arXiv subjects

Antonio Enea Romano

Publications and source records attributed to Antonio Enea Romano.

At least 55 records · Page 3Linked to original sources

arXiv2018

The MESS of cosmological perturbations

We introduce two new effective quantities for the study of comoving curvature perturbations $ζ$: the space dependent effective sound speed (SESS) and the momentum dependent effective sound speed (MESS) . We use the SESS and the MESS to derive a new set of equations which can be applied to any system described by an effective stress-energy-momentum tensor (EST), including multi-fields systems, supergravity and modified gravity theories. We show that this approach is completely equivalent to the standard one and it has the advantage of requiring to solve only one differential equation for $ζ$ instead of a system, without the need of explicitly computing the evolution of entropy perturbations. The equations are valid for perturbations respect to any arbitrary flat spatially homogeneous background, including any inflationary and bounce model. As an application we derive the equation for $ζ$ for multi-fields $KGB$ models and show that observed features of the primordial curvature perturbation spectrum are compatible with the effects of an appropriate local variation of the MESS in momentum space. The MESS is the natural quantity to parametrize in a model independent way the effects produced on curvature perturbations by multi-fields systems, particle production and modified gravity theories and could be conveniently used in the analysis of LSS observations, such as the ones from the upcoming EUCLID mission or CMB radiation measurements.

gr-qc↗

arXiv2018

Degeneracy in the spectrum and bispectrum among featured inflaton potentials

We study the degeneracy of the primordial spectrum and bispectrum of the curvature perturbation in single field inflationary models with a class of features in the inflaton potential. The feature we consider is a discontinuous change in the shape of the potential and is controlled by a couple of parameters that describe the strength of the discontinuity and the change in the potential shape. This feature produces oscillations of the spectrum and bispectrum around the comoving scale $k=k_0$ that exits the horizon when the inflaton passes the discontinuity. We find that the effects on the spectrum and almost all configurations of the bispectrum including the squeezed limit depend on a single quantity which is a function of the two parameters defining the feature. This leads to a degeneracy, i.e. different features of the inflaton potential can produce the same observational effects. However, we find that the degeneracy in the bispectrum is removed at the equilateral limit around $k=k_0$. This can be used to discriminate different models which give the same spectrum.

astro-ph.CO↗

arXiv2018

Hubble trouble or Hubble bubble?

The recent analysis of low-redshift supernovae (SN) has increased the apparent tension between the value of $H_0$ estimated from low and high redshift observations such as the cosmic microwave background (CMB) radiation. At the same time other observations have provided evidence of the existence of local radial inhomogeneities extending in different directions up to a redshift of about $0.07$. About $40\%$ of the Cepheids used for SN calibration are directly affected because are located along the directions of these inhomogeneities. We derive a new simple formula relating directly the luminosity distance to the monopole of the density contrast, which does not involve any metric perturbation. We then use it to develop a new inversion method to reconstruct the monopole of the density field from the deviations of the redshift uncorrected observed luminosity distance respect to the $ΛCDM$ prediction based on cosmological parameters obtained from large scale observations. The inversion method confirms the existence of inhomogeneities whose effects were not previously taken into account because the $2M++$ density field maps used to obtain the peculiar velocity for redshift correction were for $z\leq 0.06$, which is not a sufficiently large scale to detect the presence of inhomogeneities extending up to $z=0.07$. The inhomogeneity does not affect the high redshift luminosity distance because the volume averaged density contrast tends to zero asymptotically, making the value of $H_0^{CMB}$ obtained from CMB observations insensitive to any local structure. The inversion method can provide a unique tool to reconstruct the density field at high redshift where only SN data is available, and in particular to normalize correctly the density field respect to the average large scale density of the Universe.

astro-ph.CO↗

arXiv2018

A Simple No-Scale Model of Modulus Fixing and Inflation

We construct a no-scale model of inflation with a single modulus whose real and imaginary parts are fixed by simple power-law corrections to the no-scale K{\" a}hler potential. Assuming an uplift of the minimum of the effective potential, the model yields a suitable number of e-folds of expansion and values of the tilt in the scalar cosmological density perturbations and of the ratio of tensor and scalar perturbations that are compatible with measurements of the cosmic microwave background radiation.

