* Astronomy

Title: Essential Building Blocks of Dark Energy
Authors: Jerome Gleyzes, David Langlois, Federico Piazza, Filippo Vernizzi

We propose a minimal description of single field dark energy/modified gravity within the effective field theory formalism for cosmological perturbations, which encompasses most existing models. We start from a generic Lagrangian given as an arbitrary function of the lapse and of the extrinsic and intrinsic curvature tensors of the time hypersurfaces in unitary gauge, i.e. choosing as time slicing the uniform scalar field hypersurfaces. Focusing on linear perturbations, we identify seven Lagrangian operators that lead to equations of motion containing at most two (space or time) derivatives, the time-dependent coefficients of three of these operators being determined only by the background evolution. We then establish a dictionary that translates any existing or future model whose Lagrangian can be written in the above form into our parametrised framework. As an illustration, we show that Horndeski's-or generalized Galileon-theories can be described, up to linear order, by only six of the seven operators mentioned above. Finally, in order to make the link with observations, we provide the entire set of linear perturbation equations in Newtonian gauge, the effective Newton constant in the quasi-static approximation and the ratio of the two gravitational potentials, in terms of the time-dependent coefficients of our Lagrangian.

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Title: Dark energy from entanglement entropy
Authors: Salvatore Capozziello, Orlando Luongo

We show that quantum decoherence, in the context of observational cosmology, can be connected to the cosmic dark energy. The decoherence signature could be characterized by the existence of quantum entanglement between cosmological eras. As a consequence, the Von Neumann entropy related to the entanglement process, can be compared to the thermodynamical entropy in a homogeneous and isotropic universe. The corresponding cosmological models are compatible with the current observational bounds being able to reproduce viable equations of state without introducing {\it a priori} any cosmological constant. In doing so, we investigate two cases, corresponding to two suitable cosmic volumes, V\propto a^3 and V\propto H^{-3}, and find two models which fairly well approximate the current cosmic speed up. The existence of dark energy can be therefore reinterpreted as a quantum signature of entanglement, showing that the cosmological constant represents a limiting case of a more complicated model derived from the quantum decoherence.

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Title: How early is early dark energy?
Authors: Valeria Pettorino, Luca Amendola, Christof Wetterich

We investigate constraints on early dark energy (EDE) from the Cosmic Microwave Background (CMB) anisotropy, taking into account data from WMAP9 combined with latest small scale measurements from the South Pole Telescope (SPT). For a constant EDE fraction we propose a new parametrisation with one less parameter but still enough to provide similar results to the ones previously studied in literature. The main emphasis of our analysis, however, compares a new set of different EDE parametrisations that reveal how CMB constraints depend on the redshift epoch at which Dark Energy was non negligible. We find that bounds on EDE get substantially weaker if dark energy starts to be non-negligible later, with early dark energy fraction Omega_e free to go up to about 5% at 2 sigma if the onset of EDE happens at z < 100. Tight bounds around 1-2% are obtained whenever dark energy is present at last scattering, even if its effects switch off afterwards. We show that the CMB mainly constrains the presence of Dark Energy at the time of its emission, while EDE-modifications of the subsequent growth of structure are less important.

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Dark energy hints hidden in cosmic voids

Dark energy is thought to make up about 70 per cent of the universe, and several ideas are competing to dark energy density can change with time.
One way to test these ideas is to measure the expansion rate over time, which should leave a mark on cosmic voids. Quantum fluctuations in the early universe led to variations in the density of primordial matter. Galaxies and clusters formed in denser regions while voids emerged between them, creating the large-scale structure of the universe seen in galaxy surveys like the Sloan Digital Sky Survey (SDSS). Voids are typically up to 150 million light years across.
The expansion of the universe has stretched the nearly spherical voids into egg-shaped bubbles.
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