Matter fields from a decaying background Lambda vacuum

We suggest an alternative framework for interpreting the current state of the visible universe. Our approach is based on a dynamical ``Cosmological Constant'' and the starting point is that a decaying vacuum produces matter. As we point out, such a dynamical Lambda is not incompatible with the general requirements of general relativity. By assuming inflation and big bang nucleosynthesis we can solve for the present fractional densities of matter Omega_{m,0} and vacuum Omega_{Lambda, 0} in terms of only one parameter which we call the vacuum domination crossing redshift, z_c. We put constraints on z_c to obtain a universe that is presently vacuum dominated and with characteristic densities consistent with observations. The model points to the possible existence of newly formed dark matter in the inter-cluster voids. We argue that some of this matter could be accreting onto clusters through the latter's long range gravitational potentials. If so, then cluster dark matter halos may not manifest clear cut-offs in their radial density profiles. Furthermore, if a substantial amount of this newly produced matter has already drained onto the clusters, then the CMB power spectrum may favor lower dark matter density values than is currently observed bound in the clusters. A final feature of our approach relates to the combined effect of the matter production by a decaying vacuum and the different rates at which matter and the vacuum will dilute with the scale factor. Such combination may create conditions for a universe in which the vacuum and matter densities dilute and evolve towards comparable amplitudes. In this sense the model offers a natural and conceptually simple explanation to the Coincidence Problem.Comment: 22 pages, 1 figure, accepted for publication in Int. J. Mod. Phys. Lett.

Constraints On Cosmic Dynamics

Observationally, the universe appears virtually critical. Yet, there is no simple explanation for this state. In this article we advance and explore the premise that the dynamics of the universe always seeks equilibrium conditions. Vacuum-induced cosmic accelerations lead to creation of matter-energy modes at the expense of vacuum energy. Because they gravitate, such modes constitute inertia against cosmic acceleration. On the other extreme, the would-be ultimate phase of local gravitational collapse is checked by a phase transition in the collapsing matter fields leading to a de Sitter-like fluid deep inside the black hole horizon, and at the expense of the collapsing matter fields. As a result, the universe succumbs to neither vacuum-induced run-away accelerations nor to gravitationally induced spacetime curvature singularities. Cosmic dynamics is self-regulating. We discuss the physical basis for these constraints and the implications, pointing out how the framework relates and helps resolve standing puzzles such as "why did cosmic inflation end?", "why is Lambda small now?" and "why does the universe appear persistently critical?". The approach does, on the one hand, suggest a future course for cosmic dynamics, while on the other hand it provides some insight into the physics inside black hole horizons. The interplay between the background vacuum and matter fields suggests an underlying symmetry that links spacetime acceleration with spacetime collapse and global (cosmic) dynamics with local (black hole) dynamics.Comment: 11 page

Cosmology with Interacting Dark Energy

The early cosmic inflation, when taken along with the recent observations that the universe is currently dominated by a low density vacuum energy, leads to at least two potential problems which modern cosmology must address. First, there is the old cosmological constant problem, with a new twist: the coincidence problem. Secondly, cosmology still lacks a model to predict the observed current cosmic acceleration and to determine whether or not there is a future exit out of this state (as previously in the inflationary case). This constitutes (what is called here) a dynamical problem. In this article a framework is proposed to address these two problems, based on treating the cosmic background vacuum (dark) energy as both dynamical and interacting. The universe behaves as a vacuum-driven cosmic engine which, in search of equilibrium, always back-reacts to vacuum-induced accelerations by increasing its inertia (internal energy) through vacuum energy dissipation. The process couples cosmic vacuum (dark) energy to matter to produce future-directed increasingly comparable amplitudes in these fields by setting up oscillations in the decaying vacuum energy density and corresponding sympathetic ones in the matter fields. By putting bounds on the relative magnitudes of these coupled oscillations the model offers a natural and conceptually simple channel to discuss the coincidence problem, while also suggesting a way to deal with the dynamical problem. A result with useful observational implications is an equation of state w(t) which specifically predicts a variable, quasi-periodic, acceleration for the current universe. This result can be directly tested by future observational techniques such as SNAP

Can gravitational collapse sustain singularity-free trapped surfaces?

