Why are we here?

It is a question that has been asked many times in classrooms and lecture halls around the world, but it does have a deeper meaning in physics and cosmology. Why should it be that the Universe, with seemingly random laws and variables, should happen to suitable for the formation of life? With so many possible states of the Universe that are completely sterile, why did it just happen to form in the perfect way to provide a home for beings like us?

This is one of the big questions of modern science, known as the fine tuning problem, and the answer is that we just don't know. Life can only form if the fundamental constants of nature take on very precise values, and yet these values seem to be random.

Consider for example the strengths of the fundamental forces of nature. If the force of gravity were slightly changed, stars do not form and so the heavy elements do not form, or if changed too much the Universe itself collapses before structures such as planets and galaxies form. If the electromagnetic force were slightly stronger, or slightly weaker, then chemical reactions behave in different ways and either molecules do not form or they become too fragile for complex structures to form. Changing the strength of the weak nuclear interactions by too much will cause some elements to decay slower and others to decay faster. And yet the latter two are known from the (now proven) Higgs model to be a result of random symmetry breaking in the first moments after the Big Bang, while some speculative models suggest that the strength of gravity is also a random value selected by nature during the formation of the Universe.

There are similar (though weaker) arguments regarding the relative density of matter, dark matter, and dark energy in the Universe. Certainly if there is too much dark energy present, the Universe would tear itself apart before any complex structures can form - and there would certainly be no time for life to evolve. And a large change in the amount of dark matter or ordinary matter present in the Universe could either create too few stars to generate the heavier elements, or too many black holes that destroy the existing stars and planets. However these arguments are admittedly weaker than those regarding the fundamental forces, because they require larger changes to the fundamental constants, and involve theories and phenomena that are not well understood. It is quite possible that dark energy and dark matter have the values they do because of their as yet unknown nature, with no fine tuning required. (for example, there are models in which higher densities of dark matter decay quickly until they reach the right level)

It must also be said that these arguments are disputed. We do not fully understand all of the laws of physics, and it may be that the lifespan of the Universe and the probability that stars can form are not as sensitive to the fundamental constants of nature as current models predict. At the same time though, we do know that many of these constants are the result of random events during the Big Bang, and it is unlikely that every possible combination of constants yields a Universe suitable for life. It is actually quite unlikely that the majority of possible combinations will yield some form of life - carbon based or otherwise.

So why is the Universe perfect for life? Why did the random events during the Big Bang generate a set of fundamental laws and constants that allowed for life to evolve? 

Why are we here? In the next few pages I will try to provide a brief overview of a few of the best theories we have on this issue.