We consider a FRW cosmological model with an exotic fluid known as Chaplygin gas. We show that the resulting evolution of the universe is not in disagreement with the current observation of cosmic acceleration. The model predict an increasing value for the effective cosmological constant.Comment: 8 pages, latex. References and a new section adde

We consider a quantum corrected inflation scenario driven by a generic GUT or Standard Model type particle model whose scalar field playing the role of an inflaton has a strong non-minimal coupling to gravity. We show that currently widely accepted bounds on the Higgs mass falsify the suggestion of the paper arXiv:0710.3755 (where the role of radiative corrections was underestimated) that the Standard Model Higgs boson can serve as the inflaton. However, if the Higgs mass could be raised to ∼ 230 GeV, then the Standard Model could generate an inflationary scenario with the spectral index of the primordial perturbation spectrum ns ≃ 0.935 (barely matching present observational data) and the very low tensor-to-scalar perturbation ratio r ≃ 0.0006.

We study the role that tachyon fields may play in cosmology as compared to the well-established use of minimally coupled scalar fields. We first elaborate on a kind of correspondence existing between tachyons and minimally coupled scalar fields; corresponding theories give rise to the same cosmological evolution for a particular choice of the initial conditions but not for any other. This leads us to study a specific one-parameter family of tachyonic models based on a perfect fluid mixed with a positive cosmological constant. For positive values of the parameter one needs to modify Sen's action and use the σ-process of resolution of singularities. The physics described by this model is dramatically different and much richer than that of the corresponding scalar field. For particular choices of the initial conditions the universe, that does mimick for a long time a de Sitter-like expansion, ends up in a finite time in a special type of singularity that we call a big brake. This singularity is characterized by an infinite deceleration.PACS numbers: 98.80.Cq, 98.80.Jk

In this note two cosmological models representing the flat Friedmann Universe filled with a Chaplygin fluid, with or without dust, are analyzed in terms of the recently proposed "statefinder" parameters [1]. Trajectories of both models in the parameter plane are shown to be significantly different w.r.t. "quiessence" and "tracker" models. The generalized Chaplygin gas model with an equation of state of the form p = −A/ρ α is also analyzed in terms of the statefinder parameters. PACS numbers: 98.80. Es, 98.80.Cq, 98.80.Hw In the search for cosmological models describing the observed cosmic acceleration [2,3,4], the inspiration coming from inflation has suggested mainly models making use of scalar fields [5,6,7,8,9]. There are of course alternatives; in particular, in [10,11,12] an elementary model has been presented describing a Friedmann universe filled with a perfect fluid obeying the Chaplygin equation of statewhere A is a positive constant (for a thorough review see Ref.[13]). The interesting feature of this model is that it naturally provides a universe that undergoes a transition from a decelerating phase, driven by dust-like matter, to a cosmic acceleration at later stages of its evolution (see [10] for details). An interesting attempt to justify this model [14] makes use of an effective field theory for a three-brane universe [15]. In the flat case, the model can be equivalently described in terms of a homogeneous minimally coupled scalar field φ, with potential [10]However, since models trying to provide a description (if not an explanation) of the cosmic acceleration are proliferating, there exists the problem of discriminating between the various contenders. To this aim a new proposal introduced in [1] makes use of a pair of parameters {r, s}, called "statefinder". The relevant definition is as follows:where H ≡ȧ a is the Hubble constant and q ≡ −ä aH 2 is the deceleration parameter. The new feature of the statefinder is that it involves the third derivative of the cosmological radius.

Abstract. We suggest a novel picture of the quantum Universe -its creation is described by the density matrix defined by the Euclidean path integral. This yields an ensemble of universes -a cosmological landscape -in a mixed state which is shown to be dynamically more preferable than the pure quantum state of the Hartle-Hawking type. The latter is dynamically suppressed by the infinitely large positive action of its instanton, generated by the conformal anomaly of quantum fields within the cosmological bootstrap (the self-consistent back reaction of hot matter). This bootstrap suggests a solution to the problem of boundedness of the on-shell cosmological action and eliminates the infrared catastrophe of small cosmological constant in Euclidean quantum gravity. The cosmological landscape turns out to be limited to a bounded range of the cosmological constant Λ min ≤ Λ ≤ Λ max . The domain Λ < Λ min is ruled out by the back reaction effect which we analyze by solving effective Euclidean equations of motion. The upper cutoff is enforced by the quantum effects of vacuum energy and the conformal anomaly mediated by a special ghost-avoidance renormalization of the effective action. They establish a new quantum scale Λ max which is determined by the coefficient of the topological Gauss-Bonnet term in the conformal anomaly. This scale is realized as the upper bound -the limiting point of an infinite sequence of garland-type instantons which constitute the full cosmological landscape. The dependence of the cosmological constant range on particle phenomenology suggests a possible dynamical selection mechanism for the landscape of string vacua.

We consider the renormalization group improvement in the theory of the Standard Model Higgs boson playing the role of an inflaton with a strong non-minimal coupling to gravity. It suggests the range of the Higgs mass 135.6 GeV M H 184.5 GeV compatible with the current CMB data (the lower WMAP bound on n s ), which is close to the widely accepted range dictated by the electroweak vacuum stability and perturbation theory bounds. We find the phenomenon of asymptotic freedom induced by this non-minimal curvature coupling, which brings the theory to the weak coupling domain everywhere except at the lower and upper boundary of this range. The renormalization group running of the basic quantity A I -the anomalous scaling in the non-minimally coupled Standard Model, which analytically determines all characteristics of the CMB spectrum -brings A I to small negative values at the inflation scale. This property is crucial for the above results and may also underlie the formation of initial conditions for the inflationary dynamics in quantum cosmology.

The quantum gravitational scale of inflation is calculated by finding a sharp probability peak in the distribution function of chaotic inflationary cosmologies driven by a scalar field with large negative constant $\xi$ of nonminimal interaction. In the case of the no-boundary state of the universe this peak corresponds to the eternal inflation, while for the tunnelling quantum state it generates a standard inflationary scenario. The sub-Planckian parameters of this peak (the mean value of the corresponding Hubble constant ${\mbox{\boldmath $H$}}\simeq 10^{-5}m_P$, its quantum width $\Delta{\mbox{\boldmath $H$}}/{\mbox{\boldmath $H$}}\simeq 10^{-5}$ and the number of inflationary e-foldings ${\mbox{\boldmath $N$}}\simeq 60$) are found to be in good correspondence with the observational status of inflation theory, provided the coupling constants of the theory are constrained by a condition which is likely to be enforced by the (quasi) supersymmetric nature of the sub-Planckian particle physics model.Comment: 12 pages, latex, Alberta, Thy 16-9

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