分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The current LIGO-Virgo observing run has been pushing the sensitivity limit to touch the stochastic gravitational-wave backgrounds (SGWBs). However, no significant detection has been reported to date for any single dominated source of SGWBs with a single broken-power-law (BPL) spectrum. Nevertheless, it could equally well escape from existing Bayesian searches from, for example, two comparable dominated sources with two separate BPL spectra (double-peak case) or a single source with a doubly-BPL (DBPL) spectrum (doubly-broken case). In this paper, we put constraints on these two cases from Advanced LIGO-Virgo's first three observing runs. We found strong negative evidence for the double-peak case and hence place 95\% confidence-level (CL) upper limits $\Omega_\mathrm{BPL,1}<2.5\times10^{-7}$ and $\Omega_\mathrm{BPL,2}<9.4\times10^{-8}$ on the two BPL spectra amplitudes with respect to the unresolved compact-binary-coalescence (CBC) amplitude $\Omega_\mathrm{CBC}<5.6\times10^{-9}$. We further found weak negative evidence for the doubly-broken case and hence place 95\% CL upper limit $\Omega_\mathrm{DBPL}<1.7\times10^{-7}$ on the overall amplitude of the DBPL spectrum with respect to $\Omega_\mathrm{CBC}<6.0\times10^{-9}$. The implications of cosmological first-order phase transitions are also discussed.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The current LIGO-Virgo observing run has been pushing the sensitivity limit to touch the stochastic gravitational-wave backgrounds (SGWBs). However, no significant detection has been reported to date for any single dominated source of SGWBs with a single broken-power-law (BPL) spectrum. Nevertheless, it could equally well escape from existing Bayesian searches from, for example, two comparable dominated sources with two separate BPL spectra (double-peak case) or a single source with a doubly-BPL (DBPL) spectrum (doubly-broken case). In this paper, we put constraints on these two cases from Advanced LIGO-Virgo's first three observing runs. We found strong negative evidence for the double-peak case and hence place 95\% confidence-level (CL) upper limits $\Omega_\mathrm{BPL,1}<2.5\times10^{-7}$ and $\Omega_\mathrm{BPL,2}<9.4\times10^{-8}$ on the two BPL spectra amplitudes with respect to the unresolved compact-binary-coalescence (CBC) amplitude $\Omega_\mathrm{CBC}<5.6\times10^{-9}$. We further found weak negative evidence for the doubly-broken case and hence place 95\% CL upper limit $\Omega_\mathrm{DBPL}<1.7\times10^{-7}$ on the overall amplitude of the DBPL spectrum with respect to $\Omega_\mathrm{CBC}<6.0\times10^{-9}$. The implications of cosmological first-order phase transitions are also discussed.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The sample variance due to our local density fluctuations in measuring our local Hubble-constant ($H_0$) can be reduced to the percentage level by choosing the Hubble-flow type Ia supernovae (SNe Ia) outside of the homogeneity scale. In this Letter, we have revealed a hidden trend in this one-percent $H_0$ variation both theoretically and observationally. We have derived for the first time our $H_0$ variation measured from any discrete sample of distant SNe Ia. We have also identified a residual linear correlation between our local $H_0$ fitted from different groups of SNe Ia and their ambient density contrasts of SN-host galaxies evaluated at a given scale. We have further traced the scale dependence of this residual linear trend, which becomes more and more positively correlated with the ambient density contrasts of SN-host galaxies estimated at larger and larger scales, on the contrary to but still marginally consistent with the theoretical expectation from the $\Lambda$-cold-dark-matter model. This might indicate some unknown corrections to the peculiar velocity of the SN-host galaxy from the density contrasts at larger scales or the smoking gun for the new physics.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the inflationary implications of a novel parity-violating gravity model, which modifies the teleparallel equivalent of general relativity by introducing the Nieh-Yan term coupled to an axion-like field. The parity-violating Nieh-Yan term results in the velocity birefringence of gravitational waves (GWs) and triggers the tachyonic instability only for one of the two circular polarization states. We consider that the inflaton is identified as the coupled axion-like field with a wiggly potential characterized by steep cliffs connected by smooth plateaus. During inflation, the temporary fast roll of axion on the cliff-like region leads to the significant enhancement of the tensor perturbations in one polarization state with the wave numbers that exit the horizon around this period. In this setup, the resulting energy spectrum for GWs presents a sizable localized bump involving the contribution of only one polarization state. This chiral GW background is detectable by LISA and Taiji, and its chirality can be determined by correlating two detectors, which provide an opportunity to probe the inflation and test the gravity model.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The sample variance due to our local density fluctuations in measuring our local Hubble-constant ($H_0$) can be reduced to the percentage level by choosing the Hubble-flow type Ia supernovae (SNe Ia) outside of the homogeneity scale. In this Letter, we have revealed a hidden trend in this one-percent $H_0$ variation both theoretically and observationally. We have derived for the first time our $H_0$ variation measured from any discrete sample of distant SNe Ia. We have also identified a residual linear correlation between our local $H_0$ fitted from different groups of SNe Ia and their ambient density contrasts of SN-host galaxies evaluated at a given scale. We have further traced the scale dependence of this residual linear trend, which becomes more and more positively correlated with the ambient density contrasts of SN-host galaxies estimated at larger and larger scales, on the contrary to but still marginally consistent with the theoretical expectation from the $\Lambda$-cold-dark-matter model. This might indicate some unknown corrections to the peculiar velocity of the SN-host galaxy from the density contrasts at larger scales or the smoking gun for the new physics.