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The evolution of structure in the expanding universe has redistributed the baryons from a nearly smooth plasma at the time of light element nucleosynthesis to a variety of states - condensed, atomic, molecular, and plasma - in clouds of gas and dust, planets, stars, and stellar remnants, that are arranged in the galaxies, in groups and clusters of galaxies, and in between. The amounts of baryons in each state and form at low redshift can be compared to what is observed at higher redshift and to the total cosmic abundance predicted by the theory of element production in the early universe. Budget estimates must be informed by ideas on how structure evolves as well as by the observations, and the budget in turn is a test of these ideas. Knowledge of the baryon budget is an essential boundary condition for the analysis of how structure formed and of the nature of cosmic dark matter.

Advances in observations allow reasonably sound estimates of the amount of baryons present in a considerable variety of forms. In a previous discussion (Fukugita, Hogan, & Peebles 1996), we presented a picture for cosmic evolution suggested by the results of budget estimates. In this paper we give details of the budget calculations and update them using our choices for the current best knowledge relevant to the calculation. Persic & Salucci (1992), Gnedin & Ostriker (1992), and Bristow & Phillipps (1994) estimate baryon abundances with different emphases; the results are compared in Section 5.1.

The main focus of this paper is the baryon budget at low redshift; our accounting is presented in Section 2. The low redshift budget can be compared to the situation at z ~ 3, where quasar absorption lines allow a comprehensive accounting of the diffuse components. We comment on this in Section 3. The budgets are compared to the constraint from light element production in Section 4, and the implications for galaxy and structure formation are summarized in Section 5.

We write Hubble's constant as

Equation 1 (1)

Where not explicitly written we use solar units for mass and luminosity and megaparsecs for the length unit.

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