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    Default Universe notes

    Universe
    The universe is commonly defined as the totality of everything that exists,including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space.Definitions and usage vary and similar terms include the cosmos, the world and nature. Scientific observation of earlier stages in the development of the universe, which can be seen at great distances, suggests that the universe has been governed by the same physical laws and constants throughout most of its extent and history. There are various multiverse theories, in which physicists have suggested that our universe might be one among many universes that likewise exist.

    Structure Of Universe
    Think of a very large ball. Even though you look at the ball in three space dimensions, the outer surface of the ball has the geometry of a sphere in two dimensions, because there are only two independent directions of motion along the surface. If you were very small and lived on the surface of the ball you might think you weren't on a ball at all, but on a big flat two-dimensional plane. But if you were to carefully measure distances on the sphere, you would discover that you were not living on a flat surface but on the curved surface of a large sphere.The idea of the curvature of the surface of the ball can apply to the whole Universe at once. That was the great breakthrough in Einstein's theory of general relativity. Space and time are unified into a single geometric entity called spacetime, and the spacetime has a geometry, spacetime can be curved just like the surface of a large ball is curved.
    When you look at or feel the surface of a large ball as a whole thing, you are experiencing the whole space of a sphere at once. The way mathematicians prefer to define the surface of that sphere is to describe the entire sphere, not just a part of it. One of the tricky aspects of describing a spacetime geometry is that we need to describe the whole of space and the whole of time. That means everywhere and forever at once. Spacetime geometry is the geometry of all space and all time together as one mathematical entity.


    Age Of The Universe
    The age of the universe is the time elapsed since the Big Bang. The best current estimate of the age of the universe is 13.75 ± 0.11 billion years(4.339 ± 0.035 ×1017 seconds) within the Lambda-CDM concordance model. The uncertainty range of 0.11 billion years has been obtained by the agreement of a number of scientific research projects, such as microwave background radiation measurements by Wilkinson Microwave Anisotropy Probe and other probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time.


    Space
    Have you ever gone out at on a cloudless night, when there is no bright Moon in the sky or city lights to obscure your view, and been blown away by the astonishing number of stars in the sky? Probably for as long as people have been around, to look up into the sky, we have wondered what the many spots of light in the sky mean. Thousands of years ago, the earliest civilizations observed the heavens. Early man observed the heavens because the Sun, Moon and stars gave indication of coming seasons to farmers and hunters. The sky aided in navigation especially for nomads and sailors. And many ancient civilizations thought the sky gave signs of life, war, earthquakes, the fate of kingdoms...and more. Since the invention of the telescope, we have been able to "see" further away and study stars and galaxies, as well as many of the more mysterious objects in our Universe.


    Galaxy
    A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter.The word galaxy is derived from the Greek galaxias literally "milky", a reference to the Milky Way galaxy. Examples of galaxies range from dwarfs with as few as ten million (107) stars to giants with a hundred trillion (1014) stars, each orbiting their galaxy's own center of mass.
    Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.


    Milky Way Galaxy
    The Milky Way is the galaxy that contains the Earth.This name derives from its appearance as a dim "milky" glowing band arching across the night sky, in which the naked eye cannot distinguish individual stars.The Milky Way appears like a band because it is a disk-shaped structure being viewed from inside. The fact that this faint band of light is made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s, observations by astronomer Edwin Hubble showed that the Milky Way is just one of many galaxies.The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200–400 billion stars. It may contain at least as many planets, with an estimated 10 billion of those orbiting in the habitable zone of their parent stars.The Solar System is located within the disk, around two thirds of the way out from the Galactic Center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The stars in the inner ≈10,000 light-years are organized in a bulge and one or more bars. The very center is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole. The Galaxy rotates once every 15 to 50 million years. The Galaxy as a whole is moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster.

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