Spike Profile Research Articles

Effect of Einasto spike and isothermal dark matter density profile on the geometry of wormhole in [formula omitted] gravity

In the present article, we uncover striking properties of traversable wormhole solutions generated under a newly developed f(ℜ,Lm) gravitational theory. Two distinct dark matter density profiles—the Einasto spike and the Pseudo-isothermal model—are employed to demonstrate the existence of stable wormhole geometries with throat radii of 1.18 km and 1.41 km, respectively. Through a rigorous analytical and plot-based survey, we delve deeper into the exotic matter characteristics of these wormhole structures and analyze their matter content in the context of energy conditions. Impressively, both wormhole models exhibit an adiabatic index greater than 1.34 (Γ>43), indicating their inherent stability. What is more, our study of the anisotropy parameter reveals the attractive nature of these wormhole solutions, a crucial property for their potential traversability. This novel work not only broadens our understanding of modified gravity theories but also highlights the remarkable possibilities of traversable wormholes, opening up new avenues in theoretical physics and in the exploration of exotic spacetime geometries.

Exploring dark matter spike distribution around the Galactic centre with stellar orbits

ABSTRACT Precise measurements of the stellar orbits around Sagittarius A* have established the existence of a supermassive black hole (SMBH) at the Galactic centre (GC). Due to the interplay between the SMBH and dark matter (DM), the DM density profile in the innermost region of the Galaxy, which is crucial for the DM indirect detection, is still an open question. Among the most popular models in the literature, the theoretical spike profile proposed by Gondolo & Silk (GS hereafter) is well adopted. In this work, we investigate the DM spike profile using updated data from the Keck and Very Large Telescope (VLT) telescopes considering that the presence of such an extended mass component may affect the orbits of the S-stars in the GC. We examine the radius and slope of the generalized Navarro–Frenk–White (NFW) spike profile, analyse the Einasto spike, and discuss the influence of DM annihilation on the results. Our findings indicate that an initial slope of γ ≳ 0.92 for the generalized NFW spike profile is ruled out at a 95 per cent confidence level. Additionally, the spike radius Rsp larger than 21.5 pc is rejected at 95 per cent probability for the Einasto spike with α = 0.17, which also contradicts the GS spike model. The constraints with the VLT/GRAVITY upper limits are also projected. Although the GS NFW spike is well constrained by the Keck and VLT observation of S2, an NFW spike with a weak annihilation cusp may still be viable, as long as the DM annihilation cross-section satisfies $\left\langle \sigma v \right\rangle \gtrsim 7.7\times 10^{-27}~{\rm cm^3\, s^{-1}} (m_{\rm DM}/100~{\rm GeV})$ at 95 per cent level.

Stability and dynamics of spike-type solutions to delayed Gierer-Meinhardt equations

<p style='text-indent:20px;'>For a specific set of parameters, we analyze the stability of a one-spike equilibrium solution to the one-dimensional Gierer-Meinhardt reaction-diffusion model with delay in the components of the reaction-kinetics terms. Assuming slow activator diffusivity, we consider instabilities due to Hopf bifurcation in both the spike position and the spike profile for increasing values of the time-delay parameter <inline-formula><tex-math id="M1">\begin{document}$ T $\end{document}</tex-math></inline-formula>. Using method of matched asymptotic expansions it is shown that the model can be reduced to a system of ordinary differential equations representing the position of the slowly evolving spike solution. The reduced evolution equations for the one-spike solution undergoes a Hopf bifurcation in the spike position in two cases: when the negative feedback of the activator equation is delayed, and when delay is in both the negative feedback of the activator equation and the non-linear production term of the inhibitor equation. Instabilities in the spike profile are also considered, and it is shown that the spike solution is unstable as <inline-formula><tex-math id="M2">\begin{document}$ T $\end{document}</tex-math></inline-formula> is increased beyond a critical Hopf bifurcation value <inline-formula><tex-math id="M3">\begin{document}$ T_H $\end{document}</tex-math></inline-formula>, and this occurs for the same cases as in the spike position analysis. In all cases, the instability in the profile is triggered before the positional instability. If however the degradation of activator is delayed, we find stable positional oscillations can occur in this system.</p>

