Universe is big is a well-known fact. But when it comes to being big, how big? How is it measured? It is big with reference to what? Is universe really infinite? There should always be a reference point which helps infer how big our universe is. If our universe is infinite then why is it termed as big? Scientists have proposed that universe is expanding. The size of universe includes the number of elements existed in that and their sizes.
The distances between objects in space are way beyond your imagination. The reference with which the universe is measured is called as Supernova. Edmund Halley discovered a method to find out the distance between sun and Venus. This was discovered around three decades earlier. He postulated that Venus very rarely passes between sun and earth. It happens some where once in 121 years. There is transit of Venus which is a very rare event. Knowing the approximate distance between the sun and the earth helped in determining the scale of our solar system.
Based on the distance between and the light year calculations astronomers have predicted the age of universe to be 13.8 billion years. Light takes time to travel. While the light is travelling the universe is expanding is what astronomers have inferred. So when it comes to finding out how big the universe is, there is no single reference with which the distances can be measured. Generally astronomers concentrate on the red shift measure by z, than just finding the distances in the spaces. It is like if the red shift is more the distance is also more. By using the red shift we can calculate how much time the light takes to travel. Based on the astronomic calculation the universe is 29 billion light years away.
Universe has no limit. Beyond our Milky Way galaxy there is a huge expansion of galaxies. More and more galaxies are discovered deep in space. Finding the distance between the galaxies is a tough job. But it can be done to some extent with the help of exploding star called supernova. Based on the brightness of the star the distance can be estimated.
So scientists say that it is difficult to say how big the universe is. If the universe is infinite there is no point in finding out how big universe would be. No one knows whether the universe is infinite or not. Even though it is expanding there is no clue about in what pace it is expanding.