1 Light Year = 9,500,000,000,000 km
Scientists have determined the age of the universe is 13.7 Billion years old.
Then a question arises how big should be a visible universe?
Then a simple thing comes to our mind that we can't see before its beginning that is before the big bang. Therefore the observable universe consists of a sphere with a radius of 13.7 billion light years. Because when the big bang happened only light was there, which then traveled through the cosmos. Now we are seeing it arrive which means the light may travel 13.7 Billion light years before hits the earth.
However, we are talking about the universe no logic in the universe is that simple.
The thing may be that simple if the universe is static. Then the distance between cosmic objects isn't changing.
However, the truth is the universe itself is expanding. After the very next seconds of the big bang the universe looks like microwave background radiation everywhere. Imagine the current location of Earth at that time. Let that be the point around which a sphere is there (Earth Point). The sphere is the origin of the cosmic microwave background radiation hitting the earth right now. That radiation moves towards the earth at the speed of light. In addition to this, it took 13.7 Billion years to get to the point. This may be called the critical sphere.
If we take a sphere smaller than that sphere the light has already passed the Earth and we will be unable to see it. If we take a bigger sphere, then the light from it doesn't touch the earth's point yet. So we are unable to see it either.
When this light was emitted shortly after the Big Bang the sphere radius was about 43 Million light years away. In a simple way, we expected that the light would reach the earth point in 43 Million years. However, it took 13.7 Billion years to reach the earth point. The reason it took more time is the universe itself is expanding. One more thing the universe must travel much faster than the light. Because for a distance of 42 Million light years, light takes 13.7 billion years of time.
Now remember the critical sphere from which the microwave background radiation is originally emitted also expanded its size and now the radius becomes 46 billion light years. That means that line is 41 Billion light years away from the earth point.
We can currently see object having a distance 46 billion light years away but we see them as they were in the past distance and in the form of how it was then. now those radiation or energy becomes starts we can't see those.
Any cosmic object that is currently outside of the sphere of radius 15 billion light years, we will never be able to see now no matter how long we wait. One more worst thing is we constantly lose approximately 20,000 stars per second. That means we are able to see the stars' emitted photons now but unable to see the stars' emitted photons after the very next seconds.
