arXiv subjects

Oriol Pujolas

Publications and source records attributed to Oriol Pujolas.

52 records · Page 3Linked to original sources

arXiv2008

Strongly Coupled Radiation from Moving Mirrors and Holography in the Karch-Randall model

Motivated by the puzzles in understanding how Black Holes evaporate into a strongly coupled Conformal Field Theory, we study particle creation by an accelerating mirror. We model the mirror as a gravitating Domain Wall and consider a CFT coupled to it through gravity, in asymptotically Anti de Sitter space. This problem (backreaction included) can be solved exactly at one loop. At strong coupling, this is dual to a Domain Wall localized on the brane in the Karch-Randall model, which can be fully solved as well. Hence, in this case one can see how the particle production is affected by A) strong coupling and B) its own backreaction. We find that A) the amount of CFT radiation at strong coupling is not suppressed relative to the weak coupling result; and B) once the boundary conditions in the AdS_5 bulk are appropriately mapped to the conditions for the CFT on the boundary of AdS_4, the Karch-Randall model and the CFT side agree to leading order in the backreaction. This agreement holds even for a new class of self-consistent solutions (the `Bootstrap' Domain Wall spacetimes) that have no classical limit. This provides a quite precise check of the holographic interpretation of the Karch-Randall model. We also comment on the massive gravity interpretation. As a byproduct, we show that relativistic Cosmic Strings (pure tension codimension 2 branes) in Anti de Sitter are repulsive and generate long-range tidal forces even at classical level. This is the phenomenon dual to particle production by Domain Walls.

hep-th↗

arXiv2008

Non Pauli-Fierz Massive Gravitons

We study general Lorentz invariant theories of massive gravitons. We show that, contrary to the standard lore, there exist consistent theories where the graviton mass term violates Pauli-Fierz structure. For theories where the graviton is a resonance this does not imply the existence of a scalar ghost if the deviation from Pauli-Fierz becomes sufficiently small at high energies. These types of mass terms are required by any consistent realization of the DGP model in higher dimension.

hep-th↗

arXiv2008

Dressed Domain Walls and Holography

The cutoff version of the AdS/CFT correspondence states that the Randall Sundrum scenario is dual to a Conformal Field Theory (CFT) coupled to gravity in four dimensions. The gravitational field produced by relativistic domain walls can be exactly solved in both sides of the correspondence, and thus provides one further check of it. We show in the two sides that for the most symmetric case, the wall motion does not lead to particle production of the CFT fields. Still, there are nontrivial effects. Due to the trace anomaly, the CFT effectively renormalizes the domain wall tension. On the five dimensional side, the wall is a codimension 2 brane localized on the Randall-Sundrum brane, which pulls the wall in a uniform acceleration. This is perceived from the brane as a domain wall with a tension slightly larger than its bare value. In both cases, the deviation from General Relativity appears at nonlinear level in the source, and the leading corrections match to the numerical factors.

hep-th↗

arXiv2007

Cascading DGP

We present a higher codimension generalization of the DGP scenario which, unlike previous attempts, is free of ghost instabilities. The 4D propagator is made regular by embedding our visible 3-brane within a 4-brane, each with their own induced gravity terms, in a flat 6D bulk. The model is ghost-free if the tension on the 3-brane is larger than a certain critical value, while the induced metric remains flat. The gravitational force law `cascades' from a 6D behavior at the largest distances followed by a 5D and finally a 4D regime at the shortest scales.

hep-th↗

arXiv2007

Bubbles in the Self-Accelerating Universe

We revisit the issue of the stability in the Dvali-Gabadadze-Porrati model, by considering the nucleation of bubbles of the conventional branch within the self-accelerating branch. We construct an instanton describing this process in the thin wall approximation. On one side of the bubble wall, the bulk consists of the exterior of the brane while on the other side it is the interior. The solution requires the presence of a 2-brane (the bubble wall) which induces the transition. However, we show that this instanton cannot be realized as the thin wall limit of any smooth solution. Once the bubble thickness is resolved, the equations of motion do not allow O(4) symmetric solutions joining the two branches. We conclude that the thin wall instanton is unphysical, and that one cannot have processes connecting the two branches, unless negative tension bubble walls are introduced. This also suggests that the self-accelerating branch does not decay into the conventional branch nucleating bubbles. We comment on other kinds of bubbles that could interpolate between the two branches.

hep-th↗

arXiv2007

Consistent Lorentz Violation in Flat and Curved Space

Motivated by the severity of the bounds on Lorentz violation in the presence of ordinary gravity, we study frameworks in which Lorentz violation does not affect the spacetime geometry. We show that there are at least two inequivalent classes of spontaneous Lorentz breaking that even in the presence of gravity result in Minkowski space. The first one generically corresponds to the condensation of tensor fields with tachyonic mass which in turn is related to ghost-condensation. In the second class, realized by the DGP model or theories of massive gravitons, spontaneous Lorentz breaking is induced by the expectation value of sources. The generalization to de-Sitter space is also discussed.

