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Questions tagged [general-relativity]

Questions about relativistic theory of gravity. General relativity describes gravity as a geometrical property of space and time.

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Is the hypothetical '(Don) Page curve' of evaporating black holes equal in time, so to speak, on both sides of the curve?

I have heard that Hawking radiation is slow at first, but rapidly increases when the black hole becomes much smaller... (I forget the formula...) Does that mean that the second half of Page's curve, ...
Kurt Hikes's user avatar
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What exactly is a 'gravastar'? How is it different from a 'regular' black hole?

The Wikipedia article on gravastar says that it has a 'regular' black hole metric (Schwarzschild, I presume?) on the outside, but a de Sitter metric on the inside.... What exactly does that mean? Don'...
Kurt Hikes's user avatar
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Does an infalling observer ever cross the black hole event horizon in finite proper time?

I am interested in how an infalling observer perceives the plunge into a black hole. Let me assume that actually we have three spaceships: C being at a constant distance to the event horizon, and A ...
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Cosmic massive graviton bounds

In the paper, Relativistic theory of gravitation and the graviton rest mass. Theoretical and Mathematical Physics, 65(1), 971–979. doi:10.1007/bf01028629, the mass bound $$m_g<6.4\cdot 10^{-66}g=6....
riemannium's user avatar
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Expanding voids ending being collapsed?

According to some papers I've found 1, 2 expanding voids can be foun inside clouds of denser materials that can cause them to eventually collapse. I have two questions about this: Overdensities ...
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Is gravitational lensing "additive" in a line of sight?

As usually depicted, gravitational lensing implies, at least, three objects: a distant source, an observer, and a "massive enough" object in the middle, where enough is the mass needed to ...
nuwe's user avatar
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Shouldn't the Friedmann equation take into account the massive extra distance traveled during the time for gravity to travel across the universe? [closed]

I’ve got a question that has been bothering me for the past few years that of the few sources that are only somewhat related to this, none of them say anything about this should be wrong so I can’t ...
Gref's user avatar
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Can we measure how fast space is expanding from how quickly gravitationally lensed galaxies or stars are blurring or getting sharper?

A galaxy that lenses another celestial body should as it moves away from Earth alter the sharpness of what it lenses by gravity. Could this be used to determine how quickly space is expanding?
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Is the difference of redshift parameter z able to prove expansion is accelerating in flat FLRW universe model?

Sorry for my bad english, this is not my mother tongue. Recently, I've encountered a problem in cosmology class that is about FLRW universe. Assuming that the universe we inhabit is composed of non-...
ALLin's user avatar
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Does dark matter exist because as Einstein said the laws of physics should be the same inside and outside a black hole?

Spacetime is said to be time like inside a black hole ( two events can happen at the same place at different times). Spacetime is called space- like outside a black hole ( two events can happen ...
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$v_{GW}$ near a binary star?

In empty space, at large distances from masses and at small amplitudes, GWs propagate at the speed of light. This does not apply near the source, because there is a large mass and the amplitude of the ...
9herbert9's user avatar
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How can Kerr black holes have a 'speed limit' to how fast they can spin?

Obviously, the speed of light is a presumed limit, at least for 'physical' objects moving 'within' ('through'?) spacetime, but... In recent news some scientists believe they have figured out the ...
Kurt Hikes's user avatar
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Question about how it's possible for black holes to have gravity and a possible resolution [duplicate]

1. Assumptions 1.a: gravity propagates at c (maximum) 1.b: gravitational fields cause time dilation 1.c: escape velocity at the event horizon is c 1.d: gravity "warps" space-time, but in ...
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Which static spherically symmetric perfect fluid solutions of Einstein field equations are known to be suitable for stellar models?

Wikipedia's (Static spherically symmetric perfect fluid) short list ends in 2005 and lists only in total 5 such solutions. Does somebody know more?
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Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?

I was having a discussion with an undergraduate student of physics about binaries and their interactions with external celestial bodies (which could cause the ejection of one of the members in the ...
vengaq's user avatar
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Gravitational recoil with stars/planets...?

When two black holes are merging, the resulting merge can be ejected if one of the black holes had less mass than the other one, so the gravitational waves emitted by both of them is unbalanced, and ...
vengaq's user avatar
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If astronomers didn't know about redshift could we still determine the universe is expanding?

