* Astronomy

Synthetic universes: How simulations will help search for dark energy

The Dark Energy Survey is one of the most ambitious astrophysics experiments ever launched. For five years, a custom-designed camera mounted on a telescope in Chile will collect images of distant galaxies in the southern sky over an area of 5,000 square degrees, corresponding to roughly one-eighth of the visible universe.
That project will generate petabytes (thousands of terabytes) of data that must be painstakingly analysed by the collaboration of scientists from 27 institutions to find answers about the nature of dark energy, dark matter and the forces that shape the evolution of the universe.
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Title: Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle
Authors: Sean February, Chris Clarkson, Roy Maartens (Cape Town and Western Cape)

A suggested solution to the dark energy problem is the void model, where accelerated expansion is replaced by Hubble-scale inhomogeneity. In these models, density perturbations grow on a radially inhomogeneous background. This large scale inhomogeneity distorts the spherical Baryon Acoustic Oscillation feature into an ellipsoid which implies that the bump in the galaxy correlation function occurs at different scales in the radial and transverse correlation functions. We compute these for the first time, under the approximation that curvature gradients do not couple the scalar modes to vectors and tensors. The radial and transverse correlation functions are very different from those of the concordance model, even when the models have the same average BAO scale. This implies that if the models are fine-tuned to satisfy average BAO data, there is enough extra information in the correlation functions to distinguish a void model from LCDM. We expect these new features to remain when the full perturbation equations are solved, which means that the radial and transverse galaxy correlation functions can be used as a powerful test of the Copernican principle.

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Title: Birth of Dark Energy by Quantum Metric Fluctuations in Imaginary Time
Authors: Leonid Marochnik

We show that in imaginary time quantum metric fluctuations of empty space form a self-consistent De Sitter gravitational instanton that can be thought of as describing the tunnelling from "nothing" into De Sitter space of real time (no cosmological constant or scalar fields are needed). The first time, this mechanism is activated to give birth to a flat inflationary Universe. The second time, it is turned on to complete cosmological evolution of the Universe after energy density of matter drops below the threshold (energy density of instantons). An accelerated expansion takes over after the scale factor exceeds this threshold, which marks the birth of dark energy at the redshift 1+z=1.44 and provides a possible solution to the "coincidence problem". The number of gravitons which tunnelled into the Universe must be of the order of 10^122 to create the observational value of the Hubble constant. This number has nothing to do with vacuum energy, which is a possible solution to the "old cosmological constant problem". The emptying Universe should possibly complete its evolution by tunnelling back to "nothing". After that, the entire scenario is repeated, and it can happen endlessly.

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Title: Natural Quintessence in String Theory
Authors: Michele Cicoli, Francisco G. Pedro, Gianmassimo Tasinato

We introduce a natural model of quintessence in string theory where the light rolling scalar is radiatively stable and couples to Standard Model matter with weaker-than- Planckian strength. The model is embedded in an anisotropic type IIB compactification with two exponentially large extra dimensions and TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the scale of the gravitino mass is of the order of the observed value of the cosmological constant. The quintessence field is a modulus parameterising the size of an internal four-cycle which naturally develops a potential of the order (gravitino mass)^4, leading to a small dark energy scale without tunings. The mass of the quintessence field is also radiatively stable since it is protected by supersymmetry in the bulk. Moreover, this light scalar couples to ordinary matter via its mixing with the volume mode. Due to the fact that the quintessence field is a flat direction at leading order, this mixing is very small, resulting in a suppressed coupling to Standard Model particles which avoids stringent fifth-force constraints. On the other hand, if dark matter is realised in terms of Kaluza-Klein states, unsuppressed couplings between dark energy and dark matter can emerge, leading to a scenario of coupled quintessence within string theory. We study the dynamics of quintessence in our set-up, showing that its main features make it compatible with observations.

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