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A Supernova Sheds Light on Dark Energy  

A discovery by astronomers using the Hubble Space Telescope supports the notion that the Universe is filled with a mysterious form of energy pushing galaxies apart at an ever-increasing rate.

by Dr Tony Philips

NASA's Hubble Space Telescope has spotted a burst of light from an exploding star located much farther from Earth than any previously seen - a supernova blast in the early Universe that is casting light on a mystery of truly cosmic scale.

This stellar explosion is extraordinary not only because of its tremendous distance - 10 billion light-years from our planet - but also because it greatly bolsters the case for the existence of a mysterious form of "dark energy" pervading the cosmos. The concept of dark energy, which shoves galaxies away from each other at an ever-increasing speed, was first proposed, then discarded, by Albert Einstein early in the last century.

The Hubble discovery also reinforces the startling idea that the universe only recently began speeding up -- it offers the first tantalising observational evidence that gravity began slowing down the expansion of the universe after the Big Bang, and only later did the repulsive force of dark energy win out over gravity's grip.

The team of astronomers, led by Adam Riess of the Space Telescope Science Institute (STScI), made the discovery by analysing hundreds of images taken by Hubble to study how galaxies formed.

"This supernova appears to be one of a special class of explosions that allows astronomers to understand how the universe's expansion has changed over time, much as the way a parent follows a child's growth spurts by marking a doorway," said Riess. "It shows us the universe is behaving like a driver who slows down approaching a red stoplight and then hits the accelerator when the light turns green."


Astronomers know the universe is expanding because of the "red shift" seen in the light from distant galaxies. Due to the Doppler Effect, light waves are compressed when a light source is moving toward you and stretched when it's moving away. This happens to sound waves too, which is why the pitch of a police car's siren drops as the car passes you. With light, however, frequency equates with colour, not pitch. Higher frequencies look bluer and lower frequencies look redder. So the "red shift" in the light from the galaxies around ours means those galaxies are moving away from us. How could all the galaxies be moving away from us? Only if the universe itself is expanding, as demonstrated by the balloon below.


The record-breaking supernova appears relatively bright, a consequence of the Universe slowing down in the past (when the supernova exploded) and accelerating only recently.

"Long ago, when the light left this distant supernova, the universe appears to have been slowing down due to the mutual tug of all the mass in the universe," explained Riess. "Billions of years later, when the light left more recent supernovas, the universe had begun accelerating, stretching the expanse between galaxies and making objects in them appear dimmer."


Albert Einstein revamped humanity's understanding of the universe in several fundamental ways. Special Relativity overhauled our notions of space and time, and General Relativity modified those ideas to include an explanation of the hitherto mysterious force of gravity. Now it appears his conjecture that the universe is pervaded by an expansive force called the "cosmological constant" was also correct.

"Hubble's ability to find titanic stellar explosions at these extreme distances is what it takes to confirm this theory that the universe must have been slowing down before it switched into high gear," said Dr. Anne Kinney, Director of NASA's Origins program at NASA Headquarters, Washington, DC. "Later this year astronauts will install a new camera on Hubble that will give us 10 times better resolution than the current camera, which will give us even better capability to find answers to grand cosmic questions like this."

Observations of several distant supernovae by two teams of astronomers in 1998 led to the theory that the universe got the "green light" to accelerate when it was half its present age. Astronomers say the new Hubble findings rule out other explanations. 

Shortly afterwards, astronomer Edwin Hubble made the celebrated discovery that the universe was expanding. He assumed that the universe must be slowing down under gravity and might even come to a halt, leading Einstein later to say that his cosmological constant was the biggest blunder of his career. Now it appears Einstein was on the right track after all.

The source of the repulsive gravity may be something akin to Einstein's cosmological constant -- referred to as the energy of the "quantum vacuum," a subatomic netherworld pervading space -- or it may be something entirely new and unexpected.

"While we don't know what dark energy is we are certain that understanding it will provide crucial clues in the quest to unify the forces and particles in the universe, and that the route to this understanding involves telescopes, not accelerators," said astrophysicist Michael Turner of the University of Chicago.


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First Science 2014