Unexplainable astronomy? Part 2: Of pigeons and cosmic uniformity

By Sam Jarman

Ask just about any scientist, and they will tell you that the very best discoveries they can make are the ones which force us to change how we think about the world completely. That’s exactly what happened in 1923, when Edwin Hubble looked out at a small, hazy nebula from his mountaintop telescope in California.

Without Henrietta Leavitt’s work, we might still think of galaxies like Andromeda as funny-looking nebulae which don’t make any sense | Image: NASA

Based on previous research by Henrietta Leavitt, Hubble was looking for Cepheid variable stars in the nebula, which would tell him exactly how far away it was. He managed to find a variable star easily enough, but it revealed a pretty big problem. The star showed Hubble that the nebula he was looking at was not even a nebula after all, but a vast cluster of stars situated very, very far away.

The distance was greater than any we had thought possible for thousands of years. For scientists, that meant that we needed to completely rethink our model for the structure of the universe. With the discovery of the Andromeda galaxy, the fundamental idea of billions of galaxies occupying a vast, ever-expanding universe was born. It was a lot for the world of astronomy to take in.

In the decades following Hubble’s discovery, theoretical physicists struggled to discern the scientific rules which could possibly govern such a newly colossal, mysterious universe. Some of their theories seemed promising, but In the meantime, the imaginations of the media and the public ran wild. If our understanding of the universe could be so shaken by one discovery like Hubble’s, they reasoned, what other revelations could be hiding in plain sight? And, of course, the biggest question: could intelligent life be out there somewhere?

Apprehension had grown strong by the 60s. Theoretical physicists had some convincing ideas about the nature of the universe, but at the time, technology just wasn’t sophisticated enough to prove experimentally which were correct, and which were misguided. Their ideas so often disagreed with each other that it seemed like no-one had really got anywhere at all. But change was coming. Improved telescopes meant that by 1964, mysterious signals, the likes of which would make or break previously speculative cosmological theories, began to come in.

A dodgy radio receiver? 

Pigeons. It had to be. Since making their home in the cosy hovel that was the Holmdel Horn Antenna, the birds had been busy and painting the walls with the remains of their dinner, which was giving off who knows what kind of radiation. How else could radio astronomers Arno Penzias and Robert Woodrow Wilson explain the uniform, unwanted microwave signal, which wouldn’t go away no matter where in the sky they pointed the telescope?

Whenever you’re feeling down, just remember that a sad metal toilet in New Jersey once became one of the most important telescopes of the 20th century| Image: NASA

The pair cleared out the avian intruders, and scrubbed the inner surface of the antenna clean. They checked the equipment thoroughly yet again, making sure the receiver was still cooled to just above absolute zero, minimising any interference from within the telescope. But still, the low, intense noise hindering their experiment persisted. Clearly, the radiation was originating from something more interesting than pigeon droppings. Something, they realised, beyond our own galaxy.

Coinciding with Penzias and Wilson’s predicament, a group of astrophysicists at Princeton University were working on a theory they believed could explain what was happening in the earliest moments of the universe’s existence. Based on earlier work by cosmologist Ralph Alpher, their idea hinged on the concept that the universe had all started with a rapid, high-energy expansion from a single point: a Big Bang. They proposed that if the Big Bang had really happened, it must have released a colossal surge of radiation, still observable to this day with the right equipment. The radiation would have cooled significantly by now, but if its remnants were detected, it would be definitive proof that the scientists’ notion was correct.

The idea was in direct conflict with the earlier-proposed Steady State theory, which suggested that the universe had no beginning, meaning no Big Bang, at all. Instead, the universe had existed for an eternity beforehand, and had never changed significantly. The two theories were radically different, and both appeared to have equal weight. But without any hard evidence, neither could be validated over the other. That all changed in 1964, when Penzias and Wilson caught word of the Princeton scientists’ research.

The Colossal Clang and Tremendous Toot were also considered, but never really caught on| Image: NASA/WMAP Science Team

Through a friend, Penzias got access to a preview of the Princeton paper. What he read astounded him: using first principles, the astrophysicists had argued that a surge of electromagnetic radiation had permeated the entire universe following the Big Bang. As the universe expanded, so too did the wavelength of the radiation, until reaching its current wavelength of 7.35 cm. That was the very same radiation Penzias and Wilson had attributed to pigeon poop – almost exactly. The pair got in touch with the Princeton scientists as soon as they could.

The rest of the story is history. The two groups teamed up to publish not only a theory of how the Big Bang could have hypothetically played out, but indisputable experimental proof of the Big Bang itself on top of it. The Steady State theory had been disproved, and was consigned to history. In 1975, Penzias and Wilson earned a Nobel Prize for their ground-breaking discovery.

