Questions tagged [redshift]

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2answers
33 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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244 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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1answer
55 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
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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1answer
111 views

Is there any way to differentiate between a redshift caused by recessional motion vs. gravity?

How certain, if at all, are astronomers that the redshift they are measuring is relating to recessional motion as opposed to gravity?
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2answers
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Calculating the redshift in cosmological constant dominated universe

How can the redshift be found when $\rho_m = \rho_\Lambda$ and when the cosmological constant begins to dominate from matter? Given that $\Omega_M \approx 0.3$ and $\Omega_\Lambda \approx0.7$ . I've ...
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2answers
255 views

Size of the Universe at end of recombination (~477000 years)?

What was the size of the Universe at start of recombination (377000 years) when the Universe started to become transparent and release of trapped photons into what we now know as the CMB and till end ...
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2answers
454 views

Finding the cosmological redshift of a galaxy in the expanding Universe

Firstly, I understand what the Doppler effect is when it comes to sound or light waves. From everything that I've read, we are told that the universe is constantly expanding since the all the ...
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1answer
114 views

Cause of observed galactic red shift or acceleration in expansion of the universe: velocity vs gravity.

Scientist look out at the universe and observe red shifts for galaxies that are roughly proportionate from their distance from us and attribute it to a velocity away from us. Also there is an ...
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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
49 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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1answer
30 views

Potential as a function of redshift [closed]

I have a cosmology project where I will plot the potential U as a function of redshift. Is there a formula for this?
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5answers
235 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
88 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
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
59 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
77 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
41 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
69 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
39 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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2answers
69 views

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
51 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
67 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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2answers
138 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
47 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
62 views

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
150 views

How can the angle to a pulsar be determined by measuring its redshift?

Suppose we (a spacecraft, probe, or similar) know our location only to within a few billion km (a few Tm) in any direction in 3D space, and we want to determine our location to a higher degree of ...
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2answers
159 views

Light and Gravity

The gravitational force does not affect the speed of light rather affects the frequency of light (hence light changes colors, red to blue and vice-versa). I know this has been verified but I have 2 ...
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2answers
179 views

Event horizon and particle horizon

The furthest distance that we can see is defined by the radius of the particle horizon, which is nearly 46 Gly. However, the event horizon is nearly 16 Gly. Does this mean the galaxies that are ...
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2answers
313 views

Is the expansion of the universe really accelerating? Or does it just look like it?

I pulled this quote from an article on the Hubble Constant: "...for example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a ...
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1answer
93 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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2answers
54 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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1answer
70 views

How does the redshift - distance graph show the rate of expansion of the universe at every moment in time?

By plotting the graph of redshift against the distance of the object from earth, we are able to obtain a best-fit curve showing the relationship of redshit against the distance. How does this ...
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0answers
26 views

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
76 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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4answers
69 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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0answers
45 views

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
39 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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1answer
39 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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6answers
1k views

Redshift of distant galaxies: why not a doppler effect?

How can I explain to my 17 year old pupils that the observed redshift of distant galaxies cannot be interpreted as a doppler effect and inescapably leads to the conclusion that space itself is ...
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2answers
88 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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0answers
101 views

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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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
150 views

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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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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1answer
31 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
54 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
1k views

Kerr black hole horizons and infinite redshift surfaces

In the Kerr black holes we have infinite-redshift surfaces (where a infalling body is still according to the asymptotic observer) and event horizons (the escape velocity becomes greater than the speed ...

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