Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, \ensuremath{\Lambda}; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is old. This would alleviate the classical problem of the curious energy scale of a millielectron volt associated with a constant \ensuremath{\Lambda}. Dark energy may have been detected by recent cosmological tests. These tests make a good scientific case for the context, in the relativistic Friedmann-Lema\^{\i}tre model, in which the gravitational inverse-square law is applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one-quarter of the critical Einstein--de Sitter value. The case for detection of dark energy is not yet as convincing but still serious; we await more data, which may be derived from work in progress. Planned observations may detect the evolution of the dark-energy density; a positive result would be a considerable stimulus for attempts at understanding the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

/pdf/the-cosmological-constant-and-dark-energy-1w78acsak0.pdf

The Cosmological Constant and Dark Energy

We review the current status of our knowledge of the chemical composition of the Sun, essentially derived from the analysis of the solar photospheric spectrum. The comparison of solar and meteoritic abundances confirms that there is a very good agreement between the two sets of abundances. They are used to construct a Standard Abundance Distribution.

Standard Solar Composition

amount of energy, a tiny fraction of which is sufficient to explode the star as a supernova. The authors examine our current understanding of the lives and deaths of massive stars, with special attention to the relevant nuclear and stellar physics. Emphasis is placed upon their post-helium-burning evolution. Current views regarding the supernova explosion mechanism are reviewed, and the hydrodynamics of supernova shock propagation and ‘‘fallback’’ is discussed. The calculated neutron star masses, supernova light curves, and spectra from these model stars are shown to be consistent with observations. During all phases, particular attention is paid to the nucleosynthesis of heavy elements. Such stars are capable of producing, with few exceptions, the isotopes between mass 16 and 88 as well as a large fraction of still heavier elements made by the r and p processes.

/pdf/the-evolution-and-explosion-of-massive-stars-57jc6k6s1i.pdf

The evolution and explosion of massive stars

Stellar multiplicity is an ubiquitous outcome of the star formation process Characterizing the frequency and main characteristics of multiple systems and their dependencies on primary mass and environment is therefore a powerful tool to probe this process While early attempts were fraught with selection biases and limited completeness, instrumentation breakthroughs in the last two decades now enable robust analyses In this review, we summarize our current empirical knowledge of stellar multiplicity for Main Sequence stars and brown dwarfs, as well as among populations of Pre-Main Sequence stars and embedded protostars Clear trends as a function of both primary mass and stellar evolutionary stage are identified that will serve as a comparison basis for numerical and analytical models of star formation

Stellar Multiplicity

Past surveys have revealed that the large-scale distribution of galaxies in the universe is far from random: it is highly structured over a vast range of scales. Surveys being currently undertaken and being planned for the next decades will provide a wealth of information about this structure. The ultimate goal must be not only to describe galaxy clustering as it is now, but also to explain how this arose as a consequence of evolutionary processes acting on the initial conditions that we see in the cosmic microwave background anisotropy data. In order to achieve this we need to build mathematically quantifiable descriptions of cosmic structure. Identifying where scaling laws apply and the nature of those scaling laws is an important part of understanding which physical mechanisms have been responsible for the organization of clusters of galaxies, superclusters, and the voids between them. Finding where these scaling laws are broken is equally important since this indicates the transition to different underlying physics. In describing scaling laws it is helpful to make analogies with fractals, mathematical constructs that can possess a wide variety of scaling properties. We must beware, however, of saying that the universe is a fractal on some range of scales: it merely exhibits a specific kind of fractal-like behavior on those scales. The richness of fractal scaling behavior is an important supplement to the usual battery of statistical descriptors. This article reviews the history of how we have learned about the structure of the universe and presents the data and methodologies that are relevant to an understanding of any scaling properties that structure may have. The ultimate goal is to have a complete understanding of how that structure emerged. We are getting close!

/pdf/scaling-laws-in-the-distribution-of-galaxies-2j7mbr1swi.pdf

Scaling laws in the distribution of galaxies

Twelve lines of evidence bearing on the determination of the distance to the Crab nebula and NP 0532 are discussed. Some of these depend on the dynamics of the nebular expansion, some on physical processes in the supernova or its remnant, and some on the nature of the intervening interstellar medium. An unweighted average of the various methods yields a distance of about 1930 pc. Key words: Crab nebula - NP 0532- distances to cosmic objects

https://iopscience.iop.org/article/10.1086/129507/pdf

The distance to the crab nebula and np 0532

Stellar interiors : physical principles, structure, and evolution

The basic scheme of nucleosynthesis (building of heavy elements from light ones) has held up very well since it was first proposed more than 30 years ago by E.M. Burbidge, G.R. Burbidge, A.G.W. Cameron, W.A. Fowler, and F. Hoyle. Significant advances in the intervening years include (a) observations of elemental and a few isotopic ratios in many more extrasolar-system sites, including metal-poor dwarf irregular galaxies, where very little has happened, and supernovae and their remnants, where a great deal has happened, (b) recognition of the early universe as good for making all the elements up to helium, (c) resolution of heavy element burning in stars into separate carbon, neon, oxygen, and silicon burning, with fine tuning of the resulting abundances by explosive nucleosynthesis in outgoing supernova shock waves, (d) clarification of the role of Type I supernovae, (e) concordance between elements produced in short-lived and long-lived stars with those that increased quickly and slowly over the history of the galaxy, and (f) calibration of calculations of the evolution and explosion of massive stars against the detailed observations of SN 1987A. The discussion presupposes a reader (a) with some prior knowledge of astronomy at the level of recognizing what is meant by an A star and an AGB star and (b) with at least a mild interest in how we got to where we currently are.

/pdf/the-origin-and-abundances-of-the-chemical-elements-revisited-42dbfw4dmq.pdf

The origin and abundances of the chemical elements revisited

This has been the Year of the Baryon. Some low temperature ones were seen at high redshift, some high temperature ones were seen at low redshift, and some cooling ones were (probably) reheated. Astronomers saw the back of the Sun (which is also made of baryons), a possible solution to the problem of ejection of material by Type II Supernovae (in which neutrinos push out baryons), the production of R Coronae Borealis stars (previously-owned baryons), and perhaps found the missing satellite galaxies (whose failing is that they have no baryons). A few questions were left unanswered for next year, and an attempt is made to discuss these as well.

Virginia Trimble

Papers

Scaling laws in the distribution of galaxies

The distance to the crab nebula and np 0532

Stellar interiors : physical principles, structure, and evolution

The origin and abundances of the chemical elements revisited

Astrophysics in 2002