The study of the universe on the largest scales of length and time, particularly the propounding of theories concerning the origin, nature, structure, and evolution of the universe. A cosmology is any model said to represent the observed universe. Western cosmology is entirely scientific in its approach, and has produced two famous models in modern times: the 'big bang' and the steady state hypotheses. The study of the origin and mode of formation of various celestial objects is known as cosmogony.
Questions concerning the nature of the Universe as a whole were until recently, the province of philosophy and superstition only. There was no way to examine the fabric of the heavens to see what it was made of - until the invention of spectroscopy and the construction of powerful telescopes in the past century. The data collected have been analysed with sophisticated mathematical techniques, and models have been developed which help us to understand how this Universe may have come to be how it is. Cosmology draws on the physical sciences - especially mathematics, physics, and astronomy.
Modern cosmology is the scientific study of the origin, development, and large-scale structure of the Universe. It seeks to answer such questions as How did the Universe begin? How will it end, if ever? What physical forces control its development? What is it made of?
Before the 20th century, there was little hope or expectation of being able to answer such questions scientifically. How could you possibly know what a star was made of, without a sample in the laboratory? And, since the Universe appeared to be static (on the large scale), questions of beginnings and endings could only be speculative or mythological.
This all changed with the founding of scientific method and the rapid development of science and mathematics in their modern forms, especially astronomy, astrophysics, physics, chemistry; and the development of their associated instruments such as telescopes, spectroscopes, detectors for radio waves, gamma rays, X rays; rocketry and artificial satellites, and computers. Perhaps the most important milestones in cosmology were Galileo and Newton's developments of astronomical telescopes, Einstein's relativity theories, Hubble's discovery of the expansion of the Universe, and the Hubble Space Telescope.
The Cosmological Principle
Observations of the distributions of objects such as galaxies show that on a 'small' scale (a few million light years) there is clustering, and clusters of clusters; on larger scales, the Universe is isotropic (on average, the same in all directions) and homogeneous (on average, all places are alike at any time). These conditions are known as the cosmological principle.
The cosmological principle applied to an expanding universe implies uniform expansion such that the separations of all pairs of objects must increase proportionately in any given time period; so further objects recede faster. To help you visualise this, imagine many dots painted on a balloon, which is then blown up. Dots close together separate slower than dots further apart.
In the Big Bang Theory, the observable universe began with an instantaneously expanding point, roughly 15 billion years ago. Since then, the universe has continued to expand, gradually increasing the distance between our Galaxy and external galaxies. This expansion also cools the microwave background radiation (heat left over from the Big Bang), which today has a temperature of 2.728 Kelvin.
Gravity and Space-Time
On the vast energy, distance, and time scales that govern the evolution of the universe, gravity dominates all other forces. Therefore, cosmology is very much concerned with the study of gravity and its interaction with energy and matter.
Albert Einstein's theory of special relativity, published in 1905, showed that energy and matter are different aspects of the same phenomenon and can be transformed into each other in terms of the relationship E = mc2 (where m is the mass of an object, and c is the velocity of light).
Einstein's Theory of General Relativity, published in 1917, leads to field equations that don't allow a static universe, in their simplest form (The field equations describe the curvature of space-time by matter). They provided a new way to view gravity as a warping of the four-dimensional space-time continuum by the presence of matter. If space-time is imagined as a rubber sheet, then massive objects such as stars and galaxies create deformations in it. Thus the shape of space-time determines the behavior of matter/energy. At the same time the presence of matter/energy determines the shape of space-time. Many of the major questions of cosmology concern the geometry of space-time itself.
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The universe is everything there is that can be detected by physical effects, from the Earth beneath our feet, to the Sun in our sky, to other worlds in the farthest reaches of Space. It contains billions of 'island universes' called Galaxies, which are huge clusters of stars.
The galaxies are all moving away from each other, because the universe is expanding as a result of the Big Bang - a kind of 'explosion' which happened some 15 billion years ago. This singular event created space, time, and matter/energy.
There are at least 100 billion galaxies, each with approximately 1 billion stars. How many of those stars have planets? How many of those planets have intelligent life? These are some of the questions of astronomy and SETI.
The scientific study of the universe is called cosmology. Some scientists speculate that there is more than one universe; this concept of the multiverse (or parallel universes) arises from quantum physics.