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Re: [Phys-l] Cosmic background radiation



On 11/05/2006 10:54 PM, Aker, Leanna wrote:

I teach 9th grade physical science. We are getting ready to discuss the Big Bang Theory and I
was wondering if someone could explain to me just how the cosmic background radiation is
supporting evidence? I know that the fact it is constant throughout the universe is the key, but
I'm foggy on just how this provides supporting evidence for the Big Bang.

The question is premised on some very shaky assumptions.

First of all, as Karl Popper and others have discussed, we rarely (if
ever) have evidence /supporting/ this-or-that theory. Usually all we
can say is that the theory is _consistent_ with the presently-available
evidence, and that certain competing theories are _ruled out_ by the
evidence.

To say the same thing more formally, consider the following syllogism:
A implies B.
We observe B.
This is supporting evidence for A. ??!!??!!

This is fallacious reasoning, and has been recognized as such for 2000
years.

Returning to the cosmological example, by the early 1960s the Hubble
expansion was well documented. This could be explained by a big-bang
model *or* by a steady-state model. The observation of the microwave
background radiation put the kibosh on the steady-state model, since
the big-bang model could explain the radiation but the steady-state
model could not.

Let's be clear: When a theory survives a test like this, the theory
becomes more credible, but not because the test is evidence for the
theory, but because the test is evidence /against/ some of the
alternative theories.

I know that the fact it is constant throughout the universe is the key,

I wouldn't say it was "the key". It has been known since 1992 that the
background radiation is slightly anisotropic.
http://en.wikipedia.org/wiki/Timeline_of_cosmic_microwave_background_astronomy

The fact that the radiation is /approximately/ isotropic is unsurprising;
if (by way of approximation) we say the early universe was in thermal
equilibrium, then the light was everywhere _then_ ... and therefore it
should be everywhere _now_, just red shifted.

Again: This is unsurprising, but nowhere near being a proof of the
correctness of the big bang model(s).

(I have great lab activities showing red shift, etc.)

Which red shift is that? In the lab you most likely deal with the Doppler
red shift. That is, to first order, the same as the cosmological red shift,
but this equivalence is not obvious ... certainly not obvious to 9th graders.
There are some tricky concepts that need to be understood, concerning what
expands -- and what doesn't expand -- when the fabric of spacetime expands.
http://math.ucr.edu/home/baez/physics/Relativity/GR/hubble.html

===================================

Here is a terse catalog of some of the main ideas connected to the background
radiation:
*) Observation: Red shift versus distance.
*) Interpretation: Expansion of the universe.
*) Basic theory: Free particles are "pulled" away from each other
by the expansion of the universe. It looks like a tidal stress;
the particles accelerate away from each other. OTOH atoms, rulers,
stars, and even galaxies are sufficiently firmly bound that they
can resist this stress, so they do not expand. This is important,
because if rulers expanded along with the universe, the expansion
of the universe would be unmeasurable and indeed meaningless.
*) Basic theory & observation: Gasses cool as they expand.
This includes relativistic gasses including photons.
*) Steady state theory: Universe expands, but the density and
temperature remain the same, because new matter is somehow
created to "fill in the gaps".
-- No way to predict background radiation.
*) Big bang theory: The universe cools as it expands.
-- If you run the movie backwards, you see that at earlier times
the universe must have been much hotter. At some early time t0,
it must have been hot enough to ionize all the atoms. Such a
cloud of ions is a plasma, and is opaque to electromagnetic
radiation over a wide range. This particular cloud is called the
/cosmic fireball/.
-- Suppose you look at the sky in some random direction. Follow
your line of sight outward until you hit something. You might hit
something foreground object, such as a star, but if you don't, you
will eventually hit the cosmic fireball.
-- If your line of site passes through interstellar gas, that doesn't
matter. Unionized gas is highly transparent to microwaves.
-- The background radiation is just the light from the surface of
the cosmic fireball, duly redshifted.
-- You can't use light or microwaves to see anything earlier than
the cosmic fireball, because the plasma is opaque.
-- To the extent that the fireball was everywhere _then_, the
background radiation will be everywhere _now_.

If you have questions about any of these items, please ask a more
specific question.