Quantum Effects Of Matter Research Articles

Quantum (in)stability of maximally symmetric space-times

Classical gravity coupled to a CFT4 (matter) is considered. The effect of the quantum dynamics of matter on gravity is studied around maximally symmetric spaces (flat, de Sitter and Anti de Sitter). The structure of the graviton propagator is modified and non-trivial poles appear due to matter quantum effects. The position and residues of such poles are mapped as a function of the relevant parameters, the central charge of the CFT4, the two R2 couplings of gravity as well as the curvature of the background space-time. The instabilities induced are determined. Such instabilities can be important in cosmology as they trigger the departure from de Sitter space and in some regions of parameters are more important than the well-known scalar instabilities. It is also determined when the presence of such instabilities is unreliable if the associated scales are larger than the “species” cutoff of the gravitational theory.

Torsion Fields Generated by the Quantum Effects of Macro-bodies

We generalize Einstein’s General Relativity (GR) by assuming that all matter (including macro-objects) has quantum effects. An appropriate theory to fulfill this task is Gauge Theory Gravity (GTG) developed by the Cambridge group. GTG is a “spin-torsion” theory, according to which, gravitational effects are described by a pair of gauge fields defined over a flat Minkowski background spacetime. The matter content is completely described by the Dirac spinor field, and the quantum effects of matter are identified as the spin tensor derived from the spinor field. The existence of the spin of matter results in the torsion field defined over spacetime. Torsion field plays the role of Bohmian quantum potential which turns out to be a kind of repulsive force as opposed to the gravitational potential which is attractive. The equivalence principle remains and essential in this theory so that GR is relegated to a locally approximate theory wherein the quantum effects (torsion) are negligible. As a toy model, we assume that the macro matter content can be described by the covariant Dirac equation and apply this theory to the simplest radially symmetric and static gravitational systems. Consequently, by virtue of the cosmological principle, we are led to a static universe model in which the Hubble redshifts arise from the torsion fields.

Matter quantum corrections to the graviton self-energy and the Newtonian potential

We revisit the calculation of matter quantum effects on the graviton self-energy on a flat Minkowski background, with the aim to acquire a deeper understanding of the mechanism that renders the graviton massless. To this end, we derive a low-energy theorem which directly relates the radiative corrections of the cosmological constant to those of the graviton mass to all orders in perturbation theory. As an illustrative example, we consider an Abelian Higgs model with minimal coupling to gravity and show explicitly how a suitable renormalization of the cosmological constant leads to the vanishing of the graviton mass at the one-loop level. In the same Abelian Higgs model, we also calculate the matter quantum corrections to the Newtonian potential and present analytical formulae in terms of modified Bessel and Struve functions of the particle masses in the loop. We show that the correction to the Newtonian potential exhibits an exponential fall-off dependence on the distance $r$, once the non-relativistic limit with respect to the non-zero loop mass is carefully considered. For massless scalars, fermions and gauge bosons in the loops, we recover the well-known results presented in the literature.

The Geometrodynamic Nature of the Quantum Potential

The de Broglie–Bohm theory allows us to have got a satisfactory geometrodynamic interpretation of quantum mechanics. The fundamental element, which creates a geometrodynamic picture of the quantum world in the non-relativistic domain, a relativistic curved space-time background, and the quantum gravity domain, is the quantum potential. It is shown that, in the non-relativistic domain, the geometrodynamic nature of the quantum potential followsfrom the fact that it is an information potential containing a space-like active information on the environment; the geometric properties of the space expressed by the quantum potential determine non-local correlations between subatomic particles. Moreover, in the de Broglie–Bohm theory in a curved space-time, it is shown that the quantum, as well as the gravitational, effects of matter have geometric nature and are highly related: the quantum potential can be interpreted as the conformal degree of freedom of the space-time metric, and its presence is equivalent to the curved space-time. It is shown on the basis of some recent research that, in quantum gravity, we have a generalized geometric unification of gravitational and quantum effects of matter; Bohm's interpretation shows that the form of a quantum potential and its relation to the conformal degree of freedom of the space-time metric can be derived from the equations of motion.