Impact of relativistic corrections on the detectability of dark-matter spikes with gravitational waves

Black holes located within a dark matter cloud can create overdensity regions known as dark matter spikes. The presence of spikes modifies the gravitational-wave signals from binary systems through changes in the gravitational potential or dynamical friction effects. We assess the importance of including relativistic effects in both the dark matter distribution and the dynamical friction. As a first step we numerically calculate the particle dark matter spike distribution in full general relativity, using both Hernquist and Navarro-Frenk-White profiles in a Schwarzschild background, and we produce analytical fits to the spike profiles for a large range of scale parameters. Then we use a post-Newtonian prescription for the gravitational-wave dephasing to estimate the effect of relativistic corrections to the spike profile and to the dynamical friction. Finally we include the torques generated by dynamical friction in fast-to-generate relativistic models for circular extreme mass-ratio inspirals around a nonspinning black hole. We find that both types of relativistic corrections positively impact the detectability of dark matter effects, leading to higher dephasings and mismatches between gravitational-wave signals with and without dark matter spikes.

Primordial black hole dark matter in the presence of p-wave WIMP annihilation

We study the allowed primordial black hole (PBH) dark matter abundance in the mixed dark matter scenarios consisting of PBHs and self-annihilating weakly interacting massive particles (WIMPs) with a velocity dependent annihilation cross section. We first briefly illustrate how the WIMP dark matter halo profile changes for the velocity suppressed p-wave annihilation scenarios, compared with the familiar s-wave annihilation scenarios, and then discuss the PBH mass dependent upper bound on the allowed PBH dark matter abundance. The WIMPs can accrete onto a PBH to form an ultracompact minihalo with a spiky density profile. Such a spike is moderated in the central region of a halo because the WIMPs are annihilated away and this moderation is less effective for a smaller annihilation cross section. The WIMP core density becomes larger while the core radius becomes smaller for a velocity suppressed p-wave annihilation cross section than those for the s-wave annihilation scenarios. The annihilation cross section is dependent on the velocity which varies across the halo, and, in addition to the change of the WIMP density profile, another interesting feature is the PBH mass dependent bound on PBH dark matter abundance. This is in stark contrast to the s-wave annihilation scenarios where the PBH abundance bound is independent of the PBH mass. The allowed PBH dark matter fraction (with respect to the total dark matter abundance) is of order f PBH ≲ \U0001d4aa(10−7)(M⊙/MPBH)(−6+2γsp)/(3γsp+3) for the thermal relic p-wave dark matter with the mass 100 GeV where γsp is the slope index of the spike profile, to be compared with f PBH ≲ \U0001d4aa(10−9) for the corresponding thermal relic s-wave dark matter scenarios.

Vertical Characteristics of Winter Ozone Distribution within the Boundary Layer in Shanghai Based on Hexacopter Unmanned Aerial Vehicle Platform

Ozone is an important secondary air pollutant and plays different significant roles in regulating the formation of secondary organic aerosols. However, the characteristics of winter vertical ozone distributions have rarely been studied. In the winter of 2017, field experiments were performed in Shanghai, China using hexacopter unmanned aerial vehicle (UAV) platforms. The vertical profiles of ozone were obtained from 0–1200 m above the ground level. Results show that the UAV observations were reliable to capture the vertical variations of ozone. Vertical ozone profiles in the winter are classified into four categories: (1) well-mixed profile, (2) altitudinal increasing profile, (3) stratification profile, and (4) spike profile. Results show that although the average surface ozone level was relatively low, strong ozone variability and high ozone concentrations occurred at the upper air. The maximum observed ozone concentration was 220 ppb. In addition, using meteorological profiles and backward trajectories, we found that the ozone elevation aloft can be attributed to the downward transport of air flow from higher altitudes. Furthermore, ozone accumulation in the winter could be influenced by the horizontal transport of air masses for the northern part of China. This study successfully used hexacopter UAV platforms to perform vertical observations within the boundary layer. This provides systematic classification of winter ozone distribution within the boundary layer.

