Billions of years have passed since the big bang that created our universe, to be precise 13.75 billion years, but still, we can’t observe the whole of the universe. As the universe is continually expanding a photon emitted at the moment of the big bang could have traveled 46.1 billion light years. So, we can’t observe any object in our universe which is farther than 46.1 billion light years, and a universe beyond 46.1 billion light years is an unobservable universe. 46.1 billion light years is the cosmic limit of the observable universe. Beyond the cosmic limit lies the unobservable universe. An observable universe is finite but an unobservable universe can be infinite because our universe is continually expanding. The diameter of the observable universe is 92 billion light years.
The extent of the observable universe is increasing. With time many galaxies which were not visible before have become visible. The big bang happened just 13.75 billion light years ago but objects as far away as 46.1 billion light years are becoming visible now as light from these objects are reaching us only now. This is because the expansion of the universe has been increasing the distance between Earth and such objects continually. Hypothetically speaking, if the distance of Earth from any such object was 13.75 million light-years at the moment of the big bang then it will be 46.1 billion light years now, due to the expansion of the universe. The expansion of the universe has also pushed many galaxies and stars out of the observable universe into an unobservable universe. These galaxies have not evaporated but have gone out of the cosmic limit of the observable universe.
As the observable universe is finite it has an edge but beyond the edge lies the unobservable universe. As an unobservable universe can’t be observed it is impossible to say that it also has an edge just like a finite observable universe. If someday it is proved that the unobservable universe also has an edge then the world would easily infer that our universe has an edge. Till then the infinite status of the universe will be secure. Till now nothing more than hypotheses are available about the unobservable universe. One hypothesis is that an unobservable universe is just like an observable universe but the universe is infinite. Another hypothesis is that there are multiple universes like ours and collectively they form a multiverse.
There is ample evidence that an unobservable universe exists beyond the cosmic limits of the observable universe. Otherwise, new galaxies would appear from nowhere, and similarly visible galaxies would not disappear into thin air. That part of the universe from where light has not reached us is an unobservable universe and chunks of it become part of the observable universe as and when light reaches us from said chunks.
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