Open Questions: Dark Energy

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See also: The cosmological constant -- The vacuum -- Dark matter

Introduction

Reading the redshift

Measuring cosmic distances

The cosmological constant

Quintessence

Modified general relativity


Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books

Introduction



Recommended references: Web sites

Site indexes


Sites with general resources

Supernova/Acceleration Probe
Home page of proposed satellite mission to measure supernova distances very precisely in order to determine rate of acceleration of expansion of the universe. This would involve a more precise determination of the cosmological constant and matter density of the universe.
Resource Book on Dark Energy
A directory of survey papers on dark energy, focused on the nature of dark energy and the study of it by means of supernova observations. Includes other references. Maintained by Eric V. Linder.
SuperNova Legacy Survey
Home page of research group producing better measurements of the universe's expansion. "The Canada-France-Hawaii Telescope Legacy Survey Supernova Program aims primarily at measuring the equation of state of Dark Energy. It is designed to precisely measure several hundred high-redshift supernovae." Important results were announced November 22, 2005.
HETDEX - The Hobby-Eberly Telescope Dark Energy Experiment
A research project to investigate dark energy using a ground-based telescope to map the expansion of the universe. "HETDEX is a project designed to understand the evolutionary history of dark energy. ... The goal is to use results from HETDEX to precisely describe how the Universe expands, thereby illuminating the nature of dark energy."
Evidence for Cosmic Acceleration from Supernovae
Slide presentation by Robert Kirshner at the 2001: A Spacetime Odyssey conference.
The Runaway Universe
Companion Web site to the November 2000 PBS documentary.
High Redshift Supernova Search
Home page of the Berkeley Supernova Cosmology Project.
The High-Z SN Search
Home page of Harvard-based research group searching for high red-shift (very distant) supernovae in order to determine rate of expansion of the universe.


