arXiv subjects

Ramzi R. Khuri

Publications and source records attributed to Ramzi R. Khuri.

At least 19 recordsLinked to original sources

arXiv2023

Inertial Spinner Swarm Experiments: Spin Pumping, Entropy Oscillations and Spin Frustration

We present here an inertial active spinning swarm consisting of mixtures of opposite handedness torque driven spinners floating on an air bed with low damping. Depending on the relative spin sign, spinners can act as their own anti-particles and annihilate their spins. Rotational energy can become highly focused, with minority fraction spinners pumped to very high levels of spin angular momentum. Spinner handedness also matters at high spinner densities but not low densities: oscillations in the mixing spatial entropy of spinners over time emerge if there is a net spin imbalance from collective rotations. Geometrically confined spinners can lock themselves into frustrated spin states.

cond-mat.soft↗

arXiv2019

Remark on Protein Collapse from a Random Walk

I use a mean-field approximation to show the rapid collapse of heteropolymers from random walk size to a much smaller, molten globule state due to hydrophobic interactions, to be followed by a slower annealing process in which there is little change in overall size.

physics.bio-ph↗

arXiv2003

Dark Matter as Dark Energy

Velocity-dependent interactions in a fundamental-string dominated universe lead quite naturally, with reasonable assumptions on initial conditions, to an accelerating expanding universe without assuming the existence of a cosmological constant. This result also holds generically for a universe dominated by moving extremal black holes, owing to a repulsive velocity- dependent force. This interaction, however, does not preclude structure formation. Here we discuss a toy model including both ordinary and extremal matter, in which the latter accounts for dark matter while simultaneously acting as effective dark energy. Eternal acceleration is once more seen to arise in this case.

astro-ph↗

arXiv2002

Remarks on Fundamental String Cosmology

In recent work, it was shown that velocity-dependent forces between moving strings or branes lead to an accelerating expanding universe without assuming the existence of a cosmological constant. Here we show that the repulsive velocity-dependent force arises in more general contexts and can lead to cosmic structure formation.

hep-th↗

arXiv2002

Strings, Fivebranes and an Expanding Universe

It was recently shown that velocity-dependent forces between parallel fundamental strings moving apart in a $D-$dimensional spacetime implied an accelerating expanding universe in $D-1$-dimensional space-time. Exact solutions were obtained for the early time expansion in $D=5,6$. Here we show that this result also holds for fundamental strings in the background of a fivebrane, and argue that the feature of an accelerating universe would hold for more general $p$-brane-seeded models.

hep-th↗

arXiv2002

Velocity-Dependent Forces and an Accelerating Universe

In recent work, it was shown that velocity-dependent forces between parallel fundamental strings moving apart in a D-dimensional spacetime implied an expanding universe in D-1-dimensional spacetime. Here we expand on this work to obtain exact solutions for various string/brane cosmological toy models.

hep-th↗

arXiv2001

Remarks on Black Hole Degrees of Freedom in String Theory

The Bekenstein-Hawking black hole area entropy law suggests that the quantum degrees of freedom of black holes may be realized as projections of quantum states unto the event horizon of the black hole. In this paper, we provide further evidence for this interpretation in the context of string theory. In particular, we argue that increase in the quantum entropy due to the capture of infalling fundamental strings appears in the form of horizon degrees of freedom.

hep-th↗

arXiv2001

Fundamental Strings and Cosmology

We show that the velocity-dependent forces between parallel fundamental strings moving apart in a D-dimensional spacetime imply an expanding universe in (D-1)-dimensional spacetime.

hep-th↗

arXiv2000

Entropy and String Black Hole Correspondence

We make some observations regarding string/black hole correspondence with a view to understanding the nature of the quantum degrees of freedom of a black hole in string theory. In particular, we compare entropy change in analogous string and black hole processes in order to support the interpretation of the area law entropy as arising from stringy constituents.

hep-th↗

arXiv1999

Worldvolume Theories, Holography, Duality and Time

Duality transformations involving compactifications on timelike as well as spacelike circles link M-theory, the 10+1-dimensional strong coupling limit of IIA string theory, to other 11-dimensional theories in signatures 9+2 and 6+5 and to type II string theories in all 10-dimensional signatures. These theories have BPS branes of various world-volume signatures, and here we construct the world-volume theories for these branes, which in each case have 16 supersymmetries. For the generalised D-branes of the various type II string theories, these are always supersymmetric Yang-Mills theories with 16 supersymmetries, and we show that these all arise from compactifications of the supersymmetric Yang-Mills theories in 9+1 or 5+5 dimensions. We discuss the geometry of the brane solutions and, for the cases in which the world-volume theories are superconformally invariant, we propose holographically dual string or M theories in constant curvature backgrounds. For product space solutions $X\times Y$, there is in general a conformal field theory associated with the boundary of $X$ and another with the boundary of $Y$.

