Cosmological Constant Behavior in DBI Theory

Cosmological constant behavior can be realized as solutions of the Dirac-Born-Infeld (DBI) action within Type IIB string theory and the AdS/CFT correspondence. We derive a family of attractor solutions to the cosmological constant that arise purely from the "relativistic" nature of the DBI action without an explicit false vacuum energy. We also find attractor solutions with values of the equation of state near but with $w\ne-1$; the forms for the potential arising from flux interactions are renormalizable and natural, and the D3-brane tension can be given by the standard throat form. We discuss present and future observational constraints on the theory.Comment: 4 pages, 2 figure

Statistical inference of the mechanisms driving collective cell movement

Numerous biological processes, many impacting on human health, rely on collective cell movement. We develop nine candidate models, based on advection-diffusion partial differential equations, to describe various alternative mechanisms that may drive cell movement. The parameters of these models were inferred from one-dimensional projections of laboratory observations of Dictyostelium discoideum cells by sampling from the posterior distribution using the delayed rejection adaptive Metropolis algorithm (DRAM). The best model was selected using the Widely Applicable Information Criterion (WAIC). We conclude that cell movement in our study system was driven both by a self-generated gradient in an attractant that the cells could deplete locally, and by chemical interactions between the cells

Fitting inverse power-law quintessence models using the SNAP satellite

We investigate the possibility of using the proposed SNAP satellite in combination with low-z supernova searches to distinguish between different inverse power-law quintessence models. If the true model is that of a cosmological constant, we determine the prospects of ruling out the inverse power-law potential. We show that SNAP combined with e.g. the SNfactory and an independent measurement of the mass energy density to 17% accuracy can distinguish between an inverse power-law potential and a cosmological constant and put severe constraints on the power-law exponent.Comment: 5 pages, 6 figure

The Robustness of Quintessence

Recent observations seem to suggest that our Universe is accelerating implying that it is dominated by a fluid whose equation of state is negative. Quintessence is a possible explanation. In particular, the concept of tracking solutions permits to adress the fine-tuning and coincidence problems. We study this proposal in the simplest case of an inverse power potential and investigate its robustness to corrections. We show that quintessence is not affected by the one-loop quantum corrections. In the supersymmetric case where the quintessential potential is motivated by non-perturbative effects in gauge theories, we consider the curvature effects and the K\"ahler corrections. We find that the curvature effects are negligible while the K\"ahler corrections modify the early evolution of the quintessence field. Finally we study the supergravity corrections and show that they must be taken into account as $Q\approx m_{\rm Pl}$ at small red-shifts. We discuss simple supergravity models exhibiting the quintessential behaviour. In particular, we propose a model where the scalar potential is given by $V(Q)=\frac{\Lambda^{4+\alpha }}{Q^{\alpha}}e^{\frac{\kappa}{2}Q^2}$. We argue that the fine-tuning problem can be overcome if $\alpha \ge 11$. This model leads to $\omega_Q\approx -0.82$ for $\Omega_{\rm m}\approx 0.3$ which is in good agreement with the presently available data.Comment: 16 pages, 7 figure

Quintessence with two energy scales

We study quintessence models using low energy supergravity inspired from string theory. We consider effective supergravity with two scales m_S, the string scale, and m_PL, the Planck scale and show that quintessence naturally arises from a supersymmetry breaking hidden sector. As long as supersymmetry is broken by the $F$-term of a Polonyi-like field coupled to the quintessence field in the K\"ahler potential we find that the Ratra-Peebles potential and its supergravity version are generic predictions. This requires that the string scale decouples from the Planck scale, m_S << m_PL. In the context of supergravity, the potential possesses a minimum induced by the supergravity corrections to the Ratra-Peebles potential at low redshifts. We study the physical consequences of the presence of this minimum.Comment: 16 pages, 9 figures, minor changes matching published version. Accepted for publication in PR

Outline of a sensory-motor perspective on intrinsically moral agents

This is the accepted version of the following article: Christian Balkenius, Lola Cañamero, Philip Pärnamets, Birger Johansson, Martin V Butz, and Andreas Olson, ‘Outline of a sensory-motor perspective on intrinsically moral agents’, Adaptive Behaviour, Vol 24(5): 306-319, October 2016, which has been published in final form at DOI: https://doi.org/10.1177/1059712316667203 Published by SAGE ©The Author(s) 2016We propose that moral behaviour of artificial agents could (and should) be intrinsically grounded in their own sensory-motor experiences. Such an ability depends critically on seven types of competencies. First, intrinsic morality should be grounded in the internal values of the robot arising from its physiology and embodiment. Second, the moral principles of robots should develop through their interactions with the environment and with other agents. Third, we claim that the dynamics of moral (or social) emotions closely follows that of other non-social emotions used in valuation and decision making. Fourth, we explain how moral emotions can be learned from the observation of others. Fifth, we argue that to assess social interaction, a robot should be able to learn about and understand responsibility and causation. Sixth, we explain how mechanisms that can learn the consequences of actions are necessary for a robot to make moral decisions. Seventh, we describe how the moral evaluation mechanisms outlined can be extended to situations where a robot should understand the goals of others. Finally, we argue that these competencies lay the foundation for robots that can feel guilt, shame and pride, that have compassion and that know how to assign responsibility and blame.Peer reviewedFinal Accepted Versio

The Cosmological Constant

This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.Comment: 50 pages. Submitted to Living Reviews in Relativity (http://www.livingreviews.org/), December 199

Model-independent dark energy test with sigma_8 using results from the Wilkinson Microwave Anisotropy Probe

By combining the recent WMAP measurements of the cosmic microwave background anisotropies and the results of the recent luminosity distance measurements to type-Ia supernovae, we find that the normalization of the matter power spectrum on cluster scales, sigma_8, can be used to discriminate between dynamical models of dark energy (quintessence models) and a conventional cosmological constant model (LCDM).Comment: 5 pages, 6 figures. Additional discussion and reference, matches PRD accepted versio

f(R) theories

Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in Relativity, Published version, Comments are welcom

Exhaustive Study of Cosmic Microwave Background Anisotropies in Quintessential Scenarios

Recent high precision measurements of the CMB anisotropies performed by the BOOMERanG and MAXIMA-1 experiments provide an unmatched set of data allowing to probe different cosmological models. Among these scenarios, motivated by the recent measurements of the luminosity distance versus redshift relation for type Ia supernovae, is the quintessence hypothesis. It consists in assuming that the acceleration of the Universe is due to a scalar field whose final evolution is insensitive to the initial conditions. Within this framework we investigate the cosmological perturbations for two well-motivated potentials: the Ratra-Peebles and the SUGRA tracking potentials. We show that the solutions of the perturbed equations possess an attractor and that, as a consequence, the insensitivity to the initial conditions is preserved at the perturbed level. Then, we study the predictions of these two models for structure formation and CMB anisotropies and investigate the general features of the multipole moments in the presence of quintessence. We also compare the CMB multipoles calculated with the help of a full Boltzmann code with the BOOMERanG and MAXIMA-1 data. We pay special attention to the location of the second peak and demonstrate that it significantly differs from the location obtained in the cosmological constant case. Finally, we argue that the SUGRA potential is compatible with all the recent data with a standard values of the cosmological parameters. In particular, it fits the MAXIMA-1 data better than a cosmological constant or the Ratra-Peebles potential.Comment: 18 pages, 20 figures, submitted to PR