What do we know about the universe? Where does it come from and where is it ending to ?Does it have a beginning and if yes, what happened before it? Will it come to an end? These are some of the questions that mankind asks since ancient times.
In 340 AC ,the greek philosopher Aristotel , offered arguments in his book "About Heavens" that the earth is a round sphere and not a disc.(he realized for example that the moon eclipses were produced by the earth, which was located between the sun and moon. The shadow of the earth on the moon was always round ,this being true only if the earth was sphere shaped.)
Aristotel also believed that the earth was immovable and the sun ,the planets and the stars were all moving around it on circular orbits. This idea was elaborated by Ptolomeu in the 2 nd century BC ,in a complex cosmological model.
Centuries later, in 1514,a polish priest, Nicholas Copernic, proposed a simpler model. His idea was that the sun was stationary in the center and that the all of the planets were moving on circular orbits around the sun. .It took almost a century before 2 astronomers Johannes Kepler and Galileo Galilei started to support Copernic's theory, in spite of the fact that the orbits that he predicted didn't exactly matched with what was being observed. The final blow to the aristotel-ptolemeu theory was given in 1609, when Galilei started observing Jupiter with the new invention called the telescope. He observed that Jupiter had satellites that were spinning around the planet. This meant that not every planet had to spin around the earth, like Aristotel and Ptolemeu believed. In the same time, Kepler modified Copernic's theory ,suggesting that the planets weren't moving on circular orbits, but on elliptical ones. Now finally the predictions matched the observations.
But an explication of why the planets move so was given much later in 1687, when Sir Isaac Newton published his book Philosophiae Naturalis Principia Mathematica. Not only did Newton presented a theory about the way that planets move in space and time, but he also introduced the mathematical device necessary for analyzing these movements. He also postulated a universal gravity law according to which every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masse of the particles and inversely proportional to the square of the distance between them.Translating into an equation ,we have Fg=G* m1* m2/r2 (law of gravity);G is a fundamental physical constant called the gravitational constant.The presently accepted value (in SI) is G=6.673(10)*10-11N*m2/kg2
Before the 20 th century, none suggested that the universe was expanding or shrinking. It was generally accepted that the universe existed all the time in an non-altered condition or it was created at a certain time in the past ,more or less in the way we see it now. Even those who understood that Newton's theory predicted that the universe can't be static never thought of suggesting that it can be expanding. But in 1929, Edwin Hubble made the crucial observation that ,wherever we look, the galaxies situated at a far distance are moving away from us. In other words, the universe is expanding. This means that, at the beginning, everything was situated in one spot. In fact ,12 billions years ago ,the universe was concentrated in one spot, with an infinite density. This discovery finally explained the beginning of the universe.
Scientists from our days describe the universe with the help of 2 theories - general theory of relativity and quantum mechanics .They represent the great intellectual achievements of the first part of the century. The general theory of relativity describes the gravity force and structure if the universe at a large scale, from a few kilometers to millions of millions of millions of millions of kilometers. On the other hand, quantum mechanic works at a very small scale ,something like 1 /1012 of a centimeter.
One of the major efforts of today's physics is to search for a theory that combines the both- a quantum theory of gravity.