16Jul2017

Where Is The Centre Of The Universe?

Processed with MaxIm DL

Because other galaxies are rushing away from us, it might seems as if Earth must lie close to the centre of the universe but in fact, this is completely wrong. In fact there is no centre. The pattern of expansion Hubble discovered has the special form that every cluster of galaxies moves away from every other cluster. Whichever cluster you were situated on, the pattern of motion around you would look the same. An analogy might help. Suppose by some magic our planet were to expand noticeably every day.

The distance between cities would then steadily increase. Manchester would move away from London, New York would move away from Chicago and Beijing would move away from Hong Kong. Pick any two cities, and the distance between them would increase. Viewed from any particular city, all the others would seem to be retreating. But in fact no city is special. None lies at the centre of the pattern.

Notice also that if the Earth expanded uniformly (rather than fast here and slow there), then a city twice as far away would retreat twice as fast: Manchester would move away from London roughly twice as fast as Birmingham.This is precisely the pattern that Hubble observed: galaxies twice as far away retreat at twice the speed. Hence we can conclude that the universe is expanding at the same rate everywhere, and no galaxy lies at the centre of the expansion.

But mustn’t there be a centre somewhere, even if we can’t spot it?

Well… It’s perfectly logical to have a truly infinite expanding universe, the same everywhere on average. Then there would be no centre and no edge at all. However, this would be indistinguishable from a universe that is merely stupendously big, with us located well away from the edge.

If it is just very big but finite, couldn’t we build a super-duper telescope and see the edge?

There is a snag with this strategy. A telescope is also a time-scope. An astronomer looking at a galaxy, say, five billion light years away sees the galaxy as it was five billion years ago, not as it is now. That’s because light travels at a finite speed (one light year per year). Physicists are convinced that the speed of light is a cosmic speed limit – nothing can go faster. So no matter how fancy our instruments, there is a limit to how far we can peer into space.

balloon

As you blow the balloon the ‘Galaxies’ move further apart

This limit is set by the age of the universe. If it’s 13.7 billion years old, we can’t detect anything more than 13.7 billion light years away.

So we are left wondering whether the patch of universe we see is typical of the whole – and it just goes on for ever and ever in every direction looking much the same – or whether, sooner or later, it would come to an end – or an edge – and change into something else.

And if so, what? That is the basis of the recent popular “multiverse” idea: that there is an edge out there somewhere, but probably a very very long way away. From what I have written, it’s clear we will not discover the multiverse by peering yet deeper into space. Evidence for it rests on other arguments.

Why do newspapers often talk about astronomers seeing to the edge of the universe?

This is just sloppy talk. “The edge of the universe” can sometimes refer to the limit of visibility using current instruments – so not a physical edge in any sense. Or it can refer to factor I have just mentioned – the limit imposed by the finite speed of light. This isn’t an abrupt cut off. What happens is this. The farther into space astronomers look, the redder the galaxies become. At about 13 billion light years the universe is too young for proper galaxies with stars.

http://josepardina.files.wordpress.com/2010/08/pauldavies1.jpg?w=450&h=320

Professor Paul Davies

This is the so-called cosmic Dark Age. Before that is just glowing gas, spread evenly through space, and seen by us as hugely red-shifted to a temperature of only 2.7 degrees above absolute zero. This “wall of light” lies about 380,000 light years from the “edge” defined as the limit due to the finite speed of light. This “light” edge is known technically as our particle horizon. It no more represents a true edge of the universe than a horizon on Earth represents an edge to the planet. Rather, the horizon is merely a fundamental limit of visibility. The particle horizon increases in radius with time, by one light year per year.

The above discussion is predicated on the assumption that the big bang was a true origin of time, so that nothing can reach us from the epoch before it (because, by definition, there was no “before”). That is the case with the normal, orthodox cosmological model. However, more recent models, based on eternal inflation and predicting a multiverse, treat the big bang as a local, not a global origin.

So the evidence is strong?

In these more elaborate models, our particle horizon is no longer a true causal boundary, but it remains an observational boundary, since the big bang completely reconfigured spacetime, and reprocessed matter and energy so thoroughly that it would not be possible to look back through it to a preceding cosmic epoch. Largely this is because inflation, by its very nature, erases all information about prior epochs.

So the distinction between the orthodox big bang and the more elaborate inflation/multiverse models is essentially a conceptual rather than an observational one. Source: Paul Davies

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