A theory that describes how matter produces and responds to the geometry of space and time. It was first published by Einstein in 1915 and is currently used to study the structure and evolution of the universe, as well as having practical applications like GPS.

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193 views

Metric tensor in General Relativity or otherwise [closed]

What is the metric tensor? How can this be a covariant and contravariant tensor, or a mixed tensor, by raising and lowering indices? How it relates to distance function (metric) and angles? How ...
0
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134 views

Maximum entropy Universe?

An attempt to understand the Friedmann equation describing a flat Universe using the principle of maximum entropy together with the laws of thermodynamics. This argument implies that the mass inside ...
3
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1answer
162 views

Fulvio Melia's linear Universe

I would be interested in what people think of Fulvio Melia's argument for a linearly expanding Universe. I realize that the experimental evidence seems to be pointing to an accelerating Universe ...
4
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1answer
114 views

Kerr Metric in Orthogonal form

I've seen the Kerr metric usually presented in the Boyer-Lindquist coordinates where there is a cross term in the $d\phi$ and $dt$ term. I've done a good bit of searching and cannot find any ...
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1answer
77 views

Is the fraction of radioactive isotopes on the near side of the moon higher than on the far side?

As time passes more slowly in a region of space close to the source of a gravitational field, shouldn't the moon, which always has one side facing towards the earth, have a higher fraction of ...
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1answer
56 views

Newtonian approximation in cosmology

In Peeble's "Large scale structure of the Universe" the Newtonian approximation in cosmology is examined by looking at a transformation that makes the metric locally Minkowski. On pg 38 it is stated ...
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190 views

Einstein's Cosmic Speed Limit $c$ challenged by Black Holes/Massive Objects [duplicate]

To even suggest the great Einstein is incorrect on anything he has written is inconceivably absurd. Being fully aware of this I do indeed dare to not only suggest he is wrong but I cannot believe ...
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2answers
170 views

How much physics a mathematician needs to know to study GR? [duplicate]

I'm intending to study General Relativity on my own. The thing is, my physics background is not very strong. I know classical mechanics and I know some electromagnetism. I'm familiar with Gauss' law, ...
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1answer
147 views

Can entanglement be explained as a consequence of conservation laws?

This article at NewScientist magazine (subscription required) describes entangling photons by passing them through a half silvered mirror. ...
3
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1answer
166 views

How much Energy to create a 'warp field' according to White?

An Alcubierre Drive is a hypothetical device that can move a place someplace else faster than the speed of light without violating known laws of physics. This paper provides some equations as to the ...
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1answer
103 views

Red shift and the expanding universe

Given that the universe is expanding because space itself is expanding, is that expansion occuring in all places and on all scales? Consider a photon emitted from a source billions of years ago. As it ...
0
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1answer
117 views

General Relativity and the effect of mass on time [duplicate]

My question is related to the first page of this article regarding the big bang theory and refers to this specific sentence: Armed with the best physics of the 20th century, Albert Einstein came ...
19
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2answers
617 views

Gravitational slingshot of light using a black hole/massive object

Wikipedia has this page on gravity assists using planets. In some cases this effect was used to accelerate the spacecraft to a higher velocity. This diagram shows this in a very oversimplified manner. ...
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1answer
462 views

Is the total angular momentum of the universe zero? [duplicate]

I know that having a net angular momentum will contradict isotropy of the universe by preferring a specific direction. But is there any experimental data on the total angular momentum of the universe ...
0
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1answer
91 views

Age-ing due to Time Dilation

Will a person on top of hill will age faster than one at sea level due to Time Dilation?
1
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2answers
96 views

Time evolution of the worldlines of 2 particles

Suppose I have a lab frame that is freely falling in a gravitational field of the Earth -- assume non-homogeneity-- and a uniform constant electric field. There are 2 test particles in the frame -- ...
1
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1answer
152 views

Retarded potential in gravitational field?

