Questions tagged [redshift]

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Has anyone considered a gradual change in Gravitational Potential across the universe as a cause for cosmic red shift?

Since the formation of fundamental particles at the beginning of the universe (roughly the time of the CMB emission, and after the inflationary epoch of Big Bang theory), the gravity wave front of all ...
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Would light emitted from a white hole be red-shifted or blue-shifted?

if it's not that straight-forward, what factors would effect the resulting appearance of light emerging from a white hole?
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Why can we say that $dt_{obs}=(1+z)dt$?

I'm studying how the luminosity of a distant source changes as viewed from an observer far away from it because of the effects of the expansion of the universe. Now, apparently, if a point source ...
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57 views

A violation of energy conservation?

Imagine a universe which contains a single sun and nothing else. The sum of the energy of the univese is then simply the energy contained in the sun and the photons it emits. If I have a theoretical ...
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Is redshift acting on pulses from pulsars and how it changes the diffraction of the pulse and so its brightness?

Is redshift acting on pulses from pulsars and how it changes the diffraction of the pulse and so its brightness? As far as I know the diffraction becomes higher while wavelength increases as moving ...
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1answer
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Hubble parameter data from Cosmic Chronometers. Has time dilation been taken into account?

The expansion history of the Universe can found by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter H(z) as a function of redshift. ...
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Is it possible to create a mirror that redshifts light?

Mirrors are able to reflect light but are not perfect and after a number of reflections, light loses intensity. However I wonder, during the reflection by a different type of mirror, could the light ...
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5answers
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Cosmology - an expansion of all length scales

From the link Is non-mainstream physics appropriate for this site? "a question that proposes a new concept or paradigm, but asks for evaluation of that concept within the framework of current (...
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1answer
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Red shift for an observer outside the gravitational field

Assuming there are no forces around,consider the situation: A multi storey laboratory is accelerating in free space, for convenience say vertically. In it a light pulse is generated. The people in ...
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1answer
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Proof of an accelerated expansion of the universe [duplicate]

Are there any more "proofs" of an accelerated expansion of the universe other than: $1:$ that galaxies that are farther away from us are also more redshifted. $2:$ using "Baryon ...
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1answer
60 views

Cosmological redshift due to recession speed or expansion of space?

I am somewhat confused regarding the factors that cause cosmological redshift. This wiki states that the amound of cosmological redshift coming from let’s say, a star, is calculated by $$z=\frac{H_0 D}...
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What happens to the energy of photon red shifted by expansion of universe?

There are many similar questions but the answers don't satisfy me. I mean it as asked in the title. Some answers given before give the explanation based on gravitational potential energy but consider ...
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2answers
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Solar sails and reflectors - decrease in photon number or redshift?

Electromagnetic waves carry linear momentum. So when they strike a solar sail (or any reflective surface), the object experiences radiation pressure and net energy gain. The reflected EM waves must ...
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Gravitational redshift and special relativity

Consider a redshift experiment in the Earth’s gravitational field and assume that a special relativistic theory of gravity exists, which need not be further specified here. For such an experiment we ...
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Why is speed measured along the line of sight (LOS) a factor $\sqrt{3} $ smaller for randomly distributed velocities?

I encountered this problem from Introduction to Modern Cosmology by Andrew Liddle. I had the exact same doubt as the OP of that question had, and the first answer makes sense but I don't understand ...
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When an object crosses the future horizon, what do you see? [closed]

First, there should be a redshift due to the velocity of the object before it crosses the horizon. And then, does the object's light just shut off? Note: There are several cosmological horizon that we ...
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1answer
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Ambiguity in measuring the speeds of galaxies

People measure the speeds of galaxies via the redshift effect. However, considering our spacetime as a non-flat manifold implies that measuring the speeds of any two objects not in the same tangent ...
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1answer
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How to tell the difference between redshift by motion of stellar objects and the expansion of space? [duplicate]

By observing how much the say hydrogen spectrum line was shifted we can tell whether the source is moving closer or further away, but then clearly cosmological redshift do not work this way so how can ...
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1answer
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How do you know that objects are accelerating from each other in space?

I know that Hubble redshift comes from the relation between the distances of objects and the Hubble parameter: $$H_0 = \frac{\dot{R}}{R}.$$ But how we differentiate between redshifts effects from the ...
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Why is there “red/blue shift” instead of “light/dark shift”?

In physics class today we were learning about the doppler effect for sound, and the teacher said that blue-shift and red-shift are the exact same thing but for light. This does not make sense to me. ...
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1answer
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Redshift time relation

I was reading that the relation $dt=\frac{dz}{H(z)(1+z)} $ between time and redshift (H is the Hubble constant) holds. I don't understand this. I thought the relation between time and redshift is $z=H(...
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How, if at all, do Tachyons experience redshift?

I recently attempted to answer this question, to which I felt qualified because the possibility of Tachyons had been discussed when I took a course on Special Relativity. In that same course I learned ...
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Derivation of redshift of photon emitted from edge of schwarzschild black hole

It seems that there is no nice derivation (either on this website or elsewhere on the web) of the standard formula for the redshift of a photon emitted in the Schwarzschild metric as observed by an ...
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What is the meaning of “current” CMB rest frame or the CMB rest frame at redshift $z$?

As I know that the definition of the CMB rest frame is the comoving frame of the plasma emitting CMB. And the anisotropy of CMB is due to the velocity difference between the earth frame and the CMB ...
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1answer
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Redshift and speed $c$

If the speed of light in vacuum is constant to every observer then how is the shifting of light color possible? I am assuming that a single particle of light (a photon?) has the capability to contain ...
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2answers
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Does the universe lose energy when spacetime expands?

