# A basic question about the big bang

## Recommended Posts

We speculate that stars are moving away. They seem to merge to a single point if we reverse their motions. My question is which of the following caused the moving away.

1) The big bang is an explosion, such that they are moving away due to the explosion

2) They are accelerated away due to a force, the imaginary dark energy

3) It's due to a space expansion. That even the stars can be stationary, it gives a moving effect along with the expansion of universe. This is used to be illustrated by dots on the surface of a balloon.

So which one or more than one of the above caused the moving away of the stars.

##### Share on other sites
7 minutes ago, Courage said:

3) It's due to a space expansion. That even the stars can be stationary, it gives a moving effect along with the expansion of universe. This is used to be illustrated by dots on the surface of a balloon.

This one.

A few important things to note...

The expansion of space only happens at very large scales. At the level of galaxies and clusters of galaxies, gravity stops things moving apart.

The surface of a balloon comparison only works if you consider the just the surface of the balloon being a 2D analogy for 3D space. This also explains how a finite universe can have no boundary: the surface of the balloon (or the Earth) has a finite area, but has no edge.

Dark energy is an extra factor that causes the rate of expansion to increase (this started about 4 billions years ago, I think)

The speed at which things move apart is related to how far apart they are. This is nothing special, it is just because expansion is a scaling effect. Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and the same between all of them).

At time 0, they are 1 unit apart:
A.B.C.D.E.F

After some time they are 2 units apart:
A..B..C..D..E..F

After the same time again, they are 3 units apart:
A...B...C...D...E...F

And so on:
A....B....C....D....E....F

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. (We can see galaxies that are receding faster than light.)

13 minutes ago, Courage said:

We speculate that stars are moving away.

We observe that (distant) galaxies are moving away.

##### Share on other sites
Posted (edited)

Though change those from stars and specify galaxies.

Edit just realized Strange mentioned that above

Edited by Mordred

##### Share on other sites

Thanks guys. It's very clearly explained.

Thanks Strange and Mordred.

##### Share on other sites
On 7/17/2019 at 10:38 AM, Mordred said:

Though change those from stars and specify galaxies.

Edit just realized Strange mentioned that above

To be exact, only superclusters of galaxies are moving apart.  By definition, dark energy causes superclusters to move apart.  Dark energy has no impact at all on individual galaxies.

##### Share on other sites
9 minutes ago, Airbrush said:

To be exact, only superclusters of galaxies are moving apart.  By definition, dark energy causes superclusters to move apart.  Dark energy has no impact at all on individual galaxies.

They would move apart anyway; dark energy is accelerating the rate at which they move apart.

##### Share on other sites
On 7/22/2019 at 10:18 AM, Strange said:

They would move apart anyway; dark energy is accelerating the rate at which they move apart.

Yes you are correct.  Dark energy does not cause superclusters to move apart it only accelerates the motion that began with the big bang.  Thanks for the correction.

Whenever you think of dark energy, its' effect only works uniformly on entire superclusters of galaxies.  I heard that eventually all the galaxies in a supercluster (which is a cluster of galaxy clusters) will merge into a single galaxy and eventually all galaxies in a supercluster will end up in a single supermassive black hole, in trillions of years.

##### Share on other sites

Not as long as the universe continues expanding they won't. To be clear expansion isn't accelerating the BB.

Thus can get complex but I will keep it brief. If you consider the expansion rate in terms of units of Mpc. Then the rate of expansion is slowing down Ie at CMB time the expansion rate per Mpc is roughly 22,000 times what it is today. Per Mpc it's roughly 70 km/s/Mpc.

Now if you simply consider expansion in terms of the separation distance to the cosmological event horizon then expansion is indeed accelerating. The reason thus works is as expansion occurs volume increases so you gain units of Mpc.