What other evidence is there for the expansion of the universe? Would the equations of physics look wrong without it?
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Modelling dark energy as a potential?

Dark energy is the cosmological constant.However can we model dark energy as some form of field with a corresponding potential?
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bent space-time by Energy can radiate gravitational-wave?

as we know , both mass and energy can bend space-time , and we know that accelerate mass can radiate gravitational-wave , but my question is that : bent space-time by energy can also radiate ...
Hamidreza Abdollahi's user avatar
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What's the relation between the parallax distance and the luminosity distance?

I have read that Riess and his team are able to measure $H_0$ from supernovae calibrated using Cepheid in a model independent way. from what I have gathered they find the absolute luminosity of ...
Alucard's user avatar
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What is the minimum amount of dark matter to start controlled nuclear fusion in nature?

In large spaces where there is a lot of dark matter, Stars are born in groups and not alone, so what is the minimum amount of dark matter to start controlled nuclear fusion in nature?
z.10.46's user avatar
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2 answers
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How far can a gravitational-wave pass through space?

Is there any equation which shows how far a train of gravitational waves will travel from a source until it loses its ripples and amplitude? The wave train will lose energy as it travels. It has a ...
Hamidreza Abdollahi's user avatar
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139 views

Does the other side of the Big Bang factor into JWST observations?

Our (roughly) 13.6 billion light year view to the point of origin (big bang) is just along a radial axis. Assuming most matter ejected in a (roughly) spherical pattern, the diameter of the universe is ...
TonyG's user avatar
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Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The (Davis and Lineweaver (2003)) "Expanding Confusion" paper states that "the expected change in redshift due to cosmological acceleration or deceleration is only ∆z ∼ 10^(−8) over 100 ...
Glycoversi's user avatar
3 votes
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When two galaxies with dark matter halos merge do they provide evidence for fermionic or bosonic dark matter?

If dark matter is made from fermions these should collide and cause dark matter to become denser in some places than others Bosons wont collide in this way so there should a different effect .So is ...
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When gravitational waves pass through a star or gas cloud do they cause mixing of plasma and gas?

Does the compression and stretching of space stir up molecules and atoms?
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4 votes
1 answer
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Gravitational wave intereference

Is it possible to conduct a hypothetical experiment where gravitational wave interference occurs and where areas of constructive and destructive gravitational wave interference are present? If such ...
Vicken Vorperian's user avatar
3 votes
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450 views

Gravitational-Wave Strain and Power (watt/square metre)

If we detect a gravitational wave with a strain of, for example, $h=10^{-20}$, what is the flux of power carried by this wave, in SI units, $W/m^2$ ? How can flux of power be calculated for a given ...
Hamidreza Abdollahi's user avatar
2 votes
2 answers
170 views

Does MOND change general relativity?

Newton's inverse square law appears in General relativity for small masses . Since MOND uses 1/radius instead of 1/radius squared has General relativity been modified to accommodate this?
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Does the (relative) distance of the photon sphere from the center of a black hole vary according its mass?

I know that the photon circle or photon sphere is located at around 1.5 times the radius of the Schwarzschild event horizon, but wouldn't it be comparatively further out for smaller black holes? After ...
Kurt Hikes's user avatar
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Do you need to understand the detailed mathematics of general relativity to be a professional astronomer?

Could I become an astronomer by understanding how to use the main results of general relativity such as cosmological redshift or would I need to understand the whole theory?
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8 votes
2 answers
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Do tidal forces on moons cause them to emit gravitational waves?

As moons orbit planets, they get squashed by uneven gravitational forces acting on them. Does this make a moon emit gravitational waves?
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4 votes
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What does it exactly mean for spacetime to have no global symmetries?

Are there spacetimes or metrics with no global symmetries? Spacetimes/metrics with no global Poincaré, Lorentz, diffeomorphism, CPT, translational and gauge invariances? And if there are, what does it ...
vengaq's user avatar
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Non-homogeneous and anisotropic metric and laws of physics...?