Is there a retarded potential concept in gravitational field similar to electromagnetic radiation?
2
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1answer
90 views

Relation between $\Lambda$ and $\Omega_\Lambda$ in $\Lambda\mathrm{CDM}$

In minimal $\Lambda\mathrm{CDM}$, there is a parameter labeled $\Omega_\Lambda$, and current fits place it at around $\left( \Omega_\Lambda \sim 0.73\right)$. Meanwhile, $\Lambda$ enters the Einstein ...
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243 views

Mass of empty AdS$_5$

Five dimensional empty AdS$_5$ space has mass $$ E = \frac{3 \pi \ell^2}{32 G}. $$ Is the above equation correct? Let's do some dimensional analysis to confirm. In natural units, in 5 dimensions ...
3
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2answers
727 views

What is the shape of a black hole?

I was thinking; what shape does a black hole have?. By 'Shape', I mean its form (e.g, circle , cylinder, sphere, torus, etc..). We usually think of black holes as if they're plugholes (e.g, a flat ...
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1answer
68 views

How much does the global structure of a Lorentzian spacetime restrict the metric? And vice versa

E.g. if I know that my topology is that of a hyperboloid, how much freedom do I have left for my choice of the metric? And the other way around: if my metric is some conformal factor times the unit ...
2
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3answers
698 views

Quantum Gravity and the Holographic Mass?

So on the net (one forum) I stumbled upon this, and article by Nassim Haramein. Now, I researched this guy a bit, and I couldn't find any peer reviewed article (other than this in 'Physical Review ...
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62 views

Closed linear cosmology implies G M / R = c^2?

I have a question about a linear FRW cosmology with $k=+1$. Assuming zero cosmological constant the first Friedmann equation can be written: $$\left(\frac{\dot R}{R}\right)^2 + ...
0
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0answers
74 views

How to get general relativity from linear gravity theory?

I know someone had done this study. Namely the field approach to general relativity. We can easily get an linear gravity theory. But it will be very complicated when we consider the ...
2
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1answer
85 views

How to calculate mass of a closed Universe?

How does one integrate the mass density over a closed Universe (a 3-sphere?) to obtain the total mass of that Universe? Is this the correct integral? $$M = R(t)^3 \rho\int_0^1 4 \pi r^2 ...
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108 views

How to obtain the free energy of the canonical ensemble in Euclidean general relativity?

If the gravitational field couples with matter fields, such as a charged scalar field, I know the partition function of the grand canonical ensemble naturally relates to the path-integral expression ...
2
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2answers
133 views

Tensors in general relativity

This is a question on the nitty-gritty bits of general relativity. Would anybody mind teaching me how to work these indices? Definitions: Throughout the following, repeated indices are to be summed ...
3
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1answer
262 views

What are the general relativity equations for relativistic constant acceleration?

At constant acceleration in special relativity, the time differs for a stationary observer and the astronaut. see the following article for an in-depth explanation: Relativistic Rocket However, when ...
4
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1answer
136 views

Components of the Ricci Tensor

Is there any interpretation of what each of the components of the Ricci tensor corresponds to? For example, for the stress-energy tensor, $T_{00}$ corresponds to energy density, $T_{0i}$ is the ...
2
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2answers
100 views

Are orbits reversible in general relativity?

It seems if I reverse velocities then things begin orbiting backwards, at least in classical mechanics. From here: Every orbit and trajectory outside atmospheres is in principle reversible, i.e., ...
3
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1answer
314 views

energy momentum tensor and covariant derivative

In field theory, the energy momentum defined as the functional derivative wrt the metric $T_{\mu\nu}=\frac{2}{\sqrt{-g}}\frac{\delta S}{\delta g^{\mu\nu}}$ (up to a sign depending on ...
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1answer
48 views

Tighten rope around cylindricaly shaped space

Imagine we live on cylinder(we are 2d creature), put a rope around that cylinder and start pulling both ends of the rope against each another. Will the space get deformed? I guess it will, I have to ...
0
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1answer
181 views

If time stops for an object, does that object feel gravity?

As far as I understand The GTR, it is said that Mass bends space-time which causes gravity. So every Mass in this universe is flowing through space-time example earth is moving along with space-time ...
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1answer
84 views

Axial symmetry constraints on the metric

I am reading the following paper (link) Here is a quote(s) from that paper Throughout this paper, we shall suppose that the 4-space is axially symmetric and reflexion symmetrical.... ...... ...
3
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3answers
161 views

What would be the effect on gravity if space expanded at > $c$?