Black holes, dark matter spikes, and constraints on simplified models with t -channel mediators

The density spike of dark matter (DM) in the subparsec region near the supermassive black hole at the Galactic Center can provide potentially observable gamma-ray signals coming from DM annihilations. Taking Fermi-$LAT$ data for the gamma-ray flux from the point source 3FGL J1745.6-2859c (Sgr A*), we calculate the resulting constraints on generic models of DM, allowing for the possibility of a non-negligible velocity-dependent component of the annihilation cross section. We consider a variety of selections for the astrophysical parameters that describe the spike profile and find that the gamma-ray flux is strongly dependent on these selections, particularly the modelling of spike depletion effects due to gravitational interactions with baryons, which affect the spike radius and steepness profile. We calculate constraints on the DM parameter space for both attenuated and idealized spikes, considering different choices for the steepness profiles in each case. We find that for the most conservative selection of parameters, corresponding to a depleted spike with an NFW cusp profile, the gamma-ray flux for a 100 GeV thermal relic is lower than current observational constraints by several orders of magnitude. For more optimistic choices of parameters corresponding to spikes that have not been attenuated, bounds on DM masses can be obtained for a variety of choices of steepness profiles. We then specialize to a class of simplified models of fermionic DM that annihilate dominantly through the $t-$channel exchange of two scalar mediators with arbitrary mixing angle $\alpha$, and calculate the indirect detection constraints coming from the DM spike. Along the way, we discuss constraints on the astrophysical parameters describing the DM spike, taking the Galactic Center excess as a signal of DM annihilation.

Variations in Fusion Pore Formation in Cholesterol-Treated Platelets

Exocytosis is a highly regulated intercellular communication process involving various membrane proteins, lipids, and cytoskeleton restructuring. These components help control granule fusion with the cell membrane, creating a pore through which granular contents are released into the extracellular environment. Platelets are an ideal model system for studying exocytosis due to their lack of a nucleus, resulting in decreased membrane regulation in response to cellular changes. In addition, platelets contain fewer granules than most other exocytosing cells, allowing straightforward measurement of individual granule release with carbon-fiber microelectrode amperometry. This technique monitors the concentration of serotonin, an electroactive molecule found in the dense-body granules of platelets, released as a function of time, with 50 μs time resolution, revealing biophysical characteristics of the fundamental exocytotic process. Variations in fusion pore formation and closure cause deviations from the classic current versus time spike profile and may influence diffusion of serotonin molecules from the site of granule fusion. Physiologically, the delivery of smaller packets of chemical messengers or the prolonged delivery of chemical messengers may represent how cells/organisms tune biological response. The goals of this work are twofold: 1) to categorize secretion features that deviate from the traditional mode of secretion and 2) to examine how changing the cholesterol composition of the platelet membrane results in changes in the pore formation process. Results herein indicate that the expected traditional mode of release is actually in the minority of granule content release events. In addition, results indicate that as the cholesterol content of the plasma membrane is increased, pore opening is less continuous.

Tailored ramp wave generation in gas gun experiments

Gas guns are traditionally used as platforms to introduce a planar shock wave to a material using plate impact methods, generating states on the Hugoniot. The ability to deliver a ramp wave to a target during a gas gun experiment enables access to different regions of the equation-of-state surface, making it a valuable technique for characterising material behaviour. Previous techniques have relied on the use of multi-material impactors to generate a density gradient, which can be complex to manufacture. In this paper we describe the use of an additively manufactured steel component consisting of an array of tapered spikes which can deliver a ramp wave over ∼ 2 μs. The ability to tailor the input wave by varying the component design is discussed, an approach which makes use of the design freedom offered by additive manufacturing techniques to rapidly iterate the spike profile. Results from gas gun experiments are presented to evaluate the technique, and compared with 3D hydrodynamic simulations.

