The mysterious cosmic presence called dark energy, which is accelerating the expansion of the universe, might be lurking in hidden dimensions of space. The idea would explain how these dimensions remain stable a big problem for the unified scheme of physics called string theory. Ever since astronomers discovered in the mid-1990s that other galaxies are accelerating away from us, physicists have struggled to explain why. Their favourite suggestion is quantum vibrations in the vacuum of space (called vacuum energy or the cosmological constant) that could produce repulsive gravity. According to the calculations, however, these vibrations should either possess a ridiculously high energy density 122 orders of magnitude larger than are observed or cancel out to exactly zero. To make them almost-but-not-quite cancel, in agreement with astronomical observations, means fudging the quantum field equations. Unless, that is, the quantum vibrations are stuck in a small space

Title: Self-similar cosmological solutions with dark energy II: black holes, naked singularities and wormholes Authors: Hideki Maeda, Tomohiro Harada, B.J. Carr

We use a combination of numerical and analytical methods, exploiting the equations derived in an accompanying paper, to classify all spherically symmetric self-similar solutions which are asymptotically Friedmann at large distances and contain a perfect fluid with equation of state p=(\gamma -1)\mu with 0<\gamma<2/3. The expansion of the Friedmann universe is accelerated in this case. We find a one-parameter family of self-similar solutions representing a black hole embedded in a Friedmann background. This suggests that, in contrast to the positive pressure case, black holes in a universe with dark energy can grow as fast as the Hubble horizon if they are not too large. There are also self-similar solutions which contain a central naked singularity with negative mass. We also find various kinds of self-similar wormhole solutions; these represent a Friedmann universe connected to either another Friedmann universe or some other cosmological model. These wormholes are generally traversable, where we define a wormhole throat as a two-dimensional sphere with minimal area on a spacelike hypersurface.

Title: Constraints on oscillating dark energy models Authors: Aleksandra Kurek, Orest Hrycyna, Marek Szydlowski

The oscillating scenario of route to Lambda was recently proposed by us (PDF) as an alternative to a cosmological constant in a explanation of the current accelerating universe. In this scenario phantom scalar field conformally coupled to gravity drives the accelerating phase of the universe. In our model \LambdaCDM appears as a global attractor in the phase space. In this paper we investigate observational constraints on this scenario from recent measurements of distant supernovae type Ia, CMB R shift, BAO and H(z) observational data. The Bayesian methods of model selection are used in comparison the model with concordance \LambdaCDM one as well as with model with dynamical dark energy parameterised by linear form. We conclude that \LambdaCDM is favoured over FRW model with dynamical oscillating dark energy. Our analysis also demonstrate that FRW model with oscillating dark energy is favoured over FRW model with decaying dark energy parameterised in linear way.

Title: Dark Matter and Dark Energy Authors: Marc Kamionkowski

This is a short review, aimed at a general audience, of several current subjects of research in cosmology. The topics discussed include the cosmic microwave background (CMB), with particular emphasis on its relevance for testing inflation; dark matter, with a brief review of astrophysical evidence and more emphasis on particle candidates; and cosmic acceleration and some of the ideas that have been put forward to explain it. A glossary of technical terms and acronyms is provided.

Title: Holographic tachyon model Authors: Jingfei Zhang, Xin Zhang, Hongya Liu

We propose in this paper a holographic model of tachyon dark energy. A connection between the tachyon scalar-field and the holographic dark energy is established, and accordingly, the potential of the holographic tachyon field is constructed. We show that the holographic evolution of the universe with c >= 1 can be described completely by the resulting tachyon model in a certain way.

Title:The Accelerated Expansion of the Universe Challenged by an Effect of the Inhomogeneities. A Review Authors: Marie-Noëlle Célérier (Observatoire de Paris-Meudon) (Version v2)

Since its discovery during the late 90's, the dimming of distant SN Ia apparent luminosity has been mostly ascribed to the influence of a mysterious dark energy component. Formulated in a Friedmannian cosmological modelling framework based upon the cosmological ''principle'' hypothesis, this interpretation has given rise to the ''concordance'' model. However, a caveat of such a reasoning is that the cosmological "principle'' derives from a philosophical Copernican assumption and has never been tested. Furthermore, a weakness of its conclusion, i. e., the existence of a negative-pressure fluid or a cosmological constant, is that it would have profound implications for the current theories of physics. This is why we have proposed a more conservative explanation, ascribing the departure of the observed universe from an Einstein-de Sitter model to the influence of inhomogeneities. This idea has been independently developed by other authors and further enlarged to the reproduction of different cosmological data. We review here the main proposals which have been put forward to deal with this purpose and present some prospects for future developments.

Do magnetic fields drive dark energy We aren't even sure such things exist. But that hasn't stopped two cosmologists from proposing that huge magnetic fields spanning the great voids of the universe could explain where dark energy comes from. Dark energy is thought to be causing the expansion of the universe to accelerate, but its origin is a mystery. Now Ioannis Contopoulos and Spyros Basilakos at the Academy of Athens in Greece are proposing an unorthodox explanation. They believe that the energy stored in a patchwork of magnetic fields stretched across the universe could be driving the acceleration.

Title: Dark Energy and Gravity Authors: T. Padmanabhan

I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremising this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections.

Title: Constraining dark energy via baryon acoustic oscillations in the (an)isotropic light-cone power spectrum Authors: Christian Wagner, Volker Müller, Matthias Steinmetz

The measurement of the scale of the baryon acoustic oscillations (BAO) in the galaxy power spectrum as a function of redshift is a promising method to constrain the equation-of-state parameter of the dark energy w. In order to measure precisely the scale of the BAO a huge volume has to be surveyed. We test whether light-cone effects become important and whether the scaling relations used to compensate for a wrong reference cosmology are accurate enough in this case. We compare two different fitting methods to extract the scale of the BAO. Further, we analyse the advantage of using the two-dimensional anisotropic power spectrum. Finally, we estimate the uncertainty with which an effectively constant w can be measured with proposed surveys around redshifts of z=3 and z=1, respectively. We find that light-cone effects for the simulated survey are negligible and that the simple scaling relations used to correct for the cosmological distortions work well even for such large survey volumes. The two different fitting approaches deliver consistent results and both should be considered further. The analysis of the two-dimensional anisotropic power spectra allows independent determination of the apparent scale of BAO perpendicular and parallel to the line of sight but it does not significantly lower the uncertainty of an effectively constant w. Nevertheless, for less constrained models of w independent measurements of the apparent scale of BAO perpendicular and parallel to the line of sight are essential. We estimate that with planned surveys around z=3 and z=1 one will be able to measure an effectively constant w with sigma_w ~ 4% in both cases. Read more (113kb, PDF)

Title: Solar System Constraints on Gauss-Bonnet Mediated Dark Energy Authors: Luca Amendola, Christos Charmousis, Stephen C. Davis

Although the Gauss-Bonnet term is a topological invariant for general relativity, it couples naturally to a quintessence scalar field, modifying gravity at solar system scales. We determine the solar system constraints due to this term by evaluating the post-Newtonian metric for a distributional source. We find a mass dependent, 1/r^7 correction to the Newtonian potential, and also deviations from the Einstein gravity prediction for light-bending. We constrain the parameters of the theory using planetary orbits, the Cassini spacecraft data, and a laboratory test of Newton's law, always finding extremely tight bounds on the energy associated to the Gauss-Bonnet term. We discuss the relevance of these constraints to late-time cosmological acceleration.