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 radio-

astronomy 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

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elements by nuclear fusion processes in the stars) as an alternative to the early-

universe 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

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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

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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

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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 black-

body, 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.

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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 steady-

state 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.

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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

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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....

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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

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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)

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