The UNiverse seems less smooth than theory

If so, there will need some new understanding. Since Einstein's application of his grand theory, GRT, to comprehending the cosmos, a lot of observations have been achieved during the past years, especially about the cosmological structure on large scales. This allows one to make relatively accurate estimate about the mass and energy distribution, by calibration with GRT. Great fitting has been found if dark matter and dark energy are presumed up till now, when people find that, the universe is much more clumpier than expected on larger scale.

Thomas et al. use publicly-released catalogs from the Sloan Digital Sky Survey to select more than 700,000 galaxies whose observed colors indicate a significant redshift and are therefore presumed to be at large cosmological distances. They use the redshift of the galaxies, combined with their observed positions on the sky, to create a rough three-dimensional map of the galaxies in space and to assess the homogeneity on scales of a couple of billion light years. One complication is that Thomas et al. measure the density of galaxies, not the density of all matter, but we expect that fluctuations of these two densities about their means to be proportional; the constant of proportionality can be calibrated by observations on smaller scales. Indeed, on small scales the galaxy data are in good agreement with the standard model. On the largest scales, the fluctuations in galaxy density are expected to be of order a percent of the mean density, but Thomas et al. find fluctuations double this prediction. This result then suggests that the universe is less homogeneous than expected. [http://physics.aps.org/articles/v4/47]

LambdaDCM or MOND ?

Yesterday I encountered an article [http://physics.aps.org/articles/v4/23] and today I attended a public lecture, both on dark matter, which, if it exists, seems invisible to most of the forces known to us. But it does participate in gravitation and plausibly in weak interaction. Their nature is crying to be unveiled. In the article, the author explains the observed v-R curve in galaxies, which is the most direct motivation for dark matter, using the so-called MOND (Modified Newtonian Dynamics) and the dark matter is subsidiary. On the other hand, Harry Nelson, the today's lecturer, dismisses the MOND as unnecessary complications. He cited the "Bullet Cluster" [http://en.wikipedia.org/wiki/Bullet_Cluster] as evidences. All in all, I feel that, the MOND idea can hardly withstand this. The point is that, a good idea should explain not only the v-R curve but also many other relevant things, such as the "Bullet Cluster". On the other hand, what is dark matter ? This is regarded as one of the biggest questions in today's science.

Dark Matter Particles Remain Dark

Here is a controversy about dark matter particles, whose properties are definitely quite elusive. Most we know about them are just speculative. Bear in mind how a scientific conclusion has to go through scrutinies !

How does black holes sette in galaxies

Every galaxy is believed to harbor a compact massive body-black hole- at its center. How it gets in? A natural way is through galaxy evolution based on the Big Bang model. However, there is the other way around, which is said being enigmatic [Nature,469:305].

Bulges and their black holes seem to be a natural consequence of structure formation in the hot Big Bang theory of the expanding Universe. According to this theory, galaxies grew by gravitational assembly of matter into clumps that gathered into larger clumps, and so on to galaxies. In galaxies with bulges, including ellipticals, which have bulges and no disks, the mass of the central black hole correlates not only with the mass of the bulge, but also, as Kormendy, Bender and Cornell¹ note ( page 374), with the average spread of velocities of the bulge stars (see Fig. 2a on page 375). The plausible explanation is that part of the gas out of which bulge stars formed settled instead near to the black hole, in part increasing its mass and in part fuelling explosions that blew the gas away and suppressed bulge-star formation. That is, the growth of bulge and black hole may have controlled each other. The timing looks right. Bulge stars are old: they formed when the expanding Universe was roughly a third of its present size (redshift about 2). This is when the rate of star formation per unit of matter was near its maximum (more than 10 times the present rate³). It is also when quasars — explosions powered by the central black holes — were most abundant (100 times more common than now⁴), probably an explosive result of overfeeding of the black holes as the early generations of stars were forming.

........

In theory, galaxies both with and without bulges were growing by the gravitational collection of clumps of matter when the star-formation rate was near its peak. That would suggest that the clumps contained stars; a recent discussion puts roughly comparable masses in stars and gas⁶. So where are these early generations of stars? Not in disks, because there is nothing that would slow the motion of a star to allow it to settle onto a disk. Bulges contain old stars, and it has been suggested that this is where the early stars ended up. But we now see that this is not plausible: why would these old stars have avoided our bulgeless Galaxy and settled instead in the bulge of our neighbour M31? Maybe the old stars are in diffuse stellar haloes. If so, it seems curious that the stellar halo of our Galaxy is much less prominent than that of M31. But more studies of other nearby galaxies will be required to check for inventories of stars that are old enough and abundant enough to account for stars that formed before disks.