In this popular science article, they say that if our universe resulted to be non-uniform (that is highly anisotropic and inhomogeneous) then the fundamental laws of physics could change from place to ...
vengaq's user avatar
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1 vote
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Spacetimes where symmetries vary from place to place?

Are there spacetimes or metrics where symmetries (like Poincaré, Lorentz, diffeomorphism, translational... invariances) are only local and the symmetries of one local neighbourhood are not, a priori, ...
vengaq's user avatar
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2 votes
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Are there non-smooth metrics for spacetime (that don't involve singularities)?

I found this statement in a discussion about the application of local Lorentz symmetry in spacetime metrics: Lorentz invariance holds locally in GR, but you're right that it no longer applies ...
vengaq's user avatar
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13 votes
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How would "dark matter", subject only to gravity, behave?

If we were to hypothesise that the Universe contained a significant mass of "dark matter" particles subject only to gravity, presumably general relativity would give us a good idea of how ...
mikado's user avatar
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1 vote
1 answer
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Other Hubble spheres with no Lorentz symmetry?

Perhaps this may be a stupid question but anyways here it goes... If the Lorentz symmetry is not global but rather local, wouldn't that mean that is possible that other Hubble spheres outside our ...
vengaq's user avatar
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1 vote
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296 views

Does the expansion of space stretch the space-time fabric?

This question appeared to me when I was thinking about General Relativity. During the expansion of space, the space in which particles reside is expanded. Now, general relativity states gravity is the ...
Naveen V's user avatar
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Problem with the Orbital Precession predicted by the Relativistic Acceleration equation used in the JPL D440-D441 Ephemerides - please advise

I have previously encountered a problem with the equation used by Yeomans and others for determining the gravitational acceleration of comets (see update in my answer to the previous question Can ...
steveOw's user avatar
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7 votes
2 answers
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How do two black holes merge?

Since at the Event Horizon, time stops completely, how do two black holes merge together? Shouldn't they should stop moving due to time dilation when they get closer to each other's Schwarzschild ...
Shounak Das's user avatar
3 votes
0 answers
95 views

Kinetic energy in cosmology?

Spacetime expands at an accelerated rate due to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy ...
vengaq's user avatar
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3 votes
3 answers
234 views

Viewing black holes?

How could you possibly see a black hole when there are stars etc in front of it and then the matter being drawn into it , all you would view surely would be the objects in front of the black hole ?
Big4's user avatar
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Can objects join the Hubble flow for a given amount of time and then somehow abandon it?

When two objects (e.g. two galaxies) are sufficiently far apart, they join the Hubble flow and they get further away from each other. Normally, this would last "forever" (until, from the ...
vengaq's user avatar
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6 votes
2 answers
284 views

Light or neutrinos graze or pass through the Sun and arrive at Earth - need an expression for Sun's gravitational effect on observation direction/time

Skyfield's Github has discussion Jupiter hiccup #815 which then links back to to Non-physical gravitational deflection corrections for Solar System bodies #734. The script and plot from #815 are shown ...
uhoh's user avatar
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4 votes
1 answer
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Observable Speed of objects orbiting a black hole

This is a mostly hypothetical question and thought experiment, and I am aware of some aspects plausibility of it are open for debate. Also, this is Version 2 of the question, as the first question was ...
Confused Merlin's user avatar
3 votes
0 answers
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Deviations of conservation laws in the context of cosmological evolution?

If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an accelerating expanding spacetime (like it happens in our Universe), are ...
vengaq's user avatar
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2 votes
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Parker Solar Probe and clock correction

Due to the dynamic changes in speed and the value of the gravitational field in which the Parker Solar Probe stays, the clocks should have the errors due to relativistic effects. https://physics....
user1785960's user avatar
3 votes
1 answer
175 views

Planck epoch and time dilation

I guess this question can be broken down into three consecutive parts: Does general relativity apply during the Planck Epoch? If yes, does the gravitational time dilation prediction apply during the ...
seamux's user avatar
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Order of magnitude of this phase space invariant?

So I wanted know the order of magnitude of this phase space invariant (equation $3.31a$ )$R$ in our universe (FLRW metric): $$R = \int_{p} \frac{\mathcal{N}(x,p,t)}{\mathcal{E}} dp_x dp_y dp_z$$ ...
More Anonymous's user avatar

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