arXiv subjects

Diana López Nacir

Publications and source records attributed to Diana López Nacir.

33 records · Page 2Linked to original sources

arXiv2018

Long distance behavior of $O(N)$-model correlators in de Sitter space and the resummation of secular terms

We analyze the long distance behavior of the two-point functions for an interacting scalar $O(N)$ model in de Sitter spacetime. Following our previous work, this behavior is analyzed by analytic continuation of the Euclidean correlators, which are computed by treating the homogeneous zero mode exactly and using a partial resummation of the interactions between the zero and the non-zero modes. We focus on massless fields and present an alternative derivation of our method, which involves a double expansion in $1/N$ and the coupling constant of the theory. This derivation is simpler than the previous one and can be directly extended for fields with negative squared-mass. We extend our previous results by computing the long wavelength limit of the two-point functions at next-to-leading order in $1/N$ and at leading order in the coupling constant, which involves a further resummation of Feynman diagrams (needed when the two-point functions are analitically continued). We prove that, after this extra resummation, the two-point functions vanish in the long distance limit.

hep-th↗

arXiv2018

Vector Fuzzy Dark Matter, Fifth Forces, and Binary Pulsars

We study the secular effects that an oscillating background ultralight (fuzzy) cosmological vector field has on the dynamics of binary systems; such effects appear when the field and the binary are in resonance. We first consider the gravitational interaction between the field and the systems, and quantify the main differences with an oscillating background scalar field. If the energy density of such a field is sufficiently large, as required if it is supposed to be all of the dark matter, we show that the secular effects could yield potentially observable signatures in high precision time of arrival measurements of binary pulsars. We then analyse the secular effects that arise when the field is directly coupled to the bodies in the binary. We show that this study is particularly relevant for models where fuzzy dark matter mediates a baryonic force $B$ (or $B-L$, with $L$ the lepton number), due to the stellar amount of nucleons present in the stars. The constraints we obtain from current data are already competitive with (or even more constraining than) laboratory tests of the equivalence principle.

astro-ph.CO↗

arXiv2017

Failures of homogeneous and isotropic cosmologies in Extended Quasi-Dilaton Massive Gravity

We analyze the Extended Quasi-Dilaton Massive Gravity model around a Friedmann-Lemaître-Robertson-Walker cosmological background. We present a careful stability analysis of asymptotic fixed points. We find that the traditional fixed point cannot be approached dynamically, except from a perfectly fine-tuned initial condition involving both the quasi-dilaton and the Hubble parameter. A less-well examined fixed-point solution, where the time derivative of the 0-th Stückelberg field vanishes $\dotϕ^0=0$, encounters no such difficulty, and the fixed point is an attractor in some finite region of initial conditions. We examine the question of the presence of a Boulware-Deser ghost in the theory. We show that the additional constraint which generically allows for the elimination of the Boulware-Deser mode is only present under special initial conditions. We find that the only possibility corresponds to the traditional fixed point and the initial conditions are the same fine-tuned conditions that allow the fixed point to be approached dynamically.

astro-ph.CO↗

arXiv2016

Massless Interacting Scalar Fields in de Sitter space

We present a method to compute the two-point functions for an $O(N)$ scalar field model in de Sitter spacetime, avoiding the well known infrared problems for massless fields. The method is based on an exact treatment of the Euclidean zero modes and a perturbative one of the nonzero modes, and involves a partial resummation of the leading secular terms. This resummation, crucial to obtain a decay of the correlation functions, is implemented along with a double expansion in an effective coupling constant $\sqrtλ$ and in $1/N$. The results reduce to those known in the leading infrared approximation and coincide with the ones obtained directly in Lorentzian de Sitter spacetime in the large $N$ limit. The new method allows for a systematic calculation of higher order corrections both in $\sqrtλ$ and in $1/N$.

hep-th↗

arXiv2016

$O(N)$ model in Euclidean de Sitter space: beyond the leading infrared approximation

We consider an $O(N)$ scalar field model with quartic interaction in $d$-dimensional Euclidean de Sitter space. In order to avoid the problems of the standard perturbative calculations for light and massless fields, we generalize to the $O(N)$ theory a systematic method introduced previously for a single field, which treats the zero modes exactly and the nonzero modes perturbatively. We compute the two-point functions taking into account not only the leading infrared contribution, coming from the self-interaction of the zero modes, but also corrections due to the interaction of the ultraviolet modes. For the model defined in the corresponding Lorentzian de Sitter spacetime, we obtain the two-point functions by analytical continuation. We point out that a partial resummation of the leading secular terms (which necessarily involves nonzero modes) is required to obtain a decay at large distances for massless fields. We implement this resummation along with a systematic double expansion in an effective coupling constant $\sqrtλ$ and in 1/N. We explicitly perform the calculation up to the next-to-next-to-leading order in $\sqrtλ$ and up to next-to-leading order in 1/N. The results reduce to those known in the leading infrared approximation. We also show that they coincide with the ones obtained directly in Lorentzian de Sitter spacetime in the large N limit, provided the same renormalization scheme is used.

