detected these giant cavities before.
The latest research by Zhuravleva and her colleagues provides new insight into how energy can be transferred from these cavities to the surrounding gas. The interaction of the cavities with the gas may be generating turbulence, or chaotic motion, which then disperses to keep the gas hot for billions of years.
"Any gas motions from the turbulence will eventually decay, releasing their energy to the gas," said co-author Eugene Churazov of the Max Planck Institute for Astrophysics in Munich, Germany. "But the gas won't cool if turbulence is strong enough and generated often enough."
The evidence for turbulence comes from Chandra data on two enormous galaxy clusters named Perseus and Virgo. By analyzing extended observation data of each cluster, the team was able to measure fluctuations in the density of the gas. This information allowed them to estimate the amount of turbulence in the gas.
"Our work gives us an estimate of how much turbulence is generated in these clusters," said Alexander Schekochihin of the University of Oxford in the United Kingdom. "From what we've determined so far, there's enough turbulence to balance the cooling of the gas.
These results support the "feedback" model involving supermassive black holes in the centers of galaxy clusters. Gas cools and falls toward the black hole at an accelerating rate, causing the black hole to increase the output of its jets, which produce cavities and drive the turbulence in the gas. This turbulence eventually dissipates and heats the gas.
While a merger between two galaxy clusters may also produce turbulence, the researchers think that outbursts from supermassive black holes are the main source of this cosmic commotion in the dense centers of many clusters

Sunday, 26 October 2014

ICE FOUND ON MERCURY, THE CLOSEST PLANET TO THE SUN - Posted by Tim De Chant on Thu, 23 Oct 2014 in

Astronomers have confirmed that ice exists on one of the least likely places in the solar system—Mercury.
The planet is among the solar system’s hottest—only Venus has higher average temperatures—thanks to the sun’s searing proximity, which raises temperatures to as high as 800˚ F. But hidden in the frigid shadows—where temperatures can sink as low as -280˚ F—are frozen patches that are the likely remains of icy comets which have blasted the rocky terrain over the last 100 million years.
Scientists have long suspected that Mercury is home to solid H2O, but recent photos taken by the MESSENGER probe confirm it. Here’s Michael Lemonick, reporting for Time:
These are the first optical images, and nobody is entirely sure how the ice got there. One idea is that it was released from water-bearing rock in Mercury’s crust. But the leading theory suggests it arrived instead in the form of impacts from icy comets, which may well be the same way Earth got its oceans. “It’s a fair amount of ice,” Chabot said, “about equivalent to the water in Lake Ontario, so if it was one comet, it was a pretty sizable one.” More likely, she said, it would have been a series of smaller comets, falling over billions of years.
Mercury’s extreme temperature fluctuations are thanks to its extremely thin atmosphere. The planet’s small mass means it has a hard time holding onto gas particles, and the sun’s intense rays do their best to blast away what little that does cling to the rocky world. If the planet had a denser atmosphere, it would more closely resemble Venus, where the thick sky keeps average temperatures above 860˚ F.
Mercury’s thin atmosphere gives scientists another unique opportunity. Next spring, MESSENGER will be able to fly just 12 miles above the surface without burning up. During that pass, it will take extraordinarily high-resolution images of the planet, giving scientists a unique window into a world of extremes.

