FRW Cosmology with Non-positively Defined Higgs Potentials

We discuss the classical aspects of dynamics of scalar models with non-positive Higgs potentials in the FRW cosmology. These models appear as effective local models in non-local models related with string field theories. After a suitable field redefinition these models have the form of local Higgs models with a negative extra cosmological term and the total Higgs potential is non-positively defined and has rather small coupling constant. The non-positivity of the potential leads to the fact that on some stage of evolution the expansion mode gives place to the mode of contraction, due to that the stage of reheating is absent. In these models the hard regime of inflation gives place to inflation near the hill top and the area of the slow roll inflation is very small. Meanwhile one can obtain enough e-foldings before the contraction to make the model under consideration admissible to describe inflation.Comment: 40 pages, 20 figures, typos correcte

Gravitational Waves from Mesoscopic Dynamics of the Extra Dimensions

Recent models which describe our world as a brane embedded in a higher dimensional space introduce new geometrical degrees of freedom: the shape and/or size of the extra dimensions, and the position of the brane. These modes can be coherently excited by symmetry breaking in the early universe even on ``mesoscopic'' scales as large as 1 mm, leading to detectable gravitational radiation. Two sources are described: relativistic turbulence caused by a first-order transition of a radion potential, and Kibble excitation of Nambu-Goldstone modes of brane displacement. Characteristic scales and spectral properties are estimated and the prospects for observation by LISA are discussed. Extra dimensions with scale between 10 \AA and 1 mm, which enter the 3+1-D era at cosmic temperatures between 1 and 1000 TeV, produce backgrounds with energy peaked at observed frequencies in the LISA band, between $10^{-1}$ and $10^{-4}$ Hz. The background is detectable above instrument and astrophysical foregrounds if initial metric perturbations are excited to a fractional amplitude of $10^{-3}$ or more, a likely outcome for the Nambu-Goldstone excitations.Comment: Latex, 5 pages, plus one figure, final version to appear in Phys. Rev. Let

Nonsingular global string compactifications

We consider an exotic `compactification' of spacetime in which there are two infinite extra dimensions, using a global string instead of a domain wall. By having a negative cosmological constant we prove the existence of a nonsingular static solution using a dynamical systems argument. A nonsingular solution also exists in the absence of a cosmological constant with a time-dependent metric. We compare and contrast this solution with the Randall-Sundrum universe and the Cohen-Kaplan spacetime, and consider the options of using such a model as a realistic resolution of the hierarchy problem.Comment: 8 pages revtex, 1 figure : References added and equation correcte

Non-Gaussianity of the density distribution in accelerating universes

According to recent observations, the existence of the dark energy has been considered. Even though we have obtained the constraint of the equation of the state for dark energy ($p = w \rho$) as $-1 \le w \le -0.78$ by combining WMAP data with other astronomical data, in order to pin down $w$, it is necessary to use other independent observational tools. For this purpose, we consider the $w$ dependence of the non-Gaussianity of the density distribution generated by nonlinear dynamics. To extract the non-Gaussianity, we follow a semi-analytic approach based on Lagrangian linear perturbation theory, which provides an accurate value for the quasi-nonlinear region. From our results, the difference of the non-Gaussianity between $w = -1$ and $w= -0.5$ is about 4% while that between $w = -1$ and $w= -0.8$ is about $0.9$. For the highly non-linear region, we estimate the difference by combining this perturbative approach with N-body simulation executed for our previous paper. From this, we can expect the difference to be more enhanced in the low-$z$ region, which suggests that the non-Gaussianity of the density distribution potentially plays an important role for extracting the information of dark energy.Comment: 15 pages, 4 figures, accepted for publication in JCAP; v2: smoothing scale has been change

Homogeneity, Flatness and "Large" Extra Dimensions

We consider a model in which the universe is the direct product of a (3+1)-dimensional Friedmann, Robertson-Walker (FRW) space and a compact hyperbolic manifold (CHM). Standard Model fields are confined to a point in the CHM (i.e. to a brane). In such a space, the decay of massive Kaluza-Klein modes leads to the injection of any initial bulk entropy into the observable (FRW) universe. Both Kolmogoro-Sinai mixing due to the non-integrability of flows on CHMs and the large statistical averaging inherent in the collapse of the initial entropy onto the brane smooth out any initial inhomogeneities in the distribution of matter and of 3-curvature on any slice of constant 3-position. If, as we assume, the initial densities and curvatures in each fundamental correlation volume are drawn from some universal underlying distributions independent of location within the space, then these smoothing mechanisms effectively reduce the density and curvature inhomogeneities projected onto the FRW. This smoothing is sufficient to account for the current homogeneity and flatness of the universe. The fundamental scale of physics can be \gsim 1TeV. All relevant mass and length scales can have natural values in fundamental units. All large dimensionless numbers, such as the entropy of the universe, are understood as consequences of the topology of spacetime which is not explained. No model for the origin of structure is proffered.Comment: minor changes, matches version published in Phys. Rev. Let

