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The Question
(Submitted October 29, 2004)
Before recombination at 380,000 yrs ABB the universe was opaque
to radiation due to Compton scattering.
Then why isn't it opaque today afer the reionization that occurrred
between 500 million and 1 billion years ago.?
I don't believe it is because there is not enough hydrogen ions
and free electrons to do the Compton scattering, because when the
Hydrogen was a neutron before reionization it caused the Lyman alpha
forest.
But I know that the IGM is not opaque because we can clearly see distant
quasars and galaxies as in the Hubbble Deep Field. Hence my confusion.
The Answer
This is a very interesting and important question (and in
researching it I certainly learned a lot more about reionization and
cosmology than I knew before). First of all, a brief history of
the universe up to the time of reionization (restating some of the
points you make, but in chronological order). In the earliest
universe, up to around 300,000 years after the Big Bang (or z~1000),
the universe was completely ionized, filled with a hot (T > 104 K)
and very dense plasma. Any light emitted was immediately scattered
by a free electron. At the time of recombination, neutral hydrogen
atoms formed and the universe became more transparent. However,
neutral hydrogen does absorb light at particular wavelengths, most
prominently at the Lyman alpha transition of 121.6 nm. Since this
wavelength is redshifted by the expansion of the universe, the
absorption feature is spread out over a wide part of the
electromagnetic spectrum to form the Lyman alpha forest as seen at
http://antwrp.gsfc.nasa.gov/apod/ap030126.html
and explained in
http://astron.berkeley.edu/~jcohn/lya.html (http://astron.berkeley.edu/%7Ejcohn/lya.html)
. Then in the epoch starting between 0.5 and 1 billion years after
the Big Bang (z~6),
the neutral hydrogen was reionized probably by quasars and hot
stars. It is only after this time that the universe has been truly
transparent. You can see an excellent graphic of this process at
http://www.astro.caltech.edu/~george/reion/ (http://www.astro.caltech.edu/%7Egeorge/reion/)
and links therein. The
epoch of reionization is of great interest currently in astronomy
with many questions including when and how it all happened. See for
example
http://www.ast.cam.ac.uk/~rtnigm/reion/RTNreionization.html (http://www.ast.cam.ac.uk/%7Ertnigm/reion/RTNreionization.html).
Now on to your question. The primary reason that the
reionized universe is transparent is that because of the expansion
of the universe the intergalactic medium (IGM) is much less dense on
average than the plasma before recombination. There are some
localized regions of dense ionized hydrogen (or H-II regions) such
as the Orion Nebula and in galaxy clusters where temperatures are
very high (T > 108 K) and X rays are emitted. Everywhere else the
IGM is cold and extremely tenuous, so there just aren't very many
electrons to scatter light from distant galaxies and quasars.
Cheers,
Hans Krimm and Jay Norris
for "Ask a High Energy Astronomer"
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