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Edwin Hubble confirmed through analysis of galactic redshifts in 1929 that galaxies are indeed drifting apart this is important observational evidence for an expanding universe. Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, which he called the "primeval atom". Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang theory and various observations indicate that this excess gravitational potential is not created by baryonic matter, such as normal atoms. Besides these primordial building materials, astronomers observe the gravitational effects of an unknown dark matter surrounding galaxies.
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Giant clouds of these primordial elements-mostly hydrogen, with some helium and lithium-later coalesced through gravity, forming early stars and galaxies, the descendants of which are visible today. Īfter its initial expansion, an event that is by itself often called "the Big Bang", the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787 ± 0.020 billion years ago, which is considered the age of the universe. Extrapolating this cosmic expansion backwards in time using the known laws of physics, the theory describes an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named " the Big Bang singularity"). The model offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.Ĭrucially, the theory is compatible with Hubble–Lemaître law-the observation that the farther away a galaxy is, the faster it is moving away from Earth. It is the prevailing cosmological model explaining the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. The Big Bang theory describes how the universe expanded from an initial state of high density and temperature.