(February 4, 2018 at 7:16 am)Rhondazvous Wrote: I've read some (I suppose they were) scientists argue that there’s not enough gravity to reverse inflation and cause the universe to collapse back to where it was before the Big Bang.
Gravity affects matter, but according to ye ol’ raisin bread dough theory it’s not matter but space that’s expanding. Why would scientists use the properties of matter and gravity to understand something that has nothing to do with either?
If we understand the properties of “space” and what’s causing it to expand, we will be able to predict whether it will one day (or eon) collapse.
Think of it this way, expansion of space is like water flowing outwards.
Gravity can propel mass to move through space. It is as if the masses floating in water, while subject to the force of the moving water, also move independently through the water, as if propelled by stretched rubber bands attached to each other.
As soon as the Big Bang occurred, space streams outwards at high velocity. But the gravitational attraction of all mass with each other also caused all the mass to grandually start swimming against the flow of the expansion of space in an attempt to come together. However, When the masses first start to move against the expansion of the universe, the initial speed of the expansion of the universe is still dominates, so masses are still moving away from each other, as they move further from each other their mutural gravitation weakens, so the force propelling them to move against the flow weakens.
The question is, were there enough mass in the universe such that, before the flow of expansion of the universe carries the masses so far from each other that their mutural granvitation weakens to the point of being no longer capable of overcoming the pressure of the outward flow of space, the gravity can already completely cancel out the effect of the outward flow and cause all mass to come together against the current of the outward flow?
The answer is no. Gravity is causing masses to move against the current of the outward expansion of space, but not fast enough so that the the outward velocity of the masses will come to zero before the objects are carries so far apart by the expansion that mutural granvitational attraction will go to zero.
Where does dark energy come in? In our analogy, in the classical model of Big Bang, the outward flow of space is constant. If there were no granvity pulling masses together, then after the Big Bang, the universe will expand for ever at constant velocity, and all masses will move away from each other at constant velocity.
But in reality, the outward flow of the universe is accelerating. Even as gravity attempt to propel masses to move upstream against the the flow of,the expansion of the universe, the flow is actually getting stronger and faster. The mysterious force that accelerates the outward flow is called dark energy.
