Source : bbc
There is another reason why the speed of light is such a natural unit to measure of distance for astronomers. Information about the universe comes to us almost exclusively through different forms of light, and all light travel at the speed of light, i.e. 1 light year per year. This places a limit on how quickly we can learn about events in the universe. If a star is 100 light years away, the light we see tonight left that star 100 years ago and is now reaching our neighborhood. We can learn about every change in this star is 100 years later. For a star 500 light years away, the light we discovered tonight is 500 years away, bringing news from 500 years ago.
Since many of us are used to instant messaging from the internet, it can be frustrating for some. When i see up there, “you ask”, won’t I know what is really going on for another 500 years?
However, this is not the most sensible way of thinking about the situation. For astronomers, light is now reaching us here on earth. There is no way of knowing anything about this star (or any other object) until its light reaches us.
But what may seem like a huge frustration at first is actually a huge benefit. If astronomers really want to reconstruct what has happened in the universe since it was formed, they need to find evidence for every epoch (or period) in the past. There we can find evidence today of cosmic events that took place billions of years ago?
The delayed arrival of light provides an answer to this question: the further we look into space, the longer it takes for light to get here and the longer it has left its place of origin. Astronomers look back billions of years. In this way we can reconstruct the history of the cosmos and get an idea of how it has evolved over time.
This is one of the reasons astronomers are eager to build telescopes that can collect more and more faint light in the universe. The more light we collect, the weaker or fainted the objects that we can observe. On average, weaker(fainted) objects are further away and can therefore tell us about even deeper periods in the past. like the Hubble Space Telescope and the Very Large Telescope in Chile gives astronomers better views of space and deep time than we have ever had before.
