Distant Galaxies Unlock New Secrets of Dark Matter
Clara Moskowitz, SPACE.com Staff Writer, SPACE.com
To weigh the universe, scientists use two kinds of cosmic scales: one to measure all the regular matter out there, and another to deduce how much invisible dark matter remains hidden underneath.
These calculations have been taken further than ever before by a new study that tallied both types of mass in smaller and more distant groups of galaxies than any previous projects. The project found that these faraway galactic clusters have roughly the same proportion of dark matter to regular matter as the closer galaxy groups do.
The findings could help astronomers understand more about dark matter, as well as its even stranger sibling – dark energy.
Dark matter is a form of stuff that does not interact with light, so cannot be seen, but makes its presence felt by exerting a gravitational pull on normal matter.
Astronomers measure how much dark matter lies in galaxies by a fluke of physics called gravitational lensing. This phenomenon, predicted by Einstein's theory of general relativity, causes light to curve as it flies through space-time that has been dented by the gravity of large bodies of mass.
For example, groups of massive galaxies will gravitationally warp the space-time around them, forcing light to bend as it passes through, and causing them to look distorted when their light reaches our telescopes. Scientists can tell how much total mass there is by how much of this distortion occurs.
Next, researchers calculate how much normal matter is in a cluster of galaxies by looking at its X-ray light, since the light must be coming from only the regular stars and gas that make up the cluster.
Comparing these two calculations — the total matter to just the regular matter — gives a ratio astronomers call the mass-luminosity relation. So far, the mass-luminosity relation has been measured well for nearby, large galaxy clusters, but there has not been good enough X-ray data to probe farther or smaller, dimmer clusters of galaxies.
"We can map out the big cities, but no one's been able to map out the villages yet," said Alexie Leauthaud of the Lawrence Berkeley National Laboratory in Berkeley, Calif., leader of the new study.
Astronomers used observations from the European Space Agency's XMM-Newton satellite and from NASA's Chandra satellite, as well as data from the Hubble Space Telescope's Cosmic Evolution Survey (COSMOS). These ultra-high resolution photos allowed the scientists to extend the mass-luminosity relation further than ever before.
With such dim objects, the gravitational lensing wasn't immediately apparent. So researchers used a statistical analysis to measure the orientation and shape of the galaxies to find small distortions due to so-called weak lensing.
They found that the same general ratio of dark matter to normal matter prevailed in these distant, small clusters as for nearby, larger clusters.
"We didn't know what to expect going down to lower masses or [farther distances], and we find this nice simple relationship," Leauthaud told SPACE.com. "Now the aim is to find out why we find this nice, simple relationship."
Dark energy enigma
The finding may help shed light on an even more bizarre aspect of the universe — dark energy. Dark energy is the name given to whatever mysterious force is causing the universe to accelerate as it expands.
"We want to try to understand the properties of dark energy," Leauthaud said. "One way to measure properties of dark energy is to measure the number of structures that have formed for a given amount of dark matter."
Dark energy basically works against gravity in a tug-of-war. While gravity constantly pulls mass inward, encouraging things to clump together and condense into smaller space, dark energy does the opposite. This force somehow pulls everything apart, causing everything in the universe to move away from everything else at ever-increasing speeds.
When mass clumps together enough to form galaxies, it means that gravity has won on those scales, helping things to stick together despite the pull of dark energy. So the more astronomers can measure when and how structures formed in the universe, the better they can understand just how far dark energy's pull reaches.
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五個假設有「暗物質」的理由 – A. Yoho
Five Reasons We Think Dark Matter Exists
No other idea explains even two of these.
Amanda Yoho, Scienceblogs, 08/14
Any recent article about the remaining mysteries of the Universe will include dark matter close to the very top of the list of unsolved problems. What is it? Where is it? And if it’s there, how do we measure it? These are important questions still at the forefront of research in Cosmology. But this elusive substance that affects the motion of our galaxy and is the reason that galaxies exist with the properties they have, has only been detected indirectly, and has yet to be measured via direct detection. Earlier this year, the most sensitive dark matter experiment to date, LUX, released its results showing no direct evidence for dark matter and failing to confirm potential detections by two groups of experiments, DAMA/Libra and CoGeNT and Super-CDMS.
Despite this, fellow scientists are pushing forward, determined to measure direct evidence of dark matter. The U.S. Department Of Energy and National Science Foundation are on board with this plan, as they recently announced a new round of funding for 3 upcoming dark matter experiments: LZ (the successor to LUX), SuperCDMS-SNOLAB, and ADMX-Gen2. So if we haven’t measured dark matter directly yet, what is keeping researchers on the scent and funding agencies interested?
The idea of dark matter is very well motivated by other observations. Completely independent cosmological and astrophysical phenomena that aren’t explained within other theoretical frameworks can be solved by the existence of dark matter alone. Here are five of the most compelling reasons we think* dark matter exists:
1.) Galaxy Clusters
Image credit: Paul Tankersley’s astrophotography, of the Coma Cluster of galaxies 321 million light-years away, via http://ptank.blogspot.com/2010/05/abell-1656.html.
Throughout space, astrophysical objects of all sizes swirl and orbit: planets revolve around our sun, stars orbit around our galactic center, and individual galaxies in groups whiz around themselves. To keep these objects tightly bound together, the gravitational pull felt by an object must be strong enough to balance the energy it has due to its motion. A fast-moving object with more kinetic energy is harder to keep gravitationally bound.
In 1933, Fritz Zwicky (below) was studying the nearest very large cluster of galaxies to us in space: the Coma cluster (above).
He used the virial theorem, an equation which relates the average kinetic energy of a system to it’s total potential energy, to infer the gravitational mass of the cluster. He then compared that to the mass inferred from the bright, luminous matter (stars and gas) in the galaxies. You’d expect those two numbers — gravitational mass and mass due to luminous matter — to match, wouldn’t you? But instead, he found that the mass from the luminous matter was not enough to keep the cluster bound, and was several times smaller than the inferred gravitational mass. Assuming that the luminous matter constituted all of the mass in each galaxy, they should have been flying apart! He thus coined the term “dark matter” for the material that must therefore be present, quietly holding the galaxy cluster tightly together.
