Since the baryon-to-photon ratio \eta _ { 10 } is in some doubt at present , we ignore the constraints on \eta _ { 10 } from big bang nucleosynthesis ( BBN ) and fit the three key cosmological parameters ( h, \Omega _ { M } , \eta _ { 10 } ) to four other observational constraints : Hubble parameter ( h _ { o } ) , age of the universe ( t _ { o } ) , cluster gas ( baryon ) fraction ( f _ { o } \equiv f _ { G } h ^ { 3 / 2 } ) , and effective shape parameter ( \Gamma _ { o } ) .
We consider open and flat CDM models and flat \Lambda CDM models , testing goodness of fit and drawing confidence regions by the \Delta \chi ^ { 2 } method .
CDM models with \Omega _ { M } = 1 ( SCDM models ) are accepted only because we allow a large error on h _ { o } , permitting h < 0.5 .
Open CDM models are accepted only for \Omega _ { M } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } % \hbox { $ > $ } } } 0.4 . \Lambda CDM models give similar results .
In all of these models , large \eta _ { 10 } ( \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 6 ) is favored strongly over small \eta _ { 10 } ( \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 2 ) , supporting reports of low deuterium abundances on some QSO lines of sight , and suggesting that observational determinations of primordial ^ { 4 } He may be contaminated by systematic errors .
Only if we drop the crucial \Gamma _ { o } constraint are much lower values of \Omega _ { M } and \eta _ { 10 } permitted .