Publications

Title: Revising the observable consequences of slow-roll inflation.

Authors: Ivan Agullo, Jose Navarro-Salas, Gonzalo J. Olmo, Leonard Parker.

Journal-ref: Phys.Rev.D 81, 043514(2010)

Abstract: We study the generation of primordial perturbations in a (single-field) slow-roll inflationary universe. In momentum space, these (Gaussian) perturbations are characterized by a zero mean and a non-zero variance D²(k, t). However, in position space the variance diverges in the ultraviolet. The requirement of a finite variance in position space forces one to regularize D²(k, t). This can (and should) be achieved by proper renormalization in an expanding universe in a unique way. This affects the predicted scalar and tensorial power spectra (evaluated when the modes acquire classical properties) for wavelengths that today are at observable scales. As a consequence, the imprint of slow-roll inflation on the CMB anisotropies is significantly altered. We find a non-trivial change in the consistency condition that relates the tensor-to-scalar ratio r to the spectral indices. For instance, an exact scale-invariant tensorial power spectrum, n_t=0, is now compatible with a non-zero ratio $r\approx 0.12\pm0.06$, which is forbidden by the standard prediction (r=-8n_t). The influence of relic gravitational waves on the CMB may soon come within the range of planned measurements, offering a non-trivial test of the new predictions.

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Title: New Phenomenology for Palatini f(R) Theories: Non-singular Universes.

Authors: Gonzalo J. Olmo

Journal-ref: AIP proceedings series. "The Invisible Universe International Conference", Paris (France) June 29 - July 03, 2009.

Abstract: We study modified theories of gravity of the f(R) type in Palatini formalism. We first consider the stability of atoms when the Palatini gravitational interaction is taken into account in the derivation of the non-relativistic Schrodinger equation. We show that theories with infrared curvature corrections are ruled out by the mere existence of atoms. In particular, we carry out fully perturbative calculations that, for the first time, convincingly rule out the 1/R model of Carroll et al. in its Palatini version. We then study the Planck scale corrected quadratic model f(R)=R+R^2/R_P and show that it can avoid the big bang singularity for matter sources which satisfy all the energy conditions. We comment on the mechanisms that cure this singularity and point out that they are closely related to non- perturbative terms also present in the atomic Hamiltonian of infrared corrected models.

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Title: Inflation, quantum fields, and CMB anisotropies.

Authors: I.Agulló, J.Navarro-Salas, Gonzalo J. Olmo, Leonard Parker.

Journal-ref: Gen.Rel.Gravit.

Abstract: Inflationary cosmology has proved to be the most successful at predicting the properties of the anisotropies observed in the cosmic microwave background (CMB). In this essay we show that quantum field renormalization significantly influences the generation of primordial perturbations and hence the expected measurable imprint of cosmological inflation on the CMB. However, the new predictions remain in agreement with observation, and in fact favor the simplest forms of inflation. In the near future, observations of the influence of gravitational waves from the early universe on the CMB will test our new predictions.

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Title: Dynamical Aspects of Generalized Palatini Theories of Gravity.

Authors: Gonzalo J. Olmo, Helios Sanchis-Alepuz, Swapnil Tripathi

Journal-ref: Phys. Rev. D 80, 024013 (2009)

Abstract: We study the field equations of modified theories of gravity in which the lagrangian is a general function of the Ricci scalar and Ricci-squared terms in Palatini formalism. We show that the independent connection can be expressed as the Levi-Civita connection of an auxiliary metric which, in particular cases of interest, is related with the physical metric by means of a disformal transformation. This relation between physical and auxiliary metric boils down to a conformal transformation in the case of f(R) theories. We also show with explicit models that the inclusion of Ricci squared terms in the action can impose upper bounds on the accessible values of pressure and density, which might have important consequences for the early time cosmology and black hole formation scenarios. Our results indicate that the phenomenology of f(R_{ab}R^{ab}) theories is much richer than that of f(R) and f(R_{ab}R^{ab}) theories and that they also share some similarities with Bekenstein's relativistic theory of MOND.

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Title: Bouncing Cosmologies in Palatini f(R) Gravity

Authors: Carlos Barragan, Gonzalo J. Olmo, Helios Sanchis-Alepuz.

