Is there a black hole minimum mass?

Applying the first and generalised second laws of thermodynamics for a realistic process of near critical black hole formation, we derive an entropy bound, which is identical to Bekenstein's one for radiation. Relying upon this bound, we derive an absolute minimum mass $\sim0.04 \sqrt{g_{*}}m_{\rm Pl}$, where $g_{*}$ and $m_{\rm Pl}$ is the effective degrees of freedom for the initial temparature and the Planck mass, respectively. Since this minimum mass coincides with the lower bound on masses of which black holes can be regarded as classical against the Hawking evaporation, the thermodynamical argument will not prohibit the formation of the smallest classical black hole. For more general situations, we derive a minimum mass, which may depend on the initial value for entropy per particle. For primordial black holes, however, we show that this minimum mass can not be much greater than the Planck mass at any formation epoch of the Universe, as long as $g_{*}$ is within a reasonable range. We also derive a size-independent upper bound on the entropy density of a stiff fluid in terms of the energy density.Comment: 4 pages, accepted for publication in Physical Review D, minor correctio

Hawking Radiation from Fluctuating Black Holes

Classically, black Holes have the rigid event horizon. However, quantum mechanically, the event horizon of black holes becomes fuzzy due to quantum fluctuations. We study Hawking radiation of a real scalar field from a fluctuating black hole. To quantize metric perturbations, we derive the quadratic action for those in the black hole background. Then, we calculate the cubic interaction terms in the action for the scalar field. Using these results, we obtain the spectrum of Hawking radiation in the presence of interaction between the scalar field and the metric. It turns out that the spectrum deviates from the Planck spectrum due to quantum fluctuations of the metric.Comment: 35pages, 4 figure

Casimir force for cosmological domain walls

We calculate the vacuum fluctuations that may affect the evolution of cosmological domain walls. Considering domain walls, which are classically stable and have interaction with a scalar field, we show that explicit symmetry violation in the interaction may cause quantum bias that can solve the cosmological domain wall problem.Comment: 15 pages, 2figure

Redshift Drift in LTB Void Universes

We study the redshift drift, i.e., the time derivative of the cosmological redshift in the Lema\^itre-Tolman-Bondi (LTB) solution in which the observer is assumed to be located at the symmetry center. This solution has often been studied as an anti-Copernican universe model to explain the acceleration of cosmic volume expansion without introducing the concept of dark energy. One of decisive differences between LTB universe models and Copernican universe models with dark energy is believed to be the redshift drift. The redshift drift is negative in all known LTB universe models, whereas it is positive in the redshift domain $z \lesssim 2$ in Copernican models with dark energy. However, there have been no detailed studies on this subject. In the present paper, we prove that the redshift drift of an off-center source is always negative in the case of LTB void models. We also show that the redshift drift can be positive with an extremely large hump-type inhomogeneity. Our results suggest that we can determine whether we live near the center of a large void without dark energy by observing the redshift drift.Comment: 16 pages, 2 figure

Enhanced spin Hall effect by tuning antidot potential: Proposal for a spin filter

We propose an efficient spin filter including an antidot fabricated on semiconductor heterostructures with strong spin-orbit interaction. The antidot creates a tunable potential on two-dimensional electron gas in the heterostructures, which may be attractive as well as repulsive. Our idea is based on the enhancement of extrinsic spin Hall effect by resonant scattering when the attractive potential is properly tuned. Numerical studies for three- and four-terminal devices indicate that the efficiency of the spin filter can be more than 50% by tuning the potential to the resonant condition.Comment: 11 pages, 10 figure

Long-range heteronuclear J-coupling constants in esters: Implications for 13C metabolic MRI by side-arm parahydrogen-induced polarization

Side-arm parahydrogen induced polarization (PHIP-SAH) presents a cost-effective method for hyperpolarization of 13C metabolites (e.g. acetate, pyruvate) for metabolic MRI. The timing and efficiency of typical spin order transfer methods including magnetic field cycling and tailored RF pulse sequences crucially depends on the heteronuclear J coupling network between nascent parahydrogen protons and 13C, post-parahydrogenation of the target compound. In this work, heteronuclear nJHC (1 < n ≤ 5) couplings of acetate and pyruvate esters pertinent for PHIP-SAH were investigated experimentally using selective HSQMBC-based pulse sequences and numerically using DFT simulations. The CLIP-HSQMBC technique was used to quantify 2/3-bond JHC couplings, and 4/5-bond JHC ≲ 0.5 Hz were estimated by the sel-HSQMBC-TOCSY approach. Experimental and numerical (DFT-simulated) nJHC couplings were strongly correlated (P < 0.001). Implications for 13C hyperpolarization by magnetic field cycling, and PH-INEPT and ESOTHERIC type spin order transfer methods for PHIP-SAH were assessed, and the influence of direct nascent parahydrogen proton to 13C coupling when compared with indirect homonuclear TOCSY-type transfer through intermediate (non-nascent parahydrogen) protons was studied by the density matrix approach

NMR Analysis of Poly(Lactic Acid) via Statistical Models

The physical properties of poly(lactic acid) (PLA) are influenced by its stereoregularity and stereosequence distribution, and its polymer stereochemistry can be effectively studied by NMR spectroscopy. In previously published NMR studies of PLA tacticity, the NMR data were fitted to pair-addition Bernoullian models. In this work, we prepared several PLA samples with a tin catalyst at different L,L-lactide and D,D-lactide ratios. Upon analysis of the tetrad intensities with the pair-addition Bernoullian model, we found substantial deviations between observed and calculated intensities due to the presence of transesterification and racemization during the polymerization processes. We formulated a two-state (pair-addition Bernoullian and single-addition Bernoullian) model, and it gave a better fit to the observed data. The use of the two-state model provides a quantitative measure of the extent of transesterification and racemization, and potentially yields useful information on the polymerization mechanism

Growth of primordial black holes in a universe containing a massless scalar field

The evolution of primordial black holes in a flat Friedmann universe with a massless scalar field is investigated in fully general relativistic numerical relativity. A primordial black hole is expected to form with a scale comparable to the cosmological apparent horizon, in which case it may go through an initial phase with significant accretion. However, if it is very close to the cosmological apparent horizon size, the accretion is suppressed due to general relativistic effects. In any case, it soon gets smaller than the cosmological horizon and thereafter it can be approximated as an isolated vacuum solution with decaying mass accretion. In this situation the dynamical and inhomogeneous scalar field is typically equivalent to a perfect fluid with a stiff equation of state $p=\rho$. The black hole mass never increases by more than a factor of two, despite recent claims that primordial black holes might grow substantially through accreting quintessence. It is found that the gravitational memory scenario, proposed for primordial black holes in Brans-Dicke and scalar-tensor theories of gravity, is highly unphysical.Comment: 24 pages, accepted for publication in Physical Review