The Meaning of Eros/Macho

Most of the mass density in the Universe---and in the halo of our own galaxy---exists in the form of dark matter. Overall, the contribution of luminous matter (in stars) to the mass density of the Universe is less than 1\%; primordial nucleosynthesis indicates that baryons contribute between 1\% and 10\% of the critical density (0.01h^{-2}\la \Omega_B\la 0.02h^{-2}; $h=$ the Hubble constant in units of 100\kms\Mpc^{-1}); and other evidence indicates that the total mass density is at least 10\% of critical density, and likely much greater. If the universal density is as low as 10\% of the critical density there may be but one kind of dark matter. More likely, the universal density is greater than 10\%, and there are two kinds of dark matter, and thus two dark matter problems: In what form does the baryonic dark matter exist? and In what form does the nonbaryonic dark matter exist? The MACHO and EROS collaborations have presented evidence for the microlensing of stars in the LMC by $10^{-1\pm 1}M_\odot$ dark objects in the halo of our own galaxy and may well have solved {\it one} of the dark matter puzzles by identifying the form of the baryonic dark matter.Comment: 15 pages Latex + 3 Figs available on request, FNAL---Pub-93/298-

Cosmological Parameters

The discussion of cosmological parameters used to be a source of embarrassment to cosmologists. Today, measurements of the cosmological parameters are leading the way into the era of precision cosmology. The CMB temperature is measured to four significant figures, T_0=2.7277+/-0.002 K; the Hubble constant is now determined with a reliable error estimate, H_0=(65+/-5) km sec^-1 Mpc^-1; the mass density of baryons is precisely determined by big-bang nucleosynthesis Omega_B = (0.019+/-0.001) h^-2; and the age of the Universe inferred from the ages of the oldest stars is 14+/-1.5 Gyr, which is consistent the expansion age. Further, we have the first full accounting of matter and energy in the Universe, complete with a self consistency check. Expressed as a fraction of the critical density it goes like this: neutrinos, between 0.3% and 15%; stars, between 0.3% and 0.6%; baryons (total), 5+/-0.5%; matter (total),40% +/- 10%; smooth, dark energy, 80% +/- 20%; totaling to the critical density (within the errors).Comment: 27 pages LaTeX with 8 eps figures. To be published in The Proceedings of Particle Physics and the Universe (Cosmo-98), edited by David O. Caldwell (AIP, Woodbury, NY

The Case for Omega_M = 0.33 +/- 0.035

For decades, the determination of the mean density of matter(Omega_M) has been tied to the distribution of light. This has led to a ``bias,'' perhaps as large as a factor of 2, in determining a key cosmological parameter. Recent measurements of the physical properties of clusters, cosmic microwave background (CMB) anisotropy and the power spectrum of mass inhomogeneity now allow a determination of Omega_M without ``visual bias.'' The early data lead to a consistent picture of the matter and baryon densities, with Omega_B = 0.039 +/- 0.0075 and Omega_M = 0.33 +/- 0.035.Comment: 4 ApJ LaTeX. Submitted to Astrophys J Lett. Less provocative title, same conclusion

Cosmology Solved? Quite Possibly!

The discovery of the cosmic microwave background (CMB) in 1964 by Penzias and Wilson led to the establishment of the hot big-bang cosmological model some ten years later. Discoveries made in 1998 may ultimately have as profound an effect on our understanding of the origin and evolution of the Universe. Taken at face value, they confirm the basic tenets of Inflation + Cold Dark Matter, a bold and expansive theory that addresses all the fundamental questions left unanswered by the hot big-bang model and holds that the Universe is flat, slowly moving elementary particles provide the cosmic infrastructure, and quantum fluctuations seeded all the structure seen in the Universe today. Just as it took a decade to establish the hot big-bang model after the discovery of the CMB, it will likely take another ten years to establish the latest addition to the standard cosmology and make the answer to ``Cosmology Solved?'', ``YES!'' Whether or not 1998 proves to be a cosmic milestone, the coming avalanche of high-quality cosmological data promises to make the next twenty years an extremely exciting period for cosmology.Comment: 19 pages LaTeX including 5 eps figures. Presented at Great Debate: Cosmology Solved?, October 4, 1998, Baird Auditorium, Smithsonian Natural History Museum, Washington, DC. To be published in Proc. Astron. Soc. Pacific, February 199

Windows on the axion

Peccei-Quinn symmetry with attendant axion is a most compelling, and perhaps the most minimal, extension of the standard model, as it provides a very elegant solution to the nagging strong CP-problem associated with the theta vacuum structure of QCD. However, particle physics gives little guidance as to the axion mass; a priori, the plausible values span the range: 10(-12)eV is approx. less than m(a) which is approx. less than 10(6)eV, some 18 orders-of-magnitude. Laboratory experiments have excluded masses greater than 10(4)eV, leaving unprobed some 16 orders-of-magnitude. Axions have a host of interesting astrophysical and cosmological effects, including, modifying the evolution of stars of all types (our sun, red giants, white dwarfs, and neutron stars), contributing significantly to the mass density of the Universe today, and producting detectable line radiation through the decays of relic axions. Consideration of these effects has probed 14 orders-of-magnitude in axion mass, and has left open only two windows for further exploration: 10(-6)eV is approx. less than m(a) is approx. less than 10(-3)eV and 1eV is approx. less than m(a) is approx. less than 5eV (hadronic axions only). Both these windows are accessible to experiment, and a variety of very interesting experiments, all of which involve heavenly axions, are being planned or are underway