May The Fifth Force Be With You

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.

Space 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.

Quantum force

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.

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