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Hubble tension seems to be a crisis with $\sim5\sigma$ discrepancy between the most recent local distance ladder measurement from type Ia supernovae calibrated by Cepheids and the global fitting constraint from the cosmic microwave background data. To narrow down the possible late-time solutions to the Hubble tension, we have used in a recent study [Phys. Rev. D 105, L021301 (2022)] an improved inverse distance ladder method calibrated by the absolute measurements of the Hubble expansion rate at high redshifts from the cosmic chronometer data, and found no appealing evidence for new physics at the late time beyond the $\Lambda$CDM model characterized by a parametrization based on the cosmic age. In this paper, we further investigate the perspective of this improved inverse distance ladder method by including the late-time matter perturbation growth data. Independent of the dataset choices, model parametrizations, and diagnostic quantities ($S_8$ and $S_{12}$), the new physics at the late time beyond the $\Lambda$CDM model is strongly disfavored so that the previous late-time no-go guide for the Hubble tension is further strengthened.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Hubble tension, if not caused by any systematics, could be relieved or even resolved from modifying either the early-time or late-time Universe. The early-time modifications are usually in tension with either galaxy clustering or galaxy lensing constraints. The late-time modifications are also in conflict with the constraint from the inverse distance ladder, which, however, is weakened by the dependence on a sound-horizon prior and some particular approximation for the late-time expansion history. To achieve a more general no-go argument for the late-time scenarios, we propose to use a global parametrizationbased on the cosmic age (PAge) to consistently use the cosmic chronometers data beyond the Taylor expansion domain and without the input of a sound-horizon prior. Both the early-time and late-time scenarios are therefore largely ruled out, indicating the possible ways out of the Hubble tension from either exotic modifications of our concordance Universe or some unaccounted systematics.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Values of the Hubble constant between the direct measurements from various independent local observations and that inferred from the cosmic microwave background with the $\Lambda$-cold-dark-matter model are in tension with persistent significance. We propose a late-time inhomogeneous resolution suggesting that a chameleon field coupled to a local overdensity of matter could be trapped at a higher potential energy density as an effective cosmological constant driving the local expansion rate faster than that of the background with lower matter density. We illustrate this mechanism in a toy model in which a region with only $20\%$ overdensity of matter is sufficient to resolve the Hubble tension, and the Hubble constant measured by the local distance ladders could be accommodated by the chameleon coupled to the observed overdensities from the large-scale structure surveys.
分类: 核科学技术 >> 裂变堆工程技术 提交时间: 2023-06-01
摘要: Recent reactor antineutrino experiments have observed that the neutrino spectrum changes with the reactor core evolution and that the individual fissile isotope antineutrino spectra can be decomposed from the evolving data, providing valuable information for the reactor model and data inconsistent problems. We propose a machine learning method by building a convolutional neural network based on a virtual experiment with a typical short-baseline reactor antineutrino experiment configuration: by utilizing the reactor evolution information, the major fissile isotope spectra are correctly extracted, and the uncertainties are evaluated using the Monte Carlo method. Validation tests show that the method is unbiased and introduces tiny extra uncertainties.
分类: 物理学 >> 核物理学 提交时间: 2024-03-08
摘要: Neutron radiography is a crucial nondestructive testing technology widely used in the aerospace, military, andnuclear industries. However, because of the physical limitations of neutron sources and collimators, the resultingneutron radiographic images inevitably exhibit multiple distortions, including noise, geometric unsharpness,and white spots. Furthermore, these distortions are particularly significant in compact neutron radiography systemswith low neutron fluxes. Therefore, in this study, we devised a multi-distortion suppression network thatemploys a modified generative adversarial network to improve the quality of degraded neutron radiographic images.Real neutron radiographic image datasets with various types and levels of distortion were built for the firsttime as multi-distortion suppression datasets. Thereafter, the coordinate attention mechanism was incorporatedinto the backbone network to augment the capability of the proposed network to learn the abstract relationshipbetween ideally clear and degraded images. Extensive experiments were performed; the results show that theproposed method can effectively suppress multiple distortions in real neutron radiographic images and achievestate-of-the-art perceptual visual quality, thus demonstrating its application potential in neutron radiography.
