How can galaxies exist with the expansion of space?

[ACG52]

 

Can you really accept any of these responses as answering the question?

 

 

They've all answered the question, when you allow for the distortions in your rephrasing.

 

 

1. Gravity "overwhelms the expansion of space".

"Overwhelms is hardly a measure of anything"

 

Overwhelms is the description of what happens. The attractive force of gravity is much, much, much stronger than the expansive force. Why does this seem so difficult for you to understand?

 

 

2. At a distance of 199 million light years

expansion has no effect. At 201 million

light year it has the effect of driving galaxies apart.

Weird!

 

The hubble constant, which is the current rate of expansion, is 69.32 m/s/megaparsec. At a distance of 199mlys (to continue your ridiculous argument) from us, the force of expansion and the force of gravity are balanced on a cusp, no red-shift, no blue-shift. Another 2 million lys and the rate of expansion, which continues to increase with distance is enough to overcome the attractiveness of gravity and space between us and the point 201million lys expands.

 

 

3. Referred to Wikipedia's "ants on arubber band"

analogy. As useless as the pennies on a balloon analogy.

I didn't think analogies proved much.

 

I'm not real fond of analogies either, because most people confuse them with an actual model, rather than a simplified, dumbed down explanation, and just don't understand them.

 

4. The distance between objects doesn't change the effect of space expansion.

Doesn't set well with the contention effects only occur when objects

are 200 million light years apart

 

I don't know where you got point 4, other than by totally misunderstanding something you were told. The rate of expansion increases with the distance between points in space at the rate of 69.32 meters per second for each 3.26 million light years. That's very, very little, which is why it is easily overcome by gravity. But the farther away you get, the greater the rate of increase becomes and the weaker gravity becomes. At some point, (outside the local galactic supergroup, about 200 million lys away) gravity is weak enough so that the distance between objects increases.

 

 

5. You are a dummy, go away. May be so.

 

At this point number 5 is gaining.

Edited February 8, 2013 by ACG52

[zapatos]

I will try one more time to get someone to take an objective look at why galaxies

don't expand with the expansion of space.

 

It is only logical that if space is expanding the distance between objects should

increase.

 

The reasons that I received are:

 

1. Gravity "overwhelms the expansion of space".

"Overwhelms is hardly a measure of anything"

 

2. At a distance of 199 million light years

expansion has no effect. At 201 million

light year it has the effect of driving galaxies apart.

Weird!

 

3. Referred to Wikipedia's "ants on arubber band"

analogy. As useless as the pennies on a balloon analogy.

I didn't think analogies proved much.

 

4. The distance between objects doesn't change the

effect of space expansion.

Doesn't set well with the contention effects only occur when objects

are 200 million light years apart.

 

5. You are a dummy, go away. May be so.

 

Can you really accept any of these responses as answering the question?

And the arguments we received from you describing why we are wrong consist of:

 

1. Nuh-uh.

 

 

It really pisses me off that you ask for help, we take the time to explain it, and rather than make any effort to understand it, argue against it, or simply acknowledge it, you just ignore it. You then have the gall to give us one more chance to give you an answer you will be happy with?

 

Go away. I have better things to do than coddle those who act like petulant children.

[michel123456]

As much as i know the following;

 

Forces of nature are transparent.

 

At each level of dimension, all interactions apply. Even at the infinitesimal scale, gravity exist together with weak & strong interactions. Simply at those levels gravity is negligible.

 

I guess, and I have read it somewhere, that even at the very smallest scale, expansion of space exists and acts but is soooooo weak that it is negligible. Its effects are even weaker than gravity.

 

At a larger scale, the weak & strong interactions become negligible and other interactions take over; electromagnetism and gravity.

Over very large scales, gravity takes over.

Over veeeeery laaaarge scales, expansion of space takes over.

 

Gravity gets weaker as a function of the square of distance. Expansion of space gets weaker as a function of distance.

Edited February 8, 2013 by michel123456

[pwagen]

Using the same logic, shouldn't airplanes be impossible? The effects of gravity works on you or me or my car, but it doesn't work on a 747? What's up with that?