QUANTUM EFFECTS AND STABILITY OF CHAMELEON COSMOLOGY

One possibility to explain the current accelerated expansion of the universe may be related with the presence of cosmologically evolving scalar whose mass depends on the local matter density (chameleon cosmology). We point out that matter quantum effects in such scalar–tensor theory produce the chameleon scalar field dependent conformal anomaly. Such conformal anomaly adds higher derivative terms to chameleon field equation of motion. As a result, the principal possibility for instabilities appears. These instabilities seem to be irrelevant at small curvature but may become dangerous in the regions where gravitational field is strong.

Bulk versus boundary (gravitational Casimir) effects in the quantum creation of an inflationary brane-worlduniverse

The role of bulk matter quantum effects (via the corresponding effective potential discussed using the example of a conformal scalar) and of boundary matter quantum effects (via the conformal anomalyinduced effective action) is considered in brane-world cosmology.A scenario is used where brane tension is not a free parameter,and the initial classical bulk-brane action is defined by some considerations. The effective bulk-brane equations of motion arealso analysed.The quantum creation of a four-dimensional de Sitter or anti-de Sitter (AdS) braneuniverse living in five-dimensional AdS space is possible when quantum bulkand/or branematter is taken into account. The consideration of only conformal field theory (CFT) living on the brane admits the full analytical treatment. The bulk gravitational Casimir effect leads to deformations of five-dimensional AdS spaceand spherical/hyperbolic brane shapes. The generalization of the above scenario to dominant bulk quantum gravity naturally representsthis scenario as self-consistent warped compactification within an AdS/CFT set-up.

Dilatonic brane-world black holes, gravity localization, and Newton’s constant

The family of brane-world solutions of d+1-dimensional dilatonic gravity is presented. It includes flat brane with small cosmological constant and (anti) de Sitter brane, dilatonic brane-world black holes (Schwarzschild-(anti-) de Sitter, Kerr, etc). Gravitational and dilatonic perturbations around such branes are found. It is shown that near dilatonic brane-world black hole the gravity may be localized in a standard form. The brane corrections to Newton law are estimated. The proposal to take into account the dilaton coupled brane matter quantum effects is made. The corresponding effective action changes the structure of 4d de Sitter wall. RG flow of four-dimensional Newton constant in IR and UV is briefly discussed.

NONMINIMAL SCALAR–TENSOR THEORIES AND QUANTUM GRAVITY

Recently1it has been shown that the quantum effects of matter determine the conformal degree of freedom of the space–time metric. This was done in the framework of a scalar–tensor theory with one scalar field.2,3A point with that theory is that the form of quantum potential is preassumed. Here we present a scalar–tensor theory with two scalar fields, and no assumption on the form of quantum potential. It is shown that using the equations of motion one gets the correct form of quantum potential plus some corrections.

QUANTUM, GRAVITY AND GEOMETRY

Recently,1–3 it is shown that, the quantum effects of matter are well described by the conformal degree of freedom of the space–time metric. On the other hand, it is a well-known fact that according to Einstein's gravity theory, gravity and geometry are interconnected. In the new quantum gravity theory,1–3 matter quantum effects completely determine the conformal degree of freedom of the space–time metric, while the causal structure of the space–time is determined by the gravitational effects of the matter, as well as the quantum effects through back reaction effects. This idea, previously, is realized in the framework of scalar–tensor theories. In this work, it is shown that quantum gravity theory can also be realized as a purely metric theory. Such a theory is developed, its consequences and its properties are investigated. The theory is applied, then, to black holes and the radiation-dominated universe. It is shown that the initial singularity can be avoided.

Quantum annihilation of Anti-de Sitter universe

We discuss the role of conformal matter quantum effects (using large N anomaly induced effective action) to creation-annihilation of an Anti-de Sitter Universe. The arbitrary GUT with conformally invariant content of fields is considered. On a purely gravitational (supersymmetric) AdS background, the quantum effects act against an (already existing) AdS Universe. The annihilation of such a Universe occurs, what is common for any conformal matter theory. On a dilaton-gravitational background, where there is dilatonic contribution to the induced effective action, the quantum creation of an AdS Universe is possible assuming fine-tuning of the dilaton.

CONFORMAL TRANSFORMATIONS AND QUANTUM GRAVITY

Recently,1 it was shown that quantum effects of matter could be identified with the conformal degree of freedom of the space–time metric. Accordingly, one can introduce quantum effects either by making a scale transformation (i.e. changing the metric), or by making a conformal transformation (i.e. changing all physical quantities). These two ways are investigated and compared. Also, it is argued that, the ultimate formulation of such a quantum gravity theory should be in the framework of the scalar–tensor theories.