Spin and Torsion in GravitationV PREFACE This book deals with spin and torsion in gravitation. The earlier on "Introduction to Gravitation" (World Scientific, 1985) gave a detailed exposition of the General Theory of Relativity developed by Einstein and others: in this book (of the year 1985) there is a chapter on the Einstein-Cartan theory and now we like to extend exhaustively this theory, that is the introduction of the spin in General Relativity, considering all possible physical consequences, spin being another universal attribute of matter besides the mass.The spin of elementary particles was to play a profound role in atomic, nuclear and particle physics. The unity of special relativity and quantum mechanics through the Dirac equation led to spectacular new results in physics: the prediction of antiparticles and the intrinsic magnetic moment of elementary particles, to name e few. The spin of elementary particles manifested itself in several new effects in fundamental interactions such as splitting of nuclear energy levels and non-degeneracy of hadronic states in strong interactions, parity violation in weak interactions etc. The fact that the effects of spin when considered in gravitational interactions can also lead to several interesting physical phenomena in both the micro and macroworld is not so well known. The literature is mostly confined to specialized articles read by only those few directly working on the subject. Even most physicists working on gravitation theory are not much aware of the interesting consequences of spin modified gravitational effects, especially those caused by torsion which is the geometric effect of spin in space-time (analogously to mass causing space-time curvature). Spin and Torsion in Gravitation VIThe book we are contemplating could fill this gap and give an exposition of both the old and new results of spin and torsion effects on gravitational interactions with implications for particle physics, cosmology etc.The stress would be more on the physical aspects with a discussion of measurable effects in relation to other areas of physics (with a discussion of orders of magnitude and number involved). We would thus have for instance a discussion of the analogy between torsion and magnetism (with consequences for astrophysics and cosmology), we will consider the Dirac equation in general relativity with torsion developed in a gauge theoretic manner with its implications for weak interactions and for strong interactions. We could have clear cut alternative ways of unifying gravity with electroweak and strong interactions by an energy dependent spin torsion coupling constant. The idea that all interactions can be understood as originating in spin curvature coupling is discussed.The Maxwell equations when coupled to gravity in a space-time with torsion also gives rise to novel effects, there being effects due to polarisation of photons and neutrinos. We have analogy of Faraday polarisation, and other gravimagnetic effects distinct from Lense-Thirring precession, wmith is of...

The nature of dark matter (DM) and dark energy (DE) which is supposed to constitute about 95% of the energy density of the universe is still a mystery. There is no shortage of ideas regarding the nature of both. While some candidates for DM are clearly ruled out, there is still a plethora of viable particles that fit the bill. In the context of DE, while current observations favour a cosmological constant picture, there are other competing models that are equally likely. This paper reviews the different possible candidates for DM including exotic candidates and their possible detection. This review also covers the different models for DE and the possibility of unified models for DM and DE. Keeping in mind the negative results in some of the ongoing DM detection experiments, here we also review the possible alternatives to both DM and DE (such as MOND and modifications of general relativity) and possible means of observationally distinguishing between the alternatives.Comment: 55 page

A Weyl type of action which is scale free and quadratic in the curvature is suggested for strong gravity. The corresponding field equations have solutions which imply confinement. At the QCD scale, the scale invariance is broken inducing a Hilbert type term which describes the short distance behavior. This approach may provide a strong gravity basis for QCD.

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Analogies between the properties of black holes (in the framework of strong gravity) and those of elementary particles are discussed especially in connection with recent works on black holes with gauge charges and blackhole thermodynamics.Recent renewal' ' of interest in general relativity playing some role in elementary-particle physics has drawn attention to certain striking resemblances between the properties of black holes and those of elementary particles. For instance, black holes are characterized by only a few observable parameters such as mass, angular momentum, and charge. These are the measurable parameters for an elementary particle where these quantities occur in discrete or quantized units. One could picture particles as quantum black holes of the strong gravity field (i.e. , mediated by massive spin-2) particles as was done in Refs. 2, 3, 6, 7, and 8. Moreover, it is known that a charged rotating black hole, a Kerr-Newman black hole, has a gyromagnetic ratio of 2, the same value as that for an elementary particle. Although they have a magnetic moment, charged black holes with angular momentum do not have an electric dipole moment, which is also true for elementary particles. In a recent paper, Tennakone' has pictured the proton to be a black-hole singularity of the Reissner-Nordstrom metric in the strong gravitational field, assuming that the usual, results of general relativity are applicable in the case of strong gravity.If the structure of space-time in the immediate vicinity of hadrons is presumed to be determined by strong gravity, it is natural to replace the Newtonian constant G"by the strong gravitational coupling constant Gf, the dimensionless constant then being of the same magnitude as the strong-interaction dimensionless constant. Now the Einstein field equations G""=KT "relate a geometrical invariant quantity (i.e. , the Einstein tensor G"") on the left-hand side to an invariant physical quantity (i.e. , the conserved energy-momentum tensor T ") on the right-hand side through a proportionality (coupling) constant v. It must be emphasized that the derivation of the equation places no restriction whatsoever on the numerical value of the constant For instance, in the standard derivation of the field equation from an action principle with the Lagrangian density 2 = v 'Rl-g+ Z [where R is the curvature scalar and g = det(g, "), 2 = matter Lagrangian density], x is a factor of dimensions g 'cm ' sec' whose numerical value is entirely undetermined at this stage. It is only when one uses the field equations as the basis for a relativistic theory of gravitation that one relates to the Newtonian constant G". Since Einstein used his field equations to describe a. theory of gravitation, he chose~= 8mG"/c', so a.s to be consistent with Newtonian gravitation theory. Following this it has become customary to always relate K to the Newtonian constant as a matter of habit since all applications of general relativity have hitherto been to macrophysics.Apart from this there is no other compelling reason ...