Universe expansion speed vs gravitational waves speed

In summary, the conversation discusses the concept of dark energy and its role in the expansion of the universe. The use of terms like "fabric of spacetime" and "color charge" are questioned and it is mentioned that string theory has not been proven to have real-world effects. The discussion also touches on the relationship between distance and expansion, and the idea that galaxies beyond our cosmic horizon are causally disconnected from us. It is emphasized that the speed of light remains constant regardless of the speed of the emitting object. The potential effect of dark energy on local phenomena is debated.
  • #1
Mectaresh
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From my own(simplistic)perspective,dark energy is expanding the universe by creating further spacetime at a velocity faster than the speed of light,if gravitational waves propagate at the speed of light how is it possible that M31 and the milky way are still bound by gravity when the fabric of spacetime is constantly being created between them at speeds faster than the speed of light?

According to string theory gravitons are not attached to the membrane because of their shape (closed string).
Does this explain why gravitons don't seem to be affected by the expansion of the fabric of spacetime ?
 
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  • #2
"dark energy", the name we give to whatever it is that is causing the acceleration of the expansion of the universe, is unbelievably weak and on the scale of galactic clusters and smaller, it has no effect. It's like an ant pushing on a house. It isn't that the ant's tiny force has a tiny effect on the house, it's that it has NO effect. It just isn't strong enough to shift the house on its foundations by any amount.

String theory is at present nothing more than a mathematical exercise and has not been shown to be responsible for, or descriptive of, any real-world effects.

"Fabric" is a terrible word to use when talking about space-time. It has an English-language meaning that does not translate well to cosmology.
 
  • #3
phinds said:
"Fabric" is a terrible word to use when talking about space-time. It has an English-language meaning that does not translate well to cosmology.
Interesting!
How do you feel about the use of color charge,flavor and black hole?
 
  • #4
Mectaresh said:
Interesting!
How do you feel about the use of color charge,flavor and black hole?

I think they are perfectly reasonable and widely accepted. "Fabric" is neither (unless you count television "science" in which case "fabric" is just fine)
 
  • #5
Mectaresh said:
Interesting!
How do you feel about the use of color charge,flavor and black hole?

Well, neither color nor flavor lend themselves to the sort of misunderstanding (no one is likely to think that quarks of different flavors will affect the taste buds on their tongue differently) that the "fabric of spacetime" metaphor does... And even though the space near the event horizon of a black hole may be excitingly bright, the black hole itself likely still appears in the visible wavelengths as a black disk, so that's not so bad... But if you want an example of a really unfortunate term, it's hard to beat quantum-mechanical "spin".

At some point, you have to move past the natural-language hand-waving and look at the math. The question you asked back in post #1 of this thread about the binding between our galaxy and M31 has a perfectly natural answer in the framework of general relativity.
 
  • #6
I remember the old days before the Fabric of the Universe was wash-and-wear. We were constantly ironing and pressing, but it never looked quite right.
 
  • #7
The rate that the distance is increasing between us and another galaxy is proportional to the distance to that galaxy. The Hubble Constant give this relationship precisely. A nearby galaxy like M31 is too close for its distance to be increasing faster than light. In fact, its distance is really not changing at all because it is gravitationally bound to us, like phinds said above.
A galaxy more than 14 billion light years away is receding from us at a speed faster than light due to the expansion of the universe. So that galaxy can not affect us in any way, not by gravity or by light or anything else.
 
  • #8
Mectaresh said:
From my own(simplistic)perspective,dark energy is expanding the universe by creating further spacetime at a velocity faster than the speed of light,if gravitational waves propagate at the speed of light how is it possible that M31 and the milky way are still bound by gravity when the fabric of spacetime is constantly being created between them at speeds faster than the speed of light?

To an observer in (some far away galaxy that is receding faster than the speed of light), we are receding at greater than the speed of light, but we still light that only travels at ... the speed of light ... from that galaxy. If it is no problem for light to overcome, it shouldn't be a problem for gravitational waves either ;)
 
  • #9
Gravitational waves travel at c. This was demonstrated by Hulse-Taylor to within 1%.
 
