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Notes: 1. I'm in 8th grade so i don't have in-depth schooling on this. Lengthier explanations with more extra information that would improve understanding would be much appreciated. 2. I asked a question on this topic earlier so this is an expansion of it. 3. English isn't perfect yet, so if anyone can give either Russian, Arabic, or Mandarin translations of complex terms, this would also help very much.

Many of the responses mentioned the big bang, and I thought that maybe it's continuous expansion, if the expansion is throughout and not new mass added on the edge of expansion, is this what balances out gravity? And if so, can its rate of movement (due to the loss of energy over time) decrease to a point that gravity overpowers it, and all mass would collapse in on itself?

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The mere fact that the universe is expanding doesn't mean that it can't collapse, any more than a stone moving upward means it can't turn around and come back down. The universe eventually would stop expanding and collapse back to a Big Crunch if it had enough matter and no dark energy.

But there apparently is dark energy, and it is making the universe expand faster and faster, not slower and slower! The anti-gravity of dark energy has been the dominant force in the universe for the last five or six billion years. Matter is no longer particularly important. According to the standard Lambda-CDM model of cosmology, dark energy now constitutes 69.11% percent of the energy density of the universe, and matter (both visible and dark) only 30.89%.

According to the model, the universe will continue to expand forever, faster and faster, with the Friedmann scale factor $a(t)$ that determines its size eventually doubling every 11.6 billion years. The fraction of the energy density due to matter will approach 0%, while the fraction of the energy density due to dark energy will approach 100%. There is no Big Crunch in our future, according to the model.

Note: There is no "edge" to the expanding universe. Galaxies are not expanding into empty space "beyond the edge". Instead, space is expanding everywhere between galaxies, and there is no distance far from us at which you stop finding galaxies. Also, there is no new mass being added.

If there were no dark energy, the universe would be expanding slower and slower rather than faster and faster. If there was enough matter, it would turn around and collapse. If there was not enough matter, it would keep expanding. It would be like a rocket launched from Earth. If a rocket isn't launched with escape velocity, it falls back to Earth. If it has exactly escape velocity, it can reach infinity with zero velocity. If it has more than escape velocity, it can reach infinity with positive velocity. This is similar to how the universe would work if there were no dark energy.

CORRECTION

11.6 billion years is actually the eventual $e$-folding time of the scale factor $a(t)$. The doubling time is $\ln{2}$ times this, or 8.04 billion years.

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    $\begingroup$ According to this model, isn't the big rip (where the distance between any two finite points becomes infinite) the final outcome? $\endgroup$ – forest Mar 24 at 7:02
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    $\begingroup$ @forest Note that a big rip means that that would happen at a finite time (in this model the distance would only go to infinity as time goes to infinity). A big rip requires an even more exotic form of dark energy than a cosmological constant. $\endgroup$ – Winther Mar 24 at 12:28
  • $\begingroup$ Winther is right. The standard Lambda-CDM model doesn’t have a Big Rip. See en.wikipedia.org/wiki/Big_Rip $\endgroup$ – G. Smith Mar 24 at 15:58
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I will try to give a simple example:

You know that all mass attracts all other mass, but in such a weak way, that only with large bodies can strong enough gravitational fields exist, as with the earth, which can hold the moon in an orbit around it with their mutual attraction.

When throwing a ball , your muscles supply kinetic energy, and it moves, but the earths gravitational pull makes the track a parabola. In space far from the earth, a thrown ball would go straight. The gravitational pull between small masses of each other is very weak , because of the small numbers in front of the formula:

$F=G{m_1m_2}/r^2$

Where F is the force, m1 and m2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant, a very small number.

Because the force for small masses is very weak , when the masses are moving, i.e. have kinetic energy, the effect of the gravitational pull on the trajectory of the small masses can be ignored.

On the surface of the earth we just have a constant g , and only use the attraction of masses to the huge mass of earth.

Take a bomb and trigger it to explode. Why does not the gravity of the individual pieces exploding pull back together? Because chemical energy was supplied and turned into kinetic energy and the pieces flying away have a very small attraction to each other due to their gravity, so small, that the tracks of the break up are straight lines until the earth's large gravity gets them.

