Anisotropic Sky Research Articles

The Impact of Anisotropic Sky Sampling on the Hubble Constant in Numerical Relativity

We study the impact of nearby inhomogeneities on an observer’s inference of the Hubble constant. Large-scale structures induce a dependence of cosmological parameters on observer position as well as an anisotropic variance of those parameters across an observer’s sky. While the former has been explored quite thoroughly, the latter has not. Incomplete sampling of an anisotropic sky could introduce a bias in our cosmological inference if we assume an isotropic expansion law. In this work, we use numerical relativity simulations of large-scale structure combined with ray tracing to produce synthetic catalogs mimicking the low-redshift Pantheon supernova (SN) data set. Our data contain all general-relativistic contributions to fluctuations in the distances and redshifts along geodesics in the simulation. We use these synthetic observations to constrain H 0 for a set of randomly positioned observers. We study both the dependence on observer position as well as the impact of rotating the sample of SNe on the observer’s sky. We find a 1%–2% variance in H 0 between observers when they use an isotropic sample of objects. However, we find the inferred value of H 0 can change by up to 3%–6% when observers simply rotate their Pantheon data set on the sky. While the variances we find are below the level of the “Hubble tension,” our results may suggest a reduction in the significance of the tension if anisotropy of expansion can be correctly accounted for.

SIMPLE: Simple Intensity Map Producer for Line Emission

We present the Simple Intensity Map Producer for Line Emission (Simple), a public code for quickly simulating mock line-intensity maps, and an analytical framework for modeling intensity maps including observational effects. Simple can be applied to any spectral line sourced by galaxies. The Simple code is based on lognormal mock catalogs of galaxies including positions and velocities and assigns luminosities following the luminosity function. After applying a selection function to distinguish between detected and undetected galaxies, the code generates an intensity map, which can be modified with anisotropic smoothing, noise, a mask, and sky subtraction, and it calculates the power spectrum multipoles. We show that the intensity auto-power spectrum and the galaxy-intensity cross-power spectrum agree well with the analytical estimates in real space. We derive and show that the sky subtraction suppresses the intensity autopower spectrum and the cross-power spectrum on scales larger than the size of an individual observation. As an example application, we make forecasts for the sensitivity of an intensity mapping experiment similar to the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) to the cross-power spectrum of Lyα-emitting galaxies and the Lyα intensity. We predict that HETDEX will measure the galaxy-intensity cross-power spectrum with a high signal-to-noise ratio on scales of 0.04 h Mpc−1 < k < 1 h Mpc−1.

Model-independent search for anisotropies in stochastic gravitational-wave backgrounds and application to LIGO-Virgo’s first three observing runs

A stochastic gravitational-wave (GW) background consists of a large number of weak, independent and uncorrelated events of astrophysical or cosmological origin. The GW power on the sky is assumed to contain anisotropies on top of an isotropic component, i.e., the angular monopole. Complementary to the LIGO-Virgo-KAGRA (LVK) searches, we develop an efficient analysis pipeline to compute the maximum-likelihood anisotropic sky maps in stochastic backgrounds directly in the sky pixel domain using data folded over one sidereal day. We invert the full pixel-pixel correlation matrix in map-making of the GW sky, up to an optimal eigenmode cutoff decided systematically using simulations. In addition to modeled mapping, we implement a model-independent method to probe spectral shapes of stochastic backgrounds. Using data from LIGO-Virgo's first three observing runs, we obtain upper limits on anisotropies as well as the isotropic monopole as a limiting case, consistent with the LVK results. We also set constraints on the spectral shape of the stochastic background using this novel model-independent method.

