Å®ÉúСÊÓƵ

Distant quasars appear to be spread very evenly across the sky, according
to the latest surveys. Because their light has taken many billions of years
to reach us, this is equivalent to saying that matter was evenly distributed
in the early Universe. But today matter is in clumps, with galaxies apparently
distributed around bubble-like ‘voids’.

This poses problems for the the ‘top down’ model of galaxy formation,
according to Michael Longo of the University of Michigan in Ann Arbor. The
theory predicts that large structures formed first after the big bang and
that these later broke up into galaxies and stars.

The ‘frothy’ nature of the galaxy distribution was discovered by surveys
at small red shifts – about 0.1. An object’s red shift is a measure of its
distance, with higher red shifts corresponding to greater distances. The
clusters of galaxies and voids in our vicinity are on a scale of about a
hundred megaparsecs (1 parsec is about 3.25 light years).

Quasars, the bright cores of ‘active’ galaxies, can be seen farther
away than ordinary galaxies. But few surveys have attempted to catalogue
the positions of all the quasars in patches of the sky large enough to reveal
structures of the scale seen in galaxy surveys. Longo studied two such surveys
– one of 216 quasars and the other of 127 quasars. Each survey of a particular
portion of the sky includes all the quasars with red shifts around 2.2.

It is not easy to compare the patterns seen in the quasar distribution
with those in the galaxy distribution at lower red shifts, because you have
to take account of the expansion of the Universe between a red shift of
2.2 and a red shift of about 0.1. But the subtleties of such calculations
do not have to be considered in order to appreciate Longo’s most dramatic
finding: ‘All the correlations for both quasar samples are consistent with
zero, so there is no evidence for clustering’ (Astrophysical Journal, vol
403, p 466).

Strictly speaking, the lack of evidence for clustering only rules out
the existence of structures more than a third the size of the structures
seen at lower red shifts. But this is still sufficient to rule out the
possibility that today’s galaxy conglomerations arose from the top downwards.

Longo does not discuss how these findings might relate to the ‘ripples’
in the structure of the cosmic microwave background radiation found by
NASA’s COBE satellite, the cosmic background explorer. These correspond
to still earlier epochs. But Longo’s results clearly suggest that structure
in the Universe formed from the bottom up, with galaxies first forming clusters,
then superclusters, and so on. This is the way structure arises in the
widely favoured ‘cold dark matter’ scenarios, which have been shown to
agree well with the COBE data.