关于暗物质的进展

夏凉 · 发表于 2010-7-19 00:04:40

新浪科技讯北京时间7月13日消息，据国外媒体报道，一项最新研究显示，太阳也许是网罗暗物质的大网。如果暗物质恰好具有某种特定形态，它将能够在这颗距离我们最近的恒星内部积聚，并以一种能被我们观测到的形式改变热量在太阳内部的传递方式。

　　暗物质是一种神秘的物质，它构成宇宙中物质总量的83%，但却不和任何电磁力发生作用。虽然暗物质的量要比我们通常意义上的常规物质多5倍，但人类的肉眼以及任何一种已发明的望远镜都无法看到它。物理学家之所以知道它的存在，完全是因为它对于常规物质施加的引力影响。暗物质使星系快速旋转而不致被自身离心力撕裂，并且对诸多宇宙大尺度结构的形成产生影响。

　　目前的暗物质探测器所寻找的目标是弱相互作用重粒子(WIMP)，它仅和弱核力和引力作用有关。依据广为接受的理论，大多数实验设备都旨在寻找一种较质子质量大100倍左右的粒子。但事实上另一类粒子也不能排除，那就是弱相互作用重粒子的反粒子，当两颗弱相互作用重粒子相遇，他们将发生湮灭反应，消失无踪。

　　“这是一个常常困扰我的问题”，来自牛津大学的宇宙粒子物理学家苏比尔·萨卡尔(Subir Sarkar)说。如果大爆炸时产生了相同数量的物质和反物质，它们应当早已经“相互消灭”了。“很显然这并没有发生，我们在这里就是最好的证明”，他说，“因此必定有某种机制使物质产生的量胜过了反物质的量，从而使得在反物质全部消失之后还能有一小部分物质幸存下来。”

　　萨卡尔认为，不管是什么机制，既然它对物质战胜反物质产生影响，那也应当对暗物质起到同样的作用。如果暗物质的演化历程和常规物质类似，那么它应当要比现在实验预料的质量要轻的多，大约仅有5个质子质量。这是一个非常有提示性的数字，萨卡尔说。“如果它(指暗物质)的质量增加5倍，那么其丰度也会增加5倍，这就是暗物质，”他说道，“这就是在我看来对暗物质最简单的解释”。

　　但问题在于，这些更轻的粒子更难使用现有实验设备进行探测。在发表于7月2日的《物理评论快报》上的一篇论文中，萨卡尔及其牛津大学的同事麦兹·弗兰德森(Mads Frandsen)提出了另一种寻找这种更轻暗物质的方法：去太阳里找。

　　因为较轻质量的暗物质相遇时不会相互湮灭，因此太阳应当可以收集到很多粒子，就如同滚雪球可以越滚越大一样。“太阳已经绕着银河系旋转了50亿年了，他会在运行过程中吸附很多的暗物质”，萨卡尔说。暗物质的集聚可以解决太阳物理中的一个困惑，即“太阳组分问题”。对太阳表面震动的精密观测显示太阳内部热量传导至表面所用的时间要比标准模型预计的时间短。而仅和同类粒子发生反应的暗物质的参与也许可以解释这一现象。常规物质的光子和粒子在它们向太阳表层运动时会相互碰撞，因此光和热要花上数十亿年的时间才能逃离太阳。但对暗物质而言，其他常规物质都虚若无物，因此它们在往太阳表层运动时遇到的阻力就小，也因此能更有效的传递热量。“当我们进行计算时，我们惊喜的发现这是正确的”，萨卡尔说，“暗物质可以传递足够的热量，从而解决太阳组分问题”。

　　接下来，萨卡尔和弗兰德森计算了充斥暗物质的假设会对太阳释放出的中微子数量产生何种影响。他们发现中微子的流量将出现数个百分点的变化。这并不大，萨卡尔说，但却足以被两个不同的中微子探测器探测到——一个在意大利，叫“太阳中微子实验”(Borexino)，另一个位于加拿大，叫萨德伯里中微子天文台(SNO+)——它们不久即将投入运行。“这是一个猜想，但却是可以验证的”，萨卡尔说。“而且用来验证它的设备很快就要完工了，我们不必为此等上20年。””关于轻质量暗物质影响太阳行为的想法“在我看来，并不十分离谱”，来自伊利诺伊州费米国家实验室的物理学家丹·霍普( Dan Hooper)说。“我看了他们的数据，看起来很不错。”

