Is dark energy causing different acceleration speeds?

[MarkE]

It's known that all galaxies are moving away faster from each other, due to dark energy. The further away these galaxies are from each other, the faster their movement is. 

Newton's law of motion (which is about gravity of course, not dark energy) states that, on earth, a falling object will accelerate, but, after a while this acceleration stops, due to its mass, and a steady falling speed remains without any further acceleration.

Again, this is gravity, not dark energy, so why this example? Well, I'm wondering if it has yet been detected that galaxies far away from us (outside our Local Group, to exclude a gravitationally bound galaxy like Andromeda) have decreased their acceleration speed to a steady acceleration, just like a falling object on earth. If not, than I guess it must have been detected that all galaxies are continuously increasing their acceleration speed at the same rate, no matter what their location in space is, NOT like a falling object on earth would behave. Is this true?

[Strange]

7 minutes ago, MarkE said:

Newton's law of motion (which is about gravity of course, not dark energy) states that, on earth, a falling object will accelerate, but, after a while this acceleration stops, due to its mass, and a steady falling speed remains without any further acceleration.

Newton's law of gravity says that a body will continue to accelerate (until it hits the ground). The only thing that stops a falling body accelerating is air resistance. So, in a vacuum, acceleration will continue.

(Which, I guess, makes the rest of your post moot?)

[EdEarl]

Terminal velocity on Earth is determined by wind resistance. A feather falls much slower than a cannon ball.

In a vacuum, the speed of light and relativity limit terminal velocity, I believe.

Moreover, dark and regular matter is not distributed evenly around the Universe. At large scale the distribution is even, but locally it is lumpy. Thus, at local scales, the acceleration from matter varies. Thus, the force of gravity affects things differently at local scales. Thus, local galaxy groups are collapsing on one another, while dark energy is causing the Universe to expand faster and faster. Both forces affect the acceleration of bodies.

Edited September 6, 2017 by EdEarl

[Strange]

12 minutes ago, MarkE said:

Again, this is gravity, not dark energy, so why this example? Well, I'm wondering if it has yet been detected that galaxies far away from us (outside our Local Group, to exclude a gravitationally bound galaxy like Andromeda) have decreased their acceleration speed to a steady acceleration

My understanding is that acceleration started a few billion years ago and has continued since then. (Not sure if it is constant or increasing though ...)

Before that, it was decelerating due to gravity.

[swansont]

32 minutes ago, MarkE said:

It's known that all galaxies are moving away faster from each other, due to dark energy. The further away these galaxies are from each other, the faster their movement is. 

That's Hubble expansion: velocity is proportional to distance. Dark energy is causing that to accelerate, i.e. distant objects are moving even faster than Hubble's law predicts (and not slower, as was expected, from gravitational attraction)

https://www.e-education.psu.edu/astro801/content/l10_p9.html 

[Tub]

1 hour ago, swansont said:

That's Hubble expansion: velocity is proportional to distance. Dark energy is causing that to accelerate, i.e. distant objects are moving even faster than Hubble's law predicts (and not slower, as was expected, from gravitational attraction)

https://www.e-education.psu.edu/astro801/content/l10_p9.html 

Just a couple of questions:  as it accelerates, is the total mass of a galaxy increasing and, if it is, does this mean that the Dark Energy will also have to increase to provide more and more momentum to sustain that rate of acceleration? Also, if velocity is proportional to distance, does this imply that different regions of Space/Time are expanding at different velocities? Is that at all possible?

[Strange]

18 minutes ago, Tub said:

as it accelerates, is the total mass of a galaxy increasing

No. If something "local" is accelerated then its increase in kinetic energy can be interpreted as an increase in (relativistic) mass, as described by SR. That does not apply to galaxies moving apart from one another which is not [local] motion and there is no increase in relative energy.

Quote

Also, if velocity is proportional to distance, does this imply that different regions of Space/Time are expanding at different velocities?

Yes, because expansion is a scaling effect, not a speed. So it is just simple arithmetic that the speed of separation is proportional to distance. For example, 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.

[swansont]

23 minutes ago, Tub said:

Just a couple of questions:  as it accelerates, is the total mass of a galaxy increasing and, if it is, does this mean that the Dark Energy will also have to increase to provide more and more momentum to sustain that rate of acceleration?

Don't know.

23 minutes ago, Tub said:

Also, if velocity is proportional to distance, does this imply that different regions of Space/Time are expanding at different velocities? Is that at all possible?

That's what happens, so yes. If space is being added at some small amount per light year of distance per unit time (I think it's 16 cm), then the more light years you are away from something, the faster the expansion speed.

[Tub]

12 minutes ago, Strange said:

No. If something "local" is accelerated then its increase in kinetic energy can be interpreted as an increase in (relativistic) mass, as described by SR. That does not apply to galaxies moving apart from one another which is not [local] motion and there is no increase in relative energy.

