17-03-2007 The Universe: (Article 77)

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Simon Kenny
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Location: Shannon, Co. Clare, Ireland

17-03-2007 The Universe: (Article 77)

Post by Simon Kenny » Wed Feb 04, 2009 10:47 am

The Universe: a Puzzle inside a Conundrum (Article 77)

“Bright star, would I were as steadfast as thou art”

The poet John Keats gives us the traditional view of the stars as serene, unchanging and eternal. The last 50 years of astronomical discovery is unveiling a different cosmos: mysterious, changeable and violent, a monster lurking behind the bland innocence of the night sky.

For the past 500 years, gravity is recognised as a key force in the universe. Its influence is seen everywhere, but its nature is poorly understood. Every object has a built-in gravity and the more mass it has, the stronger its gravity. It is the invisible tether that ties us to the earth.

In a wider context gravity’s effects are rather different. While the Earth is attracted by the Sun’s gravity, it does not collide with the Sun, because its motion around the Sun tries to force it away from the Sun. The Earth’s orbit is the balance of these opposing forces. On a much larger scale though, gravity does cause collisions. Several examples of galaxies in various stages of collision can be seen in any amateur’s telescope. Our own Milky Way galaxy and our large neighbour, the Andromeda galaxy, are closing at about half a million kilometres per hour due to their gravitational attraction and will eventually merge. In case you’re alarmed- don’t be! Merging Galaxies behave like fluids: it’s like pouring water into a pond. The water molecules slide past each other without damage. So it is with the stars and planets in merging galaxies. Perhaps more importantly for us, the big merger is a long way off: about two billion years from now!

While the Earth orbits the sun, the Sun orbits the Milky Way galaxy at the leisurely pace of once every 225 million years. Scientists studying the orbital motions of stars in many galaxies are puzzled by what they see: the stars are travelling too fast, so fast that the galaxies should break apart, as there isn’t enough gravity in the observable matter in the galaxies to hold everything together! Clearly, that isn’t happening, so the question is: where is all that extra matter that makes the extra gravity?

This problem is now engaging cosmologists worldwide. There are two possible sources to the problem: one is that there is a lot more mass in every galaxy that can’t be seen by telescopes, or that there is something very wrong with how scientists calculate the relationship between mass, motion and gravity: the Equivalence
Principle, which is linked to Einstein’s Theory of General Relativity. For now, scientists assume Einstein is right and their observations are inadequate. Several theories are being proposed about invisible forms of ‘dark matter’ making up over 20% of the universe and the source of all that extra gravity around galaxies. So the search continues for answers, and the theories multiply. We are still a long way from understanding our universe. Clear skies!

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