28,456 research outputs found

### A New Model of Holographic Dark Energy with Action Principle

We propose a new model of holographic dark energy with an action. It is the
first time that one can derive a HDE model from the action principle. The
puzzles of causality and circular logic about HDE have been completely solved
in this model. The evolution of the universe only depends on the present state
of the universe, clearly showing that it obeys the law of causality.
Furthermore, the use of future event horizon as a present cut-off is not an
input but automatically follows from equations of motion. Interestingly, this
new model is very similar to the initial one of Li except a new term which may
be explained as dark radiation.Comment: 11 pages, no figures, typo corrected, English improve

### Degrees of freedom of $f(T)$ gravity

We investigate the Hamiltonian formulation of $f(T)$ gravity and find that
there are five degrees of freedom. The six first class constraints
corresponding to the local Lorentz transformation in Teleparallel gravity
become second class constraints in $f(T)$ gravity, which leads to the
appearance of three extra degrees of freedom and the violation of the local
Lorentz invariance in $f(T)$ gravity. In general, there are $D-1$ extra degrees
of freedom for $f(T)$ gravity in $D$ dimensions, and this implies that the
extra degrees of freedom correspond to one massive vector field or one massless
vector field with one scalar field.Comment: 18 pages, some references added, minor revision to appear in JHE

### Quantum Corrections to Noncommutative Solitons

Noncommutative solitons are easier to find in a noncommutative field theory.
Similarly, the one-loop quantum corrections to the mass of a noncommutative
soliton are easier to compute, in a real scalar theory in 2+1 dimensions. We
carry out this computation in this paper. We also discuss the model with a
double-well potential, and conjecture that there is a partial symmetry
restoration in a vacuum state.Comment: 19 pages, harvmac. v2: a ref. added, minor typos correcte

### Energy-Momentum Conservation and Holographic S-Matrix

We investigate the consequence of the energy-momentum conservation law for
the holographic S-matrix from AdS/CFT correspondence. It is shown that the
conservation law is not a natural consequence of conformal invariance in the
large N limit. We predict a new singularity for the four point correlation
function of a marginal operator. Only the two point scattering amplitude is
explicitly calculated, and the result agrees with what is expected.Comment: 16 pages, harvma

### Comments on Supersymmetric Yang-Mills Theory on a Noncommutative Torus

D0-brane theory on a torus with a nonvanishing B field is embedded into a
string theory in the weak coupling limit. It is shown that the usual
supersymmetric Yang-Mills theory on a noncommutative torus can not be the whole
story. The Born-Infeld action survives the noncommutative torus limit.Comment: 11 pages, two attached figures, harvma

### A proposal on the topological sector of 2d string

The field content of the two dimensional string theory consists of the
dynamical tachyon field and some nonpropagating fields which consist in the
topological sector of this theory. We propose in this paper to study this
topological sector as a spacetime gauge theory with a simple centrally extended
$w_\infty$ algebra. This $w_\infty$ algebra appears in both the world sheet
BRST analysis and the matrix model approach. Since the two dimensional
centrally extended Poincar\'e algebra is naturally embedded in the centrally
extended $w_\infty$ algebra, the low energy action for the metric and dilaton
appears naturally when the model is truncated at this level. We give a
plausible explanation of emergence of discrete states in this formulation. This
theory is again the effective theory at zero slope limit. To include higher
order $\alpha'$ corrections, we speculate that the whole theory is a gauge
theory of a deformed $w_\infty$ algebra, and the deformation parameter is just
$\alpha'$.Comment: no major changes in revisio

### Correlators of Special States in c=1 Liouville Theory

We generalize the ground ring structure to all special BRST invariant
operators in the right branch in the c=1 Liouville theory. We also discuss
correlation functions of special states on the sphere.Comment: 17 page

### Gluino Condensation in F-theory

Coupling of bilinears of gauginos on D-branes to anti-symmetric tensor fields
can be deduced by comparing F-theory compactified to 8 dimensions with the
heterotic string on $R^8\times T^2$. Application to SUSY breaking in F-theory
triggered by gaugino condensation is discussed. Compactification to four
dimensions with a warp factor is briefly discussed. In this case some 3-branes
whose world-volume coincides with the open spacetime must be introduced, in
addition to 7-branes.Comment: 11 pages, harvmac. One mistake and some typos corrected, two
references adde

### Black Holes and Spacetime Physics in String/M Theory

In addition to briefly reviewing recent progress in studying black hole
physics in string/M theory, we describe several robust features pertaining to
spacetime physics that one can glean by studying quantum physics of black
holes. In particular, we review 't Hooft's S-matrix ansatz which results in a
noncommutative horizon. A recent construction of fuzzy AdS2 is emphasized, this
is a nice toy model for fuzzy black hole horizon. We demonstrate that this
model captures some nonperturbative features of quantum gravity.Comment: 33 pages, harvma

### Dirichlet String Theory and Singular Random Surfaces

We show that string theory with Dirichlet boundaries is equivalent to string
theory containing surfaces with certain singular points. Surface curvature is
singular at these points. A singular point is resolved in conformal coordinates
to a circle with Dirichlet boundary conditions. We also show that moduli
parameters of singular surfaces coincide with those of smooth surfaces with
boundaries. Singular surfaces with saddle points indeed arise in the strong
coupling expansion in lattice QCD. The kind of saddle point, which may be the
origin of a singular point we need, is of infinite order.Comment: 8 pages, BROWN-HET-923, a minor error is correcte

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