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Essay review - A few of my favourite thingsAndrew J. BensonPublished online: 08 Nov 2010.

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Essay review

A few of my favourite things

ANDREW J. BENSON

A review of The Universe at Midnight. Observations

Illuminating the Cosmos. By K. CROSSWELL. (Free Press,

2001). Pp. xii� 338. US$27.00 (hbk). ISBN 0 684 85931 9.

Scope: popular survey. Level: general reader.

Is this yet another popular cosmology book? Book shops

are already over¯owing with them; so do we really need

another? There is room, I believe, for a well-written book,

aimed at the intelligent layman, which covers the exciting

advances of the last few years, describing the details

without getting bogged down in technicalities. The Universe

at Midnight by Ken Croswell might just satisfy those

criteria. This is a book that I enjoyed reading. Croswell

aims to educate the reader quickly about the history of

cosmology, bringing them up to date with current thinking

and then to focus on the quantum leaps made in cosmology

in the last decade. As he guides us through this story, we

learn that much of modern cosmology has been dominated

by the quest for the values of three characters, namely O0, land H0, and the quest itself seems to have been dominated

by some rather unusual characters, namely the cosmolo-

gists themselves.

Astronomy, and perhaps cosmology in particular, is a

rather strange science; I have heard that some of the more

vociferous practitioners of other branches of physics claim

that it is not really a science at all. In nearly every other

science, study proceeds through controlled experiments, the

system is set up in idealized conditions, a single variable is

varied and its consequences are recorded. Furthermore, to

be certi®ed as scienti®c fact, the experiment must be

repeatable. In cosmology we have no ability to control the

experiment, all we can do is observe it and have little hope

of repeating it unless someone were to invent a Big Bang

machine. Even worse, the Universe seems to have taken

perverse pleasure in hiding most of itself from us, and, quite

probably, in making most of itself from vacuum energy

rather than matter.

These simple facts explain, I believe, two important

points which should be kept in mind when reading this

book. Firstly, it is only in very recent years that ®rm

quantitative results have become available in cosmology.

Secondly, confounded for years in their searches for

observations free from systematic biases which would

allow them to understand the Universe, most cosmologists

have become at least slightly, let us say, eccentric.

This latter point is evidenced in several of the chapters,

perhaps none more so than that discussing the Hubble

constant. Ever since Hubble ®rst realized that the Universe

is expanding, scores of astronomers have attempted to

measure the rate of this expansion, encapsulated in the

constant bearing Hubble's name. The Hubble constant is

crucial since it sets both the size and the age of our

Universe. Currently, all cosmological measures of distances

involve an unknown factor of this Hubble constant, and

this propagates into many other measurements, such as the

luminosity of distant galaxies. Croswell does a ®ne job of

describing the decades of back-and-forth arguments over

the value of the Hubble constant, interspersed with vitriolic

quotes from those astronomers involved. Perhaps my

favourite quote in the whole book is from Allan Sandage

responding to criticism of his observations which turn up a

lower value of the Hubble constant than certain other

astronomers obtain: `many other people have said, we will

always get 55 regardless of what the data say. That's

because [the Hubble constant] is 55!' More importantly,

although this chapter more than any other really demon-

strates why it is so hard to obtain robust results in

cosmology, the whole debate about the value of the Hubble

constant is due to the di�culty in measuring something

seemingly so trivial as the distances to galaxies. Measure-

ment of distances requires climbing the rungs of the

infamous distance ladder, using a patchwork of di�erent

techniques to measure ever greater distances. Just one weak

rung and the whole ladder collapses.

Although the measurement of the three basic cosmolo-

gical parameters has been the `holy grail' of cosmology for

several decades, it is clear to anyone who has ever lookedMr A. Benson is at the California Institute of Technology, MC105-24, 1200E. California Boulevard, Pasadena, California 91125, USA

Contemporary Physics, 2003, volume 44, number 1, pages 73±75

Contemporary Physics ISSN 0010-7514 print/ISSN 1366-5812 online # 2003 Taylor & Francis Ltdhttp://www.tandf.co.uk/journalsDOI: 10.1080/00107510210167036

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through a telescope (and I did at least once) that three

numbers are not quite enough to describe the Universe

completely. Even Martin Rees' Just Six Numbers are

probably not su�cient. The cosmic microwave background

which proved the long-held belief that the Universe is

isotropic and homogeneous on the largest scales (the so-

called `cosmological principle') contains the blueprints of

the very inhomogeneous Universe that we see around us.

Three chapters in the middle of the book describe the quest

to measure these ripples in the microwave background and

the vast structures of galaxies that we see around us:

®laments, clusters, superclusters and voids. The topic of

in¯ation, now almost universally accepted as a component

of the Big Bang model, is also discussed in some detail. The

idea that galaxies, and so ultimately ourselves, grew from

quantum ¯uctuations generated 10735 s seconds after the

Universe began is almost as romantic as that old chestnut

about us being made of stardust! While it would be naõÈ ve to

think that we have heard the last word on the basic

cosmological parameters, most of cosmology is now

shifting its focus towards understanding those ripples in

the microwave background and their descendants.

In some sense this is resulting in a subtle and yet

fundamental shift in the way that observers and theorists

interact. In the past the theoretical predictions were all

rather easy to make (e.g. the spectrum of cosmic microwave

background ripples depends only on well-understood and,

crucially, linear physical processes) while the observers

were forced to struggle to measure the properties of the

entire Universe using only that small fraction of its mass

which happens to emit light, and even that in a not

particularly well understood way. Now, however, the

observers get their revenge. New instrumentation is

allowing beautifully detailed data to be acquired, spanning

huge ranges of wavelengths and reaching back over most of

the age of the Universe. The theorists meanwhile are having

to contend with messy astrophysical processes, highly

nonlinear systems and subtle feedback loops to name but

a few of the complexities.

