28 votes
Accepted

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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16 votes
Accepted

If I live on a planet that is heavy enough, would the CMB get blue shifted to be in the visible spectrum?

The cosmic background radiation has a wavelength about 2000 times longer than visible light. So you would need to be sitting deep in a gravitational well such that local time progresses 2000 times ...
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  • 2,482
13 votes

Does light lose energy in transit?

Yes the effect is real, potentially at least, but no it's not measurable. As an aside, the redshift of light by its gravitational interaction with (homogeneous and isotropic) dust is exactly what the ...
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13 votes
Accepted

Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted?

The answer to your question is yes, but the gravitational redshift could not be confused with cosmological redshift because it is small. The basics of gravitational redshift can be grasped by the ...
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  • 113k
12 votes

Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted?

Yes, you're quite correct that galaxies in a region of the universe with greater than average density would appear redshifted to observers outside that region. In fact this is the origin of the Sachs-...
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12 votes
Accepted

How accurately has general relativity's prediction of gravitational redshift been measured?

A recent test (2018) using atomic clocks aboard two satellites found general relativity's gravitational redshift prediction to be accurate to $(+0.19 \pm 2.48)\times10^{-5}$. This was not a planned ...
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  • 274
10 votes

Is it possible that galaxies' redshift is caused by something else than the expansion of space?

I posted more-or-less the same thing in response to another query about whether "tired light" still holds any water. I don't think there is any doubt that the redshifts we see in the spectra of ...
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  • 113k
10 votes
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Other explanation for cosmological redshift?

Stellar motion would imply we're in the place where the Big Bang occurred while everything else is speeding away from us. I can see how this would be a worthwile explanation for a creationist, but it ...
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  • 5,982
10 votes

Do photons lose energy due to gravitational redshift? If so, where does the lost energy go?

Update: According to this paper, "On the Interpretation of the Redshift in a Static Gravitational Field", the answer I give below is a common but misleading interpretation. The classical ...
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8 votes
Accepted

What is the cosmological redshift of SN 1987A?

SN 1987A is (was?) in the Large Magellanic Cloud, which is gravitationally bound to the Milky Way. This means that its motion relative to us is only minimally affected by cosmological expansion, and ...
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8 votes
Accepted

Rainbow Blackhole?

Gravitation clearly can change wavelength and frequency, and it does that for instance for the cosmological redshift. But with the speed of light being $c$ locally, independent of frequency or ...
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  • 13.7k
7 votes

Measurement of blueshift from Andromeda galaxy

It's true that stars can be very different but the spectral lines are the same in all of them, though the relative intensities of the lines will differ from star to star. So if for example you're ...
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7 votes
Accepted

Time Dilation Equals Red Shift looking in or blue shift looking out

My answer assumes that by "at the center", you mean orbiting the supermassive black hole (SMBH) at the centre. The amount of time dilation or gravitational redshift (as measured by an observer at ...
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  • 113k
7 votes

How is the global time coordinate $t$ ("observer at infinity" time) defined operationally e.g. in the Schwarzschild metric?

The meaning of the coordinate t is much broader than that. In fact we might want to talk about events that never send or receive signals from infinity . After all signals to and ...
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6 votes

Other explanation for cosmological redshift?

Another line of thought is that there are other observations that support the notion of an expanding universe - it does not all hinge on redshift. For instance, I believe numbers 3 onwards would ...
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  • 113k
6 votes

Does the universe expansion theory take into account that light emitted by massive stars is red-shifted?

Gravitational redshift is extremely small for stars, it has no significant impact on cosmic redshift measurements of galaxies. The gravitational redshift of light emitted by a star is of the form $$ z ...
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  • 14.1k
6 votes
Accepted

Do free falling observers see gravitational blueshift?

There are two effects that will change the frequency of the light: the gravitational time dilation the Doppler shift due to the velocity of the infalling observer To a first approximation we can ...
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6 votes

What does the $O$ mean in "$O(v^4)$"?

$O(x^n)$ stands for big O notation. It means that you wrote all terms that are proportional to $x$ up to the ones that are proportional to $x^{n-1}$ and that there are some more terms proportional to $...
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  • 3,128
5 votes

Does the density of a galaxy affect time?

The answer is yes they can but the effect is trivially small. Observed gravitational redshift just depends on the difference in gravitational potential between source and observer. Roughly, $$ z \sim ...
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  • 113k
5 votes

What were the measurements taken when S2 reached periapsis that were the 'best test of GR to date'?

I'm not sure I would agree the S2 observations are the best test of GR, or even the most rigorous test. However they are a test in a regime that we have not been able to directly probe before. The ...
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5 votes

How is the global time coordinate $t$ ("observer at infinity" time) defined operationally e.g. in the Schwarzschild metric?

You raise good points, which are often not clear in pedagogy. Indeed, there is a big difference between the statements: "at $r_0 \gg 2M$, $\quad t$ is the proper time", and "at any event $(t,r,\theta,...
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4 votes

Can we change a photon's frequency in mid-air?

To my knowledge, other than red/blue shifting light, the frequency cannot be changed after it's emission. However you may be able to somehow set up some other material which would emit x-rays when ...
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4 votes

Big bang red shift verification

I'm not dealing with most of this ; the reason most physicists are convinced that something like the big bang model explains many aspects of the universe is that the model has been verified against ...
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  • 113k
4 votes
Accepted

Measurement of blueshift from Andromeda galaxy

The measurement of the velocity vector of a nearby, large galaxy is not a simple matter. If you just dropped a spectrograph slit down randomly somewhere in Andromeda then you could get a wide variety ...
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  • 113k
4 votes
Accepted

Quasars with differing emission / absorption redshifts

I had a quick look at the paper - it's mostly nonsense. The intrinsic light from a quasar is completely dominated by its emission line spectrum and a mostly featureless continuum. The observed ...
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  • 113k
4 votes

Can we change a photon's frequency in mid-air?

Point the light source up or down in a gravitational field and let Einstein take care of you. Of course, if are forced to use the Earth as the source of your gravitational field you are going to need ...
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4 votes

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

I don't think that would even be considered. If you consider Hubble's law - once we get out of our neighbourhood and into the Hubble flow, then what we see is that identically in every direction, the ...
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