hep-th↗

arXiv2015

Implications of the scalar tilt for the tensor-to-scalar ratio

We investigate the possible implications of the measured value of the scalar tilt $n_s$ for the tensor-to-scalar ratio $r$ in slow-roll, single-field inflationary models. The measured value of the tilt satisfies $n_s -1\sim 1/N_*$, where $N_* \sim 60$ is the number of $e$-folds for observationally relevant scales. If this is not a coincidence and the scaling holds for different values of $N$, it strongly suggests that either $r$ is as big as $10^{-1}$ (a possibility in tension with the recent data), or smaller than $10^{-2}$ and exponentially dependent on $n_s$. A large region of the ($n_s$,$r$) plane is not compatible with this scaling.

astro-ph.CO↗

arXiv2015

Detecting Primordial $B$-Modes after Planck

We update the forecasts for the measurement of the tensor-to-scalar ratio $r$ for various ground-based experiments (AdvACT, CLASS, Keck/BICEP3, Simons Array, SPT-3G), balloons (EBEX 10k and Spider) and satellites (CMBPol, COrE and LiteBIRD), taking into account the recent Planck data on polarized dust and using a component separation method. The forecasts do not change significantly with respect to previous estimates when at least three frequencies are available, provided foregrounds can be accurately described by few parameters. We argue that a theoretically motivated goal for future experiments is $r\sim2\times10^{-3}$, and that this is achievable if the noise is reduced to $\sim1\,μ$K-arcmin and lensing is reduced to $10\%$ in power. We study the constraints experiments will be able to put on the frequency and $\ell$-dependence of the tensor signal as a check of its primordial origin. Futuristic ground-based and balloon experiments can have good constraints on these parameters, even for $r\sim2\times10^{-3}$. For the same value of $r$, satellites will marginally be able to detect the presence of the recombination bump, the most distinctive feature of the primordial signal.

astro-ph.CO↗

arXiv2015

Extreme parameter sensitivity in quasidilaton massive gravity

We reanalyze the behavior of Friedmann-Lemaître-Robertson-Walker cosmologies in the recently proposed quasidilaton massive-gravity model, and discover that the background dynamics present hitherto unreported features that require unexpected fine-tuning of the additional fundamental parameters of the theory for an observationally consistent background cosmology. We also identify new allowed regions in the parameters space and exclude some of the previously considered ones. The evolution of the mass of gravitational waves reveals non-trivial behavior, exhibiting a mass squared that may be negative in the past, and that presently, while positive, is larger than the square of the Hubble parameter. These properties of the gravity-wave mass have the potential to lead to observational tests of the theory. While quasidilaton massive gravity is known to have issues with stability at short distances, the current analysis is a first step toward the investigation of the more stable extended quasidilaton massive gravity theory, with some expectation that both the fine-tuning of parameters and the interesting behavior of the gravity-wave mass will persist.

astro-ph.CO↗

arXiv2014

$ϕ^2$ or Not $ϕ^2$: Testing the Simplest Inflationary Potential

The simplest inflationary model $V=\frac12 m^2ϕ^2$ represents the benchmark for future constraints. For a quadratic potential, the quantity $(n_s-1)+r/4+11 (n_s-1)^2/24$ vanishes (up to corrections which are cubic in slow roll) and can be used to parametrize small deviations from the minimal scenario. Future constraints on this quantity will be able to distinguish a quadratic potential from a pseudo-Nambu-Goldstone boson with $f \lesssim 30 M_{\rm pl}$ and set limits on the deviation from unity of the speed of sound $| c_s-1| \lesssim 3\times 10^{-2}$ (corresponding to an energy scale $Λ\gtrsim 2\times 10^{16}\, \mathrm{GeV}$), and on the contribution of a second field to perturbations ($\lesssim 6 \times 10^{-2}$). The limiting factor for these bounds will be the uncertainty on the spectral index. The error on the number of e-folds will be $ΔN \simeq 0.4$, corresponding to an error on the reheating temperature $ΔT_\mathrm{rh}/T_\mathrm{rh}\simeq 1.2$. We comment on the relevance of non-Gaussianity after BICEP2 results.

