Our observable universe can be defined as the spherical region of space from which light has had time to reach us during the 14 billion years since the Big Bang. It is the part of our physical reality that contains everything that exists which we can in principle observe. But is there anything that lies beyond our observable region or our Hubble volume? Well, given that our universe may be infinite in size with an intrinsic property known as inflation, chances for the existence of the Multiverse are extremely high. So, for physicists like Max Tegmark of MIT, the question is not whether the multiverse exists but rather how many levels it has. He has classified as many as 4 distinct types of Multiverses in his book “Our Mathematical Universe”. So, let’s take a brief look at what these different levels of the Multiverse are like!

The first, and the least controversial level of our Multiverse is simply a region of space that is too far away for us to have observed yet. The furthest we can observe in any direction is currently about 46 billion lightyears which is the distance that light has been able to travel since the beginning of the Big Bang. But just because we can’t see something, doesn’t necessarily mean that it doesn’t exist. In fact, the cosmic microwave background radiation allows us to perform extremely sensitive tests for whether our universe is finite, and so far, all the evidence seems to be against it. Now, since the flat infinite models fit the observed data, it is reasonable to assume that our universe is infinite, or at least sufficiently large, which makes the level 1 multiverse a trivial consequence of these tests and the cosmological theory of inflation. Furthermore, observations of our galaxy distribution and the microwave background radiation have shown that the arrangement of matter on large scales collapses to dull uniformity with no coherent structures larger than about a billion lightyears. If this is true, then there is no reason to believe that our part of the cosmic horizon is the only one with physical stuff or matter in it. So, with an infinite amount of space and matter available, the level 1 multiverse contains an infinite number of regions similar to our own observable universe. This means that there may be an identical copy of you somewhere out there in the multiverse as we speak! But not just 1 identical copy, but infinitely many identical copies of you may exist as there may be an infinite number of Hubble volumes identical to our own in the level 1 Multiverse. In fact, Max Tegmark estimates that if you traveled in a straight line for about 10^10^118 meters, you would come across an identical copy of our observable universe. But if you wanted to meet an identical copy of you, then the distance to that would be relatively less, about 10^10^29 meters away, which is still astronomically huge. So, we can think of observers living in Level 1 parallel universes as experiencing the same effective laws of physics but with different possible histories.