arXiv subjects

Luca Amendola

Publications and source records attributed to Luca Amendola.

At least 145 records · Page 8Linked to original sources

arXiv2009

Distinguishing Between Void Models and Dark Energy with Cosmic Parallax and Redshift Drift

Two recently proposed techniques, involving the measurement of the cosmic parallax and redshift drift, provide novel ways of directing probing (over a time-span of several years) the background metric of the universe and therefore shed light on the dark energy conundrum. The former makes use of upcoming high-precision astrometry measurements to either observe or put tight constraints on cosmological anisotropy for off-center observers, while the latter employs high-precision spectroscopy to give an independent test of the present acceleration of the universe. In this paper, we show that both methods can break the degeneracy between LTB void models and more traditional dark energy theories. Using the near-future observational missions Gaia and CODEX we show that this distinction might be made with high confidence levels in the course of a decade.

astro-ph.CO↗

arXiv2009

Cosmic Parallax as a probe of late time anisotropic expansion

Cosmic parallax is the change of angular separation between pair of sources at cosmological distances induced by an anisotropic expansion. An accurate astrometric experiment like Gaia could observe or put constraints on cosmic parallax. Examples of anisotropic cosmological models are Lemaitre-Tolman-Bondi void models for off-center observers (introduced to explain the observed acceleration without the need for dark energy) and Bianchi metrics. If dark energy has an anisotropic equation of state, as suggested recently, then a substantial anisotropy could arise at $z \lesssim 1$ and escape the stringent constraints from the cosmic microwave background. In this paper we show that such models could be constrained by the Gaia satellite or by an upgraded future mission.

astro-ph.CO↗

arXiv2009

Cosmological constraints on the Hu-Sawicki modified gravity scenario

In this paper we place new constraints on a f(R) modified gravity model recently proposed by Hu and Sawicki. After checking that the Hu and Sawicki model produces a viable cosmology, i.e. a matter dominated epoch followed by a late-time acceleration, we constrain some of its parameters by using recent observations from the UNION compilation of luminosity distances of Supernovae type Ia, including complementary information from Baryonic Acoustic Oscillations, Hubble expansion, and age data. We found that the data considered is unable to place significant constraints on the model parameters and we discuss the impact of a different assumption of the background model in cosmic parameters inference.

astro-ph.CO↗

arXiv2009

Cosmic Parallax: possibility of detecting anisotropic expansion of the universe by very accurate astrometry measurements

Refined astrometry measurements allow us to detect large-scale deviations from isotropy through real-time observations of changes in the angular separation between sources at cosmic distances. This "cosmic parallax" effect is a powerful consistency test of FRW metric and may set independent constraints on cosmic anisotropy. We apply this novel general test to LTB cosmologies with off-center observers and show that future satellite missions such as Gaia might achieve accuracies that would put limits on the off-center distance which are competitive with CMB dipole constraints.

astro-ph↗

arXiv2009

Findings of the Joint Dark Energy Mission Figure of Merit Science Working Group

These are the findings of the Joint Dark Energy Mission (JDEM) Figure of Merit (FoM) Science Working Group (SWG), the FoMSWG. JDEM is a space mission planned by NASA and the DOE for launch in the 2016 time frame. The primary mission is to explore the nature of dark energy. In planning such a mission, it is necessary to have some idea of knowledge of dark energy in 2016, and a way to quantify the performance of the mission. In this paper we discuss these issues.

astro-ph.IM↗

arXiv2008

Mimicking general relativity in the solar system

In order for a modified gravity model to be a candidate for cosmological dark energy it has to pass stringent local gravity experiments. We find that a Brans-Dicke (BD) theory with well-defined second order corrections that include the Gauss-Bonnet term possess this feature. We construct the generic second order theory that gives, to linear order, a BD metric solution for a point-like mass source. We find that these theories interpolate between general relativity (GR) and BD gravity. In particular it is found that the relevant Eddington parameter, that is commonly heavily constrained by time delay experiments, can be arbitrarily close to the GR value of 1, with an arbitrary BD parameter. We find the region where the solution is stable to small timelike perturbations.

