OUR Universe had just the right 鈥渃lumpiness鈥� for stars, planets and life, say
astronomers. This is thanks to the value of the so-called density fluctuation
parameter, which was measured as 10-5 in 1992 from the size of the
鈥渃osmic ripples鈥� discovered by NASA鈥檚 Cosmic Background Explorer (COBE)
satellite.
鈥淚f the parameter had been 10 times smaller, no stars would have formed in
galaxies,鈥� says Martin Rees of the University of Cambridge. 鈥淚f it had been 10
times bigger, encounters between stars would have kicked planets out of their
orbits. Either way, we probably would not be here.鈥�
The density fluctuation parameter determined the clumpiness of the Universe
when the first structures started to congeal out of the cooling gas of the big
bang, about 300 000 years after the Universe鈥檚 birth. The bigger the parameter,
the more pronounced would have been the dense regions compared with the average,
making them grow faster by pulling in matter through gravity.
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Such density ripples may have originated in quantum fluctuations in the first
split-second of creation. But no physics predicts their size at the time of
galaxy formation. This prompted Rees and Max Tegmark of the Institute for
Advanced Study in Princeton to investigate what kind of Universe might have
arisen if the density fluctuation parameter had been slightly different.
If the density fluctuation parameter had been smaller than
10-5鈥攕ay 10-6鈥攖hen the clumps in the Universe would
have been less dense and taken far longer to gather enough mass to form
galaxies. Rees and Tegmark calculate that this would have only occurred when the
Universe was 30 times older and 10 times bigger鈥攚ith the galaxies 10 times
bigger, and having just a thousandth of the density.
This low density would have had an adverse effect on star formation. In order
to collapse under gravity to form a star, a blob of gas must lose the heat that
opposes the gravitational collapse by providing pressure. Heat loss occurs when
particles of the gas collide and radiate. However, if a gas is 1000 times less
dense, collisions are 1000 times less frequent and a blob takes 1000 times
longer to cool. 鈥淕as will just sit there and never form stars,鈥� says Rees.
If, on the other hand, the density fluctuation parameter had been bigger than
10-5 鈥攕ay 10-4鈥攇alaxies would have formed when the
Universe was a tenth of the size and a third of the age. So galaxies would be a
tenth of the size and 1000 times denser. Since stars would be so closely packed,
close encounters between them would be frequent, ripping away any planets. 鈥淚t鈥檚
not very conducive for life,鈥� says Rees.
If the density fluctuation parameter were even bigger, the researchers
calculate that the gas would collapse straight into huge black holes. They have
submitted their results to The Astrophysical Journal.
Had the parameter been different, Rees concludes, life would not have
evolved. This may not simply be coincidence. One theory of the early Universe
hints that there may be countless different regions of the Universe, each with a
slightly different density fluctuation parameter. 鈥淚t may be no accident that we
find ourselves in a region of the Universe in which the parameter is of the
right size to ensure stars, planets and humans,鈥� says Rees.