Why does the universe require relativity? Are there any known compelling reasons why our universe is governed by relativity instead of Newtonian physics? 
I recognize that an infinite $c$ would be a problem, so I'm not asking about that; I figure that so long as you still conserve energy and momentum etc., everything would be limited to arbitrarily large but finite speeds, ensuring that causality remains intact and preserving our computationally tractable universe.
I suspect that there are lots of instances where relativity informs our understanding of the operation of stars or subatomic processes or what have you, but which if any of those kinds of things couldn't be more easily explained with a Newtonian outlook?
In case it helps, my motivation here is to try and understand whether relativity makes sense from an Occam's razor perspective. And I know that relativity fits all our observations beautifully, and that it's all implied once you accept that physics are invariant in any reference frame, but that all doesn't strike me as the simplest a priori arrangement we could have had for a universe, and I find that possibility unsettling.
 A: Guth gives the answer on p.296 of the 1997 ed. of his book entitled "The Inflationary Universe":  "Draw the outline of two spheres that contain the same density, such that Sphere B has twice the radius of Sphere A.  To compare the amount of time it will take each sphere to collapse, we can compare the gravitational acceleration of the points P and Q", which he draws with point P on the smaller sphere and point Q on the larger one, before going to say that, as per Euclidean geometry, "sphere B has 8 times the volume of sphere A, and therefore 8 times the mass....On the other hand, the point Q is twice as far away the center of its sphere as the point P" is from its own sphere. "Since the force of gravitation [in Newton's theory] falls off as the square of the distance, the extra distance of the point Q implies that the force is weakened by a factor of 4.  Combining the effects of the extra mass and the extra distance, the force on Q is ...twice as great as the force on P.  It follows that the acceleration of Q will be twice...that of P, so at any given time the velocity of Q will be twice...that of P.  Since that distance that Q must move while collapsing to the center is also twice as large...both spheres will collapse in exactly the same amount of time!"  
So, although Newtonian physics looks simpler, and is even used for the outline of some orbits by NASA, it would result in a universe that would collapse to a point, REGARDLESS OF ITS SIZE, as soon as it would reach ANY size at all!  That's NOT what Olbers, Hubble, and everyone else have always been seeing!  (I know it's surprising that this consideration would've been overlooked for a couple of centuries, but such oversights do happen:  Olbers' Paradox, which is much simpler and is described on Wikipedia, was only formulated in writing 6 or 8 times before the 1929 discovery of the Hubble expansion!  That's why physics is such a great field to work in!) 
