arXiv subjects

M. M. Sheikh-Jabbari

Publications and source records attributed to M. M. Sheikh-Jabbari.

At least 145 records · Page 8Linked to original sources

arXiv2013

Gauge-flation: Inflation From Non-Abelian Gauge Fields

Inflationary models are usually based on dynamics of one or more scalar fields coupled to gravity. In this work we present a new class of inflationary models, gauge-flation or non-Abelian gauge field inflation, where slow-roll inflation is driven by a non-Abelian gauge field. This class of models are based on a gauge field theory with a generic non-Abelian gauge group minimally coupled to gravity. We then focus on a particular gauge-flation model by specifying the action for the gauge theory which allows for a successful slow-roll inflation. This model has two parameters the value of which can be fixed using the CMB and other cosmological data. These values are within the natural range of parameters in generic grand unified theories of particle physics.

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arXiv2013

Non-Abelian Gauge Field Inflation

In [arXiv:1102.1513] we introduced an inflationary scenario, Non-Abelian Gauge Field Inflation or gauge-flation for short, in which slow-roll inflation is driven by non-Abelian gauge field minimally coupled to gravity. We present a more detailed analysis, both numerical and analytical, of the gauge-flation. By studying the phase diagrams of the theory, we show that getting enough number of e-folds during a slow-roll inflation is fairly robust to the choice of initial gauge field values. In addition, we present a detailed analysis of the cosmic perturbation theory in gauge-flation which has many special and interesting features compared the standard scalar-driven inflationary models. The specific gauge-flation model we study in this paper has two parameters, a cutoff scale Lambda and the gauge coupling g. Fitting our results with the current cosmological data fixes Λ\sim 10 H \sim 10^{15} GeV (H is the Hubble parameter) and g\sim 10^{-4}, which are in the natural range of parameters in generic particle physics beyond standard models. Our model also predicts a tensor-to-scalar ratio r>0.05, in the range detectable by the Planck satellite.

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arXiv2013

Gauge Fields and Inflation

The isotropy and homogeneity of the cosmic microwave background (CMB) favors "scalar driven" early Universe inflationary models. Non-scalar fields, and in particular gauge fields, are on the other hand commonplace in all high energy particle physics models proposed to be at work at the upper bound on energy scale of inflation set by the current CMB observations. In this review we consider the role and consequences, theoretical and observational, that gauge fields can have during inflationary era. Gauge fields may be turned on in the background during inflation, or may become relevant at the level of cosmic perturbations. There have been two main class of models with gauge fields in the background, models which show violation of cosmic no-hair theorem and those which lead to isotropic FLRW cosmology, respecting the cosmic no-hair theorem. Models in which gauge fields are only turned on at the cosmic perturbation level, may source primordial magnetic fields. We also review specific observational features of these models on the CMB and/or the primordial cosmic magnetic fields. Our discussions will be mainly focused on the inflation period, with only a brief discussion on the post inflationary (p)reheating era.

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arXiv2012

Revisiting Cosmic No-Hair Theorem for Inflationary Settings

In this work we revisit Wald's cosmic no-hair theorem in the context of accelerating Bianchi cosmologies for a generic cosmic fluid with non-vanishing anisotropic stress tensor and when the fluid energy momentum tensor is of the form of a cosmological constant term plus a piece which does not respect strong or dominant energy conditions. Such a fluid is the one appearing in inflationary models. We show that for such a system anisotropy may grow, in contrast to the cosmic no-hair conjecture. In particular, for a generic inflationary model we show that there is an upper bound on the growth of anisotropy. For slow-roll inflationary models our analysis can be refined further and the upper bound is found to be of the order of slow-roll parameters. We examine our general discussions and our extension of Wald's theorem for three classes of slow-roll inflationary models, generic multi-scalar field driven models, anisotropic models involving U(1) gauge fields and the gauge-flation scenario.

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arXiv2012

Gauge-flation Vs Chromo-Natural Inflation

Gauge-flation, non-Abelian gauge field inflation, which was introduced in arXiv:1102.1513 and analyzed more thoroughly in arXiv:1102.1932, is a model of inflation driven by non-Abelian gauge fields minimally coupled to Einstein gravity. In this model certain rotationally invariant combination of gauge fields play the role of inflaton. Recently, the chromo-natural inflation model was proposed arXiv:1202.2366 which besides the non-Abelian gauge fields also involve an axion field. In this short note we show that the model involving axions, indeed allows for various slow-roll trajectories for different values of its parameters: A specific trajectory discussed in arXiv:1202.2366 starts from a "small axion" region, while the trajectory considered in arXiv:1102.1513 corresponds to a "large axion" region.

