Questions tagged [redshift]

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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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How do someone calculate the wavelength of a receding object in space?

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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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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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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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
33 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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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
42 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
159 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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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
106 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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2answers
56 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
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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
61 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
141 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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197 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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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
48 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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120 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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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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69 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 extra 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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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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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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2answers
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Why does the expansion of space lead to cosmic redshift?

I have always thought of light as a point-like object in space, with the EM field strengths dependent on time. However, stretching of space would not affect a point-like object. Thus, I assume that ...
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1answer
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does dark mass/energy change cosmic red shift [closed]

Virtually all cosmology is based on the observation that galactic light is red-shifted. The Hubble Law defines that the red-shift is proportional to distance photons traveled. Further, this ...
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1answer
51 views

How do we determine both the distance to and speed of distant stars?

My understanding is that we can measure the distance to far away stars by using their color to infer the absolute magnitude of the light they emit and compare that to the observed magnitude of the ...
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1answer
106 views

Redshift of a black hole [duplicate]

Is that ring the "redshift" of a black hole ?
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1answer
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How would distant objects in a contracting universe look?

Our current universe is expanding, and distant galaxies are redshifted, with a redshift that increases as the distance to the galaxy increases. After a certain distance, the galaxies move faster than ...
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1answer
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Would the Doppeler effect be incorrect to use as evidence to support an expanding universe?

In our science class we have a question to answer, which is 'explain how redshift supports the idea of an expanding universe'. I am not sure how to answer this question, as I know that there are '...
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3answers
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Cosmological Inflation: If Photon expands, why not other matter?

I keep repeatedly reading in many Stack Exchange and Quora questions that space of universe expands but particles (matter) don't, see e.g. this Phys.SE post. The reason given is that particles are ...
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Bad or valid expression for gravitational redshift

In my lecture of General Relativity, teacher takes the following metric : $g_{\mu\nu}(x)=\eta_{\mu\nu}+h_{\mu\nu}(x),~ {\rm with}\ h_{\mu\nu}\ll1$ Qquestion 1) I understand its first demonstration ...
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What wavelength was the CMB originally?

What wavelength was the light that became the CMB after factoring out redshift? Is it safe to assume that what we're seeing in the CMB was emitted at a distance of approx: ...
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What can I see if I hold a ruler that stretches into a distant receding galaxy? [closed]

If my clones were to form a line along that metric ruler that bridges Milky Way galaxy and a still red shifting galaxy far far away, would any of me notice anything about the scale of the ruler? Now ...
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What is the current most accurate measurement of the cosmological transition redshift

Does anyone know what the current most accurate measurement of the redshift for when the Universe's expansion changed from decelerating to accelerating (with a reference)?
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Why does the black hole's matter ring in the image released on April 10th seem to lack blue/red shift? [duplicate]

If the matter around a black hole is spinning around it at nearly the speed of light, then the blue/red shift on each side should be quite visible (even though we're watching at an angle). But in this ...
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1answer
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Can expanding space stretch the wavelength of GWs? [duplicate]

I have read this question: Redshifting of Light and the expansion of the universe Now analogously, we could talk about GWs traveling in the empty voids of space, where the expansion of space is ...
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When space expands where do the energy of the gravitational wave dissipate to? [duplicate]

I think it is safe to say that gravitational wave do not dissipate unless it met matter along the way imparting some of it's energy to cause some distortion or when it's frequency is stretched even ...
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Why hasn't anybody done a two-telescope test on the speed of starlight?

It seems that even though it wouldn't be too hard, nobody has taken the trouble to use a pair of telescopes to confirm that light originating from red-shifted / distant stars is indeed traveling at c. ...
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On Distant Lanterns and Relative Speed [closed]

It is said, that by measuring distant sources, we can gather information about the past state of the universe. The further we measure, the faster things appear to recede, which brings me to my ...