On Thursday, December 28, 2017 at 2:00:43 AM UTC-6, [email protected]
wrote:
>
>
>
> On Wednesday, December 27, 2017 at 6:37:16 PM UTC-7, Brent wrote:
>>
>>
>>
>> On 12/27/2017 5:21 PM, [email protected] wrote:
>>
>>
>>
>> On Wednesday, December 27, 2017 at 3:43:05 PM UTC-7, Lawrence Crowell
>> wrote:
>>>
>>> The CMB is composed of radiation with a black body distribution peaked
>>> at around 1000nm. The radiation is now in the microwave band at about 1 mm
>>> wavelength. The IR photons were spread by the expansion of spacetime by a
>>> factor of 1000, and the actual z factor for the CMB is z = 1100. With the
>>> exponential expansion of the universe this z will increase until the CMB is
>>> in the radio wave band > 1m wavelength, and then eventually expanded beyond
>>> the dimensions of any practical antenna. It will take about 10 billion
>>> years for that to happan.
>>>
>>> LC
>>>
>>
>> There's a huge difference between photons falling below our threshold of
>> detection, and those which can never reach us because they left their
>> source after it had crossed our event horizon. AG
>>
>>
>> Not really. From the event horizon they are redshifted to zero energy,
>> which is the threshold of detection. LC isn't talking about some current
>> technological limit on detection.
>>
>> Brent
>>
>
> LC refers to a "practical antenna" so he must mean something related to
> our detection capabilities. Moreover, if a source of light is receding
> faster than c due to expansion, it must mean that photons leaving it will
> simply never reach us. Not a case IMO of being redshifted to zero energy.
> AG
>
Confusion reigns! The photons that we detect in the CMB did leave
propagate towards us before crossing the horizon relative to our position
now. In fact if we could detect gravitons emitted during the first
10^{-35}sec of the cosmos the same would be the case. The region the CMB
photons were emitted from are now 46 billion light years away on the Hubble
frame, which means this region is now outside our causal domain. Remember,
photons from the CMB occur along a light cone or null geodesic from a
region in the past, not on the Hubble frame now.
If we could peer beyond the CMB, say with neutrinos or gravitons
(gravitational radiation etc), these would be extremely redshifted say z ~
10^{26} or more. The CMB has a mere z = 1100. This means weak gravitons or
now classical gravitational waves from the end of inflation with a
wavelength ~ 1m would be hundreds of millions of light years in scale. We
can't detect these directly, but the CMB can act as a sort of detector with
B-modes. This was the intended result from BICEP-II. The signal is now
known to be muddled up with EM polarization from galactic dust.
The same in a sense holds in the distant future. The CMB will be observable
to any IGUS or observers 10 billion years from now. However, it will no
long be 1mm microwaves but radio waves in the meter length scale. So
observers would find a curious radio noise in the background that would be
the CMB. Observing finer detail would be more difficult in ways due to the
larger wavelength of radiation, but the data could be teased out. Galaxies
outside of any local cluster could only be observed in the infrared.
Further out in time much the same happens, and the CMB would be redshifted
to kilometer waves and eventually waves measuring larger than any stellar
system. The CMB is still in principle observable, but observers say 100
billion or a trillion years from now (assuming they exist( would most
likely not find it. The universe would be dark and cold and local galaxies
would be dim with mostly feeble red dwarf stars.
LC
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