　　来自暗物质探测器的一些令人困惑的结果暗示这些轻质暗物质也许已经被探测到了。今年早些时候，明尼苏达州一个矿井中的设备：相干锗中微子技术 (CoGeNT)探测到一个7倍于质子质量的粒子信号，虽然他们目前还不能确定这是否是暗物质。而另一个位于意大利的设备“暗物质”(DAMA)也报告了类似的结果。

　　“有说服力的证据正在不断累积”，那就是暗物质仅仅是几倍于质子质量的粒子，霍普说。“现在还不能下定论，但如果这些数据是正确的，也许明年我们就能更有把握一些了。”

　　最新数据：常规的物质占宇宙物质密度的5%，暗物质占25%(5倍于常规物质)。剩下的70%是暗能量。

夏凉 · 发表于 2010-8-3 02:21:22

Dark Fluid: Dark Matter And Dark Energy May Be Two Faces Of Same Coin

ScienceDaily (Feb. 1, 2008) — Astronomers at the University of St Andrews believe they can "simplify the dark side of the universe" by shedding new light on two of its mysterious constituents.

Dr HongSheng Zhao, of the University's School of Physics and Astronomy, has shown that the puzzling dark matter and its counterpart dark energy may be more closely linked than was previously thought.

Only 4% of the universe is made of known material - the other 96% is traditionally labelled into two sectors, dark matter and dark energy.

A British astrophysicist and Advanced Fellow of the UK's Science and Technology Facilities Council, Dr Zhao points out, "Both dark matter and dark energy could be two faces of the same coin.

"As astronomers gain understanding of the subtle effects of dark energy in galaxies in the future, we will solve the mystery of astronomical dark matter at the same time. "

Astronomers believe that both the universe and galaxies are held together by the gravitational attraction of a huge amount of unseen material, first noted by the Swiss astronomer Fritz Zwicky in 1933, and now commonly referred to as dark matter.

Dr Zhao reports that, "Dark energy has already revealed its presence by masking as dark matter 60 years ago if we accept that dark matter and dark energy are linked phenomena that share a common origin."

In Dr Zhao's model, dark energy and dark matter are simply different manifestations of the same thing, which he has considered as a 'dark fluid'. On the scale of galaxies, this dark fluid behaves like matter and on the scale of the Universe overall as dark energy, driving the expansion of the Universe. Importantly, his model, unlike some similar work, is detailed enough to produce the same 3:1 ratio of dark energy to dark matter as is predicted by cosmologists.

Efforts are currently underway to hunt for very massive dark-matter particles with a variety of experiments. The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Geneva is a particle accelerator that amongst other objectives, could potentially detect dark matter particles.

According to Dr Zhao, these efforts could turn out to be fruitless. He said, "In this simpler picture of universe, the dark matter would be at a surprisingly low energy scale, too low to be probed by upcoming Large Hadron Collider.

"The search for dark-matter particles so far has concentrated on highly-energetic particles. If dark matter however is a twin phenomenon of dark energy, it will not show up at instruments like the LHC, but has been seen over and over again in galaxies by astronomers."

However, the Universe might be absent of dark-matter particles at all. The findings of Dr Zhao are also compatible with an interpretation of the dark component as a modification of the law of gravity rather than particles or energy.

Dr Zhao concluded. "No matter what dark matter and dark energy are, these two phenomena are likely not independent of each other."

Background

Theories of the physics of gravity were first developed by Isaac Newton in 1687 and refined by Albert Einstein’s theory of General Relativity in 1916 which stated that the speed of gravity is equal to the speed of light. However, Einstein was never fully decided on whether his equation should add an omnipresent constant source, now called dark energy in general.