The original Big Bang model is about elementary particles and radiation exploding ( energy provided not known) into space. The model , after the energy is dissipated could end up into falling back into itself, in what is the Big Crunch model.

So what is keeping the universe at our time stable is the kinetic energy that masses acquired at the Big Bang, with trajectories which are affected by each other making galaxies etc, but the over all effect still is that clusters of galaxies are moving away from each other , not converging, in a continuous explosive like expansion of space.

Since you are interested in the subject you could read the links I have provided.

For an accurate explanation you should try to understand the answer by G.Smith.

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  • $\begingroup$ I’m jealous that you have admirers who upvote even before you have explained anything! $\endgroup$ – G. Smith Mar 24 at 4:38
  • $\begingroup$ @G.Smith :) I think the upvote is that I try to preempt the closure by a place holder , which closure is sometimes too precipitate for new users. it is a downvote to closure ;) $\endgroup$ – anna v Mar 24 at 5:02
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    $\begingroup$ @G.Smith the reason I do place holders is because it has happened too often that I have worked on an answer, and suddenly the question is closed.As I spend some time in locating inks and figures etc, I feel cheated, so I use place holders and then I can edit even after closure. $\endgroup$ – anna v Mar 24 at 6:16
  • $\begingroup$ Yes, I’ve done placeholders too when an answer takes some time to construct. $\endgroup$ – G. Smith Mar 24 at 6:17
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This is actually a subtle question since - as you surmise - if it is merely expanding outward under its own momentum, gravity could, indeed, start pulling everything back together again. Not only "could" it, but it would, because the gravity would rob the expanding matter of outward kinetic energy, and as a result it would eventually stop and then collapse again. And the fact we see expansion now does not negate this - it could be that the Big Bang threw the Universe outward with enough force that, or gravity is weak enough that, we would have happened to be here during the "upward" part of the trajectory and in the future it would reverse to a "downward" trajectory.

However, we can be reasonably sure that this is not what is happening - and the reason for that is that were it the case, we would expect the expansion we have now to be slowing down. Just as if you throw a ball upward, it will be at its fastest the moment it leaves your hand, and thereafter - until it stops - it will start slowing down, the same would happen with the Universe.

Given that, astronomical observations allow us to measure how quickly the Universe is moving. We can do this by looking at objects, such as galaxies, very far away, and measure how that the light from them is distorted - much like how that the engine sound of a car moving away from you sounds lower than when it is at rest (though there is more than just this going on here), and with that, we can get the speed at which the object is departing us.

And what we find out is that the expansion is not slowing. Instead, objects seem to be moving away more quickly and worse - they are moving away more quickly the further away they are. This behavior makes no sense at all if the Universe was simply expanding out on a sort of ballistic, unpowered motion against gravity. Instead, the actual effect seems to be that the distances between objects are increasing in a way reminiscent of blowing up a bitmap: as time goes on, the distance between every object (above a suitably large size) multiplies by a constant factor, just as the distances on a picture on your computer multiply by a constant factor when you repeatedly click the zoom button. In particular, the rate is about $2.3\ \mathrm{Es}^{-1}$, or 2.3 per exasecond, meaning that in the span of one exasecond - a huge unit of time (one quintillion seconds) in the metric system that is about twice as long as the current age of the Universe - the distances multiply by a factor of 2.3: imagine hitting a "2.3x" zoom button over and over. This rate is called the "Hubble constant", and denoted $H_0$. Because of this, gravity can be defeated - it doesn't have the ability to cause an opposite contraction that is literally exponential, and it will eventually cause everything to be so widely separated that if there are any people-like creatures in the distant future, they will not ever be able to know there are other galaxies "out there" or even that the Big Bang happened at all!

The reason for this is not known for sure, but the simplest explanation seems to be that there is something called "dark energy" which is basically a uniform field of energy that fills up space - a specific, constant amount of energy filling every parcel of volume, across the entire Universe, that seems to be "how much a chunk of space is worth in energy", i.e. it's an intrinsic energy associated with space simply existing. And that energy has the effect of creating a constant (unlike gravity, which weakens with distance) "negative pressure" against gravity: effectively, genuine suction that is an actual force pulling things apart instead of just the absence of something pushing them. There are other models that are more complex than this, but this is the simplest one and so far, it seems to work.

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