Chapter 4: A cosmic debate
The big bang vs the steady-state model
In this chapter we encounter a new view of the universe, the steady-state model. A great debate arose
between the big bang and steady-state models, an argument that was resolved by the advent of radioastronomy and the discovery of the cosmic microwave background .
The application of nuclear physics to Lemaitre’s theory of the early universe led
Gamow et al. to a model of a universe that started as a hot, dense primordial soup of
elementary particles and radiation (now known as the hot big bang model). This
model made two new predictions; a universe that is dominated by the lightest
elements hydrogen and helium, and a universe that might contain remnants of
primordial cosmic radiation.
The steady-state universe
We saw in the last chapter that the study of cosmology went into sharp decline over
the next fifteen years. However, this is not to say that physicists abandoned the
subject altogether. In fact, a completely new model of the universe emerged in Britain
in the late 1940s. At Cambridge University, a trio of physicists, Fred Hoyle, Hermann
Bondi and Thomas Gold, became interested in a model of an expanding universe that
did not originate in a primeval fireball.
Like Gamow, Fred Hoyle was a brilliant nuclear scientist who was also a rather
colourful character. The son of a Yorkshire wool merchant, he went up to Cambridge
as a scholarship student. In 1936, he won the Mayhew Prize in the Cambridge
Mathematical Tripos and went on to a glittering postgraduate career, culminating in
his election to a fellowship at St John's College in 1939. Hoyle was a noticeable
figure around Cambridge as he was rather argumentative and pugnacious, with a
marked tendency to rub polite Cambridge dons up the wrong way.
As a result of war work, Hoyle became friends with Bondi and Gold, two Austrian
refugees from Hitler’s Europe who were also at Cambridge. The trio had a common
interest in astronomy and cosmology and they often discussed developments in the
field together. All three were familiar with the works of Lemaitre and the Gamow
group, but they were unconvinced by their models of the early universe. The
Cambridge physicists were particularly interested in the problem of the age of the
universe; it seemed to them that Lemaitre’s use of the cosmological constant to
overcome the problem was quite contrived. An emerging specialist in nuclear physics,
Hoyle was also unimpressed by the model of primordial nucleosynthesis of Gamow et
al., pointing out that their theory could only account for the lightest elements. Finally,
there was the old problem of the singularity; if the universe really began as the
Friedmann/Lemaitre model suggested, at what point did the laws of physics become
the laws we know today?
These questions led Hoyle and his colleagues to consider a very different model of the
universe. The catalyst was when all three viewed the film The Dead of Night in a
Cambridge cinema. The film features a plot that repeats itself endlessly (not unlike the
more recent American film Groundhog Day) and it led Gold to a daring hypothesis:
what if the universe also cyclic? The trio set to work on the idea, assuming at first
that it would be easily ruled out1.
2
Many hours later, it seemed the question was not trivial. The core of the question was
whether an expanding universe could somehow remain essentially the same, just as a
river is unchanging but not static. A key characteristic of such a ‘steady-state’
universe would be that the density of matter remains constant – in contrast with the
evolving model of Lemaitre where the density of matter decreases rapidly as space
expands. But how could this happen? Hoyle’s daring insight was that to suppose that
if matter is continually created, one could have a universe that is expanding but not
changing – and such a steady-state universe need not have an origin. Of course, the
continuous creation of matter might seem a rather far-fetched idea, but Hoyle was
able to show that the amount of matter needed is extremely small – one atom of
hydrogen for every cubic meter of space2 !
The steady-state model extended Einstein’s cosmological principle (that the universe
is both homogeneous and isotropic on the largest scales) to a perfect cosmological
principle – that the universe is also the same at all times. This view was nicely in line
with classical views in science and philosophy of an eternal universe3. In addition, the
steady-state model avoided the empirical problem of the age of the universe, and the
theoretical problem of the singularity in the Friedmann-Lemaitre models. Finally, the
new model addressed an old puzzle concerning the expansion; although relativity
predicts an expanding universe, the physical reason for the expansion is not obvious.
In the model of Hoyle et al., it is the process of continuous creation that forces space
to expand in order to make room for new matter4.
Hoyle became convinced that he and his colleagues were on the right track, and in
consequence he set about an analysis of stellar nucleosynthesis (the formation of the
3
elements by nuclear fusion processes in the stars) as an alternative to the earlyuniverse nucleosynthesis of the Gamow group. He made several important advances
in this field during the 1950s. In particular, he came up with a brilliant solution to the
riddle of how carbon is formed in the stars, a problem that had dogged the field for
years5. The result was a theory that successfully described how the heavier elements
are formed in stars and supernovae, a model that is still in use today (Burbidge,
Burbidge Hoyle and Fowler 1954).