  • #10
If a galaxy is so far away that it is receding faster than light, then we can't see it at all. No light or gravity from it will ever reach us. And no light or gravity from us with reach that galaxy. It is beyond our cosmic horizon.
We are causally disconnected from that galaxy.
 
  • #11
kochanskij said:
If a galaxy is so far away that it is receding faster than light, then we can't see it at all. No light or gravity from it will ever reach us. And no light or gravity from us with reach that galaxy. It is beyond our cosmic horizon.
We are causally disconnected from that galaxy.

Galaxies WITHIN our observable universe (which we CAN see, by definition) are receding from us at about 3c, so you have that one wrong.

The light from these galaxies was emitted before their recession velocity got so high, and they were closer.

The point you are missing is that it does not matter what the speed of the object is, the light it omits is always traveling at c, so what matters is only how FAR it is from us.
 
  • #12
All good answers above:

The point you are missing is that it does not matter what the speed of the object is, the light it omits is always traveling at c, so what matters is only how FAR it is from us.

I would say "traveling at c locally" for clarity...in curved spacetime, different observers may record different speeds. As phinds implies, the light was originally emitted at c and remains at that fixed speed for all local observers along the way.

Whether dark energy has ANY effect locally, that is cosmological expansion, say between adjacent galaxies, is still debatable. The expansion is based on the homogeneous and isotropic cosmological [FLRW} model. We debated this previously and a participating cosmologist said "The model doesn't apply locally." Others stuck with the description above...that it is too small to matter...unimaginably too small to be observed so far.
 
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  • #13
According to string theory gravitons are not attached to the membrane because of their shape (closed string). Does this explain why gravitons don't seem to be affected by the expansion of the fabric of spacetime ?

not really...depending on just what you mean: gravitons are 'not affected' [and neither are photons] in a sense. In another sense, they are affected: acceleration of either an observer or a cosmological horizon causes different particle counts to be measured...see 'Unruh effect' for example.

String theory lives in static spacetime, not the dynamic evolving space of cosmology.

From Michio Kaku:

Strings moves in space time and execute a complicated set of motions...which require a string to obey a large set of self consistency conditions...they place extraordinary restrictions on space-time.

From Lee Smolin:

Because string theory is a background dependent theory...by choosing different background geometries we got technically different theories...these (geometric) constraints are part of the description of how strings propagate and interact with each other

What these guys are implying is that just as in the Standard Model of particle physics we have to pick some free parameter values from observations, like electric charge size and electron mass, in string theory you have to pick a particular shape of the extra dimensions...say particular Calabi-Yau manifolds. So far as I know in the theory, when the geometry changes, change size or shape for example, the particle characteristics take on values we don't observe.


One string theory with gravity is David Gross's heterotic string theory...I don't know much about it...but it utilizes a type of closed string
 

1. What is the current understanding of the relationship between universe expansion speed and gravitational waves speed?

The current understanding is that the speed of gravitational waves is equal to the speed of light, while the expansion of the universe is believed to be accelerating, meaning that the expansion speed is increasing over time.

2. How are gravitational waves and universe expansion related?

Gravitational waves are ripples in the fabric of space-time caused by the acceleration of massive objects, while universe expansion refers to the stretching of space itself. The two are related in that gravitational waves can be used to study the expansion of the universe and its rate of acceleration.

3. Can the speed of gravitational waves affect the rate of universe expansion?

While the speed of gravitational waves is constant, their detection and study can provide valuable information about the expansion of the universe and its rate. However, it is not believed that the speed of gravitational waves has a direct effect on the expansion of the universe.

4. How do scientists measure the speed of universe expansion and gravitational waves?

The speed of universe expansion is measured through various methods, such as the redshift of distant galaxies, the cosmic microwave background radiation, and the use of standard candles like Type Ia supernovae. Gravitational wave speed is measured using interferometers, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), which can detect the tiny distortions in space-time caused by passing gravitational waves.

5. Is there a limit to the speed of universe expansion and gravitational waves?

According to current theories, there is no limit to the speed of universe expansion or gravitational waves. However, the expansion of the universe is believed to be limited by the speed of light, while gravitational waves are also thought to travel at the speed of light. This means that objects at the edge of the observable universe may be moving away from us at speeds greater than the speed of light, but we are unable to detect them due to the limitations of the speed of light.

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