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Ph. Brax

Publications and source records attributed to Ph. Brax.

At least 37 records · Page 2Linked to original sources
arXiv2003

Gauge Coupling Variation in Brane Models

We consider the space-time variation of gauge couplings in brane-world models induced by the coupling to a bulk scalar field. A variation is generated by the running of the gauge couplings with energy and a conformal anomaly while going from the Jordan to the Einstein frame. We indicate that the one-loop corrections cancel implying that one obtains a variation of the fine structure constant by either directly coupling the gauge fields to the bulk scalar field or having bulk scalar field dependent Yukawa couplings. Taking into account the cosmological dynamics of the bulk scalar field, we constrain the strength of the gauge coupling dependence on the bulk scalar field and relate it to modifications of gravity at low energy.

astro-ph↗
arXiv2003

On Tachyon kinks from the DBI action

We consider solitonic solutions of the DBI tachyon effective action for a non-BPS brane in the presence of an electric field. We find that for a constant electric field $\tilde E\le 1$, regular solitons compactified on a circle admit a singular and decompactified limit corresponding to Sen's proposal provided the tachyon potential satisfies some restrictions. On the other hand for the critical electric field $\tilde E=1$, regular and finite energy solitons are constructed without any restriction on the potential.

hep-th↗
arXiv2003

Branes at low energy

Brane models at low energy are described by an effective action involving gravity and two moduli fields associated with the brane positions. The scalar-tensor nature of the effective theory and the coupling to matter are explicitly spelt out. We discuss the post-Newtonian gravitational constraints on the models, some applications to quintessence and the variations of constants.

hep-ph↗
arXiv2003

Brane World Moduli and the CMB

The evolution of moduli fields, which naturally appear in higher dimensional models such as brane worlds, and their effects on the anisotropies of the cosmological microwave background radiation is discussed.

hep-ph↗
arXiv2003

CMB Anisotropies in the Presence of Extra Dimensions

We discuss the effect of the time evolution of extra dimensions on CMB anisotropies and large-scale structure formation. We study the impact of scalar fields in a low-energy effective description of a general class of brane world models on the temperature anisotropy power spectrum. We show that when the coupling between these scalar fields and matter evolves over cosmological timescales, current observations of the CMB anisotropies can constrain primordial values of the fields in a manner complementary to local, late-time tests of gravity. We also present the effect of these fields on the polarization anisotropy spectra and the growth of large-scale structure, showing that future CMB observations will constrain theories of the Universe involving extra dimensions even further.

astro-ph↗
arXiv2003

Tachyon kinks on non BPS D-branes

We consider solitonic solutions of the DBI tachyon effective action for a non-BPS brane. When wrapped on a circle, these solutions are regular and have a finite energy. We show that in the decompactified limit, these solitons give Sen's infinitely thin finite energy kink -- interpreted as a BPS brane -- provided that some conditions on the potential hold. In particular, if for large $T$ the potential is exponential, $V = e^{-T^a}$, then Sen's solution is only found for $a<1$. For power-law potentials $V = 1/T^b$, one must have $b>1$. If these conditions are not satisfied, we show that the lowest energy configuration is the unstable tachyon vacuum with no kinks. We examine the stability of the solitons and the spectrum of small perturbations.

hep-th↗
arXiv2003

On the AdS/CFT Dual of Deconstruction

We consider a class of non-supersymmetric gauge theories obtained by orbifolding the N=4 super-Yang-Mills theories. We focus on the resulting quiver theories in their deconstructed phase, both at small and large coupling, where a fifth dimension opens up. In particular we investigate the rôle played by this extra dimension when evaluating the rectangular Wilson loops encoding the interaction potential between quarks located at different points in the orbifold. The large coupling potential of the deconstructed quiver theory is determined using the AdS/CFT correspondence and analysing the corresponding minimal surface solution for the dual gravitational metric. At small coupling, the potential between quarks decreases with their angular distance while at strong coupling we find a linear dependence at large distance along the (deconstructed) fifth dimension.

hep-th↗
arXiv2003

Cosmological Evolution of Brane World Moduli

We study cosmological consequences of non-constant brane world moduli in five dimensional brane world models with bulk scalars and two boundary branes. We focus on the case where the brane tension is an exponential function of the bulk scalar field, $U_b \propto \exp{(αϕ)}$. In the limit $α\to 0$, the model reduces to the two-brane model of Randall-Sundrum, whereas larger values of $α$ allow for a less warped bulk geometry. Using the moduli space approximation, we derive the four-dimensional low-energy effective action from a supergravity-inspired five-dimensional theory. For arbitrary values of $α$, the resulting theory has the form of a bi-scalar-tensor theory. We show that, in order to be consistent with local gravitational observations, $α$ has to be small (less than $10^{-2}$) and the separation of the branes must be large. We study the cosmological evolution of the interbrane distance and the bulk scalar field for different matter contents on each branes. Our findings indicate that attractor solutions exist which drive the moduli fields towards values consistent with observations. The efficiency of the attractor mechanism crucially depends on the matter content on each branes. In the five-dimensional description, the attractors correspond to the motion of the negative tension brane towards a bulk singularity, which signals the eventual breakdown of the four-dimensional description and the necessity of a better understanding of the bulk singularity.