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arXiv2011

Emergent IR dual 2d CFTs in charged AdS5 black holes

We study the possible dynamical emergence of IR conformal invariance describing the low energy excitations of near-extremal R-charged global AdS5 black holes. We find interesting behavior especially when we tune parameters in such a way that the relevant extremal black holes have classically vanishing horizon area, i.e. no classical ground-state entropy, and when we combine the low energy limit with a large N limit of the dual gauge theory. We consider both near-BPS and non-BPS regimes and their near horizon limits, emphasize the differences between the local AdS3 throats emerging in either case, and discuss potential dual IR 2d CFTs for each case. We compare our results with the predictions obtained from the Kerr/CFT correspondence, and obtain a natural quantization for the central charge of the near-BPS emergent IR CFT which we interpret in terms of the open strings stretched between giant gravitons.

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arXiv2011

1/N Resolution to Inflationary eta-Problem

We observe that the dominant one loop contribution to the graviton propagator in the theory of N (N>>1) light scalar fields ϕ_a (with masses smaller than M_{pl}/\sqrt{N}) minimally coupled to Einstein gravity is proportional to N while that of graviton-scalar-scalar interaction vertex is N independent. We use this to argue that the coefficient of the Rϕ_a^2 term appearing at one loop level is 1/N suppressed. This observation provides a resolution to the η-problem, that the slow-roll parameter ηreceives order one quantum loop corrections for inflationary models built within the framework of scalar fields minimally coupled to Einstein gravity, for models involving large number of fields. As particular examples, we employ this to argue in favor of the absence of η-problem in M-flation and N-flation scenarios.

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arXiv2011

Gauge-flation and Cosmic No-Hair Conjecture

Gauge-flation, inflation from non-Abelian gauge fields, was introduced in [1,2]. In this work, we study the cosmic no-hair conjecture in gauge-flation. Starting from Bianchi-type I cosmology and through analytic and numeric studies we demonstrate that the isotropic FLRW inflation is an attractor of the dynamics of the theory and that the anisotropies are damped within a few e-folds, in accord with the cosmic no-hair conjecture.

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arXiv2011

EVH Black Holes, AdS3 Throats and EVH/CFT Proposal

Within class of generic black holes there are extremal black holes (with vanishing Hawking temperature T) and vanishing horizon area Ah, but with finite Ah/T ratio,the Extremal Vanishing Horizon (EVH) black holes. We study the near horizon limit of a four dimensional EVH black hole solution to a generic (gauged) Einstein-Maxwell dilaton theory and show that in the near horizon limit they develop a throat which is a pinching orbifold limit of AdS3. This is an extension of the well known result for extremal black holes the near horizon limit of which contains an AdS2 throat. We show that in the near EVH near horizon limit the pinching AdS3 factor turns to a pinching BTZ black hole and that this near horizon limit is indeed a decoupling limit. We argue that the pinching AdS3 or BTZ orbifold is resolved if the near horizon limit is accompanied by taking the 4d Newton constant G4 to zero such that the Bekenstein-Hawking entropy S = Ah/(4G4) remains finite. We propose that in this limit the near horizon EVH black hole is dual to a 2d CFT. We provide pieces of evidence in support of the EVH/CFT correspondence and comment on its connection to the Kerr/CFT proposal and speculations how the EVH/CFT may be used to study generic e.g. Schwarzchild-type black holes.

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arXiv2011

Near Horizon Limits of Massless BTZ and Their CFT Duals

We consider the massless BTZ black hole and show that it is possible to take its "near horizon" limit in two distinct ways. The first one leads to a null self-dual orbifold of AdS3 and the second to a spacelike singular AdS3/Z_K orbifold in the large K limit, the "pinching orbifold". We show that from the dual 2d CFT viewpoint, the null orbifold corresponds to the p^+=0 sector of the Discrete Light-Cone Quantisation (DLCQ) of the 2d CFT where a chiral sector of the CFT is decoupled, while the pinching orbifold corresponds to taking an infinite mass gap limit in both the right and left sectors of the 2d CFT, essentially leaving us with the states L_0=\bar L_0=c/24 only. In the latter case, one can combine the near horizon limit with sending the 3d Planck length l_P to zero, or equivalently the dual CFT central charge c to infinity. We provide preliminary evidence that in that case some nontrivial dynamics may survive the limit.

