Can sterile neutrinos be the dark matter?

We use the Ly-alpha forest power spectrum measured by the Sloan Digital Sky Survey and high-resolution spectroscopy observations in combination with cosmic microwave background and galaxy clustering constraints to place limits on a sterile neutrino as a dark matter candidate in the warm dark matter scenario. Such a neutrino would be created in the early Universe through mixing with an active neutrino and would suppress structure on scales smaller than its free-streaming scale. We ran a series of high-resolution hydrodynamic simulations with varying neutrino masses to describe the effect of a sterile neutrino on the Ly-alpha forest power spectrum. We find that the mass limit is m(s) >13 keV at 95% C.L. (9 keV at 99.9%), which is above the upper limit allowed by x-ray constraints, excluding this candidate from being all of the dark matter in this model.     

Can non-propagating hydrodynamic solitons be forced to move?

Development of technologies based on localized states depends on our ability to manipulate and control these nonlinear structures. In order to achieve this, the interactions between localized states and control tools should be well modelled and understood. We present a theoretical and experimental study for handling non-propagating hydrodynamic solitons in a vertically driven rectangular water basin, based on the inclination of the system. Experiments show that tilting the basin induces non-propagating solitons to drift towards an equilibrium position through a relaxation process. Our theoretical approach is derived from the parametrically driven damped nonlinear Schr?dinger equationwhich models the system. The basin tilting effect is modelled by promoting the parameters that characterize the system, e.g. dissipation, forcing and frequency detuning, as space dependent functions. A motion law for these hydrodynamic solitons can be deduced from these assumptions. The model equation, which includes a constant speed and a linear relaxation term, nicely reproduces the motion observed experimentally.     

Can Quantum Theory be Applied to the Universe as a Whole?

I argue that quantum theory can, and in fact must, be applied to the Universe as a whole. After a general introduction, I discuss two concepts that are essential for my chain of arguments: the universality of quantum theory and the emergence of classical behaviors by decoherence. A further motivation is given by the open problem of quantum gravity. I then present the main ingredients of quantum cosmology and discuss their relevance for the interpretation of quantum theory. I end with some brief epistemological remarks.     

Can Quantum Gravity be Exposed in the Laboratory?

I propose an experiment that may be performed, with present low temperature and cryogenic technology, to reveal Wheeler’s quantum foam. It involves coupling an optical photon’s momentum to the center of mass motion of a macroscopic transparent block with parameters such that the latter is displaced in space by approximately a Planck length. I argue that such displacement is sensitive to quantum foam and will react back on the photon’s probability of transiting the block. This might allow determination of the precise scale at which quantum fluctuations of space–time become large, and so differentiate between the brane-world and the traditional scenarios of spacetime.     

Can massive gravitons be an alternative to dark energy?

In this work, we explore some cosmological implications of the model proposed by M. Visser in 1998. In his approach, Visser intends to take into account mass for the graviton by means of an additional bimetric tensor in the Einstein?s field equations. Our study has shown that a consistent cosmological model arises from the Visser?s approach. The most interesting feature is that an accelerated expansion phase naturally emerges from the cosmological model, and we do not need to postulate any kind of dark energy to explain the current observational data for distant type Ia supernovae (SNIa).     

Can the photon velocity be derived from the Klein-Gordon equation?

In their most recent article, Grado-Caffaro et al. have addressed the question of the ‘photon velocity’. They have expressed the photon velocity in terms of the wavefunctions of the Klein-Gordon equation (Grado-Caffaro and Grado-Caffaro [4]). In this note, we closely follow their work and explicitly obtain the photon velocity using the free solutions of the Klein-Gordon equation. It is shown that the plane wave solutions give rise to six possible values of the photon velocity. Two of these solutions are the most expected (v=±c). The remaining four solutions, the real pair ±0.786c and the imaginary pair ±1.272ic are difficult to comprehend.     

Can the states of the W-class be suitable for teleportation?

Entangled states of the W-class are considered as a quantum channel for teleportation of an entangled state and as well the state to be teleported via a multiparticle quantum channel. Using an introduced unitary transformation in the teleportation schemes based on the multiparticle Greenberger–Horne–Zeilinger channel it is found a set of protocols main feature of which is a collection of non-local recovering operators.     

