The fine-tuned Universe is the idea that conditions that allow life in the Universe can only occur with the tightly restricted values of the universal physical constants, and that if any of several fundamental constants were only slightly different the universe would be unlikely to be conducive to the establishment and development of matter, astronomical structures, elemental diversity, or life as it is presently understood.
The arguments relating to the fine-tuned universe concept involve the anthropic principle, which states that any valid theory of the universe must be consistent with our existence as human beings at this particular time and place in the universe. In other words, even if the actual probability of our universe that supports intelligent life may be very low, the conditional probability of supporting intelligent life, given our existence in it, is 1. Even if there could be other universes, less "fine-tuned" and so devoid of life, there would be no one there to observe them.
Fine-tuned Universe proponents argue that deep-space structures such as the Eta Carinae Nebula would not form in a universe with significantly different physical constants. Photo: HST / NASA / ESA.The premise of the fine-tuned universe assertion is that a small change in several of the approximately 26 dimensionless fundamental physical constants would make the universe radically different: if, for example, the strong nuclear force were 2% stronger than it is (i.e. if the coupling constant representing its strength were 2% larger), diprotons would be stable and hydrogen would fuse into them instead of deuterium and helium. This would drastically alter the physics of stars, and presumably prevent the universe from developing life as it is currently observed on the earth. However, many of the 26 constants describe the properties of the unstable strange, charmed, bottom and top quarks and mu and tau leptons which seem to play little part in the universe or the structure of matter. It seems unlikely that the precise values of these constants are important for life; at any rate they are not included in the usual discussion of fine-tuning.
Larry Abbott wrote: "the small value of the cosmological constant is telling us that a remarkably precise and totally unexpected relation exists among all the parameters of the Standard Model of particle physics, the bare cosmological constant and unknown physics." Victor Stenger has suggested that the fine-tuned universe concept can be interpreted as a "claim of evidence for divine cosmic plan" ": "As the argument goes, the chance that any initially random set of constants would correspond to the set of values that we find in our universe is very small and the universe is exceedingly unlikely to be the result of mindless chance. Rather, an intelligent, purposeful Creator must have arranged the constants to support life". Stenger in that paper is critical of the claims of the fine-tuning advocates and provides his own explanations highlighting the flaws in those claims and concludes that "The universe is not fine-tuned for humanity. Humanity is fine-tuned to the universe".
As modern cosmology developed, various hypotheses have been proposed. One is an oscillatory universe or a multiverse where physical constants are postulated to resolve themselves to random values in different iterations of reality. Therefore separate parts of reality would have wildly different characteristics. In such scenarios the issue of fine-tuning does not arise at all, as only those "universes" with constants hospitable to life (such as what we observe) would develop life capable of asking the question.
There are fine tuning arguments that are naturalistic, however, the assertion that the universe was designed to be fine-tuned is largely put forward by advocates of intelligent design and other forms of creationism. This apparent fine-tuning of the universe is cited by William Lane Craig as an evidence for the existence of God or some form of intelligence capable of manipulating (or designing) the basic physics that governs the universe. Craig argues, however, "that the postulate of a divine Designer does not settle for us the religious question."
Alvin Plantinga argues that random chance, applied to a single and sole universe, only raises the question as to why this universe could be so "lucky" as to have precise conditions that support life at least at some place (the Earth) and time (within millions of years of the present).
Based upon the Anthropic principle, physicist Robert H. Dicke proposed the "Dicke coincidence" argument that the structure (age, physical constants, etc) of the universe as seen by living observers is not random, but is constrained by biological factors that require it to be roughly a "golden age".
Critics argue that the fine-tuned universe assertion and the anthropic principle are essentially tautologies. The fine-tuned universe argument has also been criticized as an argument by lack of imagination because it assumes no other forms of life, based upon alternative biochemistry, are possible. In addition, critics argue that humans are adapted to the universe through the process of evolution, rather than the universe being adapted to humans. They also see it as an example of the logical flaw of hubris or anthropocentrism in its assertion that humans are the purpose of the universe.-source wikipedia