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The cosmological constant and dark energy
 Reviews of Modern Physics
, 2003
"... Physics invites the idea that space contains energy whose gravitational effect approximates that of Einstein’s cosmological constant, Λ; nowadays the concept is termed dark energy or quintessence. Physics also suggests the dark energy could be dynamical, allowing the arguably appealing picture that ..."
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Physics invites the idea that space contains energy whose gravitational effect approximates that of Einstein’s cosmological constant, Λ; nowadays the concept is termed dark energy or quintessence. Physics also suggests the dark energy could be dynamical, allowing the arguably appealing picture that the dark energy density is evolving to its natural value, zero, and is small now because the
Cosmological Constant  the Weight of the Vacuum
 Phys. Rept
, 2003
"... Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G�/c 3) ≈ 10 −123. This review discusses several aspects of the cosmological constant both from the cosmological (sections 1–6) and field theoretical (sections 7–11) perspectives. The f ..."
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Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G�/c 3) ≈ 10 −123. This review discusses several aspects of the cosmological constant both from the cosmological (sections 1–6) and field theoretical (sections 7–11) perspectives. The first section introduces the key issues related to cosmological constant and provides a brief historical overview. This is followed by a summary of the kinematics and dynamics of the standard Friedmann model of the universe paying special attention to features involving the cosmological constant. Section 3 reviews the observational evidence for cosmological constant, especially the supernova results, constraints from the age of the universe and a few others. Theoretical models (quintessence, tachyonic scalar field,...) with evolving cosmological ‘constant ’ are described from different perspectives in the next section. Constraints on dark energy from structure formation and from CMBR anisotropies are discussed in the next two sections. The latter part of the review (sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant. Section 7 provides some alternative interpretations of the cosmological constant which could have a bearing on the possible solution to the problem. Several relaxation mechanisms have been suggested in the literature to reduce the cosmological constant to the currently observed value and some of these attempts are described in section 8. Next section gives a brief description of the geometrical structure of the de Sitter spacetime and the thermodynamics of the de Sitter universe is taken up in section 10. The last section deals with the role of string theory in the cosmological constant problem.
Nonsymmetric gravity theories: inconsistencies and a cure, Phys
 Rev. D
, 1993
"... Motivated by the apparent dependence of string σ–models on the sum of spacetime metric and antisymmetric tensor fields, we reconsider gravity theories constructed from a nonsymmetric metric. We first show that all such “geometrical ” theories homogeneous in second derivatives violate standard physic ..."
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Motivated by the apparent dependence of string σ–models on the sum of spacetime metric and antisymmetric tensor fields, we reconsider gravity theories constructed from a nonsymmetric metric. We first show that all such “geometrical ” theories homogeneous in second derivatives violate standard physical requirements: ghostfreedom, absence of algebraic inconsistencies or continuity of degreeoffreedom content. This nogo result applies in particular to the old unified theory of Einstein and its recent avatars. However, we find that the addition of nonderivative, “cosmological” terms formally restores consistency by giving a mass to the antisymmetric tensor field, thereby transmuting it into a fifthforcelike massive vector but with novel possible matter couplings. The resulting macroscopic models also exhibit “van der Waals”type gravitational effects, and may provide useful phenomenological foils to general relativity. I. Introduction. It is a remarkable historical coincidence that modern string theory can be interpreted as reviving an ancient attempt at geometric unification of forces. Specifically, we note that (ignoring the 1 dilaton for simplicity) the string σ–model action in conformal gauge is just
On the interpretation of the redshift in a static gravitational field
 Am. J. Phys
"... The classical phenomenon of the redshift of light in a static gravitational potential, usually called the gravitational redshift, is described in the literature essentially in two ways: on the one hand the phenomenon is explained through the behaviour of clocks which run the faster the higher they a ..."
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The classical phenomenon of the redshift of light in a static gravitational potential, usually called the gravitational redshift, is described in the literature essentially in two ways: on the one hand the phenomenon is explained through the behaviour of clocks which run the faster the higher they are located in the potential, whereas the energy and frequency of the propagating photon do not change with height. The light thus appears to be redshifted relative to the frequency of the clock. On the other hand the phenomenon is alternatively discussed (even in some authoritative texts) in terms of an energy loss of a photon as it overcomes the gravitational attraction of the massive body. This second approach operates with notions such as the “gravitational mass ” or the “potential energy ” of a photon and we assert that it is misleading. We do not claim to present any original ideas or to give a comprehensive review of There are two kinds of photon redshift known in the literature, a gravitational and a cosmological one. Though in the General Relativity framework the two shifts can be described very similarly, in the literature they are usually discussed separately. The cosmological redshift
Axiomatic Geometric Formulation of Electromagnetism with only one axiom: the Field Equation for the bivector Field F with an Explanation of the TroutonNoble Experiment
"... In this paper we present an axiomatic, geometric, formulation of electromagnetism with only one axiom: the field equation for the Faraday bivector field F. This formulation with F field is a selfcontained, complete and consistent formulation that dispenses with either electric and magnetic fields o ..."
