🚀 go-pugleaf

"Joseph.D.Warner" <jwarner@grc.nasa.gov> writes:

> Matt Carey wrote:
> 
>> <snip>
>> 2) What is the "Room Temperature" of the universe?  The bright
>> boys at Bell Labs measured the background radiation of the universe
>> at about 3K (2.77K?) as I recall.  About 0.01 what we consider
>> room temperature.  kbT is about 1/40 eV at RT, so the background
>> "temperature" of the universe would be about 1/4000 eV.  Hardly
>> enough to make a neutrino relativistic.
> 
> No one, as I understand, has said that neutrinos are in thermal
> equalibrium with rest of the universe

The majority of the neutrinos left over from the Big Bang *were* in 
thermal equilibrium at the time they decoupled, i.e., at the time the
Universe became essentially ``transparent'' to neutrinos ca. 1 sec 
after the Big Bang (``ABB'' :-); however, since then cosmic expansion 
has ``cooled'' them in the same sense that photons have ``cooled,'' 
and like photons, most of them are not in thermal equilbrium with
_ANYTHING_ anymore, since the average neutrino (or BB photon!) now has 
a negligible probability of EVER interacting with ANYTHING ever again...
(Unless, of course, the ``Accelerating Universe'' claims turn out 
to be wrong, and the Universe re-collapses after all!). 

Because neutrinos decoupled earlier than photons, and also because photons
are believed to have been ``reheated'' when the electron/positron plasma
recombined ca. 15 sec ABB (whereas nothing comparable happened neutrinos),
the BB relic neutrinos are now ``colder'' than BB photons; hence, the BB
relic neutrinos are expected to now have a temperature of something like 
2 Kelvin, as opposed to the current measurement of 2.725 Kelvin for the 
BB relic photons.

See <http://casswww.ucsd.edu/public/tutorial/BB.html> for a tutorial and
timeline on the events believed to have occurred just after the Big Bang.


-- Gordon D. Pusch   

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