Astronomers following Fred Zwicky have also speculated additional sources to Einstein's equation in the form of non-light emitting material, called dark matter in general. Apart from very light neutrinos neither dark sources have been confirmed experimentally.

Dr Zhao and his collaborators' findings have recently been published by Astrophysical Journal Letters in December 2007, and Physics Review D. 2007.

夏凉 · 发表于 2010-8-3 03:37:26

Did 'Dark Gulping' Generate Black Holes In Early Universe?

ScienceDaily (Apr. 23, 2009) — A process called ‘dark gulping’ may solve the mystery of the how supermassive black holes were able to form when the Universe was less than a billion years old.
Dr Curtis Saxton will be presenting the study at the European Week of Astronomy and Space Science at the University of Hertfordshire in Hatfield.

Dr Saxton and Professor Kinwah Wu, both of UCL’s Mullard Space Science Laboratory, developed a model to study the gravitational interactions between the invisible halo of dark matter in a cluster of galaxies and the gas embedded in the dark matter halo.  They found that the interactions cause the dark matter to form a compact central mass, which can be gravitationally unstable, depending on the thermal properties of the dark matter.  If the cluster is disturbed, the dark matter central mass would undergo a very rapid collapse, without a trace of electro-magnetic radiation being emitted.  This fast dynamical collapse of the unstable dark-matter is called dark gulping.

The affected dark mass in the compact core is compatible with the scale of supermassive black holes in galaxies today.  There are several theories for how supermassive black holes form: one possibility is that a single large gas cloud collapses, another is that a black hole formed by the collapse of a giant star swallows up enormous amounts of matter; another possibility is that a cluster of small black holes merge together. However, all these options take many millions of years and are at odds with recent observations that suggest that black holes were present when the Universe was less than a billion years old.  Dark gulping may provide a solution to how the slowness of gas accretion was circumvented, enabling the rapid emergence of giant black holes.

“Dark matter appears to gravitationally dominate the dynamics of galaxies and galaxy clusters.  However, there is still a great deal of conjecture about origin, properties and distribution of dark particles.  We can only be certain that dark matter is non-interactive with light, but it interacts with ordinary matter via gravity.  Previous studies have ignored the interaction between gas and the dark matter but, by factoring it into our model, we’ve achieved a much more realistic picture that fits better with observations and may also have gained some insight into the presence of early supermassive black holes,” said Dr Saxton.

According to the model, the development of a compact mass at the core is inevitable. Cooling by the gas causes it to flow gently in towards the centre. The gas can be up to 10 million degrees at the outskirts of the halos, which are few million light years in diameter, with a cooler zone towards the core, which surrounds a warmer interior a few thousand light years across. The gas doesn't cool indefinitely, but reaches a minimum temperature, which fits well with X-ray observations of galaxy clusters.

The model also investigates how many dimensions the dark particles move in, as these determine the rate at which the dark halo expands and absorbs and emits heat, and ultimately affect the distribution of dark mass the system.

“In the context of our model, the observed core sizes of galaxy cluster halos and the observed range of giant black hole masses imply that dark matter particles have between seven and ten degrees of freedom,” said Dr Saxton.  “With more than six, the inner region of the dark matter approaches the threshold of gravitational instability, opening up the possibility of dark gulping taking place.”

The findings have been published in the Monthly Notices of the Royal Astronomical Society.

夏凉 · 发表于 2010-8-3 03:38:25

New Evidence for Cold Dark Matter

Posted in: Uncategorized by Fraser Cain (Comments Off)
A new image taken by the Chandra X-ray observatory is helping astronomers to understand the composition of dark matter in the Universe. Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas – and a mass of dark matter equal to a hundred trillion Suns. The X-ray data shows that the density of the dark matter increases smoothly all the way to the centre of the galaxy, which matches predictions of the "cold dark matter" model. This model gets its name from the assumption that the dark matter particles were moving slowly when galaxies first formed, and interact with normal matter only through gravity.

