# Since the gravitational force is only attractive, why does matter not just concentrate into a small sphere?

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?

• maybe it will happen: en.wikipedia.org/wiki/Big_Crunch – pentane May 8 '15 at 14:03
• There are other repulsive forces and other mechanisms that keep matter from collecting into one big lump. Additionally, the expansion of the universe makes most matter move away from other matter – Jim May 8 '15 at 14:04
• For one thing, the universe is not a sphere. That's just a very poor illustration they keep showing (for no good reason other than the limited attention span of the audience) on tv. For another, the "size" of the universe may be quite small at all times, we simply don't know what the relevant size scale for its evolution is. – CuriousOne May 8 '15 at 14:14
• Somewhat related: physics.stackexchange.com/q/182679 – dmckee --- ex-moderator kitten May 8 '15 at 17:24
• @ACuriouaJim I just wanted what these repulsive forces were. – Aditya Kumar May 8 '15 at 17:28

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.

• If gravitational force of repulsion doesn't exist then how could the universe expand? – Aditya Kumar Jun 18 '15 at 8:26

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.