gr-qc↗

arXiv2008

Deconstructing Baryon Acoustic Oscillations: A Comparison of Methods

The Baryon Acoustic Oscillations (BAOs) or baryon wiggles which are present in the galaxy power spectrum at scales 100-150Mpc/h are powerful features with which to constrain cosmology. The potential of these probes is such that these are now included as primary science goals in the planning of several future galaxy surveys. However, there is not a uniquely defined BAO Method in the literature but a range of implementations. We study the assumptions and cosmological performances of three different BAO methods: the full Fourier space power spectrum [P(k)], the `wiggles only' in Fourier space and the spherical harmonics power spectrum [C(l)]. We contrast the power of each method to constrain cosmology for two fiducial surveys taken from the Dark Energy Task Force (DETF) report and equivalent to future ground and space based spectroscopic surveys. We find that, depending on the assumptions used, the dark energy Figure of Merit (FoM) can change by up to a factor of 35 for a given fiducial model and survey. We compare our results with the DETF implementation and, discuss the robustness of each probe, by quantifying the dependence of the FoM with the wavenumber range. The more information used by a method, the higher its statistical performance, but the higher its sensitivity to systematics and implementations details.

astro-ph↗

arXiv2008

Mapping the galactic gravitational potential with peculiar acceleration

It has been suggested recently that the change in cosmological redshift (the Sandage test of expansion) could be observed in the next generation of large telescopes and ultra-stable spectrographs. In a recent paper we estimated the change of peculiar velocity, i.e. the peculiar acceleration, in nearby galaxies and clusters and shown it to be of the same order of magnitude as the typical cosmological signal. Mapping the acceleration field allows for a reconstruction of the galactic gravitational potential without assuming virialization. In this paper we focus on the peculiar acceleration in our own Galaxy, modeled as a Kuzmin disc and a dark matter spherical halo. We estimate the peculiar acceleration for all known Galactic globular clusters and find some cases with an expected velocity shift in excess of 20 cm/sec for observations fifteen years apart, well above the typical cosmological acceleration. We then compare the predicted signal for a MOND (modified Newtonian dynamics) model in which the spherical dark matter halo is absent. We find that the signal pattern is qualitatively different, showing that the peculiar acceleration field could be employed to test competing theories of gravity. However the difference seems too small to be detectable in the near future.

astro-ph↗

arXiv2008

Dark Energy Phenomenology

We discuss the phenomenology of the dark energy in first order perturbation theory, demonstrating that the dark energy cannot be fully constrained unless the dark matter is found, and that there are two functions that characterise the observational properties of the dark sector for cosmological probes. We argue that measuring these two functions should be an important goal for observational cosmology in the next decades.

astro-ph↗

arXiv2007

Phantom crossing, equation-of-state singularities, and local gravity constraints in f(R) models

We identify the class of f(R) dark energy models which have a viable cosmology, i.e. a matter dominated epoch followed by a late-time acceleration. The deviation from a LambdaCDM model (f=R-Lambda) is quantified by the function m=Rf_{,RR}/f_{,R}. The matter epoch corresponds to m(r=-1) simeq +0 (where r=-Rf_{,R}/f) while the accelerated attractor exists in the region 0 1, i.e. within reach of observations. If we add local gravity constraints, the bound on m becomes very stringent, with m several orders of magnitude smaller than unity in the region whose density is much larger than the present cosmological density. The representative models that satisfy both cosmological and local gravity constraints take the asymptotic form m(r)=C(-r-1)^p with p>1 as r approaches -1.

astro-ph↗

arXiv2007

Power-laws f(R) theories are cosmologically unacceptable

In a recent paper [1] (PRL 98,131302,2007) we have shown that f(R)=R + mu R^{n} modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the accelerated stage the cosmic expansion is given by a sim t^{1/2} rather than a sim t^{2/3}, where a is a scale factor and t is the cosmic time. A recent work [2] (PLB 639,135-143,2006) by Capozziello et al. criticised our results presenting some apparent counter-examples to our claim in f(R)= mu R^{n} models. We show here that those particular R^{n} models can produce an expansion as a sim t^{2/3} but this does not connect to a late-time acceleration. Hence, though acceptable f(R) dark energy models may exist, the R^{n} models presented in Capozziello et al. are not cosmologically viable, confirming our previous results in Ref. [1].