A cosmic debate
The steady-state model emerged soon after the model of the Gamow group and it
made some impact amongst the small community of relativists, astronomers and
physicists interested in cosmology. Here was a model that avoided the need for a
psychotic beginning for the universe, gave a physical explanation for the expansion of
space and could explain the formation of most of the chemical elements in terms of
stellar processes. Coupled with the problem of the age of the universe, serious doubts
were raised concerning Gamow’s white-hot infant universe. A gifted science
communicator, Hoyle lost no opportunity to promote his own model. Indeed, it was he
who first coined the term ‘big bang’ in a comparison of the two models on BBC radio.
The term stuck, although it is one of science’s great misnomers; as we saw in chapter
3, the model of the Gamow group says nothing about the bang itself.
The debate between a big bang and a steady-state universe lasted more than a decade.
It probably helped revive interest in cosmology as it is the sort of debate that scientists
like best. After all, the universe is either changing in time (big bang model) or it isn’t
(steady-state). In particular, any evidence that our universe was in fact different in the
4
past would effectively rule out the steady-state model. This principle of falsifiability is
very important in science; as the science philosopher Karl Popper pointed out, science
mainly progresses by ruling things out6. It was soon realised that astronomy could
provide the answer – as the great telescopes gaze at the most distant objects in the
sky, they also look back in time because of the finite time it takes light to travel vast
distances. By comparing measurements of the most distant galaxies with those close
by, could astronomers settle the debate?
Astronomy to the rescue once more
They could, but not before another important discovery was made. In 1952, Walter
Baade, Hubble’s successor at the Mount Wilson observatory, announced that
Hubble’s original measurements of stellar distance contained a significant systematic
error. Hubble had underestimated the distances to the galaxies by at least a factor of
two! (The problem was that there are two different types of Cepheid variable stars, a
fact Hubble was unaware of). By 1956, further work by Humason, Mayall and
Sandage suggested a Hubble constant almost three times smaller than that estimated
in 1929. In consequence, the Hubble graph now predicted an age of at least 6 billion
years for the universe, in reasonable agreement with the age of the stars as estimated
from astrophysical processes. The paradox of the age of the universe that had so
bedevilled the big bang model was no more!
At around the same time, the advent of radio astronomy (where physicists study the
sky at radio rather than optical wavelengths) allowed astronomers to peer deeper into
space than ever before. With the great cosmic debate above in mind, the Cambridge
physicist Martin Ryle set about cataloguing all the new radio sources that were being
5
discovered in the sky. By 1955, it seemed that the number of these sources was
significantly higher in the furthest reaches of space. This was the first tentative
evidence that the early universe was indeed different from that of the present.
However, the results were somewhat controversial as the nature of the radio-sources
was not fully understood. More detailed studies undertaken in 1959 and 1962 made it
clear that Ryle’s results were essentially correct. By the early 1960s, there was
compelling evidence that there is an excess of radio sources at the largest distances
observable – in clear contradiction with the predictions of the steady-state model. The
importance of this finding was recognized when Ryle and his Cambridge colleague
Anthony Hewish became the first astronomers to win the Nobel prize in 1974.
An interesting spinoff of the radio-astronomy program was the discovery of quasars –
bright sources at extreme redshifts, indicating incredibly powerful sources at extreme
distances – and pulsars (stars that pulsate in an incredibly regular fashion). Again,
these exotic objects were only seen in the most distant galaxies, suggesting a clear
difference between the young universe and the present one. By the mid-1960s, most
physicists considered that radio-astronomy offered strong support for the big bang
model and cast serious doubt on the steady-state theory. Best of all, as so often
happens in science, the new astronomy led to an unexpected discovery that
revolutionized the field.
The discovery of the cosmic microwave background
In 1963, Arno Penzias and Robert Wilson, two physicists at Bell Laboratories in New
Jersey who had both trained as astronomers, became interested in the problem of
detecting weak signals at radio and microwave wavelengths. This problem had arisen
6
as a result of the nascent satellite communication industry, and Penzias and Wilson set
about the task of constructing an instrument that could act as a sensitive radio
receiver. For this project, they used a unique 20-foot horn-shaped receiver previously
used at Bell as part of the Echo satellite communications program (the giant horn
shields the radio antenna from noise, see figure 6).
Using their highly sensitive instrument, the astronomers detected a ubiquitous, faint
signal in the microwave region of the spectrum at the extremely low temperature of 3
Kelvin (radiation picked up by a radio receiver at a given wavelength is usually
measured in units of temperature – the temperature at which an ideal black body emits
at this wavelength). Taking the signal to be background noise, the duo spent a great
deal of time trying to get rid of it. They did not succeed, and they eventually came to
the conclusion that the signal was of astronomical origin7. At this point, they heard
that a group at Princeton University were working on a theory of cosmic radiation
emanating from the early universe, and they contacted the eminent theoretician Robert
Dicke at Princeton.