Journal-ref: Phys.Rev.D 80, 024016, (2009)

Abstract: We consider the early time cosmology of f(R) theories in Palatini formalism and study the conditions that guarantee the existence of homogeneous and isotropic models that avoid the Big Bang singularity. We show that for such models the Big Bang singularity can be replaced by a cosmic bounce without violating any energy condition. In fact, the bounce is possible even for pressureless dust. We give a characterization of such models and discuss their dynamics in the region near the bounce. We also find that power-law lagrangians with a finite number of terms may lead to non-singular universes, which contrasts with the infinite-series Palatini f(R) lagrangian that one needs to fully capture the effective dynamics of Loop Quantum Cosmology. We argue that these models could also avoid the formation of singularities during stellar gravitational collapse.

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Title: Insensitivity of Hawking radiation to an invariant Planck- scale cutoff.

Authors: Ivan Agullo, Jose Navarro-Salas, Gonzalo J. Olmo, Leonard Parker

Journal-ref: To appear in Phys.Rev.D

Abstract: A disturbing aspect of Hawking's derivation of black hole radiance is the need to invoke extreme conditions for the quantum field that originates the emitted quanta. It is widely argued that the derivation requires the validity of the conventional relativistic field theory to arbitrarily high, trans-Planckian scales. We stress in this note that this is not necessarily the case if the question is presented in a covariant way. We point out that Hawking radiation is immediately robust against an invariant Planck-scale cutoff. This important feature of Hawking radiation is relevant for a quantum gravity theory that preserves, in some way, the Lorentz symmetry.

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Title: Revising the predictions of inflation for the cosmic microwave background anisotropies.

Authors: Ivan Agullo, Jose Navarro-Salas, Gonzalo J. Olmo, Leonard Parker

Journal-ref:

Abstract: Inflationary cosmology predicts that, due to quantum effects, small density perturbations are generated in the very early universe with a nearly "scale-free" spectrum. The detection and analysis of anisotropies in the cosmic microwave background has spectacularly confirmed this prediction. Moreover, inflation also predicts the creation of primordial gravitational waves, which still remain undetectable. Forthcoming high-precision measurements of the cosmic microwave background may measure effects of relic gravitational waves, and this will be crucial to test the inflationary paradigm and strongly constrain inflationary models. Therefore, it is particularly important to scrutinize, from all points of view, the quantitative predictions of inflation. In this work we point out that if quantum field renormalization is taken into account, the predictions of slow- roll inflation for both the scalar and tensorial power spectrum change significantly. This leads, in particular, to a change in the consistency condition that relates the tensor-to-scalar amplitude ratio r with spectral indices. Moreover, a reexamination of the chaotic potentials \phi^2, \phi^4, shows that both fall well inside the 68% confidence level region in the plane (n_s, r) of the five-year WMAP data. In contrast, the standard predictions rule out the potential \phi^4. The alternative predictions presented in this work may soon come within the range of measurement of near-future experiments.

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Title: Reexamination of Polytropic Spheres in Palatini f(R) Gravity.

Authors: Gonzalo J. Olmo

Journal-ref: Phys.Rev.D 78, 104026 (2008)

Abstract: We investigate spherically symmetric, static matter configurations with polytropic equation of state for a class of f(R) models in Palatini formalism and show that the surface singularities recently reported in the literature are not physical in the case of Planck scale modified lagrangians. In such cases, they are just an artifact of the idealized equation of state used. In fact, we show that for the models f(R)=R\pm\lambda R^2, with \lambda on the order of the Planck length squared, the presence of a single electron in the Universe would be enough to cure all stellar singularities of this type. From our analysis it also follows that the stellar structure derived from these lagrangians is virtually undistinguishable from that corresponding to General Relativity. For ultraviolet corrected models far from the Planck scale, however, the surface singularities may indeed arise in the region of validity of the polytropic equation of state. This fact can be used to place constraints on the parameters of particular models.

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Title: The role of Planck scale in black hole radiance.

Authors: Iván Agulló, José Navarro-Salas, Gonzalo J. Olmo, and Leonard Parker.

Journal-ref: Int.J.Mod.Phys.D17:489-494,2008.

Abstract:Lorentz invariance plays a pivotal role in the derivation of the Hawking effect, which crucially requires an integration in arbitrarily small distances or, equivalently, in unbounded energies. New physics at the Planck scale could, therefore, potentially modify the emission spectrum. We show, however, that the kinematic invariance can be deformed in such a way that the thermal spectrum remains insensitive to trans-Planckian physics.

