By Max Tegmark
Is there another copy of you reading this article, deciding to put it aside without finishing this sentence while you are reading on? A person living on a planet called Earth, with misty mountains, fertile fields, and sprawling cities, in a solar system with seven other planets? The life of this person has been identical to yours in every respect–until now, that is, when your decision to read on signals that your two lives are diverging.
You probably find this idea strange and implausible, and I must confess that this is my gut reaction, too. Yet it looks like we will just have to live with it, since the simplest and most popular cosmological model today predicts that this person actually exists in a galaxy about 1029 meters from here. This does not even assume speculative modern physics, merely that space is infinite and rather uniformly filled with matter. Your alter ego is simply a prediction of the concordance model of cosmology, which agrees with all current observational evidence and is used as the basis for most calculations and simulations presented at cosmology conferences. In contrast, alternative cosmological models, such as a fractal universe, a closed universe, and a multiply connected universe, have been seriously challenged by observations.
The farthest you can observe is the distance that light has been able to travel during the 14 billion years since the Big-Bang expansion began. The most distant visible objects are now about 4—1026 meters away, and a sphere of this radius defines our …