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In this paper we present an axiomatic, geometric, formulation of electromagnetism with only one axiom: the field equation for the Faraday bivector field F. This formulation with F field is a selfcontained, complete and consistent formulation that dispenses with either electric and magnetic fields or the electromagnetic potentials. All physical quantities are defined without reference frames, the absolute quantities, i.e., they are geometric four dimensional (4D) quantities or, when some basis is introduced, every quantity is represented as a 4D coordinatebased geometric quantity comprising both components and a basis. The new observer independent, expressions for the stressenergy vector T(n) (1vector), the energy density U (scalar), the Poynting vector S and the momentum density g (1vectors), the angular momentum density M (bivector) and the Lorentz force K (1vector) are directly derived from the field equation for F. The local conservation laws are also directly derived from that field equation. The 1vector Lagrangian with the F field as a 4D absolute quantity is presented; the interaction term is written in terms of F and not, as usual, in terms of A. It is shown that this geometric formulation is in a full agreement with the TroutonNoble experiment. Key words: electromagnetism with bivector field F, the TroutonNoble experiment 1.
Conformal transformations in classical gravitational theories and cosmology
, 1999
"... In recent years, the use of conformal transformation techniques has become widespread in the literature on gravitational theories alternative to general relativity, on cosmology, and on nonminimally coupled scalar fields. Tipically, the transformation to the Einstein frame is generated by a fundamen ..."
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In recent years, the use of conformal transformation techniques has become widespread in the literature on gravitational theories alternative to general relativity, on cosmology, and on nonminimally coupled scalar fields. Tipically, the transformation to the Einstein frame is generated by a fundamental scalar field already present in the theory. In this context, the problem of which conformal frame is the physical one has to be dealt with and, in the general case, it has been clarified only recently; the formulation of a theory in the “new ” conformal frame leads to departures from canonical Einstein gravity. In this article, we review the literature on conformal transformations in classical gravitational theories and in cosmology, seen both as purely mathematical tools and as maps with physically relevant aspects. It appears particularly urgent to refer the analysis of experimental tests of Brans–Dicke and scalar–tensor theories of gravity, as well as the predictions of cosmological inflationary scenarios, to the physical conformal frame, in order to have a meaningful comparison with the observations.
On radar time and the twin “paradox
"... In this paper we apply the concept of radar time (popularised by Bondi in his work on kcalculus) to the wellknown relativistic twin ‘paradox’. Radar time is used to define hypersurfaces of simultaneity for a class of travelling twins, from the ‘Immediate Turnaround ’ case, through the ‘Gradual Tu ..."
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In this paper we apply the concept of radar time (popularised by Bondi in his work on kcalculus) to the wellknown relativistic twin ‘paradox’. Radar time is used to define hypersurfaces of simultaneity for a class of travelling twins, from the ‘Immediate Turnaround ’ case, through the ‘Gradual Turnaround’ case, to the ‘Uniformly Accelerating ’ case. We show that this definition of simultaneity is independent of choice of coordinates, and assigns a unique time to any event (with which the travelling twin can send and receive signals), resolving some common misconceptions.
2004, “Electrodynamics of hyperbolically accelerated charges V. The field of a charge in the Rindler space and the Milne space,” Ann. Phys
 N.Y
"... The question whether a hyperbolically moving charge emits radiation is discussed. In order to arrive at an unambiguous answer the question is considered from several points of view. The power spectrum of the electromagnetic field due to a hyperbolically moving charge is deduced. Also we analyse the ..."
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The question whether a hyperbolically moving charge emits radiation is discussed. In order to arrive at an unambiguous answer the question is considered from several points of view. The power spectrum of the electromagnetic field due to a hyperbolically moving charge is deduced. Also we analyse the field in order to clarify whether it contains a radiation zone. Our conclusion is that a charge with constant proper acceleration emits radiation in spite of the fact that the Poynting vector of the field vanishes everywhere in the instantaneous inertial rest frame of the charge. 2000 Academic Press 1.
M.L.Ruggiero, Space Geometry of Rotating Platforms: an Operational Approach
, 207
"... We study the space geometry of a rotating disk both from a theoretical and operational approach; in particular we give a precise definition of the space of the disk, which is not clearly defined in the literature. To this end we define an extended 3space, which we call relative space: it is recogni ..."
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We study the space geometry of a rotating disk both from a theoretical and operational approach; in particular we give a precise definition of the space of the disk, which is not clearly defined in the literature. To this end we define an extended 3space, which we call relative space: it is recognized as the only space having an actual physical meaning from an operational point of view, and it is identified as the ’physical space of the rotating platform’. Then, the geometry of the space of the disk turns out to be non Euclidean, according to the early Einstein’s intuition; in particular the Born metric is recovered, in a clear and self consistent context. Furthermore, the relativistic kinematics reveals to be self consistent, and able to solve the Ehrenfest’s paradox without any need of dynamical considerations or ad hoc assumptions.