Two Component Charged Condensate in White Dwarfs

The possibility of the formation of a condensate of charged spin-0 nuclei inside white dwarf cores, studied in arXiv:0806.3692 and arXiv:0904.4267, is pursued further. It has been shown, for cores composed mainly of one element (Helium or Carbon), that after condensation phonons become massive and the specific heat drops by about two orders of magnitude. In this note we extend that analysis by considering the coexistence of the nuclei of both types (Helium and Carbon), whose condensation points are generically different. An effective field theory is developed to describe the system when both elements are condensed. The spectrum of fluctuations of this two component charged condensate possesses a collective massless mode with $\omega \propto {\bf k}^2$. Assuming that the fraction of the less abundant element is greater than about 1/100, the thermal history changes as follows: There is a softer discontinuity in the average specific heat after the condensation of first sector, resulting in slower cooling and a milder drop in luminosity function. The specific heat remains almost constant until the condensation of the second sector, then starts to declines as $T^{3/2}$.Comment: 8 pages, 1 figure; corrected typo

Lorentz-violating massive gauge and gravitational fields

We study nonlinear dynamics in models of Lorentz-violating massive gravity. The Boulware-Deser instability restricts severely the class of acceptable theories. We identify a model that is stable. It exhibits the following bizarre but interesting property: there are only two massive propagating degrees of freedom in the spectrum, and yet long-range instantaneous interactions are present in the theory. We discuss this property on a simpler example of a photon with a Lorentz-violating mass term where the issues of (a)causality are easier to understand. Depending on the values of the mass parameter these models can either be excluded, or become phenomenologically interesting. We discuss a similar example with more degrees of freedom, as well as a model without the long-range instantaneous interactions.Comment: 14 LaTex pages, 3 refs with comments added; PLB versio

ICTP Lectures on Large Extra Dimensions

I give a brief and elementary introduction to braneworld models with large extra dimensions. Three conceptually distinct scenarios are outlined: (i) Large compact extra dimensions; (ii) Warped extra dimensions; (iii) Infinite-volume extra dimensions. As an example I discuss in detail an application of (iii) to late-time cosmology and the acceleration problem of the Universe.Comment: 39 LaTex pgs; 6 ps figures; Based on lectures given at Summer School on Astroparticle Physics and Cosmology Triese, Italy, June 17 -- July 5, 200

Looking At The Cosmological Constant From Infinite--Volume Bulk

I briefly review the arguments why the braneworld models with infinite-volume extra dimensions could solve the cosmological constant problem, evading Weinberg's no-go theorem. Then I discuss in detail the established properties of these models, as well as the features which should be studied further in order to conclude whether these models can truly solve the problem. This article is dedicated to the memory of Ian Kogan.Comment: 64 pages, 4 figures; To appear in Ian Kogan Memorial Volume, ``From Fields to Strings: Circumnavigating Theoretical Physics'', M. Shifman, A. Vainshtein, and J. Wheater, eds. (World Scientific, 2004

The Big Constant Out, The Small Constant In

Some time ago, Tseytlin has made an original and unusual proposal for an action that eliminates an arbitrary cosmological constant. The form of the proposed action, however, is strongly modified by gravity loop effects, ruining its benefit. Here I discuss an embedding of Tseytlin's action into a broader context, that enables to control the loop effects. The broader context is another universe, with its own metric and dynamics, but only globally connected to ours. One possible Lagrangian for the other universe is that of unbroken AdS supergravity. A vacuum energy in our universe does not produce any curvature for us, but instead increases or decreases the AdS curvature in the other universe. I comment on how to introduce the accelerated expansion in this framework in a technically natural way, and consider the case where this is done by the self-accelerated solutions of massive gravity and its extensions.Comment: 14 pages; a brief paragraph unfolded; 3 refs added; minor improvement

Phase Transitions of Charged Scalars at Finite Temperature and Chemical Potential

We calculate the grand canonical partition function at the one-loop level for scalar quantum electrodynamics at finite temperature and chemical potential. A classical background charge density with a charge opposite that of the scalars ensures the neutrality of the system. For low density systems we find evidence of a first order phase transition. We find upper and lower bounds on the transition temperature below which the charged scalars form a condensate. A first order phase transition may have consequences for helium-core white dwarf stars in which it has been argued that such a condensate of charged helium-4 nuclei could exist.Comment: 20 pages, 3 figures. Version accepted for publication in JHE

From screening to confinement in a Higgs-like model

We investigate a recently proposed Higgs-like model (arXiv:0811.4423 [hep-th]), in the framework of a gauge-invariant but path-dependent variables formalism. We compute the static potential between test charges in a condensate of scalars and fermions. In the case of charged massive scalar we recover the screening potential. On the other hand, in the Higgs case, with a "tachyonic" mass term and a quartic potential in the Lagrangian, unexpected features are found. It is observed that the interaction energy is the sum of an effective-Yukawa and a linear potential, leading to the confinement of static charges.Comment: 14 pages, no figures. Inserted 2 new references and comments about the short-distance physical cut-off. Final version accepted for publication in PL