Dark Energy: Break-through of year 2003

Discovering the truth about ‘Dark Energy’ is the top break-through of year 2003 science arena. New evidence cemented the bizarre idea that the universe is made mostly of mysterious ‘dark matter’, being stretched apart by an unknown force called ‘Dark Energy’.
Information from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite and the Sloan Digital Sky Survey (SDSS) telescopes confirmed some of cosmologists' strangest proposals about the fate of the universe.
The implications of these discoveries about the universe are truly stunning. Cosmologists have been trying for years to confirm the hypothesis of a dark universe.
Dark energy entered the astronomical scene in 1998, after two groups of astronomers made a survey of exploding stars, or supernovas, in a number of distant galaxies. These researchers found that the supernovas were dimmer than they should have been, and that meant they were farther away than they should have been. The only way for that to happen, the astronomers realized, was if the expansion of the universe had speed up at some time in the past.
Until then, astronomers had generally believed that the cosmic expansion was gradually slowing down, due to the gravitational tugs that individual galaxies exert on one another. But the supernova results implied that some mysterious force was acting against the pull of gravity, causing galaxies to fly away from each other at ever greater speeds.
At first, other researches questioned the result; perhaps the supernovas were dimmer because their light was being blocked by clouds of interstellar dust. Or may be the supernovas themselves were intrinsically dimmer than scientists thought. But with careful checking, and more data, those explanations have largely been put aside, and the dark energy hypothesis has held up.
In one sense, the idea is not completely new. Einstein had included such an “anti-gravity effect in his theory of general relativity, in his so-called cosmological constant. But Einstein himself and later many other astronomers, came to regard this as a kind of mathematical contrivance that had little relationship to the real universe. By the 1990s no one expected that the effect would turn out to be real.
The supernova evidence suggests that the acceleration kicked in about 5 billion years ago. At that time, galaxies were far enough apart that their gravity (which weakens with distance) was overwhelmed by the relatively gentle but constant repulsive force of dark energy. Since then, dark energy's continuing push has been causing the cosmic expansion to speed up, and it seems likely now that this expansion will continue indefinitely.
Dark energy is causing quite a bit of upset for astronomers who have to adjust to an unexpected and outlandish new view of the universe. Already, scientists have had to accept the notion of dark matter, which is now thought to far outnumber ordinary matter in the universe, but which has never been detected in any laboratory. Now, the arrival of an unknown force that rules cosmic expansion has added insult to injury.
Now, we shall discuss here that what is ‘Dark Energy’---
A landmark discovery of the 1990s was that the expansion of the Universe is accelerating. The source of this mysterious force opposing gravity we call ‘dark energy’.
Because he originally thought the Universe was static. Einstein conjectured that even the emptiest possible space, devoid of matter and radiation, might still have a dark energy, which he called a “Cosmological Constant”. When Edwin Hubble discovered the expansion of the Universe, Einstein rejected his own idea, calling it his greatest blunder.
As Richard Feynman and other developed the quantum theory of matter, they realized that “empty space” was full of temporary (“virtual”) particles continually forming and destroying themselves. Physicists began to suspect that indeed the vacuum ought to have a dark form of energy, but they could not predict its magnitude.
Through recent measurements of the expansion of the Universe, astronomers have discovered that Einstein's “blunder” was not a blunder ; some form of dark energy does indeed appear to dominate the total mass-energy content of the Universe, and its weird repulsive gravity is pulling the Universe apart. We still do not know whether or how the highly accelerated expansion in the early Universe (inflation) and the current accelerated expansion (due to dark energy) are related.
A Beyond Einstein mission will measure the expansion accurately enough to learn whether this energy is a constant property of empty space as Einstein conjectured or whether it shows signs of the richer structure that is possible in modern unified theories of the forces of nature.
Several years earlier the international Supernova Cosmology Project, based at the Department of Energy's Lawrence Berkeley National Laboratory, had developed a way to find many of these bright exploding stars, once thought to occur too randomly for systematic search. By 1998, the Supernova Cosmology Project and another team using the same method, the High-Z Supernova search Team basd at the Mount Stromlo and Siding Spring Observatories in Australia, had recorded several dozen supernovae, including some so distant that their light had started toward Earth when the universe was only a fraction of its present age.
Their goal was to measure changes in the expansion rate of the universe, which in turn would yield clues to the origin, structure, and fate of the cosmos. Like everyone else, the researchers assumed that expansion had been slowing under the gravitational attraction of matter since shortly after the Big Bang and that this deceleration rate could be used to determine the average density of matter in the universe.
The last thing the two teams expected to find was that the expansion of the universe is not slowing at all. Instead, it is accelerating.
“The implications of these discoveries about the universe are stunning. Cosmologist have been trying for years to confirm the hypothesis of a dark universe”.
Last Year, WMAP took the most detailed picture ever of the cosmic microwave background - the emitted by the universe during the first instant of its existence. By analyzing patterns in this light, researchers concluded that the universe is only 4 per cent ordinary matter. Twenty-three percent is dark matter, which astrophysicists believe is made up of a currently unknown particle. The remainder, 73 percent, is dark energy.
WMAP also nailed down other basic properties of the universe, including its age (13•7 billion years old), expansion rate and density.
The SDSS, an effort to map out a million galaxies, also made major contribution to our understanding of the universe this year. By analyzing how galaxies are spread out through space, the researchers can see if the galaxies are being pulled apart by dark energy or pushed together by gravity.
In October, the SDSS team reported its analysis of the first quarter-million galaxies. Its conclusion was the same as WMAP's; the universe is dominated by dark energy.
Recent data on fluctuations in the intensity of the CMB (an all sky wash of microwave energy that is the oldest observable result of the Big Bang) first theoretical models of a universe which will continue to expand forever. Scientists had puzzled about what would keep the expansion going; dark energy seems to provide the answer.
But that doesn't mean dark energy has made life easy for cosmologists. For one thing, they still have to figure out what it is. It's important to realize that the dark energy is different from any other kind of energy we've over found.
In the meantime, astronomers look forward to observations by satellites designed to map the Cosmic Microwave Background at higher sensitivity and finer resolution. Those data should reveal more about the expansion of the universe and the role of the dark energy.
Laboratory experiments are underway at the University of Washington to test how gravity acts across small distances, which may have bearing on dark energy.
Light coming from far beyond Earth must cross space that is expanding. The effect is to stretch the light wave as surely as if it had been drawn on the skin of an expanding balloon; as it travels, its colour shifts toward the red end of the spectrum.
The red shift of astronomical objects is measured by comparing characteristic spectral lines of elements in them with spectral lines of the same elements measured in the laboratory. The higher the red shift, the more distant the object that emitted the light.
The farthest red shifted galaxies discussed by Edwin Hubble in 1929 were about 6,000,000 light years away; the light of such “close” galaxies was emitted recently, and the expansion of the universe since then has been relatively small.
Light from the most distant galaxies has traveled billions of years, giving a snapshot of the universe at a fraction of its present age. If expansion were now slowing under the influence of gravity, as astronomers expected before 1998, supernovae in distant galaxies should appear brighter and closer than their high red shifts might otherwise suggest.
The distant supernovae found so far tell a different story. At high red shifts, the most distant supernovae are dimmer than they would be if the universe were slowing under the influence of gravity; they must be located farther away than would be expected for a given red shift — larger-than-expected distances that can only be explained if the expansion rate of the universe is acceleration.
What do these observations imply about the geometry of the universe ? What if that geometry is not Euclidean, or ‘flat’ but ‘curved’ instead ? If the universe were open, with negative curvature — and if observations of supernovae were subject to some systematic distortion, such as a novel form of intergalactic dust that absorbs their light — distant supernovae might appear deceptively fainter, mimicking acceleration. To determine the curvature of the universe and to detect possible distortion are among the goals of the Supernova Cosmology Project.
While it may be too soon to rule out a negatively curved universe, there is independent evidence against it. For example, measurements of the cosmic microwave back-ground radiation hint that the universe is probably flat — its energy density equal to the critical energy density.
By far the most successful explanation for the flatness of the universe, which is otherwise extremely unlikely, is the theory known as inflation.
Closed, open, and flat once described universes destined to re-collapse, expand forever, or reach a tenuous balance. But invisible energy could propel even a closed universe to eternal expansion. Many different observations suggest that the universe is flat, not curved, and that some mechanism is forcing expansion to accelerate.

