On Dark Matter and Dark Energy

Brian Clegg is an English science writer. He is the author of popular science books on topics including light, infinity, quantum entanglement and surviving the impact of climate change, and biographies of Roger Bacon and Eadweard Muybridge. In this exclusive interview he discusses ideas relating to his latest book, ‘Dark Matter & Dark Energy: The Hidden 95% of the Universe’.

Richard Bright: Can we begin by you saying something about your background?

Brian Clegg: I studied Natural Sciences at Cambridge, specialising in experimental physics and went on to get a masters in operational research – mathematical problem solving – at Lancaster. This led to a job at British Airways, where I worked for 17 years, with roles that included developing scheduling models, setting up a PC centre and exploring emerging technologies for the airline. While at BA I trained in business creativity and left to consult independently in this field and, increasingly, to write.

RB: What started you on writing and, particular, writing about science?

BC: While at BA I had started writing for computing magazines in my spare time and enjoyed it. When I left, I dipped my toe into book writing, producing business creativity titles to support my consultancy. Writing books was something I got a lot out of, but I found business books very shallow in their approach. I thought it would be far more interesting to go back to my science roots and wrote my first popular science title, Light Years, which is still in print. I found this the ideal combination – it was a chance to get back to science and I enjoyed making science accessible to a wider audience, which I thought was essential in a world where science is so important but relatively few understand it.

Over a few years, science writing and giving talks took over from the business creativity. In principle I still run a business creativity company, and still give the occasional workshop, but in practice I’m a full-time writer.

RB: When the Laser Interferometer Gravitational Wave Observatory (LIGO) reported the detection of gravitational waves in February 2016, you stated that the announcement ‘signalled the beginning of the biggest change to astronomy since the introduction of telescopes.’ Can you say more about this?

BC: It was frustrating at the time, because most of the press coverage of the detection seemed to be about the detection proving Einstein correct, which was both trivial and wrong. In fact, it proved Einstein wrong: although he had indeed predicted the existence of gravitational waves, ripples in the fabric of spacetime itself, he also said that they would never be detected because they would be far too weak. But this wasn’t the significance of the discovery.

I mentioned telescopes because they were our first technology to improve on our eyes in our ability to see out into the universe. Over the years, we added widely to the original optical telescopes with radio, infrared, X-rays and more. But all these devices made use of the same mechanism to get information: electromagnetic waves. Using different wavelengths meant that we could see things not detectable in the visible region, but we were still faced with the limitations that electromagnetic waves encounter.

One limit, for example, is that before the universe was about 380,000 years old it was opaque to electromagnetic waves. We can see back in time as we look out into the universe, because light takes time to get to us – but there is an effective barrier here. With the exception of neutrinos, which are limited in their applicability, gravitational waves provided the first opportunity to have another, totally different vehicle to find out what was going on in the universe, one that didn’t have the limitations of electromagnetic radiation. And it has already proved valuable in our understanding of the behaviour of black holes and neutron stars.

When I did my degree, my final year project involved constructing a primitive radio telescope, and I can still remember the excitement of picking up signals from space, of ‘seeing’ the stars in a totally different way. Gravitational waves take this excitement to a whole new level.

RB: Your forthcoming book is titled Dark Matter and Dark Energy: The Hidden 95% of the Universe. What is dark matter and how is its existence and properties inferred?

BC: What dark matter is remains one of the significant mysteries of science – you ask about inference, and dark matter is, as yet, just that: an inference that is yet to be directly detected. It all started back in the 1930s when a Swiss astronomer called Fritz Zwicky noticed that there seemed to be a problem with the way that collections of galaxies rotate. By that time, they were starting to get estimates of the number of stars in galaxies and the amount of mass that should be present. Zwicky discovered that a cluster of galaxies he observed was rotating too fast for the amount of matter that was in it.

