# Astrophysics and Gravitation:

### Lensing of Black Holes Can Determine the Metric Around Them?

Bin-Nun, A. (2010). Gravitational lensing of stars orbiting Sgr A* as a probe of the black hole metric in the Galactic center Physical Review D, 82 (6) DOI: 10.1103/PhysRevD.82.064009

From the abstract:

We show that a possible astrophysical experiment, detection of lensed images of stars orbiting close to Sgr A*, can provide insight into the form of the metric around a black hole. We model Sgr A* as a black hole and add in a 1/r

^{2}term to the Schwarzschild metric near the black hole. … This knowledge will be useful in constraining any modified gravity theory that adds a similar term into the strong field near a black hole.

Sounds too good to be true? It’s hard to say, but a technique to observationally determine the spacetime metric would be awfully exciting (and huge – to classical/quantum relativists, that is).

For more, see Black Hole May Offer Clues to Extra Dimensions.

### Dark Energy and the Geometry of the Universe

Marinoni, C., & Buzzi, A. (2010). A geometric measure of dark energy with pairs of galaxies Nature, 468 (7323), 539-541 DOI: 10.1038/nature09577

From the abstract:

There is a purely geometric test of the expansion of the Universe (the Alcock–Paczynski test), which would provide an independent way of investigating the abundance () and equation of state () of dark energy. … Here we report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows us to determine that the Universe is flat…

Speaking of *observing *metrics… this is a lot less exciting, however, as it takes in many more assumptions about the basic nature of the universe, galaxy distances, and, of course, dark energy.

For more, see Distant Galaxies Confirm Dark Energy’s Existence and Universe’s Flatness, Dark Energy Theory Gets a Boost From New Galactic Measurements, Cosmology: Geometry of the Universe.

### Unified Origin of Matter and Dark Matter?

Davoudiasl, H., Morrissey, D., Sigurdson, K., & Tulin, S. (2010). Unified Origin for Baryonic Visible Matter and Antibaryonic Dark Matter Physical Review Letters, 105 (21) DOI: 10.1103/PhysRevLett.105.211304

The abstract:

We present a novel mechanism for generating both the baryon and dark matter densities of the Universe. A new Dirac fermion X carrying a conserved baryon number charge couples to the standard model quarks as well as a GeV-scale hidden sector. CP-violating decays of X, produced nonthermally in low-temperature reheating, sequester antibaryon number in the hidden sector, thereby leaving a baryon excess in the visible sector. The antibaryonic hidden states are stable dark matter. A spectacular signature of this mechanism is the baryon-destroying inelastic scattering of dark matter that can annihilate baryons at appreciable rates relevant for nucleon decay searches.

This is a surprisingly practical one (and really should be classified as *high energy*): A UBC team has proposed a new fermion that could explain dark matter, while linking to regular matter and the Standard Model. Signatures related to this *fermion X* should be detectable, in the right experiment, making it a target for future searches.

For more, see The X factor, UBC physicists make atoms and dark matter add up.

### Accurate Measurement in the Field of the Earth of the General-Relativistic Precession

Lucchesi, D., & Peron, R. (2010). Accurate Measurement in the Field of the Earth of the General-Relativistic Precession of the LAGEOS II Pericenter and New Constraints on Non-Newtonian Gravity Physical Review Letters, 105 (23) DOI: 10.1103/PhysRevLett.105.231103

From the abstract:

The pericenter shift of a binary system represents a suitable observable to test for possible deviations from the Newtonian inverse-square law in favor of new weak interactions between macroscopic objects. We analyzed 13 years of tracking data of the LAGEOS satellites with GEODYN II software but with no models for general relativity. From the fit of LAGEOS II pericenter residuals we have been able to obtain a 99.8% agreement with the predictions of Einstein’s theory

It’s always nice to see confirmations of general relativity, especially when they help put limits on possible deviations from the theory, ie. in this case, a Yukawa-like potential would have to have strength ≲1×10^{-11}.

For more, see Via satellite.

# High Energy Physics and Particles:

### LHC *Sees *Primordial Universe

From the Press Release:

Geneva, 26 November 2010.After less than three weeks of heavy-ion running, the three experiments studying lead ion collisions at the LHC have already brought new insight into matter as it would have existed in the very first instants of the Universe’s life. The ALICE experiment, which is optimised for the study of heavy ions, published two papers just a few days after the start of lead-ion running. Now, the first direct observation of a phenomenon known as jet quenching has been made by both the ATLAS and CMS collaborations.

Papers from the start of heavy-ion run are beginning to appear; it’s exciting that everything is working ideally, although *really exciting *results are still a ways off.

For more, see LHC experiments bring new insight into primordial universe, ATLAS Scientists catch glimpse of the Primordial Universe.

# General Relativity, Quantum Gravity, et al.:

### Black Hole Entropy, Loop Gravity and Polymer Physics, Oh My!

Eugenio Bianchi (2010). Black Hole Entropy, Loop Gravity, and Polymer Physics arXiv arXiv: 1011.5628v1

The abstract:

Loop Gravity provides a microscopic derivation of Black Hole entropy. In this paper, I show that the microstates counted admit a semiclassical description in terms of shapes of a tessellated horizon. The counting of microstates and the computation of the entropy can be done via a mapping to an equivalent statistical mechanical problem: the counting of conformations of a closed polymer chain. This correspondence suggests a number of intriguing relations between the thermodynamics of Black Holes and the physics of polymers.

The latest hot topic in gravity correspondences: Combining de Gennes-esque polymer physics techniques with black hole thermodynamics (as if I didn’t already have a big crush on Bianchi). There is very little about this that isn’t exciting.

*This Week in the Universe: November 23rd – November 29th* by *PLOS Blogs Network*, unless otherwise expressly stated, is licensed under a Creative Commons Attribution 4.0 International License.