New paper offers another approach to proof that dark matter exists

(PhysOrg.com) -- In yet another stab at trying to prove the existence of dark matter, Dan Hooper and colleagues have published a paper on arXiv describing what they believe to be credible evidence of the material believed to comprise most of the mass in the Universe. They say, according to a recent BBC post, that it could be that electrons are created when high energy dark matter particles crash into one another, giving rise to the synchrotron radiation that has thus far puzzled scientists here on Earth. The result, it is postulated, are the so far unexplained non-thermal radio filaments (NRFs) found near the center of our galaxy.

As the BBC article notes, Hooper gives credit for the idea to fellow team member Tim Linden; a move that may or may not instill confidence in the new theory. Hooper then goes on to say in an interview with BBC news, that the theory does seem to answer a lot of the previously unanswerable questions regarding the mysterious filaments, such as why they are so close to the center of the universe, and why they?re so bright. He says that?s likely due to there being more dark matter as you approach the center of the galaxy.

In the paper, Hooper et al, note that the filaments trace magnetic field lines, and that others have not had much success in figuring out their origin. They then go on to explain that they believe the creation of electrons and positrons that make up the filaments is due to the annihilation of some amount of light, dark matter. In addition to confirming their belief that it is this annihilation process that is responsible for the filaments, they also explain how the characteristics of such events would also explain the excess gamma-ray emissions detected by the Fermi Large Area Telescope and described by the CoGeNT collaboration and also seen in the DAMA/LIBRA experiment in Italy.

Of course, as is noted in the BBC article, basing a new theory on other unproven older theories might in some ways be interpreted as little more than speculation, even if you back up your claims with lots of fancy math.

More information: Dark Matter and Synchrotron Emission from Galactic Center Radio Filaments, arXiv:1106.5493v1 [astro-ph.HE] http://arxiv.org/abs/1106.5493

Abstract

The inner degrees of the Galactic center contain a large population of filamentary structures observed at radio frequencies. These so-called non-thermal radio filaments (NRFs) trace magnetic field lines and have attracted significant interest due to their hard (S_v ~ -0.1 +/- 0.4) synchrotron emission spectra. The origin of these filaments remains poorly understood. We show that the electrons and positrons created through the annihilations of a relatively light (~5-10 GeV) dark matter particle with the cross section predicted for a simple thermal relic can provide a compelling match to the intensity, spectral shape, and flux variation of the NRFs. Furthermore, the characteristics of the dark matter particle necessary to explain the synchrotron emission from the NRFs is consistent with those required to explain the excess gamma-ray emission observed from the Galactic center by the Fermi-LAT, as well as the direct detection signals observed by CoGeNT and DAMA/LIBRA.

2010 PhysOrg.com

How to keep lonely exoplanets snug -- just add dark matter

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Exoplanets near the galactic center could accumulate sufficient dark matter to keep them warm enough to sustain life. Image credit: ESO

Dark matter, inferred to exist from its gravitational effects on the universe, is still a mystery to scientists. But a new study says that dark matter could help life evolve and survive on distant worlds outside of our solar system.

Dark matter is perhaps not the first thing that comes to mind when considering how life can be supported on another planet, but to Dan Hooper and Jason Steffen of the Fermilab Center for Particle Astrophysics, dark matter could be a contributing factor in allowing life to evolve and survive on distant worlds outside of our solar system.

The scientists propose that dark matter particles could sink into a planet?s core, and through the annihilation of matter, release enough energy to keep the surface of the planet warm enough for liquid water, even outside the traditional habitable zone.

Dark matter was first postulated in 1933 by Fritz Zwicky, and then again in the 1970s by Vera Rubin to account for evidence of ?missing mass? in the orbital velocities of galaxies in clusters and stars at the edges of galaxies. Dark matter is inferred to exist from its gravitational effects on visible matter and background radiation, but it emits no visible light and does not interact with any other matter except by gravity. Despite elusive evidence for its existence, it has been hypothesized to make up around 23 percent of the composition of the Universe (the rest being 4 percent ?normal? matter and 73 percent dark energy, which is the unknown force driving the accelerating expansion of the Universe). Dark matter is said to influence the evolution of the Universe gravitationally, even though scientists are still in the dark as to what it is.

?The reason we don't see dark matter directly is that it is made up of very weakly interacting material,? says Hooper. He is the lead author of a paper submitted to Astrophysical Journal titled, ?Dark Matter and the Habitability of Planets,? which describes one theory for dark matter ? weakly interacting massive particles, or WIMPs. ?For that reason, it doesn't interact - or do much of anything else - very much. It?s basically inert, and thus doesn't produce much usable energy.?

