Saturday, June 23, 2007

Explained: Dark Energy

By: Astroversity

After the Big Bang, the universe began to expand. This expansion was mostly constant until around 4.5 billion years ago, when the expansion suddenly increased. Today, the universe is still expanding, but at an increasing rate. Dark Energy is the concept that explains the accelerating expansion.

Many people know that gravity is the force that keeps you on the Earth, and holds things together. Gravity can keep the Milky Way galaxy together, and it also keeps the planets revolving around the sun. Dark Energy is a force that acts like an “anti-gravity.” Dark Energy is a force that causes the expansion of space.

Most of the universe is empty space with vast distances from one galaxy to another. There is not much between these galaxies, just mostly empty space. Dark Energy increases the amount of space between galaxies. Moreover, as the amount of space increases, Dark Energy is able to overcome less gravity and then it is able to expand more space. Then with more space and less gravity, the expansion can grow even faster. This constant cycle will continue forever.

Most of the Universe is composed of Dark Energy.
Image Credit: NASA

This scenario can be looked at metaphorically. Think of dots, on an un-inflated balloon, as galaxies in space. As you blow air into the balloon, the dots seem to be moving farther away from each other. One might say that the dots are moving away from each other, but this is not the case. Instead, the space between the dots is being increased as more air is blown into the balloon. This is how Dark Energy works with the universe.

One might ask, “Then why doesn’t the space between the Earth and the Sun increase, or the space between me and my chair?” That is because things like galaxies, the Earth and Sun, and you and your chair, are gravitationally bound to each other. Gravity keeps those systems ‘tight’ to each other so the space does not change.

Since the Dark Energy cycle will continue forever, scientists now believe that all the galaxies will slowly seem to drift away from each other, and in about 3 trillion years, there will not be any visible galaxies other than our own. The universe has a date with expansion, and this expansion will continue endlessly.

The image at right, was produced by a supercomputer to show the distribution of dark energy. Clicking on the image will reveal a high-resolution version. The image shows a part of the Universe, with Galaxies shown as bright dots, surrounded by filaments of matter. Dark Energy is shown to fill in the rest of the image, in-between the galactic lines of matter.

Image Credit: James Wadsley, McMaster University, Hamilton, Ontario

Alternate Article: Dark Energy


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dark energy is explained by the presence of a cosmological constant in the general theory of relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity

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