An anisotropic parameterization scheme for longwave irradiance and its impact on radiant load in urban outdoor settings

A robust representation of the radiative properties in complex urban settings is important for accurate estimations of radiant load. Here, we present a new parameterization scheme in the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model that partitions the upper hemisphere into 153 patches. Partitioning of the upper hemisphere enables determination if longwave irradiance originates from the sky, vegetation, sunlit building surfaces, or shaded building surfaces from each patch. Furthermore, a model for anisotropic sky longwave irradiance where emissivity increases with zenith angle is included. Comparisons between observations and simulations show high correlation, with R2 and RMSE for Tmrt of 0.94 and 4.6 °C, respectively, and R2 and RMSE for longwave radiation of 0.89 and 14.1 Wm−2, respectively. Simulations show that mean radiant temperature (Tmrt) can be up to 1.5 °C higher with an anisotropic sky compared to a uniform sky as an effect of higher radiant load on the vertical of a human when sky longwave irradiance increases with zenith angle. In comparisons of simulated Tmrt with the new parameterization and old parameterization schemes, previously overestimated Tmrt under trees (high sky obstruction, sky view factor (SVF) < 0.3) can be decreased by up to 3 °C from more realistic estimations using the patches. Moreover, Tmrt close to sunlit walls (SVF ~ 0.5) is increased by up to 2–3 °C from increased exposure to sunlit surfaces. Concluding, anisotropic sky longwave radiation and directionality of longwave radiation from different sources are important in estimations of Tmrt of humans in outdoor settings.

Integrated physical approach to assessing urban-scale building photovoltaic potential at high spatiotemporal resolution

Assessing the urban-scale building photovoltaic (PV) potential is important for designing urban environments, retrofitting existing structures, or integrating PVs with grids. However, few studies have considered high-temporal-resolution simulations, the facade PV potential, and a comprehensive PV model simultaneously; thus, the overall accuracy of the estimation of PV potential may be limited. Therefore, this study developed an integrated framework to assess the urban-scale PV potential of rooftops and facades at high spatiotemporal resolution. The proposed approach integrates an anisotropic sky diffuse model, a vector-based shading calculation method, and a temperature-related PV performance model. The annual PV potential and spatial/temporal characteristics were analyzed in a case study of over 170,000 buildings in Beijing. The results showed that the estimated rooftop PV power generation was 7.55 TWh/y, whereas the facade PV power generation was 18.07 TWh/y, which was 239% of the rooftop PV yield. The integrated model estimated PV yield with higher accuracy than the simplified models by depicting more details. The proposed approach can be applied to the large-scale assessment of future energy systems with increasing penetration of PVs, and the results can support effective policies for the integration of PVs into the built environment in dense cities.

Calculating the shading reduction coefficient of photovoltaic system efficiency using the anisotropic sky scattering model

The front-row shading reduction coefficient is a key parameter used to calculate the system efficiency of a photovoltaic (PV) power station. Based on the Hay anisotropic sky scattering model, the variation rule of solar radiation intensity on the surface of the PV array during the shaded period is simulated, combined with the voltage–current characteristics of the PV modules, and the shadow occlusion operating mode of the PV array is modeled. A method for calculating the loss coefficient of front shadow occlusion based on the division of the PV cell string unit and Hay anisotropic sky scattering model is proposed. This algorithm can accurately evaluate the degree of influence of the PV array layout, wiring mode, array spacing, PV module specifications, and solar radiation on PV power station system efficiency. It provides a basis for optimizing the PV array layout, reducing system loss, and improving PV system efficiency.

A modified method to measure outdoor mean radiant temperature: Comparison between two-hemisphere method and six-direction integral method

Measurement of the outdoor mean radiant temperature (MRT) is crucial for the investigation and design of outdoor thermal environments. However, the most accurate method for measuring outdoor MRT, the six-direction integral method (SDIM), is expensive and cumbersome. Consequently, the purpose of this study is to propose a modified method (MTHM) for the less costly and more practical MRT measurement. On the basis of radiation measurements in Shanghai, China, the MRT deviation in the two-hemisphere method (THM) was quantified, and its reasons were identified by comparing the radiation and MRT differences between the THM and the SDIM. During the summer in Shanghai, the MRT deviation in the THM is found to be less than 3 °C in the shade and up to 5 °C in direct sunlight. The failure to account for anisotropic sky diffuse radiation is the primary reason for the THM error under direct sunlight. Therefore, the MTHM was generated from the THM by incorporating anisotropic sky diffuse radiation, and it was demonstrated that it could significantly enhance the accuracy and robustness of the MRT compared to the THM. This study will help the outdoor MRT measurement, consequently supporting the outdoor thermal environment planning.