Dicke was flabbergasted. Unaware of the earlier work of the Gamow group, he and
his colleague Jim Peebles had been developing a theory of cosmic background
radiation for some time, and had just reached the point where they and their
colleagues were designing an experiment to search for it. The Dicke group took a trip
to New Jersey, inspected the radio receiver and realized the Bell astronomers had hit
the jackpot!8 The experimental findings of Penzias and Wilson were published in a
historic issue of the Astrophysics Journal in 1965, next to an accompanying article by
Dicke and Peebles explaining the theoretical importance of the finding.
7
Figure 6 Penzias (L) and Wilson with their giant radio receiver in the background
The Princeton group soon followed up with their own measurement, with David
Wilkinson and Pete Roll reporting the detection of the background radiation at a
slightly different wavelength later that year. Several other experiments followed and
by mid-1966 the news had spread throughout the world of physics; remnant radiation
from the early universe had been found, strong evidence indeed for the big bang
model. In particular, the detection of ubiquitous radiation at microwave wavelengths
was in excellent accord with the relativistic picture of hot primordial radiation hugely
red-shifted and cooled by the subsequent expansion of space9. Penzias and Wilson
were later awarded the 1978 Nobel Prize in physics for their serendipitous discovery.
Soon, the search was on to measure the shape of the entire spectrum of the cosmic
microwave background (CMB). This was an important test – if the radiation was truly
of cosmic origin, it should exhibit the spectrum of a perfect black body10. For many
years, this program took the form of sending delicate instruments aboard balloons
above the atmosphere (to avoid interference from the atmosphere). These painstaking
8
experiments did yield results although they were extremely difficult11. Each could
record at one wavelength only and instruments often froze or malfunctioned at the
freezing temperatures of the stratosphere. In time, the program gave way to a new
generation of experiments where instruments were mounted on satellites that hovered
far above the atmosphere. This approach scored a spectacular success in 1992, when
instruments aboard the COBE satellite gave the first accurate measurement of the full
spectrum of the cosmic radiation – the spectrum was a perfect fit to that of a blackbody, confirming the primordial nature of the radiation (figure 7). Today, much of
modern cosmology is concerned with the study of the CMB with ever more precision,
using more and more sophisticated telescopes mounted on satellites.
Figure 7 Spectrum of the cosmic microwave background measured by the FIRAS instrument on the
COBE satellite. Squares are experimental points while the solid curve is the black body spectrum
predicted by theory.
9
On the philosophy of science
The discovery of a new scale for the Hubble graph (removing the problem of the age
of a big bang universe) and the radio-source surveys of Ryle and others (indicating
that the universe was different in the past) were significant triumphs for the big bang
model, but it was the detection of the cosmic microwave background that clinched the
deal. The finding marked a new era in cosmology; the evidence for a hot early phase
of the universe was convincing and the study of the origin of the universe moved to
centre stage in the world of physics. Cosmology was no longer an abstract,
speculative subject confined to relativists and a few astronomers, but a vibrant field of
science open to enquiry by astrophysicists, nuclear physicists, particle physicists and
everyone else.
That said, such changes in science do not happen overnight. Alternate explanations
for the background radiation were offered for some time, but none proved convincing.
Meanwhile, more sophisticated versions of the steady-state model were developed;
however, most physicists found these models very contrived. As the evidence
accumulated, the big bang scenario seemed more and more plausible and the steadystate theory less and less so. This is the way science progresses; not by abrupt changes
in world view but by a gradual process, much like a group of observers agreeing on
the nature of a distant object that is gradually coming into view. At first, several
possibilities are tenable, but as the object approaches, one becomes more and more
likely while others are gradually ruled out. Crucially, experimentalists must consider
all of the models whilst this process is ongoing, letting the data speak for itself.
Indeed, long after a particular view has become dominant, it is standard practice to
consider new data in the context of all the main theories, not just the current favourite.
10
For the experimentalist, the case is never truly closed, an approach that is in contrast
with Thomas Kuhn’s view of how paradigm shifts occur in science12 (see chapter 1).
It should be noted that Kuhn was an eminent historian and philosopher but he was not
a noted physicist, and his views are more popular in the social sciences than amongst
practising scientists.
Debates such as the big bang versus the steady-state model also act as a spur for new
discoveries. For example, Hoyle’s brilliant analysis of stellar nucleosynthesis was
spurred by his dislike of the Gamow model, while Ryle’s pioneering work on the
cataloguing of the radio sources was motivated by a desire to prove Hoyle wrong13!
