A Way to Dynamically Overcome the Cosmological Constant Problem

The Cosmological Constant problem can be solved once we require that the full standard Einstein Hilbert lagrangian, gravity plus matter, is multiplied by a total derivative. We analyze such a picture writing the total derivative as the covariant gradient of a new vector field (b_mu). The dynamics of this b_mu field can play a key role in the explanation of the present cosmological acceleration of the Universe.Comment: 5 page

Electroweak Sudakov form factors and nonfactorizable soft QED effects at NLC energies

We study the leading log infrared behavior of electroweak corrections at TeV scale energies, that will be reached by next generation of linear colliders (NLC). We show that, contrary to what happens at typical LEP energies, it is not anymore possible to disentangle ``pure electroweak'' from ``photonic'' corrections. This means that soft QED effects do not factorize and therefore cannot be treated in the usual ``naive'' way they were accounted for in the LEP-era. The nonfactorizable effects come up first at the two loop LL level, that we calculate explicitly for a fermion source that is neutral under the SU(2)$\otimes$U(1) gauge group (explicitly, a Z' decay into two fermions). The basic formalism we set up can be used to calculate LL effects at any order of perturbation theory. The results of this paper might be important for future calculations of electroweak corrections at NLC energies.Comment: LaTeX, 8 pages, 3 figure

Weak Massive Gravity

We find a new class of theories of massive gravity with five propagating degrees of freedom where only rotations are preserved. Our results are based on a non-perturbative and background-independent Hamiltonian analysis. In these theories the weak field approximation is well behaved and the static gravitational potential is typically screened \`a la Yukawa at large distances, while at short distances no vDVZ discontinuity is found and there is no need to rely on nonlinear effects to pass the solar system tests. The effective field theory analysis shows that the ultraviolet cutoff is (m M_PL)^1/2 ~ 1/\mu m, the highest possible. Thus, these theories can be studied in weak-field regime at all the phenomenologically interesting scales, and are candidates for a calculable large-distance modified gravity.Comment: 5 page

Thermodynamics of perfect fluids from scalar field theory

The low-energy dynamics of relativistic continuous media is given by a shift-symmetric effective theory of four scalar fields. These scalars describe the embedding in spacetime of the medium and play the role of St\"uckelberg fields for spontaneously broken spatial and time translations. Perfect fluids are selected imposing a stronger symmetry group or reducing the field content to a single scalar. We explore the relation between the field theory description of perfect fluids to thermodynamics. By drawing the correspondence between the allowed operators at leading order in derivatives and the thermodynamic variables, we find that a complete thermodynamic picture requires the four Stuckelberg fields. We show that thermodynamic stability plus the null-energy condition imply dynamical stability. We also argue that a consistent thermodynamic interpretation is not possible if any of the shift symmetries is explicitly broken.Comment: 25 pages, 1 figure. Few typos corrected. Accepted for publication in PR

On the 6th Mode in Massive Gravity

Generic massive gravity models in the unitary gauge correspond to a self-gravitating medium with six degrees of freedom. It is widely believed that massive gravity models with six degrees of freedom have an unavoidable ghost-like instability; however, the corresponding medium has stable phonon-like excitations. The apparent contradiction is solved by the presence of a non-vanishing background pressure and energy density of the medium that opens up a stability window. The result is confirmed by looking at linear stability on an expanding Universe, recovering the flat space stability conditions in the small wavelength limit. Moreover, one can show that under rather mild conditions, no ghost-like instability is present for any wavelength. As a result, exploiting the medium interpretation, a generic massive gravity model with six degrees of freedom is perfectly viable.Comment: Latex 17 page