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The Question

(Submitted May 30, 2000)

Does the discovery of the intergalactic "hydrogen fog," which now accounts for the missing matter in the universe, mean that the "big crunch" model is now viable or is the universe nevertheless going to expand endlessly?

The Answer

You (or local news broadcasts for that matter) may have gotten confused by the similar terminology. What they have recently announced:

is the discovery of "missing hydrogen."

To back up a step: the fate of the universe is determined by the density of the universe. In the usual cosmological units, Omega=1.0 is the critical value (Omega>1 implies Big Crunch). We can see the gravitational influence of about Omega~0.4 of matter, but the visible stars and galaxies only amount to Omega~0.05 --- the remainder, Omega~0.35 or so, is called dark matter (they are dark, not missing).

Some of the dark matter is probably plain ordinary matter: gas that never became stars, dead stars, planets, bricks, ... However, the observed ratio of helium (and several other elements) to hydrogen, mostly created in the big bang, is thought to imply that ordinary ("baryonic") matter only amounts to Omega~0.1. The rest is the exotic ("non-baryonic") dark matter.

As the press release says:

"Astronomers believe at least 90 percent of the matter in the universe is hidden in exotic dark form that has not yet been seen directly. But more embarrassing is that, until now, they have not been able to see most of the universe's ordinary, or baryonic, matter (normal protons, electrons, and neutrons)."

In summary, the news story is that scientists have at last directly seen some of the ordinary matter that they had previously detected only through indirect means. It therefore does not change our view of the fate of the universe. Other recent discoveries suggest that the universe will expand forever, however.

Best wishes,

Koji Mukai & Bram Boroson
for "Ask an Astrophysicist"

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