Universe

The universe is all of space and time and their contents, including planets, stars, galaxies, and all other space objects in forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. According to estimation of this theory, space and time emerged together 13.8 billion years ago, and the universe has been expanding ever since. While the spatial size of the entire universe is unknown it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day. The most familiar galaxy is the Milky Way which includes the Solar System; The Solar System consists of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe.

Of known laws of physics presumably broken, it is owned by the law of gravity. Thus, dense particles tend to collapse in on themselves at some point to create black holes. After some unknown force caused the expansion the force of gravity is thought to have overcome the force of expansion in local pockets to form all the particular bodies and objects seen today. Meanwhile, the laws of physics also indicate that the universe is expanding at an accelerated rate, leading to an assumption of a size far greater than that which has been detected with the most powerful telescopes in existence. Once having determined that the earth orbits the sun, the distance to the local star was determined to be about 93 million miles. In an orbit among relatively stationary local stars, geometry has been used to determine vast distances between stars and galaxies. Based on assumptions hammered out among scientists over the years, the observable universe is a sphere, from the viewers perspective. However, this is is based on measurements of expansion of visible things. Theoretically, the size is thought to be infinite.

Within this observable universe, there are an estimated 100 billion galaxies with an average of around 30 billion stars each. Around many of these stars there revolve planets such as those in the planetary system or star system. These planets often are accompanied with moons. Interspersed in the local system, at least, are smaller bodies known as asteroids and comets. Smaller bits of material, which are mostly debris from deteriorating comets, often enter the atmospheres of the planets, mostly burning up before impact with the surfaces. Impacts do occur, as is readily visible upon the earth's moon.