The present author, like many theorists, has long regarded the Einstein-de Sitter (m = 1, = 0) cosmology as the most attractive one. For one thing, of the three possible constant values for - 0, 1, and - the only one that can describe our universe is m = 1. Also, cosmic inflation is the only known solution for several otherwise intractable problems, and all simple inflationary models predict that the universe is flat, i.e. that m + = 1. Since there is no known physical reason for a non-zero cosmological constant, it was often said that inflation favors = 1. Of course, theoretical prejudice is not a reliable guide. In recent years, many cosmologists have favored m ~ 0.3, both because of the H0 - t0 constraints and because cluster and other relatively small-scale measurements have given low values for m. (For a summary of arguments favoring low 0.2 and = 0, see ;  is a review that notes that larger scale measurements favor higher m.) But the most exciting new evidence has come from cosmological-scale measurements.
Type Ia Supernovae. At present, the most promising techniques for measuring m and on cosmological scales use the small-angle anisotropies in the CMB radiation and high-redshift Type Ia supernovae (SNe Ia). We will discuss the latter first. SNe Ia are the brightest supernovae, and the spread in their intrinsic brightness appears to be relatively small. The Supernova Cosmology Project  demonstrated the feasibility of finding significant numbers of such supernovae. The first seven high redshift SNe Ia that they analyzed gave for a flat universe m = 1 - = 0.94+0.34-0.28, or equivalently = 0.06+0.28-0.34 (< 0.51 at the 95% confidence level) . But adding one z = 0.83 SN Ia for which they had good HST data lowered the implied m to 0.6±0.2 in the flat case . Analysis of their larger dataset of 42 high-redshift SNe Ia gives for the flat case m = 0.28+0.09 + 0.05-0.08 - 0.04 where the first errors are statistical and the second are identified systematics . The High-Z Supernova team has also searched successfully for high-redshift supernovae to measure m [48, 104], and their 1998 dataset of 14 + 2 high-redshift SNe Ia including three for which they had HST data (two at z 0.5 and one at 0.97) imply m = 0.32±0.1 in the flat case with their MLCS fitting method.
The main concerns about the interpretation of this data are evolution of the SNe Ia [34, 106] and dimming by dust. A recent specific supernova evolution concern is that the rest frame rise-times of distant supernovae may be longer than nearby ones . But a direct comparison between nearby supernova and the SCP distant sample shows that they are rather consistent with each other . Ordinary dust causes reddening, but hypothetical ``grey'' dust would cause much less reddening and could in principle provide an alternative explanation for the fact that high-redshift supernovae are observed to be dimmer than expected in a critical-density cosmology. Grey interstellar dust would induce more dispersion than is observed, so the hypothetical grey dust would have to be intergalactic. It is hard to see why the largest dust grains, which would be greyer, should preferentially be ejected by galaxies . Such dust, if it exists, would also absorb starlight and reradiate it at long wavelengths, where there are other constraints that could, with additional observations, rule out this scenario . Such grey dust would also produce some reddening which could be detectable via comparison of infrared vs. optical colors of supernovae; such a measurement for one high-redshift SN Ia disfavors significant grey dust extinction , and more observations could strengthen this conclusion. Yet another way of addressing this question is to collect data on supernovae with redshift z > 1, where the dust scenario predicts considerably more dimming than the cosmology. The one z > 1 supernova currently available, SCP's ``Albinoni'' (SN1998eq) at z = 1.2, favors the cosmology. More such data are needed for a statistically significant result, and both the SCP and the High-Z group are attempting to get a few more very high redshift supernovae.
CMB anisotropies. The location of the first acoustic (or Doppler, or Sakharov) peak at angular wavenumber l 200 indicated by the data available at the time of this meeting was evidence in favor of a flat universe tot m + 1 (e.g. ). New data from the BOOMERANG long-duration balloon flight around Antarctica  and the MAXIMA-1 balloon flight  confirm this, with tot = 1.11+0.13-0.12 at 95% C.L. . The preliminary BOOMERANG results  are lower around l 500 than the predictions in this second peak region in CDM-type models (e.g., ), and this could  indicate higher b than expected from Big Bang Nucleosynthesis together with the recent deuterium measurements (discussed below). However, the MAXIMA-1 data for l 500 are more consistent with expectations of standard models and the standard BBN b  (but cf. ). The BOOMERANG and MAXIMA-2 data are still being analyzed, and other experiments will have relevant data as well. Further data should be available in 2001 from the NASA Microwave Anisotropy Probe satellite.
