International Journal of Modern Physics D

This author's recent proposal of interferometric tests of Planck-scale-related properties of spacetime is here revisited from a strictly phenomenological viewpoint. The results announced previously are rederived using elementary dimensional considerations. The dimensional analysis is then extended to the other two classes of experiments (observations of neutral kaons at particle accelerators and observations of the gamma rays we detect from distant astrophysical sources) which have been recently considered as opportunities to explore "foamy" properties of spacetime. The emerging picture suggests that there is an objective and intuitive way to connect the sensitivities of these three experiments with the Planck length. While in previous studies the emphasis was always on some quantum-gravity scenario and the analysis was always primarily aimed at showing that the chosen scenario would leave a trace in a certain class of doable experiments, the analysis here reported takes as starting point the experiments and, by relating in a direct quantitative way the sensitivities to the Planck length, provides a model-independent description of the status of Planck-length phenomenology.

In this work we introduce two experimental proposals that could shed some light upon the inertial properties of intrinsic spin. In particular we will analyze the role that the gravitomagnetic field of the Earth could have on a quantum system with spin 1/2. We will deduce the expression for Rabi transitions, which depend, explicitly, on the coupling between the spin of the quantum system and the gravitomagnetic field of the Earth. Afterwards, the continuous measurement of the energy of the spin-1/2 system is considered, and an expression for the emerging quantum Zeno effect is obtained. Thus, it will be proved that gravitomagnetism, in connection with spin-1/2 systems, could induce not only Rabi transitions but also a quantum Zeno effect.

The decay of the string perturbative vacuum, if triggered by a suitable, duality-breaking dilaton potential, can efficiently proceed via the parametric amplification of the Wheeler–De Witt wave function in superspace, and can appropriately describe the birth of our Universe as a quantum process of pair production from the vacuum.

The appearance of new fundamental forces and extra-dimensional modifications to gravity in extensions of the Standard Model has motivated considerable interest in testing Newtonian gravity at ultra-short distances (≲10^-4m). New experiments which attempt to probe these distances will encounter formidable new obstacles, including large intermolecular forces, which could obscure weak new forces or small modifications to Newton's law of gravity. Here we will review the motivation for conducting such ultra-short distance gravity experiments, and discuss some of the new problems that may arise in future experiments. Finally, we suggest a schematic design for a null experiment which would address some of these problems using the "iso-electronic" effect.

Recent results of quantum field theory on a curved spacetime suggest that extremal black holes are not thermal objects and that the notion of zero temperature is ill-defined for them. If this is correct, one may have to go to a full semiclassical theory of gravity, including backreaction, in order to make sense of the third law of black hole thermodynamics. Alternatively it is possible that we shall have to drastically revise the status of extremality in black hole thermodynamics.

By exploring the relationship between the propagation of electromagnetic waves in a gravitational field and the light propagation in a refractive medium, it is shown that, in the presence of a positive cosmological constant, the velocity of light will be smaller than its special relativity value. Then, restricting again to the domain of validity of geometrical optics, the same result is obtained in the context of wave optics. It is argued that this phenomenon and the anisotropy in the velocity of light in a gravitational field are produced by the same mechanism.

Recent astrophysical observations seem to indicate that the cosmological constant is small but nonzero and positive. The old cosmological constant problem asks why it is so small; we must now ask, in addition, why it is nonzero (and is in the range found by recent observations), and why it is positive. In this essay, we try to kill these three metaphorical birds with one stone. That stone is the unimodular theory of gravity, which is the ordinary theory of gravity, except for the way the cosmological constant arises in the theory. We argue that the cosmological constant becomes dynamical, and eventually, in terms of the cosmic scale factor R(t), it takes the form Λ(t)=Λ(t₀)(R(t₀)/R(t))², but not before the epoch corresponding to the redshift parameter z~1.

We give an account of the physical behaviour of a quasiparticle horizon due to non-Lorentz invariant modifications of the effective spacetime experienced by the quasiparticles ("matter") for high momenta. By introducing a "relativistic" conserved energy–momentum tensor, we derive quasi-equilibrium states of the fluid across the "Landau" quasiparticle horizon at temperatures well above the quantum Hawking temperature. Nonlinear dispersion of the quasiparticle energy spectrum is instrumental for quasiparticle communication and exchange across the horizon. It is responsible for the establishment of the local thermal equilibrium across the horizon with the Tolman temperature being inhomogeneous behind the horizon. The inhomogeneity causes relaxation of the quasi-equilibrium states due to scattering of thermal quasiparticles, which finally leads to a shrinking black hole horizon. This process serves as the classical thermal counterpart of the quantum effect of Hawking radiation and will allow for an observation of the properties of the horizon at temperatures well above the Hawking temperature. We discuss the thermal entropy related to the horizon. We find that only the first nonlinear correction to the energy spectrum is important for the thermal properties of the horizon. They are fully determined by an energy of order E_Planck(T/E_Planck)^1/3, which is well below the Planck energy scale E_Planck, so that Planck scale physics is not involved in determining thermal quantities related to the horizon.

The chemical evolution of nascent quark matter core in a newborn compact neutron star is studied in presence of a strong magnetic field. The effective rate of strange quark production in degenerate quark matter core in presence of strong magnetic fields is obtained. The investigations show that in presence of strong magnetic field the quark matter core becomes energetically unstable and hence a first/higher order deconfinement transition to quark matter at the centre of a compact neutron star under such circumstances is absolutely impossible. The critical strength of magnetic field at the central core to make the system energetically unstable with respect to dense nuclear matter is found to be ~4.4 × 10¹³ G. This is the typical strength at which the Landau levels for electrons are populated. The other kind of phase transformations at such high density and ultra strong magnetic field environment are discussed.

We have found exact constant solutions for the cosmological density parameter using a generalization of general relativity that incorporates a cosmic time-variation of the velocity of light in vacuum and the Newtonian gravitation constant. We have determined the conditions when these solutions are attractors for an expanding universe and solved the problems of the Standard Big Bang model for perfect fluids.