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Cherenkov radiation may occur whenever the source is moving faster than the waves it generates. In a radiation dominated universe, with equation-of-state w = 1/3, we have recently shown that the Bardeen scalar-metric perturbations contribute to the linearizedWeyl tensor in such a manner that its wavefront propagates at acoustic speed √w = 1/√3. In this work, we explicitly compute the shape of the Bardeen Cherenkov cone and wedge generated respectively by a supersonic point mass (approximating a primordial black hole) and a straight Nambu-Goto wire (approximating a cosmic string) moving perpendicular to its length. When the black hole or cosmic string is moving at ultra-relativistic speeds, we also calculate explicitly the sudden surge of scalar-metric induced tidal forces on a pair of test particles due to the passing Cherenkov shock wave. These forces can stretch or compress, depending on the orientation of the masses relative to the shock front's normal. [ABSTRACT FROM AUTHOR]

We show, through an explicit calculation of the relevant Green’s functions, that the transverse-traceless (TT) portion of the gravitational perturbations of Minkowski spacetime and of spatially flat cosmologies with a constant equation-of-state w receive contributions from their isolated matter source(s) outside the past null cone of the observer. This implies the TT gravitational wave (GW) cannot be a standalone observable—despite widespread (apparent) claims in the gravitational wave literature to the contrary. About a Minkowski background, all 4 of the gauge-invariant variables—the two scalars, one vector and tensor—play crucial roles to ensure the spatial tidal forces encoded within the gauge-invariant linearized Riemann tensor are causal. These gravitational tidal forces do not depend solely on the TT graviton but rather on the causal portion of its acceleration. However, in the far zone radiative limit, the flat spacetime ‘TT’ graviton Green’s function does reduce to the causal ‘tt’ ones, which are the ones commonly used to compute gravitational waveforms. Similar remarks apply to the spin-1 photon; for instance, the electric field does not depend solely on the photon, but is the causal part of its velocity. As is known within the quantum theory of photons and linearized gravitons, there are obstacles to the construction of simultaneously gauge-invariant and Lorentz-covariant descriptions of these massless spin-1 and spin-2 states. Our results transparently demonstrate that the quantum operators associated with the helicity-1 photon and helicity-2 linear graviton both violate micro-causality: namely, they do not commute outside the light cone in flat and cosmological spacetimes. [ABSTRACT FROM AUTHOR]

Co-axial electrospraying is a simple and versatile process, achieving liquid atomization through electrical forces, to produce core–shell-structured nano/microparticles. Despite its advantages in terms of preparing particles with sizes ranging from 300–500 µm down to 50–70 nm, the optimization of the co-axial electrospraying process remains a challenge. In this study, we investigated experimentally the effects of processing parameters on the size distribution of co-axially electroprayed poly(d,l-lactide-co-glycolide) (PLGA) microparticles. The effects of various parameters, including voltage, flow rate ratio, travel distance, and polymeric concentration, were examined using a factorial experimental design. It was found that the particle size of electrosprayed core–shell microparticles decreases with the voltage and travel distance, and it increases with the flow rate of the solutions and the PLGA concentration in the solutions. Furthermore, to verify the existence of proteins in the co-axially electrosprayed microparticles, PLGA was used as the shell, and recombinant enhanced green fluorescent protein (reGFP) was employed as the core material. Transmission electron microscopy (TEM) and laser scanning confocal microscopy (LSCM) were employed to confirm the core–shell structure of the microparticles. The experimental results demonstrated that under optimum conditions, core–shell-structured microparticles can be successfully prepared using a protein with high activity at the core. [ABSTRACT FROM AUTHOR]