Origins of the Universe

The Cosmic Microwave Background and the Search for Quantum Gravity

Keith Cooper

• Publisher: Icon Books
• Year: 2020
• ISBN:
  9781785786433
• Topic: Physical Sciences
• Subtopic: Cosmology

The quest to find a theory of quantum gravity that could potentially explain everything. Nearly 60 years ago, Nobel Prize-winners Arno Penzias and Robert Wilson stumbled across a mysterious hiss of faint radio static that was interfering with their observations. They had found the key to unravelling the story of the Big Bang and the origin of our universe. That signal was the Cosmic Microwave Background (CMB), the earliest light in the universe, released 379, 000 years after the Big Bang. It contains secrets about what happened during the very first tiny increments of time, which had consequences that have rippled throughout cosmic history, leading to the universe of stars and galaxies that we live in today. This is the enthralling story of the quest to understand the CMB radiation and what it can tell us of the origins of time and space, from bubble universes to a cyclical cosmos - and possibly leading to the elusive theory of quantum gravity itself.

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THE FIRST LIGHT IN THE UNIVERSE

A Little Bit of History

Like a tune that you just can’t stop playing in your mind, the anomalous hum of radio static refused to go away.

It was June 1964, and Arno Penzias and Robert Wilson didn’t quite know what to make of their persistent radio hiss. Employed by Bell Labs, the pair of radio astronomers were gearing up to use a large, anvil-shaped contraption called the Holmdel Horn Antenna. Their plan was to use it to study the strength of radio waves emanating from distant celestial objects, such as galaxies host to active black holes, or the expanding remnants of exploded stars.

At the time, radio astronomy – that is, the observation of the Universe at radio wavelengths – was still in its formative years. It was just three decades since Karl Jansky, a previous employee at Bell Labs, had made the first astronomical observations in radio waves, when he detected emissions coming from what we now know to be charged particles moving through powerful magnetic fields around the black 6hole at the centre of our Milky Way Galaxy. Jansky’s discovery had opened up an entirely new astronomical frontier, one that Penzias and Wilson were eager to explore.

The Holmdel Horn Antenna that Penzias and Wilson used to detect the CMB.
NASA

Both astronomers were still young, Penzias just over 30, and Wilson still in his twenties, having graduated with a PhD in physics from the California Institute of Technology a few years earlier. At Caltech, Wilson had been taught by the brilliant but controversial Yorkshire-born astrophysicist Fred Hoyle, who at the time had been promoting his ‘Steady State’ theory of an eternal universe, in competition with the theory of the Big Bang. Penzias was already at Bell Labs and had recruited Wilson after getting to know him at various scientific conferences. 7

The Holmdel Horn had been designed and built a few years earlier by engineers at Bell Labs, with the intent of bouncing signals off the world’s first crude communications satellite, Echo 1, but now Penzias and Wilson were given permission to use it for radio astronomy instead. Although it wasn’t the largest radio telescope in the world – it had an aperture of 6.1 metres (20 feet), amounting to a total collecting area of 25 square metres, which is minuscule compared to the 4,560 square metres of the 76-metre Lovell Telescope built in 1957 at Jodrell Bank, in the UK’s Cheshire countryside – it had several things going for it. One was that its horn-shaped design meant that its receivers were well sheltered from any terrestrial radio interference – radio waves from space weren’t going to be drowned out by the Billboard Hot 100. Second was that Penzias and Wilson believed that all sources of ‘noise’ – i.e. radio interference from things like the telescope’s electronics – were already well known, which would assist them in making their absolute radio brightness measurements. Coupled with specially designed amplifiers, it was arguably the most sensitive radio telescope in the world, pound for pound, when observing celestial sources that filled its field of view.

However, before they could embark on their radio astronomy experiment, the antenna required some upgrades. In particular, Penzias and Wilson added a device known as a ‘cold load’, which was nothing more sophisticated than a radio-wave-emitting container filled with liquid helium at a temperature of about –270 degrees Celsius (approximately three degrees above absolute zero, which is designated as 0 kelvin/–273.15 degrees Celsius). The cold load, radiating radio waves at a wavelength correlating to its frigid temperature, was critical to what Penzias and Wilson were trying to achieve. In those early days of exploring the radio sky, 8astronomers were mainly estimating the true radio brightness of objects using a technique called the on/off method. It was quite simple – point a radio telescope at a radio-wave-emitting target, log the strength of the radio waves, and then turn the telescope to an apparently empty part of the sky and measure the strength of the radio waves in the background, which in theory should be roughly the same value in any random direction. At which point the background value could be subtracted from the target’s radio signal, to leave just the radio waves from the target.