No preprint available

Title: Effective Action for Loop Quantum Cosmology a la Palatini

Authors: Gonzalo J. Olmo, Parampreet Singh

Journal-ref: JCAP01 (2009)030

Abstract: The resolution of the big bang singularity in loop quantum cosmology can be described by quantum gravitational modifications to the Friedman dynamics without introducing any new degrees of freedom. A challenging question is to construct a covariant effective action able to reproduce that new physics emergent at the Planck scale. By enlarging our scope to the realm of non-metric theories, we show that apparent tensions with conventional approaches can be overcome. We provide an algorithm to construct an effective action for loop quantum cosmology in the Palatini framework and obtain an action which involves an infinite series in the scalar curvature of the independent connection. This effective action successfully captures non-local quantum geometric effects and the non-singular bounce at the Planck scale and leads to general relativity at low curvatures.

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Title: Reexamination of the Power Spectrum in de Sitter Inflation.

Authors: Ivan Agullo, Jose Navarro-Salas, Gonzalo J. Olmo, Leonard Parker

Journal-ref: Phys.Rev.Lett. 101,171301(2008)

Abstract: We find that the amplitude of quantum fluctuations of the invariant de Sitter vacuum coincides exactly with that of the vacuum of a comoving observer for a massless scalar (inflaton) field. We propose redefining the actual physical power spectrum as the difference between the amplitudes of the above vacua. An inertial particle detector continues to observe the Gibbons-Hawking temperature. However, although the resulting power spectrum is still scale-free, its amplitude can be drastically reduced since now, instead of the Hubble's scale at the inflationary period, it is determined by the square of the mass of the inflaton fluctuation field.

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Title: Two-point functions with an invariant Planck scale and thermal effects

Authors: Ivan Agullo, Jose Navarro-Salas, Gonzalo J. Olmo, Leonard Parker

Journal-ref: Phys.Rev.D77,124032, 2008.

Abstract: Nonlinear deformations of relativistic symmetries at the Planck scale are usually addressed in terms of modified dispersion relations. We explore here an alternative route by directly deforming the two-point functions of an underlying field theory. The proposed deformations depend on a length parameter (Planck length) and preserve the basic symmetries of the corresponding theory. We also study the physical consequences implied by these modifications at the Planck scale by analyzing the response function of an accelerated detector in Minkowski space, an inertial one in de Sitter space, and also in a black hole spacetime.

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Title: Hydrogen atom in Palatini theories of gravity

Authors: Gonzalo J. Olmo

Journal-ref: Phys.Rev.D77,084021, 2008

Abstract: We study the effects that the gravitational interaction of $f(R)$ theories of gravity in Palatini formalism has on the stationary states of the Hydrogen atom. We show that the role of gravity in this system is very important for lagrangians $f(R)$ with terms that grow at low curvatures, which have been proposed to explain the accelerated expansion rate of the universe. We find that new gravitationally induced terms in the atomic Hamiltonian generate a strong backreaction that is incompatible with the very existence of bound states. In fact, in the 1/R model, Hydrogen disintegrates in less than two hours. The universe that we observe is, therefore, incompatible with that kind of gravitational interaction. Lagrangians with high curvature corrections do not lead to such instabilities.

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Title: Acceleration radiation and the Planck scale

Authors: Iván Agulló, José Navarro-Salas, Gonzalo J. Olmo, and Leonard Parker.

Journal-ref: Phys.Rev.D77,104034, 2008

Abstract: A uniformly accelerating observer perceives the Minkowski vacuum state as a thermal bath of radiation. We point out that this field-theory effect can be derived, for any dimension higher than two, without actually invoking very high energy physics. This supports the view that this phenomenon is robust against Planck-scale physics and, therefore, should be compatible with any underlying microscopic theory.

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Title:Short distances, black holes, and TeV gravity.

Authors: Iván Agulló, José Navarro-Salas, Gonzalo J. Olmo

Journal-ref: Proceedings of the 11th Marcel Grossman Meeting

Abstract:The Hawking effect can be rederived in terms of two- point functions and in such a way that it makes it possible to estimate, within the conventional semiclassical theory, the contribution of ultrashort distances at I^+ to the Planckian spectrum. Thermality is preserved for black holes with \kappa l_P much smaller than 1. However, deviations from the Planckian spectrum can be found for mini black holes in TeV gravity scenarios, even before reaching the Planck phase.