Large-scale Measurements. The comparison of the IRAS redshift surveys with POTENT and related analyses typically give values for the parameter I m0.6 / bI (where bI is the biasing parameter for IRAS galaxies), corresponding to 0.3 m 3 (for an assumed bI = 1.15). It is not clear whether it will be possible to reduce the spread in these values significantly in the near future -- probably both additional data and a better understanding of systematic and statistical effects will be required. A particularly simple way to deduce a lower limit on m from the POTENT peculiar velocity data was proposed by , based on the fact that high-velocity outflows from voids are not expected in low- models. Data on just one nearby void indicates that m 0.3 at the 97% C.L. Stronger constraints are available if we assume that the probability distribution function (PDF) of the primordial fluctuations was Gaussian. Evolution from a Gaussian initial PDF to the non-Gaussian mass distribution observed today requires considerable gravitational nonlinearity, i.e. large m. The PDF deduced by POTENT from observed velocities (i.e., the PDF of the mass, if the POTENT reconstruction is reliable) is far from Gaussian today, with a long positive-fluctuation tail. It agrees with a Gaussian initial PDF if and only if m ~ 1; m < 1 is rejected at the 2 level, and m 0.3 is ruled out at 4 [87, 10]. It would be interesting to repeat this analysis with newer data. Analyzing peculiar velocity data without POTENT again leads to a strong lower limit m > 0.3 (99% C.L.), and together with the SN Ia constraints leads to the conclusion that m 0.5 .
Measurements on Scales of a Few Mpc. A study by the Canadian Network for Observational Cosmology (CNOC) of 16 clusters at z ~ 0.3, mostly chosen from the Einstein Medium Sensitivity Survey , was designed to allow a self-contained measurement of m from a field M/L which in turn was deduced from their measured cluster M/L. The result was m = 0.19±0.06 . These data were mainly compared to standard CDM models, and they appear to exclude m = 1 in such models.
Estimates on Galaxy Halo Scales. Work by Zaritsky et al.  has confirmed that spiral galaxies have massive halos. They collected data on satellites of isolated spiral galaxies, and concluded that the fact that the relative velocities do not fall off out to a separation of at least 200 kpc shows that massive halos are the norm. The typical rotation velocity of ~ 200 - 250 km s-1 implies a mass within 200 kpc of ~ 2 × 1012 M. A careful analysis taking into account selection effects and satellite orbit uncertainties concluded that the indicated value of m exceeds 0.13 at 90% confidence , with preferred values exceeding 0.3. Newer data suggesting that relative velocities do not fall off out to a separation of ~ 400 kpc  presumably would raise these m estimates. Weak lensing data confirms the existence of massive galactic halos [116, 125, 4, 131].
Cluster Baryons vs. Big Bang Nucleosynthesis. White et al.  emphasized that X-ray observations of the abundance of baryons in clusters can be used to determine m if clusters are a fair sample of both baryons and dark matter, as they are expected to be based on simulations . The fair sample hypothesis implies that
We can use this to determine m using the baryon abundance b h2 = 0.019±0.0024 (95% C.L.) from the measurement of the deuterium abundance in high-redshift Lyman limit systems, of which a third has recently been analyzed [66, 122] and more are in the pipeline D. Tytler, these proceedings. Using X-ray data from an X-ray flux limited sample of clusters to estimate the baryon fraction fb = 0.075h-3/2  gives m = 0.25h-1/2 = 0.3±0.1 using h = 0.65±0.08. Estimating the baryon fraction using Sunyaev-Zel'dovich measurements of a sample of 18 clusters gives fb = 0.077h-1 , and implies m = 0.25h-1 = 0.38±0.1.
Cluster Evolution. The dependence of the number of clusters on redshift can be a useful constraint on theories . But the cluster data at various redshifts are difficult to compare properly since they are rather inhomogeneous. Using just X-ray temperature data,  concludes that m 0.45±0.2, with m = 1 strongly disfavored.
Power Spectrum. In the context of the CDM class of models, two additional constraints are available. The spectrum shape parameter m h 0.25±0.05, implying m 0.4±0.1. A new measurement m = 0.34±0.1 comes from the amplitude of the power spectrum of fluctuations at redshift z ~ 3, measured from the Lyman forest . This result is strongly inconsistent with high-m models because they would predict that the fluctuations grow much more to z = 0, and thus would be lower at z = 3 than they are observed to be.