Still, that is my third most favourite thing about

cosmology; it really involves every bit of physics that you

ever learned: gravity, hydrodynamics, radiative processes,

atomic, molecular and nuclear physics, particle physics,

thermodynamics and so on. Currently, one of the most

active areas is in trying to understand the so called `dark

ages' of our Universe. Strangely, we now know much about

our Universe 100 000 years after its birth, and have a good

deal of knowledge about it from a few billion years after the

Big Bang to the present day. Looking at it logarithmically,

that leaves about four orders of magnitude in time that we

really have no information about. We know, or we think we

know, that some important events occurred in this span of

time. The very ®rst structures in the Universe began to

form, quite possibly forming bizarre objects such as

supermassive stars that have never occurred again,

although their remnants may be lurking in the present-

day Universe. We also know that at some time in this

unexplored period the ®rst few stars and quasars were able

to dissociate all the hydrogen in the Universe, returning it

to the ionized state that it had occupied since just after the

Big Bang until the Universe cooled su�ciently for it to

recombine, thereby permitting the cosmic microwave

background photons to begin their long journey towards

us. Amazingly, observations are beginning to probe this era

(spectra of the most distant quasars known in the Sloan

Digital Sky Survey are hinting that the epoch of ionization

is almost within our sights) and future instrumentation

promises to provide a wealth of data. For example, plans

are being made in both the USA and Europe for the next

generation of optical telescopes, with diameters of 30 ±

100 m, while new radio telescopes, such as the proposed

Square Kilometre Array, should allow us to probe the

distribution of neutral gas in the era directly.

Croswell's book proceeds in roughly chronological

order. After beginning by giving us a sense of scale, and

of our place in the Universe, Croswell continues by

addressing perhaps the oldest cosmological question of

them all: why is the sky dark at night? An explanation of

this fact, known today as Olber's paradox, outfoxed

astronomers for centuries, who speculated incorrectly that

distant stars would be just too faint for us to see or that

dust obscured their light from us. As Croswell explains, the

solution had to await that most famous of cosmologists

Edgar Allan Poe, who, in his Eureka, put forward the

possibility that the sky is dark because the Universe has a

®nite age. (I am not aware that Poe's work has ever gone

through the peer review process and so we should treat it

with some caution.) It is anecdotes such as this, and other

human interest stories, scattered throughout clear explana-

tions of the science that make this book a pleasure to read.

With that particular problem solved by a writer and poet

the `real' cosmologists take over. Croswell guides us

through a century of advancement in cosmological under-

standing, beginning with the ®ght between Big Bang and

Steady State pictures, a ®ne example of how both the

scienti®c method proceeds and cosmology invokes almost

religious questions, proceeding through the gradual accep-

tance that the Universe is made mostly of unseen matter,

and covering in some detail the recent and much heralded

birth of `quantitative cosmology'. Quantitative cosmology

means that we now know the values of the three key

fundamental cosmological parameters with reasonable

accuracy (only in cosmology would numbers known to at

best 10% be called `accurate'!).

In fact, they are now known accurately enough for us to

make an educated guess at what the future has in store for

our Universe, the focus of the ®nal chapter. As it turns out,

it is fairly bleak if we accept the reality of the somewhat

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unpalatable l, the cosmological constant, as observations

of distant supernovae suggest. A Universe dominated by

this vacuum energy will expand in an exponentially fast

manner, rapidly diluting the density of interesting cosmo-

logical objects such as galaxies. Eventually nearly all the

galaxies nearby will vanish. Croswell seems to be an

optimistic sort of person, however, speculating that

humanity might be able to outlive the death of our Sun

and even the death of all the stars in our Galaxy.

Nevertheless, it seems that l killed the cosmologist, or at

least it will eventually. When all the nearby galaxies pass

beyond our horizon, and the cosmic microwave back-

ground becomes undetectably faint, there will be no more

need for cosmologists. That certainly inspires me to work a

little harder; time is short.

Croswell generally does a very good job of explaining

the science, the techniques and the caveats relevant to the

topics discussed (I encountered only one statement that I

thought was just plain wrong). As I bemoaned at the start

of this essay, there are, of course, innumerable popular

cosmology books out there; so why should one buy this

book in particular? Perhaps its strongest point is that it is

actually readable! Too many such books either trivialize

the subject matter, getting the science wrong, or else are so

dry as to be useful only as a mild sedative. Croswell gets

the balance just about right, keeping the focus on

explaining the science, while mixing in enough entertain-

ment to keep you turning the pages. The book is nicely

produced with clear error-free text, and an extensive

bibliography and glossary. The book ends with a set of

tables, allowing armchair cosmologists everywhere to pick

their own favourite values of the cosmological parameters

and see just how old and large the Universe should be. The

only thing that I thought was lacking were pictures. I

imagine colour pictures increase greatly the production

costs of a book, but it would certainly be worth it; my

second most favourite thing about cosmology is how

beautiful the pictures are. So, I can happily recommend

investing in The Universe at Midnight as it clearly

demonstrates that, although cosmology is one of the oldest

sciences, it remains active and ever changing. It also makes

it quite clear that we remain a long way from having a full

understanding of our Universe. That is my favourite thing

about cosmology.

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