Gravity Waves

[Ivanov2000]

Ok, so if gravity propagates in waves at c, I have few questions.

 

1. Does the cosmic horizon also apply to gravity?

 

2. Is gravity 'red shifted' by expansion of space?

 

3. Is this taken into account in current models of expanding universe?

 

4. Would this explain dark matter/energy to some degree?

 

[Strange]

Ok, so if gravity propagates in waves at c, I have few questions.

 

1. Does the cosmic horizon also apply to gravity?

 

2. Is gravity 'red shifted' by expansion of space?

 

3. Is this taken into account in current models of expanding universe?

 

4. Would this explain dark matter/energy to some degree?

 

 

 

Note that gravity does not propagate as waves. There are no waves involved in pulling an apple to the Earth, for example. But waves can be generated by specific types of moving masses.

 

1. Yes, I think so. (To gravitational waves, at least)

 

2. Gravitational waves are, yes. (This had to be taken into account in calculating the actual orbiting frequency of the black holes that generated the decade detected waves)

 

3. I assume you mean the speed at which gravity propagates. This is implicitly taken into account as it is part of the same theory used to describe expansion.

 

4. I don't believe so (see 3)

BTW, gravity waves are something completely different (to do with fluid dynamics).

Edited October 7, 2016 by Strange

[Janus]

Ok, so if gravity propagates in waves at c, I have few questions.

 

1. Does the cosmic horizon also apply to gravity?

 

2. Is gravity 'red shifted' by expansion of space?

 

3. Is this taken into account in current models of expanding universe?

 

4. Would this explain dark matter/energy to some degree?

"Gravity" does not propagate as a wave, although there are such things as gravitational waves.

 

Gravitational waves bear the same relationship to the attraction between masses as light (electromagnetic waves) has to the attraction of two opposite electrical charges or magnetic poles.

 

In other words, gravitational waves can not be used to pull a mass towards you anymore than shining a flashlight at a piece of iron will draw it towards you. Conversely, two masses will still attract each other gravitationally even if neither of them is emitting gravitational waves.

[Ivanov2000]

"gravitational waves can not be used to pull a mass towards you anymore than shining a flashlight at a piece of iron will draw it towards you"

 

Thanks, I think this line made it clear.

 

However, I have read that if the sun were to vanish, the change in gravity would not affect earth for 8 minutes. If this is correct, then a cosmic horizon does exist for gravity? If this horizon is expanding, would that not result in more distant space expanding at a greater rate?

It also explains how gravity can 'leave' a black hole (I assume gravitation waves cannot).

[Strange]

However, I have read that if the sun were to vanish, the change in gravity would not affect earth for 8 minutes. If this is correct, then a cosmic horizon does exist for gravity?

 

 

It certainly exists for changes in gravity, like that example.

 

But the overall "static" curvature of spacetime within the observable universe does (I think) still depend on the mass outside the horizon. This is why measures of the overall "flatness" of the universe give us a lower limit on the total size of the universe.

 

 

 

If this horizon is expanding, would that not result in more distant space expanding at a greater rate?

 

I'm not sure why. Remember that the distant objects will have their own horizon, the same distance around them. So a galaxy 13 billion light years away, will have a cosmological horizon that extends 13 billion light years around it. So it will have the same gravitational environment as galaxies that are nearby.

 

 

It also explains how gravity can 'leave' a black hole (I assume gravitation waves cannot).

 

Good point.

[Ivanov2000]

Yes, changes in gravity. If the mass/energy content of the universe is constant, I can see that the overall curvature is not going to change much, and that curvature will expand with space time, beyond the horizon. This would suggest that it was fixed at birth of universe. Any changes would be local, within the horizon(s) - but would there in fact be any changes at all?

[Mordred]

The last post is almost correct but I note you need some details on what constitutes curvature. Curvature is a relationship between critical density to actual density. Critical density is a calculated value showing when an expanding universe will halt and start contracting.

 

It doesn't require "outside the horizon"

 

If the universe outside the horizon was a different mass density. This would cause a flow that is anistropic. A preferred direction.

However we know there is no flow. Isotropic (no preferred direction ).

 

here I wrote this several years back.

 

http://cosmology101.wikidot.com/universe-geometry

page 2 with the metrics.

http://cosmology101.wikidot.com/geometry-flrw-metric/

Edited October 8, 2016 by Mordred

[Ivanov2000]

H is a mystery...

 

I'm thinking Hubble constant in 1st link.

 

in 2nd link, I spent about half hour wondering what H was, then realised it should be sinh.

 

Correct?

[Mordred]

H is a mystery...

 

I'm thinking Hubble constant in 1st link.

 

in 2nd link, I spent about half hour wondering what H was, then realised it should be sinh.

 

Correct?

The first answer is correct. Hubbles constant.

 

https://en.m.wikipedia.org/wiki/Hubble%27s_law

Edited October 9, 2016 by Mordred