hep-ph↗

arXiv2017

A discrete estimation of the uncertainty of the background redshift and its effects on cosmological parameters

We develop a discrete model to account for the effects of inhomogeneities on the redshift of photons. Using this model we compute the probability distribution of the observed redshift respect to the background value, obtaining that its dispersion around the central value is proportional the square root of the comoving distance. This implies that data analysis should include a contribution to the total error budget which depends on the distance of the source. Assuming large scale inhomogeneities with a power spectrum given by primordial curvature perturbations the effect is expected to be small at low red-shift, and to become important only at very high redshift. We then consider what are the general implications for the estimation of background cosmological parameters, giving some example for the case of quantities related to the luminosity distance such as the Hubble parameter and the cosmological constant. The model correctly reproduces the expected stochastic properties of the propagation of photons in inhomogeneous media and due to its computational simplicity it could be particularly suitable for the numerical estimation of the effects of inhomogeneities on cosmological observables using Montecarlo methods.

gr-qc↗

arXiv2017

Reconstructing the metric of the local Universe from number counts observations

Number counts observations available with new surveys such as the Euclid mission will be an important source of information about the metric of the Universe. We compute the low red-shift expansion for the energy density and the density contrast using an exact spherically symmetric solution in presence of a cosmological constant. At low red-shift the expansion is more precise than linear perturbation theory prediction. We then use the local expansion to reconstruct the metric from the monopole of the density contrast. We test the inversion method using numerical calculations and find a good agreement within the regime of validity of the red-shift expansion. The method could be applied to observational data to reconstruct the metric of the local Universe with a level of precision higher than the one achievable using perturbation theory.

astro-ph.CO↗

arXiv2017

The maximum sizes of large scale structures in alternative theories of gravity

The maximum size of a cosmic structure is given by the maximum turnaround radius -- the scale where the attraction due to its mass is balanced by the repulsion due to dark energy. We derive generic formulae for the estimation of the maximum turnaround radius in any theory of gravity obeying the Einstein equivalence principle, in two situations: on a spherically symmetric spacetime and on a perturbed Friedman-Robertson-Walker spacetime. We show that the two formulae agree. As an application of our formula, we calculate the maximum turnaround radius in the case of the Brans-Dicke theory of gravity. We find that for this theory, such maximum sizes always lie above the \LCDM value, by a factor $1 + \frac{1}{3ω}$, where $ω\gg 1$ is the Brans-Dicke parameter, implying consistency of the theory with current data.

astro-ph.CO↗

arXiv2017

CMB anomalies and the effects of local features of the inflaton potential

Recent analysis of the WMAP and Planck data have shown the presence of a dip and a bump in the spectrum of primordial perturbations at the scales $k=0.002$ Mpc${}^{-1}$ and $k=0.0035$ Mpc${}^{-1}$ respectively. We analyze for the first time the effects a local feature in the inflaton potential to explain the observed deviations from scale invariance in the primordial spectrum. We perform a best fit analysis of the cosmic microwave background (CMB) radiation temperature and polarization data. The effects of the features can improve the agreement with observational data respect to the featureless model. The best fit local feature affects the primordial curvature spectrum mainly in the region of the bump, leaving the spectrum unaffected on other scales.

astro-ph.CO↗

arXiv2017

General background conditions for K-bounce and adiabaticity

We study the background conditions for a bounce uniquely driven by a single scalar field model with a generalized kinetic term $K(X)$, without any additional matter field. At the background level we impose the existence of two turning points where the derivative of the Hubble parameter $H$ changes sign and of a bounce point where the Hubble parameter vanishes. We find the conditions for $K(X)$ and the potential which ensure the above requirements. We then give the examples of two models constructed according to these conditions. One is based on a quadratic $K(X)$, and the other a $K(X)$ which is avoiding divergences of the second time derivative of the scalar field, which may otherwise occur. An appropriate choice of the initial conditions can lead to a sequence of consecutive bounces, or oscillations of $H$. In the region where these models have a constant potential they are adiabatic on any scale and because of this they may not conserve curvature perturbations on super-horizon scales. While at the perturbation level one class of models is free from ghosts and singularities of the classical equations of motion, in general gradient instabilities are present around the bounce time, because the sign of the squared speed of sound is opposite to the sign of the time derivative of $H$. We discuss how this kind of instabilities could be avoided by modifying the Lagrangian by introducing Galileion terms in order to prevent a negative squared speed of sound around the bounce.

