The Size of the Universe Compared to the Probability of Life Existing

When considering the size of the observable universe and the probability of life existing, we must first understand the hard numbers at play. The observable universe, the part of the cosmos visible from Earth or any other celestial body, is indeed finite and has a specific size. Its diameter, which is double the proper distance to the edge of the observable universe, is 91 billion light-years. This understanding is based on precise measurements and calculations derived from the age of the universe, currently estimated to be 13.8 billion years, and the equations of General Relativity.

Understanding the Probability of Life

The probability of life existing is often interpreted as 1.0 or 100%, because we know that life is indeed present on Earth. However, the question we might want to explore is more specific: what is the probability that life could evolve given the parameters of the universe and the laws of physics?

Universe of Variables

According to the Standard Model encapsulated in the Lagrangian, several free parameters define the universe's fundamental forces and interactions. Of these, 17 free parameters are particularly significant in determining whether the universe could harbor complex chemistry and, by extension, life.

If these parameters were randomly chosen from a uniform density, the probability of them resulting in values that permit the evolution of complex chemistry and life (or, for that matter, stars) would be extremely low. The estimated probability is as low as one in (10^{120}). To put this into perspective, consider a scenario where 200 dice are rolled; landing all on the maximum (boxcars) is about as likely.

Interpreting Fine-Tuning

Such a low probability has led many to discuss the fine-tuning problem. Essentially, why are the parameters of the universe so precisely tuned? There are several views on this:

God: Some posit that it's a divine creation with specific parameters designed to support life. It happened. We're here: This interpretation suggests that if the parameters were not finely tuned, no one would be here to ask the question. Essentially, any universe with life will eventually form a life form to ask these questions. Non-uniform Probabilities: Another perspective is that the probabilities are not uniformly distributed. With a more complex distribution, the parameters just had to fall within the right range. This view is supported by physicists like David Gross, who argues that as we discover more about the underlying physics, we will find that the parameters simply had to be this way. Multiverse Theory: Leonard Susskind's The Cosmic Landscape posits that the universe has undergone multiple iterations, with a vast number of "experimental runs." In this scenario, through sheer volume, some of these runs would have the right conditions for life.

Conclusion

While the universe's vast size and the specific parameters that allow for the existence of life are fascinating, the answers to these questions are not yet fully settled. Most physicists believe that either the non-uniform probability distribution (view 3) or the multiverse theory (view 4) is more plausible, but they argue that views 1 and 2 are too much like cheating, abandoning the search for fundamental answers.

The true scale and nature of other universes, if they exist, remain unclear. However, the ongoing research into the cosmic microwave background offers hope for better understanding the origins and characteristics of our universe.

The mystery and wonder inherent in these questions continue to drive scientific exploration and philosophical inquiry.