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Hi Dave, news to me too --I thought I understood this as a simple Doppler s=
hift phenomenon, but as the two replies indicate, not so. This useful (?),=
seemingly appropriate raisin bread analogy comes from:
www.esa.int/Our_Activities/Space_Science/What_is_red_shift<http://www.esa.i=
nt/Our_Activities/Space_Science/What_is_red_shift>
"However, to be accurate, the red shifts observed in distant objects are no=
t exactly due to the Doppler phenomenon, but are rather a result of the exp=
ansion of the Universe.
Doppler shifts arise from the relative motion of source and observer throug=
h space, whereas astronomical redshifts are 'expansion redshifts' due to th=
e expansion of space itself.
Two objects can actually be stationary in space and still experience a red =
shift if the intervening space itself is expanding.
A convenient analogy for the expansion of the Universe is a loaf of unbaked=
raisin bread. The raisins are at rest relative to one another in the dough=
before it is placed in the oven. As the bread rises, it also expands, maki=
ng the space between the raisins increase.
If the raisins could see, they would observe that all the other raisins wer=
e moving away from them although they themselves were stationary within the=
loaf. Only the dough - their 'Universe' - is expanding."
Steve
---------------------------------------------
On Apr 7, 2019, at 4:13 PM, David Webster <dwebster@glinx.com<mailto:dwebst=
er@glinx.com>> wrote:
Hi Burkhard & All,
Thanks Burkhard. You have answered my question. Not really an argument;=
just need for clarification.
The red shift I now understand is measured not by their being less ligh=
t of shorter wavelength but by displacement of spectral lines, presumably t=
o a longer than normal wavelength. So the spectral lines of e.g. hydrogen =
in some state would all be shifted to a somewhat longer wavelength.
But I do have problems with the concept of everything having started wi=
th an explosive expansion of some kernel of infinite density; just after Go=
d said "Let there be light"; aka big bang.
Dave
On 2019-04-07 12:13 p.m., Burkhard Plache wrote:
Hi Dave,
your last question makes the correct observation that of all the light
emitted by a distant object, the shorter wavelength light will be
preferentially scattered away, leaving more of the original red light
than blue light arriving at our doorsteps. Your implied, though not
stated, assumption seems to be that redshift is measured by 'relative
amounts of light' or 'the light looking more red'. - However, redshift
is measured by looking at spectral lines, which are not modified by
Rayleigh scattering. - Could you clearly state what your argument is?
Thanks,
Burkhard
On Sun, Apr 7, 2019 at 10:53 AM David Webster <dwebster@glinx.com<mailto:dw=
ebster@glinx.com>> wrote:
On 2019-04-07 8:45 a.m., Burkhard Plache wrote:
Hi David,
to correct a common misrepresentation: The cosmological red shift of
light is not due to the source moving away but due to the space
expanding. Two very different phenomena.
Also, Rayleigh scattering is not changing the wavelength of the
scattered light, hence is not contributing to redshift.
Burkhard
On Sun, Apr 7, 2019 at 8:17 AM David Webster <dwebster@glinx.com<mailto:dwe=
bster@glinx.com>> wrote:
Hi Burkhard,
Thanks. It seems to me that we are getting tangled up in semantics.
If space expands then it makes objects appear to be moving away.
And, indeed, scattering does not destroy shorter wavelengths but it
does deflect them so they are partially or entirely culled from those
waves which are moving from source to observer. Thus, at the local
level; blue skies, white clouds, red sunsets and that green flash which
one sometimes sees from the cockpit when landing and facing west near
sunset.
The above are all effects of our atmosphere. But there is ample
evidence of cosmic dust, ranging from particles to atoms, so one would
expect scattering of shorter wavelengths throughout space to increase
with distance between observer and source; greater opportunity for
scattering.
So rephrasing my question in current jargon, are red shifts of
light due to expansion of space, distinct from red shifts which might be
due to Rayleigh scattering whereby shorter wavelengths from a source are
less likely to reach an observer ?
Or more directly, why is the observed increase in red shift with
distance between source and observer attributed to an expansion of
space as opposed to greater opportunity for scattering of shorter
wavelengths of light as this distance increases ?
Dave
Dear All, but especially astrophysics experts,
Is the red shift of light, which would be due to the source moving
away at great speed, intrinsically unlike the red shift due to Rayleigh
scattering (which selectively scatters shorter wavelengths; 1/[length to
the fourth power]) ?
With ample dust in space, ranging from particles to atoms, one
would expect the red shift due to scattering to also be a function of
distance to source.
Dave Webster, Kentville
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<p><font color=3D"#4d22b3">Hi Dave, news to me too --I thought I understood=
this as a simple Doppler shift phenomenon, but as the two replies indicate=
, not so. This useful (?), seemingly appropriate raisin bread analogy=
comes from:</font></p>
<p><a href=3D"http://www.esa.int/Our_Activities/Space_Science/What_is_red_s=
hift">www.esa.int/Our_Activities/Space_Science/What_is_red_shift</a></p>
<p>"However, to be accurate, the red shifts observed in distant object=
s are not exactly due to the Doppler phenomenon, but are rather a result of=
the expansion of the Universe.
</p>
<p>Doppler shifts arise from the relative motion of source and observer thr=
ough space, whereas astronomical redshifts are 'expansion redshifts' due to=