Can the baryon number density and the cosmological constant be interrelated?

A toy model is proposed in which the cosmological constant and the baryon number density of the Universe are interrelated. The model combines the mechanism of Dimopoulos and Susskind [S. Dimopoulos, L. Susskind, Phys. Rev. D 18 (1978) 4500] in which the baryon number density of the Universe is generated by the time-dependence of the phase of a complex scalar field, i.e. its ‘angular momentum’ in the two-dimensional complex field space, with that of Yoshimura [M. Yoshimura, Phys. Lett. B 608 (2005) 183, hep-ph/0410183] in which the ‘centrifugal force’ due to the ‘angular momentum’ pushes the vacuum expectation value of the scalar field out of a negative potential minimum and provides a small but positive cosmological constant. Unfortunately, our model fails to relate the smallness of the two numbers directly, requiring a fine-tuning of the negative potential minimum.     

Can a Higgs boson be produced at the LHC?

A simple model is designed to simulate, by using the mean free path method, the probability of Higgs boson production at the Large Hadron Collider (LHC). The probability that the colliding particles could get close to a given distance with different colliding energies is discussed in this model. Calculated results imply that the probability of producing a Higgs boson is near zero according to the existing theoretical mechanism for Higgs boson production.     

Can a light beam be considered to be the sum of a completely polarized and a completely unpolarized beam?

In a classic paper that may be regarded as the starting point of polarization optics, G. G. Stokes [Trans. Cambridge Philos. Soc.9, 399 (1852)] presented a theorem according to which any light beam is equivalent to the sum of two light beams, one of which is completely polarized and the other completely unpolarized. We show that Stokes' proof of this theorem is flawed. We present a condition for the theorem to be valid.     

Can Spacetime be a Condensate?

We explore further the proposal [Hu, B. L. (1996). General relativity as geometro-hydrodynamics. (Invited talk at the Second Sakharov Conference, Moscow, May 1996); gr-qc/9607070.] that general relativity is the hydrodynamic limit of some fundamental theories of the microscopic structure of spacetime and matter, i.e., spacetime described by a differentiable manifold is an emergent entity and the metric or connection forms are collective variables valid only at the low-energy, long-wavelength limit of such micro-theories. In this view it is more relevant to find ways to deduce the microscopic ingredients of spacetime and matter from their macroscopic attributes than to find ways to quantize general relativity because it would only give us the equivalent of phonon physics, not the equivalents of atoms or quantum electrodynamics.It may turn out that spacetime is merely a representation of certain collective state of matter in some limiting regime of interactions, which is the view expressed by Sakharov [Sakharov, A. D. (1968). Soviet Physics-Doklady 12, 1040–1041; Sakharov, A. D. (1967). Vacuum quantum fluctuations in curved space and the theory of gravitation. Doklady Akad. Nauk S.S.R. 177, 70; Adler, S. L. (1982). Reviews of Modern Physics 54, 729]. In this talk, working within the conceptual framework of geometro-hydrodynamics, we suggest a new way to look at the nature of spacetime inspired by Bose–Einstein condensate (BEC) physics. We ask the question whether spacetime could be a condensate, even without the knowledge of what the‘atom of spacetime’ is. We begin with a summary of the main themes for this new interpretation of cosmology and spacetime physics, and the ‘bottom-up’ approach to quantum gravity. We then describe the ‘Bosenova’ experiment of controlled collapse of a BEC and our cosmology-inspired interpretation of its results. We discuss the meaning of a condensate in different context. We explore how far this idea can sustain, its advantages and pitfalls, and its implications on the basic tenets of physics and existing programs of quantum gravity.     

Can Hawking temperatures be negative?

It has been widely believed that the Hawking temperature for a black hole is uniquely determined by its metric and positive. But, I argue that this might not be true in the recently discovered black holes which include the exotic black holes and the black holes in the three-dimensional higher curvature gravities. I argue that the Hawking temperatures, which are measured by the quantum fields in thermal equilibrium with the black holes, might not be the usual Hawking temperature but the new temperatures that have been proposed recently and can be negative. The associated new entropy formulae, which are defined by the first and second laws of thermodynamics, versus the black hole masses show some genuine effects of the black holes which do not occur in the spin systems. Some cosmological implications and physical origin of the discrepancy with the standard analysis are noted also.