Decades later, the radiation observed for the first time by Penzias and Wilson – now known as Cosmic Background Radiation – has been studied in meticulous detail. It has been found to be incredibly uniform, but not perfectly so. In the very earliest moments of the universe, some regions emerged where matter was densely clumped together, whereas it was sparser in others. As the regions formed, the surge of radiation created by the Big Bang was slightly distorted by the clumps of matter.

Of all the scientific results which were first thought to be literally crap, the Cosmic Microwave Background has got to be the most impressive | Image: NASA

Billions of years later, the denser regions have become areas abundant in galaxies, while the sparser regions are now vast, empty voids in space. The Cosmic Microwave Background has retained the distortion it underwent at the very start. By picking up the subtle variations in the radiation, we can now create maps of the structure of the universe with detail which must have been unthinkable just a few decades ago.

So from a mysterious error first attributed to pigeons with poor housekeeping skills, came another fundamental theory in cosmology. But the 60s weren’t over, and there were yet to be more discoveries which would prove to shake up our understanding of how the universe works. In 1967 came another bizarre signal, this time with a somewhat more exciting misguided idea about where it came from.

Unexplainable Astronomy? Part 1: The Wow! Signal

By Sam Jarman

The scribble that started it all | Image: Big Ear Radio Observatory, NAAPO

Monday was dragging for Jerry R. Ehman, in a way that only a volunteer astronomer at the SETI project could truly understand. The frequency data printed out by the IBM 1130 computer was, as always, infuriatingly ordinary. The stream of paper he needed to analyse by hand was seemingly endless. Coffee in a Styrofoam cup was slowly growing lukewarm. Then… something different.


On August 15 1977, Ohio State University’s Big Ear radio telescope picked up an intense and unexpected burst of radio waves, which lasted for a full 72 seconds. Stunned by the break in the monotony of his task, Ehman couldn’t help but briefly forget his deeply engrained skills in data analysis. Taking a bright red marker, he drew a circle around the numbers and letters indicating the burst, and wrote one word next to it: “Wow!”

Ehman wasted no time in letting his fellow members of SETI know about his discovery. The implications of the burst were not lost on any of them. The SETI volunteers knew there and then that the burst could not be explained by any current science. On each of their minds was the possibility that the purpose of their project had at long last been fulfilled: evidence of intelligent life beyond the Solar System.

 It doesn’t sound much like ‘We come in peace’, or ‘As you no doubt will be aware, the plans for the development of the outlying regions of the western spiral arm of the galaxy require the building of a hyperspace express route through your star system and, regrettably, your planet is one of those scheduled for demolition’, but the Wow! Signal could be the closest thing we have to a message from aliens

Hey, I know that frequency!

To understand why Ehman and his colleagues were so excited by the burst, we need to look at the characteristics of the particular frequency their telescope was picking up. The Big Ear radio telescope had been programmed to detect signals with the very specific frequency of 1420.406 Megahertz. That number might not roll off the tongue, but it’s incredibly important in astrophysics, and even has its own name: the Hydrogen Line frequency.

The red colours shows where hydrogen gas has been detected within the Milky Way. As it turns out, there’s a lot of it | Image: NASA

Hydrogen gas is extremely common in the wide expanses of space which lie between the stars. Atoms of the gas, consisting of an electron orbiting a single proton, are normally very stable and pretty uninteresting. But occasionally, the electron will flip over spontaneously due to quantum processes – a phenomenon known as spin flip transition, making the system unstable. To return to normal, the electron needs to flip back around, releasing a flash of light with a specific energy – a radio photon – in the process.

This process is incredibly rare for individual hydrogen atoms, but when hydrogen gas is gathered in clouds which span many light years, enough radio photons are given off by spontaneously flipping electrons that they can be easily picked up by radio telescopes on Earth. These photons make up the radio signal of the Hydrogen Line, which is important both to radio astronomers and to those hopeful that other technologically advanced civilisations could be out there somewhere.

The Hydrogen Line frequency is useful to astrophysicists, as they can use it to detect the exact locations of clouds hydrogen gas within our galaxy, and in those beyond. The clouds will always be densest in galactic arms, allowing radio astronomers to map out the distinctive structures of galaxies. They can also use the Doppler Effect to measure how fast hydrogen gas at various distances from the centre of galaxies is moving. From this, they can then create rotation curves for different galaxies, which currently give the clearest evidence we have for the existence of Dark Matter (but that’s a whole other story!)

