All Questions
Tagged with cosmology special-relativity
127
questions
41
votes
5
answers
13k
views
Is light actually faster than what our present measurements tell us?
It is well established that the light speed in a perfect vacuum is roughly $3\times 10^8 \:\rm m/s$. But it is also known that outer space is not a perfect vacuum, but a hard vacuum. So, is the speed ...
user182521
20
votes
5
answers
3k
views
Wouldn't the cosmic background radiation (CMB) produce drag and thus create a preferential inertial frame?
Because the CMB is everywhere and is isotropic, if an object would have a certain velocity, it could have a pressure differential produced by the CMB which would produce drag till it would stop with ...
- 2,035
19
votes
2
answers
1k
views
Limitations on how far one can travel in the universe
Someone once incorrectly told me that, given the speed of light is the speed limit of the universe, aliens would have to live for hundreds of years if they are to travel distances of hundreds of light ...
- 5,611
18
votes
2
answers
426
views
Fate of largest scale structures?
In $\Lambda\mathrm{CDM}$, structures form "bottom up" with larger structures forming later. Structures are generally speaking supported by the velocity dispersion of their constituent objects (e.g. ...
- 18.5k
16
votes
6
answers
2k
views
Question on Roger Penrose's argument on using particles as clocks
In Roger Penrose's book Cycles of Time under section 2.3 (space-time, null cones, metrics, conformal geometry), Penrose makes the following argument which states that only particles with mass can be ...
- 498
16
votes
4
answers
3k
views
How would we see a near-lightspeed object emitting light?
Consider an object travelling near the speed of light relative to us (let it be a spaceship or a star), which is emitting light (consider it monochromatic resulting from a two level electronic ...
- 1,334
12
votes
6
answers
2k
views
How large is the universe?
We know that the age of the universe (or, at least the time since the Big Bang) is roughly 13.75 billion years. I have heard that the size of the universe is much larger than what we can see, in other ...
- 3,439
12
votes
2
answers
843
views
Do the standard cosmology models spontaneously break Lorentz symmetry?
In standard cosmology models (Friedmann equations which your favorite choice of DM and DE), there exists a frame in which the total momenta of any sufficiently large sphere, centered at any point in ...
- 1,869
9
votes
2
answers
804
views
Violation of Lorentz invariance (Lagrangian for particle)
I'm trying to get the relativistic action (or Lagrangian) for a free particle in the case of violation of Lorenz invariance.
Suppose we have the modified dispersion relation:
$$
E^{2}=\Omega^{2}(p^{2}...
- 1,565
8
votes
3
answers
522
views
Why are particles described with Poincaré symmetry even though space seems inhomogeneous?
Poincaré transformation consists of translation, rotation, and boosting. And by assuming the physical quantities are invariant and equations are covariant under the transformations, we build the ...
- 479
8
votes
2
answers
660
views
Is the FRW metric physically distinguishable from a metric with a speed of light that changes over time?
There are many questions on this site that ask whether the expansion of space could instead be interpreted as a speed of light that changes over time, e.g.:
Has the speed of light changed over time?
...
- 48.4k
7
votes
2
answers
4k
views
Is 2.5x speed of light possible between two objects?
These news are in Finnish:
http://www.hs.fi/ulkomaat/Kaukaisin+havaittu+galaksi+et%C3%A4%C3%A4ntyy+maasta+valoa+nopeammin/a1305618680897
The main excerpt is:
"Distant galaxy moves away from us as ...
- 225
7
votes
1
answer
2k
views
Is the observable universe equivalent to 'our' light cone?
All the objects we can observe (stars, galaxies, ...) must be in our past light cone, since otherwise we couldn't see them. Presumably there are more objects located outside of our observable universe ...
- 1,438
7
votes
1
answer
492
views
Frequency of cosmic microwave background
Some people refer to cosmic microwave background's (CMB) frame of reference as an absolute one. If I understand correctly, we can measure 'absolute' velocity in this frame by using the Doppler effect. ...
- 135
7
votes
1
answer
417
views
Do cosmologists see length contraction in the stars?
Wouldn't we be able to see length contraction in objects, like stars, that we can see and are moving at relativistic speeds?
- 317