This Week in the Universe: September 9th – September 15th

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Astrophysics and Gravitation:

Astrophysics, by General Mills

Vella, D., & Mahadevan, L. (2005). The “Cheerios effect” American Journal of Physics, 73 (9) DOI: 10.1119/1.1898523

FOX News and a few other news agencies have discovered the Cheerios Effect this week (unfortunately, MSNBC beat them to this scope by about five years).  Back in 2005, Vella and Mehadevan described the “Cheerios Effect”, the tendency for small, wet, objects to attract each other.

From the abstract:

Objects that float at the interface between a liquid and a gas interact because of interfacial deformation and the effect of gravity.

Makes sense, but what does this have to do with astrophysics? Well, just like how the few remaining Cheerios left in the bowl have a tendency to group together in the milk, galaxies have a tendency to group together in space.  While the forces involved are quite different, the outcome is visually rather similar, and, astrophysics has a long history of using fluid models for approximations.  Is this big news? No, but it makes a very good classroom demonstration (as the authors originally wanted).

For more, see Cereal And Saturday Morning Physics.

PAMELA Results on the Cosmic-Ray Antiproton Flux

O. Adriani, & et al. (2010). PAMELA Results on the Cosmic-Ray Antiproton Flux from 60 MeV to 180 GeV in Kinetic Energy Phys. Rev. Lett., 105 (12), 1101-1106 : 10.1103/PhysRevLett.105.121101

The abstract:

The satellite-borne experiment PAMELA has been used to make a new measurement of the cosmic-ray antiproton flux and the antiproton-to-proton flux ratio which extends previously published measurements down to 60 MeV and up to 180 GeV in kinetic energy. During 850 days of data acquisition approximately 1500 antiprotons were observed. The measurements are consistent with purely secondary production of antiprotons in the Galaxy. More precise secondary production models are required for a complete interpretation of the results.

Earlier this year, PAMELA observed an usually large flux of positrons within the cosmic rays they were seeing from some unknown, beyond the atmosphere, source.  Naturally, like all unusual cosmic ray results, people thought “dark matter”.  Now, with more data, the PAMELA team has confirmed these observations, but still can’t suggest a cause.  Observation supports some “secondary production” mechanism for their creation, ie. the positrons being produced by some annihilation interaction with some unknown particles, but it’s still quite open what that might be.

From Tevong You:

Secondary production is referring to predictions of expected antiparticle abundance from models of cosmic rays interacting with interstellar nuclei producing antiparticles from known physics, and the excess or lack thereof is with respect to these models. The positron excess is countered by the antiproton measurement not showing any disagreement with the expectation from secondary production, which is what is being confirmed.

For more, see Uncertain Sources.

Hungry Hungry Stars

Credit: X-ray (NASA/CXC/RIT/J.Kastner et al), Optical (UCO/Lick/STScI/M.Perrin et al); Illustration: NASA/CXC/M.Weiss

NASA’s Chandra X-ray Observatory has recently seen evidence of star cannibalism.  It appears that star BP Piscium (BP Psc – a red giant phase star once probably the size of our Sun) may have absorbed a companion star in the past (or perhaps a large planet).

David Rodriguez from UCLA:

BP Psc shows us that stars like our Sun may live quietly for billions of years, but when they go, they just might take a star or planet or two with them.

For more, see Chandra Finds Evidence for Stellar Cannibalism.

Dark Matter in the Sun (again?)

Lopes, I., & Silk, J. (2010). Neutrino Spectroscopy Can Probe the Dark Matter Content in the Sun Science DOI: 10.1126/science.1196564

From the abstract:

After being gravitationally captured, low mass cold dark matter particles (mass range 5 to ~50 x 109 electron volts) are thought to drift to the center of the Sun and affect its internal structure. Solar neutrinos provide a way to probe the physical processes occurring in the Sun’s core. Solar neutrino spectroscopy, in particular, is expected to measure the neutrino fluxes produced in nuclear reactions in the Sun. Here, we show how the presence of dark matter particles inside the Sun will produce unique neutrino flux distributions…

Ah, more speculation that dark matter could be found inside the sun.  Is this based on observation? Well no, but we still can’t quite explain the expected composition of the sun with its mass and spectrum, so there is room for dark matter.  Is it science? Not yet.

For more, see Searching the Sun for dark matter.

General Relativity, Quantum Gravity, et al.:

Fun with Five-Dimensional Black Strings

Lehner, L., & Pretorius, F. (2010). Black Strings, Low Viscosity Fluids, and Violation of Cosmic Censorship Physical Review Letters, 105 (10) DOI: 10.1103/PhysRevLett.105.101102

The Abstract:

We describe the behavior of 5-dimensional black strings, subject to the Gregory-Laflamme instability. Beyond the linear level, the evolving strings exhibit a rich dynamics, where at intermediate stages the horizon can be described as a sequence of 3-dimensional spherical black holes joined by black string segments. These segments are themselves subject to a Gregory-Laflamme instability, resulting in a self-similar cascade, where ever-smaller satellite black holes form connected by ever-thinner string segments. This behavior is akin to satellite formation in low-viscosity fluid streams subject to the Rayleigh-Plateau instability. The simulation results imply that the string segments will reach zero radius in finite asymptotic time, whence the classical space-time terminates in a naked singularity. Since no fine-tuning is required to excite the instability, this constitutes a generic violation of cosmic censorship.

Video: Frans Pretorius: Apparent horizon embedding diagram of an unstable 5 dimensional black string

Pretorius and Lehner investigate five dimensional black strings to make sense of singularities through quantum gravity.  Through simulation, they showed that black strings evolve into black holes (eventually leading to naked singularities) connected by thin string segments.  If these simulations are correct, it appears we get a violation of cosmic censorship.  A result like this certainly suggests that quantum gravity, formulated along these lines, would lead to violations of causality and a definite change to the causal structure of the universe.  Does this mean that the simulations are wrong or that our notions of causal structure are wrong? At this point, we can’t say either way (and how cool is that?).

For more, see Physicists investigate fate of five-dimensional black strings.

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