2.) Galactic Rotation Curves
Similar evidence was observed within galaxies themselves. From standard Newtonian dynamics, we expect the velocity of stars to fall as you move from the near the center of mass of a galaxy to its outer edges. But when studying the Andromeda galaxy in the 1960s, Vera Rubin and Kent Ford found something very different: the velocity of stars remained approximately constant, regardless of how far they were from the galactic center.
This and many future observations of the velocities of stars in spiral galaxies hinted that the mass of the galaxy must not be entirely defined by the objects we could see with our telescopes, which Rubin and Ford presented at an American Astronomical Society meeting in 1975. If instead a large fraction of the galaxy’s mass resided in a diffuse dark matter ‘halo’ that extended well beyond the edges of the luminous matter, the observed galactic rotation curves could be explained.
3.) The Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is the earliest photograph of our Universe. The patterns that we see in observations of the CMB were set up by competition between two forces acting on matter; the force of gravity causing matter to fall inward and an outward pressure exerted by photons (or particles of light). This competition caused the photons and matter to oscillate into-and-out-of dense regions. But if the Universe consisted partially of dark matter in addition to normal matter, that pattern would be affected dramatically. The existence of dark matter leaves a characteristic imprint on CMB observations, as it clumps into dense regions and contributes to the gravitational collapse of matter, but is unaffected by the pressure from photons.
We can predict these oscillations in the CMB with and without dark matter, which we often present in the form of a power spectrum. The power spectrum of the CMB shows us the strength of oscillations at different sizes of the photons and matter. The Wilkinson Microwave Anisotropy Probe (WMAP) was the first instrument to measure the CMB power spectrum through the first peak of oscillations, and showed that the existence of dark matter is favored.
4.) The Bullet Cluster
In 2006, astronomers working on the Hubble Space Telescope and the Chandra X-ray Observatory released exciting information about an object known as the bullet cluster. This cluster is actually two galaxy clusters which have recently undergone a high-speed collision, forcing the contents of each cluster to merge together. Observations from the two telescopes allowed us to measure the location of the cluster mass after the collision using two methods: optical observations of X-ray emission and gravitational lensing.
One way we can tell two clusters have just collided is through X-ray astronomy. An extremely hot gas of particles pervades the space between each galaxy in a cluster, which accounts for for about 90% of the mass from ordinary matter (rather than stars). When two galaxy clusters collide, the gas particles become even hotter from crashing into each other, causing an increase in brightness of the X-ray emission. From this we can tell how energetic the gas is and where it is located.
Gravitational lensing occurs because matter isn’t the only thing that feels the effects of gravity: light does as well. This means that a massive object can act as a lens; a background source that emits light in all directions will have some of that light focused if it passes by a massive object. By measuring these focused images, we can infer the location and mass of the lens between us and the source.
If the clusters were entirely comprised of ordinary matter, the location of mass from the optical observations and the location calculated from gravitational lensing in the bullet cluster should overlap. Instead, the observations showed a glaring inconsistency. The optically visible matter told us the mass should be concentrated near the center of the image shown, highlighted in red. The mass distribution from gravitational lensing, highlighted in blue, shows that the concentration of mass is actually in two pieces, just outside of the luminous matter in the galaxy! Invoking dark matter, this behavior is easy to explain as follows:
a.) Dark matter interacts with its surroundings significantly less frequently than ordinary matter.
b.) During the cluster collision, the dark matter of one cluster would have slipped through all of the objects in the other cluster with relative ease.
c.) The luminous matter, on the other hand, would have bounced off of other particles around it, causing it to slow and separate from the dark matter.
The net result? High-velocity collisions between galaxy clusters should have the majority of their mass — in the form of dark matter — pass through one another unimpeded, while the normal matter collides, slows down, and heats up, emitting X-rays.
5.) Large-Scale Structure Formation
When telescopes like the Sloan Digital Sky Survey map the locations the galaxies in the Universe, with the biggest features being referred to as large-scale structure, it sees a set of patterns that couldn’t happen with only the gravity due to ordinary matter at work. We know that before the CMB, ordinary matter wasn’t able to efficiently clump into dense objects due to the oscillations from the competing forces of gravity and pressure from radiation. The structure we observe is much more advanced in its evolution given the amount of time available for objects to gravitationally collapse after the time of the CMB.
Instead, dark matter provides a reasonable explanation. Because dark matter didn’t undergo the same oscillations with matter and light, it was free to collapse on its own to form dense regions that helped structure formation get a head start, and allowed the distribution of galaxies and clusters to be what we observe today.
These five independent pieces of evidence, when taken all together, provide a compelling reason that dark matter must exist. Reading through each explanation again, there is a common theme: gravity. Each piece of the puzzle relies on the way dark matter affects things around it via the gravitational force.
If I had to place bets, my money would fully be on the “dark matter” square. At conferences and seminars, astronomers, astrophysicists, and cosmologists speak about dark matter as though it’s a certainty (and most think it is). So why do I say “five reasons we think dark matter exists”? Since we haven’t measured it directly yet, and the evidence for dark matter’s existence centers on its gravitational interactions, a responsible scientific community would ask “what if we just don’t understand gravity as well as we think we do?” Some research groups have been tackling that question, investigating theories like MOND (MOdified Newtonian Dynamics), which are often grouped together under the umbrella “modified gravity.” So far, these theories have had successes in describing one of these peculiarities: galactic rotation curves, but have not yet provided an explanation for the complete set of observations like dark matter does.
Modifying the theory of gravity is no easy game. We have fantastically precise measurements of gravity’s influence on objects throughout our solar system which fit precisely within the current understanding of gravity from General Relativity (a fact that underpins the precision of modern GPS). If you want to change the theory of gravity, you have to preserve its behavior as we’ve already measured it in the solar system. Further, the idea of modified gravity extends beyond trying to explain away dark matter. Modified gravity is an incredibly active field of research, with many ideas trying to explain the even more elusive phenomenon of dark energy. Often, these theories still require dark matter of some sort to exist.