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Title: Limit to General Relativity in f(R) theories of gravity

Authors: Gonzalo J. Olmo

Journal-ref: Phys.Rev.D. 75, 023511 (2007)

Abstract:We discuss two aspects of f(R) theories of gravity in metric formalism. We first study the reasons to introduce a scalar- tensor representation for these theories and the behavior of this representation in the limit to General Relativity, f(R) going to R. We find that the scalar-tensor representation is well behaved even in this limit. Then we work out the exact equations for spherically symmetric sources using the original f(R) representation, solve the linearized equations, and compare our results with recent calculations of the literature. We observe that the linearized solutions are strongly affected by the cosmic evolution, which makes very unlikely that the cosmic speedup be due to f(R) models with correcting terms relevant at low curvatures.

Preprint - Published version

Title: Violation of the Equivalence Principle in Modified Theories of Gravity

Authors: Gonzalo J. Olmo - See this article in the news.

Journal-ref: Phys. Rev. Lett. 98, 061101 (2007)

Abstract:We show that the metric in f(R) theories of gravity in Palatini formalism can be solved as the product of a rank-two tensor times a scalar function which is very sensitive to the local energy- momentum densities. This local dependence of the metric generates new gravitationally-induced microscopic interactions, which eventually would lead to self-accelerated test body trajectories. These facts make very unlikely the viability of Palatini f(R) models designed to change the late-time cosmic evolution.

Preprint - Published version

Title:Short-distance contribution to the spectrum of Hawking radiation

Authors: I. Agullo, J. Navarro-Salas, Gonzalo J. Olmo, Leonard Parker

Journal-ref: Phys.Rev.D 76,044018(2007)

Abstract:The Hawking effect can be rederived in terms of two- point functions and in such a way that it makes it possible to estimate, within the conventional semiclassical theory, the contribution of ultrashort distances to the Planckian spectrum. For Schwarzschild black holes of three solar masses the analysis shows that Hawking radiation is very robust up to frequencies of 96 T_H or 270 T_H for bosons and fermions, respectively. For primordial black holes (with masses around 10^{15} g) these frequencies turn out to be of order 52T_H and 142 T_H. Only at these frequencies and above do we find that the contribution of Planck distances is of order of the total spectrum itself. Below this scale, the contribution of ultrashort distances to the spectrum is negligible. This suggests that only above these frequencies could an underlying quantum theory of gravity potentially predict significant deviations from Hawking's semiclassical result

Preprint

Title:Black hole radiance, short distances, and TeV gravity

Authors: Iván Agulló, José Navarro-Salas, Gonzalo J. Olmo

Journal-ref: Phys.Rev.Lett. 97 (2006) 041302

Abstract:Using a derivation of black hole radiance in terms of two-point functions one can provide a quantitative estimate of the contribution of short distances to the spectrum. Thermality is preserved for black holes with \kappa l_P much smaller than 1. However, deviations from the Planckian spectrum can be found for mini black holes in TeV gravity scenarios, even before reaching the Planck phase.

Preprint - Published version

Title:Semiclassical zero-temperature corrections to Schwarzschild spacetime and holography

Authors: A. Fabbri, S. Farese, J. Navarro-Salas, Gonzalo J. Olmo, H. Sanchis-Alepuz

Journal-ref: Phys.Rev. D73 (2006) 104023

Abstract: Motivated by the quest for black holes in AdS braneworlds, and in particular by the holographic conjecture relating 5D classical bulk solutions with 4D quantum corrected ones, we numerically solve the semiclassical Einstein equations (backreaction equations) with matter fields in the (zero temperature) Boulware vacuum state. In the absence of an exact analytical expression for T_mn in four dimensions we work within the s-wave approximation. Our results show that the quantum corrected solution is very similar to Schwarzschild till very close to the horizon, but then a bouncing surface for the radial function appears which prevents the formation of an event horizon. We also analyze the behavior of the geometry beyond the bounce, where a curvature singularity arises. In the dual theory, this indicates that the corresponding 5D static classical braneworld solution is not a black hole but rather a naked singularity.

Preprint - Published version

Title: Static quantum corrections to the Schwarzschild spacetime

Authors: A. Fabbri, S. Farese, J. Navarro-Salas, Gonzalo J. Olmo, H. Sanchis-Alepuz

Journal-ref: J.Phys.Conf.Ser.33:457-462,2006

Abstract: We study static quantum corrections of the Schwarzschild metric in the Boulware vacuum state. Due to the absence of a complete analytic expression for the full semiclassical Einstein equations we approach the problem by considering the s-wave approximation and solve numerically the associated backreaction equations. The solution, including quantum effects due to pure vacuum polarization, is similar to the classical Schwarzschild solution up to the vicinity of the classical horizon. However, the radial function has a minimum at a time-like surface close to the location of the classical event horizon. There the g_{00} component of the metric reaches a very small but non-zero value. The analysis unravels how a curvature singularity emerges beyond this bouncing point. We briefly discuss the physical consequences of these results by extrapolating them to a dynamical collapsing scenario.

