Subsequent evolution The planets were originally thought to have formed in or near their current orbits. From that, a minimum mass of the nebula i.e. the protoplanetary disc was derived that was necessary to form the planets – the minimum mass solar nebula. It was derived that the nebula mass must have exceeded 3585 times that of the Earth. However, this has been questioned during the last 20 years. Currently, many planetary scientists think that the Solar System might have looked very different after its initial formation: several objects at least as massive as Mercury were present in the inner Solar System, the outer Solar System was much more compact than it is now, and the Kuiper belt was much closer to the Sun. Terrestrial planets At the end of the planetary formation epoch, the inner Solar System was populated by 50–100 Moon- to Mars-sized planetary embryos . Further growth was possible only because these bodies collided and merged, which took les...
Big bounce models have a venerable history [ further explanation needed ] and were endorsed on largely aesthetic grounds [ which? ] [ when? ] by cosmologists including Willem de Sitter , Carl Friedrich von Weizsäcker , George McVittie and George Gamow (who stressed that "from the physical point of view we must forget entirely about the precollapse period"). By the early 1980s, the advancing precision and scope of observational cosmology had revealed that the large-scale structure of the universe is flat , homogenous and isotropic , a finding later accepted as the Cosmological Principle to apply at scales beyond roughly 300 million light-years . It was recognized that it was necessary to find an explanation for how distant regions of the universe could have essentially identical properties without ever having been in light-like communication. A solution was p...
Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density , then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch. Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving white dwarfs , neutron stars , and black holes . Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero —a Big Freeze .Moreover, if the proton were unstable , then baryonic mat...
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