1)why would the universe contract or expand if there is gravity?

2)how can the universe expand? doesn't it defy gravity?

3)what would be the effecr of a repulsive gravitational force at greater distances on an infinte number of stars?

thnks:)

9 Answers

Relevance
  • 1 decade ago
    Favorite Answer

    There is no proof as of yet that the universe is actually expanding, just of planetary bodies moving away from Earth. Let's assume, however, that it is expanding.

    The universe supposedly came into being through the Big Bang, an explosion of such force, it sent all matter hurling out in different directions. The universe continues to move because, in the vacuum of space, there is nothing to stop it. Theoretically, gravity should have stopped the process by now, but there are a few things keeping that from happening. The farther away from a planet, such as Earth, something is, the less it is affected by gravity. Astronauts hovering just over the Earth's atmosphere are so minutely affected by gravity that they are weightless. Now, think of how far away stars and planets are from each other. They would not be affected by each others' pull except in extreme cases where they are very close together. Thus, gravity has no effect of the expansion of the universe.

    That was lengthy, but I hope it helped.

    • Commenter avatarLogin to reply the answers
  • 1 decade ago

    If you jump up, you continue up until gravity stops you then you will gain speed till you hit the earth again. If you are assisted with your jump, with a rocket, you can jump so high and so fast the gravity of Earth can't stop you.

    When the universe started, it was explosion assisted. It already had enough speed to continue to expand free from the cumulative effect of gravity. It is possible that gravity did not begin to have an effect until the expansion was well under way. Or it is possible there is some anti-gravity force we have not identified yet.

    In either event, the expansion seems to be speeding up, not slowing down.

    Gravity is always an attractive force. If the universe stood still, it would begin to fall back on itself. If it is expanding faster than gravity can slow it, it will expand forever.

    The always attractive force of gravity ensures the universe must expand or contract. It can not stand still.

    The expansion does not defy gravity. The acceleration of the expansion does seem to defy gravity. Or, there might be some force at work we do not understand yet.

    If gravity is attractive at close range and repulsive at a far distance, we would see what we see today. Galaxies and stars bound by gravity into clumps and clusters and those clumps and cluster being driven apart.

    Source(s): One theory of dark energy
    • Commenter avatarLogin to reply the answers
  • Bob G
    Lv 6
    1 decade ago

    It's almost easier to consider what happens with two objects, then extend that to all of the objects in the universe.

    In an elliptical orbit, the orbiting object gets closer to the orbited object at perigee, then further away at apogee. You could think of it as the two-body system is contracting, then expanding, then contracting, then expanding, etc, forever. If the mass of the universe is great enough that gravity overcomes the initial velocity from the Big Bang, then this is what will happen to the universe. Expanding or contracting just tells you where you are in the cycle, but the overall cycle of expansion/contraction goes on forever.

    If an object is going fast enough (escape velocity), gravity can never slow the object's radial velocity to zero and the object just gets further away, never closer. It's radial velocity decreases, but the rate of decrease is always decreasing as well (it's radial acceleration). The two-body system is always expanding, but the rate of expansion is always slowing down. If the initial velocity of the Big Bang was great enough to overcome the gravitational force of the mass of the universe, the universe expands forever even though the expansion rate decreases.

    If the expansion rate increases (as seems to be the case from observations), then some other force besides gravity must be acting on the universe. A repulsive gravitational force, dark energy, etc. are all things haven't been observed directly, but have to exist in some form to explain observations - in other words, they're theories to explain why we see an increasing expansion rate - something that can't happen due to gravity alone. Either something exists to cause an increasing expansion rate or else our observations are wrong for some reason (if the observations are wrong, then the reason for them being wrong would also have to be explained, so there's no free pass by just saying the observations are wrong).

    • Commenter avatarLogin to reply the answers
  • 1 decade ago

    1. The universe would expand if there was too little gravity too pull it together after the initial start of expansion after the Big Bang. The universe would contract and collapse when there is enough gravity to counteract expansion.

    2. The universe expands because of a form of energy called dark energy that acts as a from of antigravity that could cause acceleration of the universe. The universe is filled with 74% of this stuff.

    3. A repulsice gravitational force? Something like dark energy? Well, if the force had to be used on an infinite number of stars, it would hardly affect an inevitable collapse of the stars by gravitational pull.

    • Commenter avatarLogin to reply the answers
  • How do you think about the answers? You can sign in to vote the answer.
  • 1 decade ago

    The expansion of the universe could have been a prediction of Einstein's theory of gravity. In fact, his original equation predicted exactly that,but he had a philosophical belief that the universe had to be stable, so he added another term to his equation to allow that. He later called this his biggest mistake.

    One way to look at the original Big Bang is like throwing a ball upward. The gravity does not immediately cause the ball to go back down. In fact, if the ball is thrown fast enough, the ball may not every go back down (this speed is called the escpae velocity). For the universe, the amount of matter determines how fast the expansion slows down or if the universe will expand forever.

    Unfortunately, this simple explanation is not the whole story. Recentevidence shows that the expansion is speedingup, not slowing down. This seems to be caused b a pressure of space itself. This is related to 'dark energy'.

    The upshot is that for 1) and 2) gravity alone would suggest the expansion could occur, but it should slow down.

    As for 3), there does appear to be such a force and it is making the expansion speed up.

    • Commenter avatarLogin to reply the answers
  • 1 decade ago

    the universe does not contract it only expands,that is due to the big bang.The force and the energy during big bang is greater then the gravity.gravity is considered only for individual bodies not for the whole system like universe.There would be no effect on the stars on an infinite stars if the magnitude of the repulsive gravitational force is less..

    • Commenter avatarLogin to reply the answers
  • 1 decade ago

    Your mindset is that of the closed universe concept. Try shifting your point of view to the open universe concept. The former is like a container where we and all the heavenly bodies exist. The latter views our universe as a localized containment in a great and boundless realm. Take a swimming pool for example. The water supply is like the stars while the drain is like the black holes. There are countless of universes and dimensions outside our own.

    • Commenter avatarLogin to reply the answers
  • Anonymous
    1 decade ago

    The expansion of space is conceptually different from other kinds of expansions and explosions that are seen in nature. Our understanding of the "fabric of the universe" (spacetime) requires that what we see normally as "space", "time", and "distance" are not absolutes, but are determined by a metric that can change. In the metric expansion of space, rather than objects in a fixed "space" moving apart into "emptiness", it is the space that contains the objects which is itself changing. It is as if without objects themselves moving, space is somehow "growing" in between them.

    Because it is the metric defining distance that is changing rather than objects moving in space, this expansion (and the resultant movement apart of objects) is not restricted by the speed of light upper bound that results from special relativity.

    • Commenter avatarLogin to reply the answers
  • Anonymous
    1 decade ago

    The universe does not expand or contract, that's a dogmatic belief of science.

    • Commenter avatarLogin to reply the answers
Still have questions? Get your answers by asking now.