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Cosmology is a branch of metaphysics that deals with the nature of the universe, a theory or doctrine describing the natural order of the universe. [36] The basic definition of Cosmology is the science of the origin and development of the universe. In modern astronomy, the Big Bang theory is the dominant postulation.
The physical universe is defined as all of space and time [a] (collectively referred to as spacetime) and their contents. [10] Such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space.
Observations made by Edwin Hubble during the 1930s–1950s found that galaxies appeared to be moving away from each other, leading to the currently accepted Big Bang theory. This suggests that the universe began very dense about 13.787 billion years ago, and it has expanded and (on average) become less dense ever since. [1]
Time is infinite with a cyclic universe, where the current universe was preceded and will be followed by an infinite number of universes. [16] [17] The different states of matter are guided by eternal kala (time), which repeats general events ranging from a moment to the lifespan of the universe, which is cyclically created and destroyed. [18]
A group of physicists have theorized a mechanism for "cosmological collapse" which predicts the universe will at some point stop expanding.
The heat death of the universe (also known as the Big Chill or Big Freeze) [1] [2] is a hypothesis on the ultimate fate of the universe, which suggests the universe will evolve to a state of no thermodynamic free energy, and will therefore be unable to sustain processes that increase entropy.
The universe is a four-dimensional spacetime, but within a universe that obeys the cosmological principle, there is a natural choice of three-dimensional spatial surface. These are the surfaces on which observers who are stationary in comoving coordinates agree on the age of the universe .
The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies ever photographed at that time. Except for the few stars in the foreground (which are bright and easily recognizable because only they have diffraction spikes), every speck of light in the photo is an individual galaxy, some of them as old as 13.2 billion years; the observable universe is ...