Saturday, 25 October 2014


Here’s one reason libraries hang on to old science journals: A paper from an experiment conducted 32 years ago may shed light on the nature ofdark matter, the mysterious stuff whose gravity appears to keep the galaxies from flying apart. The old data put a crimp in the newfangled concept of a "dark photon" and suggest that a simple bargain-basement experiment could put the idea to the test.
No one really knows what dark matter is. Since the 1980s, theorists' best hunch has been that it consists of so-called weakly interacting massive particles, or WIMPs. If they exist, WIMPs would have a mass between one and 1000 times that of a proton. They would interact only through the feeble weak nuclear force—one of two forces of nature that ordinarily flex their muscle only within the atomic nucleus—and could disappear only by colliding and annihilating one another. So if the infant universe cooked up lots of WIMPs, enough of them would naturally survive to produce the right amount of dark matter today. But physicists have yet to spot WIMPs, which every now and then should ping off atomic nuclei in sensitive detectors and send them flying.
More recently, theorists have explored other ideas, such as self-interacting dark matter. This would consist of a particle, known as a χ (pronounced chi), with a mass between 1/1000 and one times that of the proton. Those particles would interact with one another through a force like the electromagnetic force, which produces light. That force would be conveyed by a massive particle called a dark photon—a dark matter version of a particle of light—that might "mix" slightly with the ordinary ones. So with some small probability, a dark photon might interact with ordinary charged particles such as electrons and atomic nuclei—just as ordinary photons do.
Self-interacting dark matter has attractive properties. In particular, a dark photon could also explain a particle physics puzzle. A particle called the muon appears to be very slightly more magnetic than theory predicts, and that discrepancy could be resolved if the muon interacts with dark photons lurking in the vacuum. However, χs and dark photons would be hard to detect with WIMP detectors; with their low masses, they couldn't whack a nucleus hard enough to create a signal.
But archival data already rule out dark photons with certain combinations of properties, argues Rouven Essig, a theoretical physicist at Stony Brook University in New York, and his colleagues. The data come from E137, a "beam dump" experiment that ran from 1980 to 1982 at SLAC National Accelerator Laboratory in Menlo Park, California. In the experiment, physicists slammed a beam of high-energy electrons, left over from other experiments, into an aluminum target to see what would come out. Researchers placed a detector 383 meters behind the target, on the other side of a sandstone hill 179 meters thick that blocked any ordinary particles. They then looked for hypothetical particles called axions, which would have pierced the earth and reached the detector—and saw none.
But electrons hitting the target should also have produced a beam of high-energy χs. A χ could have traversed the hill and interacted with an electron in the detector through a dark photon, blasting it into motion. The fact that E137 saw no recoiling electrons enabled Essig and his colleagues to nix some possible combinations of the dark photon's mass and the strength of its mixing with ordinary photons, as they report this week in Physical Review Letters. The results do not prove that the dark photon cannot exist at all, but they do put limits on its possible properties.
Other physicists have used archival data to test new dark matter theories. Last year, Philip Schuster, a theorist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and a colleague used the result from another beam dump experiment at SLAC that ran in 1994 and 1995 to probe self-interacting dark matter. But the millicharge, or mQ, experiment was sensitive to χs sending atomic nuclei flying and set somewhat looser limits. "The electron-recoil limit looks a little better," Schuster says.
With certain assumptions, the analysis disfavors a dark photon with the properties needed to explain the muon's magnetism. But those assumptions could be loosened and the idea more thoroughly tested with a new experiment, Schuster says. He and roughly 80 other physicists hope to build a new beam dump experiment called BDX, which would look at 100 times as many events as E137 did. They have submitted a letter of intent to the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, although the experiment could be staged elsewhere.
Compared with some particle physics experiments, BDX would be small and cheap, says Marco Battaglieri of Italy's National Institute for Nuclear Physics in Genoa and co-spokesman for the BDX team. "We are not talking about thousands of tons of detector," he says. "We are talking about a 1-ton detector." BDX would cost a few million dollars, Battaglieri says.
The study also suggests it's not so easy to dream up models of dark matter that don't run afoul of data already taken, Schuster says: "All of this has to be done in a very tight straitjacket."

Posted in Physics Dark Matter

Thursday, 23 October 2014

AUSTRALIA’S FIRST DARK MATTER EXPERIMENT A proposed dark matter experiment would use two underground detectors, one in each hemisphere - Glenn Roberts Jr.

Physicists are hoping to hit pay dirt with a proposed experiment—the first of its kind in the Southern Hemisphere—that would search for traces of dark matter more than a half mile below ground in Victoria, Australia.
The current plan, now being explored by an international team, is for two new, identical dark matter experiments to be installed and operated in parallel—one at an underground site at Grand Sasso National Laboratory in Italy, and the other at the Stawell Gold Mine in Australia.
“An experiment of this significance could ultimately lead to the discovery of dark matter,” says Elisabetta Barberio of the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP) and the University of Melbourne, who is Australian project leader for the proposed experiment.
The experiment proposal was discussed during a two-day workshop on dark matter in September. Work could begin on the project as soon as 2015 if it gathers enough support. “We’re looking at logistics and funding sources,” Barberio says.
The experiments would be modeled after the DAMA experiment at Gran Sasso, now called DAMA/LIBRA, which in 1998 found a possible sign of dark matter.
DAMA/LIBRA looks for seasonal modulation, an ebb and flow in the amount of potential dark matter signals it sees depending on the time of year.
If the Milky Way is surrounded by a halo of dark matter particles, then the sun is constantly moving through it, as is the Earth. The Earth’s rotation around the sun causes the two to spend half of the year moving in the same direction and the other half moving in opposite directions. During the six months in which the Earth and sun are cooperating, a dark matter detector on the Earth will move faster through the dark matter particles, giving it more opportunities to catch them.
This seasonal difference appears in the data from DAMA/LIBRA, but no other experiment has been able to confirm this as a sign of dark matter.
For one thing, the changes in the signal could be caused on other factors that change by the season.
“There are environmental effects—different characteristics of the atmosphere—in winter and summer that are clearly reversed if you go from the Northern to the Southern hemisphere,” says Antonio Masiero, vice president for the Italian National Institute of Nuclear Physics (INFN) and a member of the Italian delegation collaborating on the proposal, which also includes Gran Sasso Director Stefano Ragazzi. If the results matched up at both sites at the same time of year, that would help to rule out such effects.
The Australian mine hosting the proposed experiment could also house scientific experiments from different fields.
“It wouldn’t be limited to particle physics and could include experiments involving biology, geosciences and engineering,” Barberio says. “These could include neutrino detection, nuclear astrophysics, geothermal energy extraction and carbon sequestration, and subsurface imaging and sensing.”
Preliminary testing has begun at the mine site down to depths of about 880 meters, about 200 meters above the proposed experimental site. Regular mining operations are scheduled to cease at Stawell in the next few years.

The ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), the local government in the Victoria area, and the mine operators have joined forces with COEPP and INFN to support the proposal.

Thursday, 16 October 2014


Researchers have discovered a formerly undetected impact of space weather on the polar atmosphere, which may explain some previously unexplained variations in winter weather patterns. Their results, published today (Tuesday 14 October), in the journalNature Communications could have important implications for seasonal weather forecasting.
The international team from the Finnish Meteorological Institute, Otago University and British Antarctic Survey (BAS) studied data from three different satellites over an 11 year period. They found that energetic electrons (highly charged particles created by the sun) from the outer radiation belt hitting the Earth's atmosphere cause ozone loss high above the Earth.
Vast quantities of energetic electrons are found in the Earth's radiation belts, trapped there by the Earth's magnetic field. During magnetic storms, which are driven by the solar wind, the electrons accelerate to high speeds and 'rain' into the atmosphere at the poles. The temporary, but frequent, ozone loss occurring as a result of these 'rains' may explain changes in wind patterns which affect regional winter temperatures in the Northern Hemisphere by a maximum of plus or minus 5 degrees centigrade. The variation in temperature is only seen during winter because of the complex linkages from space through to the Earth's surface.

BAS co-author Dr Mark Clilverd says, "This is an exciting piece of research because it shows us a key part of the chain-reaction of how electrons affect ozone at higher latitudes and can ultimately affect weather systems – a link that we didn't truly understand before. It could contribute to data for weather forecasting, for instance to show us when Europe is likely to experience an especially cold winter."

Tuesday, 14 October 2014

How Was Uranus Formed? by Nola Taylor Redd, Contributor   |   November 30, 2012 12:18pm ET

There are two theories as to how planets in the solar system were created. The first and most widely accepted, core accretion, works well with the formation of the terrestrial planets but has problems with giant planets such as Uranus. The second, the disk instability method, may account for the creation of giant planets.
The core accretion model
Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.
With the rise of the sun, the remaining material began to clump together. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create terrestrial worlds. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants such as Uranus. In this way, asteroidscomets, planets, and moons were created.
Unlike most gas giants, Uranus has a core that is rocky rather than gaseous. The core likely formed first, and then gathered up the hydrogen, helium, and methane that make up the planet's atmosphere. Heat from the core drives Uranus' temperature and weather, overpowering the heat coming from the distant sun, which is almost two billion miles away.
The disk instability model
But the need for a rapid formation for the giant gas planets is one of the problems of core accretion. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.
According to a relatively new theory, disk instability, clumps of dust and gas are bound together early in the life of the solar system. Over time, these clumps slowly compact into a giant planet. These planets can form faster than their core accretion rivals, sometimes in as little as a thousand years, allowing them to trap the rapidly-vanishing lighter gases. They also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.
As scientists continue to study planets inside of the solar system, as well as around other stars, they will better understand how Uranus and its siblings formed.
A dangerous youth
The early solar system was a time of violent collisions, and Uranus was not exempt. While the surface of the moon and Mercury both show evidence of bombardment by smaller rocks and asteroids, Uranus apparently suffered a significant collision with an Earth-size proto planet. As a result, Uranus is tipped on its side, with one pole pointing toward the sun for half the year.
Uranus is the smallest of the gas giants, perhaps in part because it lost some of its mass during the impact.
— Nola Taylor Redd, Contributor

A COLD-ATOM AMMETER A super fluid current is only as strong as its weak link - October 8, 2014