Of particular interest is the fact that, while his steady-state model was duly ruled out,
Hoyle’s work on stellar nucleosynthesis has stood the test of time. Indeed, he also
provided a vital step in fully explaining the synthesis of helium in a big bang
universe14 . However, as the years went on, Hoyle could not accept that the big bang
model was a much better fit to the data than his own, a rather unusual attitude for a
theorist15. This situation was exacerbated when he resigned his academic position at
Cambridge over an administrative row, and he became very isolated in later years16.
And what of Gamow et al.? Their model was spectacularly vindicated by observation,
in particular the detection of the cosmic background radiation predicted by Alpher and
Herman. Yet the group received remarkably little recognition for their work. Dicke
and Peebles were almost completely unaware of their research and it was not
acknowledged in the seminal 1965 papers. Indeed, it was many years before the
Gamow group received due credit. Whatever about Hoyle, it’s quite difficult to
explain exactly why the pioneers of the winning theory were neglected. One reason
may be the decline in interest in cosmology in the 1950s (chapter 3), another that none
11
of the trio remained active in cosmology after 1955. As we saw earlier, a third reason
may have been Gamow’s reputation as a prankster and his status as a Russian émigré;
certainly, his other contributions to physics are often overlooked17. Whatever the
reasons, it is sobering to consider that none of the major pioneers of today’s big bang
model – Lemaitre, Gamow, Alpher and Herman - were honoured with a Nobel prize.
Indeed, the contribution of the Gamow group is often misrepresented to this day18.
Finally, we note that the discovery of the cosmic background radiation settled an old
debate, but also opened the door to a new era of cosmology. By the 1990s, physicists
were able to analyse the radiation in astonishing detail, using a new generation of
telescopes mounted on satellites. As so often in science, the new era was to usher in a
raft of new puzzles....
12
Notes
1.
This episode is described by Hoyle himself in the essay ‘An assessment of the evidence against
the steady-state theory’ in the book Cosmology in Restrospect (Bertoti et al 1990)
2.
Ibid
3.
The concept of an eternal universe goes back to classical antiquity and was revived during the
scientific revolution. The is nicely described in ‘The Big Bang’ (Singh, 2004)
4.
Hoyle’s continuous-creation field was hypothesised to have negative pressure, rather similar to
today’s theories of inflation.
5.
Hoyle postulated an excited state of carbon that had never been observed. The subsequent
observation of this phenomenon by a group at Caltech in 1956 established him as a theorist of
the first rank.
6.
Popper wrote extensively on this principle. Note that, contrary to what many philosophers claim,
the big bang model is also falsifiable since evidence showing an unchanging universe would rule
it out.
7.
This episode has been described many times. Possibly the best reference is that by Wilson
himself in his essay ‘Discovery of the cosmic microwave background’ in the book Cosmology in
Restrospect (Bertoti et al. 1990). It is interesting to note that the radiation may have been
detected previously, but not recognized.
8.
According to Peebles, Dicke realised immediately that they had been scooped. This story is told
in the book “Physical Cosmology”(Peebles, 1990)
9.
This point is often overlooked; the red-shifted CMB constitutes the best evidence that space is
expanding
10. A black body is an ideal emitter (and absorber) of heat. It has a characteristic spectrum that is
only seen if the body is in complete isolation
11. There is a very nice description of this period in the book “
“ by the science writer Marcus
Chown
12. Kuhn emphasised the ‘incommensurability’ of the new paradigm and the old, suggesting that
once a particular worldview becomes dominant, scientists do not examine emerging evidence in
terms of alternative models (Kuhn, 1976). Most practising scientists reject this view
13
13. Sadly, there was considerable personal animosity between the two physicists, a bitterness that
affected cosmology and astronomy at Cambridge for years
14. Hoyle’s contribution to big bang nucleosynthesis is often overlooked, yet his paper on the
subject (Hoyle and Tayler 1964) is a classic
15. Most theoreticians see their job as exploring the different paths nature might choose, with
emerging data as the ultimate arbiter - rather than putting all their eggs in one basket.
16. See ‘Fred Hoyle: A Life in Science’ (Mitton, 1992)
17. For example, it was a suggestion from Gamow that led to the historic splitting of the nucleus by
Cockroft and Walton in 1932, a prediction that he is rarely credited with. In the 1940s, he was
denied a role in the Manhattan project despite his nuclear expertise because of his status as a
Russian émigré. It is interesting that no major biography of Gamow has yet been written.
18. In the famous book ‘A Brief History of Time’ Stephen Hawking attributes the prediction of the
CMB to Gamow instead of Alpher and Herman (Hawking, 1988)
14