Over 2,000 years ago, the Greeks thought they had it all worked out. In their cosmology, the entire Universe was composed of four elements: earth, wind, fire and water. Now, despite several millennia of effort, modern cosmologists are significantly worse off. We have no idea what the bulk of the Universe is composed of. We cannot even tell whether the majority of matter in the Universe is in some microscopic form (such as axions or other exotic particles) or in some macroscopic form (such as brown dwarfs or primordial black holes). Our ignorance of the mass of the basic building blocks of the Universe spans a good 50 orders of magnitude. In a paper in the Astrophysical Journal last month, Metcalf and Silk propose that, by observing the gravitational effects of matter on the light from distant supernovae, it is possible to distinguish between microscopic and macroscopic forms of matter.
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