arXiv subjects

Luca Amendola

Publications and source records attributed to Luca Amendola.

At least 163 records · Page 9Linked to original sources

arXiv2007

Are f(R) dark energy models cosmologically viable ?

All $f(R)$ modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological evolution radically different from standard cosmology. We find that in all $f(R)$ theories that behave as a power of $R$ at large or small $R$ (which include most of those proposed so far in the literature) the scale factor during the matter phase grows as $t^{1/2}$ instead of the standard law $t^{2/3}$. This behaviour is grossly inconsistent with cosmological observations (e.g. WMAP), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.

astro-ph↗

arXiv2006

Constraints on Gauss-Bonnet Gravity in Dark Energy Cosmologies

Models with a scalar field coupled to the Gauss-Bonnet Lagrangian appear naturally from Kaluza-Klein compactifications of pure higher-dimensional gravity. We study linear, cosmological perturbations in the limits of weak coupling and slow-roll, and derive simple expressions for the main observable sub-horizon quantities: the anisotropic stress factor, the time-dependent gravitational constant, and the matter perturbation growth factor. Using present observational data, and assuming slow-roll for the dark energy field, we find that the fraction of energy density associated with the coupled Gauss-Bonnet term cannot exceed 15%. The bound should be treated with caution, as there are significant uncertainies in the data used to obtain it. Even so, it indicates that the future prospects for constraining the coupled Gauss-Bonnet term with cosmological observations are encouraging.

hep-th↗

arXiv2006

SNLS data are consistent with acceleration at z=3

We point out that the Type Ia supernovae in the SNLS dataset are consistent with an early beginning of the cosmic acceleration if dark energy interacts strongly with dark matter. We find that the acceleration could have started at redshift z =3 and higher.

astro-ph↗

arXiv2006

Challenges for scaling cosmologies

A cosmological model that aims at solving the coincidence problem should show that dark energy and dark matter follow the same scaling solution from some time onward. At the same time, the model should contain a sufficiently long matter-dominated epoch that takes place before acceleration in order to guarantee a decelerated epoch and structure formation. So a successful cosmological model requires the occurrence of a sequence of epochs, namely a radiation era, a matter-dominated era and a final accelerated scaling attractor with $Ω_ϕ \simeq 0.7$. In this paper we derive the generic form of a scalar-field Lagrangian that possesses scaling solutions in the case where the coupling $Q$ between dark energy and dark matter is a free function of the field $ϕ$. We then show, rather surprisingly, that the aforementioned sequence of epochs cannot occur for a vast class of generalized coupled scalar field Lagrangians that includes, to our knowledge, all scaling models in the current literature.

astro-ph↗

arXiv2005

Dark Matter from an ultra-light pseudo-Goldsone-boson

Dark Matter (DM) and Dark Energy (DE) can be both described in terms of ultra-light Pseudo-Goldstone-Bosons (PGB) with masses m_{DM} ~ 10^{-23}eV and m_{DE} <= 10^{-33}eV respectively. Following Barbieri et al, we entertain the possibility that a PGB exists with mass m_I intermediate between these two limits, giving a partial contribution to DM. We evaluate the related effects on the power spectrum of the matter density perturbations and on the cosmic microwave background and we derive the bounds on the density fraction, f_I, of this intermediate field from current data, with room for a better sensitivity on f_I in the near future. We also give a simple and unified analytic description of the free streaming effects both for an ultra-light scalar and for a massive neutrino.

hep-ph↗

arXiv2005

Observational Constraints on Silent Quartessence

We derive new constraints set by SNIa experiments (`gold' data sample of Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the quartessence Chaplygin model. We consider both adiabatic perturbations and intrinsic non-adiabatic perturbations such that the effective sound speed vanishes (Silent Chaplygin). We show that for the adiabatic case, only models with equation of state parameter $ |α|\lesssim 10^{-2}$ are allowed: this means that the allowed models are very close to \LambdaCDM. In the Silent case, however, the results are consistent with observations in a much broader range, -0.3<α<0.7.

astro-ph↗

arXiv2005

Phantom damping of matter perturbations

Cosmological scaling solutions are particularly important in solving the coincidence problem of dark energy. We derive the equations of sub-Hubble linear matter perturbations for a general scalar-field Lagrangian--including quintessence, tachyon, dilatonic ghost condensate and k-essence--and solve them analytically for scaling solutions. We find that matter perturbations are always damped if a phantom field is coupled to dark matter and identify the cases in which the gravitational potential is constant. This provides an interesting possibility to place stringent observational constraints on scaling dark energy models.