Still Quiet is Dark Matter

Cosmological observations suggest the existence of dark matter, which has not shown any traces of interacting with known baryonic matter. Yet, dark matter comprises over 80% of the total matter needed to explain the space-time structure. Scientists have not a clue regarding the nature of these matter. One proposal says they may be made of sort of particles, the so-called WIMPs (weakly interacting massive particles). Various experiments have been devised to detect them. NO positive results exist up to now. A latest effort came in PRL, still no activities of these particles detected. They are really quiet, should they be there. [Phys. Rev. Lett. 105, 131302 (2010)]

The XENON100 experiment, in operation at the Laboratori Nazionali del Gran Sasso in Italy, is designed to search for dark matter weakly interacting massive particles (WIMPs) scattering off 62 kg of liquid xenon in an ultralow background dual-phase time projection chamber. In this Letter, we present first dark matter results from the analysis of 11.17 live days of nonblind data, acquired in October and
November 2009. In the selected fiducial target of 40 kg, and within the predefined signal region, we observe no events and hence exclude spin-independent WIMP-nucleon elastic scattering cross sections above 3:4 10 44 cm2 for 55 GeV=c2 WIMPs at 90% confidence level. Below 20 GeV=c2, this result
constrains the interpretation of the CoGeNT and DAMA signals as being due to spin-independent, elastic, light mass WIMP interactions.

NO dark matter detected, yet

Without a wisp of exaggeration, the greatest myth in present physics might be about the so-called dark matter and dark energy. Physicists, fairly speaking, for the moment have not even the slightest definite clue about them. They were motivated for two observations: (1) the rotation velocity of a typical galaxy does not follow the pattern based on Newton's theory; (2) the universe is expanding faster and faster. Fact (1) leads to proposal of dark matter while (2) to that of dark energy. Interestingly, the dark energy term was first hypothesized by Einstein, who later on dismissed it for Hubble's discovery, to find a static and stable universe. This energy never dilute in the course of expansion. It permeates everywhere. People don't know where it comes from, although some suggested it might be vacuum energy (calculations rejected this idea). As regards the dark matter, it is usually hypothesized as some undetected particles other than baryons. They interact extremely weakly with visible matter. Some suggest these might be the so-called Weakly Interacting Massive Particles that are predicted by supersymmetric theory. Detectors have been mounted to settle this issue. A latest survey reports a failure [Phys. Rev. Lett. 105, 131302 (2010) ].

Although the above dark matter idea is popular, it is quite dubious to some physicists, who don't like extra assumptions. In 2004, a German group did a study which reveals running gravitational constant that goes bigger at astronomical scales [Physical Review D 70: 124028 (2004)]. This study might null the necessity of dark matter.

US sets dark things as cosmic priorities

This will surely give thrust to the research in dark matter and dark matter:

Over ten years, the US$465-million observatory will also build up an unprecedented 100-petabyte database for astronomers trying to discern the nature of two mysterious factors that shape the Universe. One is dark matter, thought to be an unknown particle or family of particles beyond the standard model of physics. Hidden in vast quantities among the galaxies, dark matter generates a gravitational pull that has shaped the evolution of the Universe. The other factor is dark energy, the pervasive but mysterious phenomenon that is causing cosmic expansion to accelerate. Crucial data on both factors can be derived from a three-dimensional survey of the surrounding Universe that the LSST is well suited to provide.

“Increasingly, we are able to ask new questions by querying huge databases.”

"Increasingly, we are able to ask new questions by querying huge databases," says Tyson. "The key is to populate those databases with calibrated and trusted data."

The LSST is expected to help US astronomers regain some momentum in ground-based astronomy at a time when European facilities have begun to dominate the field. To that end, the survey stresses the need for a swift decision on which of two competing mega-telescopes should receive federal funding.

The proposed Thirty Meter Telescope, on Mauna Kea in Hawaii, and the Giant Magellan Telescope, envisioned for Las Campanas in Chile, are both supported by significant private money, and would have many times the light-gathering power and resolution of today's largest telescopes. Realistically, only one project will receive federal funds, which the survey recommends should be between $257 million and $350 million. Given that Europe has also prioritized a 42-metre telescope, the European Extremely Large Telescope, a choice needs to be made now to avoid a counterproductive stalemate.

In space, the decadal survey proposes the Wide Field Infrared Survey Telescope (WFIRST), a 1.5-metre instrument that will map the whole sky at near-infrared wavelengths. Such data would contain subtle clues — in the distance–brightness relationships of supernovae, the bending of light (microlensing) from background galaxies and the three-dimensional clustering of matter in space — that can be used to independently measure dark energy.

