This doubt has been wandering in my head for a long time. The gravitational force is only attractive force not a repulsive force. Since gravitational force is only attractive force, why not the universe is concentrated as a small sphere. All the stars would be attracting each other and hence the would stick to each to attain equilibrium. Why doesn't this happen? Is it because we consider gravitational force to be only attractive not repulsive?
All the stars would be attracting each other and hence the would stick to each to attain equilibrium. Why doesn't this happen?
You are forgetting angular momentum. Consider a binary star pair. Ignoring the expansion of spacetime, and in the absence of some mechanism that removes angular momentum from the system, those stars will orbit one another forever. The distance between the stars will vary between some fixed minimum and maximum values. While gravitation holds the system together, gravitation (from a Newtonian perspective) cannot make the two stars collide.
There is a mechanism in general relativity that can remove angular momentum from the system, but this is rather small in most cases. This answer ignores gravitational waves; that's a separate question.
One of the key subjects investigated in astrophysics is the formation of galaxies, black holes, and stars. One of the key challenges is explaining how they can form at all. The formation of galaxies, black holes, and stars has a common problem: Angular momentum. The collapse of gas clouds into stars, stars and gas clouds into galaxies cannot take place unless there are mechanisms that somehow remove angular momentum from the gravitationally collapsing system. Without such mechanisms, the collapse just stops, and it would stop well before a star (or black hole, or galaxy) could form.
A lot of progress has been made in addressing the angular momentum problem, but it still remains an unsolved problem to some extent.
All the stars would be attracting each other and hence they would stick to each to attain equilibrium. Why doesn't this happen?
This is an old question. Even Newton himself had thought about this question. His idea was that in long distances or separations (say, inter-galactic distances) the force of gravity might appear to be repulsive. That's why not
the universe is concentrated as a small sphere.
There were also many contemporary thoughts on this topic.
But this idea does not work at present day. We have got some new view about the gravitational force itself. The gravitation is not considered to be a force anymore. Today physicists believe that gravitation is a manifestation of space-time curvature or the space-time curvature or distortion creates gravity, you can read about this in the following site http://www.einstein-online.info/spotlights/geometry_force If any doubt remains then you can ask. So gravity is not a force rather it is the effect of space-time geometry (you can say "Gravity is Geometry"). Now the question is what determines the geometry of space-time. The answer is the distribution of matter and energy (electromagnetic fields also) in the space-time determines its geometry. Therefore, the universe will be contracted or not depends upon the matter distribution or the distribution of stars, galaxies etc. For different distributions of matter there are three possibilities about the fate of the universe, shown as models of universe and known as Friedman models of universe, which are: the universe may always be expanding with acceleration, the universe may stop accelerating but may expand with something like uniform velocity or the universe may stop expanding at some critical point and start to contract to end up as a concentrated small sphere and known as Big Crunch. (Note: I am avoiding technicalities as much as possible). Recently it has been proved that the first model is correct. Now why the universe does not contract rather it expands. The physicist wish to explain it in terms of Dark Energy (read: http://en.wikipedia.org/wiki/Dark_energy). Physicists hope that the dark energy opposes gravity (in common language) and far more stronger than gravity that's why the net effect is against gravity and the universe is expanding rather being contracted. The energy is Dark because not more is known about this. Hope this answers your question. In case of doubt you can ask another question.
If there would be initial rotation(as we see most of all objects today are rotating around another) gravitational force is accounting for the centripetal force. I'm new here so, I don't know how to type the equations, but I hope you get my point. Further many objects are there which have many other forces, like Coulombic force(when charged bodies are involved). All objects have charges, but they are neutralized due to proper distribution, incase something erratic occurred, that could be another form of attractive and repulsive force causing new equilibrium.