Fall 2014

Cosmology Today

David Nathaniel Spergel

We seem to live in a simple but strange universe. Our basic cosmological model fits a host of astronomical observations with only five basic parameters: the age of the universe, the density of atoms, the density of matter, the initial “lumpiness” of the universe, and a parameter that describes whether this lumpiness is more pronounced on smaller physical scales. Our observations of the cosmic microwave background fluctuations determine these parameters with uncertainties of only 1 to 2 percent. The same model also provides an excellent fit to the large-scale clustering of galaxies and gas, the properties of galaxy clusters, observations of gravitational lensing, and supernova-based measurements of the Hubble relation. This model implies that we live in a strange universe: atoms make up only 4 percent of the visible universe, dark matter makes up 24 percent, and dark energy – energy associated with empty space – makes up 72 percent.

DAVID N. SPERGEL, a Fellow of the American Academy since 2012, is the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation and Professor of Astrophysical Sciences at Princeton University. He is a theoretical astrophysicist with interests ranging from the search for planets around nearby stars to the shape of the universe.

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