Preprint - Published version

Title: Post-Newtonian constraints on f(R) cosmologies in metric and Palatini formalism

Authors: Gonzalo J. Olmo

Journal-ref: Phys.Rev.D72, 083505, (2005)

Abstract: We compute the complete post-Newtonian limit of both the metric and Palatini formulations of f(R) gravities using a scalar- tensor representation. By comparing the predictions of these theories with laboratory and solar system experiments, we find a set of inequalities that any lagrangian f(R) must satisfy. The constraints imposed by those inequalities allow us to find explicit bounds to the possible nonlinear terms of the lagrangian. We conclude that in both formalisms the lagrangian f(R) must be almost linear in R and that corrections that grow at low curvatures are incompatible with observations. This result shows that modifications of gravity at very low cosmic densities cannot be responsible for the observed cosmic speed-up.

Prep-0505135 - Prep-0505136 - Published version

Title:The gravity lagrangian according to solar system experiments

Authors: Gonzalo J. Olmo

Journal-ref: Phys.Rev.Lett. 95 (2005) 261102

Abstract: In this work we show that the gravity lagrangian f(R) at relatively low curvatures in both metric and Palatini formalisms is a bounded function that can only depart from the linearity within the limits defined by well known functions. We obtain those functions by analysing a set of inequalities that any f(R) theory must satisfy in order to be compatible with laboratory and solar system observational constraints. This result implies that the recently suggested f(R) gravity theories with nonlinear terms that dominate at low curvatures are incompatible with observations and, therefore, cannot represent a valid mechanism to justify the cosmic speed-up.

Preprint - Published version

Title: Nonlinear Gravity Theories in the Metric and Palatini Formalisms

Authors: Gonzalo J. Olmo and William Komp

Journal-ref:

Abstract: We study nonlinear gravity theories in both the metric and the Palatini (metric-affine) formalisms. The nonlinear character of the gravity lagrangian in the metric formalism causes the appearance of a scalar source of matter in Einstein's equations that can be interpreted as a quintessence field. However, in the Palatini case no new energy sources appear, though the equations of motion get modified in such a way that usual matter can lead to repulsive gravity at very low densities. Thus, the Palatini formalism could provide a mechanism to explain the recent acceleration of the universe without the necessity of dark energy sources. We also show that in contrast to the metric formalism where only the Einstein frame should be considered as physical, the Palatini formalism allows both the Einstein and the Jordan frames to be physically acceptable.

Preprint

Title: Particles and energy fluxes from a CFT perspective

Authors: A. Fabbri, J. Navarro-Salas, Gonzalo J. Olmo

Journal-ref: Phys.Rev. D70 (2004) 064022

Abstract: We analyze the creation of particles in two dimensions under the action of conformal transformations. We focus our attention on Möbius transformations and compare the usual approach, based on the Bogolubov coefficients, with an alternative but equivalent viewpoint based on correlation functions. In the latter approach the absence of particle production under full Möbius transformations is manifest. Moreover, we give examples, using the moving-mirror analogy, to illustrate the close relation between the production of quanta and energy.

Preprint - Published version

Title: Late-time correlators in semiclassical particle-black- hole scattering

Authors: A. Fabbri, D.J. Navarro, J. Navarro-Salas, Gonzalo J. Olmo

Journal-ref: Phys.Rev.D68,041502(R), (2003)

Abstract: We analyze the quantum corrected geometry and radiation in the scattering of extremal black holes by low-energy neutral matter. We point out the fact that the correlators of local observables inside the horizon are the same as those of the vacuum. Outside the horizon the correlators at late times are much bigger than those of the (thermal) case obtained neglecting the back reaction. This suggests that the corrected Hawking radiation could be compatible with unitarity.

Preprint - Published version

Title: A note on Möbius transformations and Bogolubov coefficients

Authors: A. Fabbri, J. Navarro-Salas and Gonzalo J. Olmo

Journal-ref: Proceedings of "Symmetries in Gravity and Field Theory", Workshop in honour of Prof. J. A. de Azcarraga. June 9-11, 2003. Salamanca (Spain)

Abstract: We analyze the creation of scalar massless particles in two dimensions under the action of conformal transformations. We focus our attention to Möbius transformation and clarify an apparent tension between the results obtained with the Bogolubov coefficients and those obtained within the conformal field theory approach.

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