So when astronomers point their radio telescopes at hydrogen gas clouds, it’s hardly surprising for them to observe radio waves at the Hydrogen Line frequency. But what if we observe them in places in the sky where we aren’t expecting them, or at higher-than-expected intensities? Any astronomer will agree that if this happens, something unexplained is going on.

“Huh, these aliens know about hydrogen spin-flip transition and have an in-depth understanding of their place in the galaxy… so why haven’t they invented clothes yet?” | Image: NASA

Astronomers at SETI became interested in the Hydrogen Line frequency because it is such a fundamental figure in astronomy. Clouds of hydrogen gas are so abundant in the galaxy that their signal can be picked up no matter where you go in space. SETI figured that if there are any other enlightened civilisations elsewhere in the universe, then they must realise this too. No matter how different their scientific units are to ours, if we asked them to create a radio signal at the Hydrogen Line frequency, it would be exactly the same as the signal we would create. So what better way to announce your presence to the galaxy than to send out a distinct, high-intensity transmission of the frequency?


SETI has already sent signals like this out into space, in the hope that others out there may be listening. In fact, they have a strict control over the transmission of the Hydrogen Line signal; it is now illegal to get anyone’s hopes up by transmitting the frequency yourself. But that isn’t the only method we have used to broadcast our knowledge of the Hydrogen Line. In 1972, under the efforts of Carl Sagan, Frank Drake and other journalists and astrophysicists, the famous Pioneer plaque was attached to probes Pioneer 10 and Pioneer 11.

In the top-left of the plaque is etched a diagram representing the distinctive electron flipping process which causes the Hydrogen Line. Between two representations of hydrogen atoms with electrons in the two different states is a horizontal line 21.106 cm long – the exact wavelength of a radio wave at the Hydrogen Line frequency of 1420.406 MHz. If an intelligent civilization ever finds one of the probes then it’s a safe bet that they will understand exactly what the diagram is representing, no matter their language.

 An explanation, or an alien-killing pretender?

Not long after Ehman’s bizarre discovery in 1977, the newly dubbed Wow! Signal, named after Ehman’s hasty scrawl in red pen, had taken the worlds of both science and the media by storm. While astrophysicists rushed to discover the astronomical source of the signal, journalists began to enthusiastically speculate that without any existing scientific explanation, it could have been more purposefully created in origin. For hopefuls of the existence of extra-terrestrial intelligence, the evidence was now more tantalising than ever.

Sorry it was me all along… or was it? | Image: NASA, ESA, J.-Y. Li

Yet for all the attention the Wow! Signal gained, the search for its origin proved fruitless for scientists and alien hunters alike. For over 40 years, the result sat there unexplained; frustrating some, and instilling hope in others. But in April 2017, astronomer Antonio Paris from St Petersburg College, Florida claimed in a paper to have solved the mystery once and for all.

Paris argued that on August 15, 1977, two comets inside the Solar System – 266P/Christensen and 355P/Gibbs – passed directly in front of the Big Ear radio telescope. Surrounding one of the comets was a cloud of hydrogen gas, which was given off by one of the masses of ice and rock. Naturally, the telescope picked up the Hydrogen Line signal of the cloud, but only as it passed through Big Ear’s field of view. For the first time, it looked like the mystery had been solved. But not everyone was satisfied with the new explanation.

Within weeks, Paris was receiving backlash from scientists, who had some strong criticisms of his paper. In June, Robert S. Dixon, director of the SETI project himself, published a rebuttal to Paris’ paper, claiming that the two comets weren’t in fact within Big Ear’s field of view on the day of the Wow! Signal. Other criticisms included claims that the comets were a long way from the Sun and therefore inactive, meaning neither of the comets could possibly maintain a hydrogen cloud around themselves. Some scientists even had many harsh words to say about Paris’ scientific methods in general. Paris stands by his theory, but he’s open to debate.

So for now, many astronomers still regard the Wow! Signal as a mystery, and hope remains that the true explanation of the burst could be another advanced civilization broadcasting its existence. But this isn’t the only case where strange signals in astronomy have gone unanswered for long periods, or where far-fetched and alluring explanation theories have been thought up.

Over the last century, the world of astronomy has been become famous for detecting mysterious signals, making bizarre discoveries, and throwing up seemingly unanswerable questions. Some of these questions have had mundane explanations. Others have led to new research which has come to revolutionize our understanding of the universe. And, like the Wow! Signal, still others remind us just how much we have yet to learn.

In this series, we will find out more about the signals which have both answered and created some of the most enticing questions in modern astronomy. Next time, we will turn back the clock to before 1977, when astrophysics was reeling at the revelation that our place in the universe was far less significant than we realized.

