A fundamental concept is coming back to the fore 鈥� that the universe may be fine-tuned for life. The idea is that physical laws and constants are inexplicably just right to support it; any different and we wouldn鈥檛 be around to ponder this.
The notion that this might be so has been around for decades, but has sat on the sidelines, considered idle speculation or even outside the bounds of science.
Today, theories that would shed some light on this notion are in the ascendant, along with related ideas from cosmology and particle physics. That explains why cosmologists and philosophers from across the globe flocked to the Greek island of Crete last week to discuss, and argue about, fine-tuning.
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Organised by the University of Oxford and sponsored by the John Templeton Foundation of Philadelphia, Pennsylvania, the meeting is part of a series pondering why the properties of physical laws seem 鈥渢uned鈥� to allow complexity 鈥� for example, chemical complexity, which is necessary for life to exist in our universe.
Worlds apart
What do we mean by fine-tuning? A universe with different laws of physics would not be like our own. One in which gravity were stronger would collapse before a single star was born. And a universe in which the fundamental masses of the elementary particles known as quarks were different would be a sea of neutrons, unable to bind together as atoms, molecules, and eventually living creatures. So no complexity.
In fact, most of these alternative universes would be dead and sterile. Yet we seem to find ourselves in one that is a cosmic sweet spot, with physical laws just right to allow our existence.
This 鈥渁nthropic鈥� reasoning seems almost trivial 鈥� we are in this universe because its conditions are right for us 鈥� but many key questions remain. Just how fine-tuned is our universe? And could complexity remain even if the laws of physics were radically different?
In universes where dark energy, which drives the expansion of the universe, were significantly more energetic, matter would be instantly dispersed and no stars and galaxies would ever form.
But in , we could still get something like stars. And although these stars would possess quite different temperatures and lifetimes to our own, they could still provide the power source for life.
Enter the multiverse
While there are many competing theories, the leading idea that supports the fine-tuning view relies on 鈥渆ternal鈥� inflationary cosmic expansion to produce an immense sea of individual universes in what鈥檚 known as the multiverse. This ties in with M-theory, the leading incarnation of string theory, which suggests that each of these pocket universes will have their own laws of physics.
Both the multiverse concept and M-theory are enjoying a resurgence. This is partly because predicted new elementary particles that would disprove them have been elusive so far at the Large Hadron Collider 鈥撀爐he particle smasher at CERN near Geneva, Switzerland.
If this state of affairs holds, many more questions will need answering, such as just how does M-theory write the specific laws of physics for each patch in the multiverse?
There are also those who think we can get rid of the multiverse concept altogether, with new mathematical ideas suggesting we inhabit a single universe with no choice in its fundamental laws, and so no fine-tuning to worry about.
Underlying all of these potential explanations are serious philosophical questions. Is adopting the multiverse as a solution to fine-tuning even science as we traditionally understand it, given that we can never experience other universes and hence never really test this explanation experimentally? And would this new definition of science, with the unseen and the untestable, bring it too close to religion for the comfort of most?
The apparent fine-tuning of the physical laws for complexity and life remains one of the peculiarities of our universe. We might be simply part of a much larger, much weirder , but to adopt this view of the cosmos might mean we have to change our notion of what science is 鈥� and that would be hard. Expect a lot more debate.
