How large is the observable universe is depend on how far back in time we can see?.
Universe is unimaginably large. According to our everyday experience, to say that something is more or less large involves comparing its size with a ruler to measure. Therefore, it involves measuring distances to determining the size of an object regardless of its form,.
So we can ask ourselves what is the maximum distances that are able to measure in the universe. But there are several problems. One of them is that, finding the two most distant points. To figure out these points we useses some other mechanism rather using any length measuring tools.
In astronomy, our measuring instruments are telescopes. Using these telescopes scientists collect information that comes in the form of electromagnetic radiation emitted by different objects, predominantly galaxies, spread across the vast and wide cosmos.
So, to measure how far objects are located farther from the universe, astronomers require powerful telescopes. And then they need to point them to different parts of the sky, wait long exposure times and analyze the light from objects which are located in “more remote” areas.
Surprisingly the light comes from far remote has some evenly distributed data set. Farther we go with the light captured in the telescopes, it become blurry until up to some edge.
The observable universe
The light has a finite speed, very high, but finite. The light that is emitted from a cosmic object, takes time to reach to our telescopes. Until it reaches our telescope time elapses.
For example, the Sun is eight light minutes away from us. A photon emitted from the sun, takes about eight minutes to travel the 150 million kilometers specace between the sun and the earth. The light came from the closest star to the us takes just over four years to reach. If all the stars in the Andromeda galaxy went out suddenly, still would enjoy our neighboring galaxy in the sky about two and a half million years, since it’s locates 2.5 million light years away.
According to the most widely accepted cosmological model the Big bang, universe was born 13,700 million years ago. So the light from a galaxy that is more than 13,700 million light-years away from us have not yet had time to get to any of our telescopes and therefore remains invisible to our eyes.
One could then think about we can define an imaginary sphere of 13,700 million light years, so that any object outside this border would remain, so far, inaccessible to us. We would have no direct information on the existing universe beyond this border. It could be infinitely large or not, or not even exist. It seems that he is “causally disconnected”. Of course, every year we would know a little more of it, because as time elapses and the universe grows older our imaginary border would grow.
Well, this concept is called observable universe.
And indeed it comprises the directly accessible universe but, the universe is not static. It expands.
Galaxy are moving away from each other.
Based on the calculation the edge of the observable universe also moved away from us about about 46.6 billion light years.