Dynamics and stability of spike-type solutions to a one dimensional Gierer–Meinhardt model with sub-diffusion

The dynamics and stability of spike-type patterns to a sub-diffusive Gierer–Meinhardt reaction–diffusion system is studied in a one dimensional spatial domain. A differential algebraic system (DAE) is derived to characterise the dynamics of an n-spike quasi-equilibrium pattern in the presence of sub-diffusion. With sub-diffusive effects it is shown that quasi-equilibrium spike patterns exist for diffusivity ratios asymptotically smaller than for the case of regular diffusion, and that the spikes approach their equilibrium locations at an algebraic, rather than exponential, rate in time.A new non-local eigenvalue problem (NLEP) is derived to examine the stability of an n-spike pattern. For a two spike pattern sub-diffusion has little effect on the competition instability threshold, whereas the threshold associated with an oscillatory instability of the spike profile increases significantly. Furthermore, for a multi-spike pattern it is shown that an asynchronous oscillatory instability of the spike profile, rather than a synchronous oscillatory instability characteristic of the case of regular diffusion, is the dominant instability when the anomaly index γ is below a certain threshold. Detailed numerical results are presented for the two spike case.

Antinociceptive Action of Oxytocin Involves Inhibition of Potassium Channel Currents in Lamina Ii Neurons of the Rat Spinal Cord

BackgroundGrowing evidence in the literature shows that oxytocin (OT) has a strong spinal anti-nociceptive action. Oxytocinergic axons originating from a subpopulation of paraventricular hypothalamic neurons establish synaptic contacts with lamina II interneurons but little is known about the functional role of OT with respect to neuronal firing and excitability.ResultsUsing the patch-clamp technique, we have recorded lamina II interneurons in acute transverse lumbar spinal cord slices of rats (15 to 30 days old) and analyzed the OT effects on action potential firing ability. In the current clamp mode, we found that bath application of a selective OT-receptor agonist (TGOT) reduced firing in the majority of lamina II interneurons exhibiting a bursting firing profile, but never in those exhibiting a single spike discharge upon depolarization. Interestingly, OT-induced reduction in spike frequency and increase of firing threshold were often observed, leading to a conversion of the firing profile from repetitive and delayed profiles into phasic ones and sometimes further into single spike profile. The observed effects following OT-receptor activation were completely abolished when the OT-receptor agonist was co-applied with a selective OT-receptor antagonist. In current and voltage clamp modes, we show that these changes in firing are strongly controlled by voltage-gated potassium currents. More precisely, transient IA currents and delayed-rectifier currents were reduced in amplitude and transient IA current was predominantly inactivated after OT bath application.ConclusionThis effect of OT on the firing profile of lamina II neurons is in good agreement with the antinociceptive and analgesic properties of OT described in vivo.

The stability and dynamics of a spike in the 1D Keller–Segel model

In the limit of a large mass M » 1 and on a finite interval of length 2L, an equilibrium spike solution to the classical Keller-Segel chemotaxis model with a linear chemotactic function is constructed asymptotically. By calculating an asymptotic formula for the translational eigenvalue for M » 1, it is shown that the equilibrium spike solution is unstable to translations of the spike profile. If in addition L » 1, the equilibrium spike is shown to be metastable as a result of an asymptotically exponentially small eigenvalue. For M » 1 and L » 1, an asymptotic ordinary differential equation for the metastable spike motion is derived that shows that the spike drifts exponentially slowly towards one of the boundaries of the domain. For a certain reduced Keller-Segel model, corresponding to a domain of small length, a solution with a spike at each of the two boundaries is constructed. This solution is found to be metastable, and it is shown that there is an exponentially slow exchange of mass between the two spikes that occurs over very long timescales. For arbitrary initial conditions, energy methods are used to show the global existence of solutions. The relationship between this reduced Keller-Segel model and a Burgers-type equation modelling the upward propagation of a flame front in a finite channel is emphasized. Full numerical computations are used to confirm the asymptotic results.

