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Uncovering the universe’s invisible secrets

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Taken from the August 2022 issue of Physics World, where it appeared under the headline “Out of sight, beyond imagination”. Members of the Institute of Physics can enjoy the full issue via the Physics World app.

Laura Hiscott reviews The Invisible Universe: Why There’s More to Reality than Meets the Eye by Matthew Bothwell

Dark truths Images like this, captured by the Hubble Space Telescope, let us see deep into the universe – this spiral galaxy, NGC 6217, is 60 million light-years away. (Courtesy: NASA, ESA, and the Hubble SM4 ERO Team)

Like many people, I was afraid of the dark as a child. I would struggle to sleep in a pitch-black room, my imagination projecting anything scary onto the space I couldn’t see. Perhaps there are evolutionary reasons for this common fear to do with nocturnal predators, but there’s also a very simple one: sight is the primary sense most of us rely on to collect information about our surroundings. If we can’t see what’s around us, then how can we be sure there are no monsters lurking in the dark?

In astronomy, it turns out, invisible monsters are very real – even if they aren’t the kind that most children are scared of. From black holes to the ancestors of dead galaxies, these elusive cosmic characters are the focus of The Invisible Universe: Why There’s More to Reality than Meets the Eye, by Matthew Bothwell, an astronomer and science communicator at the University of Cambridge, UK.

In the introduction, Bothwell makes a compelling case for studying the unseeable by use of a striking analogy. If we consider the spectrum of visible light to be a single octave on a piano, with red light being the note middle C and blue light, roughly half the wavelength, being the C an octave above, then how far does the full spectrum of electromagnetic (EM) radiation extend? The answer is 65 octaves, “as much as nine grand pianos standing in a line”.

It’s a humbling fact for a species that uses sight as its main sense – and it doesn’t stop there. There are astronomical spectacles that would still elude us even if we could see across all 65 octaves of light. After all, we can’t see black holes because no light of any wavelength can escape them, while dark matter doesn’t seem to interact with EM radiation at all. Meanwhile, gravitational waves are ripples in the fabric of space-time itself rather than anything to do with EM radiation, and all we know about dark energy is that it is causing the universe to expand at an accelerating rate. The Invisible Universe has chapters dedicated to all of these phenomena, describing them vividly while explaining the physics behind them in an accessible manner.

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Bothwell approaches each chapter like a detective story: he introduces some unexpected observation, follows the development of theories to explain it, and relates their triumphs or failures. One chapter that particularly swept me along is “Monsters in the dark: the quest to find the Universe’s hidden galaxies”.

Bothwell explains that there is a class of galaxies in the universe that are gargantuan even by galaxy standards – one such behemoth is “big enough to swallow the Milky Way, Andromeda, and all the space between”. But strangely, all the galaxies this size seem to be dead – they no longer actively form stars. The rest of this chapter reads like a cosmic murder mystery, as we look back in time – by looking deeper into space – in search of these galaxies’ ancestors and what killed them off.

Puzzlingly, there are no candidates visible in the images of deep space beamed back by the Hubble Space Telescope that are nearly extreme enough to be those predecessors. But non-visible sub-millimetre observations reveal a new type of galaxy in the early universe, shrouded in dust, that produced stars ten times more rapidly than even “starburst” galaxies nearby. These ancient star factories are excellent candidates for the dead galaxies’ ancestors, but how did they all die off? It is a testament to how much I enjoyed this chapter that I feel I shouldn’t give this spoiler away – but I will say that the resolution does not disappoint.

I also enjoyed the stories of the scientists and engineers that Bothwell weaves throughout the book. One I hadn’t heard before concerned Karl Jansky and Grote Reber, who were early pioneers of radio astronomy. In the early 1930s, Jansky was an engineer working at Bell Labs in the US, trying to get rid of the problem of radio static. While studying this noise, he discovered a type of signal that repeated every 23 hours and 56 minutes.

Bothwell explains that this is a tell-tale sign of something from beyond the solar system. Although we think of a day as being 24 hours long, it actually only takes 23 hours and 56 minutes for the Earth to rotate once relative to the Milky Way. During that time the Earth has moved a bit further around the Sun, so the planet needs to rotate for another four minutes for the Sun to reach the original point in the sky. But in 23 hours 56 minutes, we’re lined up with our galaxy again. Therefore a signal with this period is likely to be coming from outside of the solar system – indeed, Jansky was detecting radio waves from centre of the Milky Way.

Incredibly, the astronomy community as a whole took little notice of Jansky’s results when he first published them. Bothwell describes this as being for one simple reason: “the world of radio engineering was just too far removed from the world of astronomy”.

But Reber – an engineer who designed electric equipment for radio broadcasts – was fascinated with this noise from outer space. He built his own radio telescope in his back garden in Chicago, and, from the mid-1930s to the mid-1940s, was “the only radio astronomer in the world”.

Reber took courses in physics and astronomy at his local university, to help him understand his observations, and he was the first person to detect radio waves emanating from hot gas clouds where stars were being born. According to Bothwell, Reber was “the last of the amateur ‘outsider’ scientists”, who “through painstaking and meticulous work managed to change the scientific world”.

This story and many others throughout the book underline the crucial importance of technology for progress in astronomy – and the current technology is remarkable. For example, to detect gravitational waves, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has to be able to sense a squeeze and stretch of less than the diameter of a proton, and to image black holes’ event horizons, the Event Horizon Telescope achieves a resolution equivalent to reading a newspaper in New York using a telescope in London.

So the feats of engineering that humans have achieved to detect astronomical phenomena are perhaps as awe-inspiring as the phenomena themselves. And Bothwell reminds us that there’s more to come, with projects underway to take capabilities to the next level across all fields of astronomy. For example, the European Space Agency is currently building the first space-based gravitational wave detector. The Laser Interferometer Space Antenna (LISA) will be able to detect gravitational waves with much longer wavelengths than LIGO can because it’s mirrors will be an incredible 2.5 million kilometres apart compared to LIGO’s 4 km.

By highlighting the future of astronomy, the book pre-empts its own expiry date, well aware that it is likely an incomplete tour of the invisible, with future technology destined to uncover yet more monsters in the dark. When the book was written, the James Webb Space Telescope had not yet been launched. Now it is in space, already sending back breath-taking infrared images of the universe.

But I think the book’s timing is actually perfect. In an era when so many pioneering new projects are underway, it has reignited my excitement about what weirder and more wonderful curiosities they might find. And, even if it means the book becomes slightly out-of-date, I get the strong impression that the author is pretty excited about that too.

  • 2021 Oneworld Publications 320pp £18.99hb
  • 2022 Oneworld Publications 320pp £10.99pb

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  • Source: https://physicsworld.com/a/uncovering-the-universes-invisible-secrets/

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