Surveys, overviews, tutorials

Accelerating universe
Article from Wikipedia. See also Dark energy, Quintessence.
Dark Energy
Excellent single-page overview of dark energy by Adam Riess, leader of the High-z team, which discovered evidence that the expansion of the universe was accelerating.
Dark Energy and the Preposterous Universe
A brief overview of the place in cosmology of dark energy and the cosmological constant, by Sean M. Carroll. A shorter and more elementary page on this topic by Carroll is here. A more detailed exposition with some mathematics is here. A list of many other reviews and talks by Carroll on related topics in general relativity and cosmology is here.
What Is Dark Energy?
April 2009 Space.com article giving a brief overview of the topic.
The Cosmological Constant
2000 review paper on the physics and cosmology of the cosmological constant, by Sean M. Carroll.
What Is Dark Energy?
Very brief page about NASA's plans to investigate dark energy as part of the Beyond Einstein program.
3D map of Universe bolsters case for dark energy and dark matter
A page at Max Tegmark's site describing observations of galaxy distributions reported in October 2003 which confirm the existence of dark energy making up 70% of the universe. It is based on results obtained from the Sloan Digital Sky Survey.
Dark questions remain over dark energy
December 2009 article about whether the discovery of different kinds of type1a supernovae may affect conclusions about the existence of dark energy.
The hunt for dark energy
Brief April 2009 article on dark energy research.
Dark energy may disguise shape of universe
August 2009 New Scientist article. "As we learn more about dark energy and its effect on the expansion of space and time, we find that dark energy and the shape, or geometry, of the universe are worryingly intertwined."
Dark energy: how the paradigm shifted
June 2010 article from Physics World. "The idea that our universe is dominated by mysterious dark energy was revealed by two paradigm-shifting studies of supernovae published in 1998. Lucy Calder and Ofer Lahav explain how the concept had in fact been brewing for at least a decade before - and speculate on where the next leap in our understanding might lie."
Dark energy: the decade ahead
December 2007 article from Physics World, by Eric Linder and Saul Perlmutter. "Ten years after astrophysicists discovered that the expansion of the universe is accelerating, further measurements have given us few clues into the nature of the dark energy that drives it. But, as the authors describe, advances in observational techniques promise to shed light on this revolutionary physics in the decade ahead."
A dark future for cosmology
December 2007 article from Physics World, by Lawrence Krauss. "Even with the many observations planned over the next decade, there is a real chance that we will never understand the true nature of dark energy."
Dark Energy
December 2007 article from Physics World, by Robert Crease. "The competition between the two teams of scientists that discovered that the expansion of the universe is accelerating reveals how hard it can be to assign scientific credit - particularly when a Nobel prize may one day be at stake."
Dark energy
May 2004 article from Physics World, by Robert R. Caldwell. "New evidence has confirmed that the expansion of the universe is accelerating under the influence of a gravitationally repulsive form of energy that makes up two-thirds of the cosmos."
Quintessence
November 2000 article from Physics World, by Robert R. Caldwell and Paul J. Steinhardt. "Cosmologists have proposed that a mysterious substance called quintessence can explain why our universe is accelerating. But what is it made of?" Roughly 70% of the value of Omega requied for a flat universe may consist of some sort of "dark energy". It may be energy related to the cosmological constant, or some other form of energy that has been named "quintessence". Article has good links to other resources.
String theorist explores dark energy and our unique 'pocket' of the universe
February 2005 press release describing a talk by Leonard Susskind on dark energy, string theory, and "pocket universes".
Dark Energy: Astronomers Still 'Clueless' About Mystery Force Pushing Galaxies Apart
January 2002 article from Space.com. Presents general overview of the concept of dark energy and observations that suggest its existence.
Dark Energy Lecture
A presentation by Michael Turner, in PDF format, from an October 2001 conference on cosmology.
Dark Energy in the Accelerating Universe
A "brochure" describing the basic facts and theories about dark energy and the rate of expansion of the universe. Good explanations, marred by not so good site design.
Dark Energy and Particle Astrophysics
June 2001 lecture by Eric Linder, in PDF format.
Dark Energy
Basic information on dark energy. Part of NASA's Imagine the Universe site.
More evidence for the accelerating universe
September 1999 news article from PhysicsWeb about improved evidence for accelerating expansion of the universe.
Evidence mounts that the expansion of the Universe is accelerating
November 1998 news article from PhysicsWeb about supernova evidence that the expansion of the universe is accelerating.
The Next Frontier for Cosmologists: Dark Energy
May 2001 press release from Dartmouth College, providing a brief explanation of dark energy, by Robert Caldwell.
Starry data support revved-up cosmos
March 2001 news article in Science News about new supernova data that further support the hypothesis of dark energy.
Dark Matter and Dark Energy in the Universe
1998 technical/expository paper by Michael S. Turner.
Why Now?
November 20, 2000 Physical Review Focus article, about the coincidently similar magnitudes of the densities of matter, "dark energy", and ordinary energy in the universe.
Turning On the Universe
An article about quintessence from the Physical Review Focus (February 24, 2000).
Dark Forces at Work
April 2008 Scientific American article by David Appell. "Ten years ago two teams discovered that the universe will expand forever at an ever faster rate, thanks to an unseen energy. The leader of one of the groups, Saul Perlmutter, expects that new observations will soon illuminate the universe's dark side."
Discovering a Dark Universe: A Q&A with Saul Perlmutter
April 2008 Scientific American article. "Dark energy is pushing the universe apart at an ever faster rate. Astrophysicist Saul Perlmutter recounts the experimental approaches he took to make that discovery."
Boom or Bust
October 1999 Scientific American Science and the Citizen article, subtitled "New doubts about whether the universe's expansion is accelerating." (The doubts have since appeared to be unfounded.)
A Hundred Billion Years of Solitude
April 1999 Scientific American Science and the Citizen article, subtitled "Evidence for an accelerating universe continues to pile up."
Dark Energy Fills the Cosmos
June 1999 article by Paul Preuss in the LBL Science Beat.