The trouble was that this was all very imprecise, since the vagaries of the background sky were still uncertain and not well understood. What Penzias and Wilson intended to do was to bypass the background sky entirely, by using the cold load as an artificial source of radio waves with a precisely known output to compare against the radio emission from celestial targets in order to produce an absolute measurement of their brightness. Since the wavelength of radio emissions are related to the temperature of their source, in the sense that the hotter an object is, the shorter the wave-length of its emitted radiation, and vice versa (according to Wien’s law, developed by physicist Wilhelm Wien in 1893), the cold load has to be as chilly as possible so that any radio waves it emits are at a wavelength long enough not to drown out any of the radio signals from space.

After adapting the horn antenna for radio astronomy by adding the cold load and a microwave receiver called a radiometer, Penzias and Wilson switched it on and found to their dismay that there was something wrong: an excess of signal that they couldn’t account for. They had expected some noise – a degree from the walls of the antenna absorbing and re-radiating photons, and a few degrees from the 9background radio sky behind their target of interest – but this was something else, a radio hiss at a wavelength equivalent to a radiation temperature of 2.73 kelvin (–270.45 degrees Celsius) that the two radio astronomers could not explain. The signal was like a faint static, and whichever direction they pointed the horn antenna, day or night, it was there.

The obvious solution seemed to be that it was interference from somewhere, perhaps from the telescope itself, or, in spite of the Holmdel Horn’s design, from the environment around it. For the best part of a year, Penzias and Wilson battled away, trying to rid themselves of this annoying radio hiss so that they could get on with their astronomical experiments. At one point they even suspected a pair of pigeons that had been nesting inside the horn and leaving their droppings on the surface, which in theory could have produced a small radio signal. So they safely extracted the pigeons and mailed them away, to be released over 60 kilometres from the antenna, before sweeping out all of the pigeon droppings. Yet before Penzias and Wilson had chance to test whether this had solved the problem, the pigeons managed to find their way back to the antenna, and so more serious measures were taken, with a colleague bringing a shotgun to the antenna and unceremoniously shooting the birds.

Alas, the pigeons died in vain, as the rogue hiss didn’t go away. By April 1965 the two young astronomers were at their wits’ end, when it was recommended to Penzias that he speak to Bob Dicke, who was a physicist at nearby Princeton University, and who, it was intimated, may have some answers for him. As a last throw of the dice, Penzias picked up the telephone and made the call.

* * *

10There is a scene at the end of the film Raiders of the Lost Ark where, having handed over the Ark of the Covenant to government officials, our erstwhile heroes ask who is now studying the powerful artefact. ‘Top men,’ comes the blunt reply.

If the field of study had been physics rather than archaeology, then Bob Dicke would have been one of those ‘top men’. His work included pioneering advances in radar technology and microwave receivers during the Second World War, a patent for an infrared laser, the science of spectroscopy and testing Albert Einstein’s General Theory of Relativity. It was this latter work that brought Dicke to cosmology in the early 1960s. Working alongside fellow cosmologist Jim Peebles at Princeton, he made an important theoretical breakthrough, although what neither realised was that they’d already been beaten to this breakthrough about fifteen years earlier.

Anyone working in cosmology is already standing on the shoulders of giants, beginning with Edwin Hubble and what was, and probably still is, the greatest achievement in all of astronomy. During the 1920s Hubble turned the 2.5-metre mirror of the Hooker Telescope, atop Mount Wilson in California, towards the distant, misty patches of light that were called the spiral nebulae, and discovered that they were not nebulae in our galaxy at all, but galaxies in their own right, existing far beyond the confines of the Milky Way. Thanks to Hubble, what we thought of as the Universe had suddenly vastly increased in size, while at the same time our contextual place in that Universe had conversely grown smaller.