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arXiv2011

Gauged M-flation, its UV sensitivity and Spectator Species

In this paper we study gauged M-flation, an inflationary model in which inflation is driven by three NxN scalar field matrices in the adjoint representation of U(N) gauge group. We focus our study on the gauged M-flation model which could be derived from the dynamics of a stack of D3-branes in appropriate background flux. The background inflationary dynamics is unaltered compared to the ungauged case of [arXiv:0903.1481[hep-th]], while the spectrum of "spectator species", the isocurvature modes, differs from the ungauged case. Presence of a large number of spectators, although irrelevant to the slow-roll inflationary dynamics has been argued to lower the effective UV cutoff $Λ$ of the theory from the Planck mass, invalidating the main advantage of M-flation in not having super-Planckian field values and unnaturally small couplings. Through a careful analysis of the spectrum of the spectators we argue that, contrary to what happens in N-flation models, M-flation is still UV safe with the modified (reduced) effective UV cutoff $Λ$, which we show to be of order (0.5-1)x10^{-1} M_{pl}. Moreover, we argue that the string scale in our gauged M-flation model is larger than $Λ$ by a factor of 10 and hence one can also neglect stringy effects. We also comment on the stability of classical inflationary paths in the gauged M-flation.

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arXiv2011

Beyond Logarithmic Corrections to Cardy Formula

As shown by Cardy modular invariance of the partition function of a given unitary non-singular 2d CFT with left and right central charges c_L and c_R, implies that the density of states in a microcanonical ensemble, at excitations Delta and Delta-bar and in the saddle point approximation, is ρ_0(Δ,\barΔ;c_L, c_R)=c_L c_R \exp(2π\sqrt{{c_LΔ}/{6}})\exp(2π\sqrt{{c_R\barΔ}/{6}}). In this paper, we extend Cardy's analysis and show that in the saddle point approximation and up to contributions which are exponentially suppressed compared to the leading Cardy's result, the density of states takes the form ρ(Δ,\barΔ; c_L,c_R)= f(c_LΔ) f(c_R\barΔ)ρ_0(Δ,\barΔ; c_L, c_R), for a function f(x) which we specify. In particular, we show that (i) ρ(Δ,\barΔ; c_L, c_R) is the product of contributions of left and right movers and hence, to this approximation, the partition function of any modular invariant, non-singular unitary 2d CFT is holomorphically factorizable and (ii) ρ(Δ,\barΔ; c_L, c_R)/(c_Lc_R) is only a function of $c_R\barΔ$ and $c_LΔ$. In addition, treating ρ(Δ,\barΔ; c_L, c_R) as the density of states of a microcanonical ensemble, we compute the entropy of the system in the canonical counterpart and show that the function f(x) is such that the canonical entropy, up to exponentially suppressed contributions, is simply given by the Cardy's result \lnρ_0(Δ,\barΔ; c_L, c_R).

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arXiv2010

M-flation: Inflation From Matrix Valued Scalar Fields

We propose an inflationary scenario, M-flation, in which inflation is driven by three $N\times N$ hermitian matrices $Φ_i, i=1,2,3$. The inflation potential of our model, which is strongly motivated from string theory, is constructed from $Φ_{i}$ and their commutators. We show that one can consistently restrict the classical dynamics to a sector in which the $Φ_i$ are proportional to the $N\times N$ irreducible representations of SU(2). In this sector our model effectively behaves as an N-flation model with $3 N^2$ number of fields and the effective inflaton field has a super-Planckian field value. Furthermore, the fine-tunings associated with unnaturally small couplings in the chaotic type inflationary scenarios are removed. Due to the matrix nature of the inflaton fields there are $3N^2-1$ extra scalar fields in the dynamics. These have the observational effects such as production of iso-curvature perturbations on cosmic microwave background. Moreover, the existence of these extra scalars provides us with a natural preheating mechanism and exit from inflation. As the effective inflaton field can traverse super-Planckian distances in the field space, the model is capable of producing a considerable amount of gravity waves that can be probed by future CMB polarization experiments such as PLANCK, QUIET and CMBPOL.

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arXiv2010

Light-Like Noncommutativity, Light-Front Quantization and New Light on UV/IR Mixing

We revisit the problem of quantizing field theories on noncommutative Moyal spacetime with \emph{light-like} noncommutativity. To tackle the issues arising from noncommuting and hence nonlocal time, we argue that for this case light-front quantization procedure should be employed. In this appropriate quantization scheme we perform the non-planar loop analysis for the light-like noncommutative field theories. One of the important and peculiar features of light-front quantization is that the UV cutoff of the light-cone Hamiltonian manifests itself as an IR cutoff for the light-cone momentum, $p^+$. Due to this feature, the naive results of covariant quantization for the light-like case allude to the absence of the UV/IR mixing in the light-front quantization. However, by a careful analysis of non-planar loop integrals we show that this is not the case and the UV/IR mixing persists. In addition, we argue in favour of the perturbative unitarity of light-like noncommutative field theories in the light-front quantization scheme.