Erratum: Luminosity distance and anisotropic sky-sampling at low redshifts: A numerical relativity study [Phys. Rev. D 104 , 023525 (2021)

In this erratum we fix a bug in the generation of initial conditions for the simulations presented in our paper ``Luminosity distance and anisotropic sky sampling at low redshifts: A numerical relativity study.'' Our new simulations have overall smaller typical density contrasts, and therefore the amplitude of anisotropic effects found has reduced. However, qualitatively our results are unchanged.

Exergetic Performance Assessment of Optimally Inclined BIPV Thermal System by Considering Cyclic Nature of Insolation

Abstract The structural and architectural elements of building-integrated photovoltaic-thermal (BIPVT) systems are made up of photovoltaic (PV) modules and these are required to be fixed at an optimum inclination angle for generating maximum exergy. This work presents an attempt to determine the amount of exergy generated by an optimally inclined double-storied BIPV thermal system by considering the actual cyclic nature of insolation, surrounding air temperature, PV cell temperature, intermediate slab temperature, and the chamber temperature. The insolation value, which is computed by an anisotropic sky model along with these cyclic variables, is used for solving the set of governing differential equations for evaluating the exergy of the system. Other influencing parameters of the BIPV thermal systems such as air changes in both chambers, packing factor of PV module, the orientation of PV module, and thickness of the intermediate slab are considered for finding its effect on the total exergy of the system. Numerical results show that for packing factor more than 0.6, there is no significant change in total heat exergy with respect to the inclination angle. For packing factor more than 0.3, the generation of electrical exergy exceeds the heat exergy, and the overall exergy of BIPVT system decreases with rise in packing factor (βm) up to 0.3 and then rises nonlinearly.

Hints of a local matter underdensity or modified gravity in the low z Pantheon data

A redshift tomography of the Pantheon type Ia supernovae (SnIa) data focusing on the best fit value of the absolute magnitude $M$ and/or Hubble constant $H_0$ in the context of $\Lambda$CDM indicates a local variation ($z\lesssim 0.2$) at $2\sigma$ level, with respect to the best fit of the full dataset. If this variation is physical, it can be interpreted either as a locally higher value of $H_0$, corresponding to a local matter underdensity $\delta \rho_0/\rho_0 \simeq -0.10 \pm 0.04$ or as a time variation of Newton's constant which implies an evolving Chandrasekhar mass and thus an evolving absolute magnitude $M$ of SnIa. The local void scenario would predict an anisotropy in the best fit value of $H_0$ since it is unlikely that we are located at the center of a local spherical underdensity. Using a hemisphere comparison method we find an anisotropy level consistent with simulated isotropic datasets. We show however, that the anisotropic sky distribution of the Pantheon SnIa data induces a preferred range of directions even in simulated Pantheon data obtained in the context of isotropic $\Lambda$CDM. We thus construct a more isotropically distributed subset of the Pantheon SnIa and show that the preferred range of directions disappears. Using this subset we again find no evidence for anisotropy using either the hemisphere comparison method or the dipole fit method. In the context of the modified gravity scenario, we allow for an evolving normalized Newton's constant consistent with General Relativity (GR) at early and late times $\mu(z)=1+g_a z^2/(1+z)^2-g_a z^4/(1+z)^4$ and fit for $g_a$ assuming $L\sim G_{\rm{eff}}^b$. For $b=-3/2$ indicated by previous studies we find $g_a=-0.47 \pm 0.36$ which is more than $1.5\sigma$ away from the GR value of $g_a=0$. This weak hint for weaker gravity at low $z$ is consistent with similar evidence from growth and weak lensing data.

