August 2022

The Nation Reviewed

What the James Webb Space Telescope reveals

By Paul Davies
Illustration by Jeff Fisher
Why NASA’s new telescope is a huge step forward for understanding the universe

A hundred years ago next month, an intrepid group of astronomers set out by ship on an expedition to a remote part of Western Australia. Their mission? To make the first definitive observation of a spacewarp. The bizarre claim that space itself can be bent or distorted was the brainchild of Albert Einstein, and was a key prediction of his theory of general relativity, which had been published just a few years earlier. But could the claim be tested?

Proof that space really can be warped is now there for all to see in the amazing pictures released by NASA from its brand-new James Webb Space Telescope, launched from French Guiana last Christmas Day. Images of luminous arcs from the farthest reaches of the universe perfectly illustrate how the enormous gravitational fields of galaxies and black holes can serve as lenses, made not of glass but of space itself.

The century-old expedition, to a site called Wallal on the coast 1700 kilometres north of Perth, was conceived by a dapper Scotsman from the University of Western Australia named Alexander Ross. Its purpose was to exploit the eclipse of the sun due to occur on September 21, 1922. Under good conditions, with the sun’s glare blocked by the moon, it’s possible to see stars in the patch of sky close to the sun’s position. Einstein had predicted that the sun’s gravity would create a slight distortion in the geometry of the space around it, which would manifest itself as a tiny displacement of the stars’ apparent positions. This phenomenon is sometimes loosely described as starbeams passing close to the sun being slightly bent by the sun’s gravity.

The Wallal expedition was not the first to seek evidence of the bending of space. In 1919, the British astronomer Arthur Eddington had travelled to the island of Principe, off the west coast of Africa, to test this effect, with a parallel team going to Brazil. Sadly, their efforts were plagued by technical problems and inclement weather. The results lacked the necessary precision to be a decisive test of whether space can be warped. The Wallal project, however, was a resounding success, and dispelled any doubts.

Studying gravitational lensing is just one of a plethora of reasons astronomers have been clamouring for the James Webb Space Telescope (JWST). It is far bigger and more complex than the famous Hubble Space Telescope, giving it much greater sensitivity. As a result, it can see a lot farther and in much greater detail. But the key feature of the JWST is its ability to extend its light-gathering power into the infrared region of the electromagnetic spectrum. That’s as important an advance as going from black and white to colour.

The story of redness in astronomy is critical to this topic, and it goes back to 1909. An American businessman, Percival Lowell, was convinced Mars was inhabited by beings who had built canals on the planet’s surface, and he founded an observatory in Flagstaff, Arizona, to map them. The canals were of course a figment of his imagination, but what was definitely real was a curious observation by one of Lowell’s junior astronomers, Vesto Slipher. His project was to study the light from what we now know as galaxies, although at the time the nature of these fuzzy patches of light was disputed. Slipher found that the fainter, hence more distant, galaxies were measurably redder than the nearby ones. This systematic “red shift” was used by Edwin Hubble in his famous declaration a decade later that the universe is expanding – one of the greatest scientific discoveries of all time.

The best way of envisaging the expansion of the universe is as the stretching of space, so that over time the galaxies get farther and farther apart. Light traversing the universe gets stretched too, increasing its wavelength (analogous to the distance between successive ripples on a pond) and thus altering its colour towards the red end of the visible spectrum. The most powerful telescopes can detect galaxies more than 13 billion light years away. The wavelength of light from those sources is stretched by more than a factor of 10. Light from farther out is so stretched, or red-shifted, by the cosmic expansion it is displaced into the infrared region of the spectrum. Hence the need for JWST’s infrared capability.

Cosmologists have for decades been studying the fading afterglow of the Big Bang, which has travelled undisturbed since about 380,000 years after the universe’s explosive origin. Known as the cosmic microwave background, this primordial light has been red-shifted by a factor of a thousand. It provides a snapshot of what the universe was like at a tiny fraction of its current age. But there is a huge gap in our understanding of the universe’s history between 380,000 years after the Big Bang to the epoch when the first galaxies lit up, a few hundred million years later. It is a chasm referred to by astronomers as the cosmic dark ages. The JWST will probe that dark chasm for details on how galaxies, and the monstrous black holes that lie in the centres of many of them, first formed from the swirling gases coughed out of the Big Bang. It will also help settle disputes about how fast the universe is expanding.

Another major task of the JWST will be the study of extra-solar planets. We now know that most stars in the Milky Way have planets, and inevitably there has been speculation about whether some of them may host life. Although the presence of planets can be inferred by indirect means, actually observing them individually is hampered by the glare of the host star. But the JWST has the power to capture images of some large planets and analyse the infrared spectra of their atmospheres for telltale chemicals that may hint at life. The first tranche of data released by NASA last month includes the spectrum of a giant planet called WASP 96-b, and features clear signs of water – one of the prerequisites for life – in the atmosphere.

The James Webb Space Telescope is something of a technological wonder. With a diameter of 6.5 metres, its segmented mirror was far too large to fit in the nosecone of a rocket, so it had to be folded up to fit atop one of the European Space Agency’s Ariane 5 launch vehicles. In space, the compactified assemblage opened up, along with a parasol the size of a tennis court to shield the telescope from the sun’s heat. Unlike Hubble, which is in low-Earth orbit, JWST is parked 1.5 million kilometres away at what is known as a Lagrange point: a position in space where the gravity of Earth and the sun balance. That is far too distant for a space mission to fix any problems. So it has been a nailbiting few months waiting to see if the telescope was injected into the right orbit and all its components deployed correctly. Well, the telescope works.

Scientists have long lists of target objects for the coming months and years, not just deep field images of the early cosmos and extrasolar planets, but also gas and dust clouds, black holes and much else. A century ago, when the Wallal astronomers set out to test the theory of general relativity, they had no inkling that the universe glows brightly in every region of the spectrum, from radio and microwaves, through infrared and visible to ultra-violet, x-rays and gamma rays. All are intensively studied today. If history is any guide, however, every time astronomers have increased the power of their instruments, they have discovered something new and unexpected. What excites me the most is not so much the list of known cosmic mysteries, but – to paraphrase Donald Rumsfeld – the unknown unknowns. In the coming years it will be these that could totally transform our understanding of the universe, and our place within it.

Paul Davies

Paul Davies is a physicist and astrobiologist at Arizona State University, where he is Regents’ Professor. His latest book is What’s Eating the Universe? And Other Cosmic Questions.

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