# Tagged Questions

A theory that describes how matter interacts dynamically with 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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### Asymtotically flat spacetime applicable for spacetimes which are not diffeomorphic to $\mathbb{R}^4$

I wanted to investigate changes on a compact 4-manifold $M$. More specifically it is the K3-surface. I follow a paper by Asselmeyer-Maluga from 2012. The idea there was to make sure that the manifold ...
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### Stability condition for AdS background (when gravity coupled to matter fields)

In finding the stability condition for AdS background (when gravity coupled to matter fields), why the conserved energy should be positive?
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### When we look in different directions in the universe how do we know we're not seeing the same thing?

For my question assume: 1: Big bang happened at a point (I know it happened everywhere) but after that explosion universe started to expand in all directions so it maybe considered to happened ...
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### Two definitions of Riemann curvature tensor

I am relatively used to the coordinate free expression of the Riemann tensor: $$R(X, Y)Z=\nabla_X\nabla_Y Z - \nabla_Y\nabla_X Z - \nabla_{[X, Y]} Z,$$ where $\nabla$ is the Levi-Civita connection ...
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### Is gravity just electromagnetic attraction?

Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the ...
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### Time-like Killing vector in FRW metric?

The spatially flat FRW metric in cartesian co-ordinates is given by: $$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$ As I understand it there are Killing vectors in the $x$, $y$, $z$ directions implying ...
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### What happens when some asteroid comes in the way of warp drive?

Recently Harold white, NASA scientist shared his knowledge and the things he is working on. That is Warp drive for interstellar navigation. This spaceship compresses the space in front of it and ...
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### Does time really exist? [duplicate]

Does time really exist? or is it a human invention and convention? What is the truth? are there time particles? please remember, I'm looking for constructive answers, not condescending and void ones.
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### Infinite Redshift [closed]

My text says that the signals emitted at a physical size of horizon $l_{H}(t)$ come to an observer at time $t$ with an infinite redshift. How do I show this mathematically? My approach is as follows: ...
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### Proving one field equation leads to the other [closed]

Assume that the universe is homogenous and isotropic, and the following equation holds: $$R_{00}-\frac{1}{2}g_{00}R=8\pi GT_{00}; \space \space \nabla_{\mu}T^{\mu 0}=0.$$ ...
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### Non-stationary spacetime

What is an example for a spacetime that is non-stationary that is considered as a description of something in nature? So far all the spacetimes I encounted have always been stationary (Schwartzschild,...
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### What exactly is charge? [duplicate]

If gravity is really the bending of space/time causing objects with mass to experience acceleration, is there a similar physical meaning to 'charge' besides 'a property of matter which causes it to ...
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### Covariant derivative of a covariant tensor wrt superscript

Is it true that when you take the covariant derivative of a covariant tensor, do you always have to do with a subscript? What if you do it wrt a superscript?Does the first term (with the partial ...