In the end, all that holds a galaxy, or a cluster of galaxies, together is gravity. If the whole thing is spinning too fast, then outer parts should start flying off, like clay from an out-of-control potter’s wheel. The fact that the Coma Cluster was rotating too fast implied that there was more matter in it than had been calculated by a significant factor. Zwicky estimated that the cluster needed around 400 times more mass than it appeared to have. Since then that figure has been brought down, but it’s still speeding by a considerable factor. Zwicky assumed that this was caused by what he called dunkle Materie – translated from the German as dark matter. What he meant by this was perfectly ordinary stuff, but that wasn’t radiating electromagnetic radiation, so we couldn’t see it.

Zwicky’s results were largely ignored, but in the 1970s, American astronomer Vera Rubin found something equally unexpected. Galaxies aren’t solid rotating disks like a CD. You would expect that further out from the centre, where the gravitational attraction was weaker, stars would be orbiting more slowly. But Rubin discovered that the edges of galaxies were rotating at a similar speed to the middle. She suspected this was caused by an invisible spherical distribution of matter around the outside of the galaxy, now called a halo.

By this time, there was a better idea of how much conventional non-radiating matter there should be in a galaxy, and even combining that with the visible matter, the result was way too low. It was inferred that there was another kind of matter out there. As data was collected it became clear that there should be over five times as much of this dark matter as there was of conventional matter in the universe.

In a way, dark matter is an unfortunate piece of terminology. Dark matter isn’t dark. Something that is dark absorbs light – but light passes straight through dark matter. It’s totally transparent. As yet, dark matter is defined by its properties. It appears to be a substance that has mass, so can be detected by its gravitational effects, but is unaffected by electromagnetic radiation. But that leaves the question open of just what is producing this effect.

RB: Are there differing theories that aim to provide an explanation for dark matter?

BC: There are indeed. One potential candidate was the neutrino I mentioned earlier. These are particles that are produced in vast quantities in stars. They are pretty much unaffected by electromagnetism and do have mass (though they are very light). Unfortunately, neutrinos have been pretty much ruled out, because they show little sign of behaving in the way that dark matter does. One problem is that neutrinos move very quickly, but dark matter needs to have been moving quite slowly if it were to form the haloes that are thought to surround galaxies. Neutrinos would simply not have hung around long enough.

The main contender particles to make up dark matter were early on described as ‘MACHOs’ and ‘WIMPs’. These are respectively ‘Massive Compact Halo Objects’ and ‘Weakly Interacting Massive Particles’. MACHOs, which are now considered unlikely to make a significant contribution, were an extension of Zwicky’s original concept – ordinary matter that can’t be detected, along with black holes. They still cling on as a possibility for all of dark matter by introducing a hypothetical concept of primordial black holes – small black holes that were formed not from collapsing stars, but in the early universe, but there is as yet no evidence for these existing.

The WIMP category is broader, taking in both the existing neutrinos and several hypothetical particles, which are not part of the current ‘standard model’ of particle physics. It ought to be stressed that the ‘massive’ in ‘Weakly Interactive Massive Particles’ is not the common usage of being extremely large, but just having some mass. One possibility for WIMPs comes from a once-popular, but now uncertain concept called supersymmetry.

The current particle model divides particles into two types, fermions, which are effectively the matter particles, and bosons which carry forces. Quarks, electrons (plus some heavier equivalents) and neutrinos are fermions, while photons – the particle of light and the carrier of the electromagnetic force – the famous Higgs and others fit on the boson side. That makes for 17 particles in all, which seems enough. However, many theorists are uncomfortable with the asymmetric nature of the standard model. They point out that it would fit well with string theory, an attempt to unify the forces of nature, if each particle had a ‘supersymmetric’ partner of the opposite kind.

If such supersymmetric particles existed, some of them would be a good fit for expectations of the nature of a dark matter particle. However, there is a problem. The right kind of supersymmetric particles are expected to have a similar mass to a Higgs boson, which mean they should have been detected by the Large Hadron Collider at CERN. And they haven’t. This puts the whole of supersymmetry – and supersymmetric WIMPs in particular – in jeopardy.