WFIRST is effectively a rebranding of the Joint Dark Energy Mission, a NASA–DOE collaboration. The new name, says one survey reviewer, signals that the $1.6-billion telescope is not a one-trick pony, but a way of serving other astronomical needs as well. The survey committee stresses, for example, that WFIRST could spot microlensing events caused when exoplanets — planets outside our Solar System — pass briefly in front of background stars in the Milky Way. Although the method is unsuitable for studying individual solar systems in detail, it promises, through its sheer number of discoveries, to provide an unbiased sample of the kinds of planetary systems prevalent in the Galaxy.

Visualizing relativity theory

(1) For anyone interested in illustrating relativity effects graphically, this is a good website:

http://www.spacetimetravel.org/

(2) Here is a news article illuminating the discovery of dark energy:

http://physicsworld.com/cws/article/indepth/31908

Dark matter: a review on the phenomenology

In his latest blog, Z.z. posted an article collecting the evidences advocating the existence of dark matter.

Dark Matter: A Primer

This is a very useful review on the phenomenology of Dark Matter. It has a good review on the early and most recent evidence in support of the existence of dark matter. At the very least, it is convenient to have all of the references in one place for easy look-up.

Abstract: Dark matter is one of the greatest unsolved mysteries in cosmology at the present time. About 80% of the universe's gravitating matter is non-luminous, and its nature and distribution are for the most part unknown. In this paper, we will outline the history, astrophysical evidence, candidates, and detection methods of dark matter, with the goal to give the reader an accessible but rigorous introduction to the puzzle of dark matter. This review targets advanced students and researchers new to the field of dark matter, and includes an extensive list of references for further study.

Zz.

Posted by ZapperZ at 7:42 AM 0 comments Links to this post

where does dark matter hide ?

I'm much interested in the so-called dark matter, though about which i know only a little. It does not interact with light. Neutrino does not interact with light as either. But neutrinos can be detected by, e.g., weak interaction. But dark matter even does not lend itself to weak interaction. Nevertheless, it is supposed to engage in gravitational force, which is exactly why it is stipulated. I'm even not sure if this matter really exists, or it is just that we need new theory. Dark matter is required for the present physical theory, but perhaps dispensable with a new theory. Anyway, it is theory, it is our language, that decides what can be observed and what we can speak of.

After all, a recent work suggests that, large galaxy hides more dark matter that small ones, seeming a result of gravitational attraction.

Astrophysicists know that 83% of the matter in the universe is dark matter—invisible stuff as yet undetected. The other 17% is detectable "baryonic matter," the atoms and ions that make up stars, planets, dust, and gas. To astronomers' surprise, the ratio of baryonic matter to dark matter seems to vary from galaxy to galaxy like the ratio of chocolate chips to dough in different batches of home-baked cookies. Now, a team led by Stacy McGaugh at the University of Maryland, College Park, has determined that the proportion varies by scale: The largest galaxies have the highest percentage of baryonic matter, although not quite 17%; whereas the smallest galaxies have less than 1%.

McGaugh and colleagues compiled the ratios for more than 100 galaxies ranging from supermassive ones to dwarfs. Researchers infer the amount of dark matter in a galaxy from the motion of its stars. They estimate its baryonic mass from the amount of light the galaxy emits, which can be converted to the total mass of its stars, and a measure of atomic hydrogen in the galaxy, which provides an estimate of the interstellar gas.

"What we find is that there is a very systematic variation in the ratio with scale," says McGaugh, who presented the findings at the American Astronomical Society meeting in Washington, D.C., last week.^* "When you go to the very large galaxies, the baryonic matter can be as much as 14%. As you go down in size, you see that galaxies fall short of the cosmic fraction [17:83] by an ever-increasing amount." In galaxies the size of the Milky Way, "all the stars and gas add up to only a third of the baryonic matter you would expect," which is about 5%. And in the smallest dwarfs, baryonic matter is a hundredth of what's expected—as minuscule as 0.2%. "These are very interesting results" that quantify the "missing baryonic matter problem," says Joel Bregman, an astronomer at the University of Michigan, Ann Arbor.

Where is all the missing baryonic matter lurking? One hypothesis is that its particles are interspersed within the galaxy's dark matter halo in the form of undetectable hot gas. Another is that supernova explosions have blown it into intergalactic space. This second idea would square with McGaugh's findings: Large galaxies, with stronger gravitational pulls, would be able to retain more of their baryonic matter, whereas smaller galaxies would let more escape. But so far, McGaugh says, that explanation is just one of several lines of speculation.

Has dark matter been detected ?

