THE anthropic principle 鈥� which argues that our universe is finely tuned to support life and there is no point in asking why it is so 鈥� has been criticised as lazy, untestable science. Now there may be a way to test the theory for one of the most problematic instances of fine-tuning.
Cosmologists have observed that the expansion of the universe is accelerating, and have attributed this to an inherent energy of space-time described by the so-called cosmological constant (CC). Quantum physics predicts that the CC should be more than 10120 times larger than observed 鈥� a value so large it would blow the universe apart before stars or galaxies formed 鈥� yet instead it seems to be just right for the formation of giant galaxies such as our Milky Way, and hence for life.
鈥淚f life is common in nearby dwarf galaxies, maybe they鈥檒l tell us why the cosmological constant is so small鈥�
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This led Nobel laureate physicist Steven Weinberg to propose in 1987 an anthropic explanation for the CC. He argued that we shouldn鈥檛 be surprised to find ourselves in such an unlikely place, because it is only here that life could exist. That explanation has been gaining favour, especially since string theory suggests that our universe could be just one of 10500 possible variants, almost all less hospitable (快猫短视频, 17 December 2005, p 48).
鈥淭he anthropic principle is the best explanation for this amazing coincidence,鈥� says Alexander Vilenkin, a cosmologist at Tufts University in Medford, Massachusetts.
Others disagree. 鈥淭he anthropic principle is a way of just giving up on a real explanation,鈥� says cosmologist Neil Turok at the University of Cambridge.
Now Avi Loeb of Harvard University claims we can test whether a hypothetical universe with a CC 1000 times larger than our own could support life. In such a universe, dwarf galaxies of the kind that formed about 400 million years after the big bang would have been the largest structures around. Within the next decade we will be able to measure the infrared spectra of these distant dwarfs, says Loeb. We can then compare the spectra with those of nearby dwarf galaxies. If they match, it would mean that distant and nearby dwarf galaxies are similar in composition.
The next step would be to search for exoplanets in the nearby dwarfs. If we find many potentially habitable exoplanets, it would suggest they could also have also formed in the distant dwarfs. Therefore life could have emerged, in theory, in a universe filled with dwarf galaxies where the CC was quite different, contradicting the anthropic explanation for its value today (Journal of Cosmology and Astroparticle Physics, in press).
If so, cosmologists will have to find other explanations for the value of the CC in today鈥檚 universe. They may get some help, jokes Loeb. 鈥淚f intelligent civilisations are common in nearby dwarf galaxies, maybe they鈥檒l broadcast an explanation for why the cosmological constant is so small.鈥�
Bangs and crunches cut constant down to size
A cyclic universe, which bounces through a series of big bangs and big crunches, could solve the puzzle of why the cosmological constant (CC) is so much smaller than quantum theory predicts.
In the 1980s, physicists considered the possibility that an initially large CC could have decayed down to today鈥檚 value. But calculations showed that it would take far longer than the 13.7 billion years since the big bang for the constant to reach today鈥檚 level.
Now Paul Steinhardt at Princeton University and Neil Turok at the University of Cambridge point out that if time stretches back beyond the big bang the problem could be solved. 鈥淓ver since the 1960s people assumed that the big bang was the beginning of time, because the laws of physics seem to break down there,鈥� says Turok. However, the equations of string theory allow time to exist before the big bang, he says.
According to Steinhardt and Turok, today鈥檚 universe is part of an endless cycle of big bangs and big crunches, with each cycle lasting about a trillion years. At each big bang, the amount of matter and radiation in the universe is reset, but the cosmological constant is not. Instead, it gradually diminishes over many cycles to the small value observed today (Science DOI: 10.1126/science.1126231).