Another WIMP that is sometimes mentioned is the axion. If this sounds like a washing powder, it’s not surprising – it was named after a European detergent. These particles were conceived to solve a problem in quantum physics. They would be even lighter than neutrinos, but if they exist at all, there should be vast quantities of them. However, like neutrinos they also would tend to be too fast moving and, crucially, where neutrinos are now regularly detected, no one has ever found an axion particle, despite repeated attempts to detect them.

Some theoreticians think all this is the result of thinking small. After all, our standard physics requires 17 particles – why should we limited dark matter to a single type of particle? It has been suggested there could be a whole dark universe of different kinds of dark particles. But there is no evidence yet for this.

And there lies the big problem with dark matter. As well as developing theories, scientists have produced a whole range of experiments to detect dark matter particles, but not a single confirmed detection has occurred. Admittedly it’s not easy – but it’s also extremely tricky to detect neutrinos, and we manage just fine with those. Because of this, there is increasing interest in theories that suggest dark matter doesn’t exist at all.

At first sight that seems to contradict the evidence. We know there is an effect which we assume is caused by dark matter. But what if there’s another reason for the data, which is, after all, entirely indirect? This has led to a number of theories, the earliest known as MOND (MOdified Newtonian Dynamics) which do away with the matter part of dark matter.

The simplest example, MOND, suggests that Einstein’s very effective general theory of relativity, which builds on Newton’s ideas to give us our best description of gravity, has to be very slightly modified when dealing with objects on the scale of galaxies. This would explain some of the observed effects ascribed to dark matter. Critics point out there are still some oddities that dark matter explains, but MOND would not. However, they usually fail to note that there are also some more common oddities that MOND explains better than does dark matter.

Finally, to top it all, a mathematician has suggested that the whole effect is imaginary. This is because we don’t actually know exactly how vast numbers of bodies within a galaxy will interact gravitationally. The maximum number of interacting bodies we can calculate gravitational effects for exactly is two. Any more and we need to approximate. While those approximations are superb for, say, the number of bodies in the solar system, there are billions of stars in a galaxy, and the suggestion is that their interactions are being incorrectly modelled.

For the moment, then, the search for dark matter goes on, but alternative theories are definitely gaining ground.

RB: What is baryonic and nonbaryonic dark matter?

BC: Baryons are particles that are made up of odd-numbered combinations of the fundamental particles called quarks. The best-known baryons are the protons and neutrons that make up the nucleus of atoms. Baryonic dark matter would make up part of a MACHO contribution to dark matter – it would be ordinary matter that is not radiating electromagnetic waves, so is invisible to us. There certainly must be some baryonic dark matter, but far too little to produce the effects we ascribe to dark matter.

Non-baryonic dark matter is any of the other particle candidates for dark matter, plus black holes, which are sometimes classed as baryonic, but really shouldn’t be as they are not composed of baryons. However, dust, planets, near-invisible brown dwarf stars and neutrons stars can all contribute to the baryonic dark matter total.

RB: Dark Energy is pushing the universe to expand faster and faster. What is dark energy?

BC: This could be a very short answer: we don’t have a clue. Dark energy was a good example of a totally unexpected discovery. It has been known that the universe is expanding since the 1920s, when it was observed that most galaxies have a red shift, and the further away they are, the greater that shift is. A red shift is the optical equivalent of the Doppler effect, where a sound changes in pitch as its source moves towards or away from us. Light becomes bluer if its source heads towards us and more red if it’s moving away.

The expectation was that the detected expansion would gradually be slowed down by the effects of gravity, but for a long time it wasn’t possible to successful test this as it required a means of determining vast distances in space – billions of light years – and these methods were only developed in the 1990s. Those making these observations expected to discover just how quickly the expansion of the universe was slowing, but found to their surprise (and delight) that it is in fact accelerating.