astro-ph↗

arXiv2005

On the Effects due to a Decaying Cosmological Fluctuation

We present the initial conditions for a decaying cosmological perturbation and study its signatures in the CMB anisotropies and matter power spectra. An adiabatic decaying mode in presence of components which are not described as perfect fluids (such as collisionless matter) decays slower than in a perfect-fluid dominated universe and displays super-Hubble oscillations. By including a correlated decaying mode with a red or a scale invariant spectrum, the anisotropy pattern shows super-imposed oscillations before the first Dopplear peak, while with a blue spectrum the amplitude of the secondary peaks relative to the first one and the matter power spectrum can be altered. WMAP first year data constrain the decaying to growing ratio of scale invariant adiabatic fluctuations at the matter-radiation equality to less than 10%.

astro-ph↗

arXiv2005

Supernovae type Ia data favour coupled phantom energy

We estimate the constraints that the recent high-redshift sample of supernovae type Ia put on a phenomenological interaction between dark energy and dark matter. The interaction can be interpreted as arising from the time variation of the mass of dark matter particles. We find that the coupling correlates with the equation of state: roughly speaking, a negative coupling (in our sign convention) implies phantom energy ($w_ϕ<-1$) while a positive coupling implies ``ordinary'' dark energy. The constraints from the current supernovae Ia Hubble diagram favour a negative coupling and an equation of state $w_ϕ<-1$. A zero or positive coupling is in fact unlikely at 99% c.l. (assuming constant equation of state); at the same time non-phantom values ($w_ϕ>-1$) are unlikely at 95%. We show also that the usual bounds on the energy density weaken considerably when the coupling is introduced: values as large as $Ω_{m0}=0.7$ become acceptable for as concerns SNIa. We find that the rate of change of the mass $\dot{m}/m$ of the dark matter particles is constrained to be $δ_{0}$ in a Hubble time, with $-10<δ_{0}<-1$ to 95% c.l.. We show that a large positive coupling might in principle avoid the future singularity known as ``big rip'' (occurring for $w_ϕ<-1$) but the parameter region for this to occur is almost excluded by the data. We also forecast the constraints that can be obtained from future experiments, focusing on supernovae and baryon oscillations in the power spectra of deep redshift surveys. We show that the method of baryon oscillations holds the best potential to contrain the coupling.

astro-ph↗

arXiv2004

Phantom energy mediates a long-range repulsive force

Scalar field models with non-standard kinetic terms have been proposed in the context of k-inflation, of Born-Infeld lagrangians, of phantom energy and, more in general, of low-energy string theory. In general, scalar fields are expected to couple to matter inducing a new interaction. In this paper I derive the cosmological perturbation equations and the Yukawa correction to gravity for such general models. I find three interesting results: first, when the field behaves as phantom energy (equation of state less than -1) then the coupling strength is negative, inducing a long-range repulsive force; second, the dark energy field might cluster on astrophysical scales; third, applying the formalism to a Brans-Dicke theory with general kinetic term it is shown that its Newtonian effects depend on a single parameter that generalizes the Brans-Dicke constant.

hep-th↗

arXiv2004

Fitting Type Ia supernovae with coupled dark energy

We discuss the possible consistency of the recently discovered Type Ia supernovae at z>1 with models in which dark energy is strongly coupled to a significant fraction of dark matter, and in which an (asymptotic) accelerated phase exists where dark matter and dark energy scale in the same way. Such a coupling has been suggested for a possible solution of the coincidence problem, and is also motivated by string cosmology models of "late time" dilaton interactions. Our analysis shows that, for coupled dark energy models, the recent data are still consistent with acceleration starting as early as at $z=3$ (to within 90% c.l.), although at the price of a large "non-universality" of the dark energy coupling to different matter fields. Also, as opposed to uncoupled models which seem to prefer a ``phantom'' dark energy, we find that a large amount of coupled dark matter is compatible with present data only if the dark energy field has a conventional equation of state w>-1.

astro-ph↗

arXiv2004

Testing for double inflation with WMAP

With the WMAP data we can now begin to test realistic models of inflation involving multiple scalar fields. These naturally lead to correlated adiabatic and isocurvature (entropy) perturbations with a running spectral index. We present the first full (9 parameter) likelihood analysis of double inflation with WMAP data and find that despite the extra freedom, supersymmetric hybrid potentials are strongly constrained with less than 7% correlated isocurvature component allowed when standard priors are imposed on the cosomological parameters. As a result we also find that Akaike & Bayesian model selection criteria rather strongly prefer single-field inflation, just as equivalent analysis prefers a cosmological constant over dynamical dark energy in the late universe. It appears that simplicity is the best guide to our universe.