Astronomers have used NASA's Chandra X-ray Observatory to make the most detailed probe yet of the distribution of dark matter in a massive cluster of galaxies. Their results indicate that about 80 percent of the matter in the universe consists of cold dark matter – mysterious subatomic particles left over from the dense early universe.

Chandra observed a cluster of galaxies called Abell 2029 located about a billion light years from Earth. The cluster is composed of thousands of galaxies enveloped in a gigantic cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. The X-ray data show that the density of dark matter increases smoothly all the way into the central galaxy of the cluster. This discovery agrees with the predictions of cold dark matter models, and is contrary to other dark matter models that predict a leveling off of the amount of dark matter in the center of the cluster.

"I was really surprised at how well we could measure the dark matter so deep into the core of a rich cluster," said Aaron Lewis of the University of California, Irvine, lead author of a paper describing the results in a recent issue of The Astrophysical Journal. "We still have very little idea as to the exact nature of these particles, but our results show that they must behave like cold dark matter."

Cold dark matter gets its name from the assumption that the dark matter particles were moving slowly when galaxies and galaxy clusters began to form. Dark matter particles interact with each other and "normal" matter only through gravity.

The astronomers' success in placing such tight constraints on the dark matter distribution was partly due to Chandra's ability to make a high resolution intensity and temperature map, and partly due to their choice of a target. The cluster and central galaxy are unusually regular, with little or no sign of disturbance.

The hot gas in a cluster is held in the cluster primarily by the gravity of the dark matter, so the distribution of the hot gas is determined by that of the dark matter. By precisely measuring the distribution of X-rays from the hot gas, the astronomers were able to make the best measurement yet of the distribution of dark matter in the inner region of a galaxy cluster.

"While Abell 2029 might be boring for the average person to look at," said David Buote, a coauthor of the paper, "it is a pure delight for astrophysicists to study, because it allows for a very straightforward and accurate comparison of theory and observation."

As a case in point, earlier observations of the Hydra A galaxy cluster by Larry David of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and colleagues found a similar result but the evidence of explosive activity in the central galaxy made it difficult to draw definite conclusions about the nature of the dark matter. The dark matter profile deduced for Abell 2029 provides evidence that the Hydra results are reliable and is an important independent confirmation of cold dark matter predictions.

John Stocke of the University of Colorado, Boulder was also involved in this research. Chandra observed Abell 2029 with the ACIS detector for 5.6 hours on April 12, 2000. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.
http://www.universetoday.com/862 ... r-cold-dark-matter/

夏凉 · 发表于 2010-8-3 03:43:15

本帖最后由夏凉于 2010-8-3 10:46 编辑

Modifying Gravity to Account for Dark Matter

Current theories may not describe our Universe very accurately. Image credit: Brussels Museum of Fine Arts, and Space Telescope Institute. Click to enlarge
A Chinese astronomer from the University of St Andrews has fine-tuned Einstein's groundbreaking theory of gravity, creating a 'simple' theory which could solve a dark mystery that has baffled astrophysicists for three-quarters of a century.

A new law for gravity, developed by Dr Hong Sheng Zhao and his Belgian collaborator Dr Benoit Famaey of the Free University of Brussels (ULB), aims to prove whether Einstein's theory was in fact correct and whether the astronomical mystery of Dark Matter actually exists. Their research was published on February 10th in the US-based Astrophysical Journal Letters. Their formula suggests that gravity drops less sharply with distance as in Einstein, and changes subtly from solar systems to galaxies and to the universe.

Theories of the physics of gravity were first developed by Isaac Newton in 1687 and refined by Albert Einstein's general theory of relativity in 1905 to allow light bending. While it is the earliest-known force, gravity is still very much a mystery with theories still unconfirmed by astronomical observations in space.

The 'problem' with the golden laws of Newton and Einstein is whilst they work very well on earth, they do not explain the motion of stars in galaxies and the bending of light accurately. In galaxies, stars rotate rapidly about a central point, held in orbit by the gravitational attraction of the matter in the galaxy. However astronomers found that they were moving too quickly to be held by their mutual gravity – so not enough gravity to hold the galaxies together instead stars should be thrown off in all directions!

