# What is Cosmological redshift: Definition and 48 Discussions

Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the spectrum.
Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model.
The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is described by the equation v = H0D, with H0 the constant of proportionality—Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation v, i.e. the derivative of proper distance with respect to cosmological time coordinate. (See "Uses of the proper distance" for some discussion of the subtleties of this definition of "velocity".)
Hubble constant is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away, and its value is about 70 (km/s)/Mpc. However, the SI unit of H0 is simply s−1, and the SI unit for the reciprocal of H0 is simply the second. The reciprocal of H0 is known as the Hubble time. The Hubble constant can also be interpreted as the relative rate of expansion. In this form H0 = 7%/Gyr, meaning that at the current rate of expansion it takes a billion years for an unbound structure to grow by 7%.
Although widely attributed to Edwin Hubble, the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might expand, and presenting the expansion speed if that were the case. Then Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of, and distance to, distant bodies, and suggested an estimated value for the proportionality constant; this constant, when Edwin Hubble confirmed the existence of cosmic expansion and determined a more accurate value for it two years later, came to be known by his name as the Hubble constant. Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. Though the Hubble constant H0 is roughly constant in the velocity-distance space at any given moment in time, the Hubble parameter H, which the Hubble constant is the current value of, varies with time, so the term constant is sometimes thought of as somewhat of a misnomer.

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1. ### B Detecting cosmological redshift in an empty part of the Universe

I'm pretty new to the concept and I want to get a better idea about it. I've seen a video in which a light wave is stretched since the space itself is stretching. Another analogy is that cosmological redshift is like some ball bearings stuck to a rubber sheet that's stretching. Suppose we just...
2. ### I Can a scalar field model account for the cosmic redshift?

A minimally coupled scalar field can model a cosmological fluid model where And where the equation of state can be the standard ## \omega = \frac {p} {\rho}## I can see how this does a fine job modeling matter, because as the scale factor increases, the density will go as ##\frac {1} {a^3}##...
3. ### A Interpreting SDSS extragalactic data in the era of JWST

• CERN talk : indico.cern.ch/event/1153372/contributions/5200955/ • Presentation materials : bit.ly/MAGIC23AMayer • CERN MAGIC23 : indico.cern.ch/event/1153372/ Talk Description (Abstract) We present empirical evidence from the Sloan Digital Sky Survey (SDSS), including...
4. ### I Sachs-Wolfe effect in a Dark Energy universe?

The integrated Sachs-Wolfe effect occurs when a photon goes through a gravitational potential that changes due to spacetime expansion (presumably caused by dark energy). For that reason, a photon going through a gravitational well would gain energy (blueshift) when entering and it would lose...
5. ### SH2372 General Relativity - Lecture 10

0:00 Cosmological redshift 18:08 Solving the Friedmann equations 34:34 Accelerated expansion and the cosmological constant 56:10 Cosmological inflation
6. ### I Formula for redshift of photon decoupling

I have come across an old formula from my notes and I have no reference for it but it is using truncated digits in its formula to calculate the redshift for decoupling. The formula is nearly as accurate as the observed data from Planck 2018. So I would like to figure out the derivation of those...
7. ### B Experimental evidence on cosmological redshift

The space expands due the dark energy, so the light wavelength also stretches during its journey between galaxies, causing a measurable redshift. How can I argue that this is the right reasoning for the measured redshift? What if someone states that the photon maybe simple loses its energy over...
8. ### Accurate depiction of cosmological redshift?

Summary:: Accurate depiction of cosmological redshift? Hello. I'm thinking about getting a tattoo representing cosmological redshift, and just wanted to make sure my custom image is correct and accurate, and if you would recognize and be able to tell what it represents. If not then any...
9. ### I Demonstration of comoving volume 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 scale factor ##R(t)## between both). The comoving volume of a region covering a solid...
10. ### I Have I got the right picture for cosmological redshift?

Summary: I have a question on cosmological redshift which I have just learned about from Sean Carroll. After calculating it for an expanding universe he does a thought experiment to show that it is different to Doppler redshift which would be detected if two galaxies were flying away from each...
11. ### Cosmological Redshift

Homework Statement Consider a point in the intergalactic medium at some cosmic time ## t_{obs}##, the time of arrival of a photon of wavelength ##λ_{obs}## as seen by a hydrogen atom at that location. The source of this photon a comoving distance ##r## away emitted it at wavelength ##λ_{em}##...
12. ### B What is the cause of Cosmological redshift?

What causes Cosmological redshift? Can it be due to Compton scattering with free electrons in the corona/atmosphere?https://en.wikipedia.org/wiki/Corona
13. ### I Cosmological Redshift and Heisenberg Uncertainty Principle

Has anyone conjectured that the redshift associated with Hubble's constant can be explained by Heisenberg's uncertainty principle? Another words, the further in space away from us a photon is emitted from a galaxy, the longer in time it takes for the photon to reach us. Since the photon has a...
14. ### I Andromeda and the cosmological redshift

I understand that andromeda is blue shifted because it is moving towards us, but do we see any kind of redshift because of the expansion of the universe. Is andromeda less blue shifted than it should be because of cosmological redshift?
15. ### I Why are non-relativistic particles not redshifted?

