- Gott, J. Richard.
The Cosmic Web.
Princeton: Princeton University Press, 2016.
ISBN 978-0-691-15726-9.
-
Some works of popular science, trying to impress the reader with the
scale of the universe and the insignificance of humans on the
cosmic scale, argue that there's nothing special about our place
in the universe: “an ordinary planet orbiting an
ordinary star, in a typical orbit within an ordinary galaxy”, or
something like that. But this is wrong! Surfaces of planets
make up a vanishingly small fraction of the volume of the universe,
and habitable planets, where beings like ourselves are neither frozen
nor fried by extremes of temperature, nor suffocated or poisoned by
a toxic atmosphere, are rarer still. The Sun is far from an ordinary star:
it is brighter than 85% of the stars in the galaxy, and only 7.8% of stars
in the Milky Way share its spectral class. Fully 76% of stars are dim
red dwarves, the heavens' own 25 watt bulbs.
What does a typical place in the universe look like? What would you see
if you were there? Well, first of all, you'd need a space suit
and air supply, since the universe is mostly empty. And you'd see
nothing. Most of the volume of the universe consists of
great voids with few galaxies. If you were at a typical place in
the universe, you'd be in one of these voids, probably far enough
from the nearest galaxy that it wouldn't be visible to the unaided
eye. There would be no stars in the sky, since stars are only formed
within galaxies. There would only be darkness. Now look out the
window: you are in a pretty special place after all.
One of the great intellectual adventures of the last century is
learning our place in the universe and coming to understand its
large scale structure. This book, by an astrophysicist who has
played an important role in discovering that structure, explains
how we pieced together the evidence and came to learn the details
of the universe we inhabit. It provides an insider's look at how
astronomers tease insight out of the messy and often confusing
data obtained from observation.
It's remarkable not just how much we've learned, but how recently
we've come to know it. At the start of the 20th century, most
astronomers believed the solar system was part of a disc of
stars which we see as the Milky Way. In 1610, Galileo's telescope
revealed that the Milky Way was made up of a multitude of faint stars,
and since the galaxy makes a band all around the sky, that the
Sun must be within it. In 1918, by observing variable stars in
globular clusters
which orbit the Milky Way, Harlow Shapley was able to measure the size
of the galaxy, which proved much larger than previously estimated, and
determine that the Sun was about half way from the centre of the
galaxy to its edge. Still, the universe was the galaxy.
There remained the mystery of the “spiral nebulæ”.
These faint smudges of light had been revealed by photographic
time exposures through large telescopes to be discs, some with
prominent spiral arms, viewed from different angles. Some astronomers
believed them to be gas clouds within the galaxy, perhaps other
solar systems in the process of formation, while others argued they
were galaxies like the Milky Way, far distant in the universe.
In 1920 a
great
debate pitted the two views against one another, concluding that
insufficient evidence existed to decide the matter.
That evidence would not be long in coming. Shortly thereafter,
using the new
100
inch telescope on Mount Wilson in California,
Edwin Hubble was able to photograph the Andromeda Nebula and
resolve it into individual stars. Just as Galileo had done three
centuries earlier for the Milky Way, Hubble's photographs proved
Andromeda was not a gas cloud, but a galaxy composed of a multitude
of stars. Further, Hubble was able to identify variable stars
which allowed him to estimate its distance: due to details about
the stars which were not understood at the time, he underestimated
the distance by about a factor of two, but it was clear the galaxy
was far beyond the Milky Way. The distances to other nearby
galaxies were soon measured.
In one leap, the scale of the universe had become breathtakingly
larger. Instead of one galaxy comprising the universe, the Milky
Way was just one of a multitude of galaxies scattered around an
enormous void. When astronomers observed the spectra of these
galaxies, they noticed something odd: spectral lines from stars
in most galaxies were shifted toward the red end of the spectrum
compared to those observed on Earth. This was interpreted as a
Doppler shift
due to the galaxy's moving away from the Milky Way. Between 1929
and 1931, Edwin Hubble measured the distances and redshifts of
a number of galaxies and discovered there was a linear
relationship between the two. A galaxy twice as distant as another
would be receding at twice the velocity. The universe was
expanding, and every galaxy (except those sufficiently close to
be gravitationally bound) was receding from every other
galaxy.
The discovery of the redshift-distance relationship provided
astronomers a way to chart the cosmos in three dimensions. Plotting
the position of a galaxy on the sky and measuring its distance
via redshift allowed building up a model of how galaxies were
distributed in the universe. Were they randomly scattered, or
would patterns emerge, suggesting larger-scale structure?
Galaxies had been observed to cluster: the
nearest cluster, in
the constellation Virgo, is made up of at least 1300 galaxies,
and is now known to be part of a
larger supercluster
of which the
Milky Way is an outlying member. It was not until the 1970s and
1980s that large-scale redshift surveys allowed plotting the
positions of galaxies in the universe, initially in thin slices,
and eventually in three dimensions. What was seen was striking.
Galaxies were not sprinkled at random through the universe, but
seemed to form filaments and walls, with great voids containing
little or no galaxies. How did this come to be?
In parallel with this patient observational work, theorists were
working out the history of the early universe based upon
increasingly precise observations of the
cosmic
microwave background radiation, which provides a glimpse of
the universe just 380,000 years after the Big Bang. This ushered
in the era of precision cosmology, where the age and scale of the
universe were determined with great accuracy, and the tiny
fluctuations in temperature of the early universe were mapped in
detail. This led to a picture of the universe very different from
that imagined by astronomers over the centuries. Ordinary matter:
stars, planets, gas clouds, and you and me—everything we
observe in the heavens and the Earth—makes up less than 5% of the
mass-energy of the universe.
