The universe is every group of galaxies in the local area of stars.
Earth and the other planets, stars, galaxies, have space around them and the energy that comes from them are all part of what we call the universe. The universe as astronomers have calculated born between 8 and 16 billion years ago. It is formed probably 15,000 million (15 billion) years ago.
One of the most famous theory about the formation of the Universe is the Big Bang. During the Big Bang, all matter and energy, even space itself, were concentrated in a single point causing a tremendous explosion. A millionth after the Big Bang, the temperature of the Universe was over 10 million million degrees. It was contain large amount of energy in form of photons—little "packets", or particles of radiation. Under suitable conditions, high-energy photons can turn into particles of matter. And that is what happened in the early stages of the Universe. These particles of matter are protons, anti-protons, neutrons, anti-neutrons, electrons and positrons. These are some of the particles we know it today that make up the Universe.
All of the subatomic particles—protons, neutrons and electrons—had been formed by the instant it was created and the Universe began expanding and cooling. After about three minutes, its temperature had dropped to about 1,000 million degrees. Then protons and neutrons began combining to form the central cores, or nuclei, of atoms, such as helium. The Universe was then made up of radiation and matter, in the form of protons, helium nuclei and electrons.
After several hundred thousand years the temperature had fallen to about 3,000°C. The protons were now able to capture and hold on to electrons. The protons became atoms of hydrogen, and the helium nuclei became helium atoms. With fewer particles about and the Universe greatly expanded, radiation could travel over vast distances without being either absorbed or deflected. And the Universe have become transparent.
Another theory proposed is that after the Big Bang was the inflation followed by the Quark Era where quarks and electrons are formed and the quarks combine to become protons and neutrons while the electrons combine with these newly formed products of quarks to form atoms of hydrogen and helium—the basic materials of the universe. These basic materials later combine to form superclusters of galaxies which are racing away from all the the other clusters at incredible speeds.
Most astronomers agree about how the Universe began with a Big Bang and how it has evolved since then. They are not sure what will happen in the future. Certainly for many billions of years the Universe will continue to expand.
Most astronomers think that it will. They say we have an open Universe. The only thing that could prevent the galaxies flying apart forever would be gravity. For gravity to be powerful enough to do this, the Universe must have a certain mass. But there appears to be nowhere near enough mass in the stars and galaxies to halt the expansion.
However, astronomers know that stars and galaxies are not the only matter in the Universe. There is also dark matter which we are largely unable to detect. There is dark matter in the dust-clouds in space and in dead burned-out stars. There is also matter hidden in the abyssal depths of the black holes.
Other possible sources of dark matter are the atomic particles called neutrinos. Recent experiments have indicated that they might have a slight mass. If they have, they would greatly increase the density of the Universe because there are so many of them.
The Big CrunchEdit
If there is sufficient dark matter to halt the expansion of the Universe, then we have a closed Universe. Eventually the Universe will collapse in on itself and end in a Big Crunch. But things may not end here. Another Big Bang may be triggered off that will set the Universe expanding once again. In turn will come another Big Crunch, yet another Big Bang and so on. We will have an oscillating Universe.