If space were to expand at > $c$ (as in inflation) would that mean gravity would no longer have any effect on the curvature of space, since gravity can only propagate at $c$?
5
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320 views

About an Einstein equation

This is a question about an historical theory of gravitation, studied by Einstein quite a bit before he settled on General Relativity. At that time, Einstein did not know that gravity was a ...
2
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1answer
62 views

Conservation of Energy and Birkhoff's theorem

I am reading the original paper by Bondi, van der Berg and Metzner (link) regarding gravitational waves in asymptotically flat axisymmetric spacetimes. In the introduction, he makes the following ...
6
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1answer
148 views

Asymptotic Symmetry Group of General Relativity

This question is a little vague and I hope I can put across what I am looking for without too much confusion. What is the motivation behind studying asymptotic symmetry groups in the context of ...
2
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3answers
109 views

Numerical relativity coordinate system displayed

In a picture or video of a numerical relativity simulation, such as a neutron star merger into a black hole, how do they set up their coordinate system? Lets take the point in a video corresponding to ...
0
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3answers
189 views

General relativity theory [duplicate]

As I understand general relativity theory (please correct me if I'm wrong), time becomes dilated and space becomes compressed around mass, and this is responsible for gravity. I'm struggling with ...
0
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0answers
38 views

Dark energy lorentz invaraince

Dark energy (or the cosmological constant) is stated as Lorentz Invariant, form websites like: http://cerncourier.com/cws/article/cern/28917 In newtonian mechanics, this is correct. But time dilation ...
3
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2answers
260 views

Why does $8\pi/3$ appear in the equations describing cosmological expansion?

What is the significance of $8\pi/3$ in the first Friedmann Equation, and in the question concerning the time independence of the Hubble Constant? Is it the 'same' $8\pi/3$ that appears in the total ...
6
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1answer
371 views

Covariant derivative of connection coefficients

Is there a meaningful way to define the covariant derivative of the connection coefficients, $\Gamma^a_{bc}$? As in, does it make sense to define the object $\nabla_d\Gamma^a_{bc}$? Since the ...
1
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1answer
353 views

What are the Conflicting Predictions of General Relativity & Quantum Mechanics? [duplicate]

I see a lot of questions in various sites about why the 2 theories are or aren't incompatible, I'm satisfied as to why that's the case. However it has been mentioned that both theories make ...
5
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2answers
311 views

Why does the event horizon of a black hole not look like a bright sphere?

All infalling matter-energy appears to an external observer as frozen in time at the event horizon. Why then is this horizon not extremely bright due to radiation that is able to escape radially? So ...
4
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2answers
608 views

Gravity as a gauge theory

Currently, (classical) gravity (General Relativity) is NOT a gauge theory (at least in the sense of a Yang-Mills theory). Why should "classical" gravity be some (non-trivial or "special" or ...
4
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1answer
111 views

Are third derivatives of metric perturbations zero?

I'm working on a problem related to fluid perturbations of stars. I'm following this paper. They start with the Einstein equation: $$G_{\alpha \beta} = 8 \pi G T_{\alpha \beta}$$ and then perturb the ...
2
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1answer
73 views

Are there bounds or measurements on the derivative of acceleration (jerk)

The title says it all. Is there a physical maximum value to the 3rd derivative of position? Common Lore says that there is not and that jerk does not play any role in physics. My guess is that there ...
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3answers
464 views

How energy curves spacetime?

We know through General Relativity (GR) that matter curves spacetime (ST) like a "ball curves a trampoline" but then how energy curves spacetime? Is it just like matter curvature of ST?
3
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2answers
272 views

Restriction of a Lagrangian

I'm wondering if anyone could help me with the following questions. Let $M$ be the Minkowski spacetime, given $f\in C^{\infty}(M) ; f(m)=x^{0}(m)$, with $\{x^{\mu}\}$ being a global Cartesian ...