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The galactic center is believed to contain a high density of dark matter. Image credit: NASA

However, Hooper and Steffen have suggested a way in which what little energy does come from dark matter can keep planets warm. Some parts of the galaxy ? in particular the galactic center, as well as the cores of dwarf galaxies that are satellites of our own Milky Way ? contain a much higher concentration of dark matter (of the order of hundreds or thousands of times more dense) than the region of space near our Sun. Hooper and Steffen have calculated that when dark matter particles scatter with the atomic nuclei within the cores of rocky planets, they lose momentum and become gravitationally-bound and sink to the core of the planet, where they annihilate and release energy.

?If a dark matter particle passes through a planet, there is a chance that it will collide with an atom and lose some of its speed and momentum,? explains Hooper. ?Once this happens, instead of just flying off into space again, it can be stuck to the planet by the force of gravity and once that happens, it does not take very long for the dark matter particle to fall into the core of the planet, where it will stay.?

In dark matter-rich regions of space, enough dark matter can accumulate inside ?super-Earths? -- rocky planets with masses several times that of our home planet -- to provide enough energy to keep the surface of the distant world warm enough for liquid water to exist. This holds even far outside the star?s habitable zone, which is usually described as the distance from the star where temperatures, among other factors, allow water to remain as a liquid on the planet?s surface -- an essential condition for life as we know it. In principle, dark matter-rich planets could widen the habitable zone and increase the number of locations where we could find life.

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The supposed distribution of dark matter throughout the Universe. Image credit: NASA/UNC

However intriguing this is, it may be too good to be true. ?I?m dubious as to how common dark-matter-heated planets really are,? says astrobiologist Lewis Dartnell of University College London. ?Although their model is consistent with some theories of the nature and distribution of dark matter, it seems to appeal to a rather special set of conditions. And such rare planets would be virtually impossible to actually spot.?

Nevertheless, with billions of planets predicted to exist in our galaxy, it appears possible that some planets could benefit from dark matter, and these worlds would be very different to Earth.

?Life on a planet heated by dark matter would likely be very different than the life on Earth,? says Hooper. ?Many species of planets on Earth, for example, have evolved to collect energy from sunlight. On a dark-matter-powered planet, the energy would be coming from the center of the planet, and life there would have to find other ways of putting that energy to use.?

The heat provided by the annihilations of dark matter, state Hooper and Steffen in their paper, could keep a planet habitable for trillions of years, even beyond the lifetime of its star.

Despite his skepticism, Dartnell recognizes the importance of Hooper and Steffen?s work. ?It is an intriguing idea, and astrobiology as a discipline needs a healthy input of fresh, bold ideas like this one.?

Source: Astrobio.net (news : web)

When matter melts: Physicists map phase changes in quark-gluon plasma

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An ordinary proton or neutron (foreground) is formed of three quarks bound together by gluons, carriers of the color force. Above a critical temperature, protons and neutrons and other forms of hadronic matter "melt" into a hot, dense soup of free quarks and gluons (background), the quark-gluon plasma. Credit: Lawrence Berkeley National Laboratory

In its infancy, when the universe was a few millionths of a second old, the elemental constituents of matter moved freely in a hot, dense soup of quarks and gluons. As the universe expanded, this quark?gluon plasma quickly cooled, and protons and neutrons and other forms of normal matter "froze out": the quarks became bound together by the exchange of gluons, the carriers of the color force.

"The theory that describes the color force is called quantum chromodynamics, or QCD," says Nu Xu of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, the spokesperson for the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at DOE's Brookhaven National Laboratory. "QCD has been extremely successful at explaining interactions of quarks and gluons at short distances, such as high-energy proton and antiproton collisions at Fermi National Accelerator Laboratory. But in bulk collections of matter ? including the quark-gluon plasma ? at longer distances or smaller momentum transfer, an approach called lattice gauge theory has to be used."

Until recently, lattice QCD calculations of hot, dense, bulk matter could not be tested against experiment. Beginning in 2000, however, RHIC was able to recreate the extreme conditions of the early universe in miniature, by colliding massive gold nuclei (heavy ions) at high energies.

