NEW YORK: The late formation of rocky planets might explain a vexing cosmological coincidence about why intelligent life has evolved precisely at a relatively exciting point of the universe's history.

During the past ten years, cosmologists have determined the age and composition of the universe to unprecedented precision: the universe is nearly 14 billion years old and is made mostly out of 'vacuum energy', which is making the expansion of the universe accelerate.

The theory goes that empty space isn't really empty, but rather filled with virtual particles constantly popping in and out of existence. All this activity imparts energy to empty space, termed vacuum energy.

Stepping on the gas

Vacuum energy itself acts like a kind of anti-gravity pushing space-time apart. Calculations suggest that only when the density of this vacuum energy is larger than the density of matter in the universe will the expansion of space-time accelerate – but this is what we are currently observing.

The cosmic coincidence is that we are living at precisely the time when vacuum energy is starting to dominate. This drives some experts crazy because statistically, there's less than a one per cent chance that this would be the case. Why, they ask, is this momentous event happening now, precisely when humans are alive to witness it?

It's as if someone has just stepped on the accelerator of the universe, claim Charles Lineweaver and Charles Egan, astronomers at the Australian National University in Canberra and the University of New South Wales in Sydney, respectively.

In order to explain this so-called 'cosmic coincidence' problem, some scientists have proposed the idea of a 'multiverse', where different parts of the universe have different values of the vacuum energy.

This doesn't sit well with everyone, however, because it implies that physical laws can differ in different parts of the universe, and that what holds true for our corner of the cosmos might not be applicable in a different part.

"There is no coincidence"

As an alternative, Lineweaver and Egan suggest that the cosmic coincidence problem can be solved if scientists take the evolution of terrestrial planets into account. According to a new model the pair has developed, rocky planets – and the intelligent beings that inhabit them – can only exist relatively late in the universe's history.

"Our existence as observers, and probably the existence of any observers in the universe depends on the existence of terrestrial planets," Lineweaver said.

Their model, detailed in a recent issue of Astrophysical Journal, suggests that the complicated chain of events which includes the formation of heavy elements in stars, their dispersion in supernova explosions, the formation of planetary systems, and finally the evolution of intelligent beings requires several billion years.

The model predicts that about 70 per cent of the observers in the universe will live during a time when the vacuum energy dominates. "Taking this into account makes the cosmic coincidence disappear," Lineweaver said. "There is no coincidence."

Lawrence Krauss, a theorist at Case Western Reserve University in Ohio, U.S., said he is skeptical of the new findings because it assumes that humans are a typical example of intelligent life in the universe. "We do not know that we are typical of all life in our universe, or any universe," he said.

Source: http://www.cosmosmagazine.com/node/1845