For 80 years, it has eluded the finest minds in science. But tonight it appeared that the hunt may be over for dark matter, the mysterious and invisible substance that accounts for three-quarters of the matter in the universe.

In a series of coordinated announcements at several US laboratories, researchers said they believed they had captured dark matter in a defunct iron ore mine half a mile underground. The claim, if confirmed next year, will rank as one the most spectacular discoveries in physics in the past century.

Tantalising glimpses of dark matter particles were picked up by highly sensitive detectors at the bottom of the Soudan mine in Minnesota, the scientists said.

Dan Bauer, head of the Cryogenic Dark Matter Search (CDMS), said the group had spotted two particles with all the expected characteristics of dark matter. There is a one in four chance that the result is due to some other effect in the underground detectors, Bauer told a seminar at the Fermi National Accelerator Laboratory, near Chicago.

Rumours that Bauer's group was on the verge of making an announcement surfaced on physicists' blogs a few weeks ago. Though tentative, tonight's results triggered an immediate wave of excitement in the science community.

"If they have a real signal, it's a seriously big deal. The scale on which people are looking for dark matter is vast," said Gerry Gilmore at Cambridge University's institute of astronomy. "Dark matter is what created the structure of the universe and is essentially what holds it together. When ordinary matter falls into lumps of dark matter it turns into galaxies, stars, planets and people. Without it, we wouldn't be here," Gilmore said.

Scientists have debated the existence of dark matter since 1933, when the Swiss astronomer Fritz Zwicky argued that a distant cluster of galaxies would fall apart were it not for the gravitational pull of some vast but invisible cosmic substance. It was named dark matter because it does not reflect or absorb light, making it impossible to observe with telescopes.

Last year, the Hubble telescope photographed indirect evidence in the form of a ghostly halo around a distant galaxy, caused by clumps of dark matter bending light from stars as it passed by. A year before that, scientists led by the British astronomer Richard Massey, at the California Institute of Technology, published the first 3D map of dark matter, which revealed how it clung around galaxies and held clusters of them together.

Dark matter is likely to be made up of a variety of invisible particles that not only explain the missing mass of the universe, but shed light on some of the most profound mysteries in science.

Some dark matter particles could explain why ordinary matter is not radioactive, while others may help scientists understand why time – so far as we know – always runs forward.

"The real impact of this is psychological, in that it shows we're getting close to being able to do a whole new kind of physics," Gilmore said. "We know there are properties of the universe that should correspond to new families of particles. One of the great mysteries is why time only goes in one direction, and one candidate to explain that is a dark matter particle."

Many scientists believe dark matter particles will turn out to be proof of a theory called supersymmetry, which predicts that every kind of particle in the universe is paired with a heavier twin. Finding evidence for supersymmetry is one of the major goals of the Large Hadron Collider at Cern, in Switzerland.

Dark matter particles are peculiar because they pass through objects as if they were not there. Their aloof nature has led scientists to name them weakly interacting massive particles, or Wimps. Vast amounts of these are thought to be constantly moving through the Earth and everything on it, us included, as the solar system spins around our galaxy.

The detectors at the Soudan mine are buried underground to shield them from other kinds of particles that bombard Earth from space. To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detector and makes it vibrate.

Detectors in the mine will be upgraded in the new year before the search for more dark matter continues, Bauer said.

The hunt for dark matter

What is dark matter?

The night sky might seem full of stars and planets, but what we see is only 4% of the stuff of the universe. Some three-quarters is dark matter, an invisible substance that scientists believe is there because of the gravitational force it exerts.

What does dark matter do?

Dark matter stretches throughout space where it attracts ordinary matter that coalesces into galaxies of billions of stars and planets. It forms a kind of cosmic skeleton that gives the universe its structure. Many scientists believe they will find a family of invisible dark matter particles, each of which plays a different role in nature. Some may even explain why time always goes in the same direction.

Who came up with the idea?

The Swiss astronomer Fritz Zwicky postulated dark matter in 1933. He noticed that a distant cluster of galaxies would fall apart were it not for the extra gravitational pull of some mysterious unseen mass in space. Astronomers verified his prediction by showing that stars swirling around distant galaxies zipped around so fast they must be held in place by extra gravitational forces.

Does everyone believe in dark matter?

A minority of astronomers and physicists dismiss dark matter as a fudge. Instead, they suspect that the strength of gravity varies from place to place, in a way that explains why stars do not hurtle out of spinning galaxies. The theory is known as Modified Newtonian Dynamics (Mond).

• This article was amended on Friday 18 December 2009. We said dark matter accounts for three-quarters of the mass of the universe; we meant to say three-quarters of the matter of the universe. This has been corrected.