分类: 生物学 >> 生物物理学 >> 细胞生物学 提交时间: 2016-05-11
摘要: Homotypic membrane fusion of the endoplasmic reticulum is mediated by dynamin-like guanosine triphosphatases (GTPases), which include atlastin (ATL) in metazoans and Sey1p in yeast. In this paper, we determined the crystal structures of the cytosolic domain of Sey1p derived from Candida albicans. The structures reveal a stalk-like, helical bundle domain following the GTPase, which represents a previously unidentified configuration of the dynamin superfamily. This domain is significantly longer than that of ATL and critical for fusion. Sey1p forms a side-by-side dimer in complex with GMP-PNP or GDP/AlF4- but is monomeric with GDP. Surprisingly, Sey1p could mediate fusion without GTP hydrolysis, even though fusion was much more efficient with GTP. Sey1p was able to replace ATL in mammalian cells, and the punctate localization of Sey1p was dependent on its GTPase activity. Despite the common function of fusogenic GTPases, our results reveal unique features of Sey1p.
分类: 天文学 >> 天文学 提交时间: 2024-02-28 合作期刊: 《Research in Astronomy and Astrophysics》
摘要: The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear, and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes. We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor. A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model. First, the parameter distributions of the time-resolved spectrum are given as follows: the low-energy spectral index α ∼ − 0.72, high-energy spectral index β ∼ − 2.42, the peak energy Ep ∼ 221.69 keV, and the energy flux F ∼ 7.49 × 10−6 erg cm−2 s−1. More than 80% of the bursts exhibit the hardest low-energy spectral index _{\max}$ exceeding the synchrotron limit (−2/3). Second, the evolution patterns of α and Ep were statistically analyzed. The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of both Ep and α. The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs. Finally, we found a significant positive correlation between F and Ep, with half of the samples exhibiting a positive correlation between F and α. We discussed the spectral evolution of different radiation models. The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process, including photospheric radiation and synchrotron radiation. However, it may also involve only one radiation mechanism, but more complicated physical details need to be considered.
分类: 物理学 >> 核物理学 提交时间: 2023-09-11
摘要: Gamma-ray polarimetry is a new and prospective tool for studying various extreme high-energy celestial objects and is of great significance for the development of astrophysics. With the rapid development of microsatellite technology, the advantages in space exploration are becoming increasingly apparent. Therefore, in this paper, we conducted a simulation study on a soft gamma-ray polarimeter for a microsatellite in space. Here, we proposed a unique design structure for the polarimeter based on the microsatellite design concept and the Compton scattering principle. And then, the detailed Monte Carlo simulations using mono-energetic gamma-ray linear polarization sources and the Crab-like sources in the energy range of 0.1-10 MeV with full consideration of the orbital background were performed. The simulation results show that the polarimeter can exhibit excellent polarization detection performance. The modulation factor is 0.800.01, and the polarization angles are accurate within an error of 0.2 at 200 keV for on-axis incidence. For the Crab-like sources for on-axis incidence, the polarization degrees are consistent with the set values within the error tolerance, the modulation factor is 0.760.01, and the minimum detectable polarization reaches 2.4% at 3 for an observation time of 106 seconds. In addition, the polarimeter has recoil electron tracking, imaging, and powerful background suppression at a large field of view (2 sr). The polarimeter designed can meet the requirements of a space-soft gamma-ray polarization detector very well and has a bright research prospect.