[ACG52]

 

Gravity gets weaker as a function of the square of distance. Expansion of space gets weaker as a function of distance.

 

 

I think you meant that expansion gets stronger as a function of distance.

 

I attempted once to calculate the rate of expansion given the distance between earth and moon and disregarding gravity. I probably messed it up, but I got a result of about a tenth of a proton's diameter per second.

 

Using the same logic, shouldn't airplanes be impossible? The effects of gravity works on you or me or my car, but it doesn't work on a 747? What's up with that?

 

A 747 has wings. Without them, it acts the same as a rock.

[Lazarus]

Sorry ACG52, I was going to shut up but I couldn't resist commenting on your post.

 

One tenth of the diameter if a proton is about 10 to the minus 16.meters.

 

The number of seconds in 13 billion years is 60*60*24*365*13000000000 or about

4*10 to the17th power.

 

Which amounts to a few meters in 13 billion years.

 

However, space is supposed to have expanded a few hundred times in 13 billion years.

 

Warm reguards,

 

Me

[ACG52]

Sorry ACG52, I was going to shut up but I couldn't resist commenting on your post.

 

One tenth of the diameter if a proton is about 10 to the minus 16.meters.

 

The number of seconds in 13 billion years is 60*60*24*365*13000000000 or about

4*10 to the17th power.

 

Which amounts to a few meters in 13 billion years.

 

However, space is supposed to have expanded a few hundred times in 13 billion years.

 

Warm reguards,

 

Me

 

I specifically said I was examining the distance between the earth and the moon, and DISREGARDING GRAVITY. Taking gravity into account, there was no expansion due to reasons already given.

[Iggy]

Photons seem to 'hold together" pretty well but still are stretched by the expansion of space.

 

They are stretched over vast intergalactic distances.

 

If the force of gravity is equal to the space expansion at about 200 million light years and stars orbit the center of the galaxy then anything closer will fail to orbit and be sucked into tthe center.

 

Yes, it would if it wasn't in orbit. There is an attractive force toward the center, but it is balanced by a centrifugal force when it rotates around a center.

 

It is very nice to have some feedback from knowledgeable individuals. I appreciate it immensely.

 

Let me try to explain what I meant by the effect of space expansion increases with distance.

 

I would like to push Mars to 186 million miles from the Sun and fatten it to match the mass of earth to make the arithmetic simpler for me. Then use the Sun, Earth and Mars for examples. Even though I majored in mathematics a million years ago, now I have to use a calculator to add two and two so anything that helps is good.

 

The picture:

 

!

!

!

! S 1 E 1 M 2

Time 1 ! x.................................y..............................z Space

!

!

!

!

! S 1 E 1 M

Time 0 ! x...............y................z Space

------------!------------------------------------------------------------------

! Distance

!

 

At time zero the earth is orbiting with gravity and space expansion in balance.

 

The time scale is such that from T0 to T1 space has doubled.

 

In Newtonian space the gravitational force is proportional to g divided by r squared.

 

In the expanding space model gravity is Newtonian gravity plus the additional

amount needed to compensate for the expansion.

 

Say g' = g + a. a is the acceleration needed to move Earth enough to match the expansion.

 

So the force g' on Earth is proportional to (g +a) / r*r.

 

However, the force required to hold Mars in place is (g + 2a) / r*r.

 

Which implies that g' is insufficient to hold Mars in place.

 

Is this wrong?

 

Yes, it is wrong because it only accounts for the mass of earth, mars, and the sun. Over large scales the universe can be approximated as homogeneous. If you want to do a Newtonian calculation to find out if a volume is gravitationally bound you have to ask what the density of that volume is and you have to know the velocity at which the volume expands.