But wait, there’s more!
These five reasons don’t constitute the total observational evidence we have for dark matter. Big Bang Nucleosynthesis (BBN), which explains the way light elements such as Helium were formed fractions of a second after the Big Bang, tells us abundance of baryonic matter doesn’t account for the total matter content of the Universe inferred from other observations, and that dark matter can’t be just be things like protons and neutrons. Observations of molecular clouds — neutral hydrogen gas — absorbing light from background galaxies and quasars, known as the Lyman-alpha forest, gives us information about the location of dark matter clumps as well as how much energy dark matter particles are allowed to have.
In almost every place we look, the Universe seems to be hinting that dark matter exists. The indirect evidence, from the early Universe to the present day, and from galactic scales up to the largest ones observable in the Universe, all point to the same conclusion. Direct detection is the next logical step. But that may be the biggest challenge of all: we still have to find it.
* “Think” here is used in a very scientific sense. We say “think” to mean “evidence strongly shows.” It is not meant in the same sense as something like “I think I turned the oven off…” or “I think that movie starred Nicolas Cage, but it could have been John Travolta.” “We think” means “we’re very sure, but we haven’t detected it yet so we can’t say ‘we know.’”
This article was written by Amanda Yoho, a graduate student in theoretical and computational cosmology at Case Western Reserve University. You can reach her on Twitter at @mandaYoho.
Have comments? Leave them at the Starts With A Bang forum on Scienceblogs!
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最新的暗物質理論模型 - The Unive. of Kansas
Researcher advances a new model for dark matter
The University of Kansas, 09/04/14
LAWRENCE -- Astrophysicists believe that about 80 percent of the substance of our universe is made up of mysterious “dark matter” that can’t be perceived by human senses or scientific instruments.
“Dark matter has not yet been detected in a lab. We infer about it from astronomical observations,” said Mikhail Medvedev, professor of physics and astronomy at the University of Kansas, who has just published breakthrough research on dark matter that merited the cover of Physical Review Letters, the world’s most prestigious journal of physics research.
Medvedev proposes a novel model of dark matter, dubbed “flavor-mixed multicomponent dark matter.”
“Dark matter is some unknown matter, most likely a new elementary particle or particles beyond the Standard Model,” Medvedev said. “It has never been observed directly, but it reveals itself via gravity it produces in the universe. There are numerous experiments around the world aimed at finding it directly.”
Medvedev’s theory rests on the behavior of elementary particles that have been observed or hypothesized. According to today’s prevalent Standard Model theory of particle physics, elementary particles -- categorized as varieties of quarks, leptons and gauge bosons -- are the building blocks of an atom. The properties, or “flavors,” of quarks and leptons are prone to change back and forth, because they can combine with each other in a phenomenon called flavor-mixing.
“In everyday life we’ve become used to the fact that each and every particle or an atom has a certain mass,” Medvedev said. “A flavor-mixed particle is weird -- it has several masses simultaneously -- and this leads to fascinating and unusual effects.”
Medvedev compared flavor-mixing to white light that contains several colors and can generate a rainbow.
“If white was a particular flavor, then red, green and blue would be different masses -- masses one, two and three -- that mix up together to create white,” he said. “By changing proportions of red, green and blue in the mix, one can make different colors, or flavors, other than white.”
Medvedev said that dark matter candidates are also theorized to be flavor-mixed -- such as neutralinos, axions and sterile neutrinos.
“These are, in fact, the most preferred candidates people speak about all the time,” Medvedev said.
“Previously we discovered that flavor-mixed particles can ‘quantum evaporate’ from a gravitational well if they are ‘shaken’ -- meaning they collide with another particle,” he said. “That's a remarkable result, as if a spacecraft made of flavor-mixed matter and hauled along a bumpy road puts itself into space without a rocket or any other means or effort by us.”
Medvedev included the physics process of quantum evaporation in a “cosmological numerical code” and performed simulations using supercomputers.
“Each simulation utilized over a 1,000 cores and ran for a week or so,” he said. “This yearlong project consumed about 2 million computer hours in total, which is equal to 230 years.”
Medvedev said that dark matter may interact with normal matter extremely weakly, which is why it hasn’t been revealed already in numerous ongoing direct detection experiments around the world. So physicists have devised a working model of completely collisionless (noninteracting), cold (that is, having very low thermal velocities) dark matter with a cosmological constant (the perplexing energy density found in the void of outer space), which they term the “Lambda-CDM model.”
But the model hasn’t always agreed with observational data, until Medvedev’s paper solved the theory’s long-standing and troublesome puzzles.
“Our results demonstrated that the flavor-mixed, two-component dark matter model resolved all the most pressing Lambda-CDM problems simultaneously,” said the KU researcher.
Medvedev performed the simulations using XSEDE high-performance computation facilities, primarily Trestles at the San Diego Supercomputer Center and Ranger at the Texas Advanced Computing Center.
The University of Kansas is a major comprehensive research and teaching university. The university's mission is to lift students and society by educating leaders, building healthy communities and making discoveries that change the world. The KU News Service is the central public relations office for the Lawrence campus.
The top photo, published in the Physics Review Letters, represents the distribution of dark matter in the universe computed within the two component flavor-mixed dark matter paradigm. The bottom photo, published in a previous paper, represents the effect of "quantum evaporation." (請至原網頁觀賞相關圖片)
天文學家無法解釋的新發現 - M. Cooper-White
This Mysterious Signal 'Could Not Be Explained By Known Physics,' Astronomers Say
Macrina Cooper-White, The Huffington Post, 07/28/14
When astronomers detected a strange signal in a massive galaxy cluster millions of light years from Earth, they knew they had stumbled upon something big.
"I couldn't believe my eyes," Esra Bulbul, of the Harvard Center for Astrophysics, said in a written statement. "What we found, at first glance, could not be explained by known physics."
