How can galaxies exist with the expansion of space?

[zapatos]

My understanding of the interaction of gravity and space

expansion from this thread just is not in agreement with

the consensus.

What does the consensus say?

[Lazarus]

What does the consensus say?

I am assuming that the "consensus" is that gravity offsets space expansion as

our observation indicates and will continuously change to match our observations.

[ACG52]

Of course both gravity on the cosmological scale and the hubble constant change over time. As the universe expands, the density of matter/energy decreases, and so does the gravitational field. And rather than being called a constant, it makes more sense to call it the hubble scaling factor.

[Lazarus]

Of course both gravity on the cosmological scale and the hubble constant change over time. As the universe expands, the density of matter/energy decreases, and so does the gravitational field. And rather than being called a constant, it makes more sense to call it the hubble scaling factor.

So actually, we are all really in agreement on how the theory says that

the universe works.

 

One inferrence is that electrostatic force must work the same way or

atoms would be growing.

 

Great thread.

[michel123456]

Thought I might add this:

 

You can modify Newton's Law with a Cosmological Constant term to approximate weak-field gravity:

 

[math]F=m\left (-\frac{GM}{r^2} + \frac{c^2 \Lambda}{3}r \right )[/math]

 

Since the CC term is proportional to distance while the mass term is inversely proportional to the square of distance, you can see that eventually the second term will overwhelm the first. This means that at some distance from a mass gravity actually becomes repulsive.

 

As a quick example, let's say that the Milky Way is the only galaxy in the universe. The MW has a mass on the order of 6×10⁴² kg. We know the cosmological constant is of the order of 10^-52 m^-2. Using these values, we can determine that at a distance of about 5.4 million light-years = 1.7 Mpc away from the MW gravity becomes repulsive.

I can't figure the consequences. Doesn't that work for an extremely small mass too?

[elfmotat]

I can't figure the consequences. Doesn't that work for an extremely small mass too?

 

It works exactly the same way that the usual law of gravitation works. So it works for all macroscopic objects that aren't particularly dense, and which are moving slowly compared to light.

Edited February 28, 2013 by elfmotat

[michel123456]

It works exactly the same way that the usual law of gravitation works. So it works for all macroscopic objects that aren't particularly dense, and which are moving slowly compared to light.

For a hydrogen atom? what is the distance?

[elfmotat]

For a hydrogen atom? what is the distance?

 

Now you're getting too small to be considering gravity, but if you plug in M=proton mass then the prediction is that gravity will become repulsive at ~1.1 feet away from the proton. But, like I said, now we've moved into the realm where Newtonian physics and gravity no longer apply.

[ACG52]

Plug in 10^-27 kg for m, 10^-15 meters for r, and you get a gravitational force on the order of 10^-34 Newtons.

 

 

[michel123456]

Now you're getting too small to be considering gravity, but if you plug in M=proton mass then the prediction is that gravity will become repulsive at ~1.1 feet away from the proton. But, like I said, now we've moved into the realm where Newtonian physics and gravity no longer apply.

So that should be testable in laboratory.

[elfmotat]

So that should be testable in laboratory.

 

Did you read the part of my post where I said, "Now you're getting too small to be considering gravity... we've moved into the realm where Newtonian physics and gravity no longer apply"?

[michel123456]

Did you read the part of my post where I said, "Now you're getting too small to be considering gravity... we've moved into the realm where Newtonian physics and gravity no longer apply"?

Yes I read that.

 

So, is it testable by some experiment somehow? You choose the mass so that "the realm where Newtonian physics and gravity do apply"

[Lazarus]

Now you're getting too small to be considering gravity, but if you plug in M=proton mass then the prediction is that gravity will become repulsive at ~1.1 feet away from the proton. But, like I said, now we've moved into the realm where Newtonian physics and gravity no longer apply.

Do electrostatic forces follow the same rules as gravity?

[elfmotat]

Yes I read that.

 

So, is it testable by some experiment somehow? You choose the mass so that "the realm where Newtonian physics and gravity do apply"

 

In the realm where Newtonian physics applies, the effect of the Cosmological Constant is far too small to be measurable.

 

Do electrostatic forces follow the same rules as gravity?

