What do we know about the Universe?

The Universe is everything we can see, touch, feel, sense, measure or detect. It includes living things, planets, stars, galaxies, dust clouds, light, and even time. Before the birth of the Universe, time, space and matter did not exist.

There are billions of galaxies in the Universe, galaxies are made of millions of stars. Perhaps, most of these stars have planets around them, just like our Solar System. The space between galaxies is almost empty: only few atoms of hydrogen per cubic centimeter. Space also has radiation, like the light and heat, magnetic fields and high energy particles,

The Universe is incredibly huge. It would take a modern airplane more than a million years to reach the nearest star to the Sun. Travelling at the speed of light (300,000 km per second), it would take 100,000 years to cross our Milky Way galaxy alone.

No one knows the exact size of the Universe, because we cannotsee the edge – if there is one. The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at very high speed. So the area of space we now see is billions of times bigger than it was when the Universe was very young. The galaxies are also moving further apart as the space between them expands.

(Adapted from ESA Kids.)

As you have read the Universe is so big that we need new units of measurement; Two of them are the light year and the astronomical unit. What is a light year? What is an astronomical unit?

Theories about the Universe

Long time ago, ancient Greek placed the Earth at the center of the Universe. This theory was supported by two observations. The first one was that the stars, the Sun, planets and stars appear to revolve around the Earth each day, making our planet the center of that system. The second one was that the Earth does not seem to move for people.

They put the Moon and planets on concentric crystalline spheres that revolved at different velocities around the Earth. There was also a big sphere with all the fixed stars.

The order of these bodies was: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn and fixed stars. Today we know that there are more planets, as Uranus and Neptune, but Greeks couldn’t observe them because they are too far from us.

But soon, as observations improved, this simple model could not explain the movement of the planets: they usually moved in one direction, but sometimes they seemed to go back in the sky.
In the Sixteenth Century, a new theory called heliocentrism offered new explanations. Can you tell us who discovered it? What are the most important statements of that theory? I hope your answers.