Hey! I was reading some script and when it comes to the cosmological redshift, it says, that only relativistic particles are affected by cosmological redshift. This does feel quite natural, however, I haven't been able to come up with an explanation that shows it with proper physics and...
16. ### I Cosmological Redshift: Distinguishing From Doppler Effect

In studying the chapters on cosmology in Misner/Thorne/Wheeler (what a textbook!), I see that the cosmological redshift is different than Doppler. If I understand correctly, it is due to the expansion of the (Friedman, FLRW) universe during the photon’s long journey to us, rather than being due...
17. ### Cosmological Redshift distance to where it begins?

what is thought to be the closest star whose light visible to us now is/has cosmological redshift? perhaps the same answer but in case not: at what LY distance does the prevalent cosmological redshift of distant starlight begin? TY, r
18. ### B Cosmological Redshift: Where Has the Energy Gone?

As the Universe expands and the photons stretch to longer wavelengths (say from UV photons to the CMBR now) where has the energy gone since longer wavelength photons have less energy? Regards, G.
19. ### I What does cosmological redshift do to light

I understand it's definition but i am unsure how it affects light. Does it slow light down? (is this even possible?) Does it stretch light's wavelength? Does it convert light into infrared? Does it make light brighter or dimmer? hmm. thank you for your kind help!
20. ### How is the redshift of gravitational wave events measured?

LIGO reported the announced event with a redshift of 0.6 < z < 1.3. With no em radiation event reported (AFAIK), does anyone know how they have determined the redshift? I understand how they could measure the pre-merger, chirp and ring-down frequencies with good accuracy. In order to determine...
21. ### Relativistic Doppler and Hubble's law

I have a quick question about the Special Relativity. Non-relativistically, we expect no doppler shift in the wavelength of the emitted light if the source is moving at right angles to the line of sight to the observer. However there is a transverse doppler shift even in this case, caused by...
22. ### Cosmological Redshift: Calculating Wavelengths

Hi all! I've got a question about the cosmological redshift. We're given the metric ds^2 = c^2\,dt^2 - a(t)^2 \left[ dr^2 + r^2\,d\theta^2 + r^2\sin^2 \theta\,d\varphi^2 \right] Now light moves on null geodesics, so c^2\,dt^2 - a(t)^2\,dr^2 for radially moving light. For a GR exercise, we are...
23. ### Why is the Cosmological Redshift z Confusing?

I am confused about the physical meaning of the redshift. Let say the non-relativistic one z=v/c. When I read Barbara Ryden, Intro to cosmology, she demonstrate the z is not related to the space expansion between the source and the receiver, but 'it does tell us what the scale factor was at the...
24. ### Cosmological redshift: Where does the energy go?

A few small questions: Observationally, why do we conclude that the metric of space is increasing, rather than that light increases in wavelength as it travels cosmological distances? Or are these two conclusions isomorphic? Since wavelength is negatively correlated with energy, where does...
25. ### Cosmological Redshift: Electron Energy Loss in Expanding Universe?

So I have been thinking. Light gets redshifted because of the cosmological expansion of the Universe. This would mean that other, material particles, should get "cosmologically redshifted" as well right? So, for example, if an electron were flying towards us from some distant galaxy (and we...
26. ### Cosmological redshift as kinematical Doppler effect

In http://arxiv.org/abs/0808.1081 / Am.J.Phys.77:688-694,2009 Bunn and Hogg explain how cosmological redshift can be interpreted as accumulation of infinitesimal Doppler shifts. This suggested to invert z = z(v) and interpret v(z) as relative velocity of two objects with redshift z. It seems...
27. ### Cosmological Redshift Question

Let's say that an event releases a 1 second burst of EM radiation that propagates outward from the source in a spherical wavefront. If the EM wave travels long enough so that expansion doubles its wavelength, is that 1 second burst received as a 2 second burst? Has the total energy of the EM...
28. ### Cosmological redshift interpretation

Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands? This explanation seems closer to the truth than the popular idea that a photon's wavelength somehow expands while it travels to us from a distant galaxy. Metric expansion...
29. ### Cosmological redshift, how fast can the universe expand?

