Unexpected discoveries are nothing new in cosmology. There is a fairly heavy trail of discarded beliefs and challenging new theories starting from early in the 20th century. In 1905 Albert Einstein described a new way of understanding the universe in his General Theory of Relativity, loosening the hold that Newton had on the description of planetary motion since the late 17th century. The Big Bang itself was a new idea - alongside the concept that the universe was expanding.

The nature of galaxies like our own was only settled in the 1920's when new telescopes were able to resolve individual stars within the outer arms of galaxies, or 'spiral nebulae' as they were called at the time. This leads us only to about 1930 when Pluto was discovered. Pulsars, quasars, neutron stars, extra-solar planets the list goes on. Black holes - much different than the recently discovered hole mentioned above - were theorised based on Einstein's general relativity and were the subject of a public bet between Stephen Hawking, Kip Thorne and John Preskill. Preskill argued that it was possible for black holes to radiate. The outcome? No surprise that it remains partly undecided as Hawking conceded the bet in 2004 and agreed with Preskill while Kip Thorne is still holding out.

Returning to the newly discovered billion light year hole in the universe, we can follow the history of it’s discovery and see that it stemmed from scientists reviewing data from a study of the Cosmic Microwave Background radiation (CMB). The CMB is the remnant radiation from the Big Bang, revealing the large scale features of the universe approximately 300,000 years after it’s beginning – over 13 billion years in the past. The CMB radiation studies done by NASA in 2001 revealed a ‘cold spot’ in the sky in the constellation of Eridanus. But why a cold spot? Is it due to a feature of the CMB radiation? Or to something else? Scientists believe that the mysterious force known as ‘dark energy’ is at work in this situation.

Dark energy is the description given to a force which is causing an accelerated expansion of the universe over time. It’s believed that when the CMB radiation from the early Big Bang passes through space containing either normal matter or dark matter it speeds up as it approaches the matter and then slows down as it passes it – both effects due to gravity. In theory, the speed of the radiation should be the same leaving the vicinity of the matter after passing it. In many ways, much the same as if you were driving your car down a small dip in the road and up the other side. Imagine taking your foot off the accelerator just before coming to the downward slope of the dip– even though the engine is no longer pushing your car it would continue to speed up until it reached the bottom – the lowest point - then, climbing the other side of the dip, your car would slow down. If the dip you’ve driven through was the same width and size on both sides from the bottom (and forgetting friction and air resistance!) your car should be going the same speed once it emerges from the dip as just before you started down. What would happen if you put your foot on the accelerator as you were going up the other side? Your car would emerge from the dip in the road going faster than it was when it first started. This is similar to what happens when the CMB radiation passes a gravitational mass - dark energy adds an additional ‘repulsive’ push so that the radiation speeds away at a greater speed that it would have ordinarily. So, if you were driving your car down a flat road - one without a dip – (and again, you must take your foot off the accelerator and assume that there is no friction or wind resistance!) this is the same as radiation moving through completely empty space – your car will continue at the same speed. Since there is no dip in the road, or mass to provide a gravitational pull, neither your car nor the radiation will change speed. You will, however, both receive an extra ‘push’ from dark energy, and so, if both roads were side-by-side, one with a dip and one without, the car which went down the hill will end up going slightly faster as it’s gained speed.

Radiation which passes through a void containing no matter at all does not receive the initial boost in velocity from approaching a gravitationally attracting mass – and so retains it’s initial velocity appearing as a ‘cold spot’ in the CMB map.

So at least we’ve now learned what caused the cold spot in the CMB data – a void. Now we have to work on what caused the void!

For more information

NASA – Cosmology and WMAP
http://map.gsfc.nasa.gov/m_uni.html

A history of Cosmology
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Cosmology.html