Conclusions

Prior to the publication of the data from recent microwave background experiments, I had investigated the power spectrum of anisotropies which would be expected for a purely baryonic universe devoid of CDM (McGaugh 1999mypred). Such a cosmology predicts a small amplitude for the second peak. This prediction is consistent with the subsequently published data (de Bernardis et al. 2000Boom; Hanany et al. 2000maxima).

The data are well described by a model in which all cosmological parameters except the geometry are fixed to values measured by independent means. Once the position of the first peak is fixed, no tuning of any of the many other parameters is required to explain the low observed amplitude of the second peak. This is not surprising; it is simply what is expected in a purely baryonic universe.

Consideration of a purely baryonic universe is motivated by the recent successes (e.g., McGaugh & de Blok 1998bMBb) of the hypothesized alternative to dark matter known as MOND (Milgrom 1983M83). Such a modification to conventional dynamics does appear to be viable. Taken in sum, the data suggest a universe in which $\Om = \Ob$ and $\OL \approx 1$.

I thank Cole Miller for conversations about reionization and scattering, Rabi Mohapatra for discussions about neutrino masses, and Glen Starkman for insights into geometry. I thank Cole Miller and Bob Sanders for a careful reading of the manuscript, and Jim Peebles, Greg Aldering, Jerry Sellwood, Eric Gawiser, and Arthur Kosowsky for their comments.

[Fig1.ps]The predictions of (a) and (b) purely baryonic models plotted against the data. The amplitudes of the models are arbitrary and are scaled to match the amplitude of the first peak. Solid lines in (a) are the models of McGaugh (1999) with baryon fractions f_b = 0.05, 0.10, 0.15 ( models 1, 2, and 3 of Table 1) in order of decreasing amplitude of the second peak. These are illustrative of reasonable models. The dotted line shows a reproduction of all the parameters of ``standard'' (e.g., Turner 1999). The low amplitude of the second peak was unexpected: all reasonable variations of the parameters of the model which were considered before the results predicted a second peak considerably larger in amplitude than allowed by the data. In contrast, the data are consistent with the predictions for a purely baryonic universe containing no CDM. The solid lines are identical to the previously published $\Ob = 0.01$, 0.02, and 0.03 models of McGaugh (1999) with geometry scaled to match the position of the first peak (Table 1). Also shown is a model (dotted line) with the baryon density given recently by Tytler et al. (2000). The data are consistent with a purely baryonic universe devoid of CDM.