A simultaneous reconstruction of three functions describing the expansion of the Universe and gravitational effects on light and matter shows the extent to which modified gravity can address... Show moreA simultaneous reconstruction of three functions describing the expansion of the Universe and gravitational effects on light and matter shows the extent to which modified gravity can address tensions between the standard cosmological model and a large body of observations.There has been substantial interest in modifications of the standard ? cold dark matter (?CDM, where ? is the cosmological constant) cosmological model prompted by tensions between certain datasets, most notably the Hubble tension. The late-time modifications of the ?CDM model can be parameterized by three time-dependent functions describing the expansion history of the Universe and gravitational effects on light and matter in the large-scale structure. We perform a joint Bayesian reconstruction of these three functions from a combination of recent cosmological observations, utilizing a theory-informed prior built on the general Horndeski class of scalar-tensor theories. This reconstruction is interpreted in light of the well-known Hubble constant, clustering amplitude S-8 and lensing amplitude A(L) tensions. We identify the phenomenological features that alternative theories would need to have to ease some of these tensions, and deduce important constraints on broad classes of modified gravity models. Among other things, our findings suggest that late-time dynamical dark energy and modifications of gravity are not likely to offer a solution to the Hubble tension, or simultaneously solve the A(L) and S-8 tensions. Show less
The theoretical explanation of cosmic acceleration is nowadays one of the biggest puzzles in cosmology. Within the standard cosmological model (LCDM) the expansion is sourced by the vacuum energy... Show moreThe theoretical explanation of cosmic acceleration is nowadays one of the biggest puzzles in cosmology. Within the standard cosmological model (LCDM) the expansion is sourced by the vacuum energy associatedto the Cosmological Constant L. Despite its simplicity, the Cosmological Constant presents various unresolved problems from both the theoretical and the observational side.However, even if we dismiss these puzzles, the study of theoretical alternatives to LCDM is still of primary importance. In fact, the wealth and quality of cosmological data that we are expecting for thenext decade will allow us to test gravity on cosmological scales with unprecedented accuracy. This will give us the chance to investigate many of our theoretical ideas and to assess the strength of the standard model of cosmology on the largest scales.In this thesis we present different approaches that we can adopt to study modifications of gravity by means of cosmology. Show less
Hogg, N.B.; Bruni, N.; Crittenden, R.; Martinelli, M.; Peirone, S. 2020
We place observational constraints on the Galileon ghost condensate model, a dark energy proposal in cubic-order Horndeski theories consistent with the gravitational-wave event GW170817. The model... Show moreWe place observational constraints on the Galileon ghost condensate model, a dark energy proposal in cubic-order Horndeski theories consistent with the gravitational-wave event GW170817. The model extends the covariant Galileon by taking an additional higher-order field derivative X2 into account. This allows for the dark energy equation of state wDE to access the region −2 Show less
Peirone, S.; Benevento, G.; Frusciante, N.; Tsujikawa, S. 2019
We study observational constraints on a specific dark energy model in the framework of Gleyzes-Langlois-Piazza-Vernizzi theories, which extends the Galileon ghost condensate (GGC) to the domain of... Show moreWe study observational constraints on a specific dark energy model in the framework of Gleyzes-Langlois-Piazza-Vernizzi theories, which extends the Galileon ghost condensate (GGC) to the domain of beyond Horndeski theories. In this model, we show that the Planck cosmic microwave background (CMB) data, combined with datasets of baryon acoustic oscillations, supernovae type Ia, and redshift-space distortions, give the tight upper bound |α(0)H|≤O(10−6) on today’s beyond-Horndeski (BH) parameter αH. This is mostly attributed to the shift of CMB acoustic peaks induced by the early-time changes of cosmological background and perturbations arising from the dominance of αH in the dark energy density. In comparison to the Λ cold dark matter (ΛCDM) model, our BH model suppresses the large-scale integrated-Sachs-Wolfe tail of CMB temperature anisotropies due to the existence of cubic Galileons, and it modifies the small-scale CMB power spectrum because of the different background evolution. We find that the BH model considered fits the data better than ΛCDM according to the χ2 statistics, yet the deviance information criterion (DIC) slightly favors the latter. Given the fact that our BH model with αH=0 (i.e., the GGC model) is favored over ΛCDM even by the DIC, there are no particular signatures for the departure from Horndeski theories in current observations. Show less