Just check out the video above, released this week by Science@NASA, to learn more.
In 2012, Bulbul and her colleagues examined data collected by NASA's Chandra X-Ray Observatory on the Perseus Cluster, an array of thousands of galaxies in the constellation Perseus. The Perseus Cluster is surrounded by a cloud of superheated gas, which contains ions that each emit their own "line" in the x-ray spectrum.
When the astronomers analyzed the cluster's "spectral signature," they found a mysterious spike that they couldn't explain.
"A line appeared at 3.56 keV (kilo-electron volts) which does not correspond to any known atomic transition," Bulbul said in the statement. "I have re-analyzed the data; split the data set into different sub groups; and checked the data from four other detectors on board two different observatories. None of these efforts made the line disappear."
The astronomers believe the emission may result from the decay of so-called sterile neutrinos, hypothetical particles that may make up dark matter, a mysterious substance that constitutes 80 percent of the mass of universe.
“We know that the dark matter explanation is a long shot, but the pay-off would be huge if we're right,” Bulbul said in an earlier statement released by NASA last month. “So we're going to keep testing this interpretation and see where it takes us.”
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你不知道的暗物質研究現況 – K. Jepsen
Four things you might not know about dark matter
How much do you really know about dark matter? Symmetry looks at one of the biggest remaining mysteries in particle physics.
Kathryn Jepsen, The symmetry, 12/17/13
Not long after physicists on experiments at the Large Hadron Collider at CERN laboratory discovered the Higgs boson, CERN Director-General Rolf Heuer was asked, “What’s next?” One of the top priorities he named: figuring out dark matter.
Dark matter is five times more prevalent than ordinary matter. It seems to exist in clumps around the universe, forming a kind of scaffolding on which visible matter coalesces into galaxies. The nature of dark matter is unknown, but physicists have suggested that it, like visible matter, is made up of particles.
Dark matter shows up periodically in the media, often when an experiment has spotted a potential sign of it. But we are still waiting for that Nobel-Prize-triggering moment when scientists know they finally have it.
Here are four facts to get you up to speed on one of the most exciting topics in particle physics:
1. We have already discovered dark matter
At this moment, several experiments are on the hunt for dark matter. But scientists actually discovered its existence decades ago.
In the 1930s, astrophysicist Fritz Zwicky was observing the rotations of the galaxies that form the Coma cluster, a group of more than 1000 galaxies located more than 300 million light years from Earth. He estimated the mass of these galaxies, based on the light they emitted. He was surprised to find that, if this estimate were correct, at the speed at which the galaxies were moving, they should have flown apart. In fact, the cluster needed at least 400 times the mass he had calculated to hold itself together. Something mysterious seemed to have its finger on the scale; an unseen “dark” matter seemed to be adding to the mass of the galaxies.
The idea of dark matter was largely ignored until the 1970s, when astronomer Vera Rubin saw something that gave her the same thought. She was studying the velocity of stars moving around the center of the neighboring Andromeda galaxy. She anticipated that the stars at the edge of the galaxy would move more slowly than those at its axis because the stars closest to the bright -- and therefore massive -- cluster of stars in the center would feel the most gravitational pull. However, she found that stars on the margins of the galaxy moved just as quickly as those in the middle. This would make sense, she thought, if the disc of visible stars were surrounded by an even larger halo made of something she couldn’t see: something like dark matter.
Other astronomical observations have since confirmed that something strange is going on with the way galaxies and light move through space. It’s possible that our confusion stems from a flaw in our understanding of gravity -- Rubin herself said she favors this idea. However, if it’s true that dark matter exists, we’ve already seen its effects.
2. We have possibly already observed dark matter
Several experiments are searching for dark matter, and some of them may have even already found it. The problem is that no experiment has been able to make that claim with enough confidence to convince the wider scientific community -- either due to statistics or an inability to rule out alternative possible explanations. And no two claims have lined up quite convincingly enough for scientists to declare any result confirmed.
In 1998 scientists on the DAMA experiment, a dark matter detector buried in Italy’s Gran Sasso mountain, saw a promising pattern in their data. The rate at which the experiment detected hits from possible dark matter particles changed over the course of the year -- climbing to its peak in June and dipping to its nadir in December.
This was exactly what DAMA scientists were looking for. If our galaxy is surrounded by a dark matter halo, the Earth is constantly moving through that halo as it orbits the sun -- and the sun is constantly moving through the dark matter as it orbits the center of the Milky Way. During half of the year, the Earth is moving in the same direction as the sun. During the other half, it is moving in the opposite direction. When the Earth and the sun are moving in tandem, their combined velocity through the dark matter halo is faster than the Earth’s velocity when it and the sun are at odds. DAMA’s results seemed to reveal that the Earth really was moving through a dark matter halo.
However, some loopholes exist; the particles the DAMA detector has been seeing could be something other than dark matter, something else the Earth and sun are constantly moving through. Or something else could be changing in the nearby environment. The DAMA experiment, now called DAMA/LIBRA, has continued to see this annual modulation, but the results are not conclusive enough for most scientists to consider it a dark-matter discovery.
It’s going to be difficult for any one experiment to convince scientists that they’ve found dark matter. It might be that people will come around only when several experiments start to see the same thing. But that will depend on what they find, says theorist Neal Weiner, director of the Center for Cosmology and Particle Physics at New York University. Dark matter could turn out to be something stranger or more complicated than we expect.
“If dark matter turns out to be something totally garden-variety, then maybe it will only take one experiment for people to be excited about it -- and two for people to be borderline convinced,” he says. “But if something unexpected shows up, it might take more than that to persuade people.”
In 2008 the space-based PAMELA experiment detected an excess of positrons -- a possible result of dark matter particles colliding and annihilating one another. In 2013 the AMS-02 experiment, attached to the International Space Station, found the same result with even more certainty. But scientists remain unconvinced, arguing that the positrons could also come from pulsars.
