Wednesday, 13 August 2014

Why is the Night Sky Dark?

Good man that Edgar Allan Poe

Consider the night sky and contemplate the myriad of stars as they scintillate, in a jewel bedecked, black dome. Well, not really, most of us have difficulty contemplating the night sky due to extraneous light from street lamps, neighbouring houses and burning buildings. In Nuzzyland there are still places where you can lie on your back and view the stars in er, resplendent splendour (arse). Under perfect circumstances you can observe several thousand stars with the unaided eye and even glimpse the lazy arc of the milky way as it meanders across the sky. If you are unfortunate enough to be sober, the stars twinkle, if not, they prance and reel incoherently across the night sky. Hic.

But have you wondered why the night sky is dark? We live in a universe full of stars. There are billions of stars in our galaxy and billions of galaxies in our universe, each consisting of billions of stars. That is a lot of stars. The night sky should, in reality, be incandescent bright. It wont do to say that stars thousands/millions/billions of light years away are simply too dim to make a difference. An 18th century astronomer, Jean-Philippe de Cheseaux showed by some simple, but neat geometry, that if you consider stars to be grouped in concentric shells around us, then those stars situated close will shine relatively brightly, whilst those stars in the outer shell shine less brightly. However, this difference in relative brightness is offset by the fact that there are so many stars in the outer heavens. Collectively, those stars in the outer 'rings' will shine as bright as those fewer, brighter stars, positioned closer. This is a simple model, but it illustrates the fundamental problem very clearly.      

Although the problem of why the night sky is dark was known to earlier astronomers, it was Heinrich Olbers in 1823 who posed the problem in a paper and offered a reasonable solution. Perhaps the vast interstellar gas clouds which  haunt the universe could be acting as a barrier, filtering out the light from trillions of stars. But no. Photons from all these shielded stars would, if given enough time, cause the gas clouds to glow as bright as the stars they hid.

The answer to this apparent paradox lies with the big bang  The big bang occurred 14 billion years ago and ever since, the universe has been expanding. Because the universe is 14 billion years old and light travels at a finite speed, we can only perceive those galaxies which are close enough for their light to have reached us. The rest, and most, of the universe is out of sight and therefore unable to contribute its light to our the night sky.

The solution to this problem is a 20th century solution. It took several branches of astronomy and modern  physics, including Einstein's insights into the connection between mass, space and time to solve this conundrum.

But there is a twist in the tale/tail, like all good science stories. Edgar Allan Poe penned this prophetic and insightful passage in 1848, a hundred years before science came up with a viable answer for the 'dark sky' paradox.  Of course, Poe was no scientist, and arrived at his conclusion through intuitive, poetic genius. Here is the passage; read and weep.

"Were the succession of stars endless, then the background of the sky would present us a a uniform luminosity, like that displayed by the Galaxy- since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we  could comprehend the voids, which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all." 


No comments:

Post a Comment