Dark energy and the accelerating expansion of the universe
If the universe is expanding at an accelerating rate, some invisible, unidentified energy must be offsetting gravity. In the beginning, when matter was close together and the universe was dense, gravitational attraction was much stronger. Now that matter is far apart and the density of the universe is low, this mysterious energy is pushing space itself outward at an accelerating rate. The source of this mysterious force opposing gravity we call “dark energy”. Because Einstein originally thought the Universe was static, he figured that even the emptiest possible space, devoid of matter and radiation, might still have a dark energy. When Hubble discovered the expansion of the Universe, and developed the quantum theory of matter Einstein rejected his own idea, calling it his greatest blunder. As physicists realized that "empty space" was full of temporary particles continually forming and destroying themselves, they 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 idea wasn’t a blunder, and 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. It is still not known whether or how the highly accelerated expansion is due to dark energy. Measure the expansion accurately enough to learn whether or not this energy is a constant property of empty space, as Einstein thought. Better observations of more supernovae over a wider range of redshifts must be plotted before the question of what is causing expansion to accelerate can be answered with confidence. About all that is known about dark energy is that it is a force that works against the force of gravity. Because dark energy is everywhere, its antigravity "is causing the universe to try and flee from itself. After the Big Bang, some scientists think, when matter in the universe was closer together, the force of gravity exerted a strong pull on objects. But gravity\'s strength dwindles as objects get farther apart. So as the universe, galaxies and all, continued to expand over billions of years, the pull of gravity lost strength. At some point, dark energy, antigravity, took over from gravity and started to make the universe expand faster. It\'s possible that dark energy might decay and change into some other form that halts the acceleration, or dark energy might accelerate the universe’s expansion forever, increasing in strength as it fills in the ever-expanding space. In that case, dark energy could end up pushing the stars so far apart that even the ones nearest Earth wouldn\'t be visible. According to the standard view of cosmology, the once infinitesimal universe has expanded in volume ever since the Big Bang, but the mutual gravitational tug of all the matter in the cosmos has gradually slowed that expansion. In 1998, however, scientists reported that a group of distant supernovas were dimmer, and therefore farther from Earth, than the standard theory indicated. It was as if, in the billion or so years it took for the light from these exploded stars to arrive at Earth, the space between the stars and our planet had stretched out more than expected. That would mean that cosmic expansion has somehow sped up, not slowed down. Throw a ball into the sky, and after it reaches a certain height, it will come back down Now imagine throwing another ball upward and finding that instead of it falling back down, it somehow keeps moving up faster and faster. For that to happen, there would have to be some force pushing upward on the ball strongly enough to overcome gravity\'s downward tug. Astronomers have come to believe that the anti-gravity force of dark energy is doing In its simplest version, dark energy would be a true constant, equally distributed throughout the universe and continuously exerting the same amount of force as the universe expands.

Physicists have been returning to Einstein’s rejected idea to explain the expansion’s acceleration. Because the cosmological constant would exist even in the absence of matter or radiation, its origins might lie within empty space itself. This property could tie dark energy to