Yes, because expansion is a scaling effect, not a speed. So it is just simple arithmetic that the speed of separation is proportional to distance. For example, 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.

Thanks, Strange. Nice clear answer.

 

Just now, swansont said:

 

That's what happens, so yes. If space is being added at some small amount per light year of distance per unit time (I think it's 16 cm), then the more light years you are away from something, the faster the expansion speed.

Thank you too, swansont.

 

[MarkE]

Thanks @swansont and @Strange!

Is this also the reason why there is no center of the universe?

[Strange]

Pretty much, yes. Everything is moving away from everything else rather than away from some central point. The latter would not match what we observe. 

This us based on the idea that the universe is homogeneous and isotropic. 

[MarkE]

On 08/09/2017 at 11:31 AM, Strange said:

Everything is moving away from everything else rather than away from some central point.

If we know the distances between stars and galaxies, than why aren't we able to calculate which galaxy in space emerged first, in order to find out chronologically what the universe's family tree looks like, and where the location of the big bang must have been?

Edited September 30, 2017 by MarkE

[Strange]

7 minutes ago, MarkE said:

If we know the distances between stars and galaxies, than why aren't we able to calculate which galaxy in space emerged first, in order to find out chronologically what the universe's family tree looks like, and where the location of the big bang must have been?

If you look far enough in any direction you will see first generation galaxies. This confirms that the "location of the Big Bang" is everywhere. In other words, everywhere takes part in the expansion, pretty much equally (on large enough scales).

[MarkE]

But...the big bang happened only once, right? So there must have been one location for it. Or, if the Big Bang happened everywhere, then what about people on earth? Did the Big Bang happen in all of us at the same time as well? (Or does this 'everywhere' only account for galaxies, not planets/organisms?)

Edited September 30, 2017 by MarkE

[Strange]

11 minutes ago, MarkE said:

But...the big bang happened only once, right? So there must have been one location for it.

That "one location" was the entire universe (which was much, much smaller then). You seem to be thinking of it as explosion that happened somewhere in space.

(The whole idea of the Big Bang as an "event that happened" is mistaken, anyway; it is about the universe expanding. The whole universe expands so it is happening everywhere. On large enough scales.)

12 minutes ago, MarkE said:

Did the Big Bang happen in all of us at the same time as well?

Were people around 13.8 billion years ago? Surviving in a plasma at several million degrees?

[MarkE]

If you state that the Big Bang happened in our galaxy, than why isn't it possible to be more specific about where it happened within that galaxy? 'Everywhere' within a galaxy as well? I'm not allowed to conclude that it therefore must have happened in all organisms, agree, but is there a possibility to more specific than just stating 'in the Milky Way'?

5 minutes ago, Strange said:

That "one location" was the entire universe (which was much, much smaller then).

Do you mean that all the stuff that galaxies are made of were already present (only closer) at the Big Bang?

[swansont]

1 hour ago, MarkE said:

If you state that the Big Bang happened in our galaxy, than why isn't it possible to be more specific about where it happened within that galaxy? 'Everywhere' within a galaxy as well? I'm not allowed to conclude that it therefore must have happened in all organisms, agree, but is there a possibility to more specific than just stating 'in the Milky Way'?

Do you mean that all the stuff that galaxies are made of were already present (only closer) at the Big Bang?

The big bang did not happen in any particular galaxy. Galaxies did not exist at the time of the big bang.

galaxy ≠ universe

[MarkE]

8 minutes ago, swansont said:

The big bang did not happen in any particular galaxy. Galaxies did not exist at the time of the big bang.

galaxy ≠ universe

If that is the case, than I don't understand want Strange meant by this:

2 hours ago, Strange said:

If you look far enough in any direction you will see first generation galaxies. This confirms that the "location of the Big Bang" is everywhere. In other words, everywhere takes part in the expansion, pretty much equally (on large enough scales).

Everywhere outside galaxies?

[Strange]

1 hour ago, MarkE said:

Do you mean that all the stuff that galaxies are made of were already present (only closer) at the Big Bang?

There were no galaxies. Just a uniform plasma filling all of space. That space then expanded and cooled. Eventually clouds of gas collapsed (because of gravity) to form galaxies and stars. 

[swansont]

8 hours ago, MarkE said:

If that is the case, than I don't understand want Strange meant by this:

Everywhere outside galaxies?

The BB took place everywhere, which is why we see these galaxies (which formed later and after some expansion) in all directions.

[MarkE]

On 30/09/2017 at 10:37 PM, swansont said:

The BB took place everywhere, which is why we see these galaxies (which formed later and after some expansion) in all directions.

There were multiple Big Bangs?

[Strange]

2 hours ago, MarkE said:

There were multiple Big Bangs?

There is no evidence for that (and it is not implied by swansont's comment).

[swansont]

2 hours ago, MarkE said:

There were multiple Big Bangs?

No. It took place everywhere. All of space was created.

[Eise]

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