Received: by alpheratz.cpm.aca.mmu.ac.uk id UAA29559 (8.6.9/5.3[ref firstname.lastname@example.org] for cpm.aca.mmu.ac.uk from email@example.com); Sat, 26 Jan 2002 20:37:25 GMT Message-Id: <200201262033.g0QKX9B12100@terri.harvard.edu> Subject: Re: necessity of mental memes Date: Sat, 26 Jan 2002 15:33:17 -0500 x-sender: firstname.lastname@example.org x-mailer: Claris Emailer 2.0v3, Claritas Est Veritas From: "Wade T. Smith" <email@example.com> To: "Memetics Discussion List" <firstname.lastname@example.org> Content-Type: text/plain; charset="US-ASCII" Sender: email@example.com Precedence: bulk Reply-To: firstname.lastname@example.org
Hi Dace -
>It's common knowledge among physicists, astronomers, cosmologists, etc.,
>that light didn't appear until 300,00 years after the big bang
>(though I recently saw this figure upped to 400,000,
>unfortunately without any explanation for the revised figure).
The cosmic background radiation that we can detect, yes, began at that
time. It is a space-time boundary caused by the scattering qualities of
photons and the nature of the early universe. It's not that light
(photons) was not there, but that it was undetectable.
The behavior of CMB photons moving through the early universe is
analogous to the propagation of optical light through the Earth's
atmosphere. Water droplets in a cloud are very effective at scattering
light, while optical light moves freely through clear air. Thus, on a
cloudy day, we can look through the air out towards the clouds, but can
not see through the opaque clouds. Cosmologists studying the cosmic
microwave background radiation can look through much of the universe back
to when it was opaque: a view back to 400,000 years after the Big Bang.
This "wall of light" is called the surface of last scattering since it
was the last time most of the CMB photons directly scattered off of
matter. When we make maps of the temperature of the CMB, we are mapping
this surface of last scattering.
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