astro-ph↗

arXiv2007

Solar System Constraints on Gauss-Bonnet Mediated Dark Energy

Although the Gauss-Bonnet term is a topological invariant for general relativity, it couples naturally to a quintessence scalar field, modifying gravity at solar system scales. We determine the solar system constraints due to this term by evaluating the post-Newtonian metric for a distributional source. We find a mass dependent, 1/r^7 correction to the Newtonian potential, and also deviations from the Einstein gravity prediction for light-bending. We constrain the parameters of the theory using planetary orbits, the Cassini spacecraft data, and a laboratory test of Newton's law, always finding extremely tight bounds on the energy associated to the Gauss-Bonnet term. We discuss the relevance of these constraints to late-time cosmological acceleration.

astro-ph↗

arXiv2007

Constraining the growth factor with baryon oscillations

The growth factor of linear fluctuations is probably one of the least known quantity in observational cosmology. Here we discuss the constraints that baryon oscillations in galaxy power spectra from future surveys can put on a conveniently parametrized growth factor. We find that spectroscopic surveys of $5000 deg^2$ extending to $z \approx 3$ could estimate the growth index $γ$ within 0.06; a similar photometric survey would give $Δγ\approx 0.15$. This test provides an important consistency check for standard cosmological model and could constrain modified gravity models. We discuss the errors and the figure of merit for various combinations of redshift errors and survey size.

astro-ph↗

arXiv2007

Peculiar acceleration

It has been proposed recently to observe the change in cosmological redshift of distant galaxies or quasars with the next generation of large telescope and ultra-stable spectrographs (the so-called Sandage-Loeb test). Here we investigate the possibility of observing the change in peculiar velocity in nearby clusters and galaxies. This ``peculiar acceleration'' could help reconstructing the gravitational potential without assuming virialization. We show that the expected effect is of the same order of magnitude of the cosmological velocity shift. Finally, we discuss how to convert the theoretical predictions into quantities directly related to observations.

astro-ph↗

arXiv2007

Observational constraints on the linear fluctuation growth rate

Several experiments in the near future will test dark energy through its effects on the linear growth of matter perturbations. It is therefore important to find simple and at the same time general parametrizations of the linear growth rate. We show that a simple fitting formula that generalizes previous expressions reproduces the growth function in models that allow for a growth faster than standard, as for instance in scalar-tensor models. We use data from galaxy and Lyman-$α$ power spectra to constrain the linear growth rate. We find $γ=0.6_{-0.3}^{+0.4}$ for the growth rate index and $η=0.0_{-0.2}^{+0.3}$ for the additional growth parameter we introduce.

astro-ph↗

arXiv2007

Growing Matter

We investigate quintessence cosmologies with a matter component consisting of particles with an increasing mass. While negligible in early cosmology, the appearance of a growing matter component has stopped the evolution of the cosmon field at a redshift around six. In turn, this has triggered the accelerated expansion of the Universe. We propose to associate growing matter with neutrinos. Then the presently observed dark energy density and its equation of state are determined by the neutrino mass.

astro-ph↗

arXiv2007

Measuring the dark side (with weak lensing)

We introduce a convenient parametrization of dark energy models that is general enough to include several modified gravity models and generalized forms of dark energy. In particular we take into account the linear perturbation growth factor, the anisotropic stress and the modified Poisson equation. We discuss the sensitivity of large scale weak lensing surveys like the proposed DUNE satellite to these parameters. We find that a large-scale weak-lensing tomographic survey is able to easily distinguish the Dvali-Gabadadze-Porrati model from LCDM and to determine the perturbation growth index to an absolute error of 0.02-0.03.

astro-ph↗

arXiv2007

Conditions for the cosmological viability of f(R) dark energy models

We derive the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. We show that the cosmological behavior of f(R) models can be understood by a geometrical approach consisting in studying the m(r) curve on the (r, m) plane, where m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f with f_{,R}=df/dR. This allows us to classify the f(R) models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. The existence of a viable matter dominated epoch prior to a late-time acceleration requires that the variable m satisfies the conditions m(r) approx+0 and dm/dr>-1 at r approx-1. For the existence of a viable late-time acceleration we require instead either (i) m=-r-1, (sqrt{3}-1)/2 0 and n<-1 and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor a propto t^{1/2}. We also find that f(R) models can have a strongly phantom attractor but in this case there is no acceptable matter era.

gr-qc↗