分类: 生物学 >> 生物物理学 提交时间: 2016-05-12
摘要: The NLR family apoptosis inhibitory proteins (NAIPs) bind conserved bacterial ligands, such as the bacterial rod protein PrgJ, and recruit NLR family CARD-containing protein 4 (NLRC4) as the inflammasome adapter to activate innate immunity. We found that the PrgJ-NAIP2-NLRC4 inflammasome is assembled into multisubunit disk-like structures through a unidirectional adenosine triphosphatase polymerization, primed with a single PrgJ-activated NAIP2 per disk. Cryo-electron microscopy (cryo-EM) reconstruction at subnanometer resolution revealed a similar to 90 degrees hinge rotation accompanying NLRC4 activation. Unlike in the related heptameric Apaf-1 apoptosome, in which each subunit needs to be conformationally activated by its ligand before assembly, a single PrgJ-activated NAIP2 initiates NLRC4 polymerization in a domino-like reaction to promote the disk assembly. These insights reveal the mechanism of signal amplification in NAIP-NLRC4 inflammasomes.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: Porous silicon/carbon (Si/C) structure has been proved of great help to enhance the electrochemical cycling performance of Si-based anode materials. Current fabrication of porous Si based anodes usually relies on the complicated chemical etching process with unavoidable pollution problem. Herein, a na- noporous Si/C composite was successfully fabricated by a facile spray drying and carbonization process. The environmentally benign NaCl is used as the sacrificing template for the generation of embedded nanopores, which can be easily removed by routine washing process. The as-prepared nanoporous Si/C anode exhibits a high reversible specific capacity of around 900 mAh g 1 with 78.2% capacity retention after 100 cycles. This synthetic method paves a simple and environment-friendly way to build porous Si based anode with improved electrochemical performance which is significantly important for the in- dustrial production.
提交时间: 2017-05-02
摘要: Polyimide/reduced graphene oxide [PI/r-GO] core-shell structured microspheres were fabricated by in situ reduction of graphene oxide [GO], which was coated on the surface of PI microspheres via hydrogen bonding and pi-pi stacking interaction. The highly o
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: Quantitative investigation is conducted on the resistance sources of the components in the NiOeYSZ/ YSZ/GDC/LSCFeGDC solid oxide electrolysis cell (SOEC) stack at the H2O/H2 ratios of 70/30, 80/20 and 90/ 10 at 750 C. The results indicate that the cell resistance accounts for 76.3e66.7% of that of the stack repeating unit (SRU), the contact resistance (CR) between the air electrode current-collecting layer (AECCL) and the interconnect accounts for 23.6e27.0%, the CR between the hydrogen electrode current- collecting layer (HECCL) and the interconnect only accounts for 2.3e3.2%, and the resistance of the interconnect can be neglected. Duration test of the stack is conducted at 0.8 A cm 2 for 380 h, the cell resistance increase is found to be the major contribution of the SRU degradation (82.2% of the SRU degradation) while the air electrode CR increase and the hydrogen electrode CR increase are other two important factors.
提交时间: 2017-05-02
摘要: Mn-doped La0.8Sr0.2CoO3 perovskite oxides [La0.8Sr0.2Co1-xMnxO3; x = 0, 0.1, 0.3, 0.5] were synthesized by a modified sol-gel method. The phase-pure oxides were obtained. CoO and carbonates were formed on the surface of La0.8Sr0.2CoO3. With increasing dop
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: Hydrogen is an emerging energy source/carrier for oil refining and fuel cell applications. The development of an efficient and stable catalyst to produce hydrogen-rich gas is required for industrial application. The Ni@yolk-ZrO2 catalyst could be a potential solution to tackle the challenges in hydrogen production. The catalyst was characterized using a combination of XRD, TEM, AAS, TPR, BET, and XPS. In this study, the amount of micropores in ZrO2 hollow shells was demonstrated to influence the catalytic performance. Ni@yolk-ZrO2 catalysts were evaluated for 48 hours under steam reforming of methane and their porosity effect in ZrO2 hollow shells was identified. From the characterization of BET and catalytic evaluation, the physical information of the ZrO2 hollow shell was established, which affected the catalytic performance in steam reforming of methane. Furthermore, the results from XPS and TEM showed that Ni particles were controlled under a ZrO2 yolk–shell structure framework and showed the characteristic of moderately strong hydrothermal stability after the steam reforming test. The catalysts were studied at a GHSV of 50 400 mL gcat 1 h 1 and S/C 1⁄4 2.5 at 750 C and they remained stable with methane conversion more than 90% for 48 hours.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: Two series of poly(ester imide)s derived from bis(trimellitic acid anhydride) phenyl ester (TAHQ) and bis[(3,4- dicarboxylic anhydride) phenyl] terephthalate (PAHP), as well as poly(ether imide)s based on hydroquinone diphthalic anhydride (HQDPA), were synthesized with aromatic diamines via solution polycondensation. These polyimide films were transparent with an ultraviolet-visible absorption cut-off wavelength below 375 nm, and with tensile strengths of 42.0–83.8 MPa, tensile moduli of 2.5–4.7 GPa and elongations at break of 2.1–5.4%. Compared with the poly(ether imide)s, the poly(ester imide)s showed higher glass transition temperatures (Tg), lower water absorption (WA) and lower temperature of 5% weight loss (Td5%). Moreover, the poly(ester imide)s derived from PAHP with a low electron affinity of 2.04 eV by theoretical calculation achieved better transparency, lower WA and slightly lower Tg than the corresponding TAHQ- based poly(ester imide)s.