Cosmological Imprint of an Energy Component with General Equation of State

We examine the possibility that a significant component of the energy density of the universe has an equation-of-state different from that of matter, radiation or cosmological constant ($\Lambda$). An example is a cosmic scalar field evolving in a potential, but our treatment is more general. Including this component alters cosmic evolution in a way that fits current observations well. Unlike $\Lambda$, it evolves dynamically and develops fluctuations, leaving a distinctive imprint on the microwave background anisotropy and mass power spectrum.Comment: revised version, with added references, to appear in Phys. Rev. Lett. (4 pages Latex, 2 postscript figures

Stretching the Inflaton Potential with Kinetic Energy

Inflation near a maximum of the potential is studied when non-local derivative operators are included in the inflaton Lagrangian. Such terms can impose additional sources of friction on the field. For an arbitrary spacetime geometry, these effects can be quantified in terms of a local field theory with a potential whose curvature around the turning point is strongly suppressed. This implies that a prolonged phase of slow-roll inflation can be achieved with potentials that are otherwise too steep to drive quasi-exponential expansion. We illustrate this mechanism within the context of p-adic string theory.Comment: 4 page

Interacting dark energy, holographic principle and coincidence problem

The interacting and holographic dark energy models involve two important quantities. One is the characteristic size of the holographic bound and the other is the coupling term of the interaction between dark energy and dark matter. Rather than fixing either of them, we present a detailed study of theoretical relationships among these quantities and cosmological parameters as well as observational constraints in a very general formalism. In particular, we argue that the ratio of dark matter to dark energy density depends on the choice of these two quantities, thus providing a mechanism to change the evolution history of the ratio from that in standard cosmology such that the coincidence problem may be solved. We investigate this problem in detail and construct explicit models to demonstrate that it may be alleviated provided that the interacting term and the characteristic size of holographic bound are appropriately specified. Furthermore, these models are well fitted with the current observation at least in the low red-shift region.Comment: 20 pages, 3 figure

Angular Inflation from Supergravity

We study supergravity inflationary models where inflation is produced along the angular direction. For this we express the scalar component of a chiral superfield in terms of the radial and the angular components. We then express the supergravity potential in a form particularly simple for calculations involving polynomial expressions for the superpotential and Kahler potential. We show for a simple Polonyi model the angular direction may give rise to a stage of inflation when the radial field is fixed to its minimum. We obtain analytical expressions for all the relevant inflationary quantities and discuss the possibility of supersymmetry breaking in the radial direction while inflating by the angular component.Comment: 7 pages, one figure. Final version. Title changed, two figures droppe

Measuring deviations from a cosmological constant: a field-space parameterization

Most parameterizations of the dark energy equation of state do not reflect realistic underlying physical models. Here, we develop a relatively simple description of dark energy based on the dynamics of a scalar field which is exact in the limit that the equation of state approaches a cosmological constant, assuming some degree of smoothness of the potential. By introducing just two parameters defined in the configuration space of the field we are able to reproduce a wide class of quintessence models. We examine the observational constraints on these models as compared to linear evolution models, and show how priors in the field space translate into priors on observational parameters.Comment: 5 pages, 6 figures. Final versio

Precision Cosmology? Not Just Yet

The recent announcement from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite experiment combined with other recent advances in observational cosmology verifies key components of the standard cosmological model. However, we argue that there remain some significant open issues regarding the basic history and composition of the Universe and uncertainties in some of the most important parameters.Comment: 2 pages, 2 figures. Online journal version http://www.sciencemag.org/cgi/content/full/299/5612/153

A phason disordered two dimensional quantum antiferromagnet

We examine a novel type of disorder in quantum antiferromagnets. Our model consists of localized spins with antiferromagnetic exchanges on a bipartite quasiperiodic structure, which is geometrically disordered in such a way that no frustration is introduced. In the limit of zero disorder, the structure is the perfect Penrose rhombus tiling. This tiling is progressively disordered by augmenting the number of random "phason flips" or local tile-reshuffling operations. The ground state remains N\'eel ordered, and we have studied its properties as a function of increasing disorder using linear spin wave theory and quantum Monte Carlo. We find that the ground state energy decreases, indicating enhanced quantum fluctuations with increasing disorder. The magnon spectrum is progressively smoothed, and the effective spin wave velocity of low energy magnons increases with disorder. For large disorder, the ground state energy as well as the average staggered magnetization tend towards limiting values characteristic of this type of randomized tilings.Comment: 5 pages, 7 figure

Where in the String Landscape is Quintessence

We argue that quintessence may reside in certain corners of the string landscape. It arises as a linear combination of internal space components of higher rank forms, which are axion-like at low energies, and may mix with 4-forms after compactification of the Chern-Simons terms to 4D due to internal space fluxes. The mixing induces an effective mass term, with an action which {\it preserves} the axion shift symmetry, breaking it spontaneously after the background selection. With several axions, several 4-forms, and a low string scale, as in one of the setups already invoked for dynamically explaining a tiny residual vacuum energy in string theory, the 4D mass matrix generated by random fluxes may have ultralight eigenmodes over the landscape, which are quintessence. We illustrate how this works in simplest cases, and outline how to get the lightest mass to be comparable to the Hubble scale now, $H_0 \sim 10^{-33} {\rm eV}$. The shift symmetry protects the smallest mass from perturbative corrections in field theory. Further, if the ultralight eigenmode does not couple directly to any sector strongly coupled at a high scale, the non-perturbative field theory corrections to its potential will also be suppressed. Finally, if the compactification length is larger than the string length by more than an order of magnitude, the gravitational corrections may remain small too, even when the field value approaches $M_{Pl}$.Comment: 8 pages RevTeX; added references, matches published versio