Stringy models of modified gravity: space-time defects and structure formation

Abstract

Starting from microscopic models of space-time foam, based on brane universes propagating in bulk space-times populated by D0-brane defects ("D-particles"), we arrive at effective actions used by a low-energy observer on the brane world to describe his/her observations of the Universe. These actions include, apart from the metric tensor field, also scalar (dilaton) and vector fields, the latter describing the interactions of low-energy matter on the brane world with the recoiling point-like space-time defect (D-particle). The vector field is proportional to the recoil velocity of the Dparticle and as such it satisfies a certain constraint. The vector breaks locally Lorentz invariance, which however is assumed to be conserved on average in a space-time foam situation, involving the interaction of matter with populations of D-particle defects. In this paper we clarify the role of fluctuations of the vector field on structure formation and galactic growth. In particular we demonstrate that, already at the end of the radiation era, the (constrained) vector field associated with the recoil of the defects provides the seeds for a growing mode in the evolution of the Universe. Such a growing mode survives during the matter dominated era, provided the variance of the Dparticle recoil velocities on the brane is larger than a critical value. We note that in this model, as a result of specific properties of D-brane dynamics in the bulk, there is no issue of overclosing the brane Universe for large defect densities. Thus, in these models, the presence of defects may be associated with large-structure formation. Although our string inspired models do have (conventional, from a particle physics point of view) dark matter components, nevertheless it is interesting that the role of "extra" dark matter is also provided by the population of massive defects. This is consistent with the weakly interacting character of the D-particle defects, which predominantly interact only gravitationally.

Fundamental physics with the Square Kilometre Array

“…The latter implies the emergence of vector-like excitations that can lead to an early era of accelerated expansion, in the absence of an inflaton field, and contribute to LSS (enhancing the DM component) and galaxy formation (Ferreras et al 2008(Ferreras et al , 2009Mavromatos et al 2009). The D-material universe has been shown to be in agreement with gravitational lensing phenomenology (Mavromatos et al 2013). Moreover, the medium of D-particles leads to recoil velocity field condensates that induce an effective mass for the graviton (Elghozi et al 2017), in agreement with the constraints imposed from the Advanced LIGO interferometric data (Abbott et al 2016a,b;Abbott et al 2017).…”

Constraining D-foam via the 21-cm line

“…where dσ/dΩ is the differential cross-section in the solid angle Ω, which is inversely proportional to the fourth power of v, exactly as in the case of millicharged dark matter/-baryon scattering [21], but with the electric charge of the baryon replaced by the couplingg V,b (2.11), and the charge ǫe of the millicharged dark matter particle replaced bỹ 14) where µ N,b = mN m b mN +m b is the reduced mass of the sterile neutrino/baryon system, m N (m b ) being the mass of the sterile neutrino (baryon). The quantity α V,N was defined in (2.9), and α = e 2 4π is the fine structure constant of electromagnetism.…”

“…where σ 2 < 1 a phenomenological stochastic fluctuation parameter of the D-foam, that sets the scale for the 'fuzziness' of space-time. If one assumes that the ensemble of D-particles on our brane Universe are themselves thermalised at a temperature T , at least at late eras of the Universe [14], then one can define the following stochastic parameter related to their thermal velocity:…”

“…As the three-brane world and the D-particles move in the bulk, they may encounter each other, in which case an observer on the three-brane perceives the D-particle defects as flashing on and off, giving the 3+1-dimensional space-time a foamy structure. In such a scenario, ordinary matter and radiation are represented by open strings with their ends attached on the three-brane.Some of the D-particles may be trapped on the three-brane, in which case they would act as dark matter [13,14]. However, depending on the dynamics of the bulk and the three-brane, the density of D-particles on the three-brane may evolve differently from the conventional dust-like dilution of matter density as the Universe expands.…”

“…This is a rather drastic assumption, but is motivated by the fact that thermalised D-matter reproduces the correct large-scale structure of the Universe, if the D-matter constitutes a dominant dark matter component at large scales[14] 10. We recall that the size of the hydrogen atom is significantly larger than the Compton wavelength of a D-particle, whose mass Ms/gs is in the multi-TeV range.…”