Method to improve performance of building integrated photovoltaic thermal system having optimum tilt and facing directions

A building integrated photovoltaic (BIPV) system can be installed at optimum tilt and orientation angle to maximize the electrical output. PV output is negatively correlated with the increase in the PV panel temperature. This paper aims at investigating the influence of water flow over the BIPV roof of a two-storied single-family house on the performance of the proposed optimally tilted semi-transparent BIPV thermal system having different facing directions. The periodic nature of insolation, ambient air temperature, BIPV cell temperature, slab temperature, water flow temperature and room temperature have been considered while solving energy equilibrium differential equations. The insolation values used in the energy equilibrium equations are computed employing periodic HDKR (Hay, Davies, Klucher and Reindl) model, which is based on anisotropic sky concept. Results indicate that the average BIPV cell temperature is reduced annually by approximately 10 °C and efficiency is enhanced by 6% when a water flow rate of 1.1 kg s−1 is provided over the BIPV roof. It is also observed that on the most critical day of the year, the temperature of outlet water is 11.5 °C higher than that of inlet water temperature which resulted in 4 kW extraction of exergy.

The influence of anisotropic diffuse shortwave radiation on mean radiant temperature in outdoor urban environments

During clear weather conditions the main part of the shortwave irradiance derives from sun direct-beam radiation. However, part of the shortwave radiation also originates from all-sky diffuse radiation (D), with a large part being circumsolar in origin. Many radiation models considers the sky as isotropic when estimating D. Here we implement an anisotropic model for D into the SOLWEIG model to examine the spatial patterns of D in a built-up environment, as well as its influence on mean radiant temperature (Tmrt), a variable essential for estimating outdoor human thermal comfort. Comparisons between the anisotropic and the isotropic models indicates that the D in the isotropic model is overestimated in shaded areas and underestimated in areas close to sunlit walls. This is explained by the circumsolar origin of D during clear and semi-cloudy conditions and solar altitude. These over- and underestimations, consecutively, have implications for Tmrt, which can differ by up to 3 °C. The deviations in D thus signify the importance of using an anisotropic model when estimating D and Tmrt, especially since the areas with the highest reported radiant loads receive even more radiation considering an anisotropic diffuse sky, i.e. the hottest areas are even hotter than previously reported.

Thermal performance of BIPV system by considering periodic nature of insolation and optimum tilt-angle of PV panel

In the present study, the thermal performance has been evaluated for optimally tilted semi-transparent BIPV panel, which is fixed as a roof system on a two-storied single room sample building. In the mathematical model, the periodic nature of insolation, ambient air temperature, BIPV cell temperature, intermittent slab temperature, and room temperature have been considered while solving energy equilibrium differential equations. The insolation values used in the model are computed by using periodic HDKR (Hay, Davies, Klucher and Reindl) model, which is based on anisotropic sky concept. The influencing parameters i.e., air changes in both rooms of the two-storied BIPV thermal system, packing factor of the BIPV panel and the thickness of slab between rooms are considered for finding the thermal performance. It is observed that the most critical day of the year i.e., on 15th May, the upper storey and lower storey of the proposed BIPV system attains the highest room temperature of 47.3 °C and 42.6 °C, respectively. The air changes in the rooms reduce the above peak temperature for making it more comfortable. The BIPV cell temperature also reaches up to 65 °C resulting reduction of the efficiency by 18% than that of standard test condition.