astro-ph.CO↗

arXiv2014

$ϕ^2$ Inflation at its Endpoint

In the simplest inflationary model $V=\frac12 m^2ϕ^2$, we provide a prediction accurate up to $1\%$ for the spectral index $n_s$ and the tensor-to-scalar ratio $r$ assuming instantaneous reheating and a standard thermal history: $n_s = 0.9668\pm0.0003$ and $r=0.131\pm 0.001$. This represents the simplest and most informative point in the $(n_s,r)$ plane. The result is independent of the details of reheating (or preheating) provided the conversion to radiation is sufficiently fast. A slower reheating or a modified post-inflationary evolution push towards smaller $n_s$ (and larger $r$), so that our prediction corresponds to the maximum $n_s$ (and minimum $r$) for the quadratic potential. We also derive similar results for a general $V \propto ϕ^p$ potential.

astro-ph.CO↗

arXiv2014

Nonlinear effects of dark energy clustering beyond the acoustic scales

We extend the resummation method of Anselmi & Pietroni (2012) to compute the total density power spectrum in models of quintessence characterized by a vanishing speed of sound. For standard $Λ$CDM cosmologies, this resummation scheme allows predictions with an accuracy at the few percent level beyond the range of scales where acoustic oscillations are present, therefore comparable to other, common numerical tools. In addition, our theoretical approach indicates an approximate but valuable and simple relation between the power spectra for standard quintessence models and models where scalar field perturbations appear at all scales. This, in turn, provides an educated guess for the prediction of nonlinear growth in models with generic speed of sound, particularly valuable since no numerical results are yet available.

astro-ph.CO↗

arXiv2010

Counterterms in semiclassical Horava-Lifshitz gravity

We analyze the semiclassical Hořava-Lifshitz gravity for quantum scalar fields in 3+1 dimensions. The renormalizability of the theory requires that the action of the scalar field contains terms with six spatial derivatives of the field, i.e. in the UV, the classical action of the scalar field should preserve the anisotropic scaling symmetry ($t \to L^{2z}t,$ $\vec{x} \to L^2 \vec{x}$, with $z=3$) of the gravitational action. We discuss the renormalization procedure based on adiabatic subtraction and dimensional regularization in the weak field approximation. We verify that the divergent terms in the adiabatic expansion of the expectation value of the energy-momentum tensor of the scalar field contain up to six spatial derivatives, but do not contain more than two time derivatives. We compute explicitly the counterterms needed for the renormalization of the theory up to second adiabatic order and evaluate the associated $β$ functions in the minimal subtraction scheme.

hep-th↗

arXiv2009

On the renormalization procedure for quantum fields with modified dispersion relation in curved spacetimes

We review our recent results on the renormalization procedure for a free quantum scalar field with modified dispersion relations in curved spacetimes. For dispersion relations containing up to $2s$ powers of the spatial momentum, the subtraction necessary to renormalize $<ϕ^2>$ and $ $ depends on $s$. We first describe our previous analysis for spatially flat Friedman-Robertson-Walker and Bianchi type I metrics. Then we present a new power counting analysis for general background metrics in the weak field approximation.

hep-th↗

arXiv2007

New counterterms induced by trans-Planckian physics in semiclassical gravity

We consider free and self-interacting quantum scalar fields satisfying modified dispersion relations in the framework of Einstein-Aether theory. Using adiabatic regularization, we study the renormalization of the equation for the mean value of the field in the self-interacting case, and the renormalization of the semiclassical Einstein-Aether equations for free fields. In both cases we consider Bianchi type I background spacetimes. Contrary to what happens for {\it free} fields in {\it flat} Robertson-Walker spacetimes, the self-interaction and/or the anisotropy produce non-purely geometric terms in the adiabatic expansion, i.e terms that involve both the metric $g_{μν}$ and the aether field $u_μ$. We argue that, in a general spacetime, the renormalization of the theory would involve new counterterms constructed with $g_{μν}$ and $u_μ$, generating a fine-tuning problem for the Einstein-Aether theory.

gr-qc↗

arXiv2007

Backreaction in trans-Planckian cosmology: renormalization, trace anomaly and selfconsistent solutions

We analyze the semiclassical Einstein equations for quantum scalar fields satisfying modified dispersion relations. We first discuss in detail the renormalization procedure based on adiabatic subtraction and dimensional regularization. We show that, contrary to what expected from power counting arguments, in 3+1 dimensions the subtraction involves up to the fourth adiabatic order even for dispersion relations containing higher powers of the momentum. Then we analyze the dependence of the trace of the renormalized energy momentum tensor with the scale of new physics, and we recover the usual trace anomaly in the appropriate limit. We also find selfconsistent de Sitter solutions for dispersion relations that contain up to the fourth power of the momentum. Using this particular example, we also discuss the possibility that the modified dispersion relation can be mimicked at lower energies by an effective initial state in a theory with the usual dispersion relation.

gr-qc↗