skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: The Expanding Universe: Dark Energy

Abstract

In 1998, observations of distant supernovae led physicists that not only was the universe expanding, but the expansion was speeding up. In this article, we describe the evidence for an expanding universe and describe what physicists and cosmologists have learned in the intervening years. The target audience for this article is high school physics teachers and college physics professors at teaching institutions.

Authors:
ORCiD logo [1];  [1]
  1. Fermilab
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1415780
Report Number(s):
FERMILAB-FN-0987-AE
1313549
DOE Contract Number:
AC02-07CH11359
Resource Type:
Journal Article
Resource Relation:
Journal Name: Phys.Teacher; Journal Volume: 52; Journal Issue: 6
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

Lincoln, Don, and Nord, Brian. The Expanding Universe: Dark Energy. United States: N. p., 2014. Web. doi:10.1119/1.4893086.
Lincoln, Don, & Nord, Brian. The Expanding Universe: Dark Energy. United States. doi:10.1119/1.4893086.
Lincoln, Don, and Nord, Brian. Mon . "The Expanding Universe: Dark Energy". United States. doi:10.1119/1.4893086. https://www.osti.gov/servlets/purl/1415780.
@article{osti_1415780,
title = {The Expanding Universe: Dark Energy},
author = {Lincoln, Don and Nord, Brian},
abstractNote = {In 1998, observations of distant supernovae led physicists that not only was the universe expanding, but the expansion was speeding up. In this article, we describe the evidence for an expanding universe and describe what physicists and cosmologists have learned in the intervening years. The target audience for this article is high school physics teachers and college physics professors at teaching institutions.},
doi = {10.1119/1.4893086},
journal = {Phys.Teacher},
number = 6,
volume = 52,
place = {United States},
year = {Mon Sep 01 00:00:00 EDT 2014},
month = {Mon Sep 01 00:00:00 EDT 2014}
}
  • At present the expanding universe is observed to be dominated by the not fully understood concepts of dark energy and matter, in a conceived almost flat Euclidian geometry. As one of the possible efforts to understand its global behaviour, the present paper attempts to explain these concepts in terms of the pressure force and gravity of a spherical photon gas cloud of zero point energy, in flat geometry. A difficult point concerns the frequency distribution of the zero point energy oscillations which leads to the unacceptable result of an infinite total energy. A modification of this distribution is therefore proposedmore » which results in finite energy density. A corresponding equilibrium is investigated, as well as small dynamic deviations from it, to form a basis for a model of the expanding universe. Provided that the crucial points of the present approach hold true, the model satisfies the requirements of cosmic linear dimensions, results in an estimated acceleration of the expansion being of the order of the observed one, presents a possible solution of the coincidence problem of dark energy and matter, and provides one of the possible explanations of the observed excess of high-energy electrons and positrons in recent balloon and satellite experiments.« less
  • We investigate the structure of cold dark matter halos using advanced models of spherical collapse and accretion in an expanding universe. These are based on solving time-dependent equations for the moments of the phase-space distribution function in the fluid approximation; our approach includes non-radial random motions and, most importantly, an advanced treatment of both dynamical relaxation effects that take place in the infalling matter: phase-mixing associated with shell crossing and collective collisions related to physical clumpiness. We find self-similar solutions for the spherically averaged profiles of mass density {rho}(r), pseudo phase-space density Q(r), and anisotropy parameter {beta}(r). These profiles agreemore » with the outcomes of state-of-the-art N-body simulations in the radial range currently probed by the latter; at smaller radii, we provide specific predictions. In the perspective provided by our self-similar solutions, we link the halo structure to its two-stage growth history and propose the following picture. During the early fast collapse of the inner region dominated by a few merging clumps, efficient dynamical relaxation plays a key role in producing closely universal mass density and pseudo phase-space density profiles; in particular, these are found to depend only weakly on the detailed shape of the initial perturbation and the related collapse times. The subsequent inside-out growth of the outer regions feeds on the slow accretion of many small clumps and diffuse matter; thus the outskirts are only mildly affected by dynamical relaxation but are more sensitive to asymmetries and cosmological variance.« less
  • A simple evolution equation is derived for b(t), the ratio of gravitational correlation energy to kinetic energy in the expanding universe, which gives the observed asymptotic value of 0.7 and also predicts the behavior of the ratio at earlier times. The equation is obtained by combining the cosmic energy equation with the BBGKY clustering hierarchy. The relationship of the solutions of b(t) to the thermodynamic description of gravitational clustering in the expanding universe is briefly discussed. 13 references.
  • We study a quantum scalar field in a de Sitter universe. The initial state of the system is of adiabatic order four, and not necessarily de Sitter invariant. We define an adiabatic number basis in which the energy-momentum tensor has a quasi-classical form in terms of the number of particles present. Numerical results are presented for a massive field with {xi}=1/6 (conformal coupling) and {xi}=0 (minimal coupling). {copyright} {ital 1999 American Institute of Physics.}