The inflaton particle is credited with generating the universe and fuelling its inflation. It has yet to be discovered, but it is fast running out of hiding places, thanks to the theoretical framework known as supersymmetry (SUSY).
Enormous and mainly extinct, supersymmetric particles are the dinosaurs of particle physics. Each of these “sparticles” is the partner of a known particle, and they have already solved several cosmological problems, including smoothing the way for a long-sought grand unified theory of physics.
Now two theories suggest that some sparticles might also be components of the elusive inflaton, which is thought to have driven space-time apart at the dawn of the universe.
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If either theory turns out to be correct, it would constitute a first glimpse of the cosmic process of inflation. What’s more, one of the new theories will soon be put to the test by collisions occurring at the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland.
Most of the inflation period is shrouded in mystery. All we are confident about is that in a fraction of a nanosecond after the big bang, the universe expanded from a size smaller than a proton to somewhere “between a football and a football field”, says of the Max Planck Institute for Physics in Munich, Germany.
The energy which drove this faster-than-light expansion is believed to have been stored in a field, similar to a magnetic or gravitational field. Every field has an associated particle, according to the standard model of physics – the theory that successfully encompasses all known particles as well as three of the four forces that act on them. In the case of the field thought to have driven inflation, physicists have aptly dubbed this particle the inflaton.
Although little is known about it, if it did exist, the inflaton must have generated all the matter in the universe from the energy stored in its field, so Antusch calls it the “mother of the universe”. It would also have to be consistent with the standard model. As a result, some physicists trying to piece together the identity of the inflaton have turned to SUSY (see “Desperately seeking SUSY”), which is an extension of the standard model.
“The inflaton must have generated all the matter in the universe from the energy stored in its field”
It had previously been suggested that inflation was driven by the Higgs particle – the particle thought to give all others mass- plus a supersymmetric accomplice (New Scientist, 19 January 2008, p 11).
Now this notion has strong competition from two more SUSY-based models of the inflaton, both presented at the in July.
One of these, devised by Antusch’s group, assumes the grand unification that SUSY enables: in this version of the early universe, ultra-high energies mean the electromagnetic force is unified with the strong and weak nuclear forces, while every particle and sparticle becomes indistinguishable from all other particles. This “unified” particle is a good potential candidate for the inflaton.
Until recently, however, this idea was still missing a crucial ingredient. To push space-time apart, the inflaton’s field must maintain a potential energy in apparently empty space, known as “vacuum” energy. But physicists were long convinced that a unified particle would be too quick to give up the energy of its field, resulting in no inflation.
Now Antusch and colleagues have found a way to extend the time in which the field has a high vacuum energy, by prolonging the period in which the unified particle is nearly massless ().
Their method, which is due to be published in the , relies on a mathematical symmetry often found in string theories. This makes it elegant and logical, and therefore attractive. But of the University of Maryland in College Park has doubts. “Symmetries are hard to test,” he warns.
The cosmic microwave background (CMB) – relic radiation from the big bang- might offer some clues, though. Observations by the Planck satellite, which measures the CMB, may reveal signs of gravitational waves produced during inflation. In this case, such signs would rule out Antusch’s theory because its predicted waves are too small for Planck to detect, but future gravitational wave detectors might be able to spot them.
For the rival SUSY-based model, developed by at the University of New Mexico in Albuquerque, and his colleagues, testability isn’t restricted to deep-space phenomena- it can be probed through particle collisions already occurring at the LHC. That is because, unlike in Antusch’s model, inflation takes place far below the energy scales needed for grand unification. This sets limits on the inflaton’s mass between 0.1 and 1 trillion electronvolts, well within the LHC’s 14 trillion electronvolt capacity ().
It also means that the particles are not unified as in Antusch’s model, but are separate entities that can be thought of as energetic points in different fields. So every electron can be thought of as an excitation in the electron field and every selectron – the electron’s supersymmetric partner – as an excitation of the selectron field.
Allahverdi’s team hypothesised that the inflaton might have been produced by an excitation in a combination of known sparticle and particle fields, so they set about finding a combination that could maintain a sufficiently high energy for long enough to drive the universe apart.
They found two options, both of which involved sparticle fields: one made from the superpartners of electron-like particles, and another made from the superpartners of the quarks that make up a neutron.
In either scenario, the component fields are strongly linked during inflation, providing the energy needed to drive the expansion. Eventually the fields become disconnected, inflation ends and the inflaton radiates energy in the form of particles, generating the contents of the universe. The latest version of their model is due to be published in Physical Review D.
at the University of Minnesota in Minneapolis can’t quite stomach the degree to which the theory rests on very precise settings for parameters that are themselves still largely unknown. “While it would be nice to be able to associate inflation with ‘low energy’ physics, the models come with too high of a price,” he says.
But Mohapatra points out the theory’s strength: “They have been able to connect very abstract ideas of the early universe to experimental tests.” The LHC won’t find the inflaton per se, but it may reveal the masses of the sparticles thought to compose it, which would allow Allahverdi’s theory to be tested.
“They have been able to connect very abstract ideas of the early universe to experimental tests”
Mohapatra characterises the two new theories as opposing camps, each with its own merits: Allahverdi’s builds the inflaton up from particles that still exist today, while Antusch’s derives it “top-down” from conditions thought to have existed in the early universe. Results favouring either will not only help unravel the process of inflation, but also shed light on the nature of those strange particle dinosaurs.
In the version of this article printed in New Scientist magazine, the first three paragraphs read:
“ENORMOUS and mainly extinct, yet capable of transforming into lighter beings that survive today, supersymmetric particles are the dinosaurs of particle physics.
“Via the theoretical framework known as supersymmetry (SUSY), these “sparticles” – each of which is the partner of a known particle – already solve several cosmological problems, including smoothing the way for a long-sought grand unified theory of physics.
“Now two theories suggest that some sparticles might also be components of the elusive inflaton, the particle credited with driving space-time apart at the dawn of the universe.”
When this article was first posted, the reference to Physical Review D, DOI: 10.1103/PhysRevD.82.035012 was incorrectly cited as arxiv.org/abs/1007.0708
Desperately seeking SUSY
BORN in the wreckage of a proton collision, the gluino is propelled into the detector- the – only to get stuck.
Trapped in either the iron discs that carry the magnetic field or the silicon and crystal of the inner detectors, the particle hangs around for crucial moments. Then it decays into gluons and quarks, which produce detectable “jets” that appear as cones of energy on the CMS event displays. But the process is out of sync with other collisions- making the gluino’s signal stand out.
Many proponents of the theory of supersymmetry (SUSY) hope this scenario will play out within a year at the Large Hadron Collider (LHC), at CERN near Geneva, Switzerland, CMS’s home. It would constitute the first hard evidence for SUSY, which gives a hidden “superpartner” to each known particle and promises to solve some big mysteries (see main story).
The gluino is the hypothetical superpartner of the gluon, which holds nuclei together. It is just one of many superparticles that might materialise in the LHC. But its expected clear signal and potentially high abundance mean that it may be glimpsed earlier than the rest. “This is one reason why we chose to study gluinos – we knew that they would be produced at a sufficiently high rate to be seen with early data,” says of Brown University in Providence, Rhode Island.
Already, CMS has begun to provide data allowing physicists to mass- a first step towards identifying it or ruling out its existence. Chou says that once the LHC gets up to design capacity, which could happen as early as next year, the gluino could be found in just a month’s worth of CMS data.
