The State of Inflation

Cosmologists (scientists who study the origin and structure of the universe) have long been intrigued by the fact that regardless where they point their telescopes what they find is that the universe is almost completely uniform, that is, its matter and energy are the same everywhere in the universe. Why that should be is a puzzle.

Another puzzle is that the expansion rate of the universe is almost perfectly balanced by the universe's mass density which exerts gravitational pull. This means that the universe should neither tear itself apart in an out of control expansion nor collapse back on itself but eventually reach a state of eternal equilibrium. This is called "flatness", and flatness requires an incredible degree of fine-tuning.

One theory that has been advanced to account for these remarkable features is cosmic inflation - the hypothesis that at the very beginning of the universe when it was only miniscule fractions of a second old, it underwent incredibly rapid expansion for a miniscule fraction of a second.

Inflation is still the consensus view among cosmologists but it's being subjected of late to some tough criticism. If the critics prevail it would have serious consequences for not only inflationary cosmology but also for the multiverse hypothesis. It would also powerfully strengthen the fine-tuning argument for the existence of God.

There's an interesting article on all this by Denyse O'Leary at Evolution News. Here's her lede:

Two features of our universe puzzle cosmologists: One is the horizon problem: The universe looks the same in all directions and the cosmic microwave background radiation is about the same temperature everywhere. As String Theory for Dummies puts it, “This really shouldn’t be the case, if you think about it more carefully.” Assuming that current measurements are correct, the radiation must have exceeded the speed of light if it really communicated in this way, but that is forbidden by the standard Big Bang model of the universe.

Then there is the “flatness problem”: “The matter density and expansion rate of the universe appear to be nearly perfectly balanced, even 14 billion years later when minor variations should have grown drastically” (Dummies). Inconveniently, the apparent 1:10^66 fine-tuning of the Big Bang, of which horizon and flatness are features, is frequently used as an argument for the existence of God.

Cosmic inflation theory, first proposed by Alan Guth in 1981, modified the Big Bang theory (the Standard Model) by proposing that the universe, instead of unfolding at a steady pace, expanded rapidly shortly after it was created, which could account for apparent fine-tuning.

The article goes on to discuss some alternatives to inflation while acknowledging that inflation remains the most plausible explanation for the fine-tuning of the Big Bang. It's the most plausible explanation, that is, if one's metaphysical commitments require that intentional design be ruled out as a possible option a priori. As O'Leary puts it:

All parties to the dispute assume, as a metaphysical stance, that science cannot address the possibility that the universe shows evidence of design. Even if design turns out to be the best explanation and the most fruitful for progress, it cannot be accepted, as a matter of first principles.

The words of the philosopher/psychologist William James come to mind here. James wrote that "any rule of thinking that would prevent me from finding a truth, if that truth were really there, is an irrational rule." And so it is.

You can read a summary of the state of the debate about inflation at the link. It's not too technical and is quite accessible to the layperson.