Dark matter,
which interacts with ordinary matter only through gravitation, makes
up 26.8% of the universe. It can be detected through its gravitational
effects on the motion of stars and galaxies, but at present we don't
have any idea what it's composed of. (It would be more accurate to
call it “transparent matter” since it does not interact with
light, but “dark matter” is the name we're stuck with.)
The balance of the universe, 68.3%, is
dark energy,
a form of energy filling empty space and causing the expansion of
the universe to accelerate. We have no idea at all about the nature
of dark energy. These three components: ordinary matter, dark matter,
and dark energy add up to give the universe a flat topology. It is
humbling to contemplate the fact that everything we've learned in
all of the sciences is about matter which makes up less than 5% of
the universe: the other 95% is invisible and we don't know anything
about it (although there are abundant guesses or, if you prefer,
hypotheses).
This may seem like a flight of fancy, or a case of theorists making up
invisible things to explain away observations they can't otherwise
interpret. But in fact, dark matter and dark energy, originally
inferred from astronomical observations, make predictions about the
properties of the cosmic background radiation, and these predictions
have been confirmed with increasingly high precision by successive
space-based observations of the microwave sky. These observations
are consistent with a period of
cosmological
inflation in which a tiny portion of the universe expanded to encompass
the entire visible universe today. Inflation magnified tiny quantum
fluctuations of the density of the universe to a scale where they
could serve as seeds for the formation of structures in the present-day
universe. Regions with greater than average density would begin to
collapse inward due to the gravitational attraction of their contents,
while those with less than average density would become voids as
material within them fell into adjacent regions of higher density.
Dark matter, being more than five times as abundant as ordinary
matter, would take the lead in this process of gravitational collapse,
and ordinary matter would follow, concentrating in denser regions
and eventually forming stars and galaxies. The galaxies formed would
associate into gravitationally bound clusters and eventually
superclusters, forming structure at larger scales. But what does
the universe look like at the largest scale? Are galaxies
distributed at random; do they clump together like meatballs in
a soup; or do voids occur within a sea of galaxies like the holes
in Swiss cheese? The answer is, surprisingly, none of the above,
and the author explains the research, in which he has been a
key participant, that discovered the large scale structure of
the universe.
As increasingly more comprehensive redshift surveys of galaxies
were made, what appeared was a network of filaments which connected
to one another, forming extended structures. Between filaments were
voids containing few galaxies. Some of these structures, such as the
Sloan Great Wall,
at 1.38 billion light years in length, are 1/10 the radius of
the observable universe. Galaxies are found along filaments, and where
filaments meet, rich clusters and superclusters of galaxies are
observed. At this large scale, where galaxies are represented by single
dots, the universe resembles a neural network like the human brain.
As ever more extensive observations mapped the three-dimensional
structure of the universe we inhabit, progress in computing allowed
running increasingly detailed simulations of the evolution of structure
in models of the universe. Although the implementation of these
simulations is difficult and complicated, they are conceptually
simple. You start with a region of space, populate it with particles
representing ordinary and dark matter in a sea of dark energy with
random positions and density variations corresponding to those observed
in the cosmic background radiation, then let the simulation run,
computing the gravitational attraction of each particle on the others
and tracking their motion under the influence of gravity. In 2005,
Volker Springel and the Virgo Consortium ran the
Millennium
Simulation, which started from the best estimate of the initial
conditions of the universe known at the time and tracked the motion
of ten billion particles of ordinary and dark matter in a cube
two billion light years on a side. As the simulation clock ran, the
matter contracted into filaments surrounding voids, with the filaments
joined at nodes rich in galaxies. The images produced by the
simulation and the statistics calculated were strikingly similar
to those observed in the real universe. The behaviour of this and
other simulations increases confidence in the existence of dark matter
and dark energy; if you leave them out of the simulation, you get
results which don't look anything like the universe we inhabit.
At the largest scale, the universe isn't made of galaxies sprinkled
at random, nor meatballs of galaxy clusters in a sea of voids, nor
a sea of galaxies with Swiss cheese like voids. Instead, it resembles
a sponge of denser filaments and knots interpenetrated by less dense
voids. Both the denser and less dense regions percolate: it
is possible to travel from one edge of the universe to another
staying entirely within more or less dense regions. (If the universe
were arranged like a honeycomb, for example, with voids surrounded by
denser walls, this would not be possible.) Nobody imagined this
before the observational results started coming in, and now we've
discovered that given the initial conditions of the universe after the
Big Bang, the emergence of such a structure is inevitable.
All of the structure we observe in the universe has evolved from a
remarkably uniform starting point in the 13.8 billion years since the
Big Bang. What will the future hold? The final chapter explores
various scenarios for the far future. Because these depend upon the
properties of dark matter and dark energy, which we don't understand,
they are necessarily speculative.
The book is written for the general reader, but at a level
substantially more difficult than many works of science
popularisation. The author, a scientist involved in this research for
decades, does not shy away from using equations when they illustrate
an argument better than words. Readers are assumed to be comfortable
with scientific notation, units like light years and parsecs, and
logarithmically scaled charts. For some
reason, in the Kindle edition dozens of
hyphenated phrases are run together without any punctuation.
May 2016 