Hi,

The following question has been troubling my tiny brain, please could
one of you boffins answer it for me:

If the Cosmic Background Radiation is leftover from the Big Bang,
which is also the source of all the galaxies then, given that almost
all the galaxies are moving away from us, why isn't the CBR also
moving away from us (in which case, according to my underdeveloped
noodle, should mean that we can't detect it)?

Thanks,
Paul

Wasn't it Paul Jones who wrote:
>Hi,
>
>The following question has been troubling my tiny brain, please could
>one of you boffins answer it for me:
>
>If the Cosmic Background Radiation is leftover from the Big Bang,
>which is also the source of all the galaxies then, given that almost
>all the galaxies are moving away from us, why isn't the CBR also
>moving away from us (in which case, according to my underdeveloped
>noodle, should mean that we can't detect it)?

Relativity.

Electromagnetic radiation always travels at the speed of light relative 
to any observer. So radiation that set off from the big bang 13.7 
billion years ago arrives at the speed of light. In exactly the same way 
that light from a star arrives at the speed of light, even if the star 
is travelling away from us at half the speed of light.

The speed of the em-radiation is constant, but the speed difference 
between source and observer causes the wavelength to be shifted. In the 
case of the cosmic background, the red shift is so great that it's gone 
right though the visible wavelengths and down into the microwave region.

-- 
Mike Williams
Gentleman of Leisure

On 15 Sep, 14:05, Mike Williams  kindly
replied to say:

> Relativity.
>
> Electromagnetic radiation always travels at the speed of light relative
> to any observer. So radiation that set off from the big bang 13.7
> billion years ago arrives at the speed of light.

This reflects my lack of understanding. My question is not really
about how fast it arrives but how does it arrive here? If I went off
into space and shone a beam of coherent light from my laser in the
diametrically opposite direction to where an observer was positioned,
then he/she wouldn't be able to detect it (would he?). Shouldn't all
the photons from the big bang are travelling away from us like the
galaxies?

> In exactly the same way
> that light from a star arrives at the speed of light, even if the star
> is travelling away from us at half the speed of light.

I think I grasp this even though it doesn't make "common sense" to my
limited terrestially evolved brain. As I understand it, it's all in
the maths.

> The speed of the em-radiation is constant, but the speed difference
> between source and observer causes the wavelength to be shifted.

Doppler Shift, check.

> In the
> case of the cosmic background, the red shift is so great that it's gone
> right though the visible wavelengths and down into the microwave region.

I'm up to speed on this as well, but I end up returning to the
question, why does it arrive here?

Thanks again,
Paul

Wasn't it Paul Jones who wrote:
>On 15 Sep, 14:05, Mike Williams  kindly
>replied to say:
>
>> Relativity.
>>
>> Electromagnetic radiation always travels at the speed of light relative
>> to any observer. So radiation that set off from the big bang 13.7
>> billion years ago arrives at the speed of light.
>
>This reflects my lack of understanding. My question is not really
>about how fast it arrives but how does it arrive here? If I went off
>into space and shone a beam of coherent light from my laser in the
>diametrically opposite direction to where an observer was positioned,
>then he/she wouldn't be able to detect it (would he?). Shouldn't all
>the photons from the big bang are travelling away from us like the
>galaxies?

The flash from the big bang wasn't coherent.

At one time the universe was full of incandescent opaque ionised plasma. 
Photons were being created randomly by the hot ions, but they didn't get 
far because they got absorbed by other ions in the plasma.

As the universe expanded and cooled, it reached a point where the ions 
were able to form into neutral atoms (hydrogen and a bit if helium) 
producing a transparent gas. Many of the photons that were in flight at 
that time, travelling from every point in the universe in every possible 
direction, were suddenly free to travel immense distances.

Some of the photons from regions that are now 13.7 billion light years 
away from here happened to be pointing in our direction.

-- 
Mike Williams
Gentleman of Leisure

Thanks for this very clear explanation, Mike.

Cheers,
Paul

"Paul Jones"  wrote in message 
news:02ee48bc-f34c-4449-9642-6b65b27aec84@a70g2000hsh.googlegroups.com...
> Hi,
>
> The following question has been troubling my tiny brain, please could
> one of you boffins answer it for me:
>
> If the Cosmic Background Radiation is leftover from the Big Bang,
> which is also the source of all the galaxies then, given that almost
> all the galaxies are moving away from us, why isn't the CBR also
> moving away from us (in which case, according to my underdeveloped
> noodle, should mean that we can't detect it)?
>
> Thanks,
> Paul

Adding to other explanations, the cosmological red shift (not Doppler shift) 
of the microwave radiation is around 1,089, that is the wavelengths are 
1,090 times longer than they were when the radiation was emitted (or set 
free by the recombination of electrons and protons).  So we still see it. 
When it was emitted it was optical radiation about the same temperature as a 
cool M-type star, but missing the absorption lines of the metals since none 
had yet formed aside from a little Li and Be.  (maybe a speck of boron).

When recombination occurred the temperature was around 3,000 K; now it is 
2.7 K.

-- 
Mike Dworetsky

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