Expanding Universe
Expanding universe is the current understanding of the state of the universe. It states that the universe is not fixed. It is gradually expanding. It is based on the finding that all galaxies are moving away from each other. Application of general relativity to cosmology, along with the detection of red shifted light coming from galaxies outside the Milky Way Galaxy, led to the realization in the 1920s that all galaxies are receding (see Edwin Hubble). It is unknown whether the universe will expand indefinitely (open universe) or eventually collapse (closed universe) into an extremely dense, congested state, as it began, according to the big-bang model.
Friedman’s universe is the model of universe developed in 1922 by the Russian meteorologist and mathematician Aleksander Friedmann (1888–1925).
He believed that Albert Einstein's general theory of relativity required a theory of the universe in motion, as opposed to the static universe that scientists until then had proposed. He hypothesized a big bang followed by expansion, then contraction and an eventual big crunch. His model supposes a closed universe, but similar solutions involve an open universe (which expands infinitely) or a flat universe (in which expansion continues infinitely but gradually approaches a rate of zero).
Milky way is a large spiral galaxy (roughly 150,000 light-years in diameter) that contains Earth's solar system.
It includes the multitude of stars whose light is seen as the Milky Way, the irregular luminous band that encircles the sky defining the plane of the galactic disk. The Milky Way system contains hundreds of billions of stars and large amounts of interstellar gas and dust. Because the dust obscures astronomers' view of many of its stars, large areas could not be studied before the development of infrared astronomy and radio astronomy (see radio and radar astronomy). Its precise constituents, shape, and true size and mass are still not known; it is believed to contain large amounts of dark matter and a massive black hole at its core. The Sun lies in one of the Galaxy's spiral arms, about 27,000 light-years from the centre.
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