Browsing by Author "Bertacca, Daniele"

Now showing 1 - 7 of 7
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    Clustering of quintessence on horizon scales and its imprint on HI intensity mapping
    (IOP Science, 2013) Duniya, Didam G.A.; Bertacca, Daniele; Maartens, Roy
    Quintessence can cluster only on horizon scales. What is the effect on the observed matter distribution? To answer this, we need a relativistic approach that goes beyond the standard Newtonian calculation and deals properly with large scales. Such an approach has recently been developed for the case when dark energy is vacuum energy, which does not cluster at all. We extend this relativistic analysis to deal with dynamical dark energy. Using three quintessence potentials as examples, we compute the angular power spectrum for the case of an HI intensity map survey. Compared to the concordance model with the same small-scale power at z = 0, quintessence boosts the angular power by up to 15% at high redshifts, while power in the two models converges at low redshifts. The difference is mainly due to the background evolution, driven mostly by the normalization of the power spectrum today. The dark energy perturbations make only a small contribution on the largest scales, and a negligible contribution on smaller scales. Ironically, the dark energy perturbations remove the false boost of large-scale power that arises if we impose the (unphysical) assumption that the dark energy is smooth.
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    Cosmology on the largest scales with the SKA
    (Proceedings of Science, 2014) Camera, Stefano; Raccanelli, Alvise; Bull, Philip; Bertacca, Daniele; Chen, Xuelei; Ferreira, Pedro G.; Kunz, Martin; Maartens, Roy; Mao, Yi; Santos, Mario G.; Shapiro, Paul R.; Viel, Matteo; Xug, Yidong
    The study of the Universe on ultra-large scales is one of the major science cases for the Square Kilometre Array (SKA). The SKA will be able to probe a vast volume of the cosmos, thus representing a unique instrument, amongst next-generation cosmological experiments, for scrutinising the Universe’s properties on the largest cosmic scales. Probing cosmic structures on extremely large scales will have many advantages. For instance, the growth of perturbations is well understood for those modes, since it falls fully within the linear régime. Also, such scales are unaffected by the poorly understood feedback of baryonic physics. On ultra-large cosmic scales, two key effects become significant: primordial non-Gaussianity and relativistic corrections to cosmological observables. Moreover, if late-time acceleration is driven not by dark energy but by modifications to general relativity, then such modifications should become apparent near and above the horizon scale. As a result, the SKA is forecast to deliver transformational constraints on non-Gaussianity and to probe gravity on super-horizon scales for the first time.
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    Matter bispectrum in cubic Galileon cosmologies
    (IOP Science, 2013) Bartolo, Nicola; Bellini, Emilio; Bertacca, Daniele; Matarrese, Sabino
    In this paper we obtain the bispectrum of dark matter density perturbations in the frame of covariant cubic Galileon theories. This result is obtained by means of a semi- analytic approach to second-order perturbations in Galileon cosmologies, assuming Gaussian initial conditions. In particular, we show that, even in the presence of large deviations of the linear growth-rate w.r.t. the CDM one, at the bispectrum level such deviations are reduced to a few percent.
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    Modelling the growth of large-scale structure with interacting fluids
    (University of the Western Cape, 2015) Onchong’a, Okeng’o Geoffrey; Maartens, Roy; Bertacca, Daniele; Genga, R. O.; Awuor, J. B.
    Prevailing astronomical and astrophysical observations suggest that we live in a spatially flat cold dark matter (CDM) universe - currently going through a period of accelerated expansion possibly driven by “dark energy” in form of a cosmological constant. Within the standard cosmological paradigm, dark energy and dark matter are the dual dominant sources in the evolution of the late-time universe contributing about 70% and 25% respectively to the total energy density in the Universe, but these are only currently detected via their gravitational interaction. There could be a non-gravitational interaction within the “dark sector” without violating current observational data, thus giving rise to changes in the dark equations of state and affecting the process of galaxy formation. In this thesis, we investigate two new interesting large-scale structure formation scenarios using interacting fluids. Firstly, in departure from the standard approach in which dark matter is treated as a single independent fluid, we split the dark matter fluid into two interacting components: a strongly clustered “halo” component and a weakly clustered “free” component- accreted by the halos. By defining the fraction of the matter inside CDM “halos” to the total matter as a time evolving function of the total matter density F (ρm), we derive the governing background and perturbation equations and the energy-momentum transfer four-vectors. We then perform numerical calculations for three models for F (ρm) that are in agreement with recently published results from halo theory of N-body simulations, and compare our results to the standard ΛCDM model. Our results show that, whereas there’s a good agreement between our model and the ΛCDM model, the perturbations are much more sensitive to the interaction and can deviate strongly from the standard case for large interaction strengths. Secondly, motivated by our current poor knowledge on the underlying “dark- sector” physics and the need to understand the nature of the two most dominant components of our universe: dark energy and dark matter; we investigate a new scenario in which the two dark components interact via an energy-momentum exchange. By re-writing the evolution equations in a more suitable form, we eliminate previously reported singularities in interacting dark energy models in which dark energy is tested to be vacuum energy with w → −1. This makes it possible to numerically integrate the resulting background and perturbation equations, comparing our results to the standard model. We show that this treatment, yields a simple model that provides a good natural extension to the standard ΛCDM model. We go further to explore in detail the cosmological implications of the interaction strength and the direction of the energy-momentum transfer in vacuum interacting dark energy. This thesis provides useful insights on the possible significance of a dark sector interaction in structure formation and shows that such an interaction provides a good natural explanation for the high value of the Hubble parameters measured by BOSS and SDSS surveys. Indeed a small and positive coupling is shown to alleviate the well-known cosmological coincidence problem.
