Once Before Time: A Whole Story of the Universe
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In his introduction to a revolutionary theory of the cosmos, Martin Bojowald shows how the big bang theory may give way to the big bounce theory, which describes our universe as an eternal series of expansions and contractions, with no beginning and no end.
In 2000, Bojowald, then a twenty-seven-year-old postdoctoral student at Pennsylvania State University, used a relatively new theory called loop quantum gravity—a cunning combination of Einstein’s theory of gravity with quantum mechanics—to create a simple model of the universe. Loop quantum cosmology, or LQC, was born, and with it, a theory that managed to do something even Einstein’s general theory of relativity had failed to do—illuminate the very birth of the universe.
slightest use in everyday life. Large-scale predictions of cosmological interest would, however, be conceivable, and thus the uniqueness question is of importance. Intellectually, this outlook represents an opportunity just as enticing for an understanding of the universe. But even if we are concerned only with the clarification of uniqueness rather than its greedy exploitation, many questions remain open once we leave the range of the simplest models. To describe the real world, many extensions
for the unit of length. So far, these thoughts have not played a technological role, due in part to the extreme smallness of the Planck length. Compared to the radius of an atom of about 0.000000001 meter (a billionth of a meter) or even that of a nucleus of 0.000000000000001 meter (a quadrillionth of a meter), the Planck length is infinitesimal. Inserting the known values of the speed of light and the gravitational and Planck constants, one obtains a value of about
of space-time by its links. The denser it is, the more continuous space appears to be. The gray scale indicates excitation levels of loops. Deriving laws for the temporal change of a quantum gravitational universe was not supposed to have taken that long. The electromagnetic force in its quantum form, with photons as the elementary building blocks, was essentially understood in the 1930s. All other fundamental forces except gravity followed with a coherent mathematical description soon after
example of values of the volume in dependence of a parameter controlling the asymmetry. Although much headway has recently been made by the calculations of Johannes Brunnemann and David Rideout, no complete picture is known for the general case. Even spatial structures that appear uniform on large cosmic scales are fundamentally constructed in an atomic way, just as a material body consists of the well-known atoms. To construct a macroscopic object—be it a crystal or just a piece of empty
of whether a black hole has a ground state akin to that of an atom, and what space-time could represent this state. At the end of the Hawking process one should, by the atom analogy, expect the black hole to be in the state of lowest energy; its form is thus decisive for the question of whether Hawking evaporation indeed destroys all information, or whether there is a compact core for storage. Then again, the analogy to atoms may in the end lead to wrong conclusions even though it can describe