hep-th↗

arXiv1999

Self-Gravitating Strings and String/Black Hole Correspondence

In a recent essay, we discussed the possibility of using polymer sizing to model the collapse of a single, long excited string to a black hole. In this letter, we apply this idea to bring further support to string/black hole correspondence. In particular, we reproduce Horowitz and Polchinki's results for self-gravitating fundamental strings and speculate on the nature of the quantum degrees of freedom of black holes in string theory.

hep-th↗

arXiv1998

Black Holes and Strings: the Polymer Link

Quantum aspects of black holes represent an important testing ground for a theory of quantum gravity. The recent success of string theory in reproducing the Bekenstein-Hawking black hole entropy formula provides a link between general relativity and quantum mechanics via thermodynamics and statistical mechanics. Here we speculate on the existence of new and unexpected links between black holes and polymers and other soft-matter systems.

gr-qc↗

arXiv1998

On Generalized Axion Reductions

Recently interest in using generalized reductions to construct massive supergravity theories has been revived in the context of M-theory and superstring theory. These compactifications produce mass parameters by introducing a linear dependence on internal coordinates in various axionic fields. Here we point out that by extending the form of this simple ansatz, it is always possible to introduce the various mass parameters simultaneously. This suggests that the various ``distinct'' massive supergravities in the literature should all be a part of a single massive theory.

hep-th↗

arXiv1997

String Solitons and Black Hole Thermodynamics

We discuss the role of string solitons in duality and examine the feature of compositeness, which allows for the interpretation of general solutions as bound states of supersymmetric fundamental constituents. This feature lies at the heart of the recent success of string theory in reproducing the Bekenstein-Hawking black hole entropy formula. Talk given at 19th annual MRST meeting, Syracuse, NY, May 12-13, 1997.

hep-th↗

arXiv1997

Solitons, Black Holes and Duality in String Theory

These lectures are intended as an introduction to some of the basic aspects of string solitons, duality and black holes. We begin with a discussion of the role of classical solutions in duality, then focus on string/string duality and fundamental membranes. Finally, we examine the feature of compositeness of string solitons, and its implications for bound states and black hole thermodynamics. As these lectures are aimed primarily at those less familiar with this field, technical details are minimized.

hep-th↗

arXiv1996

Supersymmetry, Duality and Bound States

$P$-brane solutions of low-energy string actions have traditionally provided the first evidence for the existence of string dualities, in which fundamental and solitonic $p$-branes are identified with perturbative and non-perturbative BPS states. In this talk we discuss the composite nature of solutions, which allows for the interpretation of general solutions as bound states or intersections of maximally supersymmetric fundamental constituents. This feature lies at the heart of the recent success of string theory in reproducing the Beckenstein-Hawking black hole entropy formula.

hep-th↗

arXiv1996

Algebraic Renormalization of $N=1$ Supersymmetric Gauge Theories

The complete renormalization procedure of a general N=1 supersymmetric gauge theory in the Wess-Zumino gauge is presented, using the regulator free ``algebraic renormalization'' procedure. Both gauge invariance and supersymmetry are included into one single BRS invariance. The form of the general nonabelian anomaly is given. Furthermore, it is explained how the gauge BRS and the supersymmetry functional operators can be extracted from the general BRS operator. It is then shown that the supersymmetry operators actually belong to the closed, finite, Wess-Zumino superalgebra when their action is restricted to the space of the ``gauge invariant operators'', i.e. to the cohomology classes of the gauge BRS operator. An erratum is added at the end of the paper.

hep-th↗

arXiv1996

Entropy of 4D Extremal Black Holes

We derive the Bekenstein-Hawking entropy formula for four-dimensional Reissner-Nordstrom extremal black holes in type II string theory. The derivation is performed in two separate (T-dual) weak coupling pictures. One uses a type IIB bound state problem of D5- and D1-branes, while the other uses a bound state problem of D0- and D4-branes with macroscopic fundamental type IIA strings. In both cases, the D-brane systems are also bound to a Kaluza-Klein monopole, which then yields the four-dimensional black hole at strong coupling.

hep-th↗