astro-ph↗

arXiv2004

Constraints on perfect fluid and scalar field dark energy models from future redshift surveys

We discuss the constraints that future photometric and spectroscopic redshift surveys can put on dark energy through the baryon oscillations of the power spectrum. We model the dark energy either with a perfect fluid or a scalar field and take into account the information contained in the linear growth function. We show that the growth function helps to break the degeneracy in the dark energy parameters and reduce the errors on $w_0,w_1$ roughly by 30% making more appealing multicolor surveys based on photometric redshifts. We find that a 200 square degrees spectroscopic survey reaching $z = 3$ can constrain $w_0,w_1$ to within $Δw_0=0.21,Δw_1=0.26$ and to $Δw_0=0.39,Δw_1=0.54$ using photometric redshifts with absolute uncertainty of 0.02. In the scalar field case we show that the slope $n$ of the inverse power-law potential for dark energy can be constrained to $Δn=0.26$ (spectroscopic redshifts) or $Δn=0.40$ (photometric redshifts), i.e. better than with future ground-based supernovae surveys or CMB data.

astro-ph↗

arXiv2004

Skewness as a test of the equivalence principle

The skewness of the large scale distribution of matter has long been known to be a probe of gravitational clustering. Here we show that the skewness is also a probe of violation of the equivalence principle between dark matter and baryons. The predicted level of violation can be tested with the forecoming data from the Sloan Digital Sky Survey.

astro-ph↗

arXiv2003

Linear and non-linear perturbations in dark energy models

I review the linear and second-order perturbation theory in dark energy models with explicit interaction to matter in view of applications to N-body simulations and non-linear phenomena. Several new or generalized results are obtained: the general equations for the linear perturbation growth; an analytical expression for the bias induced by a species-dependent interaction; the Yukawa correction to the gravitational potential due to dark energy interaction; the second-order perturbation equations in coupled dark energy and their Newtonian limit. I also show that a density-dependent effective dark energy mass arises if the dark energy coupling is varying.

astro-ph↗

arXiv2003

N-body simulations for coupled dark energy: halo mass function and density profiles

We present the results of a series of N-body simulations in cosmologies where dark matter (DM) is coupled to dark energy (DE), so easing the cosmic coincidence problem. The dark-dark coupling introduces two novel effects in N-body dynamics: (i) DM particle masses vary with time; (ii) gravity between DM particles is ruled by a constant $G^{*}$, greater than Newton's constant $G$, holding in other 2-body interactions. As a consequence, baryons and DM particle distributions develop a large scale bias. Here we investigate DE models with Ratra-Peebles (RP) potentials; the dark-dark coupling is set in a parametric range compatible with observations, for as concern background and linear perturbation properties. We study the halo mass function, the halo density profile and the behavior of the non-linear bias. We find that non-linear dynamics puts additional constraints to the coupling parameter. They mostly arise from density profiles, that we find to yield higher concentrations, in coupled RP models, with respect to (uncoupled) dynamical DE cosmologies. Such enhancement, although being a strong effect in some coupling parameter range, is just a minor change for smaller but significant values of the coupling parameter. With these further restrictions, coupled DE models with RP potential are consistent with non-linear observables.

astro-ph↗

arXiv2003

Tracking and coupled dark energy as seen by WMAP

The satellite experiment WMAP has produced for the first time a high-coverage, high resolution survey of the microwave sky, releasing publicly available data that are likely to remain unrivalled for years to come. Here we compare the WMAP temperature power spectrum, along with an exhautive compilation of previous experiments, to models of dark energy that allow for a tracking epoch at the present, deriving updated bounds on the dark energy equation of state and the other cosmological parameters. Moreover, we complement the analysis by including a coupling of the dark energy to dark matter. The main results are: a) the WMAP data alone constrains the equation of state of tracking dark energy to be w_ϕ<-0.67(-0.49) to 68%(95%) (confining the analysis to w_ϕ>-1), which implies for an inverse power law potential an exponent α<0.99(2.08); b) the dimensionless coupling to dark matter is |β|<0.07(0.13). Including the results from the supernovae Ia further constrains the dark energy equation of state.

astro-ph↗

arXiv2002

Acceleration at z>1?

Most models of dark energy predict the beginning of the accelerated epoch at z <= 1. However, there are no observational or theoretical evidences in favor of such a recent start of the cosmic acceleration. In fact, a model of dark energy coupled to dark matter is explicitely constructed that a) is accelerated even at high z; b) allows structure formation during acceleration; and c) is consistent with the type Ia supernovae Hubble diagram, including the farthest known supernova SN1997ff at z~1.7 . It is shown that the accelerated epoch in this model could have started as early as z~5.

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