 

I'm not sure what you mean. They both follow an inverse square law, but the electrostatic force between two electrons is of the order of 10⁴² times greater than the gravitational force. This is such a stupendously large number that gravity has a completely immeasurable effect for most fundamental interactions.

[Lazarus]

In the realm where Newtonian physics applies, the effect of the Cosmological Constant is far too small to be measurable.

 

 

I'm not sure what you mean. They both follow an inverse square law, but the electrostatic force between two electrons is of the order of 10⁴² times greater than the gravitational force. This is such a stupendously large number that gravity has a completely immeasurable effect for most fundamental interactions.

The electrostatic force between electrons and protons should have the same

problems with the expansion of space that gravity does.

 

Doesn't the sizeable expansion of space over a few billion years have any effect

within atoms and solids held together by electrostatic forces?

[elfmotat]

The electrostatic force between electrons and protons should have the same

problems with the expansion of space that gravity does.

 

Doesn't the sizeable expansion of space over a few billion years have any effect

within atoms and solids held together by electrostatic forces?

 

No. It seems to be your trend that you think expansion should somehow be more powerful than other forces. It isn't.

[ACG52]

The electrostatic force between electrons and protons should have the same

problems with the expansion of space that gravity does.

 

Doesn't the sizeable expansion of space over a few billion years have any effect

within atoms and solids held together by electrostatic forces?

Gravity is stronger than the force of expansion, out to 200 million lys.

 

Electromagnetic forces are 10²⁹ times STRONGER than gravity.

 

You figure it out.

Edited March 4, 2013 by ACG52

[zapatos]

Doesn't the sizeable expansion of space over a few billion years have any effect

within atoms and solids held together by electrostatic forces?

Why would time be a consideration?

 

If you push on the side of a building for a long enough time, will it eventually fall over?

[Lazarus]

Gravity is stronger than the force of expansion, out to 200 million lys.

 

Electromagnetic forces are 10²⁹ times STRONGER than gravity.

 

You figure it out.

 

I see what you mean.

 

Since the force of gravity at short distances is greater than

what would be necessary for stable orbits without the expansion

of space, the electrostatic force must be greater than would

be required to hold solids together without space expansion.

 

There should be no effect at greater distances unless some

galaxies have a significant excess of deficiency of electrons.

 

Why would time be a consideration?

 

If you push on the side of a building for a long enough time, will it eventually fall over?

 

Pleae forgive the irrelevant reply but I couldn't resist.

 

Telsa showed that "pushing on a building" at the right

frequency could eventually bring the building down.

Mythbusters seemed to verify it by shaking a big bridge

with an 8 pound weight.

[beefpatty]

Telsa showed that "pushing on a building" at the right

frequency could eventually bring the building down.

Mythbusters seemed to verify it by shaking a big bridge

with an 8 pound weight.

 

Galaxies are hardly solids, and thus don't have a constant resonant frequency that would break the entire galaxy apart. Besides, the expansion of space is not periodic to begin with. I don't mean to sound condescending, but the fact that forces are additive is one of the most elementary topics in physics. The net force determines the dynamics of the system. If I push a block with constant force to the left, and a smaller force is applied to the right, the block will continue to accelerate to the left at a constant rate. It's only when the force applied to the right overcomes the opposing force that the block will accelerate in the opposite direction.

[Lazarus]

Galaxies are hardly solids, and thus don't have a constant resonant frequency that would break the entire galaxy apart. Besides, the expansion of space is not periodic to begin with. I don't mean to sound condescending, but the fact that forces are additive is one of the most elementary topics in physics. The net force determines the dynamics of the system. If I push a block with constant force to the left, and a smaller force is applied to the right, the block will continue to accelerate to the left at a constant rate. It's only when the force applied to the right overcomes the opposing force that the block will accelerate in the opposite direction.

 

Dear Beefpatty,

 

With this thread winding down, I don't want it to appear that there are unresolved

issues. There are none.

 

The original question has been answered clearly with good explanations.

 

The last posts by ZAPATOS and ACH52 were absolutely valid.

 

I tried to make if very clear that my post about Telsa was not an argument for or

against anything.

 

The discussion you referred to was about atoms, not galaxies.

 

Your contribution is appreciated.

 

It was a pleasure to read all the excellent posts to this thread by knowledgeable

individuals.