No Arabic abstract
A density-dependent conformal killing vector (CKV) field is attained from a conformally transformed action composed of a unique constraint and a Klein-Gordon field. The CKV is re-expressed into an information identity and studied in its integro-differential form for both null and time-like geodesics. It is conjectured that the identity corresponds to a generalized second law of thermodynamics which holographically relates the covariant entropy contained within a volumetric $n$- and $(n-1)$-form, starting from an $(n-2)$-spatial area. The time-like geodesics inherit an effective `geometric spin while the null geodesics are suggested to obey the generalized covariant entropy bound so long as they conform to Einsteins equation of state. To then comply with the equation of state, a metriplectic system is introduced, whereby a newly defined energy functional is derived for the entropy. Such an entropy functional mediates the Casimir invariants of the Hamiltonian and therefore preserves the symplectic form of quantum mechanics. For null geodesics, the Poisson bracket of the entropy functional with the Hamiltonian is shown to elegantly result in Einsteins energy-mass relation.
We construct a double field theory coupled to the fields present in Vasilievs equations. Employing the semi-covariant differential geometry, we spell a functional in which each term is completely covariant with respect to $mathbf{O}(4,4)$ T-duality, doubled diffeomorphisms, $mathbf{Spin}(1,3)$ local Lorentz symmetry and, separately, $mathbf{HS}(4)$ higher spin gauge symmetry. We identify a minimal set of BPS-like conditions whose solutions automatically satisfy the full Euler-Lagrange equations. As such a solution, we derive a linear dilaton vacuum. With extra algebraic constraints further supplemented, the BPS-like conditions reduce to the bosonic Vasiliev equations.
Applying Clausius relation, $delta Q=TdS$, to apparent horizon of a FRW universe with any spatial curvature, and assuming that the apparent horizon has temperature $T=1/(2pi tilde {r}_A)$, and a quantum corrected entropy-area relation, $S=A/4G +alpha ln A/4G$, where $tilde {r}_A$ and $A$ are the apparent horizon radius and area, respectively, and $alpha$ is a dimensionless constant, we derive modified Friedmann equations, which does not contain a bounce solution. On the other hand, loop quantum cosmology leads to a modified Friedmann equation $H^2 =frac{8pi G}{3}rho (1-rho/rho_{rm crit})$. We obtain an entropy expression of apparent horizon of FRW universe described by the modified Friedmann equation. In the limit of large horizon area, resulting entropy expression gives the above corrected entropy-area relation, however, the prefactor $alpha$ in the logarithmic term is positive, which seems not consistent with most of results in the literature that quantum geometry leads to a negative contribution to the area formula of black hole entropy.
A classical upper bound for quantum entropy is identified and illustrated, $0leq S_q leq ln (e sigma^2 / 2hbar)$, involving the variance $sigma^2$ in phase space of the classical limit distribution of a given system. A fortiori, this further bounds the corresponding information-theoretical generalizations of the quantum entropy proposed by Renyi.
In recent work, we introduced Picard-Lefschetz theory as a tool for defining the Lorentzian path integral for quantum gravity in a systematic semiclassical expansion. This formulation avoids several pitfalls occurring in the Euclidean approach. Our method provides, in particular, a more precise formulation of the Hartle-Hawking no boundary proposal, as a sum over real Lorentzian four-geometries interpolating between an initial three-geometry of zero size, {it i.e}, a point, and a final three-geometry. With this definition, we calculated the no boundary amplitude for a closed universe with a cosmological constant, assuming cosmological symmetry for the background and including linear perturbations. We found the opposite semiclassical exponent to that obtained by Hartle and Hawking for the creation of a de Sitter spacetime from nothing. Furthermore, we found the linearized perturbations to be governed by an {it inverse} Gaussian distribution, meaning they are unsuppressed and out of control. Recently, Diaz Dorronsoro {it et al.} followed our methods but attempted to rescue the no boundary proposal by integrating the lapse over a different, intrinsically complex contour. Here, we show that, in addition to the desired Hartle-Hawking saddle point contribution, their contour yields extra, non-perturbative corrections which again render the perturbations unsuppressed. We prove there is {it no} choice of complex contour for the lapse which avoids this problem. We extend our discussion to include backreaction in the leading semiclassical approximation, fully nonlinearly for the lowest tensor harmonic and to second order for all higher modes. Implications for quantum de Sitter spacetime and for cosmic inflation are briefly discussed.
We review the covariant canonical formalism initiated by DAdda, Nelson and Regge in 1985, and extend it to include a definition of form-Poisson brackets (FPB) for geometric theories coupled to $p$-forms, gauging free differential algebras. The form-Legendre transformation and the form-Hamilton equations are derived from a $d$-form Lagrangian with $p$-form dynamical fields $phi$. Momenta are defined as derivatives of the Lagrangian with respect to the velocities $dphi$ and no preferred time direction is used. Action invariance under infinitesimal form-canonical transformations can be studied in this framework, and a generalized Noether theorem is derived, both for global and local symmetries. We apply the formalism to vielbein gravity in $d=3$ and $d=4$. In the $d=3$ theory we can define form-Dirac brackets, and use an algorithmic procedure to construct the canonical generators for local Lorentz rotations and diffeomorphisms. In $d=4$ the canonical analysis is carried out using FPB, since the definition of form-Dirac brackets is problematic. Lorentz generators are constructed, while diffeomorphisms are generated by the Lie derivative. A doubly covariant hamiltonian formalism is presented, allowing to maintain manifest Lorentz covariance at every stage of the Legendre transformation. The idea is to take curvatures as velocities in the definition of momenta.