100 views

Is there any physical difference between a receding body and a moving body? [duplicate]

Edited version. A body $A$ is receding at acceleration $\vec {a}$ with respect to a point $P$ because of the expansion of the universe. Another body $B$ is accelerating at the rate of $\vec{a}$ with ...
113 views

Is it accurate to say that nothing can travel faster than $c$ in a GR context, where more space can be created? [duplicate]

Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My ...
• 231
56 views

Superluminal speed at cosmic horizon [duplicate]

The observable universe is limited by a cosmic horizon. Galaxies beyond the horizon move away from us faster than light, so we cannot see them. Imagine a spaceship in the vicinity of our horizon. If ...
• 12.4k
11k views

Why is the observable universe so big?

The observable universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
• 3,157
6k views

Why is velocity a vector?

Velocity has a magnitude and a direction and thus it is considered a vector. But from linear algebra perspective, a vector is an element of a vector space. A set of mathematical objects can be a ...
• 1,219
2k views

No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light. There is also speculation that it might be possible to open wormholes or travel ...
• 191
1k views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=\dfrac{v}{c}$ won't apply)?
• 1,305
829 views

Interpretation of cosmological redshift

I was trying to understand why we cannot explain the observed redshift of distant galaxies using special relativity and I came upon this article by Davis and Lineweaver. Unfortunately when I arrive ...
• 300
756 views

How is the expansion of space distinguishable from objects moving away?

Suppose you are standing 5 feet (1.5 m) away from me. Then I move 10 feet (3.0 m) further away. Now you are at 15 feet (4.5 m) distance from me. You say I moved. I say no; the space between us has ...
200 views

Why can't the speed of gravitational waves be greater than the speed of light if the universe can expand faster than the speed of light?

Since the expansion speed of the universe can be greater than the speed of light, why can't gravitational waves, which also uses space as the medium, travel faster than the speed of light?
• 123
521 views

Does length contraction apply to the far galaxies moving due to expansion?

From Wikipedia: Universe in an expanding sphere. The galaxies farthest away are moving fastest and hence experience length contraction and so become smaller to an observer in the centre. Does ...
• 520
714 views

Can light in a medium travel faster than speed of light $c$ in vacuum?

Refractive index of medium $m$, $n_m$, is given by $$n_m = \sqrt{\mu_r\epsilon_r}$$where $\mu_r$ and $\epsilon_r$ are permittivity and permeability of the medium respectively. And, velocity of light ...
1k views

How to interpret the velocity from relativistic Doppler effect/equation for the redshift of cosmic background radiation? [closed]

This question has a moderation and downvotes issue: One of moderators keeps deleting one last single comment with the crucial piece of information, that clearly explains, why the given answer is wrong ...
488 views

How can light be stretched (redshifted) if the speed of light is constant?

If light travels at the same speed how can it be stretched to provide red shift that is used in measuring the expansion of the universe?