Discovery of Cosmic Fractals by Yurij Baryshev and Pekka Teerikorpi, World Scientific, $38/拢26, ISBN 9810248725 Reviewed by Marcus Chown
FRACTAL structures are everywhere 鈥� in clouds, in snowflakes, even in Islamic art, says Beno卯t Mandelbrot, godfather of fractals. So why not throughout the Universe? Why not indeed?
In Discovery Of Cosmic Fractals, astronomers Yurij Baryshev and Pekka Teerikorpi set out the evidence for fractal structures everywhere from interstellar clouds to galactic dark-matter haloes. Intriguingly, they say, a fractal distribution of matter can squirrel away 鈥渂aryonic dark matter鈥� (which consists of familiar particles such as neutrons and protons, rather than exotic ones) in small molecular clouds that might so far have escaped detection. But the most controversial idea by far is that the entire Universe is fractal.
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Recall that Einstein鈥檚 fiendishly complex equations of gravity can be solved exactly only if we assume that the Universe on the large-scale is homogeneous 鈥� that is, it looks the same from every place. This assumption, enshrined in the cosmological principle, leads to the Friedman-Robertson-Walker solutions, the big bang models. Abandon that assumption and everything we thought we knew about the Universe gets jettisoned, as New Scientist has pointed out (21 August 1999, p 22).
No one can deny that the galaxy distribution is fractal in our cosmic neighbourhood. The 鈥済eometry of chaos鈥� is apparent from scales of about 300,000 light years up to at least 300 million light years. But what about beyond? Here lies the battleground.
While mainstream astronomical opinion maintains that galaxy surveys probing even farther out into the Universe will reveal the fractal distribution giving way to homogeneity, a vocal minority led by Luciano Pietronero of the University of Rome claim the Universe will be found to be as self-similar as a snowflake on all observable scales.
Mandelbrot, who has written the introduction to this book, has even proposed the conditional cosmological Principle, which states that the Universe looks the same from all locations, provided they contain a galaxy and not a void. This subtle variation on the Cosmological Principle implies a profoundly different cosmology from the standard one.
Mandelbrot and Pietronero face many problems. They must somehow extract cosmological solutions from Einstein鈥檚 equations. And they must also explain the Hubble law that states that the recession velocity of galaxies is proportional to their distances. In the standard picture of the Universe, this is natural if the Universe expands and remains uniform.
This is a stimulating book, more than half of which stands alone as a first-rate historical primer on astronomy and cosmology. It is a bold and controversial claim indeed that the fractal structure of the Universe is a key cosmological discovery to stand alongside Hubble鈥檚 discovery of the expanding Universe, and Penzias and Wilson鈥檚 of the 鈥渁fterglow鈥� of the big bang. Time alone will tell whether it is a claim too far.