The Anthropic Principle

Earthlings may be extreme latecomers to a universe full of life, with alien microbes possibly teeming on exoplanets beginning just 15 million years after the Big Bang, new research suggests. Traditionally, astrobiologists keen on solving the mystery of the origin of life in the universe look for planets in habitable zones around stars. Also known as Goldilocks zones, these regions are considered to be just the right distance away from stars for liquid water, a pre-requisite for life as we know it, to exist. But even exoplanets that orbit far beyond the habitable zone may have been able to support life in the distant past, warmed by the relic radiation left over from the Big Bang that created the universe 13.8 billion years ago, says Harvard astrophysicist Abraham Loeb.

For comparison, the earliest evidence of life on Earth dates from 3.8 billion years ago, about 700 million years after our planet formed. Based on his findings, Loeb also challenges the idea in cosmology known as the anthropic principle. This concept attempts to explain the values of fundamental parameters by arguing that humans could not have existed in a universe where these parameters were any different than they are. So while there might be many regions in a bigger "multiverse" where the values of these parameters vary, intelligent beings are supposed to exist only in a universe like ours, where these values are exquisitely tuned for life.

For instance, Albert Einstein identified a fundamental parameter, dubbed the cosmological constant, in his theory of gravity. This constant is now thought to account for the accelerating expansion of the universe. Also known as dark energy, this constant can be interpreted as the energy density of the vacuum, one of the fundamental parameters of our universe. Anthropic reasoning suggests that there might be different values for this parameter in different regions of the multiverse — but our universe has been set up with just the right cosmological constant to allow our existence and to enable us to observe the cosmos around us. Loeb disagrees.

 

He says that life could have emerged in the early universe even if the cosmological constant was a million times bigger than observed, adding that "the anthropic argument has a problem in explaining the observed value of the cosmological constant."


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