Rolling Dilaton Research Articles

Moduli Stabilization in Brane Gas Cosmology

Recently, there have been works on dynamical compactification of string cosmology that can produce inflation consistent with current observations. Moduli, scalar fields characterizing the shape and the size of the extra dimensions, including dilaton and axion, play a key role in string cosmology. Since the vacuum expectation value of the dilaton determines the strength of the gravitational coupling, a rolling dilaton implies a changing gravitational constant. Also, since the extra dimensions should be compact and small through the cosmological evolution of the early universe, the volume modulus of the extra dimensions (radion) should be stabilized in any cosmological models based on string theory. To stabilize the moduli fields in string theory, various models and mechanisms were introduced [1]. One way to stabilize the moduli is using the framework of string or brane gas cosmology based on the mechanism of Brandenberger and Vafa [2,3]. A gas of extended objects (strings and branes) can affect the evolution of the universe in the presence of a nontrivial topology where winding modes are allowed. Strings or branes can be coupled to a cosmological background in the same way as a gas of point particles is coupled to a background of standard Einstein gravity. Many works to develop a stringy mechanism to stabilize the volume or shape moduli have been done in the brane gas formalism [4–18]. The key problem in fixing the dilaton and the radion is that their potential is flat and their vacuum expectation value cannot be determined perturbatively. Some nonperturbative effects must be introduced to stabilize them. For example, a nonperturbatively-generated potential from gaugino condensation was considered to stabilize the dilaton and the radion [19]. Some other factors

Constraining super-critical string/brane cosmologies with astrophysical data

We discuss fits of unconventional dark energy models to the available data from high-redshift supernovae, distant galaxies and baryon oscillations. The models are based either on brane cosmologies or on Liouville strings in which a relaxation dark energy is provided by a rolling dilaton field (Q-cosmology). Such cosmologies feature the possibility of effective four-dimensional negative-energy dust and/or exotic scaling of dark matter. We find evidence for a negative-energy dust at the current era, as well as for exotic-scaling (a−δ) contributions to the energy density, with δ ≊ 4, which could be due to dark matter coupling with the dilaton in Q-cosmology models. We conclude that Q-cosmology fits the data equally well with the ΛCDM model for a range of parameters that are in general expected from theoretical considerations.

Observational evidence for negative-energy dust in late-times non-standard cosmologies

We perform fits of unconventional dark energy models to the available data from high-redshift supernovae and differential galaxy ages. The models are based either on brane cosmologies or on Liouville strings in which a relaxation dark energy is provided by a rolling dilaton field (Q-cosmology). An interesting feature of such cosmologies is the possibility of effective four-dimensional negative-energy dust and/or exotic scaling of dark matter. An important constraint that can discriminate among models is the evolution of the Hubble parameter as a function of the redshift, H( z). We perform fits using a unifying formula for the evolution of H( z), applicable to different models. We find evidence for a negative-energy dust at the current era, as well as for exotic-scaling ( a − δ ) contributions to the energy density, with δ ≳ 3. The latter could be due to dark matter coupling with the dilaton in Q-cosmology models, but it is also compatible with the possibility of dark radiation from a brane Universe to the bulk in brane-world scenarios, which could also encompass Q-cosmology models. The best-fit model seems to include an a −2-scaling contribution to the energy density of the Universe, which is characteristic of the dilaton relaxation in Q-cosmology models, not to be confused with the spatial curvature contribution of conventional cosmology. We conclude that Q-cosmology fits the data equally well with the ΛCDM model for a range of parameters that are in general expected from theoretical considerations.

NON-CRITICAL STRING COSMOLOGIES

Non-critical String Cosmologies are offered as an alternative to Standard Big Bang Cosmology. The new features encompassed within the dilaton dependent non-critical terms affect the dynamics of the Universeś evolution in an unconventional manner being in agreement with the cosmological data. Non-criticality is responsible for a late transition to acceleration at redshifts z =0.2. The role of the uncoupled rolling dilaton to relic abundance calculations is discussed. The uncoupled rolling dilaton dilutes the neutralino relic densities in supersymmetric theories by factors of ten, relaxing considerably the severe WMAP Dark Matter constraints, while at the same time leaves almost unaffected the baryon density in agreement with primordial Nucleosynthesis.

Supercritical stability, transitions, and (pseudo)tachyons

Highly supercritical strings ($c\ensuremath{\gg}15$) with a timelike linear dilaton provide a large class of solutions to string theory, in which closed string tachyon condensation is under control (and follows the world sheet renormalization group flow). In this note we analyze the late-time stability of such backgrounds, including transitions between them. The large friction introduced by the rolling dilaton and the rapid decrease of the string coupling suppress the backreaction of naive instabilities. In particular, although the graviton, dilaton, and other light fields have negative effective mass squared in the linear dilaton background, the decaying string coupling ensures that their condensation does not cause large backreaction. Similarly, the copious particles produced in transitions between highly supercritical theories do not back-react significantly on the solution. We discuss these features also in a somewhat more general class of time-dependent backgrounds with stable late-time asymptotics.

Confronting dark energy models with astrophysical data: Non-equilibrium vs. conventional cosmologies

We discuss fits of cosmological dark energy models to the available data on high-redshift supernovae. We consider a conventional model with cold dark matter and a cosmological constant (ΛCDM), a model invoking super-horizon perturbations (SHCDM) and models based on Liouville strings in which dark energy is provided by a rolling dilaton field (Q-cosmology). We show that a complete treatment of Q-cosmology requires a careful discussion of non-equilibrium situations (off-shell effects). The two main high-redshift supernova data sets give compatible constraints on ΛCDM and the other models. We recover the well-known result that ΛCDM fits very well the combined supernova data sets, as does the super-horizon model. We discuss the model-dependent off-shell corrections to the Q-cosmology model that are relevant to the supernova data, and show that this model fits the data equally well. This analysis could be extended to other aspects of cosmological phenomenology, in particular to the CMB and baryon acoustic oscillations, which have so far been treated using on-shell models.

Current acceleration from the dilaton and stringy cold dark matter

We argue that string theory has ingredients to provide us with candidates for cold dark matter and explain the current acceleration of our universe. In any generic string compactification the dilaton plays an important role, as it couples to the standard model and other heavy nonrelativistic degrees of freedom such as string winding modes and wrapped branes, which we collectively call stringy cold dark matter. These couplings are nonuniversal, which results in an interesting dynamics for a rolling dilaton. Initially, it can track radiation and matter while beginning to dominate the dynamics recently, triggering a phase of acceleration. This scenario can be realized as long as the dilaton also couples strongly to some heavy modes. We furnish examples of such modes. We provide analytical and numerical results and compare them with the current supernovae result. We also mention some of the challenges for the success of our model favoring certain stringy candidates.

DILATON STABILIZATION IN BRANE GAS COSMOLOGY

Brane Gas Cosmology is an M-theory motivated attempt to reconcile aspects of the standard cosmology based on Einstein's theory of general relativity. Dilaton gravity, when incorporating winding p-brane states, has verified the Brandenberger–Vafa mechanism —a string-motivated conjecture which explains why only three of the nine spatial dimensions predicted by string theory grow large. Further investigation of this mechanism has argued for a hierarchy of subspaces, and has shown the internal directions to be stable to initial perturbations. These results, however, are dependent on a rolling dilaton, or varying strength of Newton's gravitational constant GN. In these proceedings we show that it is not possible to stabilize the dilaton and maintain the stability of the internal directions within the standard Brane Gas Cosmology setup.