gr-qc↗

arXiv2016

Global adiabaticity and non-Gaussianity consistency condition

In the context of single-field inflation, the conservation of the curvature perturbation on comoving slices, $\R_c$, on super-horizon scales is one of the assumptions necessary to derive the consistency condition between the squeezed limit of the bispectrum and the spectrum of the primordial curvature perturbation. However, the conservation of $\R_c$ holds only after the perturbation has reached the adiabatic limit where the constant mode of $\R_c$ dominates over the other (usually decaying) mode. In this case, the non-adiabatic pressure perturbation defined in the thermodynamic sense, $δP_{nad}\equivδP-c_w^2δρ$ where $c_w^2=\dot P/\dotρ$, usually becomes also negligible on superhorizon scales. Therefore one might think that the adiabatic limit is the same as thermodynamic adiabaticity. This is in fact not true. In other words, thermodynamic adiabaticity is not a sufficient condition for the conservation of $\R_c$ on super-horizon scales. In this paper, we consider models that satisfy $δP_{nad}=0$ on all scales, which we call global adiabaticity (GA), which is guaranteed if $c_w^2=c_s^2$, where $c_s$ is the phase velocity of the propagation of the perturbation. A known example is the case of ultra-slow-roll(USR) inflation in which $c_w^2=c_s^2=1$. In order to generalize USR we develop a method to find the Lagrangian of GA K-inflation models from the behavior of background quantities as functions of the scale factor. Applying this method we show that there indeed exists a wide class of GA models with $c_w^2=c_s^2$, which allows $\R_c$ to grow on superhorizon scales, and hence violates the non-Gaussianity consistency condition.

gr-qc↗

arXiv2016

Effects of local features of the inflaton potential on the spectrum and bispectrum of primordial perturbations

We study the effects of a class of features of the potential of slow-roll inflationary models corresponding to a step symmetrically dumped by an even power negative exponential factor, which we call local features. Local-type features differ from other branch-type features considered previously, because the potential is only affected in a limited range of the scalar field value, and are symmetric respect to the location of the feature. This type of features only affects the spectrum and bispectrum in a narrow range of scales which leave the horizon during the time interval corresponding to the modification of the potential. On the contrary branch-type features have effects on all the perturbation modes leaving the horizon when the field value is within the interval defining the branch, introducing for example differences in the power spectrum between large and small scale which are absent in the case of local-type features. The spectrum and bispectrum of primordial curvature perturbations are affected by oscillations around the scale $k_0$ exiting the horizon at the time $τ_0$ corresponding to the feature. We also compute the effects of the features on the CMB temperature and polarization spectra, showing the effects or different choices of parameters.

astro-ph.CO↗

arXiv2016

Directional dependence of the local estimation of $H_0$ and the non perturbative effects of primordial curvature perturbations