Experimentalists at RHIC, working with theorist Sourendu Gupta of India's Tata Institute of Fundamental Research, have recently compared lattice-theory predictions about the nature of the quark-gluon plasma with certain STAR experimental results for the first time. In so doing they have established the temperature boundary where ordinary matter and quark matter cross over and change phase. Their results appear in the journal Science.

Phase diagrams

The aim of both the theoretical and experimental work is to explore and fix key points in the phase diagram for quantum chromodynamics. Phase diagrams are maps, showing, for example, how changes in pressure and temperature determine the phases of water, whether ice, liquid, or vapor. A phase diagram of QCD would map the distribution of ordinary matter (known as hadronic matter), the quark-gluon plasma, and other possible phases of QCD such as color superconductivity.

"Plotting a QCD phase diagram requires both theory calculations and experimental effort with heavy-ion collisions," says Xu, who is a member of Berkeley Lab's Nuclear Science Division and an author of the Science paper. Experimental studies require powerful accelerators like RHIC on Long Island or the Large Hadron Collider at CERN in Geneva, while calculations of QCD using lattice gauge theory require the world's biggest and fastest supercomputers. Direct comparisons can achieve more than either approach alone.

One of the basic requirements of any phase diagram is to establish its scale. A phase diagram of water might be based on the Celsius temperature scale, defined by the boiling point of water under normal pressure (i.e., at sea level). Although the boiling point changes with pressure ? at higher altitudes water boils at lower temperatures ? these changes are measured against a fixed value.

The scale of the QCD phase diagram is defined by a transition temperature at the zero value of "baryon chemical potential." Baryon chemical potential measures the imbalance between matter and antimatter, and zero indicates perfect balance.

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The "current conjecture" for the QCD phase diagram. The boundary between the normal (hadronic) low-temperature phase and the high-temperature quark-gluon plasma phase is marked in black. The square box on the solid line indicates the yet-to-be-found critical point where phases can co-exist; RHIC is the only heavy-ion collider whose energy can be tuned across the region where it is likely to lie. Neutrons and protons and other ordinary matter particles (including antimatter particles) are detected after they "freeze out" of fireballs caused by heavy-ion collisions like those at RHIC, indicated by the dotted line. To the right is a possible region of "color superconductivity." Credit: Gupta et al.

Through extensive calculations and actual data from the STAR experiment, the team was indeed able to establish the QCD transition temperature. Before they could do so, however, they first had to realize an equally significant result, showing that the highly dynamical systems of RHIC's gold-gold collisions, in which the quark-gluon plasma winks in and out of existence, in fact achieve thermal equilibrium. Here's where theory and experiment worked hand in hand.

"The fireballs that result when gold nuclei collide are all different, highly dynamic, and last an extremely short time," says Hans Georg Ritter, head of the Relativistic Nuclear Collisions program in Berkeley Lab's Nuclear Science Division and an author of the Science paper. Yet because differences in values of the kind observed by STAR are related to fluctuations in thermodynamic values predicted by lattice gauge theory, says Ritter, "by comparing our results to the predictions of theory, we have shown that what we measure is in fact consistent with the fireballs reaching thermal equilibrium. This is an important achievement."

The scientists were now able to proceed with confidence in establishing the scale of the QCD phase diagram. After a careful comparison between experimental data and the results from the lattice gauge theory calculations, the scientists concluded that the transition temperature (expressed in units of energy) is 175 MeV (175 million electron volts).

Thus the team could develop a "conjectural" phase diagram that showed the boundary between the low-temperature hadronic phase of ordinary matter and the high-temperature quark-gluon phase.

In search of the critical point

Lattice QCD also predicts the existence of a "critical point." In a QCD phase diagram the critical point marks the end of a line showing where the two phases cross over, one into the other. By changing the energy, for example, the baryon chemical potential (balance of matter and antimatter) can be adjusted.

Among the world's heavy-ion colliders, only RHIC can tune the energy of the collisions through the region of the QCD phase diagram where the critical point is most likely to be found ? from an energy of 200 billion electrons volts per pair of nucleons (protons or neutrons) down to 5 billion electron volts per nucleon pair.

Says Ritter, "Establishing the existence of a QCD critical point would be much more significant than setting the scale." In 2010, RHIC started a program to search for the QCD critical point.

Xu says, "In this paper, we compared experimental data with lattice calculations directly, something never done before. This is a real step forward and allows us to establish the scale of the QCD phase diagram. Thus begins an era of precision measurements for heavy-ion physics."

Provided by Lawrence Berkeley National Laboratory (news : web)