Underground experiments -- including CoGeNT, XENON, CRESST, CDMS and LUX -- have gone back and forth supporting and disclaiming possible dark matter sightings. It seems we will need to wait until the upcoming generation of dark matter experiments is complete to get a clearer picture.
3. We don’t know what dark matter is like; there could be several kinds making up a whole “dark sector”
Scientists have come up with several models for what dark matter might be like. The current leading candidate is called a WIMP, a Weakly Interacting Massive Particle. Other possibilities include particles conveniently already predicted in models of supersymmetry, a theory that adds a new fundamental particle to correspond with each one we already know. Groups of scientists are also searching for dark-matter particles called axions.
But there’s no reason there should be only one type of dark matter particle. Visible matter, the quarks and gluons and electrons that make up all of us and everything we can see, along with an entire zoo of fundamental particles and forces including photons, neutrinos and Higgs bosons, makes up just 5 percent of the universe. The rest is dark matter -- which makes up about 23 percent -- and dark energy, a whole other story -- which claims the remaining 72 percent.
As Weiner puts it: Imagine a scientist in the dark-matter world trying to understand visible matter. Visible matter composes such a tiny fraction of what's out there; what dark-matter scientist would guess at its variety? The world we know is so diverse; why would dark matter be so simple? Scientists have wondered whether dark particles could combine into dark atoms that would interact through dark electromagnetism. Could dark chemistry be next? Scientists have begun to look for light dark-matter particles predicted in models of the “dark sector.”
4. Chances are good that we’ll observe dark matter in the next 5 to 10 years -- but we may never see it at all
These are heady times for a scientist searching for dark matter. With a number of different experimental ideas scheduled to come to fruition in the coming years, many predict that dark matter will be in our grasp within a decade.
“Really one of the exciting things is all of these techniques are coming to maturity at the same time,” says theorist Tim Tait of the University of California, Irvine. “It’s a great opportunity to play them against each other and see what’s going on.”
Scientists could find dark matter in a few different ways.
First, they could detect it directly. Direct detection involves waiting patiently with a big, sensitive experiment in a quiet, underground laboratory, as free as possible from potential interference from other particles. In the next few years, scientists will narrow down their current list of detector technologies to focus their resources on building the biggest, most sensitive generation of experiments yet.
The second way to find dark matter is to observe it indirectly -- by seeking out the effects of dark matter with space-based experiments. Updates from current experiments on satellites and the International Space Station will give scientists more data to help them determine the meaning of the possible dark-matter effects they have been seeing.
The third way to find dark matter is to produce it in an accelerator such as the Large Hadron Collider. It is possible that, when two particle beams collide in the LHC, their energy will convert into mass in the form of dark matter. The LHC is currently shut down for maintenance and upgrades, but when it restarts in 2015, it will reach almost double its previous energy, opening the door for it to make particles it has never made before.
Once scientists find dark matter using one method, they’ll be able to focus their efforts, Tait says. Once we know more about its properties, “that’ll really energize all of this activity,” he says. “Right now we’re in a dark room, fumbling around. Once you know where the thing you’re looking for is, you can study it a lot more carefully.”
But it is also possible that dark matter is out of our reach, simply too elusive to detect or produce. If scientists don’t see dark matter in the next 10 years, they might need to find a new way to look for it. Or they might need to reconsider what they know about gravity.
* Astrophysicist Fritz Zwicky calculated that the Coma cluster, one of the densest known galaxy clusters, needed to contain about 400 times its apparent mass--otherwise it would fly apart.
* The stars at the edge of the Andromeda galaxy seem like they should move more slowly than those in the center, but astronomer Vera Rubin discovered that they were traveling at about the same speed.
* The DAMA/LIBRA experiment sees an annual modulation in its signals from possible dark matter particles, which could be caused by Earth's rotation around the sun.
* The AMS experiment, attached to the International Space Station, this year found an excess of positrons that could point to dark matter.
* The LUX experiment is currently the world's most sensitive dark-matter detector.
* The LUX experiment recently released its first results, which contradicted another experiment's previous possible dark-matter detection.
* Scientists at experiments at the Large Hadron Collider hope to produce and detect dark matter particles.
The symmetry is a joint Fermilab/SLAC publication on dimensions of particle physics
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科學家積極探索物質何以存在 – N. Wolchover
Search Escalates for Key to Why Matter Exists
Natalie Wolchover, 10/15/13
It felt like the Apollo control room seconds before the moon landing. For the approximately 60 physicists crowded into a conference room at the Joint Institute for Nuclear Research in Dubna, Russia, on June 14, this was the moment of truth. After nearly a decade of work, the result of their painstaking search for one of the rarest radioactive decay processes in the universe -- if it exists -- was about to be revealed.
The hunting grounds were 15 kilograms of pure Germanium crystals kept in extreme isolation deep under a mountain in Italy. Members of the GERmanium Detector Array (GERDA) Collaboration had monitored electrical activity inside the crystals hoping to detect “neutrino-less double beta decay,” a spontaneous reshuffling of particles inside the nucleus of a Germanium-76 atom that would recast it as Selenium-76. The chemical decay could present a solution to one of the biggest mysteries in physics:
why there is something rather than nothing in the universe.
Among the bedlam of electrical activity caused by other types of decays, detector noise and rogue radiation, the physicists expected their instruments to pick up two or three spikes of background noise closely resembling the spikes from neutrino-less double beta decay. But they needed a stronger signal -- eight or 10 spikes -- to be convinced that they had really detected it.
On a large screen at the front of the room, the answer appeared: three spikes. “As soon as we saw the number, it was clear there was no signal,” said Allen Caldwell, director of the Max Planck Institute for Physics in Munich and a member of the GERDA Collaboration. But the negative finding was still a victory. Previous searches for neutrino-less double beta decay had been fouled by uncontrolled background noise. GERDA’s extreme sensitivity and spot-on background estimate allowed the researchers to definitively rule out a signal. “Everybody had their cameras out and was taking pictures of the screen and slapping each other on the back,” Caldwell said.
