In physical cosmology and astronomy, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain observations since the 1990s that indicate that the universe is expanding at an accelerating rate.
The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is one of three ambitious projects aiming to provide those missing observations [of dark energy]. The others are the Baryon Oscillation Spectroscopic Survey (BOSS), which achieved first light in September 2009 at the 2.5-meter Apache Point telescope in New Mexico, and the Dark Energy Survey (DES), which will add a digital camera with more than 500 megapixels to the Blanco four-meter telescope in Chile.
As first reported by Nature News, the researchers believe that the light dimmed when photons travelling between the galaxies and Earth transformed into a form of dark energy called chameleon field energy. Chameleon field energy was first theorized to exist in 2003, but these observations represent the first direct evidence of the phenomenon in nature.
[Says Gregg Easterbrook, a famous science writer,] “Suddenly there’s a fifth force, dark energy. And although it appears to comprise three-quarters of the universe, nobody’s noticed it till now.” Actually, although it is seductive to think that dark energy is a fifth force, it does not fairly represent mainstream physicists’ thinking, which is that dark energy is part of the stuff that fills the universe and that its effects on the expansion of the universe are gravitational–that is, entirely predictable within Albert Einstein theory of gravitation.
Dark energy is often a source of confusion even for physicists. People often ask whether the expansion of space means that galaxies—or perhaps even the solar system, even Brooklyn—are expanding. S’far as we can tell, they are not. The only thing dark energy seems able to do is make orbits imperceptibly larger than they would otherwise be. Dark energy is so dilute that it only has appreciable effects on larger scales, larger than the millions of light years of galaxy clusters.
A project called WiggleZ is helping to validate dark energy's existence. WiggleZ scientists have made a 3-D map of more than 150,000 galaxies near and far to trace the universe's evolution over time. The WiggleZ team looked at how readily galaxies clump together, how much space separates those galaxies, and how things have changed over the past seven billion years. They found that something is indeed driving things apart. And dark energy fits the bill nicely.
Despite its name, dark energy is better thought of as a repulsive force that pushes galaxies apart from each other. It was first widely invoked by cosmologists in 1998 to explain why the expansion of the universe seems to be speeding up, a finding that won the Nobel Prize in Physics last year.... David Wiltshire, a physicist at the University of Canterbury in New Zealand and a visitor to Barbour’s College Farm, thinks the reason dark energy is so mysterious is that it is an illusion.
If Einstein’s model of gravity is correct, around 96 percent of the cosmos appears to be missing. To make up the difference, cosmologists have posited two mysterious, invisible, and as yet unidentified ingredients: dark matter and dark energy, a double budget deficit that makes many scientists uncomfortable
Dark energy is a beacon that may lead physicists to an elusive "final theory": the unification of all known forces, from those that hold the components of atoms together to the gravity that shapes space. Meanwhile the notion of dark energy has helped reconcile a puzzling suite of recent observations about the shape and composition of the cosmos. In fact, the future of physics and the fate of the universe may ultimately depend on a kind of antigravity that has heretofore been a subject of mere conjecture.
Some ubiquitous, repulsive force is driving at the margins of space, stomping on the accelerator. And there are no red lights in sight. That mysterious propulsion looks a lot like ["fudge factor lambda" as Einstein initially called the cosmological constant]. Today's cosmologists are calling this force dark energy: "dark" because it may be impossible to detect, and "energy" because it's not matter, which is the only other option.
Everyone knew since Edwin Hubble that the universe was being flung apart as a consequence of the Big Bang, some 13.7 billion years ago. But for it to be speeding up? It meant something else was at work, a force much more mysterious and bizarre than anyone had thought. No one knows what this force is, but after another decade of calculations, physicists know it makes up about 74 percent of the universe. “We call it dark energy to express ignorance,” Perlmutter said.