80 votes

Red shifted to what?

A complementary answer to Chris's, the middle row is the spectrum at rest. A blue shift does not mean that the object ends up blue. It just means that the entire spectrum is shifted up in frequency....
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  • 220k
70 votes

Red shifted to what?

There are certain physical processes that always produce a light of the same wavelength. For instance, hydrogen changing from the $n=2$ to the ground state always emits a photon with an energy of $10....
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52 votes
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Red shifted to what?

Redshifts doesn't actually mean the light is red, or was ever red. That's what is confusing you. "Red" and "blue" in this context are shorthand ways to say "towards longer wavelengths/lower energies" ...
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35 votes
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How abruptly does the Doppler shift change sign?

The instantaneous change occurs when you consider the Doppler shift in only one dimension. In three dimensions you can consider the correction when the velocity vector and the separation vector are ...
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31 votes

How can the rest-frame spectrum of redshifted light be determined?

Specific wavelengths in a spectrum, whether later shifted or not, are identified by the absorption bands of common elements near the emitters. For example, the spectral absorption lines of hydrogen ...
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  • 12.7k
28 votes
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Why isn't the De-Broglie wavelength of massive particles redshifted in an expanding universe

The de Broglie wavelength of a massive particle is redshifted in an expanding universe. The de Broglie wavelength is given by: $$ \lambda = \frac{h}{p} $$ so a red shift of the de Broglie ...
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28 votes

Does the Earth's rotation affect redshift measurements?

It does have an effect, but whether or not you have to worry about it depends on how sensitive your measurements are. The rotational period of the Earth is about $24\operatorname{hours}$. Depending on ...
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22 votes
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Is it possible to create a mirror that redshifts light?

Yes, this is possible using nonlinear optics. This kind of frequency shift can be done using acousto-optic modulators and electro-optic modulators, and it is normally done in a transmission geometry. ...
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21 votes

Can we use redshift measurements to determine absolute velocity?

A red or blue shift is created when the light source is moving relative to the detector. In your thought experiment, you emit light and you receive it, so there is no red or blue shift. For your idea ...
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13 votes

What wavelength was the CMB originally?

The present cosmic microwave background was emitted when the initial plasma formed during the Big Bang had cooled down enough to convert into gas - mostly hydrogen and helium, which then was ...
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11 votes
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Galaxy getting closer faster than light

In an ideal FLRW recollapsing universe, the collapse is just the time reversal of the expansion. The Doppler shift factor at all times is $a(t_\text{emission})/a(t_\text{detection})$, and during the ...
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11 votes
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Mass of cosmologically distant object in Earth frame

The dynamics of black hole mergers contains only one length scale/time scale, set by the total mass of the binary. This means two mergers with the same parameters ( spin, mass ratio, etc.) but ...
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  • 8,059
10 votes
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Does an object's redshift actually decrease with time?

The redshift of a source actually changes in a more complicated way: when the source entered our cosmological horizon (i.e. at the moment its light reached Earth for the first time), its redshift was $...
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10 votes

Conversion between extragalactic distances

You need to solve this problem $$ H(a) = \frac{\dot{a}}{a} \tag{1} $$ where $a = 1 / (1 + z)$ is the scale factor, $H$ is the Hubble parameter and follows the Friedmann equation $$ \left(\frac{H(z)}...
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10 votes
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What is a layman explanation for why the wavelength of light increases when space itself expands?

Here is simple explanation: Suppose a light source and a observer are in an expanding space. Now think of two subsequent crests of wave emitted by the lightsource. The second crest is emitted ...
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  • 8,059
10 votes

Is there cosmological redshift within the Milky Way?

Your guess is correct. The constituents of the Milky Way, and also close dwarf galaxies like the Magellanic clouds form a gravitational bound system and have therefore, seen as a single system, ...
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  • 1,005
9 votes

If space is "expanding" in itself - why then is there redshift?

It's pretty easy to explain if we take a classical view of an electromagnetic wave. As an EM wave from a distant star propagates towards us, the space it propagates through is expanding. Since the ...
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9 votes
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Does redshift only affect particles of visible light?

Redshift is said to have happened when the wavelength of electromagnetic radiation increases. Blueshift, a sister phenomenon of redshift, is said to have happened when the wavelength of ...
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  • 6,903
9 votes

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

The exact number depends on the cosmological model and its parameters. In special relativistic models (e.g. the Milne model), the redshift at the speed of light is of course infinite. However, in all ...
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8 votes
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Is the cosmic microwave background radiation red shifted?

When talking of cosmology one needs a model, and the standard model for cosmology at present is the Big Bang Model. This developed fitting observations and taking into account General Relativity ( ...
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8 votes

Red shifted to what?

The other answers (as of this posting) stick mainly to "light"... but the same concepts exist in other forms of "waves" Red/Blue Shifts and associated "Doppler effects" I want to add associated ...
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  • 303
8 votes

Is it possible to create a mirror that redshifts light?

As Jon Custer said in a comment, the light returning from a receding mirror will be redshifted. This is easiest to see if you consider the problem in the rest frame of the mirror. A symmetry argument ...
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  • 17.3k
8 votes

Is there cosmological redshift within the Milky Way?

Even if the Milky Way is expanding with the Hubble flow (and most cosmologists believe that it isn't), the expansion would be difficult to measure. The size of the Milky Way $d$ is approximately $10^{...
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  • 13.4k
7 votes
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If space is "expanding" in itself - why then is there redshift?

What are the observational/experimental facts: 1)Atoms have definite spectra, with a fixed pattern, a fingerprint of the atom 2) The further away ( measured by luminocity) galaxies all around ours ...
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7 votes
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What is the cosmological redshift drift effect?

Redshift drift is the name given to the effect whereby as the universe expands, the redshift of an object will change with time. i.e. It will drift. A galaxy at a fixed co-moving distance will have a ...
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  • 110k
7 votes

Can we use redshift measurements to determine absolute velocity?

Your proposal is almost exactly the Michelson-Morley experiment, done in 1887. It compared the speed of light in perpendicular directions, in an attempt to detect the relative motion of matter ...
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  • 1,739
7 votes

Why doesn't the escape of electromagnetic waves from a microwave depend on the reference frame, because of the Doppler effect?

First of all, let's get a better picture of why microwave doors keep waves inside the oven in the first place. Using a generous amount of hand-waving: imagine that an electromagnetic wave is incident ...
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