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Register a new userBrane Isotropisation in Extra-Dimensional Tolman-Bondi Universe
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We consider the dynamics of a 3-brane embedded in an extra-dimensional Tolman-Bondi Universe where the origin of space plays a special role. The embedding is chosen such that the induced matter distribution on the brane respects the spherical symmetry of matter in the extra dimensional space. The mirage cosmology on the probe brane is studied, resulting in an inhomogeneous and anisotropic four dimensional cosmology where the origin of space is also special. We then focus on the spatial geometry around the origin and show that the induced geometry, which is initially inhomogeneous and anisotropic, converges to an isotropic and homogeneous Friedmann-Lemaitre 4d space-time. For instance, when a 3-brane is embedded in a 5d matter dominated model, the 4d dynamics around the origin converge to a Friedmann-Lemaitre Universe in a radiation dominated epoch. We analyse this isotropisation process and show that it is a late time attractor.
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We present a brane-world scenario in which two regions of $AdS_5$ space-time are glued together along a 3-brane with constant positive curvature such that {em all} spatial dimensions form a compact manifold of topology $S^4$. It turns out that the induced geometry on the brane is given by Einsteins static universe. It is possible to achieve an anisotropy of the manifold which allows for a huge hierarchy between the size of the extra dimension $R$ and the size of the observable universe $R_U$ at present. This anisotropy is also at the origin of a very peculiar property of our model: the physical distance between {em any two points} on the brane is of the order of the size of the extra dimension $R$ regardless of their distance measured with the use of the induced metric on the brane. In an intermediate distance regime $R ll r ll R_U$ gravity on the brane is shown to be effectively 4-dimensional, with corresponding large distance corrections, in complete analogy with the Randall-Sundrum II model. For very large distances $r sim R_U$ we recover gravity in Einsteins static universe. However, in contrast to the Randall-Sundrum II model the difference in topology has the advantage of giving rise to a geodesically complete space.
We illustrate a framework for constructing models of chaotic inflation where the inflaton is the position of a D3 brane along the universal cover of a string compactification. In our scenario, a brane rolls many times around a non-trivial one-cycle, thereby unwinding a Ramond-Ramond flux. These flux monodromies are similar in spirit to the monodromies of Silverstein, Westphal, and McAllister, and their four-dimensional description is that of Kaloper and Sorbo. Assuming moduli stabilization is rigid enough, the large-field inflationary potential is protected from radiative corrections by a discrete shift symmetry.
In self-tuning brane-world models with extra dimensions, large contributions to the cosmological constant are absorbed into the curvature of extra dimensions and consistent with flat 4d geometry. In models with conventional Lagrangians fine-tuning is needed nevertheless to ensure a finite effective Planck mass. Here, we consider a class of models with non conventional Lagrangian in which known problems can be avoided. Unfortunately these models are found to suffer from tachyonic instabilities. An attempt to cure these instabilities leads to the prediction of a positive cosmological constant, which in turn needs a fine-tuning to be consistent with observations.
Brane supersymmetry breaking is a peculiar phenomenon that can occur in perturbative orientifold vacua. It results from the simultaneous presence, in the vacuum, of non-mutually BPS sets of BPS branes and orientifolds, which leave behind a net tension and thus a runaway potential, but no tachyons. In the simplest ten-dimensional realization, the low-lying modes combine the closed sector of type-I supergravity with an open sector including USp(32) gauge bosons, fermions in the antisymmetric 495 and an additional singlet playing the role of a goldstino. We review some properties of this system and of other non-tachyonic models in ten dimensions with broken supersymmetry, and we illustrate some puzzles that their very existence raises, together with some applications that they have stimulated.
Hilltop inflation models are often described by potentials $V = V_{0}(1-{phi^{n}over m^{n}}+...)$. The omitted terms indicated by ellipsis do not affect inflation for $m lesssim 1$, but the most popular models with $n =2$ and $4$ for $m lesssim 1$ are ruled out observationally. Meanwhile in the large $m$ limit the results of the calculations of the tensor to scalar ratio $r$ in the models with $V = V_{0}(1-{phi^{n}over m^{n}})$, for all $n$, converge to $r= 4/N lesssim 0.07$, as in chaotic inflation with $V sim phi$, suggesting a reasonably good fit to the Planck data. We show, however, that this is an artifact related to the inconsistency of the model $V = V_{0}(1-{phi^{n}over m^{n}})$ at $phi > m$. Consistent generalizations of this model in the large $m$ limit typically lead to a much greater value $r= 8/N$, which negatively affects the observational status of hilltop inflation. Similar results are valid for D-brane inflation with $V = V_{0}(1-{m^{n}over phi^{n}})$, but consistent generalizations of D-brane inflation models may successfully complement $alpha$-attractors in describing most of the area in the ($n_{s}$, $r$) space favored by Planck 2018.
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