اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe Enigma of Entropy
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Deepak Dhar
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This is a light-hearted take at the the second law of thermodynamics.
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62 -
Jean-Pierre Luminet (Aix-Marseille Universite
, CNRS
, Laboratoiren dAstrophysique de Marseille
2021
In this pedestrian approach I give my personal point of view on the various problems posed by dark matter in the universe. After a brief historical overview I discuss the various solutions stemming from high energy particle physics, and the current s
tatus of experimental research on candidate particles (WIMPS). In the absence of direct evidence, the theories can still be evaluated by comparing their implications for the formation of galaxies, clusters and superclusters of galaxies against astronomical observations. I conclude briefly with the attempts to circumvent the dark matter problem by modifying the laws of gravity.
Aim. The unusual transient ATLAS17aeu was serendipitously detected within the sky localisation of the gravitational wave trigger GW170104. The importance of a possible association with gravitational waves coming from a binary black hole merger led to
an extensive follow-up campaign, with the aim of assessing a possible connection with GW170104. Methods. With several telescopes, we carried out both photometric and spectroscopic observations of ATLAS17aeu, for several epochs, between $sim 3$ and $sim 230$ days after the first detection. Results. We studied in detail the temporal and spectroscopic properties of ATLAS17aeu and its host galaxy. We detected spectral features similar to those of a broad lined supernova superposed to an otherwise typical long-GRB afterglow. Based on analysis of the optical light curve, spectrum and host galaxy SED, we conclude that the redshift of the source is probably $z simeq 0.5 pm 0.2$. Conclusions. While the redshift range we have determined is marginally compatible with that of the gravitational wave event, the presence of a supernova component and the consistency of this transient with the E$_{rm p}$-E$_{rm iso}$ correlation support the conclusion that ATLAS17aeu was associated with the long gamma-ray burst GRB170105A. This rules out the association of the GRB170105A/ATLAS17aeu transient with the gravitational wave event GW170104, which was due to a binary black hole merger.
Since the 1935 proposal by Einstein Podolsky and Rosen the riddle of nonlocality, today demonstrated by innumerable experiments, has been a cause of concern and confusion within the debate over the foundations of quantum mechanics. The present paper
tackles the problem by a non relativistic approach based on the Weyls conformal differential geometry applied to the Hamilton-Jacobi solution of the dynamical problem of two entangled spin 1/2 particles. It is found that the nonlocality rests on the entanglement of the spin internal variables, playing the role of hidden variables. At the end, the violation of the Bell inequalities is demonstrated without recourse to the common nonlocality paradigm. A discussion over the role of the % textit{internal space} of any entangled dynamical system involves deep conceptual issues, such the textit{indeterminism} of quantum mechanics and explores the in principle limitations to any exact dynamical theory when truly hidden variables are present. Because of the underlying geometrical foundations linking necessarily gravitation and quantum mechanics, the theory presented in this work may be considered to belong to the unifying quantum gravity scenario.
The Cambrian explosion is a grand challenge to science today and involves multidisciplinary study. This event is generally believed as a result of genetic innovations, environmental factors and ecological interactions, even though there are many conf
licts on nature and timing of metazoan origins. The crux of the matter is that an entire roadmap of the evolution is missing to discern the biological complexity transition and to evaluate the critical role of the Cambrian explosion in the overall evolutionary context. Here we calculate the time of the Cambrian explosion by an innovative and accurate C-value clock; our result (560 million years ago) quite fits the fossil records. We clarify that the intrinsic reason of genome evolution determined the Cambrian explosion. A general formula for evaluating genome size of different species has been found, by which major questions of the C-value enigma can be solved and the genome size evolution can be illustrated. The Cambrian explosion is essentially a major transition of biological complexity, which corresponds to a turning point in genome size evolution. The observed maximum prokaryotic complexity is just a relic of the Cambrian explosion and it is supervised by the maximum information storage capability in the observed universe. Our results open a new prospect of studying metazoan origins and molecular evolution.
Determining the distance to the open cluster M29 (NGC 6913) has proven difficult, with distances determined by various authors differing by a factor of two or more. To solve this problem, we have initiated a new photometric investigation of the clust
er in the Vilnius seven-color photometric system supplementing it with available data in the BV and JHK_s photometric systems, and spectra of the nine brightest stars of spectral classes O and B. Photometric spectral classes and luminosities of 260 stars in a 15 arcmin x 15 arcmin area down to V = 19 mag are used to investigate the interstellar extinction run with distance and to estimate the distance of the Great Cygnus Rift, 800 pc. The interstellar reddening law in the optical and near-infrared regions is found to be close to normal, with the ratio of extinction to color excess R(BV) = 2.87. The extinction A(V) of cluster members is between 2.5 to 3.8 mag, with a mean value of 2.97 mag or E(B-V) = 1.03. The average distance of eight stars of spectral types O9-B2 is 1.54 +- 0.15 kpc. Two stars from the seven brightest ones are field stars: HDE 229238 is a background B0.5 supergiant and HD 194378 is a foreground F star. In the intrinsic color-magnitude diagram, seven fainter stars of spectral classes B3-B8 are identified as possible members of the cluster. The 15 selected members of the cluster of spectral classes O9--B8 plotted on the log L/L(sun) vs. log T(eff) diagram, together with the isochrones from the Padova database, give the age of the cluster as 5 +- 1 Myr.
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