Universe is the name that we use to describe the collection of all the things that exist in space. It is made of many millions of millions of stars and planet sand enormous clouds of gas separated by a gigantic empty space which is called the universe.
Astronomers can use telescopes to look at very distant galaxies. This is how they see what the universe looked like a long time ago. This is because the light from distant parts of the universe takes a very long time to reach us. From these observations, it seems the physical law sand constants of the universe have not changed.
Physicists are currently unsure if anything existed before the Big Bang. They are also unsure whether the size of the universe is infinite.
History of the universe
Many people in history had ideas to explain the universe. Most early models had the Earth at the centre of the Universe. Some ancient Greeks thought that the Universe has infinite space and has existed forever. They thought it had a set of spheres which corresponded to the fixed stars, the Sun and various planets. The spheres circled about a spherical but unmoving Earth.
Over the centuries, better observations and better ideas of gravity led to Copernicus’s Sun centred model. This was hugely controversial at the time, and was fought long and hard by authorities of the Christian church (see Giordano Bruno and Galileo).
The invention of the telescope in the Netherlands, 1608, was a milestone in astronomy. By the mid-19th century, they were good enough for other galaxies to be distinguished. The modern optical (uses visible light) telescope is still more advanced. Meanwhile, the Newtonian dynamics (equations) showed how the Solar System worked.
The improvement of telescopes led astronomers to realize that the Solar System is in a galaxy made of billions of stars, the Milky Way, and that other galaxies exist outside it, as far as we can see. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the systematic red shift of galaxies led to the conclusion that the Universe is expanding
How big is the universe?
The universe is about 13.8 billion years old, so any light we see has to have been travelling for 13.8 billion years or less – we call this the observable universe. However, the distance to the edge of the observable universe is about 46 billion light years because the universe is expanding all of the time.
What is the universe made of?
Astronomers like to call all material made up of protons, neutrons and electrons “Bar tonic matter”. Until about thirty years ago, astronomers thought that the universe was composed almost entirely of this “Bar tonic matter”, ordinary atoms.