A Cosmic Background Radiation question.

[batema]

Thers was a suggestion that the post I put in the general chat section be moved here so I thought I would move it.

I need some clarification please as I think I am misunderstanding this completely when I thought I had it under some sort of control.I think I was of the understanding that the radiation being received has come/travelled across from 13.7 billion light years and as a result of the universe expanding has undergone a red shift stretch factor that has put this radiaiton into the microwave zone. But I have read that this radiation is the same everywhere and distributed evenly through the universe and I don't get it know. Can anyone simply explain this concept to a simple mind. I find this facinating and would like to clear my confusion. I listened to a 365 days podcast that talked about the background radiation being like a blueprint of the structure of the universe that exists today that is found in the radiation itself.

Mark:shrug::shrug::shrug::shrug:

[Brian W]

I can't explain it but I can help you to hear it. Tune between FM stations with any FM radio. About 10% of the static you will hear is the background radiation you are asking about.

At least that's what I have been told.
Brian

[sjastro]

Mark,

The redshift you mention is a cosmological redshift due to the metric expansion of space-time. When space-time expands there is no "absolute" centre. (Think of the surface of an expanding balloon, the centre can be any point on the surface).
Hence an observer anywhere in the Universe can claim they are at the "centre" as space-time is uniformly expanding in all directions.

Hence the CBR appears to be uniform in all directions to the observer.

Regards

Steven

[CraigS]

Hi Mark;

You seem to have the gist of it, just fine. I'm not quite sure of where your confusion might be.

The theorised time sequence of the Big Bang proposes that radiation (photons) decoupled from matter, when all the electrons finally recombined after the initial enormous energy involved, to form the first atoms. This is referred to as the time of last scattering, about 380,000 years after the Big Bang (BB).

The CMBR, is due to the extreme redshift of these photons which highlights their age. Its electromagnetic spectrum also corresponds nicely with the BB prediction. The observed patterns (slight intensity changes) in the CMBR today, is then thought to correspond roughly to the way the universe looked at the time of last scattering.

Under the BB theory, after the time of last scattering, the energy of the photons would have been stretched by the expansion of the universe, which would have left the spectrum with the same shape as it has today, (ie: a near blackbody spectrum profile).

The theory also predicts the temperature drop to present day, (down by a factor of about 1,100), as everything cooled over the eons. The CMBR photons now measure in the microwave range of the EM spectrum, as predicted.

CMBR comes from all parts of the sky, with almost equal intensity and has a pattern the same as would be expected if a uniform, hot, evenly distributed gas was expanded to the size of the observable universe. The observation would look the same no matter where it was viewed from, because the universe expands evenly in all directions, the further the object is, the bigger the redshift, (the faster it is receding - Hubble's Law), and the CMBR presently has the largest observable redshift (z =1089 approx).

Hope this helps.

Cheers

[bojan]

Mark, what you described is mostly correct (except distance.. the radius of observable universe is much wider).
The fact that that background noise level is more or less the same from any direction is the key factor that lead to the inflation theory.

Brian, you have been told wrong.
The static from FM radio (between stations) is almost 100% the thermal noise generated at the input circuits of the amplifier chain (first pre-amplifier) . The rest (and that amount will heavily depend on the antenna type attached to the input) will be from Sun, then a bit from Jupiter, and the tiny rest from Galactic centre and possibly other strong radio sources in Milky Way. And, you have to take into account that FM radio receives frequencies around 100MHz, and CBM's max power is in microwave band (5-10cm wavelength, corresponding roughly to 3-6GHz)
(BTW, if this noise difference were 10%, CBR would have been detected much earlier.. instead, only when relatively large antenna was used (together with much better receiver than those used in commercial FM radios) in microwave band, the discrepancy between theoretical (ah.. mathematics again!!!) and the measured noise level became measurable .. and it turned out to be CBR)
See here (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiati on) .

[batema]

Thank you all but I have one last (at this time) question. The concept of it being uniform in every part of the sky when measured, is this uniform nature due to photons travelling from the same distance or is it uniformly distributed throughout the observable universe like the spread out between here and jupiter. What I am trying to ask is what we are measuring do the photons come from cool gas that is 380 000Ly less than 13.7 biil Ly away. The image in my head is of a balloon in which the edge is the source of the CMBR and it has travelled for this time to us and due to the expanding nature of space time has stretched by a factor of 1000 and therefore appears in the microwave section of the electromagnetic spectrum. I am a high school physics teacher and want to get this right before I blow my students mind.

thanks

Mark

[CraigS]

Hi Mark;

There are times where the ballon model, as an explanation for our observations, just doesn't 'cut it'.

The present Standard Cosmological Model has been proposed to help us visualise what we observe. It may sound easy to invent a 'story', which fits everything we observe, but it isn't. Give it a try and you'll see what I mean. (Come to think of it, this would be an awesome experiment for your students to try to come up with a 'theory' which has a good fit for something which has many observable, (ie: repeatable), phenomena).

There are many, many, many empirical observations, mathematical certainties, and physical empirically demonstrable facts behind the Cosmological Model.

So with this model, the way to interpret the CMBR we observe, (and have it fit in nicely amongst other observable phenomena), would be to visualise it as being due to the extreme age of the photons. The photons would be scattered throughout the universe evenly. Now, with Hubble's Law and redshift being caused by expansion, the corollary to this would be that the things with the higher redshifts, are caused by the things moving away the fastest, and because the things moving the fastest are further away, we can infer that the CMBR is also the oldest thing in our observable universe and hence, the farthest away from us.

The ballon model fails when it comes to explaining what is really intended by 'uniform expansion' of the universe. As Steven said, all parts of the larger scale universe expand evenly, and it doesn't matter where you sit in that universe, the expansion in all directions seems the same.

My new catch cry which also applies to the Big Bang Theory, (or more precisely, the Standard Cosmological Model of the Universe) is:

'Try it on !' …. 'see if it fits (the facts) !' …. but whatever you do … 'don't believe it !'

This 'catch-cry' certainly applies in the case of the present Standard Cosmological Model. For the time being, it provides an unprecedented best fit match for most of our observations.

Hope this helps.

Cheers

[sjastro]

It's the photon's travelling from the same distance. The CMB is also known as the "last scattering surface" by plasma prior to recombination into neutral hydrogen. This is around z=1050. Any deeper (further back in time) and the Universe is opaque due to extensive photon scattering.

Regards

Steven

[batema]

Thankyou all. I will read and re read your comments to hopefully get in into my head.

Mark