Many people will tell you that the universe appears fine tuned for life. Don Page has decided to address this issue scientifically by calculating the best value for the cosmological constant needed to support life in the universe and then comparing it to our own. His conclusion is that the cosmological constant is actually an example that our universe is not fine tuned for life.
The cosmological constant is like a knob that affects how quickly the universe's expansion is accelerating or decelerating. As a rule of thumb, the more positive the constant is the faster the expansion accelerates and the more negative the more it decelerates. If it is zero, and there is just the right amount of matter, the universe just stays flat and we never experience a rapid acceleration or deceleration in expansion.
First, all positive values are bad. Now, what does this have to do with life? It turns out a positive cosmological constant, like our own, actually dilutes matter and prevents a lot of gravitational collapse making our universe less likely for life than if the constant were not positive. From the paper:
The reason is that a positive cosmological constant gives a repulsion between separate particles that reduce the ordinary gravitational attraction and leads to less gravitational condensation of matter. Therefore, other factors being equal, any positive cosmological constant decreases the fraction of baryons that condense to form galaxies and other structures that eventually form living substructures.
As an immediate consequence, no positive value of the cosmological constant (such as the observed value Λ) can maximize the fraction of baryons in lifeBut wouldn't a negative value also be bad? Yes, because if the value is too negative the universe recollapses and life doesn't have time to form. Page keeps this in mind while calculating the best value to find that Goldilocks region that is most optimal for life. That said, he does find that the optimal values for life in the universe are slightly negative on the order of Λ ~ -10-120.
So God created a Multiverse? Interestingly enough Page is very religious and so does not conclude this is evidence against God but actually evidence that God must have created a multiverse where each pocket universe has a different cosmological constant like most modern cosmology theories predict. From the paper:
It might be appropriate to note that although this paper has focused on the scientifically testable question of whether the constants of physics maximize a particular measure for life, it obviously also has theological implications. It could be taken as negative evidence for theists who expect God to ﬁne tune the constants of physics optimally for life. However, for other theists, such as myself, it may simply support the hypothesis that God might prefer a multiverse as the most elegant way to create life and the other purposes He has for His Creation.I for one am a big fan of the multiverse because all modern cosmological theories with inflation lead to a multiverse.
And religion aside, given our cosmological constant is such a bizarre value and currently seems to be best explained by multiverse models I will agree with Page that the cosmological constant seems to hint at a multiverse. (Which is why many respected theoretical physicists suggest the peculiar value for the cosmological constant is the best evidence so far for crazy multiverse models like the string landscape.)
Don N. Page (2011). Evidence Against Fine Tuning for Life E-Print arXiv: 1101.2444v1