Recent measurements of the cosmic microwave background (CMB) radiation have shown an apparent tension with the present value of the Hubble parameter inferred from local observations of supernovae, which look closer, i.e. brighter, than what is expected in a homogeneous model with a value of $H_0$ equal to the one estimated from CMB observations. We examine the possibility that such a discrepancy is the consequence of the presence of a local inhomogeneity seeded by primordial curvature perturbations, finding that a negative peak of the order of less than two standard deviations could allow to fit low red-shift supernovae observations without the need of using a value of the Hubble parameter different from $H_0^{CMB}$. The type of inhomogeneity we consider does not modify the distance to the last scattering, making it compatible with the constraints of the PLANCK mission data. Our calculation is fully relativistic and non perturbative, and for this reason shows important effects which were missed in the previous investigations using relativistic perturbations or Newtonian approximations, because the structures seeded by primordial curvature perturbations can be today highly non linear, and relativist Doppler terms cannot be neglected. Comparison with studies of local structure based on galaxy surveys and luminosity density analysis reveals that the density profile we find could in fact be compatible with the one obtained for the same region of sky where is located most of the Cepheids used to calibrate the luminosity distance of the supernovae employed for the local $H_0$ estimation, suggesting a possible directional dependence and calibration bias which could be partially attributed to the presence of the Sloan Great Wall.

astro-ph.CO↗

arXiv2016

A new large CMB non-Gaussian anomaly and its alignment with cosmic structure

We provide evidence of the detection of a new non-Gaussian anomaly in the cosmic microwave background (CMB) radiation which has larger statistical significance than the Cold Spot (CS) anomaly and comparable size. This temperature anomaly is aligned with a huge large quasar group (HLQG), and for this reason we call it HLQG anomaly. There are different physical phenomena by which the HLQG could have produced the observed temperature anomaly, such as for example the Sunyaev Zeldovich (SZ), the integrated Sachs Wolfe (ISW) or the Rees Sciama (RS) effect. The goal of this paper it is not to explain the observed alignment in terms of these effects, but to show the shape and position of the HLQG anomaly, and estimate its statistical significance, i.e. the probability that it could be just the result of primordial Gaussian fluctuations. We analyze the CMB Planck satellite temperature map of the region of sky corresponding to the angular position of the HLQG, and compute an inner and an outer temperature by averaging the CMB map over respectively the region subtended by the HLQG on the sky, and over a region surrounding it. It turns out that the inner region is warmer than the outer one, with a measured temperature difference of $ΔT^{obs} \approx 43μK$. The temperature excess is then compared with the results of Montecarlo simulations of random Gaussian realizations of the CMB map, indicating with at least a $2.3σ$ confidence level, that the measured $ΔT^{obs}$ cannot be attributed to primordial Gaussian fluctuations. Its alignment with HLQG and suggests that this could be due to the Rees-Sciama effect. Our results are stable under the choice of different simulations methods and different definitions of the inner and outer regions.

astro-ph.CO↗

arXiv2016

Low red-shift effects of local structure on the Hubble parameter in presence of a cosmological constant

In order to estimate the effects of local structure on the Hubble parameter we calculate the low-redshift expansion for $H(z)$ and $\frac{δH}{H}$ for an observer at the center of a spherically symmetric matter distribution in presence of a cosmological constant. We then test the accuracy of the formulae comparing them with fully relativistic non perturbative numerical calculations for different cases for the density profile. The low red-shift expansion we obtain gives results more precise than perturbation theory since is based on the use of an exact solution of Einstein's field equations. For larger density contrasts the low red-shift formulae accuracy improves respect to the perturbation theory accuracy because the latter is based on the assumption of a small density contrast, while the former does not rely on such assumption. The formulae can be used to take into account the effects on the Hubble expansion parameter due to the monopole component of the local structure. If the $H(z)$ observations will show deviations from the $ΛCDM$ prediction compatible with the formulae we have derived, this could be considered an independent evidence of the existence of a local inhomogeneity, and the formulae could be used to determine the characteristics of this local structure.

gr-qc↗

arXiv2016

Adiabaticity and gravity theory independent conservation laws for cosmological perturbations