In certain exotic situations, a collection of atoms can transition to a superfluid state, flouting the normal rules of liquid behavior. Unlike a normal, viscous fluid, the atoms in a superfluid flow unhindered by friction. This remarkable free motion is similar to the movement of electron pairs in a superconductor, the prefix ‘super’ in both cases describing the phenomenon of resistanceless flow. Harnessing this effect is of particular interest in the field of atomtronics, since superfluid atom circuits can recreate the functionality of superconductor circuits, with atoms zipping about instead of electrons. Now, JQI scientists have added an important technique to the atomtronics arsenal, a method for analyzing a superfluid circuit component known as a ‘weak link’. The result, detailed in the online journal Physical Review X, is the first direct measurement of the current-phase relationship of a weak link in a cold atom system.
“What we have done is invented a way to characterize a particular circuit element [in a superfluid atomtronic circuit],” says Stephen Eckel, lead author of the paper. “This is similar to characterizing a component in an ordinary electrical circuit, where one measures the current that flows through the component vs. the voltage across it.”
Properly designing an electronic circuit means knowing how each component in the circuit affects the flow of electrons. Otherwise, your circuit won’t function as expected, and at worst case will torch your components into uselessness. This is similar to the plumbing in a house, where the shower, sink, toilet, etc. all need the proper amount of water and water pressure to operate. Measuring the current-voltage relationship, or how the flow of current changes based on a voltage change, is an important way to characterize a circuit element. For instance, a resistor will have a different current-voltage relationship than a diode or capacitor. In a superfluid atom circuit, an analogous measurement of interest is the current-phase relationship, basically how a particular atomtronic element changes the flow of atoms.
Interferometric Investigations
The experiment, which took place at a JQI lab on the NIST-Gaithersburg campus, involves cooling roughly 800,000 sodium atoms down to an extremely low temperature, around a decidedly chilly hundred billionths of a degree above absolute zero. At these temperatures, the atoms behave as matter waves, overlapping to form something called a Bose-Einstein condensate (BEC). The scientists confine the condensate between a sheet-like horizontal laser and a target shaped vertical laser. This creates two distinct clouds, the inner one shaped like a disc and the outer shaped like a ring. The scientists then apply another laser to the outer condensate, slicing the ring vertically. This laser imparts a repulsive force to the atoms, driving them apart and creating a low density region known as a weak link (Related article on this group's research set-up).
The weak link used in the experiment is like the thin neck between reservoirs of sand in an hourglass, constricting the flow of atoms across it. Naturally, you might expect that a constriction would create resistance. Consider pouring syrup through a straw instead of a bottle -- this would be a very impractical method of syrup delivery. However, due to the special properties of the weak link, the atoms can flow freely across the link, preserving superfluidity. This doesn’t mean the link has no influence: when rotated around the ring, the weak link acts kind of like a laser ‘spoon’, ‘stirring’ the atoms and driving an atom current.
After stirring the ring of atoms, the scientists turn off all the lasers, allowing the two BECs to expand towards each other. Like ripples on a pond, these clouds interfere both constructively and destructively, forming intensity peaks and valleys. The researchers can use the resulting interference pattern to discern features of the system, a process called interferometry.
Gleaning useful data from an interference pattern means having a reference wave. In this case, the inner BEC serves as a phase reference. A way to think of phase is in the arrival of a new day. A person who lives on the other side of the planet from you experiences a new day at the same frequency as you do, once every 24 hours. However, the arrival of the day is offset in time, that is to say there is a phase difference between your day and the other person's day.
As the two BECs interfere, the position of the interference fringes (peaks in the wave) depends on the relative phase between the two condensates. If a current is present in the outer ring-shaped BEC, the relative phase is changing as a function of the position of the ring, and the interference fringes assume a spiral pattern. By tracing a single arm of the spiral a full 360 degrees and measuring the radial difference between the beginning and end of the trace, the researchers can extract the magnitude of the superfluid current present in the ring.
They now know the current, so what about the phase across the weak link? The same interferometry process can be applied to the two sides of the weak link, again yielding a phase difference. When coupled with the measured current, the scientists now have a measure of how much current flows through the weak link as a function of the phase difference across the link, the current-phase relationship. For their system, the group found this dependence to be roughly linear (in agreement with their model).
A different scenario, where the weak link has a smaller profile, might produce a different current response, one where non-linear effects play a larger role. Extending the same methods makes it possible to characterize these weak links as well, and could be used to verify a type of weak link called a Josephson junction, an important superconducting element, in a cold atom system. Characterizing the current-phase relationship of other atomtronic components should also be possible, broadening the capabilities of researchers to analyze and design new atomtronic systems.
This same lab, led by JQI fellow Gretchen Campbell, had recently employed a weak link to demonstrate hysteresis, an important property of many electronic systems, in a cold atom circuit. Better characterizing the weak link itself may help realize more complex circuits.  “We’re very excited about this technique,” Campbell says, “and hope that it will help us to design and understand more complicated systems in the future."
This article was written by S. Kelley/JQI.

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