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    The observed galaxy bispectrum from single-field inflation in the squeezed limit
    (IOP Publishing, 2018) Koyama, Kazuya; Umeh, Obinna; Maartens, Roy; Bertacca, Daniele
    Using the consistency relation in Fourier space, we derive the observed galaxy bispectrum from single- eld in ation in the squeezed limit, in which one of the three modes has a wavelength much longer than the other two. This provides a non-trivial check of the full computation of the bispectrum based on second-order cosmological perturbation theory in this limit. We show that gauge modes need to be carefully removed in the second-order cosmological perturbations in order to calculate the observed galaxy bispectrum in the squeezed limit. We then give an estimate of the e ective non- Gaussianity due to general-relativistic lightcone e ects that could mimic a primordial non-Gaussian signal.
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    Probing the imprint of interacting dark energy on very large scales
    (American Physical Society, 2015) Duniya, Didam, G. A.; Bertacca, Daniele; Maartens, Roy
    The observed galaxy power spectrum acquires relativistic corrections from light-cone effects, and these corrections grow on very large scales. Future galaxy surveys in optical, infrared and radio bands will probe increasingly large wavelength modes and reach higher redshifts. In order to exploit the new data on large scales, an accurate analysis requires inclusion of the relativistic effects. This is especially the case for primordial non-Gaussianity and for extending tests of dark energy models to horizon scales. Here we investigate the latter, focusing on models where the dark energy interacts nongravitationally with dark matter. Interaction in the dark sector can also lead to large-scale deviations in the power spectrum. If the relativistic effects are ignored, the imprint of interacting dark energy will be incorrectly identified and thus lead to a bias in constraints on interacting dark energy on very large scales.
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    Testing gravity using large-scale redshift-space distortions
    (Oxford University Press, 2013) Raccanelli, Alvise; Bertacca, Daniele; Pietrobon, Davide; Schmidt, Fabian; Samushia, Lado; Bartolo, Nicola; Doré, Olivier; Matarrese, Sabino; Percival, Will J.
    We use luminous red galaxies from the Sloan Digital Sky Survey (SDSS) II to test the cosmological structure growth in two alternatives to the standard cold dark matter ( CDM)+general relativity (GR) cosmological model. We compare observed threedimensional clustering in SDSS Data Release 7 (DR7) with theoretical predictions for the standard vanilla CDM+GR model, unified dark matter (UDM) cosmologies and the normal branch Dvali–Gabadadze–Porrati (nDGP). In computing the expected correlations in UDM cosmologies, we derive a parametrized formula for the growth factor in these models. For our analysis we apply the methodology tested in Raccanelli et al. and use the measurements of Samushia et al. that account for survey geometry, non-linear and wide-angle effects and the distribution of pair orientation. We show that the estimate of the growth rate is potentially degenerate with wide-angle effects, meaning that extremely accurate measurements of the growth rate on large scales will need to take such effects into account. We use measurements of the zeroth and second-order moments of the correlation function from SDSS DR7 data and the Large Suite of Dark Matter Simulations (LasDamas), and perform a likelihood analysis to constrain the parameters of the models. Using information on the clustering up to rmax = 120 h−1 Mpc, and after marginalizing over the bias, we find, for UDM models, a speed of sound c∞ ≤ 6.1e-4, and, for the nDGP model, a cross-over scale rc ≥ 340 Mpc, at 95 per cent confidence level.