The universe increasingly seems to be telling us there is an unexplained presence on the cosmic stage, says Daniel Cossins
Tasked with telling the universe’s epic story, cosmologis its have put on a compelling show. The curtain rises with a bang before a sweeping, unstoppable narrative unfolds. Stars form and explode, galaxies swirl their way into existence. Black holes munch and merge, sending out ripples through the auditorium. It is a ripping yarn – but the longer we watch it, the more it seems not quite to add up. The story is inconsistent. The pace changes arbitrarily. Some of the characters are ill-drawn, do inexplicable things or are just plain not there on cue. All in all, there is enough in this play that goes wrong to make you think someone has lost the plot.
Increasingly, we think we know how. We had assumed that just four fundamental forces keep the cosmic action bowling along. But hints from theory and experiment are combining to suggest it might not be just four, but five, six – or maybe even more. Sketchy though these indications are, even one new force would be a turn-up for the books. “It would be absolutely momentous,” says Philippe Brax at the Saclay Institute of Theoretical Physics in France.
Forces drive the cosmic narrative. They tell its various actors, from particles to planets, how to move and behave – things that would otherwise seem inexplicable (see “What is a force?”, scroll down) The four fundamental forces we know of are gravity, electromagnetism, the weak nuclear force and the strong nuclear force (see “The familiar four”, page 35). Of these, gravity is the outlier, the only one with no quantum field or particle attached to it and which can’t be described by the “standard model” of particle physics. Yet gravity, described by Albert Einstein’s space-and-time-warping general theory of relativity, determines the universe’s overarching plot lines. The problems with the story of the cosmos begin at the beginning. The big bang theory suggests that temperature and matter density in the universe should now be a hotchpotch, the result of early random quantum fluctuations being amplified as the cosmos expanded. But viewed at the grandest scales, galaxies and the like seem remarkably evenly spread. To square that circle, in the 1980s cosmologists invented cosmic inflation, a split-second burst of growth during which the primordial cosmos ballooned exponentially, flattening out its surface.
WHAT IS A FORCE ?
By holding this magazine, or swiping down a smartphone screen, you are exerting a force: one that operates between two objects that are physically touching. Drag forces such as friction and air resistance are also such “contact forces”, which influence movement and acceleration, and can be described by Isaac Newton’s laws of motion. When physicists talk about fundamental forces, it is something rather different: i
between things that are apparently not in contact. This “action at a distance” perplexed Newton when his universal law of gravitation first suggested it. It was, he wrote, “so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it”.
These days, we ascribe such mysteries to the action of fields that fill empty space. “In the modern understanding, the most basic things in the world are fields,” says theorist Frank Wilczek at the Massachusetts Institute of Technology. So what are fields? They are, says Matt Strassler at Harvard University, “a fundamental intermediary between two objects”. For three of the four fundamental forces we currently know of, they are quantum fields that come with accompanying particles, called bosons, that pop in an out of existence to mediate influences across various ranges: the massless photon, for instance, mediating the electromagnetic force.
The odd one out is the gravitational field. According to Albert Einstein’s general theory of relativity, which superseded Newton’s universal law, gravity is the product of mass warping space-time. The strength of the gravitational field at any point is essentially the degree to which a massive object is curving space-time around it. In all cases, what separates the fundamental forces from the commonor-garden ones we tend to notice is that they can’t be reduced to another force or field, as for example friction or air resistance can ultimately be reduced to electromagnetic interactions between different bits of matter. But the question of how many of these fundamental intermediaries exist remains unanswered (see main story).
"We don’t know what new actor to expect, other than it must be a quantum force”
wrinkles. A grand plot twist – but one that is currently entirely inexplicable.
Problem number two really became apparent around the same time – the 1980s, not the dawn of time – with the observation that there isn’t enough visible matter in most galaxies to exert the gravitational pull required to stop them flying apart as their components whirl around. Cosmologists’ second big invention was some additional invisible stuff, dark matter, to glue galaxies together – stuff we have failed to find.
The third implausible turn of events came in the late 1990s, when observations of far-off exploding stars known as supernovae revealed that the universe’s expansion is accelerating. Naively, with only gravity pulling things together, you might expect it to be slowing. Our best stab at explaining the “dark energy” we think is responsible for accelerated expansion invokes the power of quantum particles popping in and out of empty space. But this comes up with an answer for the size of the effect roughly 120 orders of magnitude too big. “The universe would have expanded so rapidly, everything would have been ripped apart,” says Clare Burrage at the University of Nottingham, UK.
The simplest solution to these problems might be just to say that gravity doesn’t work how we think it does. But general relativity has proved maddeningly difficult to edit, passing every test we have ever thrown at it, including the recent detection of gravitational waves produced when black holes and other massive cosmic objects collide. Meanwhile, ideas that try to alter gravity, such as modified Newtonian dynamics or MOND – a popular way to explain away dark matter – don’t square with all cosmic observations.
That adds to the yearning for a new character on the stage, and the belief that a fifth fundamental force of nature must be waiting in the wings. “We have several indications,” says Brax. “There’s definitely something there.”
The dark side But we don’t know what new actor to expect, other than a quantum force. This tallies with the idea that even if gravity can’t yet be described in quantum terms, most physicists believe it eventually will be, in a long sought after marrying of relativity and quantum field theory. “Any sensible physicist believes gravity’s force-carrying particle exists,” says Frank Wilczek, a particle theorist at the Massachusetts Institute of Technology who won a share of a Nobel prize in physics for the quantum theory behind the strong nuclear force. Follow that logic and any fifth force has to be quantum, too.