The solution to this, proposed by Fritz Zwicky in 1933, was that there was unseen material in the galaxies, making up enough gravity to hold the galaxies together. As this material emits no light astronomers call it 'Dark Matter'. It is thought to account for up to 90% of matter in the Universe. Not all scientists accept the Dark Matter theory however. A rival solution was proposed by Moti Milgrom in 1983 and backed up by Jacob Bekenstein in 2004. Instead of the existence of unseen material, Milgrom proposed that astronomers understanding of gravity was incorrect. He proposed that a boost in the gravity of ordinary matter is the cause of this acceleration.

Milgrom's theory has been worked on by a number of astronomers since and Dr Zhao and Dr Famaey have proposed a new formulation of his work that overcomes many of the problems previous versions have faced.

They have created a formula that allows gravity to change continuously over various distance scales and, most importantly, fits the data for observations of galaxies. To fit galaxy data equally well in the rival Dark Matter paradigm would be as challenging as balancing a ball on a needle, which motivated the two astronomers to look at an alternative gravity idea.

Legend has it that Newton began thinking about gravity when an apple fell on his head, but according to Dr Zhao, "It is not obvious how an apple would fall in a galaxy. Mr Newton's theory would be off by a large margin – his apple would fly out of the Milky Way. Efforts to restore the apple on a nice orbit around the galaxy have over the years led to two schools of thoughts: Dark Matter versus non-Newtonian gravity. Dark Matter particles come naturally from physics, with beautiful symmetries and explain cosmology beautifully; they tend to be everywhere. The real mystery is how to keep them away from some corners of the universe. Also Dark Matter comes hand- in-hand with Dark Energy. It would be more beautiful if there were one simple answer to all these mysteries".

Dr Zhao, a PPARC Advanced Fellow at University of St Andrews, School of Physics and Astronomy, and member of the Scottish Universities Physics Alliance (SUPA), continued "There has always been a fair chance that astronomers might rewrite the law of gravity. We have created a new formula for gravity which we call 'the simple formula', and which is actually a refinement of Milgrom's and Bekenstein's. It is consistent with galaxy data so far, and if its predictions are further verified for solar system and cosmology, it could solve the Dark Matter mystery. We may be able to answer common questions such as whether Einstein's theory of gravity is right and whether the so-called Dark Matter actually exists".

"A non-Newtonian gravity theory is now fully specified on all scales by a smooth continuous function. It is ready for fellow scientists to falsify. It is time to keep an open mind for new fields predicted in our formula while we continue our search for Dark Matter particles."

The new formula will be presented to an international workshop at Edinburgh's Royal Observatory in April, which will be given the opportunity to test and debate the reworked theory. Dr Zhao and Dr Famaey will demonstrate their new formula to an audience of Dark Matter and gravity experts from ten different countries.

Dr Famaey commented "It is possible that neither the modified gravity theory, nor the Dark Matter theory, as they are formulated today, will solve all the problems of galactic dynamics or cosmology. The truth could in principle lie in between, but it is very plausible that we are missing something fundamental about gravity, and that a radically new theoretical approach will be needed to solve all these problems. Nevertheless, our formula is so attractively simple that it is tempting to see it as part of a yet unknown fundamental theory. All galaxy data seem to be explained effortlessly".

夏凉 · 发表于 2010-8-3 03:54:37

A Connection Between Dark Energy and Dark Matter?

In the last few decades, scientists have discovered that there is a lot more to the universe than meets the eye: The cosmos appears to be filled with not just one, but two invisible constituents-dark matter and dark energy-whose existence has been proposed based solely on their gravitational effects on ordinary matter and energy.

Now, theoretical physicist Robert J. Scherrer has come up with a model that could cut the mystery in half by explaining dark matter and dark energy as two aspects of a single unknown force. His model is described in a paper titled "Purely Kinetic k Essence as Unified Dark Matter" published online by Physical Review Letters on June 30 and available online at http://arxiv.org/abs/astro-ph/0402316.

