modern physics deals with some truly mind-boggling extremes of scale. Cosmology reveals the Earth as a tiny dot in the midst of an observable universe that is a staggering 93 billion light-years across. Meanwhile, today’s particle colliders explore a microcosmic world billions of times smaller than the smallest atom.
These two extremes, the largest and smallest distances investigated by science, are separated by 47 orders of magnitude. It’s one with 47 trailing zeros, a number so ridiculously large that it’s not worth trying to wrap your head around. And yet, despite investigating such radically different distances and phenomena, cosmology and particle physics are deeply connected. Observing the motions of stars and galaxies can reveal the influence of particles not yet discovered, while studying fundamental particles in the laboratory can tell us about the birth and evolution of the cosmos.
Interestingly, both disciplines grapple with unexplained results that may point to the existence of a new force of nature. If such a new force were to be confirmed, the implications for our understanding of the universe, its history and composition would be profound.
There are four forces that we already know about. Gravity control the largest scales, compose the planets in their orbits and shape the evolution of the universe as a whole. Electromagnetic force gives rise to a wide variety of phenomena, from the Earth’s magnetic field to radio waves, visible light and X-rays, while also holding atoms, molecules and, by extension, the physical world together. Deep within the atomic nucleus, two further forces emerge: the vice-like “strong force”, which binds atomic nuclei, and the “weak force”, which, among other things, causes radioactive decay and enables the nuclear reactions that make the sun and the stars possible.
Studying these forces has changed our understanding of nature and generated revolutionary new technologies. Work on electromagnetism in the 19th century gave us the electric dynamo and radio transmissions, the discovery of the strong and weak forces in the 1930s led to nuclear energy and the atomic bomb, while the understanding of gravity made it possible to send astronauts on the moon and to develop GPS satellites that can tell us our location anywhere on Earth to within a few meters. Uncovering a fifth power would be one hell of a price.
Hints that physicists may be on the verge of making such a breakthrough have piled up over the past decade. The first piece of evidence comes from particle physics experiments here on Earth, the results of which appear to conflict with our current best theory of fundamental particles, the Standard Model.
Despite its rather uninspiring name, the standard model is one of humanity’s greatest intellectual achievements, the closest we’ve come to a theory of everything, and has passed virtually every experimental test thrown at it with flying colors. So far at least.
The BaBar experiment in California, the Belle experiment in Japan and the LHCb experiment at Cern have all spied exotic fundamental particles known as “beauty quarks” that behave in ways that contradict the predictions of the Standard Model. Meanwhile, just outside Chicago, Fermilab’s Muon g-2 experiment is studying a different kind of fundamental particle called a muon and found that it emits a slightly stronger magnetic field than expected.
The most exciting explanations for these anomalies involve hitherto unknown forces of nature that subtly change the way beauty quarks change into other particles or with the muon’s magnetism. Such a new force could help unlock a deeper structure at the basis of reality, explaining why we have the fundamental particles in nature that we do. Another tantalizing possibility is that it could act as a link to the invisible “dark universe”, made of invisible dark matter.
That said, for now the overall picture remains frustratingly murky. Just over a year ago, new results from LHCb threw cold water on the prospects of a major breakthrough, after missing biases were found in some of the earlier measurements. Meanwhile, theorists have debated how magnetic the muon should really be, leaving open the possibility that this anomaly is a computational problem.
Perhaps the most compelling evidence for a new force at work in the universe comes from the other end of the cosmic scale. For the past few years, cosmology has been going through what has come to be known as the “Hubble Crisis” – a dramatic disagreement about how fast the universe is expanding.
According to the accepted cosmological story, the universe as we know it began with the Big Bang about 13.8 billion years ago and has been expanding ever since, with galaxies being pushed further apart as the space between them stretches. Cosmologists have two ways to figure out how fast space is expanding. One involves studying a host of distant galaxies through telescopes, then determining the relationship between their distance and how fast they seem to be speeding away from us. The other exploits exquisitely precise maps of faded light from the fireball of the big bang – known as the “cosmic microwave background” – to infer the properties of the infant universe. Then you apply current cosmological theory to run the clock forward and predicted how fast the universe should be expanding today.
The fact that these two methods give different answers is the strongest evidence we have that there is more to the universe than we have thought so far. Possibilities abound. A popular proposal involves a form of energy that drove the universe to expand even faster than thought shortly after the big bang. Others involve “dark forces” operating in the hidden world of dark matter. Some have even suggested that gravity itself behaves differently across the vast spaces between galaxies.
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How the story of these deviations will end is unclear. But the wealth of emerging evidence does suggest that physics may be on the verge of something great. The discovery of a new power would mark the beginning of a new era of exploration, perhaps providing a deeper understanding of the basic building blocks of nature, or opening the door to a vast, unknown dark realm, which, although it is invisible, 95 contains % of all that exists. Such breakthroughs are always hard won, but following nature’s breadcrumb trail may soon lead to a profound new view of the universe.
Harry Cliff is the author of Space Oddities: The Mysterious Anomalies That Challenge Our Understanding of the Universe (Picador).
Further reading:
White holes: within the horizon by Carlo Rovelli (Allen Lane, £14.99)
The end of everything: (astrophysically speaking) by Katie Mack (Penguin, £10.99)
The elephant in the universe: our centenary search for dark matter by Govert Schilling (Belnap, £24.95)
