All Questions
Tagged with cosmology special-relativity
127
questions
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Somewhat silly special relativity and cosmology thought experiment [closed]
Edit: I have already tried to delete this several times. I have already posted that I would delete this if I could and it was deleted. I have three very legitimate masters degrees in physics and more ...
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1
answer
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Is the equation of state of the cosmological vacuum $P=-(1/3)\rho$?
I understand that the equation of state of the vacuum is assumed to be $P = -\rho$ due to the Lorentz invariance of its stress-energy tensor. But this argument assumes flat spacetime. We know at ...
- 5,925
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44
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Expansion of the universe and superluminal speeds - an analogy
To illustrate, let's imagine the whole universe having a coordinate grid. Essentially a great big mesh grid of interlocking meter sticks.
Now... Einstein's theory of relativity tells us that nothing ...
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3
votes
3
answers
209
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One-Way Speed of Light and the Big Bang
Variations on this question have been asked a few times (e.g. here, here, and a few YouTube videos here and here). The claim seems to be that because we can only measure the round-trip speed of light, ...
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2
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Why does the light intensity of stars distribute almost uniformly across the night sky, if earth is not special?
According to special relativity, the direction of light should change when a reference frame is moving at near-light speed. There was a question on stack exchange about this topic before: Light in ...
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What exists in the world according to the special relativity? [closed]
Before I learned about special relativity, I thought that only one 3-dimensional state of the world exists. Then, like in game of chess, in one "turn" previous state is destroyed - and the ...
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2
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130
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The twin paradox in a Finite Dark energy less universe
Let's imagine we have a universe that is finite and curves back on iself. So if you go far enough in one direction you end up back where you started. (e.g. 3-Sphere/3-torus) Then lets imagine that it ...
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Does strongly gravitating object travel along geodesic of a background field？ [closed]
That test particles travel along the geodesic is assumed in the context of GR. But does it apply to strongly gravitating object, such as black hole in an expanding universe, binary neutron star, etc.? ...
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Are cosmological Black Holes superluminal FTL phenomena? [closed]
What does it mean, "gravity is so strong that not even light can escape from a BH..."?
This can physically interpreted IMHO that beyond the event horizon of the BH, light is phase ...
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Is it important to understand the concept of a universal 'now'? [closed]
Is it important to understand the concept of a universal 'now', to explain that time is a local phenomenon, and the rate of time varies throughout the universe, always dependent on the local ...
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0
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Could inhomogeneties and/or topological defects break the fundamental symmetries of the universe?
I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, ...
- 2,462
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2
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98
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Mass matter, energy and "massless matter"
This is perhaps a rather silly question, or rather a matter of convention, but I would like to hear arguments about the appropriateness of certain definitions.
Traditionally, in chemistry and in pre-...
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46
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How to include the effect of cosmological expansion in the close to speed of light calculations of a relativistic rocket?
I calculated time ago as undergraduated and post it at my blog the calculations for traveling through the Universe in a (special) relativist way. At what level and how should we include the effect of ...
- 6,611
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132
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How to rule out that the speed of light was different in the past?
The constancy of the speed of light is a fundamental principle in modern physics, and it is supported by a wide range of current experimental evidence.
There is no evidence to suggest that the speed ...
- 489
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3
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147
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Why do free-falling particles converge onto the Hubble flow?
I'm currently reading the book Cosmology by Daniel Baumann, and in Chapter 2, I encountered a claim that I was unable to prove. To provide some context to my question, let's start with the expression ...
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1
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134
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Distance in Hubble law vs special relativity
Came across this very informative website https://astro.ucla.edu/~wright/cosmo_02.htm , and it has the following two diagrams. I'm having trouble figuring out why "But the Hubble law distance $D_{...
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What is the average recessional velocity of an object in the universe?
I’m trying to get a better grasp on cosmological horizons and have a question regarding recessional velocity. In particular:
What is the average recessional velocity of a cosmological object (...
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1
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172
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Twin paradox with black hole (based on Interstellar) [closed]
I'm an Undergrad student working on a summer project. I'm learning about Differential Geometry, Schwarzschild's Solution, General and Special Relativity. I want to include the twin paradox as well. In ...
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4
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Do objects outside the Hubble sphere violate special relativity?
According to special relativity, objects can move with speed at most c.
However, objects outside the Hubble sphere recede from us faster than the speed of light.
How can these be reconciled?
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5
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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 ...
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1
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Relativistic Doppler Effect: Intensity change
My physics prof. mentioned briefly, that in the framework of the relativistic Doppler effect, not only the frequency (alternatively, the wavelengh) changes when objects move with respect to each other ...
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0
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Why is the age of the universe $=D/v$, despite that $v$ is not constant with time?
I am watching a series of lectures by the Noble prize laureate Brian Schmidt and Paul Francis and in this episode (at 4:20) they make the simple assumption that a galaxy receding from us due to the ...
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2
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Cosmic rest frame breaking Lorentz invariance
Is it surprising given that the existence of a preferred frame in the universe (from the cosmic microwave background), the cosmic rest frame, that there are no preferred observers? (Lorentz invariance ...
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Is there a way to view Einstein's spacetime "from outside" to intuitively understand it?
Is there a way, in Einstein's relativity (special and general) to view spacetime from a point of view that sits "outside" of it, to intuitively understand it?
We humans can only see 3 ...
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175
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FLRW metric, universe expansion, and the energy-momentum relationship
This is a follow-up to a previous question of mine.