hep-th↗

arXiv2006

Domain Walls As Probes Of Gravity

We show that domain walls are probes that enable one to distinguish large-distance modified gravity from general relativity (GR) at short distances. For example, low-tension domain walls are stealth in modified gravity, while they do produce global gravitational effects in GR. We demonstrate this by finding exact solutions for various domain walls in the DGP model. A wall with tension lower than the fundamental Planck scale does not inflate and has no gravitational effects on a 4D observer, since its 4D tension is completely screened by gravity itself. We argue that this feature remains valid in a generic class of models of infrared modified gravity. As a byproduct, we obtain exact solutions for super-massive codimension-2 branes.

hep-th↗

arXiv2006

Quantum fluctuations in the DGP model and the size of the cross-over scale

The Dvali-Gabadadze-Porrati model introduces a parameter, the cross-over scale $r_c$, setting the scale where higher dimensional effects are important. In order to agree with observations and to explain the current acceleration of the Universe, $r_c$ must be of the order of the present Hubble radius. We discuss a mechanism to generate a large $r_c$, assuming that it is determined by a dynamical field and exploiting the quantum effects of the graviton. For simplicity, we consider a scalar field $Ψ$ with a kinetic term on the brane instead of the full metric perturbations. We compute the Green function and the 1-loop expectation value of the stress tensor of $Ψ$ on the background defined by a flat bulk and an inflating brane (self-accelerated or not). We also include the flat brane limit. The quantum fluctuations of the bulk field $Ψ$ provide an effective potential for $r_c$. For a flat brane, the 1-loop effective potential is of the Coleman-Weinberg form, and admits a minimum for large $r_c$ without fine tuning. When we take into account the brane curvature, a sizeable contribution at the classical level changes this picture and the potential develops a (minimum) maximum for the (non-) self-accelerated branch.

hep-th↗

arXiv2006

Critical escape velocity of black holes from branes

In recent work we have shown that a black hole stacked on a brane escapes once it acquires a recoil velocity. This result was obtained in the {\it probe-brane} approximation, {\it i.e.}, when the tension of the brane is negligibly small. Therefore, it is not clear whether the effect of the brane tension may prevent the black hole from escaping for small recoil velocities. The question is whether a critical escape velocity exists. Here, we analyze this problem by studying the interaction between a Dirac-Nambu-Goto brane and a black hole assuming adiabatic (quasi-static) evolution. By describing the brane in a fixed black hole spacetime, which restricts our conclusions to lowest order effects in the tension, we find that the critical escape velocity does not exist for co-dimension one branes, while it does for higher co-dimension branes.

hep-th↗

arXiv2006

Dynamics of domain walls intersecting black holes

Previous studies concerning the interaction of branes and black holes suggested that a small black hole intersecting a brane may escape via a mechanism of reconnection. Here we consider this problem by studying the interaction of a small black hole and a domain wall composed of a scalar field and simulate the evolution of this system when the black hole acquires an initial recoil velocity. We test and confirm previous results, however, unlike the cases previously studied, in the more general set-up considered here, we are able to follow the evolution of the system also during the separation, and completely illustrate how the escape of the black hole takes place.

hep-th↗

arXiv2005

Vacuum destabilization from Kaluza-Klein modes in an inflating brane

We discuss the effects from the Kaluza-Klein modes in the brane world scenario when an interaction between bulk and brane fields is included. We focus on the bulk inflaton model, where a bulk field $Ψ$ drives inflation in an almost $AdS_5$ bulk bounded by an inflating brane. We couple $Ψ$ to a brane scalar field $ϕ$ representing matter on the brane. The bulk field $Ψ$ is assumed to have a light mode, whose mass depends on the expectation value of $ϕ$. To estimate the effects from the KK modes, we compute the 1-loop effective potential $V_\eff(ϕ)$. With no tuning of the parameters of the model, the vacuum becomes (meta)stable -- $V_\eff(ϕ)$ develops a true vacuum at a nonzero $ϕ$. In the true vacuum, the light mode of $Ψ$ becomes heavy, degenerates with the KK modes and decays. We comment on some implications for the bulk inflaton model. Also, we clarify some aspects of the renormalization procedure in the thin wall approximation, and show that the fluctuations in the bulk and on the brane are closely related.

