# Questions tagged [redshift]

Questions related to the phenomenon whereby electromagnetic radiation (such as visible light) generated by an object moving away from an observer will have increase in wavelength (i.e. shifted toward the red end of the spectrum) once it reaches the observer.

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### How can you calculate the distance to an object knowing its cosmological redshift? (without Hubbles law)

Given some redshift z, how do we find the distance using the cosmological red shift formula (and presumably the Friedmann equations)? a = 1/(1+z), and we can assume that omega_0 = 1. I know a ...
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### CMB redshift used to determine the difference in aging between the two inertial observers

If CMB redshift tells us our cosmological time dilation and we have two inertial observers in relative motion, who measure slightly different CMB redshift at the direction of their motion relative to ...
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### Why would some sources in a cross-matched DESI/SDSS catalog have different spectroscopic redshifts?

I've crossmatched quasars from DESI DR9 and from SDSS DR16. When I plot the spectroscopic redshift from each of the individual catalogs for each source, I get: Naively, I'd expect the $z_{spec}$ to ...
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### Using prospector for jwst data

I am working with JWST data. I want to estimate the redshift of the sources I have identified with SEXtractor with the help of prospector. Can somebody guide me in this?
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### How do we work out the light travel time on a cosmic scale?

I just read this article in the AUSTRALIAN SKY & TELESCOPE magazine, Nov/Dec 2022 Issue 140, on P16, KEEP YOUR DISTANCE: How far away are the objects we see in the universe? And on P23: "And ...
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### Does a cosmological redshift occur within our local group, of galaxies?

I know that it is not supposed to be expanding but space is space and it is expanding everywhere else outside it so why not inside it too, and gravity just holds everything back. I suppose if it is, ...
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### Why are distant type Ia supernovae dimmer than expected?

Why are the (extremely) distant type Ia supernovae dimmer than expected? Relative to what? Their cosmological red shift? Well, what else. Please don't just say "Because the expansion is ...
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### Shouldn't the Friedmann equation take into account the massive extra distance traveled during the time for gravity to travel across the universe? [closed]

I’ve got a question that has been bothering me for the past few years that of the few sources that are only somewhat related to this, none of them say anything about this should be wrong so I can’t ...
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### Why does a Decision Tree produce lines in photometric redshift predictions?

In this paper, the authors compare the speed and accuracy of various machine learning algorithms to produce photometric redshift estimates: Why does the DT regressor put the estimates into bands ...
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### Is the difference of redshift parameter z able to prove expansion is accelerating in flat FLRW universe model?

Sorry for my bad english, this is not my mother tongue. Recently, I've encountered a problem in cosmology class that is about FLRW universe. Assuming that the universe we inhabit is composed of non-...
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### JWST Early Black Holes: Gas Collapsed To Metallic Hydrogen Core?

I was reading an article about early black holes found by JWST at Redshift 9. And saw that there was wonder at how large amounts of gas could collapse into a black hole as a theory. So my question is ...
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### Finding an expression for the rate of change of Comoving volume with respect to redshift

I am trying to find the expression for the rate of change of comoving volume with respect to redshift, that is $\frac{\mathrm{d}V_c}{\mathrm{d}z}$. In this paper (Hogg, David W.), the comoving volume ...
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### The "k-effect"?

I'm reading some older materials which refer to something called the "k-effect". This was described as "hot stars in the Sun’s neighborhood are moving away from us in all directions, ...
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### How does doppler shift work for distant stars [duplicate]

I dont understand how you can look at the light from a star and say that it has been doppler shifted. For example a star emits a photon in the wavelength of 500nm and by the time it reaches earth it ...
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### Why does the velocity dispersion within a galaxy cluster decrease as a function of radius?

In the paper 10.1093/mnras/sty2379, there are plots showing the velocity dispersion as a function of radius. Focusing on the right-hand plot, non-merging normal clusters, intuitively why should the ...
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### Could we (Earth, Humanity, Solar System) be falling into a black hole?

With the expansion of the universe, all objects far away from us appear to be moving away (ie: exhibit Redshift). With distance, this acceleration / redshift also seems to increase (~73,000 (m/s) / (...
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1 vote
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### If astronomers didn't know about redshift could we still determine the universe is expanding?

What other evidence is there for the expansion of the universe? Would the equations of physics look wrong without it?
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### Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The (Davis and Lineweaver (2003)) "Expanding Confusion" paper states that "the expected change in redshift due to cosmological acceleration or deceleration is only ∆z ∼ 10^(−8) over 100 ...
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### If a supernova explodes close to the centre of the milky way does its light get blueshifted by dark matter by the time it reaches the Earth?

Does the dark matter halo of the milky way cause most of the change in wavelength or is baryonic matter responsible?
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### How can we estimate distance of galaxies by only redshift when there are other several factors other than distance to cause redshift

There are different factors for red appearance of different objects in the night sky. Two reasons which I know are Star red by nature Example: red supergiants They emit red light because of their ...
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### How can redshifted light be detected?

I've been reading about redshifts and it got me really curious. Basically, I want to figure out how we know light is redshifted and what's the original emitted light. I found the following question ...
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### Could the redshift of all incoming photons be explained by a massive ring of distant masses pulling the sources of the photons away?

ChatGPT and wikipedia have informed me that the primary evidence for the theory that the universe is expanding is the fact that photons that arrive to our planet from all directions are being ...
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### How far away must galaxies be before current telescopes can no longer measure how quickly they are rotating around one another or internally?

What is the distance limit and does it depend on the size of the galaxy and on noise and interference from gas clouds and our own Milky Way?
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### Can the expansion of the universe cool my beer?

So the expansion of the universe stretches the light traveling through the void, as demonstrated by the cosmic microwave background radiation. These photons are lower energy than when they are ...
1 vote
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### Interpreting the color of galaxies in deep field images

The HST produced some deep field images in mostly visible light, which contain thousands of galaxies that range in color from white to red (e.g. see here). I understand that the interpretation of ...
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### Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

When we look in all directions, we see distant objects red-shifted, with the size of the red-shift correlated with the distance from us. As I understand it, the consensus among cosmologists is that ...
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### How did Hubble know the red shift difference between "moving away" and "old"?

My 9yo daughter is very into space at the moment and asked a question that my physics knowledge (6th form college, 20 years ago) is way too poor to answer. Her space book tells us that as stars age, ...
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### Doppler Redshift vs. Cosmological Redshift ... or Both?

There are several existing threads on the difference between cosmological and Doppler redshifts. However, I don't see that any of them answered the question below. @pela gave the following example: &...
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This is a follow up to a recent question on SE asking about the apparent suppression of radiation shortward of the (red-shifted) Ly-$\alpha$ line of a quasar at redshift $z=6.53$. The general ...