K. Sachs (1932–) discovered gravitationally induced redshifts in the cosmic background radiation Carl Sagan (1934–1996) Andrei Sakharov (1921–1989) invented the theory of twins, CPT-symmetric universes Allan Sandage (1936–2010) set the cosmological distance scale and accurately estimated the speed of expansion of the universe Brian P. Schmidt (1967–) used supernova data to measure the acceleration in the expansion of the universe David N. Schramm (1945–1997) was an expert on big bang theory and an early proponent of dark matter Dennis W. Sciama (1926–1999) studied many aspects of cosmology and supervised many other leading cosmologists Irving Segal (1918–1998) created chronometric cosmology with alternative explanation of redshift in spectra of distant sources Seleucus of Seleucia (c.190–c.150 BC) used tidal observations to support a heliocentric model Roman Ulrich Sexl (1939–1986) developed an ether-based theory of absolute simultaneity that is mathematically equivalent to special relativity Al-Sijzi (c. 945–1020) invented an astrolabe based on the Earth's rotation Joseph Silk (1942–) explained the homogeneity of the early universe using photon diffusion damping Willem de Sitter (1872–1934) developed a theory of dark matter with Einstein, found an expanding matterless solution to general relativity Vesto Slipher (1875–1969) performed the first measurements of radial velocities for galaxies, providing the empirical basis for the expansion of the universe Lee Smolin (1955–) studied quantum gravity, popularized a theory of cosmological natural selection George F. Smoot (1945–) used Cosmic Background Explorer Satellite to measure the temperature and anisotropy of the early universe David N. Spergel (1961–) used Wilkinson Microwave Anisotropy Probe satellite to measure the temperature and anisotropy of the early universe Paul Steinhardt (1952–) pioneered inflationary cosmology, introduced first example of eternal inflation, introduced quintessential dark energy, introduced the concept of strongly self-interacting dark matter, studied brane cosmology and cyclic models of the universe Abd al-Rahman al-Sufi (903–986) wrote the Book of Fixed Stars, which lists over forty constellations and the stars within them Nicholas B. Suntzeff (1952–) used supernova observations to discover acceleration in the expansion of the universe, calibrated the supernova distance scale Rashid Sunyaev (1943–) developed a theory of density fluctuations in the early universe, described how to use cosmic background distortion to observe large-scale density fluctuations Alex Szalay (1949–) was working on structure formation in a neutrino-dominated universe, biased galaxy formation in a cold dark matter dominated universe and computing the power spectrum in hot, cold and warm dark matter dominated universes T Max Tegmark (1967–) determined the