Throat singing: a storm of sound on the steppes

By Sam Jarman

The day dawns cold, clear, and still. A nomadic Mongolian herder knows that sound will carry well in these conditions, so he takes his chance. Climbing onto his horse, he rides into the wilderness, intent on finding a place where he can honour one of his people’s most sacred traditions. Using only his voice, he will imitate the song of birds, the hum of wind streaming over a rugged mountainside, and the hooves of wild galloping horses.

Finding the right spot is crucial. The herder needs to probe the nearby river valleys and mountaintops, to discover the place where his song will sound the most clearly.  If he chooses well, his voice could be heard for miles across the surrounding plains. He has learnt from his ancestors to use the Mongolian steppe as a sound studio; its open landscape providing the perfect acoustics to carry his voice. When he finds the right place, he begins his song.

His mesmerising chant often doesn’t sound like anything a human could create with their voice. The song could vary in pitch between being hauntingly deep, and as high as a flute. But astonishingly, he can sing both of these pitches, and many tones in between, at the same time.

For generations, people in Mongolia and surrounding areas in Siberia and northern China have learnt to use their voices to mimic the sounds of nature with a hypnotic accuracy. Even in the face of modernisation, the tradition remains as strong as ever today. It’s hardly surprising that the extraordinary talent of throat singing has become one of the most famous and iconic traditions in Mongolian culture.

Many Mongolians learn to throat sing from a very young age, but the techniques they use to produce such a bewildering array of sounds are anything but simple. To truly understand how they do it, we need to explore the anatomy of our own voices, and the physics of the acoustic waves we create with our vocal apparatus. Throat singers have learnt to intricately manipulate these acoustic waves to produce some astonishing sounds.

How it’s made: the human voice

Like any breath, or a normal speaking or singing voice, the song of a throat singer begins life as the lungs contract, forcing out a column of oxygenated air. The air travels upwards through the tube connecting our lungs to our throats, known as the trachea. But to distinguish the breath from a noiseless exhale, it first needs to pass through the larynx, sitting just above the trachea. The larynx contains a set of two vocal chords, which contract to a thin slit when we decide we have something to say, or rather sing.

Looking top-down: this little slit is responsible for every word you’ve ever said, even the embarrassing ones | Image: Henry Vandyke Carter

As air passes through the contracted vocal chords, they are forced to vibrate. These vibrations chop the air column up, forcing it to vibrate itself. At regular intervals, the air will be blocked entirely, and then be let through in a sudden rush. So, the air coming through the vocal chords vibrates in a pattern we are probably very familiar with: a sound wave.

The pitch, or frequency of these sound waves is ultimately decided by the size of our vocal chords. The tension in the chords, and the size of the gap between them, determine how much the sound waves they produce will vibrate over time. This gives them an important property, called the fundamental frequency – a value which entirely defines the unique pitch of each of our voices.

The fundamental frequency is heavily dependent on gender. Women’s vocal chords are usually smaller, meaning the air column in the trachea vibrates more over time, giving the sound waves a higher frequency. For men, larger chords will cause the air column to vibrate at a lower frequency. But the fact that no two voices sound the same can’t be explained by variations in pitch alone.

A sound wave in it’s purest form – you can see how air is compressed and spaced out at regular intervals. This type of wave is easy to understand, but lacks emotional depth… | Image: Pluke

On top of a fundamental primary wave in our voices, there are many smaller, higher-frequency waves also created by the vocal chords. Called overtones, each of these waves has a distinctive frequency; the first will have twice the frequency of the fundamental wave, the second will have three times the frequency, and so on. When stacked on top of each other, all of these waves create one intricate and complicated sound wave. This is the sound of our voices, as unique to each of us as our fingerprints.


…The sound waves of our speech are much more complicated

Already, these sound waves can accurately express our emotions. When listening to other people, our minds can tell if their voice indicates happiness, sadness, anger, or surprise, simply by picking up subtle patterns in its loudness and variations in frequency. But to turn the sound waves into a coherent language, the air finally passes through our mouths. The continuous movements of our throats, tongues, and lips sculpt sound waves into the words we use to communicate with each other (here’s a pretty hilarious demonstration of how it works).

We use our vocal apparatus so often that we don’t need to consciously think about how we need to manipulate it to produce the particular sounds we want. But for Mongolian throat singers, the anatomy we normally use to talk and sing has untapped potential. The secret of their captivating sound lies in an important phenomenon in acoustic physics, known as resonance.

