Question about gravitational force.

[Ceritus]

Earlier I made a long post about the universe being in a sphere or a cube. Basically a contained area that would allow everything to gather again and create a big bang.

I was thinking about it some more. With gravity always being a constant force wouldn't it be inevitable for everything to stop expanding much like a tether to the largest source of gravitational force and in infinite time it would all gather to the original source of the most gravitational pull?

From my understanding of gravity is that there is no escape from it that its pull would be experianced however slight over an infinite distance.

 

[TobiasTheViking]

Scientists aren't exactly sure what force it is. But some force is accelerating the universe. Making it expand faster.

This expanding force is larger than the gravitational attraction of the known visible matter in the universe.

So, with no dark matter. The universe will continue to expand, for ever and ever.

If we have enough dark matter. The universe will stop expanding at some point, and will stay the same size.

If we have too much dark amtter. The universe will start contracting, and a new big bang MAY happen.


Currently the first scenario is the most likely. But scientists are looking.

 

[Ceritus]

I understand that it is appearing to expand faster. But can you post a link exactly how this is derived? How can we have gathered enough data in such a short period of time if you understand my asking. Maybe it expands faster and slows down in waves but overall the max speed of expansion could have slowed down could it not?

 

[rppa]

Earlier I made a long post about the universe being in a sphere or a cube. Basically a contained area that would allow everything to gather again and create a big bang.

There would also have to be enough mass present in the universe.

I was thinking about it some more. With gravity always being a constant force wouldn't it be inevitable for everything to stop expanding much like a tether to the largest source of gravitational force and in infinite time it would all gather to the original source of the most gravitational pull?

Gravity isn't constant, it falls of as 1/r^2. The influence of the sun on Pluto is a lot less than on us.

Have you ever heard of "escape velocity"? While it's true that the gravitational force extends to infinity, if something is moving fast enough, the force is not enough to keep the object in a closed orbit. It will continue sailing outward forever.

From my understanding of gravity is that there is no escape from it that its pull would be experianced however slight over an infinite distance.

Yes, but there is still such a thing as escape velocity.

 

[Dr. Fendetestas]

I understand that it is appearing to expand faster. But can you post a link exactly how this is derived? How can we have gathered enough data in such a short period of time if you understand my asking. Maybe it expands faster and slows down in waves but overall the max speed of expansion could have slowed down could it not?

The classic Friedmann cosmologies offered three scenarios:

1. Small density, open (infinite) universe with negative curvature. Expands forever.
2. Critical density, open (infinite) universe with zero curvature. Expands forever.
3. Big density, closed (finite) universe with positive curvature. Expands and recontracts.

Note: dark matter is the same as matter when computing the 'density'.

These three possibilities are a consquence of GR, which doesn't allow a stationary universe.

Measuring the velocities of distant objects, cosmologists had concluded that 3 was the real scenario. However, more recent data suggested that we had to include a 'cosmological constant', also called 'dark energy'. Dark energy has repulsive gravitation, so its presence means that we now think we are leaving in an almost zero curvature universe that will expand forever, with increasing velocity. We have very good empirical observations that support this, so it's nowadays a universally accepted model. That doesn't mean we now what dark energy really is. We only now it must exist (or we would observe a different universe) and its large scale effect. But is a very strange thing with negative pressure and repulsive gravitation!

P.S.: The isotropy doesn't allow the possibility that 'it expands faster and slows down in waves', so that hypothesis is discarded from the very beginning.

 

[Alkatran]

Also note that just because gravity pulls 'forever' doesn't mean an object can't escape.

Theoretically, if you launch away from the earth (and nothing else exists to mess with your flight path) there's a certain speed after which you will NEVER end up back at the earth.

 

[kalen]

Also note that just because gravity pulls 'forever' doesn't mean an object can't escape.

Theoretically, if you launch away from the earth (and nothing else exists to mess with your flight path) there's a certain speed after which you will NEVER end up back at the earth.

That speed is known as the "escape velocity."