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arXiv2010

Orientifolded Locally AdS3 Geometries

Continuing the analysis of [arXiv:1003.4089[hep-th]], we classify all locally AdS3 stationary axi-symmetric unorientable solutions to AdS3 Einstein gravity and show that they are obtained by applying certain orientifold projection on AdS3, BTZ or AdS3 self-dual orbifold, respectively O-AdS3, O-BTZ and O-SDO geometries. Depending on the orientifold fixed surface, the O-surface, which is either a space-like 2D plane or cylinder, or a light-like 2D plane or cylinder one can distinguish four distinct cases. For the space-like orientifold plane or cylinder cases these geometries solve AdS3 Einstein equations and are hence locally AdS3 everywhere except at the O-surface, where there is a delta-function source. For the light-like cases the geometry is a solution to Einstein equations even at the O-surface. We discuss the causal structure for static, extremal and general rotating O-BTZ and O-SDO cases as well as the geodesic motion on these geometries. We also discuss orientifolding Poincare patch AdS3 and AdS2 geometries as a way to geodesic completion of these spaces and comment on the 2D CFT dual to the O-geometries.

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arXiv2010

O-BTZ: Orientifolded BTZ Black Hole

Banados-Teitelboim-Zanelli (BTZ) black holes are constructed by orbifolding AdS3 geometry by boost transformations of its O(2,2) isometry group. Here we construct a new class of solutions to AdS3 Einstein gravity, orientifolded BTZ or O-BTZ for short, which in general, besides the usual BTZ orbifolding, involve orbifolding (orientifolding) by a Z_2 part of O(2,2) isometry group. This Z_2 is chosen such that it changes the orientation on AdS3 while keeping the orientation on its 2D conformal boundary. O-BTZ solutions exhaust all un-oriented AdS3 black hole solutions, as BTZ black holes constitute all oriented AdS3 black holes. O-BTZ, similarly to BTZ black holes, are stationary, axisymmetric asymptotically AdS3 geometries with two asymptotic charges, mass and angular momentum..

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arXiv2009

What is a chiral 2d CFT? And what does it have to do with extremal black holes?

The near horizon limit of the extremal BTZ black hole is a``self-dual orbifold'' of AdS_3. This geometry has a null circle on its boundary, and thus the dual field theory is a Discrete Light Cone Quantized (DLCQ) two dimensional CFT. The same geometry can be compactified to two dimensions giving AdS_2 with a constant electric field. The kinematics of the DLCQ show that in a consistent quantum theory of gravity in these backgrounds there can be no dynamics in AdS_2, which is consistent with older ideas about instabilities in this space. We show how the necessary boundary conditions eliminating AdS_2 fluctuations can be implemented, leaving one copy of a Virasoro algebra as the asymptotic symmetry group. Our considerations clarify some aspects of the chiral CFTs appearing in proposed dual descriptions of the near-horizon degrees of freedom of extremal black holes.

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arXiv2009

Nearing 11d Extremal Intersecting Giants and New Decoupled Sectors in D = 3,6 SCFT's

We extend the analysis of arXiv:0801.4457 [hep-th] to charged black hole solutions of four-dimensional $U(1)^4$ gauged supergravity which carry three charges. There are two decoupling near-horizon limits, one over the near-BPS black hole solution and the other over the near-extremal, but non-BPS geometry. Taking the limit over the eleven dimensional uplift of these black hole solutions, for both of these cases we obtain a geometry which has a piece (conformal) to rotating BTZ $\times S^2$. We study the 4d, $11d$ and $5d, 3d$ black hole properties. Moreover, we show that the BTZ$\times S^2$ geometry obtained after the near-BPS (near-extremal) limit is also a solution to five-dimensional $U(1)^3$ un-gauged (gauged) STU supergravity. Based on these decoupling limits we argue that there should be sectors of 3d CFT resulting from low energy limit of theory on $N$ M2-branes ($N\to \infty)$, which are decoupled from the rest of the theory and are effectively described by a 2d CFT. The central charge of the 2d CFT in both near-BPS and near-extremal case scales as $N$. The engineering dimension of the operators in these decoupled sectors scales as $N^{4/3}$ (for near-BPS case) while as $N^{3/2}$ (for the near-extremal case). Moreover, we discuss the description of the decoupled sectors as certain deformations of $6d$ CFT residing on the intersecting M5-brane giants.

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