分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the influence of running vacuum on the baryon-to-photon ratio in running vacuum models (RVMs). When there exists a non-minimal coupling between photons and other matter in the expanding universe, the energy-momentum tensor of photons is no longer conserved, but the energy of photons could remain conserved. We discuss the conditions for the energy conservation of photons in RVMs. The photon number density and baryon number density, from the epoch of photon decoupling to the present day, are obtained in the context of RVMs by assuming that photons and baryons can be coupled to running vacuum, respectively. Both cases lead to a time-evolving baryon-to-photon ratio. However the evolution of the baryon-to-photon ratio is strictly constrained by observations. It is found that if the dynamic term of running vacuum is indeed coupled to photons or baryons, the coefficient of the dynamic term must be extremely small, which is unnatural. Therefore, our study basically rules out the possibility that running vacuum is coupled to photons or baryons in RVMs.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Primordial black holes (PBHs) could be formed if large perturbations are generated on small scales in inflation. We study a toy inflation model with a local minimum. The curvature perturbations are enhanced when the inflaton passes through the local minimum, with more efficient amplification rate than that of quasi-inflection point inflation, leading to the production of PBHs on small scales. For parameter spaces provided in this paper, the PBHs could comprise a fraction of the total dark matter around $0.1\%$--$1\%$.
分类: 材料科学 >> 材料力学 提交时间: 2019-02-01
摘要: Magnetorheological (MR) materials are a type of magnetoactive smart materials, whose physical or mechanical properties can be altered by applying a magnetic field. In usual, MR materials can be prepared by mixing magnetic particles into non-magnetic matrices. In this work, the electromagnet-induced (or non-uniform magnetic field-induced) normal stress of MR materials is studied. It shows that the stress does not vary monotonically along with the enhancement of the applied magnetic field. There exists a field-dependent reversal effect of the variation of the stress. The reversal effect is thought resulting from that the ratio of interparticle repellent of parallel magnetic particles to the particle-electromagnet attraction gets enlarged along with the enhancement of the field.
分类: 化学 >> 化学物理学 提交时间: 2018-11-28
摘要: Large-scale, high-power laser facility is currently a basis for the research of inertial confinement fusion (ICF). In the laser facility, fused silica optics plays irreplaceable role to conduct extremely high-intensity ultraviolet laser to fusion target. However, the surface fractures, such as surface pit, crack, scratch, and laser damage site, of the optics will lower the beam quality of transmitted laser, weaken the laser damage-resistance, shorten the lifetime of the optics, and thus limit the output performance of the laser facility. In this work, to mitigate the surface fractures, hydrofluoric acid-based (HF-based) etching effect on the surface fractures is experimentally and theoretically studied. From the experimental study, with the result of three-dimensional (3D) morphological evolution of surface fractures, it is shown that the surface fractures will be passivated and their profiles will get smoothed after etching, indicating HF-based etching is a promising way to improve the local surface quality of the fractures. Especially, it is found HF-based etching can greatly suppress the laser damage growth of laser damage site by mitigating its surface and subsurface fractures, and thus is a promising wet chemical method for repairing laser-induced damage optics. In the theoretical study, an explicit local-curvature dependent etching model is proposed. Based on this model, the result from finite difference time domain (FDTD) simulation agrees very well with that of experiment, which can reveal the detailed physical process of HF-based etching. It is demonstrated that the FDTD simulation can be a reliable and efficient way to predict the morphological evolution of the surface fractures during etching.