Universal expansion velocity SR or GR?

In summary, SR is not appropriate when velocities are close to the speed of light, but GR is. It is possible to calculate gravitational force from a distant mass by using its relativistic mass, but GR is only appropriate in a flat spacetime.
  • #1
duordi
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If I want to consider the velocity of distant receding galaxies which may be receding at velocities close to the speed of light can I use SR? I know you are not suppose to use SR with when velocities are close to the speed of light. If a velocity is due to universal expansion does that make a difference? It would seem not, but I am not sure if a velocity due to universal expansion is really a velocity, just like relitivistic mass is not really mass.
It is a poor example, I know, but it helps with the point.
 
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  • #2
duordi said:
If I want to consider the velocity of distant receding galaxies which may be receding at velocities close to the speed of light can I use SR?
No, I don't think you're just allowed to do that, because actually the galaxies are not (only) moving away from us, but the universe expands ("space is being created in between"). A famous analogy is that of coins on a balloon: if you inflate the balloon, the distance between the coins increases, while they are not actually moving relative to each other (if you'd draw a grid on the balloon, the coordinates of the coins would not change). But to an observer on one of those coins, it would seem that the other ones were moving away from him (in all directions, making it seem he was himself at the center of the expansion). In fact, it is possible to have them receding faster than the speed of light, I believe, because there is not actual "motion" involved in the normal sense.

I know you are not suppose to use SR with when velocities are close to the speed of light.
I think you said that the wrong way 'round (or I just understood it that way). SR is exactly meant for the regime where v ~ c.

I am not sure if a velocity due to universal expansion is really a velocity, just like relitivistic mass is not really mass.
As I explained, AFAIK it's not. But not in the same what that "relativistic mass is not really mass".
 
  • #3
A consequence of GR is that you cannot compare velocities at different spacetime points. The term "speed of recession" is not what it sounds like.
 
  • #4
Well then GR is out and SR is Ok.
Another question.

In SR, can I calculate gravitational force from a distant mass by using its rest mass or its relativistic mass in Newton’s Gravitation equation?
I don’t think I am suppose to do this.

How can I calculate an acceleration or a force caused by a very distant mass?
Is it possible?
 
  • #5
duordi said:
If I want to consider the velocity of distant receding galaxies which may be receding at velocities close to the speed of light can I use SR?
I think you must use GR because, IIRC, distant receding galaxies can be receding at velocities much greater than the speed of light. This is possible in GR, but not in SR.
 
  • #6
I think the problem with using SR is that the spacetime has to be flat, and in the cosmology we observe it is not. In a flat spacetime, say where you only had a "Big Bang" of test particles with no appreciable gravity, you might be able to get away with SR and just note that your perceptions of distant regions are highly length contracted relative to what they perceive locally (a la the "Milne" model), so if you wanted to have a test-particle equivalent of a cosmological principle, you'd have to include that carefully. You would be using a coordinatization where the recession speeds are never superluminal. Note that whether they are faster than c in our own universe is also a coordinate dependent issue-- I believe this was genneth's point that the concept of velocity is not terribly meaningful at distantly separated points, it is just a choice of coordinates. But in our universe, if you choose comoving-frame coordinates where the local coordinate charts follow the gradual separating of the galaxy clusters, the recession rate is faster than c for the most distantly observed galaxies. Could you get that by using comoving frame coordinates in flat spacetime? Perhaps you could.
 
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  • #7
Thanks for mentioning the Milne Model.
I have read several articles about it.
Although it may not be a serious contender for the real universe condition it made the difference between SR and GR results much easier to understand.
 

1. What is the difference between SR and GR in terms of universal expansion velocity?

SR, or Special Relativity, is a theory proposed by Albert Einstein that describes the behavior of objects moving at constant speeds in a straight line. GR, or General Relativity, is a more comprehensive theory that includes the effects of gravity and explains the curvature of space and time. In terms of universal expansion velocity, GR provides a more accurate and complete explanation as it takes into account the effects of gravity on the expansion of the universe.

2. How does universal expansion velocity affect the rate of expansion of the universe?

Universal expansion velocity is directly related to the rate of expansion of the universe. As the velocity increases, the rate of expansion also increases. This means that the universe is expanding at a faster rate as time goes on.

3. What is the significance of the speed of light in universal expansion velocity?

The speed of light, denoted as c, is the maximum speed at which energy, matter, and information can travel. It plays a crucial role in universal expansion velocity as it is used to measure the expansion of the universe. The expansion velocity is often expressed as a fraction of the speed of light, allowing scientists to compare the expansion to the speed limit of the universe.

4. How do scientists measure universal expansion velocity?

Scientists use a variety of techniques to measure universal expansion velocity, including redshift and cosmic microwave background radiation. Redshift is a phenomenon in which light from distant objects is shifted towards the red end of the spectrum, indicating that the object is moving away from us. The cosmic microwave background radiation is the leftover radiation from the Big Bang that can be used to map the expansion of the universe.

5. Can universal expansion velocity change over time?

Yes, universal expansion velocity can change over time. The rate of expansion of the universe is not constant and has been observed to increase over time. This increase in expansion velocity is due to the effects of dark energy, a mysterious force that is thought to be responsible for the acceleration of the expansion of the universe.

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