 

Let's say the density of the volume is [math]\rho[/math]

 

The distance between one star (at the center) and another star (at the edge of some volume) is [math]D[/math]

 

Over intergalactic distances things don't rotate, or orbit. They have a velocity directly away from a center. If the velocity is less than the escape velocity for that volume then they are gravitationally bound, if not then not. The escape velocity depends on the mass of the volume of space which is,

 

[math]m = \rho \frac{4}{3} \pi D^3 [/math]

 

The escape velocity is,

 

[math]V = \sqrt{ \frac{2GM}{D}} [/math]

 

combining the two gives,

 

[math]V = \sqrt{ 2 G \rho \frac{4}{3} \pi D^2} [/math]

 

The large density of a galaxy (or the density of a cluster of galaxies) makes the right hand side larger than the left hand side making them gravitationally bound.

 

What you end up with is typically called the critical density. If a volume of space is above the critical density then it is gravitationally bound. The actual velocity that things expand in our universe is given by the hubble constant. The velocity that a galaxy moves away is its distance times the hubble constant, so the critical density is easy to solve from the last equation given...

 

[math]V = \sqrt{2 G \rho \frac{4}{3} \pi D^2} [/math]

 

[math]H \cdot D = \sqrt{2 G \rho \frac{4}{3} \pi D^2} [/math]

 

[math]\rho = \frac{3 H^2}{8 \pi G} [/math]

 

If any volume of space comports to the hubble constant and is above that density then it will be gravitationally bound, and if not then not. Galaxies are.

 

 

 

Edited February 11, 2013 by Iggy

[Lazarus]

Iggy,

 

That is a beautiful explanation gravitational binding of galaxies.

 

I see where the force of gravity can match the expansion of space within a
sphere.

 

The force of gravity in a sphere is proportional to the reciprocal of the distance
from the center of the sphere. (1/r*r)

 

The effective mass and hence the magnitude of the force of gravity increases by the
cube of the distance. (r*r*r)

 

Combining the two, the net gravitational force is proportional to the distance from the
center. ®

 

The effect of expanding space is also proportional to the distance to the center.

 

That shows that there can be a balance between them.

 

However, galaxies are more disc shaped than spherical which seems to imply that gravity
would be weaker than space expansion at the outskirts of the galaxy.

 

Thank you very much for simplifying the concept for me.

[ACG52]

 

Combining the two, the net gravitational force is proportional to the distance from the

center. ®

 

Simply incorrect.

 

The gravitational force is INVERSELY proportional to the square of the distance.

[Iggy]

Simply incorrect.

 

The gravitational force is INVERSELY proportional to the square of the distance.

 

In a homogeneous medium the gravitational force toward the center of a sphere is proportional to distance. The universe is well approximated as homogenous at large scales.

[ACG52]

In a homogeneous medium the gravitational force toward the center of a sphere is proportional to distance. The universe is well approximated as homogenous at large scales.

 

The universe is not a sphere and has no center.

[Iggy]

The universe is not a sphere and has no center.

 

The Shell theorem allows one to mark off any arbitrary sphere and consider the gravitational force toward the center.

[Lazarus]

Since gravity exactly matches space expansion up to 200 million light years,

that seems to imply that either galaxies further away are not fleeing or space

is not homogenious.

[ACG52]

The shell theorem applies to a hollow, symmetrical sphere. The universe is not a hollow, symmetrical sphere.

 

 

2. If the body is a spherically symmetric shell (i.e. a hollow ball), no net gravitational force is exerted by the shell on any object inside, regardless of the object's location within the shell.

 

All objects within your arbitrary sphere are subject to gravitational force. The shell theorem does not apply.

Since gravity exactly matches space expansion up to 200 million light years,

that seems to imply that either galaxies further away are not fleeing or space

is not homogenious.

 

Gravity doesn't match expansion, it totally overwhelms it, out to a distance of about 200 million lys.

You're having difficulty grasping the concept, which, quite frankly, seems to be quite simple.

 

Gravity diminishes with distance, and expansion doesn't. At some point, gravity and expansion balance each other. Further than that, gravity diminishes to less than expansion, and expansion takes over. It's not that complicated an idea.

[Iggy]

The shell theorem applies to a hollow, symmetrical sphere. The universe is not a hollow, symmetrical sphere.

