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Dark Matter and Dark Energy


Dark matter and dark energy ,these two are the most interesting problems in world of astronomy.they dominate the universe like comprising almost 96 percent of mass and energy that exists in the universe.
But noone knows what they are. It's tempting to consider them products of the same unknown phenomenon, something theorist Robert Scherrer suggests. The professor of physics at Vanderbilt University says "k-essence" is behind it all.

Dark matter was invoked decades ago to explain why galaxies hold together. Given regular matter alone, galaxies might never have formed, and today they would fly apart. So there must be some unknown stuff that forms invisible clumps to act as gravitational glue.

Dark energy hit the scene in the late 1990s when astronomers discovered the universe is not just expanding, but racing out at an ever-faster pace. Some hidden force, a sort of anti-gravity, must be pushing galaxies apart from one another in this accelerated expansion.

Separate theories have been devised to try and solve each mystery.

To explain dark energy, for example, theorists have re-employed a "cosmological constant" that Einstein first introduced as a fudge factor to balance the force of gravity. Einstein called the cosmological constant a great blunder and retracted it. Yet many theorists now are comfortable re-employing it to account for the effects of dark energy. But it does not reveal what the force is.
Scherrer agrees two explanations might be necessary, but he's also bothered by that complexity.

"It is somewhat embarrassing to have two different unknown sources for the dominant forms of matter and energy in the universe," he said in an e-mail interview. "On the other hand, that may just be the way things are. We don't get to pick the universe we live in."

To explain this, Scherrer invokes an interesting energy field called scalar field. It's a bit like an electric or magnetic field, with energy and pressure and a magnitude. But a scalar field has no direction. A scalar field is thought to have been behind inflation, the less-than-a-second period after the Big Bang when the universe expanded many billions of times before settling into a more reasonable rate of growth.

Scherrer borrows from work by Princeton University's Paul Steinhardt, V. Slava Mukhanov at the University of Munich and Christian Armendáriz Picón of the University of Chicago, relying on a specific type of second-generation scalar field they envisioned called k-essence, short for kinetic-energy-driven quintessence.

K-essence changes behavior over time in Scherrer's model, clumping early on to help form galaxies, and now forcing the universe apart. Right now, dark matter has a density that decreases as the universe expands, he explained, while dark energy has a density that stays constant as the universe expands.

"That means that at very early times, the dark matter 'piece' of the k-essence is the dominant one," Scherrer said. "As the universe expands and the density of the dark matter 'piece' of the k-essence decreases, it eventually falls below the density of the dark energy 'piece,' and the k- essence behaves more like dark energy."

"Scherrer's model , not the first trying to tie dark energy and dark matter together ", was published July 2 in the online version of the journal Physical Review Letters.

drawback

Although Scherrer's model has a number of positive features, it also has some drawbacks. For one thing, it requires some extreme "fine-tuning" to work. The physicist also cautions that more study will be required to determine if the model's behavior is consistent with other observations. In addition, it cannot answer the coincidence problem: Why we live at the only time in the history of the universe when the densities calculated for dark matter and dark energy are comparable. Scientists are suspicious of this because it suggests that there is something special about the present era.

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