Generalized Chaplygin Gas in a modified gravity approach

We study the generalized Chaplygin gas (GCG) scenario in a modified gravity approach. That is, we impose that our universe has a pure dust configuration, and allow for a modification of gravity that yields a GCG specific scale factor evolution. Moreover, assuming that this new hypothetical gravity theory obeys a generalization of Birkhoff's law, we determine the Schwarzschild-like metric in this new modified gravity. We also study the large scale structure formation in this model. Both the linear and non-linear growth are studied together with the growth of the velocity fluctuation in the linear perturbation theory. We compare our results with those corresponding to the $\Lambda$CDM model and discuss possible distinguishable features.Comment: 13 pages and 4 figures. Final version to appear in PR

Is Cosmology Solved?

We have fossil evidence from the thermal background radiation that our universe expanded from a considerably hotter denser state. We have a well defined and testable description of the expansion, the relativistic Friedmann-Lemaitre model. Its observational successes are impressive but I think hardly enough for a convincing scientific case. The lists of observational constraints and free hypotheses within the model have similar lengths. The scorecard on the search for concordant measures of the mass density parameter and the cosmological constant shows that the high density Einstein-de Sitter model is challenged, but that we cannot choose between low density models with and without a cosmological constant. That is, the relativistic model is not strongly overconstrained, the usual test of a mature theory. Work in progress will greatly improve the situation and may at last yield a compelling test. If so, and the relativistic model survives, it will close one line of research in cosmology: we will know the outlines of what happened as our universe expanded and cooled from high density. It will not end research: some of us will occupy ourselves with the details of how galaxies and other large-scale structures came to be the way they are, others with the issue of what our universe was doing before it was expanding. The former is being driven by rapid observational advances. The latter is being driven mainly by theory, but there are hints of observational guidance.Comment: 13 pages, 3 figures. To be published in PASP as part of the proceedings of the Smithsonian debate, Is Cosmology Solved

Vacuum defects without a vacuum

Topological defects can arise in symmetry breaking models where the scalar field potential $V(\phi)$ has no minima and is a monotonically decreasing function of $|\phi|$. The properties of such vacuumless defects are quite different from those of the ``usual'' strings and monopoles. In some models such defects can serve as seeds for structure formation, or produce an appreciable density of mini-black holes.Comment: 11 pages, REVTeX, 1 Postscript figure. Minor changes. Final version, to appear in Phys. Rev.

Constraints on the coupled quintessence from cosmic microwave background anisotropy and matter power spectrum

We discuss the evolution of linear perturbations in a quintessence model in which the scalar field is non-minimally coupled to cold dark matter. We consider the effects of this coupling on both cosmic microwave background temperature anisotropies and matter perturbations. Due to the modification of the scale of cold dark matter as $\rho_{c} = \rho_{c}^{(0)} a^{-3 + \xi}$, we can shift the turnover in the matter power spectrum even without changing the present energy densities of matter and radiation. This can be used to constrain the strength of the coupling. We find that the phenomenology of this model is consistent with current observations up to the coupling power $n_{c} \leq 0.01$ while adopting the current parameters measured by WMAP. Upcoming cosmic microwave background observations continuing to focus on resolving the higher peaks may put strong constraints on the strength of the coupling.Comment: 23 pages, 7 figure

CMB Anisotropy in COBE-DMR-Normalized Flat Λ\Lambda CDM Cosmogony

We compute the cosmic microwave background (CMB) anisotropy in a low-density, flat, cosmological constant, cold dark matter model which is normalized to the two-year COBE DMR sky map. Although conclusions regarding model viability must remain tentative until systematic effects are better understood, there are mild indications that these models have more intermediate scale power than is indicated by presently available CMB anisotropy observational data, with old ($t_0 \geq 15-16$Gyr), high baryon density ($\Omega_B \geq 0.0175 h^{-2}$), low density ($\Omega_0 \sim 0.2-0.4$) models doing the worst.Comment: 9 pages including 2 figures, one 5 pages table: two uuencoded postscript file

Cosmological dynamics of scalar fields with O(N) symmetry

In this paper, we study the cosmological dynamics of scalar fields with O(N) symmetry in general potentials. We compare the phase space of the dynamical systems of the quintessence and phantom and give the conditions for the existence of various attractors as well as their cosmological implications. We also show that the existence of tracking attractor in O(N) phantom models require the potential with $\Gamma<1/2$, which makes the models with exponential potential possess no tracking attractor.Comment: 9 pages, 4 figures; Replaced with the version to be published in Classical and Quantum Gravity. Reference adde

Expansion, Geometry, and Gravity

In general-relativistic cosmological models, the expansion history, matter content, and geometry are closely intertwined. In this brief paper, we clarify the distinction between the effects of geometry and expansion history on the luminosity distance. We show that the cubic correction to the Hubble law, measured recently with high-redshift supernovae, is the first cosmological measurement, apart from the cosmic microwave background, that probes directly the effects of spatial curvature. We illustrate the distinction between geometry and expansion with a toy model for which the supernova results already indicate a curvature radius larger than the Hubble distance.Comment: 4 pages, 1 color figur

Quintessential inflation

We present an explicit observationally acceptable model for evolution from inflation to the present epoch under the assumption that the entropy and matter of the familiar universe are from gravitational particle production at the end of inflation. This eliminates the problem of finding a satisfactory coupling of the inflaton and matter fields. Since the inflaton potential $V(\phi)$ may be a monotonic function of the inflaton $\phi$, the inflaton energy could produce an observationally significant effective cosmological constant, as in quintessence.Comment: 6 pages, REVTeX, 1 figur

How to determine an effective potential for a variable cosmological term

It is shown that if a variable cosmological term in the present Universe is described by a scalar field with minimal coupling to gravity and with some phenomenological self-interaction potential $V(\varphi)$, then this potential can be unambiguously determined from the following observational data: either from the behaviour of density perturbations in dustlike matter component as a function of redshift (given the Hubble constant additionally), or from the luminosity distance as a function of redshift (given the present density of dustlike matter in terms of the critical one).Comment: Latex, 7 pages, JETP Lett., in press, 199

A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration

Increasing evidence suggests that most of the energy density of the universe consists of a dark energy component with negative pressure, a ``cosmological constant" that causes the cosmic expansion to accelerate. In this paper, we address the puzzle of why this component comes to dominate the universe only recently rather than at some much earlier epoch. We present a class of theories based on an evolving scalar field where the explanation is based entirely on internal dynamical properties of the solutions. In the theories we consider, the dynamics causes the scalar field to lock automatically into a negative pressure state at the onset of matter-domination such that the present epoch is the earliest possible time, consistent with nucleosynthesis restrictions, when it can start to dominate.Comment: 5 pages, 3 figure