Questions tagged [redshift]

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When an object crosses the future horizon, what do you see?

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
46 views

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
64 views

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
42 views

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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2answers
129 views

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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2answers
66 views

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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0answers
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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
37 views

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
77 views

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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6answers
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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
21 views

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
36 views

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
53 views

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
53 views

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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3answers
109 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
50 views

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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2answers
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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
38 views

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
40 views

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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4answers
161 views

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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4answers
65 views

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
115 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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51 views

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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2answers
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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
23 views

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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1answer
51 views

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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2answers
57 views

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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1answer
65 views

Why redshift of light occurs between galaxies only? [duplicate]

Edwin Hubble discovery of redshift of distant galaxies make me wonder why is this phenomenon only occurs when object is further away, is it possible that redshift happens in short distance too such as ...
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1answer
89 views

What is spectral redshift resolution of observationally detected damped Lyman Alpha clouds?

What is the spectral resolution, $\Delta z,$ when detecting damped Lyman alpha systems at different epochs, $z \sim 0-3,$ through current and/or future observational instruments? And is this redshift ...
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2answers
150 views

Voids and Hubble Constant

It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have ...
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3answers
227 views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=\dfrac{v}{c}$ won't apply)?
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1answer
51 views

Entanglement thought experiment with redshift

Thought experiment: 2 entangled photons, one escape from galaxy to galaxy without hitting an electron and another one got caught between 2 perfect mirrors. Would the redshift (wavelength becomes ...
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1answer
79 views

Which type of redshift does the equation $\Delta \lambda / \lambda = v/c$ apply to?

Which type of redshift does the equation: $$\Delta \lambda / \lambda = v/c$$ apply to, that which results from the Doppler effect, or that which results from cosmological expansion? Does it even ...
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2answers
165 views

Conservation of Charge vs. Conservation of Energy

Is conservation of charge ever violated like conservation of energy is violated during cosmological expansion? I am trying to understand this with respect to Noether's Theorem.
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2answers
63 views

Would this work as a way to get more energy from sunlight? [closed]

Plants only absorb certain wavelengths of light. This leaves a lot of light useless. But... imagine putting plants on a wheel and spin that wheel very fast. Then the light is red-shifted and so the ...
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2answers
84 views

Is the GW170817 event evidence or counter evidence for Dark Energy?

I've always figured there must be a simpler explanation for the observed lack of red shifting that has lead astronomers to hypothesize Dark Energy. Or at least there must be a simpler explanation of ...
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0answers
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How long does it take for the image of an object falling into a black hole to disappear?

From what I've read on other questions and answers, an outside observer cannot see an object fall into a black hole, but rather the image of the object will look like it is stuck on the surface of the ...
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41 views

Red shift of radio signals

If a radio signal is red-shifted does it still last the same amount of time? Knowing the signal wavelength is now longer but still is traveling at the speed of light one would assume the radio message ...
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4answers
1k views

What is a layman explanation for why the wavelength of light increases when space itself expands?

I understand that the universe is generally agreed to be expanding based on observations. What I have read is that space itself is expanding. My question is why does this expansion of space affect the ...
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2answers
101 views

If light speed determines the rate of time, could the speed of light be varying over time and we be unaware of it?

Is this a valid implication/alternative explanation of universal red-shift? I thought I'd ask this when I read this question about light clocks. I had speculated this way: If light speed determines ...
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0answers
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How do we know that Dark Energy has homogeneous energy density? [duplicate]

I’ve read places that the density of Dark Energy is the same throughout the universe. If the expansion density of dark energy were the same throughout the universe, why don’t all galaxies have the ...
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Does space expansion affect the CMB photon frequency?

I have read this question: Effect of expansion of space on CMB where Ted Bunn says: For definiteness, let's consider a wave packet of electromagnetic radiation with some fairly well-defined ...

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