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Given the initial conditions at the Big Bang, could our universe have evolved any differently? In other words, did the initial conditions predetermine every stage in the evolution of the universe to this day, or - if we "roll the tape backward" and play it again - could the universe be entirely different on the macro scale (eg. no stars and galaxies)?

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I don't think your attempt to distinguish the micro and macro scales is helpful. As it stands, the physical theories (quantum mechanics etc) are deterministic, that is the initial conditions pre-determine all future: there is no random element ("Gott würfelt nicht" -- god does not throw dice -- to quote Albert Einstein).

Of course, this is debatable, but certainly beyond astronomy, better (meta)phyiscs and philosophy ...


Edited to respond to the comment, effectively rephrasing the question to consider a "slighly different" early state of the universe.

Well, it pretty much depends what you mean by "slighly different". If the laws of physics are completely untouched, I would think that the universe would evolve statistically in exactly the same way. This is because all the statistical characteristics of the early universe (the power spectra and correlation functions of the various energy densities, for example) come about from the laws of physics (via natural instabilities, for example) rather than the initial state.

However 1, our understanding of the laws of physics and their deeper origin is still very limited. This means that it is not clear whether a slightly different early state with exactly the same laws of physics is actually at all possible. This is particularly true for the nature of gravity and the vacuum, neither of which is currently properly understood at the quantum mechanical level.

However 2, one has to bear in mind that we don't know whether the universe is finite or infinite. If it is infinite (and the observational evidence from our finite patch of the universe indicates that it has a flat or open, but not closed, spacetime), we cannot really make any sensible conclusions from our finite observable patch. This includes this answer! Except, of course, if we invoke an extended version of the cosmological principle, something like "an infinite universe must be the same everywhere", which is (meta)physical non-sense. This is the main reason why I don't like cosmology too much.

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  • $\begingroup$ I understand that the universe is deterministic. But that's not quite what I meant by the question. What I meant is, if the universe was "slightly different" - not in the composition, or the laws, but perhaps in how energy and matter were arranged initially - would the universe still go through the exact same stages that we know today, or could it evolve somewhat differently? $\endgroup$ – BigPic Sep 2 '14 at 22:00
  • $\begingroup$ You didn't ask it like that ("slightly different"). $\endgroup$ – Walter Sep 3 '14 at 7:44
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Given chaos theory, things could have been different. Given the laws of physics, there are limits to how much difference there could have been.

To put it bluntly, a subatomic particle could have zigged instead of zagging, causing a fusion event that led to the formation of a star not to take place in one spot but another.
But it would not prevent stars from forming.

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The current universe appears to have evolved from some extremely well-tuned parameters for which the long-term state of the universe is extremely sensitive in changes to. For example, the observed curvature is so close to flat that the initial conditions must have been absurdly flat: to within an accuracy of 1 in $10^{64}$. Any further from that and there would be drastic differences.

This absurd level of fine tuning is one of the motivations for inflationary models. In these the initial conditions can take a wide range of values, and then the inflation smoothes everything out to the perfect tuning we've observed.

So, yes, the universe is very sensitive to a number of things. Which causes a lot of problems. Finding reasons why we hit the perfect combination is a matter of some debate (and some would say is philosophical, not scientific, in nature).

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  • $\begingroup$ Well, the underlying hope is that all those fine tunings are only apparent but not real. In other words, the universe naturally obtains an apparently fine-tuned situation in a way still unbeknown to man. In yet other words, the universe only appear fine-tuned because of our ignorance/lack of knowledge. $\endgroup$ – Walter Sep 6 '14 at 9:36

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