But most of the available evidence points to a universe that stretches infinitely in all directions like a flat sheet of paper. Einstein's general theory of relativity permits the universe to assume a variety of shapes, from flat to spherical to saddle. The idea of a curved universe is not new. In a paper published in November's Nature Astronomy, Silk and his partners were able to make this discrepancy go away by introducing a new parameter to their equations: a slight curvature in the fabric of the universe. Yet astronomers already know how much dark matter exists, and there isn't enough to account for the gravitational lensing indicated by the Planck data. They attributed this to a phenomenon called gravitational lensing, whereby the light of the CMB is bent and diffused by gravity-in this case, the gravity exerted by dark matter, the unseen exotic material that makes up roughly a quarter of the universe. Silk and his colleagues noticed something odd about the Planck data, however: The peaks in the CMB were smoother than predicted. "We're looking at the fossil record of everything that formed subsequently." "It's a bit like archaeology," Silk explains. Image credit: Courtesy of the Planck Collaboration Image caption: Map of the cosmic microwave background temperature And those variations eventually translated into differences in the density of matter, with the hot spots giving rise to the clusters of galaxies that are now strewn across the sky like seeds bearing fruit in a celestial garden-even as traces of the seeds themselves remain fixed in the CMB. These tiny fluctuations represent variations in the energy density of the early universe. While the CMB is fairly uniform and quite dim, it nonetheless displays minute peaks and valleys in intensity, like hot and cold spots on a temperature map. Cosmologists have been probing Planck's data on the CMB-a remnant of the first light to flood the universe after the big bang more than 13 billion years ago-to better understand conditions in the early universe and reconstruct how the cosmos evolved over time. Together with colleagues in Italy and the United Kingdom, Silk recently analyzed data from the Planck Collaboration, a European Space Agency project that from 2009 to 2013 mapped the cosmic microwave background, a wash of low-level radiation that fills the sky. Now, cosmologist Joseph Silk thinks the same might be true of the entire universe. Through class discussions and extensive problem solving, you’ll learn about the principle of relativity and the light postulate, simultaneity, relativistic kinematic and dynamic calculations, light cones, k-calculus, and Minkowski space, and explore mathematical concepts underlying Einstein’s general theory of relativity and how they can help us understand the universe.More than 2,000 years ago, the ancient Greeks figured out that Earth was round rather than flat. This course explores the problems and inadequacies of Newtonian mechanics, then shifts focus to Einstein’s groundbreaking solution, the Special Theory of Relativity. It showed that an object’s length depends on its speed relative to an observer, that the passage of time depends on relative motion, and that an object’s mass varies with speed. Perhaps the most famous equation in science, E = mc2, revealed that matter and energy are equivalent, setting the stage for atomic bombs and nuclear power plants. This is just one of the discoveries revealed by Albert Einstein’s Special Theory of Relativity, which revolutionized physics. If a woman travels to a nearby star system at close to the speed of light, she will return much younger than her twin sister, who stayed home.
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