Modelling Three-Dimensional Spatiotemporal Distributions of Forest Photosynthetically Active Radiation Using UAV-Based Lidar Data

The three dimensional (3-D) spatiotemporal variations of forest photosynthetically active radiation (PAR) dictate the exchange rates of matter and energy in the carbon and water cycle processes between the plant-soil system and the atmosphere. It is still challenging to explicitly simulate spatial PAR values at any specific position within or under a discontinuous forest canopy. In this study, we propose a novel lidar-based approach to estimate both direct and diffuse forest PAR components from a 3-D perspective. An improved path length-based direct PAR estimation method was developed by incorporating the point density along a light transmission path, and we also obtained the diffuse PAR components using a point-based sky view analysis by assuming the anisotropic sky diffuse distribution. We compared the total PAR modelled using three light path length-based parameters with reference data measured by radiometers on a five-minute time scale during a daily solar course. Our results show that, in a discontinuous forest canopy, the effective path length is a feasible and powerful (R2 = 0.92, p &lt; 0.01) parameter to capture the spatiotemporal variations of total PAR along a light transmission path with a mean bias of −53.04 μmol·m−2·s−1(−6.8%). Furthermore, incorporating point density and spatial distribution factors will further improve the final estimation accuracy (R2 = 0.97, p &lt; 0.01). In the meantime, diffuse PAR tends to be overestimated by 17% at noon and underestimated by about 10% at sunrise and sunset periods by assuming the isotropic sky diffuse distribution. The proposed lidar-based 3-D PAR model will provide a solid foundation to various process-based eco-hydrological models for simulating plant physiological processes such as photosynthesis and evapotranspiration, intra-species competition and succession, and snowmelt dynamics purposes.

Optimum Tilt Angle and Orientation of Photovoltaic Thermal System for Application in Greater Toronto Area, Canada

We present a study conducted to obtain optimum tilt angle and orientation of a solar panel for the collection of maximum solar irradiation. The optimum tilt angle and orientation were determined using isotropic and anisotropic diffuse sky radiation models (isotropic and anisotropic models). The four isotropic models giving varying optimum tilt angles in the range of 37 to 44°. On the other hand, results of the four anisotropic models were more consistent, with optimum tilt angles ranging between 46–47°. Both types of models indicated that the collector tilt should be changed four times a year to receive more solar radiation. The results also indicate that the solar panel should be installed with orientation west or east of due south with a flatter tilt angle. A 15° change in orientation west or east of due south results in less than 1% reduction of the total solar radiation received. For a given optimum tilt angle, the effect of photovoltaic/thermal (PV/T) orientation west or east of due south on the outlet temperature was determined using a one-dimensional steady state heat transfer model. It was found that there is less than 1.5% decrease in outlet temperature for a PV/T panel oriented up to 15° east or west of due south from March to December. This result indicates that existing roofs with orientations angles up to 15° east or west of due south can be retrofitted with a PV/T system without changing the roof shape.

Angular power spectrum analysis on current and future high-energy neutrino data

Astrophysical neutrino events have been measured in the last couple of years, which show an isotropic distribution, and the current discussion is their astrophysical origin. We use both isotropic and anisotropic components of the diffuse neutrino data to constrain the contribution of a broad number of extra-galactic source populations to the observed neutrino sky. We simulate up-going muon neutrino events by applying statistical distributions for the flux of extragalactic sources, and by Monte Carlo method we exploit the simulation for current and future IceCube, IceCube-Gen2 and KM3NeT exposures. We aim at constraining source populations by studying their angular patterns, for which we assess the angular power spectrum. We leave the characteristic number of sources (N★) as a free parameter, which is roughly the number of neutrino sources over which the measured intensity is divided. With existing two-year IceCube data, we can already constrain very rare, bright sources with N★≲100. This can be improved to N★≲ 104–105 with IceCube-Gen2 and KM3NeT with ten-year exposure, constraining the contribution of BL Lacs (N★=6×102). On the other hand, we can constrain weak sources with large number densities, like starburst galaxies (N★ = 107), if we measure an anisotropic neutrino sky with future observations.

