THEY say it started with a bang, but in truth it misfired. The universe began as a hot speck of energy and, for an instant, remained just that. Then it blew up: from this initial seed, trillions upon trillions of times smaller than an atom, everything suddenly ballooned into the gargantuan proportions of a Tic Tac. In a mere fraction of a second, the universe expanded by nearly as many orders of magnitude as it would in the following 13.8 billion years.
Believe it or not, this burst of cosmological inflation, followed by a slower, tamer expansion, is the most sensible way to explain how the universe looks today. But there’s something missing: what did the inflating?
The answer could be everywhere, and right under our noses. When a long-sought particle finally appeared a few years back, it seemed to close a chapter in physics without giving any clue about what happens next. Read between the lines, though, as some theorists recently have, and you see that the famous Higgs boson – the particle that gives mass, or inertia, to all other particles – might have an explosive secret. “If the Higgs gives inertia to particles,” says at the Autonomous University of Madrid, “can it give inertia to the entire universe?”
Advertisement
Inflation wasn’t always in our cosmological story. For a long time, theorists assumed that the universe expanded steadily from the start, with no sudden burst. This was driven by the natural desire of energy to spread itself in all directions.
Yet something about this picture didn’t quite ring true. Look at large enough scales, and the stars, galaxies and other structures in the universe don’t appear to be scattered randomly; even matter at opposite ends of the universe seems to be distributed in the same pattern of webs and clusters wherever you look, almost as if some interaction evened them out as the universe expanded. But that’s impossible. Something would have had to travel faster than the speed of light between these distant points – a serious physics no-no.
Explosive urge
Inflation, first proposed by at the Massachusetts Institute of Technology and others in the early 1980s, offers a way out. The idea is that a minuscule fraction of the primordial cosmos ballooned exponentially in the blink of an eye. Tiny, short-lived quantum fluctuations that are always bubbling away in space-time got caught in the whirlwind expansion and amplified, becoming the seeds of the stars and galaxies we see today. The patterns didn’t evolve – they were there from the start.
There are plenty of other reasons to like inflation, but they don’t change the fact that we haven’t got a clue how it happened. Cosmologists suppose there must be an “inflaton”, an energy field with dynamite properties. But what and where is it?
We have an idea about what to look for: the inflaton must be a scalar field. This is just a mathematical way of describing a field that acts in all directions but whose strength can change over space and time. One way to think about it is like a weather map of air pressure. Air pressure varies depending on the location and day of the forecast, but unlike wind strength, say, it is directionless.
What we are searching for, then, is an invisible fluid-like substance, one that suffuses all of space and has the potential to influence everything in it. Or at least it did. The inflaton field must have generated something akin to extreme antigravity – a cosmic urge that blew up the fabric of space-time – but then quickly lost its impetus, to the point at which its influence essentially disappeared and normal expansion resumed.
In principle, there is nothing stopping us from tracking an invisible energy field that has lain low for 13.8 billion years. Particle physicists can isolate a little pocket of a field, otherwise known as a particle, by smashing other particles together to generate a momentary flash of energy. We discovered some of the most elusive fundamental particles, such as the quarks, in this way. But those particles are not associated with scalar fields. And in the decades following the proposal of inflation, our best particle colliders failed to find anything that was.
Then, in 2012, a fundamental scalar particle finally showed up: the Higgs boson. Discovered at the Large Hadron Collider (LHC) at CERN on the France-Switzerland border, the Higgs had long been predicted as the particle that endows all others with mass. Finding it was a momentous triumph.
As the world celebrated, however, a handful of theorists saw the arrival of the Higgs in a different light. Two of these were and at the Swiss Federal Institute of Technology in Lausanne (EPFL). Having anticipated the Higgs discovery for several years, they had begun to consider what attributes it might have besides the gift of mass-giving.
“A supercharged Higgs would flood the universe with extreme antigravity”
On the face of it, the Higgs and the inflaton are different in a crucial way. Although both are scalar fields, unlike the inflaton, some of the Higgs field remains when it falls into its lowest energy state. It is precisely this sticky residue that manifests as the property of mass for other fundamental particles. But that is in today’s universe. Bezrukov and Shaposhnikov realised that it is possible to tweak the properties of the Higgs field so that in the moment following the big bang, it could have mustered enough force to flood the still-minuscule cosmos with inflationary gusto.
They fiddled with the Higgs’s “potential curve” – essentially the energy a particle needs in order to have a certain effect, such as bestowing mass on other particles. Picture this as a ball on a steep-sided hill. For most particles, when the background energy is low, the ball comes to rest in the valley. The particle’s location determines its effect, and right in the middle of the valley the effect is “zero”, meaning the particle is essentially switched off.
