Red Shift

[brook]

New member saying hello to everyone,

Few months ago, I was reading about Redshifts. The magnitude of red shift value corresponds with the distance of galaxies. So far as I have read, the highest values of Redshift observed for most distant astronomical object is about 7.0 or somewhere around that. In another article, I read the CRB (cosmic Ray Background) has a redshift value of about 1000s. Is this number indicative of the distance the CBR came from or is it something else? I apologize if my question seem silly and not meet the professional level of this group. I was just curious about it.

Thanks in advance

[Ken G]

It is indeed indicative of the distance it comes from-- but it's the "CBR" (cosmic background radiation) or "CMB" (cosmic microwave background-- it started out visible but got redshifted all the way into the microwave regime). The value you are quoting is called "1+z", and it gives the factor by which "space itself has expanded" since the light was emitted. The wavelength of the light "expands with space"-- as there is really no "theory of space", these things I am putting in quotes are just quite successful pictures you can use to understand the predictions of general relativity. Space expands continuously as the universe ages, so the earlier the light was emitted, the more the universe has expanded in the mean time. Since light moves quite fast, the longer it has been traveling, the farther distance it must have traversed. However, if you stick around (and welcome), you will discover that the concept of "distance" in an expanding universe is not as simple as you might have imagined!

[forrest noble]

brook, hello to you also and welcome to BAUT on your very first posting here.

Few months ago, I was reading about Redshifts. The magnitude of red shift value corresponds with the distance of galaxies. So far as I have read, the highest values of Redshift observed for most distant astronomical object is about 7.0 or somewhere around that. In another article, I read the CRB (cosmic Ray Background) has a redshift value of about 1000s. Is this number indicative of the distance the CBR came from or is it something else? I apologize if my question seem silly and not meet the professional level of this group. I was just curious about it.

You are correct. A redshift of 7-8 is presently about as far back as we presently can observe a galaxy or supernovae. On the other hand, the Cosmic Microwave Background (CMB) is believed to have been caused by a "super-luminous" event/ remnant from the Big Bang era that is now generally omnipresent, so accordingly this is the theoretical reason why we can presently still see it at such a low frequency.

Based upon the Hubble distance formula that determines distances using observed redshifts, the relationship between observed redshifts and distances is non-linear. The result is that there is relatively a small amount of time between a redshift of 8 and a redshift of 1000 . A redshift of 8 is looking back about 13 1/4 billion years and a redshift of a thousand is looking back about 13 1/2 billion years (a difference of about 1/4 billion years) near the beginning of the universe, according to the Big Bang model.

[caveman1917]

On the other hand, the cosmic microwave background is believed to have been caused by a "super-luminous" event/ remnant from the Big Bang era that is now generally omnipresent, so accordingly this is the theoretical reason why we can presently still see it at such a low frequency.

(my bold)

Just wanted to point out it has always been omnipresent, ever since it was emitted.
What happened was that the photons were already there, but the universe consisted of free nuclei and free electrons. This made the photons constantly scatter, thus the light never reached very far. At around 360.000 years after the big bang temperatures cooled and nuclei and free electrons combined to make neutral atoms. Thus suddenly the photons were free to travel unimpeded. The universe changed from opaque to transparent, and thus everywhere a whole bunch of radiation was "emitted". It has always been omnipresent - and always will.

[WayneFrancis]

All great answers
http://hyperphysics.phy-astr.gsu.edu...ro/redshf.html
is a great little resource. It will give you some info about redshifts with calculators.

Does anyone know if there is a online table or, even better, calculator that uses the proper table to change z values into co moving distances?

[Substantia Innominata]

Does anyone know if there is a online table or, even better, calculator that uses the proper table to change z values into co moving distances?

Might this be of some help?

[Nereid]

All great answers
http://hyperphysics.phy-astr.gsu.edu...ro/redshf.html
is a great little resource. It will give you some info about redshifts with calculators.

Does anyone know if there is a online table or, even better, calculator that uses the proper table to change z values into co moving distances?

Ned Wright's cosmology calculator (BONUS: you can enter whatever values of the various cosmological parameters you wish ...)

[brook]

brook, hello to you also and welcome to BAUT on your very first posting here.
Based upon the Hubble distance formula that determines distances using observed redshifts, the relationship is non-linear. The result is that there is relatively a small amount of time between a redshift of 8 and a redshift of 1000 . A redshift of 8 is looking back about 13 1/4 billion years and a redshift of a thousand is looking back about 13 1/2 billion years (a difference of about 1/4 billion years) near the beginning of the universe, according to the Big Bang model.