Recommended references: Magazine/journal articles

Does Dark Energy Really Exist?
Timothy Clifton; Pedro G. Ferreira
Scientific American, April 2009
The Universe's Invisible Hand
Christopher J. Conselice
Scientific American, February 2007
Dark Doings
Ron Cowen
Science News, May 22, 2004
Searching for signs of a force that may be everywhere . . . or nowhere
Too Cold for Comfort
George Musser
Scientific American, May 2005
A Cosmic Conumdrum
Lawrence M. Krauss; Michael S. Turner
Scientific American, September 2004
The Expanding Universe: From Slowdown to Speedup
Adam G. Reiss; Michael S. Turner
Scientific American, February 2004
Distant supernovae are revealing the crucial time when the expansion of the universe changed from decelerating to accelerating.
Will Dark Energy Steal All the Stars?
Steve Nadis
Astronomy, March 2003, pp. 42-47
Dark energy now appears to be a result of a positive cosmological constant producing "vacuum energy" making up about 70% of the mass-energy in the universe. This produces an expansion of the universe at an accelerating rate. If this hypothesis is correct, about 100 billion years from now no part of the universe other than stars in our local group of galaxies will be visible.
Moving Right Along
Mario Livio
Astronomy, July 2002, pp. 34-39
Measurements of both the brightness of distant type Ia supernovae and temperature fluctuations in the cosmic microwave background point to a universe that is composed of about 65% "dark energy". There is, however, no theory that explains this value.
A Dark Force in the Universe
Ron Cowen
Science News, April 7, 2001, pp. 218-220
The accelerating expansion of the universe detected in 1998 has stimulated several theories of "dark energy" or "quintessence". Usually associated with the cosmological constant and the energy of the vacuum, the density of this energy may have changed over the life of the universe.
Very Dark Energy
Karen Wright
Discover, March 2001, pp. 70-76
Various recent observations indicate that the universe is both flat and also expanding at an accelerating rate. Some sort of "dark energy" distributed in empty space seems required to account for this.
The Quintessential Universe
Jeremiah P. Ostriker; Paul J. Steinhardt
Scientific American, January 2001, pp. 46-53
It now appears that dark matter of any kind can account for only about 30% of the "stuff" in the universe which determines its course of expansion. The remaining 70% is "dark energy", which could represent either a non-zero cosmological constant (vacuum energy) or some other type of energy now referred to as "quintessence".
More Than Meets the Eye
Michael S. Turner
The Sciences, November/December 2000, pp. 32-37
Attempts to determine the amount of matter in the universe, both visible and invisible, indicate that the total mass of such matter is at most 35% of the amount required for a "flat" universe. A non-zero cosmological constant, representing the presence of quantum-vacuum energy, could make up the difference.
Hubble Warrior
Sally Stephens
Astronomy, March 2000, pp. 52-59
The story of measuring the Hubble constant and its relation to the age of the universe is told through a sketch of the career of astronomer Wendy Freedman.
Revved-Up Universe
Ron Cowen
Science News, February 12, 2000, pp. 106-108
Evidence continues to accumulate that cosmic expansion is accelerating, which would imply higher estimates for the age of the universe. One line of evidence is observations of distant supernovas, which seem to be dimmer (hence more distant) than expected. Other evidence involves interpretation of density fluctuations in the cosmic microwave background. This suggests the geometry of the universe is flat, which implies additional mass in the form that would explain accelerating expansion.
White Dwarfs Confront the Universe
Ken Croswell
Astronomy, May 1996, pp. 42-47
New, more powerful telescopes are making it possible to identify white dwarf stars at increasingly large distances. In particular, we can now detect white dwarfs in the galactic halo, making it easier to determine the age of our galaxy and establish lower limits on the age of the universe itself.
Is There Really a Cosmological Crisis?
Rick Shaffer
Skeptic, Vol. 3, No. 4, 1995, pp. 42-45
If stars are older than the universe, there is a problem.


Recommended references: Books

David B. Cline, ed. -- Sources and Detection of Dark Matter and Dark Energy in the Universe
Springer-Verlag, 2001
Proceedings of a symposium held in Marina del Rey in 2000. Although intended for specialists, most of it is understandable by anyone who has mastered a college astronomy course. This is a long volume with many papers, which reflects the breadth of the subject area -- the greatest part of the universe which is not directly observable (both dark matter and dark energy). Additionally, both theoretical and observational issues are covered. It's a good place to look and browse in order to get one's arms around many of the principle open questions in this mysterious topic.
Peter Coles; Feorge F. R. Ellis - Is the Universe Open of Closed? The Density of Matter in the Universe
Cambridge University Press, 1997
This is a technical work with some mathematics that deals with evaluating the different kinds of evidence which bear on the question in the title. For readers patient enough to follow the mathematics, it shows how most observations tend to the conclusion that the density of matter is about 20% of the critical density.

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Copyright © 2002 by Charles Daney, All Rights Reserved