GLOSSARY

1.Accretion: Accumulation of dust and gas onto larger bodies such as stars, planets and moons.
2.Active Galactic Nuclei (AGN): A core region in certain galaxies that, like a powerful engine, spews large amounts of energy from its center. Believed to be powered by the accretion of matter onto black holes.
3.Baryon: Any subatomic particle of half-integral spin that interacts via the strong nuclear force. (Most commonly, these are protons and neutrons.) The term “Hadron” includes the lighter integer spin mesons as well the half-integral spin baryons.
4.Big Bang: A theory of cosmology in which the expansion of the Universe is presumed to have begun with a primeval explosion.
5.Big Bang Observer: A Beyond Einstein gravitational wave detector vision mission.
6.Brane World: A four-dimensional surface (brane) in a higher-dimensional space time.
7.Black Hole: An object whose gravity is so strong that not even light can escape from it.
8.Cosmology: The astrophysical study of the history, structure and dynamics of Universe.
9.Dark Energy: The residual energy in empty space which is causing the expansion of the Universe to accelerate. Einstein's Cosmological constant was a special form of dark energy.
10.Dark Matter: Mass whose existence is deduced from the analysis of galaxy rotation curves and other indirect evidence but which has so far escaped direct detection.
11.Doppler Effect: An observer receives sound and light from bodies moving away from her with lower frequency and longer wavelength than emitted and from bodies moving toward her with higher frequency and shorter wavelength.
12.eV (electron Volt): The energy an electron has after being accelerated by a 1 volt potential. Quanta of visible light (photons) have energies of a few eV.
13.Event Horizon: The boundary of the region around a black hole from which nothing can escape once crossed.
14.Gravitation: The quantum particle, associated with gravitational waves, which carries the gravitational force.
15.Jets: Beams of energetic particles, usually coming from an active galactic nucleus or a pulsar.
16.keV (kilo electron Volt): A unit of energy equal to one thousand eV. X-rays photons have energies of 0•1-100 keV.
17.Light Year: The distance light travels in year (9•5 million million kilometers, or 5•9 million million miles.)
18.MeV (Mega electron Volt): A unit of energy equal to one million eV. Gamma-ray photons are those with energies greater than 0•1 MeV, equal to 100 keV.
19.Parsec: The distance to an object that has a parallax of one aresecond (equivalent to 3•26 light years).
20.Pulsar: A rotating neutron star which generated regulars of radiation.
21.Spectroscopy: The study of spectral lines from different atoms or molecules that can indicate the chemical composition of stars, gas or dust.

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