When reading about the Big Bang you’ll commonly hear that the universe began “cooling down” soon after. However, if we assume the universe is a closed system, would that thermal energy not continue to ...
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How abruptly does the Doppler shift change sign?

(For context, I originally thought of this question in the context of electromagnetic Doppler shift, but I'm also curious if the same logic applies for acoustic Doppler shift.) Assume you are watching ...
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2answers
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Red- / Blueshifting and relativistic length contraction at $0.8c$ - How would it look?

I am a CG artist working on a specific shoot, that I have no clear answer to how it would look. And I seek your help. :D The idea for the shoot is a flyby of two Interstellar Vessels named ITV Europa ...
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1answer
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Interpreting recession velocity (red / blue shift)

I just had to teach something that I myself have never learned, concerning recessional velocity of galaxies. Awkward! The question was: "A line in a hydrogen spectrum has a wavelength of 486.1nm ...
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1answer
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Calculating the redshift drift

I am trying to calculate the redshift drift where its written as $$\frac{dz}{dt} = (1+z)H_0 - H \tag{1}$$ We also know that $$1+z = a(t_o) / a(t_e) \tag{2}$$ and $$H_0 = \frac{\dot{a}(t_0)}{a(t_0)}$$ $...
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2answers
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How can light coming from the receding galaxies be stretched like matter?

Edwin Hubble discovered in 1929 that the universe in expanding and the light from the receding galaxies are red-shifted; their wavelengths become longer. How can light which is an electromagnetic ...
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1answer
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What's the redshift of a return signal in a flat universe between two observers?

I'm trying to determine the redshift observed for a light signal between 2 observers (let's call them observers one and two) in a flat matter-dominated universe. Observer one sends a signal ($t_1$) at ...
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Parallax and redshift - has anyone used Gaia data to match objects with redshift data?

Given the recent Gaia parallax data out to large distances, has anyone used that data to match objects with observed redshift data? I have taken as much Gaia object data as I can and compared it with ...
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2answers
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How do someone calculate the wavelength of a receding object in space? [duplicate]

Is it possible to calculate the original wavelength of a star, say, which is subjected to cosmological redshift? if yes, how? Looking at the answers I think I need to edit it to be more specific. ...
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119 views

Does the Hubble law make sense?

The Hubble law says that if an object is at a distance $r$, it should have a velocity $Hr$ and therefore any light signal from that object will be correspondingly redshifted. However, the light ...
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3answers
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Can Hubble law be explained by galaxies moving away from one point at random constant speeds?

Let’s put a lot of material points at coordinates (0,0). Let’s give them completely random velocities. Let’s evolve this system for some time ...
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2answers
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Confusion with redshift

I got confused while reading about the cosmological redshift. Take an expanding Universe. If you look a galaxy moving with the Hubble flow, you will see the light of the galaxy redshifted. Suppose ...
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Intuitive Explanation for Doppler effect?

I was looking for an intuitive explanation as to why the Doppler effect happens. I haven't found any, but this is what I thought: -Waves emitted travel at a constant speed -The source emits a wave -If ...
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2answers
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What's the reference/original wavelength while calculating redshift?

Measuring redshift is one of the ways of analyzing distance in astronomy, since the redshift of an object is caused by the expansion of space between us and the object, the more space there is between ...
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1answer
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How astronomers estimate the redshift of galaxies clusters?

I don't understand how astronomers estimate the redsfhit of a cluster. As far as I understand a cluster of galaxies is something really "big", so I expect that different galaxies in the ...
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1answer
43 views

What if we red shift a photon of the lowest possible frequency?

Properties of photons are quantised right? So what if we red shift the photons with the lowest possible frequency?
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Why the factor $(1+z)^4$?

I am reading through Gravitation by Misner, Thorne and Wheeler. In the exercice 29.5 (page 794) it is asked to calculate the flux of energy $S$ for a distant source in a flat spacetime. The end ...
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Doppler shifts appear to violate conservation of energy

If the relative distance between an infrared source and a spectrometer is shortening at a such a rate that the spectrometer detects that radiation at a Doppler- (or blue-) shifted wavelength in the UV ...
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1answer
122 views

Help in solving a cosmology exercise and explanation of a concept

I'm trying to solve an exercise which should be easy but I'm very rusty and I don't really know how to do it. The exercise is the first one in this link, I have problems with the point 3 of this ...
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Do redshifted galaxies with $z>1$ look like bigger because red light has a larger diffraction angle?

As a laser beam starts to diverge after a certain distance and has a divergence or diffraction angle which is largest for red light somebody could observe two laser sources distant 1m from each other ...
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Time in Universe Chronology - Duration of inflation period

When you read articles about the chronology of the Universe, you read for example that the duration of the inflation era is roughly $10^{-32}$s. We also know Universe expansion causes a redshift ...
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1answer
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How to tell a population 3 star in red shifted galaxy?

I read that population 3 are metal poor stars especially those very very massive ones will quickly exhaust their fuel and goes into supernovae when their internal pressure drops due to pair ...
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Continuity equation in cosmology in terms of derivatives of redshift

I was trying to write this equation $$\dot{\rho}+3\frac{\dot{a}}{a}\rho_{m}=-\frac{\dot{\Lambda (t)}}{8\pi G}$$ as $$(1+z)^{3}\frac{d}{dz}(\frac{\rho_{m}(z)} {(1+z)^{3}})=-\frac{1}{8\pi G}\frac{d\...
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Demonstration of comoving volume probed between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
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Red shift in Schwarzschild geometry

I found the following derivation of the famous red shift formula in a textbook: Let a photon be emitted radially from $r_{e}$. A stationary observer receives the photon at a radius $r_{o}$. In ...

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