Martinelli, M.; Hogg, N.B.; Peirone, S.; Bruni, M.; Wands, D. 2019
We investigate an interacting dark sector scenario in which the vacuum energy is free to interact with cold dark matter (CDM), which itself is assumed to cluster under the sole action of gravity, i... Show moreWe investigate an interacting dark sector scenario in which the vacuum energy is free to interact with cold dark matter (CDM), which itself is assumed to cluster under the sole action of gravity, i.e. it is in freefall (geodesic), as in ΛCDM. The interaction is characterized by a dimensionless coupling qV(z), in general a function of redshift. Aiming to reconstruct the evolution of the coupling, we use cosmic microwave background data from Planck 2015, along with baryon acoustic oscillation, redshift space distortion, and Type Ia supernova measurements to constrain various parametrizations of qV(z). We present the full linear perturbation theory of this interacting scenario and use Monte Carlo Markov Chains (MCMC) sampling to study five different cases: two cases in which we have ΛCDM evolution in the distant past, until a set redshift ztrans, below which the interaction switches on and qV is the single-sampled parameter, with ztrans fixed at ztrans = 3000 and 0.9, respectively; a case where we allow this transition redshift to vary along with qV; a case in which the vacuum energy is zero for z > ztrans and then begins to grow once the interaction switches on; and the final case in which we bin qV(z) in four redshift bins to investigate the possibility of a dynamical interaction, reconstructing the redshift evolution of the function using Gaussian processes. We find that, in all cases where the high-redshift evolution is not modified, the results are compatible with a vanishing coupling, thus finding no significant deviation from ΛCDM. Show less
In the context of the effective field theory of dark energy (EFT) we perform agnostic explorations of Horndeski gravity. We choose two parametrizations for the free EFT functions, namely, a power... Show moreIn the context of the effective field theory of dark energy (EFT) we perform agnostic explorations of Horndeski gravity. We choose two parametrizations for the free EFT functions, namely, a power law and a dark energy density-like behavior on a nontrivial Chevallier-Polarski-Linder background. We restrict our analysis to those EFT functions which do not modify the speed of propagation of gravitational waves. Among those, we prove that one specific function cannot be constrained by data since its contribution to the observables is below the cosmic variance, although we show it has a relevant role in defining the viable parameter space. We place constraints on the parameters of these models by combining measurements from present-day cosmological data sets, and we prove that the next-generation galaxy surveys can improve such constraints by 1 order of magnitude. We then prove the validity of the quasistatic limit within the sound horizon of the dark field, by looking at the phenomenological functions μ and Σ, associated, respectively, with clustering and lensing potentials. Furthermore, we notice up to 5% deviations in μ, Σ with respect to general relativity at scales smaller than the Compton one. For the chosen parametrizations and in the quasistatic limit, future constraints on μ and Σ can reach the 1% level and will allow us to discriminate between certain models at more than 3σ, provided the present best-fit values remain. Show less
Frusciante, N.; Papadomanolakis, G.; Peirone, S.; Silvestri, A. 2019
A recent experiment [K. C. Lee et al., Science 334, 1253 (2011)] succeeded in detecting entanglement between two macroscopic specks of diamonds, separated by a macroscopic distance, at room... Show moreA recent experiment [K. C. Lee et al., Science 334, 1253 (2011)] succeeded in detecting entanglement between two macroscopic specks of diamonds, separated by a macroscopic distance, at room temperature. This impressive result is a further confirmation of the validity of quantum theory in (at least parts of) the mesoscopic and macroscopic domain, and poses a challenge to collapse models, which predict a violation of the quantum superposition principle, which is bigger the larger the system. We analyze the experiment in the light of such models. We will show that the bounds placed by experimental data are weaker than those coming from matter-wave interferometry and noninterferometric tests of collapse models. Show less