"One way to think of this is that the universe is filled with an invisible fluid that exerts pressure on ordinary matter and changes the way that the universe expands," says Scherrer, a professor of physics at Vanderbilt University.

According to Scherrer, his model is extremely simple and avoids the major problems that have characterized previous efforts to unify dark matter and dark energy.

In the 1970s, astrophysicists postulated the existence of invisible particles called dark matter in order to explain the motion of galaxies. Based on these observations, they estimate that there must be about 10 times as much dark matter in the universe as ordinary matter. One possible explanation for dark matter is that it is made up of a new type of particle (dubbed Weakly Interacting Massive Particles, or WIMPs) that don't emit light and barely interact with ordinary matter. A number of experiments are searching for evidence of these particles.

As if that weren't enough, in the 1990s along came dark energy, which produces a repulsive force that appears to be ripping the universe apart. Scientists invoked dark energy to explain the surprise discovery that the rate at which the universe is expanding is not slowing, as most cosmologists had thought, but is accelerating instead. According to the latest estimates, dark energy makes up 75 percent of the universe and dark matter accounts for another 23 percent, leaving ordinary matter and energy with a distinctly minority role of only 2 percent.

Scherrer's unifying idea is an exotic form of energy with well-defined but complicated properties called a scalar field. In this context, a field is a physical quantity possessing energy and pressure that is spread throughout space. Cosmologists first invoked scalar fields to explain cosmic inflation, a period shortly after the Big Bang when the universe appears to have undergone an episode of hyper-expansion, inflating billions upon billions of times in less than a second.

Specifically, Scherrer uses a second-generation scalar field, known as a k-essence, in his model. K-essence fields have been advanced by Paul Steinhardt at Princeton University and others as an explanation for dark energy, but Scherrer is the first to point out that one simple type of k-essence field can also produce the effects attributed to dark matter.

Scientists differentiate between dark matter and dark energy because they seem to behave differently. Dark matter appears to have mass and to form giant clumps. In fact, cosmologists calculate that the gravitational attraction of these clumps played a key role in causing ordinary matter to form galaxies. Dark energy, by contrast, appears to be without mass and spreads uniformly throughout space where it acts as a kind of anti-gravity, a repulsive force that is pushing the universe apart.

K-essence fields can change their behavior over time. When investigating a very simple type of k-essence field-one in which potential energy is a constant-Scherrer discovered that as the field evolves, it passes through a phase where it can clump and mimic the effect of invisible particles followed by a phase when it spreads uniformly throughout space and takes on the characteristics of dark energy.

"The model naturally evolves into a state where it looks like dark matter for a while and then it looks like dark energy," Scherrer says. "When I realized this, I thought, 'This is compelling, let's see what we can do with it.'"

When he examined the model in more detail, Scherrer found that it avoids many of the problems that have plagued previous theories that attempt to unify dark matter and dark energy.

The earliest model for dark energy was made by modifying the general theory of relativity to include a term called the cosmological constant. This was a term that Einstein originally included to balance the force of gravity in order to form a static universe. But he cheerfully dropped the constant when astronomical observations of the day found it was not needed. Recent models reintroducing the cosmological constant do a good job of reproducing the effects of dark energy but do not explain dark matter.

One attempt to unify dark matter and dark energy, called the Chaplygin gas model, is based on work by a Russian physicist in the 1930s. It produces an initial dark matter-like stage followed by a dark energy-like evolution, but it has trouble explaining the process of galaxy formation.

Scherrer's formulation has some similarities to a unified theory proposed earlier this year by Nima Arkani-Hamed at Harvard University and his colleagues, who attempt to explain dark matter and dark energy as arising from the behavior of an invisible and omnipresent fluid that they call a "ghost condensate."

Although Scherrer's model has a number of positive features, it also has some drawbacks. For one thing, it requires some extreme "fine-tuning" to work. The physicist also cautions that more study will be required to determine if the model's behavior is consistent with other observations. In addition, it cannot answer the coincidence problem: Why we live at the only time in the history of the universe when the densities calculated for dark matter and dark energy are comparable. Scientists are suspicious of this because it suggests that there is something special about the present era.

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