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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.
Some of the objects in the night sky are actually just the planets in our solar system reflecting the sun's bright light. Other objects may be comets or a whole galaxies of stars. Though these objects seem close together in the sky, we know that they are actually a vast distance apart.
The major components of the universe are the galaxies, stars and stellar groupings, and nebulae (clouds of interstellar gas and dust). Smaller constituents include the solar system and any other assemblage of planets, satellites, comets, and meteoroids revolving around a central star. In addition to such objects and scattered matter, the universe contains gravitational fields and various forms of radiation.
On a large scale, the universe is isotropic, i.e. uniform in every direction. It is expanding, at an apparently uniform rate - the galaxies are receding from us and each other. The origin, evolution, and future of the universe are modelled by several cosmological theories, notably The Big Bang, according to which the universe arose from an "explosion" of space-time some 15 billion years ago.
Expansion of the Universe
Atoms of every element emit and absorb a distinct spectrum of colors. By studying the spectral lines, astrophysicists can discover the composition of stars and other objects in space. Moreover, when a light source moves toward an observer, the light waves it emits squash together, appearing bluer. Conversely, when the source moves away, the light waves stretch out and appear redder. The expansion of the universe "stretches" light rays converting blue light into red light and red light into infrared light. Distant galaxies, which are rapidly moving away from us, appear redder.
This expansion was firmly established in 1931 by Hubble and Humason - the distances and radial velocities of galaxies increased in direct proportion - now known as Hubble's Law. Hubble used red shift measurements to deduce that galaxies are speeding away in all directions. He was not the first person to notice that the light from distant stars is red-shifted, but he was the first to notice that the farthest galaxies had the most extreme red shifts and were receding much faster than nearby galaxies. At the edge of the known universe, objects fly away at close to the speed of light.
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.
Density of the Universe
Gravity slows the expansion of the universe. Mutual gravitational attraction between matter in the Universe decelerates the expansion, but whether its enough to reverse it (to a 'Big Crunch') isn't yet known. If the universe is dense enough, the expansion of the universe will eventually reverse and the universe will collapse.
There is one critical value which would mean that the Universe will expand for a long time, gradually slowing down and then reach a steady state.
A mass less than this value will mean that the Universe will go on expanding for ever - an open universe. If the density is not high enough, then the expansion will continue forever.
A greater value will mean that the Universe will expand to a maximum size and then will start to contract -- eventually returning to a very small volume - a closed universe.
A flat universe has exactly the critical density between the other two - mathematically equivalent to a model by Einstein, later modified by W. de Sitter, called the Einstein - de Sitter universe.
The Mystery of the Missing Matter
For various reasons, cosmologists generally believe that we may only have observed a fraction of all the matter there is - the rest is known as 'dark matter'. Astronomers think that the mass of the Universe is equal to this critical value but can only `see' one tenth of the matter necessary to reach this value.
The same discrepancy is seen in the gravitational pull of individual galaxies and in clusters of galaxies. In the 1970s, astronomers led by Vera Rubin of the Carnegie Institution conducted research on the rotation of galaxies which led to the startling but seemingly inescapable conclusion that as much as 90 percent of the universe must be composed of matter that has not yet been detected. This matter has come to be called "missing mass," or "dark matter," but conceivably it may be not so much dark as transparent, signaling its presence only by its gravitational pull on other matter.
Structure of the Universe
Matter is not evenly distributed in the universe. It clumps into stars and interstellar gas clouds, which in turn group into galaxies. The galaxies congregate in "local groups," clusters, and superclusters. These clusters appear to be arranged in filaments, sheets, or bubbles (or all of these), and contain enormous voids hundreds of millions of light-years across.
Our Milky Way is part of the Virgo cluster, which may contain one thousand galaxies, and evidence exists that this cluster may itself be part of a larger entity. Study of such structures is closely connected to astrophysics, which studies the processes of star formation.
It is also linked to the search for dark matter, because astronomers are finding some galaxies that appear to have formed less than two billion years after the big bang. At that time in the evolution of the universe, gravity alone would not have provided sufficient clumping power to form such structures. Cosmologists believe that the early universe must already have had some structure or texture or that dark matter was already shaping the form of visible matter with its unseen hand, or both.
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.