100 incredible years of physics – astrophysics

With many of the innovations from World War II only just trickling down into astronomy, it was an exciting time to be in the field. An array of new ways of exploring the Universe made choosing a discipline difficult for Bell Burnell. It was therefore lucky she decided to work in the pioneering field of radio astronomy. 

Her project aimed to find more mysterious quasars. First she had to spend two years helping to build a radio telescope. Once completed, Bell Burnell got down to work, increasing the total number of quasars discovered from about 20 to about 200. But in August 1967, she noticed an odd squiggly stretch of data in the miles of paper readouts. She logged it with a question mark and moved on. However, in the days and weeks that followed, the squiggles – which represent pulses of radio waves – kept appearing. Lining all her observations up from the same part of the sky, Bell Burnell realised she might be on to something significant.

She tracked the signal for months, noting how the pulses repeated precisely every 1.337 seconds. And she found three more pulsating objects in other areas of the sky that also repeated with extreme regularity. Publishing her results with her PhD supervisor Anthony Hewish (IOP Fellow), they tentatively explained “this strange new class of radio source… in terms of the stable oscillations of white dwarf or neutron stars”. Her pulsating objects were given the name ‘pulsars’ and it was soon proven that pulsars are spinning neutron stars emitting beams of radio waves. 

Since their discovery, pulsars have become an indispensable tool in understanding physics under extreme conditions: “They are extreme on several fronts, all at the same time,” explains Bell Burnell. “So, they are used to test Albert Einstein’s theories of relativity, they are also stretching our understanding of solid state-physics, and they have got enormous electrical and magnetic fields, which is really stretching the physics in yet another dimension.”

Dealing with adversity

Making such a revolutionary discovery at a young age, the world should have been Bell Burnell’s oyster. But she found her scientific career restricted by the societal conventions and biases of 1960s and 70s Britain.  

Though she was subject to trite questions about her appearance from the media and missed out on a Nobel Prize in Physics – which was, instead, contentiously awarded to just Hewish and his collaborator Martin Ryle – what really halted Bell Burnell’s progress was the expectation of being a home-maker in the shadow of her husband: “I got married just as I finished my PhD, to somebody who worked in local government and regularly moved to another area to get promotions,” she reveals. “I spent about 20 years picking up part-time work in any old kind of astronomy, near where my husband was working… so I didn’t have a career in the conventional sense of the word.”

This meant different roles in different locations and different areas of astronomy: switching from radio to gamma-ray to X-ray and finally infrared and millimetre astronomy, until finally setting up her own astrophysics group at the Open University. 

Since the 1990s, Bell Burnell has used her own experiences as motivation for increasing the diversity of scientists and supporting students facing adversity. For example, joining the Open University as chair of the physics department in 1991, offered her a chance to inspire adult students working from home. And she was one of a group of senior female scientists whose efforts led to the creation of the Athena SWAN awards in 2005 – the world’s first gender-equality charter for university departments that challenges people to address and stop institutional sexism.

In 2018, Bell Burnell received the £2.3m Special Breakthrough Prize in Fundamental Physics for her pulsar research and a lifetime of inspiring leadership in the scientific community. Typical of her character, she donated the prize to the IOP – for which she was President between 2008 and 2010 – in order to fund PhD studentships for people currently underrepresented in physics such as female students, students from ethnic minority or disadvantaged backgrounds, or who are LGBT or disabled.  

A constantly changing Universe

Bell Burnell is championing diversity in physics as an equality issue, but also because she believes it will help increase diversity of thinking, which is essential to drive physics forward. She hopes that PhD students of the future supported by the Bell Burnell Fund go on to make discoveries that revolutionise our understanding of the inner workings of the Universe, much as pioneering students and astronomers from the past 100 years have profoundly altered awareness of our place in it. To give a concrete example, one constant – the Hubble constant – offers a microcosm of how these future astronomers might shape astrophysics by building on knowledge and techniques from the past and present. 

If we look back to 1920, astronomers had shown that the Sun was but one of a multitude of stars in our galaxy, but it was still thought by many that the Milky Way was the entire Universe. American astronomer Edwin Hubble changed this view forever. In 1929, he revealed that the Andromeda nebula was really the Andromeda galaxy, and that other, even fainter, spirals were probably also galaxies even farther away.

What’s more, Hubble’s results had another jaw-dropping consequence – they confirmed theoretician’s suspicions that all of these galaxies were moving away from us. “We certainly realised the Universe was expanding, was a lot bigger than we had thought, and a lot bigger than we were,” adds Bell Burnell. “So, our place in it shrank into perhaps a better context that it had been before.”

Hubble’s data clearly showed that the farther a galaxy, the faster it is receding from Earth, with the distance and velocity related by a constant – the Hubble constant. This constant has turned out to be one of the most important parameters in cosmology. Not only is it used to estimate the size and age of the Universe, and the rate at which the Universe is expanding from the Big Bang, the Hubble constant has also been central to one of the biggest discoveries in modern astrophysics.

In 1997, scientists already knew the Hubble constant was not constant (in time at least, but it is the same and therefore constant throughout space). In the distant past, the Universe’s expansion rate was much larger, and then it shrank as the cosmos expanded. It was therefore a complete surprise when two teams led by Brian Schmidt and Saul Perlmutter – and including several IOP members – revealed that the Universe’s expansion is actually accelerating and that its ultimate fate is to keep on expanding forever. The only way of explaining this result was to invoke an unknown phenomenon expanding space everywhere and making the Universe balloon at an ever-faster rate. Named dark energy, it accounts for approximately 68% of the cosmos.

Its namesake dark matter, meanwhile, makes up 27% of the Universe. It is an invisible substance forming a universal cosmic web that has an opposite binding effect on matter. This cosmic web is thought to help form galaxies and prevent them from spinning apart. First proposed in 1933 by Fritz Zwicky, but then ignored for four decades due to lack of evidence, it was not until Vera Rubin looked at galactic rotation curves in the 1970s that its existence was confirmed. “If I can name one person that I think was important and underrated it is Vera Rubin,” adds Bell Burnell. “She showed that galaxies were rotating far faster than they ought to, pointing to dark matter.”

Bell Burnell sees dark matter and dark energy as the two greatest mysteries future astronomers and astrophysicists need to tackle, “neither of which we understand and both of which are large components of the Universe”. Yet the Hubble constant might come to the rescue once again.

There are currently two ways of calculating the Hubble constant. Both are built on solid foundations, but they disagree. This mismatch presents a big problem for the current model of the Universe, known as Lambda Cold Dark Matter (ΛCDM), which describes the history and structure of the cosmos in terms of matter, cold dark matter and dark energy.

Yet just as she used a pioneering new type of astronomy to discover a new celestial object, Bell Burnell thinks the next generation of astronomers and astrophysicists could use gravitational wave astronomy to solve the current Hubble constant mismatch mystery: “The detection of gravitational waves is opening up a whole new spectrum, which is already delivering a lot of surprises,” she says. “With gravitational waves we will be able to get a third value for the Hubble constant, so that could turn out to be very important indeed.” Who knows, it may even provide the crucial information needed to finally illuminate the nature of dark matter and dark energy.