why is universe expanding?

[udhitsharma]

i just wonder why universe expands?
i am not able to understand this?
if it is expanding it is doing work.
and for every work we require energy in one way or another.
from where does this energy comes from?
please give me ur views.

[Igby]

I think one hypothesis is that dark energy causes the universe to expand. It's supposed to be a kind of counter-force to gravity, in that it's a repulsive rather than an attracting force.

It seems logical that a repulsive force would be at work, since the expansion of the universe is apparently accelerating.

[Qfwfq]

Originally Posted by udhitsharma

if it is expanding it is doing work.

It is expanding against the gravitational attraction between galaxies. This implies that kinetic energy should be decreasing. It is like a bullet that has been shot straight upward, once the gun has given it speed, it will rise until the kinetic energy runs out and then it falls back. If shot fast enough (faster than escape velocity), it would leave Earth and continue indefinitely.

The trouble is now that signs have been found according to which the rate of expansion would be increasing, indicating there might be some repulsive force as well as gravity. This would be like a huge cosmic spring that is pushing things apart.

[BrainForce]

In simple terms universe expands because big bang make it expand.
But universe is found to be accelerating ,from here the theory starts.

either dark energy is causing it to expand (perhaps) or something else.

while force against gravitational attraction of galaxies.
I think the baryonic matter present is much much less for such an imense void space to cause any gravitational attraction ,much less than the critical density and this all was only by the force of big bang ex. dark energy has much to do beyond.

[LJP07]

At what speed is the Universe expanding, and over what distances is it getting faster?

[Little Bang]

A slightly different possible explaination might be one like this, suppose we had a perfectly spherical cloud of dust particles with no forces acting on them except their own gravitational forces. You would think the center of the cloud would collapse toward the outside of the sphere and the outside would collapse toward the center. If the dust cloud were a million times the size of the visible universe it would appear just as it does today. I am not saying this is the way it is but it would explain the acceleration.

[LaurieAG]

Considering that our planet spins on its axis as it orbits around our sun and our sun orbits around the center of our galaxy, surely we can only make accurate galactic expansion/contraction measurements from a position that is relatively stationary with regards to our galactic center of rotation ?

[Jay-qu]

Im sure this has been taken into account.. and dont forget all motion is relative

[Tim_Lou]

it makes no sense that total kinetic energy of the universe is decreasing, 'cause that would violate the second law of thermodynamics... the temperature of the universe would decrease. Perhaps thats one of the reasons why an expanding universe is so troublesome.

[Mohit Pandey]

Originally Posted by Igby

I think one hypothesis is that dark energy causes the universe to expand. It's supposed to be a kind of counter-force to gravity, in that it's a repulsive rather than an attracting force.

It seems logical that a repulsive force would be at work, since the expansion of the universe is apparently accelerating.

I think Igby is correct because according to third law of motion, to every action there is an equal and opposite reaction. In this case, action is attraction between heavenly objects caused by gravitation and reaction is repulsion between heavenly bodies caused by dark energy(perhaps). I think this repulsion is causing expansion of the universe.