Morphological, stereological and radiological changes in pinealectomized chicken cervical vertebrae

Considerable evidence supports the hypothesis that melatonin plays an important role in osteogenesis. We carried out an experiment to investigate histological and radiological changes in the cervical vertebrae of pinealectomized chickens. Thirty new-hatched chicks were divided into two groups: pinealectomized group (n=15) and nonpinealectomized control (n=15). Surgical pinealectomies were performed in Hybro broiler chickens at the age of 3 days. At 8 wk, one animal from each group was examined using computed tomography scanner to obtain density histograms of four consecutive vertebrae between the 4th and 7th cervical vertebral segments of chickens. At the end of the study, all animals were killed for subsequent measurement of bone mineral density (BMD) and macroscopic, radiographic and histopathological evaluation of specimens. We found that the profile of the density histogram in nonpinealectomized control group showed a spike profile for the vertebral body, indicating an increase in the amount of higher density tissues in this region, while a plateau-shaped profile was obtained for the vertebral body in pinealectomy group, indicating the presence of heterogenous bone tissue. Accordingly, the mean value of BMD in pinealectomy group was significantly lower at the vertebral body in chickens compared with control subjects (P<0.001). At the end of the experiment, almost all of the chickens in pinealectomy group developed a scoliotic curvature and the mean weight and length of the cervical vertebral bodies of the pinealectomized chickens were significantly lower than those of control group (P<0.001). Although the numerical density of osteocytes and osteoblasts in pinealectomy group was significantly higher than that from the control group, total number of osteocytes but not osteoblasts in cervical vetrebrae from pinealectomized animals was significantly lower than that from nonpinealectomized control animals (P<0.05). In conclusion, the results of the current study demonstrated for the first time pinealectomy-induced histomorphometrical changes in chicken vertebral column using stereological methods, suggesting that pineal gland/melatonin may have an osteoinductive effect on bone formation, but further studies are needed to elucidate the relationship of our findings with some disorders such as postmenopausal and/or senile osteoporosis.

The existence and stability of spike equilibria in the one-dimensional Gray–Scott model: The pulse-splitting regime

The existence, stability, and pulse-splitting behavior of spike patterns in the one-dimensional Gray–Scott model on a finite domain is analyzed in the semi-strong spike-interaction regime. This regime is characterized by a localization of one of the components of the reaction near certain spike locations, while the other component exhibits a more global spatial variation across the domain. The method of matched asymptotic expansions is then used to construct k-spike equilibria in terms of a certain core problem. This core problem is studied numerically and asymptotically. For each integer k ≥ 1 , it is shown that there are two branches of k-spike equilibria that meet at a saddle-node bifurcation value. For small values of the diffusivity D of the second component, these saddle-node bifurcation points occur at approximately the same value. A combination of asymptotic and numerical methods is used to analyze the stability of these branches of k-spike equilibria with respect to both drift instabilities associated with the small eigenvalues and oscillatory instabilities of the spike profile. In this way, the key bifurcation and spectral conditions of Ei et al. [S. Ei, Y. Nishiura, K. Ueda, 2 n splitting or edge splitting? A manner of splitting in dissipative systems, Jpn. J. Ind. Appl. Math. 18 (2001) 181–205] believed to be essential for pulse-splitting behavior in a reaction–diffusion system are verified. By having verified these conditions, a simple analytical criterion for the occurrence of pulse-splitting is then formulated and confirmed with full numerical simulations of the Gray–Scott model. This criterion verifies a conjecture based on numerics and topological arguments reported in [A. Doelman, R.A. Gardner, T.J. Kaper, Stability analysis of singular patterns in the 1D Gray–Scott model: a matched asymptotics approach, Physica D 122 (1–4) (1998) 1–36]. The analytical results are compared with previously obtained results for pulse-splitting behavior.

Changes in vascularity of cartilage endplate of degenerated intervertebral discs in response to melatonin administration in rats.

