The Universe


The universe is about 93 billion light years in diameter. The Big Bang theory is the prevailing cosmological description of the development of the Universe. Under this theory, space and time emerged together about 13.799 billion years ago with a fixed amount of energy and matter that has become less dense as the Universe has expanded. There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible.

Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist. The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. A black hole is created when big stars explode. Its gravitational force is so strong that nothing can escape from it – luckily the closest black hole is about 10,000 light-years from Earth.