Publication 1: Delocalization from anomaly inflow and intersecting brane dynamics. (URL)
Publication 2: Resonance in asymmetric warped geometry. (URL)
Publication 3: Lorentz-violating massive gauge and gravitational fields. (URL)
URL of Research:
Statement of Interest:
My scientific interests are in the context of string theory, high
energy physics, particle physics and cosmology. In particular, my work
has focused on large distance modifications of gravity, on brane-world
scenarios and their holographic description, on strongly coupled field
theories and their analysis using gauge/gravity duality, and on the
physics of black holes and gravitational thermodynamics.
Various recent experimental data indicate that the Universe may be
undergoing a phase of accelerated expansion. This could be explained
by the existence of a non-zero cosmological constant, but its
smallness would require an unnatural fine-tuning. Alternative avenues
have therefore been explored, in particular models where cosmic
acceleration can be explained as a dynamical effect, stemming from a
modification of the laws of gravity on cosmological scales. In this
context, I have worked on two classes of models, where this idea is
explicitly realized: theories of massive gravity and (warped)
extra-dimensional models.
The former explores the effect of a graviton mass term. Naively the
zero-mass limit would be expected to be indistinguishable from general
relativity. However, even a tiny graviton mass is incompatible (at
linearized level) with experiments like light bending. This phenomenon
is called van Dam-Veltman-Zakharov (vDVZ) discontinuity.
Moreover, when classical non-linearities are taken into consideration,
the Hamiltonian becomes unbounded from below, hence the theory
develops what is known as Boulware-Deser (BD) instability. In order to
avoid these difficulties, particularly the last one, I explored with
G.~Gabadadze (hep-th/0412332) a class of Lorentz-violating massive
gravity models. We found that these models do not present the vDVZ
discontinuity, nor the BD instability, as their ADM Hamiltonian is
bounded from below. The explicit breaking of Lorentz invariance leads
to an instantaneous interaction resulting in superluminal propagation
of the gravitational field, but for small graviton mass this effect is
not experimentally ruled out. We showed that any attempt to remove
superluminal propagation and restore causality leads to either vDVZ
discontinuity, or BD instability, or both.
In the context of brane-world scenarios I investigated, with
G.~Gabadadze and Y.~Shang (hep-th/0604218), the presence of a massive
resonance in a detuned Randall-Sundrum (RS) model. The model consists
of a flat four-dimensional brane embedded in five dimensions, at the
interface between two slices of Anti de Sitter space. The curvature of
the bulk is set to be different on the two different sides of the
brane. The low energy graviton spectrum, from the four-dimensional
point of view, consists of two different modes: one is analogous to
the one found in the RS model; the other is a massive resonance
state. The presence of the latter is linked to the would-be
antisymmetric mode which is projected out in the RS model. Removing
the reflection symmetry with respect to the brane allows this mode to
propagate, thus to appear on top of the usual RS spectrum.
We also studied the holographic dual interpretation of this model,
along the lines of the AdS/CFT correspondence. We suggested that the
dual theory is a product of two conformal field theories with
different UV-cutoff coupled through a relevant operator. To motivate
this conjecture we evaluated the effective theory when a slab of one
of the two AdS slices is integrated out. In the dual picture, this is
equivalent to performing a renormalization group flow. We found the
effective theory to be a five-dimensional theory with localized four
dimensional gravity, equivalent to the Dvali-Gabadadze-Porrati (DGP)
model, whose spectrum contains a metastable graviton.
Recently, I have been interested in the application of the holographic
duality to investigate strongly coupled field theories. The strongly
coupled dynamics of theories like QCD is difficult to describe
analytically in terms of their fundamental degrees of freedom. Within
string theory however, the gauge/gravity duality provides a setup in
which quantities in the strongly coupled regime can be calculated
explicitly: its dynamics can be mapped into the dynamics of weakly
coupled gravity models. Although the exact gravity dual of QCD is
unknown, it is known of supersymmetric models which qualitatively
resemble QCD.
Using these ideas, in (hep-th/0611331) I explored the dynamics of
chiral symmetry breaking in the Gross-Neveu (GN) model, a
two-dimensional theory of fermions with quartic interaction. From the
field theory point of view, it is known that no Nambu-Goldstone (NG)
boson appears upon spontaneous symmetry breaking, a general property
of two-dimensional field theories. I investigated how the
corresponding phenomenon is realized in the dual gravity picture,
which consists of the near-horizon geometry of intersecting
$D$-branes, with chiral fermions localized at the intersection. In
this picture, the NG boson is described by the zero-mode of a higher
dimensional gauge field coupled to the fermions. The definite fermion
chirality yields a non-zero anomaly, which is canceled by an anomalous
topological term in the bulk. I have shown that the presence of this
term modifies the dynamics of the gauge field to the extent of forcing
it away from the intersection. Thus its zero-mode, dual to the NG
boson, is removed from the low energy spectrum.