104 research outputs found
The Kali Yuga and Camus’ \u3cem\u3eThe Plague\u3c/em\u3e
Hinduism is a religion in which scientific observations play an important role. Through extensive observation of astronomical phenomena, Hindu cosmologists created a calendar based on the relative positions of planets to the moon. This calendar played an important role understanding the Hindu cycle of the creation and destruction of the universe. This cycle is referred to as the cycle of the Yugas, in which the increasingly vice-ridden world is destroyed and replaced with a morally and religiously pristine one. Camus’ The Plague explores this cycle of human malice and immorality, which is corrected by natural or supernatural forces as an allegory for the Hindu cycle of the Yugas
Variational Inference as an alternative to MCMC for parameter estimation and model selection
Most applications of Bayesian Inference for parameter estimation and model
selection in astrophysics involve the use of Monte Carlo techniques such as
Markov Chain Monte Carlo (MCMC) and nested sampling. However, they are time
consuming and their convergence to posterior is difficult to determine. In this
work, we introduce variational inference (VI) as an alternative to solve
astrophysics problems, and demonstrate its usefullness for parameter estimation
and model selection. Variational inference converts the inference problem into
an optimization problem by approximating the posterior from a known family of
distributions and using Kullback-Leibler divergence to characterize the
difference. It takes advantage of fast optimization techniques, which make it
ideal to deal with large datasets and makes it trivial to parallelize. We
derive a new approximate evidence estimation based on variational posterior and
importance sampling technique called posterior weighted importance sampling for
evidence (PWISE), which is useful to perform Bayesian model selection. As a
proof of principle, we apply variational inference to five different problems
in astrophysics, where Monte Carlo techniques were previously used. These
include assessment of significance of annual modulation in the COSINE-100 dark
matter experiment, measuring exoplanet orbital parameters from radial velocity
data, tests of periodicities in measurements of Newton's constant ,
assessing the significance of a turnover in the spectral lag data of GRB
160625B and estimating the mass of a galaxy cluster using weak gravitational
lensing. We find that variational inference is much faster than MCMC and nested
sampling techniques for most of these problems while providing competitive
results. All our analysis codes have been made publicly available.Comment: 15 pages, 3 figures. Added one more example and introduced and
applied a new metric for calculating Bayesian evidence in Variational
Inferenc
Hemispheric sunspot numbers 1874--2020
We create a continuous series of daily and monthly hemispheric sunspot
numbers (HSNs) from 1874 to 2020, which will be continuously expanded in the
future with the HSNs provided by SILSO. Based on the available daily
measurements of hemispheric sunspot areas from 1874 to 2016 from Greenwich
Royal Observatory and NOAA, we derive the relative fractions of the northern
and southern activity. These fractions are applied to the international sunspot
number (ISN) to derive the HSNs. This method and obtained data are validated
against published HSNs for the period 1945--2020. We provide a continuous data
series and catalogue of daily, monthly mean, and 13-month smoothed monthly mean
HSNs for the time range 1874--2020 that are consistent with the newly
calibrated ISN. Validation of the reconstructed HSNs against the direct data
available since 1945 reveals a high level of consistency, with a correlation of
r=0.94 (0.97) for the daily (monthly) data. The cumulative hemispheric
asymmetries for cycles 12-24 give a mean value of 16%, with no obvious pattern
in north-south predominance over the cycle evolution. The strongest asymmetry
occurs for cycle no. 19, in which the northern hemisphere shows a cumulated
predominance of 42%. The phase shift between the peaks of solar activity in the
two hemispheres may be up to 28 months, with a mean absolute value of 16.4
months. The phase shifts reveal an overall asymmetry of the northern hemisphere
reaching its cycle maximum earlier (in 10 out of 13 cases). Relating the ISN
and HSN peak growth rates during the cycle rise phase with the cycle amplitude
reveals higher correlations when considering the two hemispheres individually,
with r = 0.9. Our findings demonstrate that empirical solar cycle prediction
methods can be improved by investigating the solar cycle dynamics in terms of
the hemispheric sunspot numbers.Comment: Accepted by Astron. Astrophys. 12 page
The mass and galaxy distribution around SZ-selected clusters
We present measurements of the radial profiles of the mass and galaxy number density around Sunyaev–Zel’dovich (SZ)-selected clusters using both weak lensing and galaxy counts. The clusters are selected from the Atacama Cosmology Telescope Data Release 5 and the galaxies from the Dark Energy Survey Year 3 data set. With signal-to-noise ratio of 62 (45) for galaxy (weak lensing) profiles over scales of about 0.2–20 h-1 Mpc, these are the highest precision measurements for SZ-selected clusters to date. Because SZ selection closely approximates mass selection, these measurements enable several tests of theoretical models of the mass and light distribution around clusters. Our main findings are: (1) The splashback feature is detected at a consistent location in both the mass and galaxy profiles and its location is consistent with predictions of cold dark matter N-body simulations. (2) The full mass profile is also consistent with the simulations. (3) The shapes of the galaxy and lensing profiles are remarkably similar for our sample over the entire range of scales, from well inside the cluster halo to the quasilinear regime. We measure the dependence of the profile shapes on the galaxy sample, redshift, and cluster mass. We extend the Diemer & Kravtsov model for the cluster profiles to the linear regime using perturbation theory and show that it provides a good match to the measured profiles. We also compare the measured profiles to predictions of the standard halo model and simulations that include hydrodynamics. Applications of these results to cluster mass estimation, cosmology, and astrophysics are discussed. © 2021 The Author(s)
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