Thank you very much, this really answered my question. I was plainly thinking in terms of distance only and not time. Now I see where my mistake is :-) Thanks again

[brook]

Thank you all very much for the elaborated answer. I understood now that distance and time really go together. My problem was that I only thought of distance and forgot time :-). Thanks again for the welcome. Lovely being here :-)

[Cougar]

On the other hand, the Cosmic Microwave Background (CMB) is believed to have been caused by a "super-luminous" event/ remnant from the Big Bang era that is now generally omnipresent....

Just to (hopefully) clarify, the CMB that we detect are microwave photons that were finally released from the very hot, dense "fog" of electrons and hydrogen and helium nuclei roughly 300,000 years after the "big bang." At this time the rapidly expanding universe had finally cooled enough for the nuclei to hold onto the electrons without them being blasted off again by the (then) very high-energy radiation. Electrons joined with nuclei are electromagnetically NEUTRAL, so the radiation (photons) could then travel freely without being scattered or absorbed/re-emitted in the dense fog of charged particles. This cooling and electron capture is the "event" referred to above.

[AriAstronomer]

It is indeed indicative of the distance it comes from-- but it's the "CBR" (cosmic background radiation) or "CMB" (cosmic microwave background-- it started out visible but got redshifted all the way into the microwave regime). The value you are quoting is called "1+z", and it gives the factor by which "space itself has expanded" since the light was emitted. The wavelength of the light "expands with space"-- as there is really no "theory of space", these things I am putting in quotes are just quite successful pictures you can use to understand the predictions of general relativity. Space expands continuously as the universe ages, so the earlier the light was emitted, the more the universe has expanded in the mean time. Since light moves quite fast, the longer it has been traveling, the farther distance it must have traversed. However, if you stick around (and welcome), you will discover that the concept of "distance" in an expanding universe is not as simple as you might have imagined!

Don't I know it!

[AriAstronomer]

Just to (hopefully) clarify, the CMB that we detect are microwave photons that were finally released from the very hot, dense "fog" of electrons and hydrogen and helium nuclei roughly 300,000 years after the "big bang." At this time the rapidly expanding universe had finally cooled enough for the nuclei to hold onto the electrons without them being blasted off again by the (then) very high-energy radiation. Electrons joined with nuclei are electromagnetically NEUTRAL, so the radiation (photons) could then travel freely without being scattered or absorbed/re-emitted in the dense fog of charged particles. This cooling and electron capture is the "event" referred to above.

That last part confused me a bit. Don't photons have a neutral charge, and therefore wouldn't be attracted to anything else, regardless of their charge? Maybe I'm misreading the answer. It's specifically that the electrons became buddy-buddy with the nuclei that allowed the photons to travel without bumping into anything, and not that they became electrically neutral.

[Cougar]

That last part confused me a bit. Don't photons have a neutral charge, and therefore wouldn't be attracted to anything else, regardless of their charge?

Hmm. Good question. The photons are electromagnetic radiation. The wiki article states that the photons were constantly interacting with the plasma through Thomson scattering, which is the elastic scattering of electromagnetic radiation by a free charged particle. The H and He nuclei and electrons prior to recombination were free charged particles. When the universe cooled enough (~3000 K at closer to 380,000 years after the BB), the nuclei captured the electrons and became neutral atoms => no more free charged particles => no more Thomson scattering => the photons could travel freely.

[Ken G]

Interestingly, what made the photons able to travel freely when the electrons became bound is actually related to Rayleigh scattering (the reason the sky is blue and the setting Sun is red-- red light travels fairly freely through the air). In general, putting an electron in a bound orbit gives it a number of "resonant frequencies" where it can interact much more strongly with light. However, the resonant frequencies of a hydrogen atom at 3000 K tend to be at much higher than the frequency of the light (which is the 3000 Kelvin CMB of the day). When the photons have much lower frequencies than the resonant frequencies, the binding forces act to restrict the electrons' ability to move in response to the electric field of the light (described classically, which suffices). This force does come from a proton and is electrostatic in nature, but the charge neutrality isn't essential-- any type of binding force would do, and even a He+ ion exhibits the same effect (indeed even moreso, because the resonant frequencies are all 4 times higher).