 

 

All objects within your arbitrary sphere are subject to gravitational force. The shell theorem does not apply.

 

I'm afraid you don't know what the shell theorem is. It means that you can disregard anything outside the sphere and only consider the mass of what is inside. It means, for example, if you want to know the gravitational force of the earth on a person you don't have to consider the rest of the universe pulling up on that person. That all cancels. The same applies to any planet or any arbitrary sphere.

[ACG52]

 

It means that you can disregard anything outside the sphere and only consider the mass of what is inside.

 

I'm afraid you don't know what the shell theorem is.

 

It means you can disregard anything INSIDE the sphere, and only consider the mass of what is outside.

[Iggy]

Ok, if you want to start a thread on the topic we could bicker.

[Lazarus]

Then where does the gravity that keeps the Milky Way and Andromeda from moving away

from each other due to the space expansion come from?

[pwagen]

Then where does the gravity that keeps the Milky Way and Andromeda from moving away

from each other due to the space expansion come from?

I'll go out on a limb and say it comes from the mass of the galaxies themselves.

Edited February 11, 2013 by pwagen

[Iggy]

I'll go out on a limb and say it comes from the mass of the galaxies themselves.

 

No, there is no "gravity that keeps the Milky Way and Andromeda from moving away from each other". They are moving towards each other. The only thing that affect's a galaxies motion (in any reference frame) is gravity and momentum (the cosmological constant is part of gravity for those who were about to object). The relative gravity and inertia of the Milky way and Andromeda galaxy tell us that it should be moving towards us, and it is. Nothing keeps them from moving away from each other and they aren't so kept.

 

The expansion of space is confused with the acceleration of expansion of space which is mistakenly left out of "gravity". Expansion is caused only by inertia. Things that once had a velocity away from each other, continue to have such an inertial velocity. What caused the initial velocity nobody knows. Whatever the Andromeda's initial velocity was... it wasn't a sufficient escape velocity from earth. Expansion, on this scale, only refers to the initial distance and time for which the Andromeda coasted away from us before gravity's acceleration turned the velocity negative.

Edited February 11, 2013 by Iggy

[pwagen]

No, there is no "gravity that keeps the Milky Way and Andromeda from moving away from each other". They are moving towards each other. The only thing that affect's a galaxies motion (in any reference frame) is gravity and momentum (the cosmological constant is part of gravity for those who were about to object). The relative gravity and inertia of the Milky way and Andromeda galaxy tell us that it should be moving towards us, and it is. Nothing keeps them from moving away from each other and they aren't so kept.

I stand corrected.

[Iggy]

I do ramble

[ACG52]

 

Expansion is caused only by inertia. Things that once had a velocity away from each other, continue to have such an inertial velocity. What caused the initial velocity nobody knows.

 

Expansion is caused by more space appearing between objects, not by the objects moving through space away from each other.

[Iggy]

Expansion is caused by more space appearing between objects, not by the objects moving through space away from each other.

 

That is a coordinate choice.

 

Are galaxies really moving away from us or is space just expanding?

 

This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift. General relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views. Part 3 of the tutorial shows space-time diagrams for the Universe drawn in both ways.

 

In the absence of the cosmological constant, an object released at rest with respect to us does not then fly away from us to join the Hubble flow. Instead, it falls toward us, and then joins the Hubble flow on the other side of the sky, as discussed by Davis, Lineweaver & Webb (2003, AJP, 71, 358). In what are arguably the most reasonable coordinates, the cosmic time t and the distance D(t) measured entirely at the cosmic time t, the acceleration is given by g = -GM(r<D)/D2 where M(r<D) is the mass contained within radius D. This gives g = -(4*pi/3)*G*(rho(t)+3P(t)/c2)*D(t). The 3P/c2 term is a general relativistic correction to the otherwise Newtonian dynamics. Galaxies all move under the influence of this acceleration and their initial position and velocity. In other words, F = ma and gravity provides the force. Nothing extra or weird is needed.

 

Also see the Relativity FAQ answer to this question.

Edited February 11, 2013 by Iggy