Abstract
In a previous publication by some of the authors (N.E.M., M.S. and M.F.Y.), we have argued that the ``D-material universe'', that is a model of a brane world propagating in a higher-dimensional bulk populated by collections of D-particle stringy defects, provides a model for the growth of large-scale structure in the universe via the vector field in its spectrum. The latter corresponds to D-particle recoil velocity excitations as a result of the interactions of the defects with stringy matter and radiation on the brane world. In this article, we first elaborate further on the results of the previous study on the galactic growth era and analyse the circumstances under which the D-particle recoil velocity fluid may ``mimic'' dark matter in galaxies. A lensing phenomenology is also presented for some samples of galaxies, which previously were known to provide tension for modified gravity (TeVeS) models. The current model is found in agreement with these lensing data. Then we discuss a cosmic evolution for the D-material universe by analysing the conditions under which the late eras of this universe associated with large-scale structure are connected to early epochs, where inflation takes place. It is shown that inflation is induced by dense populations of D-particles in the early universe, with the rôle of the inflaton field played by the condensate of the D-particle recoil-velocity fields under their interaction with relativistic stringy matter, only for sufficiently large brane tensions and low string mass scales compared to the Hubble scale. On the other hand, for large string scales, where the recoil-velocity condensate fields are weak, inflation cannot be driven by the D-particle defects alone. In such cases inflation may be driven by dilaton (or other moduli) fields in the underlying string theory.
Highlights
The latter has been attributed to particles that may exist in extensions of the standard model, such as axions, supersymmetry, string theory, higher-dimensional field theories, etc
The cosmology of TensorVector-S calar (TeVeS) theories has been developed [9] and some interesting links of the vector fields of such theories with large scale growth of the universe have been proposed [10]. Their phenomenology is not yet as well studied as the “conventional” ΛCDM (Cosmological Constant (Λ) Cold Dark Matter) model. That this might be due to the fact that the currently used TeVeS models are too simple for that purpose as they do not address the fundamental issue of the origin of the dark energy, which, if achieved, might lead to more complex theories that could fit the data better
In ref. [11] we conjectured that modified gravity models involving fundamental vector fields, but quite different from TeVeS models, may appear as the low-energy limit of certain brane theories of the type proposed in ref
Summary
As discussed in refs. [15, 18], the following four-dimensional space-time effective action expresses the interaction of stringy matter on a brane world of three longitudinal large dimensions with a medium of recoiling D-particles in the early universe. Before proceeding we should remark for completeness that a basic assumption [15] underlying (2.1) is that any mass contribution of the D-particle defects to the vacuum energy density is considered subleading, compared to the recoil and other terms present in (2.1) This is because, as discussed in refs. As we discussed in [18] and shall review below, the assumed high density of D-particles implies a constant density of D-particles on the brane, which contribute crucially to a Starobinsky-like inflation driven by strong condensate fields of the recoil velocities field strength Fμν F μν After this necessary digression, we remark that the four-dimensional DBI action (on the D3-brane world) in (2.1) can be expanded in derivatives, as appropriate for a low-energy weak-field approximation compared to the string scale Ms = 1/√α , as follows [19]. The dilaton equation of motion, obtained by varying the effective action (2.15) with respect to the dilaton field φ and setting it to a constant value φ0 at the end of the variation, reads
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