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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
and explained in
. 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
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

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.


Hans Krimm and Jay Norris
for "Ask an Astrophysicist"

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