Questions tagged [redshift]

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1answer
156 views

Momentum and energy of photon

Does the momentum and energy of photon change when it travels from distant galaxies through the Space?
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2answers
39 views

Does Redshift depends on wavelength?

I came across this equation on Wikipedia: $$z=\frac{\lambda_{\text{observed}} - \lambda_{\text{emitted}}}{\lambda_{\text{emitted}}}$$ and it got me thinking: If I measure a wavelength of $700 \;\text{...
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1answer
58 views

Is red-shifting the only proof for the expansion of the universe?

Are there other proofs for the expansion of the universe other than red-shifting? If yes, what are those? Thank you.
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1answer
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Galaxy getting closer faster than light

In cosmology, we speak of galaxies racing away from us due to the expansion of the universe. The light from these galaxies appears red-shifted. If the galaxies are far enough, then the space between ...
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1answer
31 views

Redshift graph of Type Ia supernova: Two case

The redshift graph obtained from a well-known supernova study appears in two cases in Google search. When I find the relevant data, regardless of whether the y-axis is set to distance or grade, the ...
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23 views

Gravitational and cosmological redshifts

Light that we receive today from a far-off galaxy originated in the past when universe was much smaller. If we approximate the universe to be a sphere, light originated from a much smaller sphere with ...
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1answer
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Redshift is same as Doppler effect in sound ? if not how different it is?

Is redshift same as doppler effect in sound ? if not what makes it different ?
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3answers
71 views

Can cosmological redshift only decrease photons' energy by increments (that correspond to Planck's constant)?

I have read this question: The equations that govern quantum mechanics predict that the angular momentum (that is, spinning or orbiting) in a system can't take on any value, but instead come in lumps....
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Does the cosmological redshift of the same object has a same value as it evolves

https://arxiv.org/abs/1802.02444 According to the link above, the following phrase comes out: " One must keep in mind that in the real universe, the same object( or comoving position) does not ...
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5answers
912 views

Is there cosmological redshift within the Milky Way?

Cosmological redshift is based on the idea that the universe is expanding. When the universe doubles in size, or scale factor, the wavelength of light doubles. But the Milky Way is not expanding so my ...
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2answers
41 views

What is effect of relativistic speeds on blue and red objects?

We know, As per Doppler effect, if an object travels with speed close to that of light, if appears red if it goes away from observer and it appears blue if it comes towards the observer. My question ...
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1answer
156 views

Redshift of the Cosmic Microwave Background: increasing or decreasing?

$$\dot z\equiv\frac{\mathrm d z}{\mathrm d t_{\text{obs}}}(t_0)=(1+z)H_0-H(z)$$ The picture and equation above are quoted from Liske et al. (2008). According to the equation, the redshift of the ...
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1answer
36 views

Can cosmological redshift be explained purely by the relativistic Doppler effect?

I want to preface this by saying I don't disagree with the idea of the universe's expansion, this was just a random thought that popped into my head. I also apologise in advance if it's a duplicate, ...
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How should I calculate the redshift of an object if I know its light travel distance?

If an object placed at hubble horizon i.e. 14.4 billion light years approx sends a light ray today and it reaches on earth sometimes in the future so how can I calculate its redshift when it will ...
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1answer
60 views

Distances in cosmology

I want to make sure that I understand the different distance measures is cosmology. To do that I consider the FLRW metric: $$ ds^2=dt^2-R(t)^2\left(\frac{dr^2}{1-kr^2}+r^2d\theta^2+r^2\sin^2\theta d\...
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4answers
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Cosmology: Is there any experimental evidence for the redshift scale-factor relation?

Modern cosmology relies heavily on the redshift scale-factor relation $$a=\frac{1}{1+z}$$ But what is the experimental evidence for it? It's derived in textbooks from General Relativity, but for all ...
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1answer
310 views

Is the CMB still red shifting?

If the CMB is red shifted from what used to be infrared and visible light, now shifted to microwave wavelengths, will there be a time where it red shifts so much that it becomes the cosmic radio ...
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2answers
55 views

Non-constant wavelength of particles

Is it possible for a particle's wavelength to change with respect to time? I'm thinking of a massless particle like a photon, but as it evolves through time its wavelength changes. I'm aware photons ...
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0answers
24 views

Recessional velocity have to increase during dark energy era?

Is the recessional velocity of a particular galaxy increasing at ‘now’ regardless of the observed redshift? Right now, we're experiencing an accelerated expansion. proper distance and scale factor ...
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0answers
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Hubble constant, Riess and why has $q_0 = -0.55$ been used?

Lot's of scientists, including Riess, have done great work in trying to determine the Hubble constant, both from observation and theory. As everyone will know there still exists the problem with the ...
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31 views

Cosmologic redshift in anti de Sitter metric

Let anti de Sitter metric be described by: $$ds^2 = - \Big(1 + \frac{r^2}{l^2}\Big) dt^2 + \frac{dr^2}{1 + \frac{r^2}{l^2}} + r^2 d\Omega_2^2$$ Instead of a classical: $$d s^{2}=c^{2} d t^{2}-(c b)^{2}...
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2answers
52 views

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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2answers
55 views

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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1answer
63 views

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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1answer
61 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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0answers
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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
60 views

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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8answers
3k views

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
541 views

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
23 views

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
92 views

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
85 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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1answer
85 views

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
45 views

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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1answer
84 views

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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1answer
53 views

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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1answer
54 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
71 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
50 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
149 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
241 views

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
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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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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
40 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
108 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
2k views

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
57 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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