Monday, March 5, 2007

Cosmological Constant and Dark Energy

1. What is Einstein’s cosmological constant?

“A constant term (labeled Lambda), which Einstein added to his general theory of relativity in the mistaken belief that the Universe was neither expanding nor contracting.”

The cosmological constant adds equations to the general theory of relativity to show that we live in a static universe, that is, one that is not expanding or contracting.

Later, the cosmological constant was found to be unnecessary. The observations of Edwin Hubble, as well as Saul Perlmutter, showed that the universe was indeed expanding.

However, the discovery of cosmic acceleration in the 1990s has renewed interest in a cosmological constant because cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate.


2. Why did Einstein create his cosmological constant?

“Einstein included the cosmological constant as a term in his field equations for general relativity because he was dissatisfied that otherwise his equations did not allow, apparently, for a static universe. Gravity would cause a universe which was initially at dynamical equilibrium to contract. To counteract this possibility, Einstein added the cosmological constant. However, soon after Einstein developed his static theory, observations by Edwin Hubble indicated that the universe appears to be expanding; this was consistent with a cosmological solution to the original general-relativity equations that had been found by the mathematician Friedman.”

Einstein’s own belief that the universe was static overshadowed his observations. He was so certain of this that he modified his equations introducing a cosmological constant that would support his personal beliefs

“When physicist Edwin Hubble discovered that the universe was not static but was expanding, Einstein called his cosmological constant "the greatest mistake of my life". Einstein effectively abandoned his cosmological constant when he learned of Hubble’s discoveries.
The cosmological constant does, however, have a major connection with dark energy and the expanding universe and had Einstein stuck to his guns, he could have been given credit for predicting one of the great scientific findings of the last 10 years - the accelerating universe,” states Neatorama.


3. What is dark energy?

According to the Contemporary Physics Education Project, “An energy causing the acceleration of the expansion of the universe, detectable through its gravitational effects.” “…dark energy is a hypothetical form of energy that permeates all of space and has strong negative pressure. According to the Theory of Relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales,” writes Wikipedia.

This energy could make up almost two-thirds of the Universe and since it is repulsive, it shoots galaxies away from each other with an increasing speed. Dark energy is thought to be the only thing that could cause the expansion of the universe to accelerate in this way.
It may also account for a big portion of the mass still missing in the universe.

4. Why did cosmologists invent dark energy?

Dark energy is an idea that is used to describe a different form of energy that penetrates all of space. Ordinary gravity consists of an attractive force, where as, dark energy is a repulsive force. The dark energy could also account for a larger percentage of the mysterious “missing mass” of the universe

“Cosmologists know something is driving an accelerated and repulsive expansion of the universe - something that is acting like an anti-gravity force. Because this energy has never been directly seen and its identity is as yet unknown, it is called dark energy.”

5. How is dark energy related to the cosmological constant?

“Physicists have tried to explain the acceleration in terms of “dark energy”, which boosts the expansion of the universe by counteracting the effects of gravity. The most popular explanation for dark energy draws on the “cosmological constant” first proposed by Einstein. Observations reveal that dark energy was around nine billion years ago and has been acting in a consistent way ever since. The data suggest that the effect of dark energy was rather weak until about five to six billion years ago when it defeated gravity in a “cosmic tug of war” and the rate of expansion began to increase.”

This cosmological constant added a repulsive force to counteract the affect gravity had on the universe. This gravitational affect would make the universe contract, so Einstein needed a repulsive force to cancel out the affects gravity would have on the universe thus making the universe static. This repulsive force could be dark energy.

The dark energy may be the vehicle that drives the concept of the cosmological constant.
The idea of a counteractive force equal to the gravity is the concept behind the cosmological constant. However, there seems to be more dark energy than gravity. The dark energy outweighs the gravitational affect, so there is more of a repulsive force than a contracting force. Thus the universe is expanding

NASA,
Cosmological Constant, Wikipedia,
Dark Energy, Wikipedia,<>
Cosmological Constant, Wikipedia,
Four Things Einstein Got Wrong, Neatorama, December 19th, 2006,
Contemporary Physics education Project,
Wikipedia,
NASA Selects ADEPT Space Mission To Probe Dark Energy,
Dark energy dates back nine billion years, Hamish Johnston, PhysicsWeb, <>

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