Chapter 2
Aggregation
2.1 The Origins
Inorganic forms evolve according to simpler rules than life, language, or society
but create unending variety as well. We turn now to shapes in the non-living world
before delving into deeper waters.
Gravity is intrinsically unstable. Right up until the very end of the 20th century
it remained uncertain whether our Universe will expand eternally or eventually collapse in a Big Crunch. Its ultimate fate depends on the balance between gravitational
attraction and kinetic energy driving the expansion. Neither is known for sure, since
they are largely hidden as “dark matter” and “dark energy”, but the unexpectedly
accelerating expansion of the Universe, or at least of its observable part where we
happened to reside, was proven by the efforts of two competing groups who carefully measured the redshifts of supernovas in galaxies lying billions of light-years
away, and found them to increase more slowly than linearly with distance.
The global tendency is, however, irrelevant for what is going on at a particular
location. One can become rich when everybody around is poor; indeed, if matter is
conserved, enrichment is only possible by impoverishing one’s neighbors, and this
is exactly what the gravitational instability is doing. Local gravitational collapse, a
“little crunch”, starts when a cloud of gas and dark matter is sufficiently massive and
compact, so that free fall toward its center cannot be supported by the gradient of
pressure that increases toward the denser inner regions. When exactly it would happen, depends on local energy, both visible and dark, and it is the former, measured
by local temperature, which is more relevant, since it should be far more sensitive
to local fluctuations. Static equilibrium of a cloud is unstable: it either compacts or
disperses, though compaction does not need to go all the way to a final crunch –
sucking into a black hole – at least, not straightforwardly, as stable structures may
be formed on the way.
There was little growth of perturbations when the Universe was dominated by
radiation, but small density fluctuations, about a thousandth of a percentage point,
present at the moment when the Universe became transparent, which we still see
11
© Springer Nature Switzerland AG 2020
L. Pismen, Morphogenesis Deconstructed, The Frontiers Collection,
https://doi.org/10.1007/978-3-030-36814-2_2
Aggregation
2.1 The Origins
Inorganic forms evolve according to simpler rules than life, language, or society
but create unending variety as well. We turn now to shapes in the non-living world
before delving into deeper waters.
Gravity is intrinsically unstable. Right up until the very end of the 20th century
it remained uncertain whether our Universe will expand eternally or eventually collapse in a Big Crunch. Its ultimate fate depends on the balance between gravitational
attraction and kinetic energy driving the expansion. Neither is known for sure, since
they are largely hidden as “dark matter” and “dark energy”, but the unexpectedly
accelerating expansion of the Universe, or at least of its observable part where we
happened to reside, was proven by the efforts of two competing groups who carefully measured the redshifts of supernovas in galaxies lying billions of light-years
away, and found them to increase more slowly than linearly with distance.
The global tendency is, however, irrelevant for what is going on at a particular
location. One can become rich when everybody around is poor; indeed, if matter is
conserved, enrichment is only possible by impoverishing one’s neighbors, and this
is exactly what the gravitational instability is doing. Local gravitational collapse, a
“little crunch”, starts when a cloud of gas and dark matter is sufficiently massive and
compact, so that free fall toward its center cannot be supported by the gradient of
pressure that increases toward the denser inner regions. When exactly it would happen, depends on local energy, both visible and dark, and it is the former, measured
by local temperature, which is more relevant, since it should be far more sensitive
to local fluctuations. Static equilibrium of a cloud is unstable: it either compacts or
disperses, though compaction does not need to go all the way to a final crunch –
sucking into a black hole – at least, not straightforwardly, as stable structures may
be formed on the way.
There was little growth of perturbations when the Universe was dominated by
radiation, but small density fluctuations, about a thousandth of a percentage point,
present at the moment when the Universe became transparent, which we still see
11
© Springer Nature Switzerland AG 2020
L. Pismen, Morphogenesis Deconstructed, The Frontiers Collection,
https://doi.org/10.1007/978-3-030-36814-2_2