That was in 1924, and five years later, after continued study of the spiral galaxies, Hubble arrived at another galaxy-shattering conclusion, all thanks to the same phenomenon that causes a police siren to change in pitch as the car drives past. That’s caused by the Doppler shift, whereby the 11sound waves of the passing siren are compressed and then stretched. The same occurs with light waves – a galaxy moving towards us will have its light compressed towards bluer wavelengths, while a galaxy moving away will have its light stretched to redder wavelengths. This blue- and red-shifting can tell us whether galaxies are moving towards or away from us, and Edwin Hubble discovered that almost every galaxy he looked at (with one or two notable exceptions, in particular the nearby Andromeda Galaxy) is moving away from us. Furthermore, the more distant a galaxy is, the faster it appears to be receding from us. Everywhere we look, the Universe seems to be expanding.

How the Doppler effect works, using the example of a passing emergency vehicle with its siren blaring. The wavelength of the sound waves from the siren as it moves away are stretched longer, whereas the sound waves moving ahead of the vehicle are compressed to shorter wavelengths.

Hubble left it to theoreticians to come to conclusions, but those conclusions seemed obvious: if the Universe is expanding, then in the past it must have been smaller – much smaller. And so the Big Bang theory came to the fore – the idea that long ago, the Universe, and all the matter and energy it contains, was condensed down to a single point that 12had begun to expand, and continues to do so to this day. This expansion is characterised by a simple calculation, named the Hubble–Lemaître law since it was actually first conceived by the Belgian mathematician and physicist Georges Lemaître in 1927, two years before Hubble announced his observational results. The law describes the rate of expansion, known as the Hubble constant, as being equal to the recessional velocity of a galaxy away from us divided by its distance from us.

Although the expansion was beyond doubt, evidence for the Big Bang wasn’t yet conclusive. Not every scientist was fond of the idea that the Universe may have had some kind of beginning, since it smacked of the notion of a Creator. Among those that railed against the notion of the Big Bang was Fred Hoyle, who developed a counter-model, the aforementioned Steady State theory, which described how new space was continually being created as the Universe expanded, allowing the cosmos to be eternal.

Dicke and Peebles realised that if the Big Bang theory was correct, then all the matter and energy in the Universe crushed into a microscopic volume when the Universe was still in its infancy would have created an exceptionally hot environment, well into the range of trillions of degrees Celsius. The Princeton duo calculated that the radiation left over from the hot Big Bang should still be present in the Universe, but since the Universe had been expanding for 13.8 billion years, the temperature of this radiation should now be only a few degrees above absolute zero, placing it firmly in the range of microwave wavelengths (part of the radio realm of the electromagnetic spectrum).

Neither Dicke nor Peebles had knowledge that all of this had been calculated once before, in the late 1940s by George Gamow, who was a Russian immigrant and cosmologist who 13had set up home at the George Washington University in Washington DC in the 1930s, and in particular his research students Ralph Alpher and Robert Herman. For over a decade their conclusions had been forgotten about, for several reasons; partly because Alpher and Herman were not big names in astrophysics, partly because their interest was more in the pure physics of the problem rather than the cosmological consequences, and also because it was erroneously thought to be too difficult to detect such a signal – the coldest signal detectable at the time was 20 kelvin.

Dicke, with his experience building radiometers, knew that it was possible to set about detecting such a signal, and so with his colleagues Peter Roll and David Wilkinson, and with input from Peebles, Dicke set about building a microwave telescope to detect this ‘cosmic microwave background (CMB) radiation’ – a telescope that would feature a cold load and which they believed was unique in the world. The quartet would hold regular meetings in Dicke’s office as they planned their project to discover the CMB radiation. It was during one of these meetings, at lunchtime on a spring day, that Dicke’s telephone rang. On the other end of the line was Arno Penzias.

Dicke listened intently to what Penzias said, and then, hanging up the receiver, turned to his group and, with a deep breath, famously said, ‘Well boys, we’ve been scooped.’

Of course, the microwave telescope that they were building on the roof of their department building was not as unique as they had thought, with Penzias and Wilson operating a very similar device, and only about 50 kilometres from Princeton at that; yet until that fateful telephone call neither group was a...