hep-th↗

arXiv2004

Massless scalar fields and infrared divergences in the inflationary brane world

We study the quantum effects induced by bulk scalar fields in a model with a de Sitter (dS) brane in a flat bulk (the Vilenkin-Ipser-Sikivie model) in more than four dimensions. In ordinary dS space, it is well known that the stress tensor in the dS invariant vacuum for an effectively massless scalar ($m_\eff^2=m^2+ξ{\cal R}=0$ with ${\cal R}$ the Ricci scalar) is infrared divergent except for the minimally coupled case. The usual procedure to tame this divergence is to replace the dS invariant vacuum by the Allen Follaci (AF) vacuum. The resulting stress tensor breaks dS symmetry but is regular. Similarly, in the brane world context, we find that the dS invariant vacuum generates $\tmn$ divergent everywhere when the lowest lying mode becomes massless except for massless minimal coupling case. A simple extension of the AF vacuum to the present case avoids this global divergence, but $\tmn$ remains to be divergent along a timelike axis in the bulk. In this case, singularities also appear along the light cone emanating from the origin in the bulk, although they are so mild that $\tmn$ stays finite except for non-minimal coupling cases in four or six dimensions. We discuss implications of these results for bulk inflaton models. We also study the evolution of the field perturbations in dS brane world. We find that perturbations grow linearly with time on the brane, as in the case of ordinary dS space. In the bulk, they are asymptotically bounded.

gr-qc↗

arXiv2003

Moduli stabilization in higher dimensional brane models

We consider a class of warped higher dimensional brane models with topology $M \times Σ\times S^1/Z_2$, where $Σ$ is a $D_2$ dimensional manifold. Two branes of codimension one are embedded in such a bulk space-time and sit at the orbifold fixed points. We concentrate on the case where an exponential warp factor (depending on the distance along the orbifold) accompanies the Minkowski $M$ and the internal space $Σ$ line elements. We evaluate the moduli effective potential induced by bulk scalar fields in these models, and we show that generically this can stabilize the size of the extra dimensions. As an application, we consider a scenario where supersymmetry is broken not far below the cutoff scale, and the hierarchy between the electroweak and the effective Planck scales is generated by a combination of redshift and large volume effects. The latter is efficient due to the shrinking of $Σ$ at the negative tension brane, where matter is placed. In this case, we find that the effective potential can stabilize the size of the extra dimensions (and the hierarchy) without fine tuning provided that the internal space $Σ$ is flat.

hep-th↗

arXiv2003

Moduli effective action in warped brane world compactifications

We consider a class of 5-D brane-world solutions with a power-law warp factor $a(y)\propto y^{q}$, and bulk dilaton with profile $ϕ\propto \ln y$, where $y$ is the proper distance in the extra dimension. This class includes the Heterotic M-theory brane-world of Refs. \cite{ovrut,ekpy} and the Randall-Sundrum (RS) model as a limiting case. In general, there are two moduli fields $y_{\pm}$, corresponding to the "positions" of two branes (which live at the fixed points of an orbifold compactification). Classically, the moduli are massless, due to a scaling symmetry of the action. However, in the absence of supersymmetry, they develop an effective potential at one loop. Local terms proportional to $K_\pm^4$, where $K_\pm=q/y_\pm$ is the local curvature scale at the location of the corresponding brane, are needed in order to remove the divergences in the effective potential. Such terms break the scaling symmetry and hence they may act as stabilizers for the moduli. When the branes are very close to each other, the effective potential induced by massless bulk fields behaves like $V\sim d^{-4}$, where $d$ is the separation between branes. When the branes are widely separated, the potentials for each one of the moduli generically develop a "Coleman-Weinberg"-type behaviour of the form $a^4(y_\pm)K_\pm^4 \ln(K_\pm/μ_\pm)$, where $μ_\pm$ are renormalization scales. In the RS case, the bulk geometry is $AdS$ and $K_\pm$ are equal to a constant, independent of the position of the branes, so these terms do not contribute to the mass of the moduli. However, for generic warp factor, they provide a simple stabilization mechanism. For $q\gtrsim 10$, the observed hierarchy can be naturally generated by this potential, giving the lightest modulus a mass of order $m_- \lesssim TeV$.

hep-th↗

arXiv2001

Effective potential in Brane-World scenarios

We review the stabilization of the radion in the Randall-Sundrum model through the Casimir energy due to a bulk conformally coupled field. We also show some exact self-consistent solutions taking into account the backreaction that this energy induces on the geometry.

hep-th↗

arXiv2000

Radion effective potential in the Brane-World

We show that in brane-world scenarios with warped extra dimensions, the Casimir force due to bulk matter fields may be sufficient to stabilize the radion field $ϕ$. In particular, we calculate one loop effective potential for $ϕ$ induced by bulk gravitons and other possible massless bulk fields in the Randall-Sundrum background. This potential has a local extremum, which can be a maximum or a minimum depending on the detailed bulk matter content. If the parameters of the background are chosen so that the hierarchy problem is solved geometrically, then the radion mass induced by Casimir corrections is hierarchycally smaller than the $TeV$. Hence, in this important case, we must invoke an alternative mechanism (classical or nonperturbative) which gives the radion a sizable mass, to make it compatible with observations.

hep-th↗