To analyze how this works classically, if we take an electron that is scattering light at a given rate, and ask how bright the incident light must be to achieve that scattering rate, we are talking about an electron with (classically) a given acceleration and a given radiative drag force. If the frequency of the light is much higher than the resonant frequency, then the binding force is negligible and the situation is just like Thomson scattering, as the electron is easily unbound. But if the frequency of the light is much lower than the resonant frequency, then the binding force is much larger than what you need for the acceleration we've assumed, so the force of the electric field of the light on the charge actually has to primarily offset the strong binding force, requiring a much brighter electric field to get the same response from the electron (and that field now has opposite phase, because it is fighting the acceleration of the charge, not causing it-- the residual binding force is what is causing the acceleration that makes the electron radiate). It works out that to get the electric field to fight the strong binding force sufficiently to get the given acceleration and radiation that we are assuming, the light must be brighter by the wavelength to the fourth power, which is the "Rayleigh scattering" that makes the sky blue-- to get a white sky, the incident red sunlight would need to be much brighter than the blue, but it is pretty much the same as the blue.

[AriAstronomer]

So when this incident photon energy is much lower than an electron resonant frequency, since it takes alot of energy to offset the binding energy (caused by the proton(s) acting on the electron) of the electron, and then uses up it's already half depleted energy reserves to accelerate the electron, instead it decides that it's not worth its trouble and just decides to pass on through? I'm just trying to connect your example to the sunset on earth. Blue is a bit higher frequency and is closer to the resonant frequencies of the electrons of the nitrogen atoms I'm guessing (or are all resonant frequencies of electrons constant, regardless of the atom they are apart of? My intuition says no, since the binding energy varies with atomic number), and so therefore more readily interacts with the electrons, and therefore is scattered more, while the red carries less energy, and is less likely to offset the binding energy enough to interact with the electron (although I'm sure, still does occur, these are probabilities?).
Thanks for the insight Ken n Cougar. I learn so much on these forums!

[AriAstronomer]

Also, is the maximum radiation emitted from a particle always greatest at 90 degrees (perpendicular)? Are there any scientific explanations for it, or as Feynman would say, I don't understand it, but that's just the way it is?

[Ken G]

So when this incident photon energy is much lower than an electron resonant frequency, since it takes alot of energy to offset the binding energy (caused by the proton(s) acting on the electron) of the electron, and then uses up it's already half depleted energy reserves to accelerate the electron, instead it decides that it's not worth its trouble and just decides to pass on through?

You could look at it that way. Along the way it's useful to think in terms of the force as a stepping stone to the energy. If most of the electric force from the light is needed to fight the binding force (as happens in Rayleigh scattering), then most of the force from the light is in some sense "wasted," relative to the situation for an unbound electron. That is not at all true at the resonant frequency though, because that is the frequency where the acceleration of the electron is entirely satisfied by the binding force, leaving the electric force completely free to do work in synch with the electron velocity-- and that's where the energy comes from to create mucho scattered light.

Blue is a bit higher frequency and is closer to the resonant frequencies of the electrons of the nitrogen atoms I'm guessing (or are all resonant frequencies of electrons constant, regardless of the atom they are apart of? My intuition says no, since the binding energy varies with atomic number)....,

Yes, follow your intuition here. You need the quantum mechanics to get the binding energies and resonant frequencies in each particular atom, but once you have that, you can understand the scattering pretty well classically, in terms of the forces or the equivalent descriptions involving energy (as you have done).

[Ken G]

Also, is the maximum radiation emitted from a particle always greatest at 90 degrees (perpendicular)? Are there any scientific explanations for it, or as Feynman would say, I don't understand it, but that's just the way it is?

Classically, that is easy to understand, because light is a transverse wave. So if you imagine a radial array of tight rubber bands (mimicking the electric field lines) all connected to some central object, then if you vigorously shake that central object, you will not send transverse waves through the rubber bands that extend in the direction you are shaking, but you will send strong transverse waves along the rubber bands that emanate perpendicular to that direction.

Perhaps Feynman was talking about why the same effect occurs in quantum electrodynamics, where you have a photon being emitted, not a classical electromagnetic field. So that comes under the heading of "why does the classical picture work when it sounds so different", and must get into issues of quantum field theory that I don't presently understand.

[dirty_g]

Thank you all very much for the elaborated answer. I understood now that distance and time really go together. My problem was that I only thought of distance and forgot time :-). Thanks again for the welcome. Lovely being here :-)

Was it you on yahoo questions and answers?

[AriAstronomer]

Thanks Ken, the explanations are great.

[brook]

Nope, I never used yahoo Q and A. Did someone ask for the same thing there?