Our solar system is a collection of planets orbiting a star. Our Sun is gravitationally bound to a huge collection of other stars. This collection of stars is called a galaxy. Our galaxy is called the Milky Way. There are lots of galaxies in the Universe.
There are various theories in existence about how the Universe started and what will happen to it. Most cultures have creation myths to explain the beginning of the Universe.
Most scientists today believe in a theory called the Big Bang.
The Big Bang theory
There is evidence that the Universe is expanding.
This evidence is found when we look at the electromagnetic radiation emitted from other galaxies. It is normally red-shifted. This means the wavelength of the electromagnetic radiation appears longer than it actually is.
This is a result of an effect known as the Doppler Effect. This effect makes waves appear to have a longer wavelength if their source is moving away from you. It also means the wavelength will appear shorter if something is moving towards you. You can hear this when a police car goes past you, with its siren on. As the car approaches you, the noise is a higher pitch than when the car is moving away from you.
One explanation for this is that all the energy and mass in the Universe all started in the same place and exploded apart. This flung all matter outwards and even now scientists believe it is still all moving away from the initial explosion.
Just after the Big Bang, scientists believe everything was energy, but soon after, the energy began to turn into mass making small particles - like the electron. It would have been incredibly hot at the start, but cooled down as it expanded. Scientists have measured the background temperature of the Universe with a satellite and found it to have a temperature just a couple of degrees above absolute zero (-273 degrees Celsius). Eventually, protons were able to join with electrons and hydrogen was made. This led to the birth of the first stars.
We think the Universe is between 11 billion and 18 billion years old, but scientists often change their estimate.
Scientists are still arguing about this, but there are three main possibilities:
- If there is too much matter in the Universe, the gravitational attraction between all the masses will eventually pull all the galaxies back together into a big crunch. This is called a closed universe. This may lead to another Big Bang setting up an oscillating universe that keeps expanding and contracting.
- If there is not enough matter in the Universe, everything will keep moving apart forever. This is called an open universe.
- If there is just the right amount of matter in the Universe, the expansion of the Universe will slow down until it is expanding so slowly that it will appear the Universe has stopped expanding. This is called a flat universe.