I am getting myself confused by some basic things in cosmology, so I hope whoever reading this is patient.
The Euclidean FLRW metric is given by
$$ ...
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240
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Question about momentum in the FLRW metric
I'm reading through Modern Cosmology by Dodelson and Schmidt 2nd edition, and I am wondering if anyone can comment more on the following part.
In Section 2.2, we define the Euclidean FLRW metric by $...
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Does shape dynamics introduce a temporal order on events separated by a space-like interval such that their simultaneity is not relative?
I've read some popular books and articles that reference shape dynamics (e.g., The Singular Universe and the Reality of Time, https://www.pbs.org/wgbh/nova/article/shape-dynamics/). I have also ...
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Why is it assumed that special relativity does not apply to a universe expanding faster than light?
An argument I hear repeatedly is the light-speed limit only applies where spacetime is flat, so faster-than-light speed is possible where spacetime is curved. Thus special relativity does not apply ...
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The $c$ in Ricci scalar in the RW metrc
I was trying to find the Friedmann Equations from the EFE and I am confused about the "$c$" term coming from the Ricci scalar. I guess I am missing something simple, but I cannot find any ...
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2
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142
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Why is special relativity not enough to explain the seemingly accelerating expansion of the universe?
I have only recently studied Special Relativity, and have yet to learn anything regarding General Relativity, so I may be asking this due to a lack of knowledge - please educate me.
I just read about ...
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Proving that the CMBR remains thermal after a Lorentz boost
Suppose $S$ is a frame in which the CMBR is thermal and isotropic. I'm taking this to mean it follows the Planck Blackbody distribution $$n(\omega)=\frac{1}{\pi^2 c^3}\frac{\omega^2}{e^{\beta\hbar\...
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Does expansion of space over time assume a particular space/time dichotomy?
Regarding the expansion of the Universe, Wikipedia states:
The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe ...
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3
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Does our past light-cone have some extension in three-dimensional space?
On spacetime diagrams cosmologists represent our past light-cone as a two-dimensional surface extending back in time, on which are situated all light-emitting events which we can observe today. This ...
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Is it valid to add energy densities of *interacting* perfect fluids?
In several papers on interacting perfect fluids in cosmology, the authors assume that we still can add the energy densities and pressures of the individual fluids, as if there wasn't any interaction:
\...
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6
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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 ...
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Can Lorentz symmetry be broken?
I am used to symmetry breaking in condensed matter, where we have a preferred reference frame (superconductivity, ferromagnets, etc). But what about Lorentz symmetry? What would such symmetry breaking ...
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What is meant by dark matter being non-relativistic and why is this?
What does it mean by dark matter being non-relativistic and why is it non-relativistic?
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1
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615
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Four-Vector For Number Flux of Particles
Imagine a four-velocity $U^\mu(x)$ defined all over spacetime. Define the number-flux four-vector as
$$N^\mu = nU^\mu$$
where $n$ is the number density of a collection of particles in their rest frame....
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Is the fact that we're moving with a certain speed with respect to the CMB special-relativity consistent?
As a sidenote to an exercise about the aberration of CMB at the dipole level, which scope was to find the peculiar velocity we have with respect to the cosmic background (assuming the doppler effect ...
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1
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Lower limit to the distance between two mouths of a wormhole?
This is more like a conceptual question. Wormholes are tunnels connecting two different parts of the same universe or connecting two parts in different universes. Taking the former one as an example; ...
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3
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Indirectly breaking the speed of light through inflationary phase
How do you define velocity? Rate of change of "space" between two objects-and that will give relative velocity between them. Now in inflationary phase the universe expanded faster than light ...
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1
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Comoving coordinates and frames in Cosmology
In FLRW metric, I am reading that all galaxies are given a comoving coordinate $\lbrace x^i, i=0,1, 2,3\rbrace$( Schutz) . Since GR can be done in any coordinate system, using comoving coordinates is ...
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2
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290
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Flow of time in a block universe
On asking how there could be a flow of time in a block universe one user said
"In the block universe concept
there is no physical flow of time. The flow of time is a purely mental sensation that ...
user282467
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0
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What implications does special relativity have on the accelerating expansion of the universe? [duplicate]
Under special relativity we have the Lorentz factor:
$$
\gamma = \sqrt{\frac{1}{1-\frac{v^2}{c^2}}}
$$
Which essentially mathematically describes how the relative speeds between objects can never ...
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480
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Coordinate transformation to find the Minkowski metric
I have the following cosmology exercise:
i) calculate the evolution scale factor for an open empty universe and write down its spacetime metric in terms of coordinate $\chi$
\begin{equation}
d\chi = \...
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3
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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 ...
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2
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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?
...
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Are questions about "when" an event outside the visible universe occurred meaningless?
It seems to me that the answer is yes, first of all because it is impossible in principle to use Einstein's definition of simultaneity in such cases since signals cannot pass from the event to us, and ...
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1
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Limiting the mass of the neutrino for a relativistic case
I came across a question that states
What mass would a neutrino need to still be relativistic today (T = 2.37K) ?
So for a particle to be relativistic we need $pc \gg mc^2$
Well Neutrino was ...
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1
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Why don't galaxies have larger peculiar velocities?
I am trying to reconcile two facts:
That galaxies' comoving velocities are generally close to zero, and
That the universe has no preferred reference frame.
Galaxies seem to move very little relative ...
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