Dark Energy

Almost three quarters of our universe is comprised of dark energy, the newest bewildering component of our cosmos.

Until just a decade ago, scientists were near unanimous in their assumption on the fate of the universe. They thought that the cosmic expansion triggered by the Big Bang would slow down over time or at least it would remain unchanged. Gravity, being an attractive long-range force, pulls the components of the universe together, thereby prohibiting unruly expansion. So they set out to assess the expansion speed of the universe. Contrary to their expectations, they found a universe that is accelerating.

This computer-generated image shows the simulated distribution of dark matter in a galaxy cluster formed in the universe with dark energy. The clumps are locations where galaxies form.

The evidence of the unexpected expansion of the universe came from observations of distant supernovae explosions. These are the brightest explosions that can be seen over vast distances across the universe. Telescopes measure the fine prints of such explosions.

Light and shadow in the Carina Nebula

Some unknown force is prompting the acceleration. This force is greater than that neutralizing the combined gravitational pull of normal and dark matter.

This discovery wasn’t cause for celebration among astronomers — rather it was shocking. This strange phenomenon portrays a world vastly different than the one we are used to. We expect a tossed object to fall back to earth unless a force overcomes the gravity of our planet. On a grand scale, such an unknown force is spurring the growth of the universe by flinging galaxies apart. To accommodate that large repulsive force accelerating the universe, scientists coined the term — dark energy.

Einstein’s theory of relativity hinted at a repulsive force in the universe. He incorporated the idea of cosmological constant to support the idea of static universe. He reasoned that for the universe to remain static, this repulsive nature of the cosmological constant would surpass the effect of gravity. When Edwin Hubble’s observation of galaxies proved the expansion of the universe as opposed to a static universe, Einstein retracted his original idea, calling it the “biggest blunder” of his career. Now, decades after his death, some cosmologists think that Einstein’s blunder may come to the rescue. They identify the cosmological constant as one of candidates for explaining dark energy.

The most recent observations by the WMAP satellite provide evidence of dark energy in the universe. In the absence of dark energy, the mass-energy density of the universe is not sufficient to explain the flat geometrical shape of the observable universe.

A Giant Hubble Mosaic of the Crab Nebula.

Quantum mechanics views the cosmological constant as the “energy of the vacuum,” or the energy of empty space. It is assumed that all space is filled with this form of energy. This repulsive background energy associated with the empty space could be dark energy. However, there is no compelling evidence for that claim. The theoretical calculations suggest that the amount of vacuum energy is too high for reasonable explanations.

Emptiness is not a true void as was deemed in the past. Quantum theory considers a vacuum as a pool of virtual particles rapidly popping in and out of existence. The particles and energy incarnate inside the so-called emptiness as a result of invisible interactions.

A pinpoint of light from a star that exploded over 10 billion light-years away is centered in the panel at the lower right, a cosmic snapshot

These are all scientific speculations until supporting evidence is lined up. Unfortunately, our observations and measurements depend upon something we cannot comprehend yet. We know the laws of gravity account for the behavior of our solar system pretty well. But, how do we know if our laws of gravity don’t break down over huge distances? Or even worse, it might be possible that we are unaware of some long-range forces other than gravity.

Determined to break the impasse, NASA and the U.S Department of Energy have together decided to develop a space based dark energy investigation. This strategic mission known as JDEM —Joint Dark Energy Mission — is currently in the design phase. The JDEM observatory is set to launch in 2016 to study the properties of dark energy and assess the expansion rate of the universe over different time periods during its cosmic evolution.

Until we understand, we have to confront the dark forces of nature. If this phantom energy exists, it will rip our universe apart leaving us alone in the island of Milky Way surrounded by huge emptiness. If it doesn’t, for our future generations dark energy may come to symbolize the extent of our ignorance.

How will our universe end? Recent speculation now includes a pervasive growing field of mysterious repulsive energy that rips virtually everything apart. As soon as few billion years from now, the controversial scenario holds, dark energy will grow to such a magnitude that our own Galaxy will no longer be able to hold itself together. After that, stars, planets, and then even atoms might not be able to withstand the expansive internal force.