Analysis of scalar perturbations in cosmological models with a non-local scalar field

We develop the cosmological perturbations formalism in models with a single non-local scalar field originating from the string field theory description of the rolling tachyon dynamics. We construct the equation for the energy density perturbations of the non-local scalar field in the presence of the arbitrary potential and formulate the local system of equations for perturbations in the linearized model when both simple and double roots of the characteristic equation are present. We carry out the general analysis related to the curvature and entropy perturbations and consider the most specific example of perturbations when important quantities in the model become complex.Comment: LaTeX, 25 pages, 1 figure, v2: Subsection 3.2 and Section 5 added, references added, accepted for publication in Class. Quant. Grav. arXiv admin note: text overlap with arXiv:0903.517

Osmotic pressure of matter and vacuum energy

The walls of the box which contains matter represent a membrane that allows the relativistic quantum vacuum to pass but not matter. That is why the pressure of matter in the box may be considered as the analog of the osmotic pressure. However, we demonstrate that the osmotic pressure of matter is modified due to interaction of matter with vacuum. This interaction induces the nonzero negative vacuum pressure inside the box, as a result the measured osmotic pressure becomes smaller than the matter pressure. As distinct from the Casimir effect, this induced vacuum pressure is the bulk effect and does not depend on the size of the box. This effect dominates in the thermodynamic limit of the infinite volume of the box. Analog of this effect has been observed in the dilute solution of 3He in liquid 4He, where the superfluid 4He plays the role of the non-relativistic quantum vacuum, and 3He atoms play the role of matter.Comment: 5 pages, 1 figure, JETP Lett. style, version accepted in JETP Letter

Brane Gases in the Early Universe

Over the past decade it has become clear that fundamental strings are not the only fundamental degrees of freedom in string theory. D-branes are also part of the spectrum of fundamental states. In this paper we explore some possible effects of D-branes on early Universe string cosmology, starting with two key assumptions: firstly that the initial state of the Universe corresponded to a dense, hot gas in which all degrees of freedom were in thermal equilibrium, and secondly that the topology of the background space admits one-cycles. We argue by t-duality that in this context the cosmological singularities are not present. We derive the equation of state of the brane gases and apply the results to suggest that, in an expanding background, the winding modes of fundamental strings will play the most important role at late times. In particular, we argue that the string winding modes will only allow four space-time dimensions to become large. The presence of brane winding modes with $p > 1$ may lead to a hierarchy in the sizes of the extra dimensions.Comment: 8 pages, 1 figure; typos corrected; published in PR

Universal Extra Dimensions on Real Projective Plane

We propose a six dimensional Universal Extra Dimensions (UED) model compactified on a real projective plane $RP^2$, a two-sphere with its antipodal points being identified. We utilize the Randjbar-Daemi-Salam-Strathdee spontaneous sphere compactification with a monopole configuration of an extra $U(1)_X$ gauge field that leads to a spontaneous radius stabilization. Unlike the sphere and the so-called $S^2/Z_2$ compactifications, the massless $U(1)_X$ gauge boson is safely projected out. We show how a compactification on a non-orientable manifold results in a chiral four dimensional gauge theory by utilizing 6D chiral gauge and Yukawa interactions. The resultant Kaluza-Klein mass spectra are distinct from the ordinary UED models compactified on torus. We briefly comment on the anomaly cancellation and also on a possible dark matter candidate in our model.Comment: 14 pages, 1 figure (v1); Comments and references added, 15 pages (v2); Comments and references added, 15 pages, version to appear in Phys. Lett. B (v3

Alternative Signature of TeV Strings

In string theory, it is well known that any hard scattering amplitude inevitably suffers exponential suppression. We demonstrate that, if the string scale is M_s < 2TeV, this intrinsically stringy behavior leads to a dramatic reduction in the QCD jet production rate with very high transverse momenta p_T > 2TeV at LHC. This suppression is sufficient to be observed in the first year of low-luminosity running. Our prediction is based on the universal behavior of string theory, and therefore is qualitatively model-independent. This signature is alternative and complementary to conventional ones such as Regge resonance (or string ball/black hole) production.Comment: a note added; version to appear in Phys. Rev. D; 11 pages, 1 eps figure, LaTeX2e; BibTeX with utphys style use