The null result, reported Sept. 19 in Physical Review Letters, indicates that it takes at least 30 trillion trillion years -- two thousand trillion times the age of the universe -- for half of the Germanium-76 atoms in a sample to undergo the decay, if they do it at all. If the “half-life” were much shorter, GERDA would have detected a signal. Because a longer half-life means a rarer decay, the scientists now know they need to monitor a larger sample of Germanium.
“It’s always difficult to convey why a negative result is an exciting result,” said Stefan Schönert, a physicist at Technical University of Munich and spokesman for the GERDA Collaboration. But it’s simple, he said: “Our experiment worked.”
Out of the Void
According to the Standard Model of particle physics, the universe should be empty. Matter and antimatter, which are identical except for their opposite electric charges, seem to be produced in equal parts during particle interactions and decays. However, matter and antimatter instantly annihilate each other upon contact, and so equal amounts of each would have meant a wholesale annihilation of both shortly after the Big Bang. The existence of galaxies, planets and people illustrates that somehow, a small surplus of matter survived this canceling process. If that hadn’t happened, “the universe would be void,” Schönert said. “It would be very, very boring for us, who would not exist.”
The explanation for the survival of some matter may lie in subatomic particles called neutrinos. These particles might have a special property that would give rise to neutrino-less double beta decay.
When an atom undergoes one type of beta decay, a neutron inside its nucleus spontaneously transforms into a proton, electron and antineutrino (the antimatter counterpart of the neutrino); in a type of inverse beta decay, the neutron absorbs a neutrino and morphs into a proton and electron.
Neutrinos oscillate between three flavors: electron, muon and tau, each with a combination of three unique masses. If neutrinos are Majorana particles, then each of the flavors is its own antiparticle.
In neutrino-less double beta decay, both processes would happen in tandem: The antineutrino produced by the first type of decay would serve as the neutrino that enters into the second. Such a dual reaction can occur only if neutrinos and antineutrinos are one and the same particle, as the Italian physicist Ettore Majorana hypothesized in 1937. Because neutrinos are electrically neutral, nothing forbids them from being “Majorana particles,” or both matter and antimatter at once.
“It seems natural that the neutrino is its own antiparticle,” said Bernhard Schwingenheuer, a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. “And in this case, neutrino-less double beta decay should exist.”
If the decay does exist, proving neutrinos are Majorana particles, this could explain the matter-antimatter asymmetry.
A widely supported hypothesis called the seesaw mechanism predicts that Majorana neutrinos would come in two varieties: the lightweight ones observed today and heavy ones that could have subsisted only in a high-energy environment like the newborn universe. (Their masses have an inverse relationship, like two sides of a seesaw.) The theory was originally developed to explain why neutrinos are far less massive than the other particles of the Standard Model, but it also suggests a means for the surplus of matter.
A fraction of a second after the Big Bang, those primordial heavy neutrinos would have undergone a process known as leptogenesis: Calculations show they would have decayed asymmetrically, generating slightly fewer leptons (electrons, muons and tau particles) than antileptons. By a conventional Standard Model process, the antilepton excess would then have cascaded into a one-part-per-billion excess of baryons (protons and neutrons) over antibaryons. “The baryons and antibaryons annihilated each other, and then the tiny imbalance left over is the matter we have today,” Caldwell said.
“If the neutrino is its own antiparticle, then the so-called leptogensis mechanism to explain the matter-antimatter asymmetry will be very plausible,” Schwingenheuer said.
Although alternative theories exist, it’s the most popular, straightforward, economical way to explain the asymmetry, the physicists said. And it would get a huge boost from eight or 10 electrical spikes of experimental evidence.
New Life for Decay
Physicists recognized more than half a century ago that observing neutrino-less double beta decay would prove that neutrinos are Majorana particles. But until the late 1990s, they “simply had very little idea of where to look,” said Alan Poon, a neutrino physicist at Lawrence Berkeley National Laboratory. They knew the decay could occur in an isotope like Germanium-76, which packs more energy in its nucleus than the isotope it would become, two spots over on the periodic table. But they had no idea how rare the decay might be, and consequently, how much Germanium they had to monitor or for how long. Without a range of possibilities for the half-life of the decay, their task felt like searching for “treasure at the bottom of the Atlantic,” Poon said, an ordeal made worse by the possibility that there might be nothing to find.
The half-life in Germanium-76 and other isotopes can be calculated from the mass of the lightweight neutrinos. Experiments over the past two decades have shown that these neutrinos oscillate between three “flavors” -- electron, muon and tau -- each with its own combination of three unique masses. Although the masses themselves are unknown, the rate of the oscillations determines the possible differences between them. These in turn dictate three possible ranges for the half-life of neutrino-less double beta decay, stretching between a few trillion trillion and a few thousand trillion trillion years. It’s a vast and remote range, but finite.
In July, the T2K experiment made the first definitive observation of a neutrino oscillating from one flavor to another. Detectors picked up rings of radiation, right, emitted when passing neutrinos struck water inside a giant underground tank in Japan.
“Neutrino oscillations put a light at the end of the tunnel,” Schönert said.
The results from GERDA -- one of the most sensitive searches for neutrino-less double beta decay to date --indicated that the half-life range must start at a higher point. The outcome corroborates recent results by the EXO-200 and KamLAND-Zen experiments that together put a lower limit on the half-life of the decay in Xenon-136, another isotope that may exhibit the decay, at 34 trillion trillion years. The physicists in these various collaborations can now continue methodically working their way through the range of possible half-lives.
The longer the half-life is, the rarer the decay and thus the more atoms must be monitored to see it. The upgraded GERDA Phase II experiment will begin collecting data from 40 kilograms of Germanium early next year; the decay should be seen by the end of the three-year run if its half-life is less than 100 trillion trillion years. Several more searches, including the U.S.-based Majorana Demonstrator experiment, are under construction, and a next generation of even more sensitive searches is planned. Bigger samples mean more background noise, and so each new experiment must be even more stringently controlled than the last.