hep-th↗
arXiv2002

Varying Constants in Brane World Scenarios

Higher-dimensional theories imply that some constants, such as the gravitational constant and the strength of the gauge-couplings, are not fundamental constants. Instead they are related to the sizes of the extra--dimensional space, which are moduli fields in the four--dimensional effective theory. We study the cosmological evolution of the moduli fields appearing in brane world scenarios and discuss the implications for varying constants.

hep-ph↗
arXiv2002

A comment on bouncing and cyclic branes in more than one extra-dimension

We argue that bouncing branes occur naturally when there is more than one extra-dimension. We consider three-branes embedded in space-times with a horizon and an isometry group SO(6). As soon as the brane angular momentum is large enough, a repulsive barrier prevents the branes from reaching the horizon. We illustrate this phenomenon with the case of D3-branes in an AdS_5-Schwarzschild-S_5 background and asymptotically flat space-time.

hep-th↗
arXiv2002

Non-BPS Brane Cosmology

We study cosmology on a BPS D3-brane evolving in the 10D SUGRA background describing a non-BPS brane. Initially the BPS brane is taken to be a probe whose dynamics we determine in the non-compact non-BPS background. The cosmology observed on the brane is of the FRW type with a scale factor $S(τ)$. In this mirage cosmology approach, there is no self-gravity on the brane which cannot inflate. Self-gravity is then included by compactifying the background space-time. The low energy effective theory below the compactification scale is shown to be bi-metric, with matter coupling to a different metric than the geometrically induced metric on the brane. The geometrical scale factor on the brane is now $S(τ) a(τ)$ where $a(τ)$ arises from brane self-gravity. In this non-BPS scenario the brane generically inflates. We study the resulting inflationary scenario taking into account the fact that the non-BPS brane eventually decays on a time-scale much larger than the typical inflationary time-scale. After the decay, the theory ceases to be bi-metric and COBE normalization is used to estimate the string scale which is found to be of order $10^{14}$ GeV.

hep-th↗
arXiv2001

Quintessence Model Building

A short review of some of the aspects of quintessence model building is presented. We emphasize the role of tracking models and their possible supersymmetric origin.

hep-th↗
arXiv2001

On Brane Cosmology and Naked Singularities

Brane-world singularities are analysed, emphasizing the case of supergravity in singular spaces where the singularity puzzle is naturally resolved. These naked singularities are either time-like or null, corresponding to the finite or infinite amount of conformal time that massless particles take in order to reach them. Quantum mechanically we show that the brane-world naked singularities are inconsistent. Indeed we find that time-like singularities are not wave-regular, so the time-evolution of wave packets is not uniquely defined in their vicinity, while null singularities absorb incoming radiation. Finally we stress that for supergravity in singular spaces there is a topological obstruction, whereby naked singularities are necessarily screened off by the second boundary brane.

hep-th↗
arXiv2001

Cosmological Evolution on Self-Tuned Branes and the Cosmological Constant

We consider the cosmological evolution of a bulk scalar field and ordinary matter living on the brane world in the light of the constraints imposed by the matter dominated cosmological evolution and a small cosmological constant now. We rule out models with a self-tuned minimum of the four dimensional potential as they would lead to rapid oscillations of the Hubble parameter now. A more natural framework is provided by supergravity in singular spaces where the brane coupling and the bulk potential are related by supersymmetry leading to a four dimensional run-away potential. For late times we obtain an accelerating universe due to the breaking of supersymmetry on the brane with an acceleration parameter of q_0=-4/7 and associated equation of state omega=-5/7.

hep-th↗
arXiv2001

Non-BPS Instability in Heterotic M-theory

We study the warped geometry of heterotic M-Theory in five dimensions where five-branes are included in the bulk. Five-branes wrapping holomorphic curves lead to BPS configurations where the junction conditions are automatically satisfied. We consider five-branes wrapped around non-supersymmetric cycles and show that the configuration is unstable. We describe explicitly the resulting time-dependent geometry where the bulk five-branes move towards the Horova-Witten boundary walls. The five-branes collide with the boundary walls in a finite time resulting in the restoration of supersymmetry.

hep-th↗
arXiv2001

Graviton Absorption by Non-BPS Branes

We consider the behaviour of neutral non-BPS branes probed by scalars and gravitons. We show that the naked singularity of the non-BPS branes is a {\it repulson} absorbing no incoming radiation. The naked singularity is surrounded by an infinite potential well breaking the unitarity of the scattering S-matrix. We compute the absorption cross section which is infrared divergent. In particular this confirms that gravity does not decouple for the non-BPS branes.

hep-th↗
arXiv2000

Cosmological Solutions of Supergravity in Singular Spaces

We study brane-world solutions of five-dimensional supergravity in singular spaces. We exhibit a self-tuned four-dimensional cosmological constant when five-dimensional supergravity is broken by an arbitrary tension on the brane-world. The brane-world metric is of the FRW type corresponding to a cosmological constant $Ω_Λ={5/7}$ and an equation of state $ω=-{5/7}$ which are consistent with experiment.

hep-th↗