To make such an acceleration happen takes energy. Not much on any local scale – less than a joule for each cubic metre of space – but added over the whole universe, we’re talking about a total which is the equivalent of twice the total mass of matter and dark matter predicted. (We can make mass and energy equivalent thanks to Einstein’s famous equation E=mc2, where E is energy, m mass and c the speed of light.)

Frankly, what dark energy is remains a huge mystery. One very plausible possibility of what’s driving it should be something called vacuum energy. This emerges from quantum physics, which tells us (and it has been experimentally shown to be true) that the amount of energy in an area of empty space can fluctuate wildly over very small amounts of time. This means there’s a kind of background level of energy in space. Unfortunately, though, theory tells us that if vacuum energy were the driver, it should produce an effect 10120 times (that’s 1 followed by 120 zeroes) bigger than what is actually observed.

Dark energy is a great topic for speculative theories. We have over 50 theories for how dark energy works, none of which has yet to have any supporting evidence. New data regularly rules out some theories, but because they are purely speculative it’s easy enough for theoreticians to come up with new theories to replace them. Work is underway to get more precise measurements to quantify dark energy, and to see further into space, which means further back in time, to when the universe was young. This is likely to give us better insights into how the early galaxies formed and when dark energy’s effect kicked in, perhaps shining a light on its cause.

RB:  How are scientists beginning to find solutions to both dark matter and dark energy?

BC: The simple answer is that they aren’t yet. A whole host of new detection experiments are under way or suggested, though some physicists have suggested they’re just make-work for scientists, where theoreticians dream up unlikely scenarios and experimentalists produce experiments to disprove them. In 1980, Vera Rubin predicted dark matter would be detected within ten years. It wasn’t. In 2000, the British Astronomer Royal Martin Rees again made a ten-year prediction. Again, no joy. It’s entirely possible that the current generation of detectors due to come online over the next ten years will finally break through – but equally alternatives to particle-based dark matter are gaining ground.

On the dark energy front, we certainly will see more theories disproved, but it is entirely possible that without a fundamental shift in our understanding, dark energy will have to remain a mystery. It could be that we’re looking for a whole new theory combining quantum theory and general relativity – so far failed by attempts such as string theory and its opponent loop quantum gravity – which may be needed before we can uncover what’s really going on.

RB: Finally, what do you see as the most exciting (and/or challenging) development for science in the future?

BC: Several times since around 1900 it has been suggested that science is nearly complete. I’m thankful it’s not – that we have challenges such as dark matter and dark energy to keep science interesting. I think we are likely to see some major shifts in our current physics theories, which haven’t changed in a major way since the 1970s. Equally there’s excitement to be had in the developments in everything from molecular biology to amazing 2D materials such as graphene.

There’s a remarkable statistic that around 90 per cent of the scientists who have ever lived are alive today. Science is thriving – and though at the moment dark matter and dark energy are proving elusive, I have every hope for the future.

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www.brianclegg.net

 

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2 Responses to “On Dark Matter and Dark Energy”

  1. Avatar
    Francesca Diano
    July 29, 2019 at 3:21 pm #

    Dear Mr Clegg, I read with great interest your interview and I’d like to ask you if, by any chance, dark matter and dark energy could be identified with the Greek philosopher Anaximander’s concept of “àpeiron periechon”?
    Thank you for your answer.
    Kind regards
    Francesca Diano

    • Avatar
      Brian Clegg
      July 31, 2019 at 8:22 am #

      There is no obvious similarity between dark matter/dark energy and Anaximander’s theory, which as I understand it was of an unbounded primal source from which all that we perceive was generated. I think it can be misleading to look for connections between ancient theories and modern ones – even if the ancient theories appear to prefigure the modern ones, it does not imply some kind of ancient wisdom, but rather is an inevitable result of coincidence.

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