Most neutrino physicists expect to eventually find the decay. “There’s this prejudice because of the beauty of the theory of Majorana neutrinos,” said Schönert, “but no guarantee that this is the true story.”
This article has been reprinted on ScientificAmerican.com.
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搜尋暗物質的結果在望？ - The Economist
The hunt for the missing 85% of matter in the universe is closing in on its quarry
The Economist, 04/06/13
IF YOU thought the Higgs boson was elusive, consider the case of dark matter. The Higgs -- the particle that gives other subatomic species mass -- was predicted in 1964 but actually nabbed only last year. That 48-year hunt, though, was a breeze compared with the one for dark matter. Physicists have known the stuff must exist since 1933, when Fritz Zwicky, a Swiss astro-physicist, coined the term to describe a substance which cannot be seen but without which visible galaxies would fly apart as they rotate. The latest results from the European Space Agency’s Planck satellite suggest it makes up 85% of all the matter in the universe (up from an earlier estimate of around 80%).
Like the Higgs boson, though, the actual particles of which dark matter is composed have proved elusive. Eight decades after Zwicky’s observations, and dozens of experiments later, they remain undetected. But on April 3rd an experiment called the Alpha Magnetic Spectrometer (AMS) offered the most tantalising hints yet.
Although Samuel Ting, the Nobel laureate who heads the effort, presented the findings at CERN (Europe’s, and the world’s, principal particle-physics laboratory), they did not stem from CERN’s own accelerators hidden beneath the Franco-Swiss countryside outside Geneva. In fact, they did not hail from Earth at all, for AMS sits on board the International Space Station (ISS), and is arguably the only piece of scientifically useful kit ever to grace that $100 billion contraption.
Like its ground-based cousins at CERN, AMS consists of a large magnet and an array of sensors to track a charged particle’s path. Unlike them, the particles it tracks are not the product of smashing things together in the Large Hadron Collider (LHC), humanity’s biggest particle accelerator. Instead, AMS uses the most powerful accelerator of all: the universe itself.
A matter of fact?
Space may look empty, but it is in fact abuzz with particles produced in an assortment of astrophysical processes, and known collectively as cosmic rays. One process of particular interest to dark-matter hunters involves hypothetical particles called neutralinos. These are predicted by supersymmetry, a theory which removes mathematically inelegant fiddle factors from the Standard Model, the reigning rule book of particle physics, by doubling the number of species in the particle zoo.
Neutralinos are the lightest of the predicted supersymmetric beasts, with a mass equivalent to that of a few hundred protons (the Higgs, by comparison, has a mass of about 124 protons). They do not interact with light, and are therefore invisible. They are also stable enough to stick around in space for a long time. Just the sort of properties, in other words, that dark matter is thought to possess.
To physicists’ chagrin, attempts to conjure neutralinos from the LHC have failed. But if they do exist -- and make up the bulk of dark matter in the cosmos -- they ought to leave traces that AMS can detect.
When two neutralinos bump into each other, the theory goes, they should annihilate one another and produce in their stead an electron and its antimatter equivalent, a positron. Since, as Albert Einstein showed, mass and energy are one and the same, and because electrons and positrons are equal and opposite, each carries precisely as much energy as one neutralino has mass. It is these high-energy electrons and positrons that AMS is on the lookout for.
The problem is spotting them against a backdrop of electrons from other cosmic sources, which are much more common than positrons are. To get round this, AMS examines how the ratio of positrons to electrons varies with the particles’ energy. At low energies, cosmic-ray electrons from other sources dominate. If high-energy cosmic positrons do indeed come mainly from dark-matter annihilation, however, then the “positron fraction” should rise with energy, and peak when it reaches the mass of a neutralino. Beyond that peak, the fraction should plummet, because few high-energy positrons from other sources would be expected to exist, whereas energetic electrons are abundant.
In the 18 months following AMS’s delivery to the ISS by the space shuttle Endeavour in May 2011, it recorded the passage of 30 billion cosmic rays. These included 6.4m electrons and 400,000 positrons that had energies ranging from 0.5 to 350 giga-electron-volts (GeV), measured in the esoteric units particle physicists like to use. These data show that the positron fraction does indeed rise with energy, just as theory predicts. As important, the same pattern is visible wherever AMS happened to be pointing as it orbited Earth.
This sits nicely with the notion that dark matter is strewn more or less evenly across the universe. At the same time it excludes another possible source of the particles: random cosmic events like exploding stars, which would not be so uniformly distributed, at least not over a period as cosmically brief as 1½ years.
Unfortunately, more data are needed to rule out a third possibility: that the observed particles were created by pulsars, the remnants of these stellar explosions. At the moment, AMS has not seen enough electrons and positrons with energies above 350 GeV to draw meaningful conclusions about them. If pulsars are responsible, theory predicts that the positron fraction should decline steadily at energies above this value. If neutralinos are responsible, though, at some point -- corresponding to the energy equivalent of their mass -- the positron fraction will fall off a cliff. Approximately, one proton mass corresponds to one GeV, so this could happen soon. But the theory of supersymmetry does not vouchsafe exactly what a neutralino’s mass should be, so it might not. The “few hundred” protons may turn out to be nearer 1,000. Collecting enough high-energy electrons and positrons to test that will take quite a long time.
Fortunately, AMS is in it for the long term. It is designed to last for another 20 years or so. That means (assuming the space station is not, as is currently planned, abandoned as being too costly to maintain) it may still be delivering results on the centenary of Zwicky’s discovery. But if neutralinos take that long to find, the hunt for the Higgs really will look like a doddle by comparison.
The Economist, Science and Technology Section
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暗物質更加撲朔迷離 – C. Moskowitz
Dark Matter Gets Darker: New Measurements Confound Scientists
Clara Moskowitz, SPACE.com, 10/24/11
New measurements of tiny galaxies contradict scientists' best model of dark matter, further complicating the already mysterious picture of the stuff that is thought to make up 98 percent of all matter in the universe.
Dark matter, the invisible material thought to permeate the universe, can only be indirectly detected through its gravitational pull on the normal matter that makes up stars and planets.
