Supernovae, an accelerating universe and the cosmological constant

Abstract

Observations of supernova explosions halfway back to the Big Bang give plausible evidence that the expansion of the universe has been accelerating since that epoch, approximately 8 billion years ago and suggest that energy associated with the vacuum itself may be responsible for the acceleration.For 40 years, astronomers have hoped to measure changes in the expansion rate of the universe as a way to measure the mass density of the universe and the geometry of space and to predict the future of cosmic expansion. In 1998, two groups reported plausible evidence based on supernova explosions that the expansion of the universe is not slowing down, as predicted by the simplest models, but actually accelerating. If these results are confirmed, it will require a major change in our picture for the universe. We will be forced to add another constituent to our best model for the universe, a form of vacuum energy that drives the expansion, which makes the large-scale geometry Euclidean, and which contains most of the energy density in the universe (1). This paper aims to sketch the background to this discovery, to show some of the evidence for cosmic acceleration, and to equip an interested, but skeptical, reader with the right kinds of questions to ask of astrophysical colleagues.Astronomers have known since Hubble's observations in 1929 that the universe is expanding (2). This was promptly incorporated into a dynamical picture of the universe based on general relativity, which describes how the presence of matter, or other energy forms in the universe, affect the curvature of space and the expansion of the universe. A decade before the discovery of cosmic expansion, Einstein introduced a ''cosmological constant'' into his equations, to make the universe static, in accord with the astronomical wisdom of the day. When the astronomical evidence changed, he quickly abandoned the cosmological constant and much later referred to it as his ''greatest blunder'' (3). Since 1929, it has been the burning ambition of observers of the expanding universe to determine the energy content and the curvature from astronomical measurements. In 1998, we may have achieved that long-sought-after goal.The observational problem is to discover objects that can be seen at large redshifts, so the cosmological effects are large enough to measure, and that are well enough understood so that their apparent brightness can be trusted to give a reliable measure of their distance. The long, winding path of observational cosmology is littered with the wreckage of past attempts to do this with galaxies, whose properties evolve over time much too rapidly to serve as ''standard candles'' for this work. But type Ia supernovae (SN Ia) can be seen to redshift 1, and their intrinsic scatter in brightness is small enough so that the cosmological effects on the observed brightness as a function of redshift can be measured. At a redshift of 0.5, the difference in apparent magnitude between a universe that is flat, decelerating, and just barely closed by matter, ⍀ m ...

Neutrinos and the Weak Interactions

“…These results were subsequently confirmed by the observations of Perlmutter and co-workers and other teams [7,8], while the Superkamiokande experiments also confirmed the mass of the neutrino, with the predicted value. The cosmological model referred to uses fluctuations in particle numbers and deduced moreover otherwise empirically well known supposedly mysterious, inexplicable coincidences-the so called Large Number Relations and the Weinberg formula [9].…”

From the neutrino to the edge of the universe

“…The first is the Superkamiokande experiment[1] which demonstrates a neutrino oscillation and therefore a non zero mass, whereas, strictly going by the standard model, the neutrino should have zero mass. The other finding based on distant supernovae observations [2,3,4] is that the universe will continue to expand without deceleration and infact possibly accelerating in the process. We will now demonstrate how a recent model of fractal, quantized space time arising from the underpinning of a quantum vaccuum or Zero Point Field, reconciles both the above facts, in addition to being in agreement with other experimental and observational data.…”

Construcción de un diagrama de Hubble: Una herramienta para la Enseñanza de la Astronomía - Building a Hubble Diagram: A Tool for Teaching Astronomy

“…Los primeros diagramas de Hubble que los estudiantes obtienen en la actividad son diagramas que presentan magnitud versus corrimiento al rojo que son los datos que presenta directamente la base de datos SDSS. Kirshner (1999) difundió este tipo de diagramas. El inconveniente con estos diagramas (figura 2) es la obtención de la pendiente, o parámetro de Hubble, teniendo que realizar los cambios a distancia y velocidad en cada caso.…”