Ok, I'm writing up something and I do something I always hate doing... I start arguing myself into a corner. The argument of the night is the expansion of the universe and how it applies to redshifts. Which has brought a series of questions which I can't seem to grasp right now. I) Is...
30. ### Mechanism of the cosmological redshift

Hello, I have read that cosmological redshift means a photon is being stretchedby space dilation and thus gets to have longer wavelength I am unsure if this is true and if yes, why is it true Many thanks
31. ### Derivation of the formula for cosmological redshift

I was hoping somebody could point me towards a derivation of the following formula for cosmological redshift: z = R(t0)/R(te)-1. Wikipedia just presents the formula as a fait accompli and the only explanation is a vague reference to "stretched photons", which is not helpful. I was...
32. ### Cosmological redshift alternative explanation?

Could the cosmological redshift be explained also as a change of the fundamental physical constants with distance? Say Planck's constant changing over billions of light years, instead of interpreting it as a Doppler effect due to a recession speed? Yes, I know, Occam's razor would favor the...
33. ### Cosmological Redshift: Equation & Universe History

What is the equation for the cosmological redshift as a function of the history of the universe? Basically as a function throughout all of time.
34. ### How is the observed redshift caused by a recessional velocity?

I know that the issue of cosmological redshift has been discussed in this forum before, e.g. https://www.physicsforums.com/showthread.php?t=368958", but I would appreciate any knowledgeable insights regarding the model outlined below. This model is only considering light in terms of a stream of...
35. ### Cosmological Redshift in FRW Coordinates – Energy Conservation & Time Dilation

I want to discuss energy conservation and time dilation relative to the cosmological redshift. Because the elements of the redshift are coordinate specific, I’ll focus just on the FRW metric using proper distance coordinates (not comoving coordinates). As explained in Tamara Davis’s cover...
36. ### Cosmological redshift and doppler redshift

I am a bit confused here. Is cosmological redshift the same as doppler redshift? This is from wikipedia: "The redshift z often is described as a redshift velocity, which is the recessional velocity that would produce the same redshift if it were caused by a linear Doppler effect (which...
37. ### Cosmological redshift as a Doppler effect or metric evolution?

In several threads where I've seen the redshift issue discussed there's been some confusion about this point, Must we treat cosmological redshift as a purely kinematic (relativistic)doppler effect or as the time dymamics of the metric space? Or both views can be made to converge?
38. ### Cosmological Redshift: Why Does k Remain the Same?

condidering a photon in expanding cosmos, it 's said that the wavenumber k remains unchanged, the wavelength \lambda increases, proportional to the scale factor a(t) of the universe, and the frequency w decreases in the opposite way, that is the cosmic redshift. so, why does k...
39. ### Cosmological redshift and photon energy loss in SR

This might be a tired topic but please help me to understand. Assume a photon moving in vacuum, ignore potential interstellar medium absorption and re-emission since it is not relevant to discussion of space expansion in this context. Redshifted photons will undergo energy loss between *right...
40. ### What is the role of gravity in the cosmological redshift?

I’ve made several efforts to provide a simple description of how the cosmological redshift works. Each has been closer to the mark than its predecessor, and now I think I’ve got it nailed. The premise underlying the redshift is simply that a photon must travel at a local velocity of c through...
41. ### Misconceptions re: Cosmological Redshift

Apparently my recent thread on the cosmological redshift assumed more general insight into this subject than is the case. So the purpose of this thread is to help dispel some popular misconceptions about the cosmological redshift and hopefully spur a vigorous discussion. This thread is NOT...
42. ### The Mechanism of the Cosmological Redshift

It is well established that the cosmological redshift causes both the wavelength and the separation of photons to increase in proportion with the increase in the cosmic scale factor a. The traditional explanation for the mechanism of the redshift is that ‘expanding space’ progressively forces...
43. ### Alternative hypoteses for cosmological redshift

I heard there are many alternative hypoteses for cosmological redshift, e g Fred Hoyle' s variation of mass and tired light. There are other ones? Thanks.
44. ### Kinematic Origin of Cosmological Redshift: Bunn & Hogg

arXiv:0808.1081 (cross-list from physics.pop-ph) [ps, pdf, other] Title: The kinematic origin of the cosmological redshift Authors: Emory F. Bunn, David W. Hogg Comments: 14 pages. Submitted to Am. J. Phys Subjects: Popular Physics (physics.pop-ph); Astrophysics (astro-ph); General...
45. ### Superluminal Recession & Cosmological Redshift

I'm taking the liberty of revising and restating this topic which started in a separate thread. Comments are welcome. A lively debate is underway today by mainstream cosmologists as to whether the expansion of the universe implies that empty space between galaxies is also expanding. When...
46. ### The cosmological redshift is not the result of a Lorentz transformation

We have a theoretical issue here. There is a misconception floating around PF about the relation of the cosmological redshift to present and past recession velocity. If a redshift is Doppler in origin then in the context of Special Relativity one has Einstein's correction of the Doppler...
47. ### Cosmological redshift: how much energy has gone missing?

In the cosmol. redshift thread (Q) asked [[I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?]] It is clear that a whole lot of...
48. ### Cosmological Redshift: Exploring Photon Energy Loss or Conservation

I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?