Despite not knowing exactly what dark matter is, scientists have gradually built up a good model to describe its behavior. The model envisions dark matter made up of cold, slow-moving exotic particles that clump together because of gravity.
This "cold dark matter" model has done remarkably well describing how dark matter behaves in most situations. However, it breaks down when applied to mini "dwarf galaxies," where dark matter appears more spread out than it should be, according to the theory.
In a new study, researchers calculated the mass distribution of two dwarf galaxies using a new method that did not rely on any dark matter theories. The scientists studied the Fornax and Sculptor galaxies, which orbit the Milky Way.
However, their measurements still contradict cold dark matter theory, further entrenching the problem. [Infographic Gallery: The History and Structure of the Universe]
According to the model, the centers of galaxies should be packed with dense clumps of the invisible matter. But dark matter appears to be spread evenly throughout Fornax and Sculptor, as well as other dwarf galaxies whose mass distributions have been measured in other ways.
"If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter 'pit' at the center," researcher Jorge Peñarrubia of England's University of Cambridge said in a statement. "Instead, the first two dwarf galaxies we studied are like pitless peaches."
The measurements suggest that some part of the theoretical model may have to be revised.
"Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies," said study leader Matt Walker of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data."
Dwarf galaxies like Fornax and Sculptor are especially good places to study dark matter, because they are thought to be almost entirely made up of the stuff. Only one percent of matter in a dwarf galaxy is thought to be the normal matter that makes up stars.
To determine where and how much dark matter inhabits the dwarf galaxies, the researchers studied the motions of 1,500 to 2,500 visible stars, which reflect the gravitational forces acting on them from dark matter.
Some researchers have suggested that when dark matter interacts with normal matter it may tend to spread out, thus decreasing the density of dark matter in the centers of galaxies. However, so far, the cold dark matter model doesn't predict this.
Either normal matter affects dark matter more than scientists thought, or it isn't cold and slow-moving, the researchers said.
"After completing this study, we know less about dark matter than we did before," Walker said.
The findings will be published in an upcoming issue of The Astrophysical Journal.
You can follow SPACE.com senior writer Clara Moskowitz on Twitter @ClaraMoskowitz. Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.
· 7 Surprising Things About the Universe
· Sifting Through the Cosmic Sands for Dark Matter
· Top 10 Strangest Things in Space
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暗物質暗藏在地球上？ -- C. Moskowitz
Elusive Dark Matter May Be Hidden on Earth
Clara Moskowitz, SPACE.com Staff Writer, SPACE.com
Scientists are hot on the tail of one of nature's most elusive substances, the mysterious dark matter that is thought to make up the bulk of the universe. Many scientists think dark matter might even be hiding right under our noses here on Earth.
Dark matter is especially tricky to find because of its dark nature. In fact, scientists don't know what it is. It doesn't emit or reflect any light, so the most powerful telescopes have no hope of spying it directly. It has been thought to exist since the 1970s based on observations of gravity's effects on large-scales, such as among and between galaxies – regular matter can't account for the amount of gravity at work.
And dark matter doesn't often interact with most other matter, scientists theorize. One idea is that it flies right through the Earth, your house, and your body without bouncing off atoms.
Some scientists have taken to underground searches in hopes of catching just a few out of the multitude of dark matter particles in a rare instance of actually bouncing off of a regular particle.
"They're just streaming right though us, and every once in while there's an interaction," said Angela Reisseter of the University of Minnesota, a member of a project called the Cryogenic Dark Matter Search (CDMS). She spoke this month at the meeting of the American Physical Society in Washington, D.C.
Dark matter detected...or was it?
In a recent issue of the journal Science Express, Reisseter and her colleagues reported finding two possible events that may or may not be dark matter impacts on their detectors.
"Our previous results have been no, no, no," Reisseter said. "This is our first maybe – that is all it is."
The CDMS is buried in a mine in Minnesota underneath about 766 yards (700 meters) of rock, plastic, lead, copper and other materials designed to stop everything but dark matter from reaching the experiment — thus cosmic rays and other particles that might be confused for dark matter particles will mostly be eliminated.
The detectors themselves are basically small, hockey-puck shaped blocks of the elements germanium and silicon. If the nucleus from one of the germanium or silicon atoms is hit by a dark matter particle, then it will rebound and send a signal to the detector.
However the researchers can't be absolutely sure that the two signals they measured were dark matter and not some other particle, which they call background. Two signals is just too few to be confident of, they said, because their calculations predicted about one false event from background.
"If it was one, we'd say 'Oh, it's the background.' If it was three you start to say 'Oh, it's a signal,'" Reisseter said. "We can't call it background and we can't call it signal."
The CDMS team intends to keep running their experiment at ever more sensitive levels in hopes that a more substantial signal is discovered.
Dark matter hunt goes on
Other attempts to track down dark matter on Earth have focused on powerful particle accelerators that speed up subatomic particles to close to the speed of light and then smash them together, hoping the incredibly high collision energies create exotic particles, including dark matter.
But even with our increasingly powerful atom smashers, no sign of dark matter has yet been spotted.
"You have to ask why would this be?" said Sarah Eno of the University of Maryland. "Why would the particle that makes up most of the matter in the universe never have been seen in our accelerators?"
One reason could be that they just aren't powerful enough. Scientists aren't sure how massive the dark matter particle might be, and certain possibilities require extremely high energies to create them in the laboratory. Or it might be impossible find at any accelerator.
"We don't know for a fact that the dark mater particle is a particle we would be able to produce and detect," Eno said.
The best hope now could be a new particle accelerator — the Large Hadron Collider (LHC) near Geneva, Switzerland — that is the largest ever built. It opened recently and isn't yet running at full speed. When it does, many are holding out hope that dark matter will finally be pinned down.
"It could be that now we have this new machine we'll finally have enough energy to make this dark matter particle and see it in our collisions," Eno said. She is a member of the Compact Muon Solenoid experiment team at the LHC.
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