We carefully study the implications of adiabaticity for the behavior of cosmological perturbations. There are essentially three similar but different definitions of non-adiabaticity: one is appropriate for a thermodynamic fluid $δP_{nad}$, another is for a general matter field $δP_{c,nad}$, and the last one is valid only on superhorizon scales. The first two definitions coincide if $c_s^2=c_w^2$ where $c_s$ is the propagation speed of the perturbation, while $c_w^2=\dot P/\dotρ$. Assuming the adiabaticity in the general sense, $δP_{c,nad}=0$, we derive a relation between the lapse function in the comoving sli\-cing $A_c$ and $δP_{nad}$ valid for arbitrary matter field in any theory of gravity, by using only momentum conservation. The relation implies that as long as $c_s\neq c_w$, the uniform density, comoving and the proper-time slicings coincide approximately for any gravity theory and for any matter field if $δP_{nad}=0$ approximately. In the case of general relativity this gives the equivalence between the comoving curvature perturbation $R_c$ and the uniform density curvature perturbation $ζ$ on superhorizon scales, and their conservation. We then consider an example in which $c_w=c_s$, where $δP_{nad}=δP_{c,nad}=0$ exactly, but the equivalence between $R_c$ and $ζ$ no longer holds. Namely we consider the so-called ultra slow-roll inflation. In this case both $R_c$ and $ζ$ are not conserved. In particular, as for $ζ$, we find that it is crucial to take into account the next-to-leading order term in $ζ$'s spatial gradient expansion to show its non-conservation, even on superhorizon scales. This is an example of the fact that adiabaticity (in the thermodynamic sense) is not always enough to ensure the conservation of $R_c$ or $ζ$.

gr-qc↗

arXiv2015

Effects of discontinuities of the derivatives of the inflaton potential

We study the effects of a class of features of the inflaton potential, corresponding to discontinuties in its derivatives. We perform fully numerical calculations and derive analytical approximations for the curvature pertubations spectrum and the bispectrum which are in good agreement with the numerical results. The spectrum of primordial perturbations has oscillations around the scale $k_0$ which leaves the horizon at the time $τ_0$ when the feature occurs, with the amplitude and phase of the oscillations determined by the size and the order of the discontinuity. The large scale bispectrum in the squeezed and equilateral limits have a very similar form and are linearly suppressed. Both in the squeezed and equilateral small scale limit the bispectrum has an oscillatory behavior whose phase depends on the parameters determining the discontinuity, and whose amplitude is inversely proportional to the scale. Given the generality of this class of features they could be used to model or classify phenomenologically different types of non Gaussian features encountered in observational data such as the cosmic microwave background radiation or large scale structure.

astro-ph.CO↗

arXiv2015

Can Brans-Dicke theory with $Λ>0$ describe stars?

A step-by-step approach is followed to study cosmic structures in the context of Brans-Dicke theory with positive cosmological constant $Λ$ and parameter $ω$. First, it is shown that regular stationary black-hole solutions not only have constant Brans-Dicke field $ϕ$, but can exist only for $ω=\infty$, which forces the theory to coincide with the General Relativity. Generalizations of the theory in order to evade this black-hole no-hair theorem are presented. It is also shown that in the absence of a stationary cosmological event horizon in the asymptotic region, a stationary black hole horizon can support a non-trivial Brans-Dicke hair. Even more importantly, it is shown next, that the presence of a stationary cosmological event horizon rules out any regular stationary solution, appropriate for the description of a star. Thus, to describe a star one has to assume that there is no such stationary horizon in the faraway asymptotic region. Under this implicit assumption generic spherical cosmic structures are studied perturbatively and is shown that only for $ω>0$ or $ω\lesssim -5$ their predicted maximum sizes are consistent with observations. We also point out how, many of the conclusions of this work differ qualitatively from the $Λ=0$ spacetimes.

gr-qc↗

arXiv2014

Non perturbative effects of primordial curvature perturbations on the apparent value of a cosmological constant

We study effects on the luminosity distance of a local inhomogeneity seeded by primordial curvature perturbations of the type predicted by the inflationary scenario and constrained by the cosmic microwave background radiation. We find that a local underdensity originated from a one, two or three standard deviations peaks of the primordial curvature perturbations field can induce corrections to the value of a cosmological constant of the order of $0.6%,1%,1.5%$ respectively. These effects cannot be neglected in the precision cosmology era in which we are entering. Our results can be considered an upper bound for the effect of the monopole component of the local non linear structure which can arise from primordial curvature perturbations and requires a fully non perturbative relativistic treatment.

astro-ph.CO↗