Bulk flow in the combined 2MTF and 6dFGSv surveys

We create a combined sample of 10,904 late and early-type galaxies from the 2MTF and 6dFGSv surveys in order to accurately measure bulk flow in the local Universe. Galaxies and groups of galaxies common between the two surveys are used to verify that the difference in zero-points is $<0.02$ dex. We introduce a maximum likelihood estimator ($\eta$MLE) for bulk flow measurements which allows for more accurate measurement in the presence non-Gaussian measurement errors. To calibrate out residual biases due to the subtle interaction of selection effects, Malmquist bias and anisotropic sky distribution, the estimator is tested on mock catalogues generated from 16 independent large-scale GiggleZ and SURFS simulations. The bulk flow of the local Universe using the combined data set, corresponding to a scale size of 40 h$^{-1}$ Mpc, is $288\pm24$ km s$^{-1}$ in the direction $(l,b)=(296\pm6^{\circ}, 21\pm5^{\circ})$. This is the most accurate bulk flow measurement to date, and the amplitude of the flow is consistent with the $\Lambda$CDM expectation for similar size scales.

The influence of the observatory latitude on the study of ultra high energy cosmic rays

Recent precision measurements of the Ultra High Energy Cosmic Rays (UHECR) arrival directions, spectrum and parameters related to the mass of the primary particle have been done by the HiRes, Pierre Auger and Telescope Array (TA) Observatories. In this paper, distributions of arrival directions of events in the nearby Universe are assumed to correlate with sources in the 2MASS Redshift Survey (2MRS), IRAS 1.2 Jy Survey, Palermo Swift-BAT and Swift-BAT catalogs, and the effect of the latitude of the observatory on the measurement of the energy spectrum and on the capability of measuring anisotropy is studied. The differences between given latitudes on the northern and southern hemispheres are quantified. It is shown that the latitude of the observatory: a) has an influence on the total flux measured and b) imposes an important limitation on the capability of measuring an anisotropic sky.

A Numerical Simulation of a Stationary Solar Field Augmented by Plane Reflectors: Optimum Design Parameters

In this study, a theoretical analysis of a solar field augmented by a fixed reflector placed in the front between the top of the preceding row and the bottom of the succeeding row is presented. An analytical model has been developed and used to estimate the solar irradiation. The analytical model is based on the anisotropic sky model, assuming an infinite length of collector and reflector rows. A simulation has been carried out in order to figure out the behavior of the solar field and to find the optimum design parameters of the solar field leading to a maximum solar energy augmentation. The results obtained are depicted synoptically as a relationship between the solar field design parameters and the latitude angle, and this presentation enables us to determine the optimum design parameters in order to achieve the intended percentage improvement of solar radiation incident on the solar field rows at any location on the Northern hemisphere, which presents the novelty of this research. Also we have introduced a new parameter named “the effective height of the collector”, which presents the portion of the collector’s height illuminated by the reflector. This parameter is very important especially in case of PV solar fields, because it determines the domain of the concentrated solar energy over the surface of the PV panel.

Probing cosmological isotropy with Planck Sunyaev–Zeldovich galaxy clusters

We probe the statistical isotropy hypothesis of the large-scale structure with the second Planck Sunyaev-Zeldovich (PSZ2) galaxy clusters data set. Our analysis adopts a statistical-geometrical method which compares the 2-point angular correlation function of objects in antipodal patches of the sky. Given possible observational biases, such as the presence of anisotropic sky cuts and the non-uniform exposure of Planck's instrumentation, ensembles of Monte Carlo realisations are produced in order to assess the significance of our results. When these observational effects are properly taken into account, we find neither evidence for preferred directions in the sky nor signs of large-angle features in the galaxy clusters celestial distribution. The PSZ2 data set is, therefore, in good concordance with the fundamental hypothesis of large-angle isotropy of cosmic objects.