Resonance: a perfect storm of sound

Every object in nature has a natural frequency it vibrates at. Normally, any vibrations the object interacts with won’t have the same frequency as its natural frequency. But if the two frequencies match up (with a few physical constraints), the object’s vibrations can become much larger. One of the most famous examples of resonance is the bizarre collapse of the Tacoma Narrows bridge in 1940:

The reasons why the Tacoma Narrows bridge acted so strangely before collapsing have been strongly debated, and still aren’t fully understood. But the most agreed-upon theory is that patterns in the strong winds blowing across the bridge that day had just the right frequency to induce resonance in the bridge, causing a huge standing wave to form in the it’s road. Eventually, the bridge’s suspension cables gave out, but not before giving an outlandish display of the properties of physics.

In the case of our vocal apparatus, the natural frequency is determined by the sizes of the gaps which the sound waves of our voices need to pass through. If the wavelength of the sound is equal to the size of the gap, the gap itself will be forced to vibrate, creating its own waves with the same frequency as the original sound. All of the waves add together in a ‘perfect storm’ to amplify the sound, making it much louder than it was originally – a resonant sound wave is formed.

Throat singers have learnt to manipulate parts of their vocal anatomy to produce resonant sound waves artificially. Different types of throat singing can cause resonance in different parts of the singer’s vocal apparatus. This creates a wide array of different types of throat singing, which each have their own name in the Mongolian language. The different styles are too numerous to cover in one article, but a smaller number of fundamental styles give rise to many of them.


In this style of throat singing, one or more of the smaller, higher-frequency overtone waves are caused to resonate as they pass through small, specifically-sized gaps in the singer’s throat and mouth. These waves are normally subtly engrained into our voices as individual overtones, but here, they can be heard as distinct, mesmerizingly clear sounds.

Astonishingly, the sound can be heard over the singer’s primary singing tone, meaning more than one note can be sung at a time by a single person. By the nature of our natural vocal sound waves, if we isolated all of the overtones which make up our voices, they would all be in harmony with one another. That means that the singer’s overtones create a one-person symphony without any further effort.

Typically in Khoomei, the singer will chant one continuous primary note, and then vary the overtones which are resonated. This creates a strong base sound, with a higher -pitched tune being sung above it. Mongolians believe the multiple tones of Khoomei give the impression of wind as it swirls around rocks and boulders, creating an enchanting natural chorus.

Sound: Alash Ensemble


The highest of our natural overtones have far higher frequencies than anything a human could create with their natural voice. In this style, singers create a tiny gap between their tongues and teeth, to match the wavelengths of their highest-frequency overtones. The sound is shrill and piercing – as close as the human voice can come to sounding like a flute.

Like Khoomei, this style involves a steady fundamental tone being sung, with the resonant overtone above it. Yet in Sygt, the high-pitched overtone dominates the song; the gap which sound waves need to pass through is so small that the primary wave is greatly diminished. To Mongolians, Sygt is intended to mimic the sound of birdsong, and the warm, gentle summer breezes over the steppes.

Sound: Alash Ensemble


Situated right above the vocal chords are two folds of membrane which look similar to the vocal chords, but normally serve an entirely different purpose. The ventricular folds are there to prevent food and drink from entering our airways, but throat singers have learnt to manipulate them to produce one of their most iconic sounds of all.

By manually contracting their larynx to exactly the right shape, throat singers can bring their ventricular folds and their vocal chords together. So when air passes through the vocal chords, the ventricular folds will resonate themselves, producing their own sound. However, this resonance is unlike the effects seen in other types of throat singing; here, the ventricular folds vibrate at exactly half the fundamental frequency, creating an artificial undertone.

In musical terms, halving the frequency of a sound will bring it down an entire octave. This has astonishing implications for throat singers. Kargyraa singers can reach a wide range of notes far deeper than anything they could sing using their vocal chords alone. The effect is a haunting, low-pitched sound, reminiscent of rolling thunder, or the mournful cries of a camel after losing her calf.

Sound: Alash Ensemble

Kargyraa in particular can partly explain why throat singing still remains so popular in Mongolia today, and is even facing a resurgence. In the past, social taboos meant that women weren’t allowed to practice the tradition, but now, these barriers are breaking down. Female vocal chords may be smaller, but women can still contract their ventricular folds to create a fantastically deep sound:

Now that men and women have equal opportunities to practice the art, throat singing is being taught to boys and girls across Mongolia and the surrounding regions; all eager to pass on the traditions of their ancestors. The iconic sound has made its way into concert halls and recording studios, and in the West, throat singing has gone from a mysterious, alien practice, to one which is beginning to influence our own culture.

Anatomically, there is no special adaptation among the Mongolian people that enables them to throat sing better than anyone else. It’s no easy talent to learn, but we are all capable of recreating the effects of resonance on our voices, if we practice for long enough.