In singularity generating spacetimes both the out-going and in-going expansions of null geodesic congruences $\theta ^{+}$ and $\theta ^{-}$ should become increasingly negative without bound, inside the horizon. This behavior leads to geodetic incompleteness which in turn predicts the existence of a singularity. In this work we inquire on whether, in gravitational collapse, spacetime can sustain singularity-free trapped surfaces, in the sense that such a spacetime remains geodetically complete. As a test case, we consider a well known solution of the Einstien Field Equations which is Schwarzschild-like at large distances and consists of a fluid with a $p=-\rho$ equation of state near $r=0$. By following both the expansion parameters $\theta ^{+}$ and $\theta ^{-}$ across the horizon and into the black hole we find that both $\theta ^{+}$ and $\theta ^{+}\theta ^{-}$ have turning points inside the trapped region. Further, we find that deep inside the black hole there is a region $0\leq r<r_{0}$ (that includes the black hole center) which is not trapped. Thus the trapped region is bounded both from outside and inside. The spacetime is geodetically complete, a result which violates a condition for singularity formation. It is inferred that in general if gravitational collapse were to proceed with a $p=-\rho$ fluid formation, the resulting black hole may be singularity-free.Comment: 17 pages, 3 figures, accepted for publication in International Journal of Modern Physics

The Gravitational Instability of the Vacuum: Insight into the Cosmological Constant Problem

A mechanism for suppressing the cosmological constant is developed, based on an analogy with a superconducting phaseshift in which free fermions coupled perturbatively to a weak gravitational field are in an unstable false vacuum state. The coupling of the fermions to the gravitational field generates fermion condensates with zero momentum and a phase transition induces a nonperturbative transition to a true vacuum state by producing a positive energy gap $\Delta$ in the vacuum energy, identified with $\sqrt{\Lambda}$, where $\Lambda$ is the cosmological constant. In the strong coupling limit a large cosmological constant induces a period of inflation in the early universe, followed by a weak coupling limit in which $\sqrt{\Lambda}$ vanishes exponentially fast as the universe expands due to the dependence of the energy gap on the density of Fermi surface fermions, $D({\epsilon})$, predicting a small cosmological constant in the present universe.Comment: 13 Page

Evolution of evaporating Black Holes in a higher dimensional inflationary universe

Spherically symmetric Black Holes of the Vaidya type are examined in an asymptotically de Sitter, higher dimensional spacetime. The various horizons are identified and located. The structure and dynamics of such horizons are studied. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87523/2/161_1.pd

Is cosmic dynamics self-regulating?

In this paper we discuss a cosmological model for a universe with self-regulating features. We set up the theoretical framework for the model and determine the time evolution of the scale-factor $a(t)$. It is shown that such a universe repeatedly goes through alternate periods of matter and dark energy domination. The resulting dynamics oscillates about the would-be ideal time-linear or coasting path, with monotonic expansion. When compared to dynamics of the observed physical Universe, the model recovers the observationally-established evolutionary features of the latter, from the big bang to the current acceleration, and farther. It suggests a universe that initially emerges from a non-singular state, associated with a non-inflationary acceleration, and which acceleration it exits naturally with matter-energy generation. The model does not have a horizon problem or a flatness problem. It reproduces the observed current values of standard cosmic parameters, including the age $t_{0}$, the current Hubble parameter $H_{0}$ and dark energy $\Omega_{de}\$and matter $\Omega_{m}$ density parameters. We find the dark matter density-profile generated by the model naturally leads to flat rotation curves in galaxy halos. The model is falsifiable. It makes predictions that can be tested, as suggested. Finally, we discuss the dimensionless age $(H_{0}t_{0}\simeq1)$ paradox as an example of the model's ability to address standing puzzles. The findings suggest dynamics of the physical Universe may be self-regulating and predictable.Comment: Updated version submitted for review, 5 figure