We carried out an experimental investigation of cartilage endplate vascularity of degenerated intervertebral discs produced by exogenous melatonin (MEL) treatment. Adult Swiss albino rats were divided into three groups: control, operated degeneration, and MEL treatment. There were five rats in each group and, using a posterior approach, cuts were made parallel to the endplates in the posterior annulus fibrosus in five consecutive intervertebral discs between the 5th and 10th vertebral segments of the rats' tails. At 8 weeks, five of these animals were treated with exogenous MEL (s.c. injection of 30 microg/100 g body weight daily for 4 weeks). In each experimental group, one animal was examined using CT scanner to study the density of the cartilage endplate of the disc. To evaluate the bone growth and vascularity of the cartilage endplate region, the animals were killed for subsequent histopathological evaluation. We found that the vascular channel counts and percentage areas from animals treated with MEL were significantly lower than from the operated degeneration animals. Accordingly, the density histogram in the MEL group showed a spike profile for both the vertebral body and the cartilage endplate, indicating an increase in the amount of higher density tissues in these regions. Our results demonstrate that the use of MEL reduces the cartilage endplate vascularity of degenerated intervertebral discs, suggesting that it may have an osteoinductive effect on bone formation. Further studies are needed to characterize fully the relevance of our findings for the treatment of disorders such as postmenopausal osteoporosis.

The Effects of Numerical Viscosities: I. Slowly Moving Shocks

We begin a systematical study on the effect of numerical viscosities. In this paper we investigate the behavior of shock-capturing methods for slowly moving shocks. It is known that for slowly moving shocks even a first-order scheme, such as the Godunov or Roe type methods, will generate downstream oscillatory wave patterns that cannot be effectively damped by the dissipation of these first-order schemes. The purpose of this paper is to understand the formation and behavior of these downstream patterns. Our study shows that the downstream errors are generated by the unsteady nature of the viscous shock profiles and behave diffusively. The scenario is as follows. When solving the compressible Euler equations by shock capturing methods, the smeared density profile introduces a momentum spike at the shock location if the shock moves slowly. Downstream waves will necessarily emerge in order to balance the momentum mass carried by the spike for the momentum conservation. Although each family of waves decays inl∞andl2while they preserve the same mass, the perturbing nature of the viscous or spike profile is a constant source for the generation of new downstream waves, causing spurious solutions foralltime. Higher order TVD or ENO type interpolations accentuate this problem.

On the interpretation of the correlated optical and radio outbursts in AO0235+164

A detailed analysis of the spectral properties of the correlated optical and radio outbursts occurred in the BL Lac object AO02235+164 is presented, taking the outburst in Oct.–Nov. 1975 as an example. The observed features can be interpreted reasonably in the frame of relativistic shock-jet models; (1) The background jet spectrum is determined and its structure is analysed on the basis of Königl's jet model. The distributions of electron density and magnetic field along the jet and other parameters are discussed. (2) With the background jet spectrum subtracted, the spectrum of the outburst is determined. It is shown that it can be represented by a uniform synchrotron radiation source with the optically thin spectral index α ∼ 0.25, the break frequency v b 3 × 10 12Hz and the high frequency cutoff v ̃ h ∼ 2 × 10 14 Hz . (3) The adiabatic acceleration of particles and amplification of magnetic field, and the parameters of the emitting regions (i.e. the post-shock regions) are discussed. (4) The simultaneity of the optical and radio outbursts is considered to be due to the optical thinness of the emitting region. The optically thin spectral index in radio band ( α ∼ 0.25) is smaller than that of the background jet spectrum ( α s2 ≅ 0.5) at similar frequencies (∼ 10 12 Hz >), and this can be used to explain the much larger amplitude of variations in optical band. (5) It is suggested that the difference between the spike profile of optical outburst and the broad profile of radio outburst (as an example, for the correlated optical radio outburst in 1979) could be explained by shock propagating through irregularities of energetic electrons and magnetic field along the jet, that is, the scale of the optical emission region along the jet might be much smaller than that of the radio emission region. In summary, the results given here show that the correlated optical-radio outbursts in AO0235+164 may be intrinsic to the source, not due to gravitational micro-lensing effects.