The Higgs is special, however, in that its potential curve is shaped not like a typical valley, but like the bottom of a champagne bottle, with a bump in the middle (see “Potentially suspect”). Given that it would take energy to push the ball up that central bump, when background energy is low, the Higgs comes to rest in the valley to one side, where it turns “on”. This is how the Higgs has the effect of giving mass to other particles, even when its field has no external energy to fuel it.
Bezrukov and Shaposhnikov spotted that there was nothing in known Higgs behaviour to stop them from adjusting the sides of its potential curve. What if, at some point high up on the curve, those steep sides flattened out somewhat? If the ball was hoisted up there for a brief time, the Higgs could sit in a supercharged “on” state, where it would flood the universe with extreme antigravity, enough to drive apart space-time itself.
Unexpected accomplice
True, it would require a hefty shot of energy to scale the sides in the first place. But there was an awful lot of background energy around at time-zero (). “The Higgs can make the universe expand,” says Bezrukov, now at the University of Manchester, UK. “It could be the inflaton.”
Finding out for sure would require a test of the particle’s interaction with gravity: if the Higgs interacts strongly with gravity, then the sides of the potential curve might be flattened out as the researchers propose. Unfortunately, gravity is too weak on Earth for that to be measured at the LHC, so collider data alone can’t tell us whether the Higgs potential has inflaton-capable flattened sides.
For GarcĂa-Bellido, the tidiness of the Higgs explaining both the origin of mass and inflation was too hard to resist. But the more he thought about it, the less tidy things seemed. In 2011, working with Shaposhnikov and others, GarcĂa-Bellido realised that the mathematical tweaking of the Higgs potential created an imbalance in the underlying equations, one that could only be remedied by a second scalar particle. This was a surprise, but not necessarily an unwelcome one.
Since the late 1990s, astronomers have known that the universe’s current expansion is accelerating. They suggested that some unknown source of energy is behind the acceleration, and modern observations indicate that it must account for more than two-thirds of all the energy in the universe. The only problem is that no one knows what this so-called dark energy is.
“This new particle happens to have just what it takes to solve dark energy too”
Handy, then, that the new particle inferred by Garcia-Bellido and his colleagues could have just what it takes to solve this mystery too. It wouldn’t be nearly as burly as the Higgs, but according to the team’s calculations, its field would be present in small quantities throughout the lifetime of the universe, providing just the right boost to expansion (Physical Review D, vol 84, p 123504). “That’s the beauty of this model,” says GarcĂa-Bellido. “First it solves inflation; second, the accelerated expansion of the universe. It’s extremely economical.”
The name of the new particle, the dilaton, reflects its close entwinement with Higgs physics. Specifically, it would prevent the Higgs’s mass from “dilating” too much – useful because without it we don’t have much of a clue why the Higgs mass has the value it does. So although the dilaton itself would be massless, it would be an influential background operator, fixing the mass of the Higgs and, by extension, all other fundamental particles. Dark energy would be its biggest footprint in the universe.
Bold claims indeed. Alas, not everyone is won over. at the University of Sussex, UK, thinks that the Higgs as mass-giver and inflaton is too contrived. Instead, she backs the possibility that the Higgs and the inflaton are part of a whole new family of scalar particles that we have barely begun to uncover. All other known particles reside in families, she says, so why should the Higgs be the only scalar particle?
If Sanz is right, the Higgs would be a mere spectator in the early universe, but it would be influenced by its sibling, the inflaton, and that would show up in Higgs data from the LHC. Sanz says she prefers that idea because it is easier to test with current colliders than that proposed by GarcĂa-Bellido and his colleagues. “In cosmology, there’s always a plethora of ideas that are hard to tell apart experimentally,” she adds. “I don’t like that.”
It’s a fair point: the LHC alone can’t rule out the Higgs-dilaton model, because the dilaton would hardly interact with other particles. But to see if the Higgs has what it takes to put the bang into the big bang, GarcĂa-Bellido is not relying on the LHC. Instead, he will stare into the distance to study the afterglow of the big bang, the cosmic microwave background.
The smoking gun for an inflationary Higgs would be a particular twist in the polarisation of this ancient light. The presence of a dilaton field would be trickier to spot, but not impossible. GarcĂa-Bellido thinks it should have left a mark in any gravitational waves that imprinted themselves on the background after the tumult of inflation. Broadly speaking, that means making precise measurements of differences in the levels of microwaves coming from various directions in space.
Currently, the best picture we have of the microwave background is that recorded by the European Space Agency’s Planck spacecraft in 2013. There wasn’t quite enough detail for GarcĂa-Bellido’s purposes, but a raft of new instruments ought to do the trick. They include the Simons Observatory, under development in the Atacama Desert in Chile, Japan’s forthcoming LiteBIRD satellite and the latest BICEP/Keck telescope at the South Pole.
We will have to be patient. The Simons Observatory will only begin studying the heavens in the next few years, while LiteBIRD is not due for launch until the 2020s. For GarcĂa-Bellido, however, that is not long to wait to solve two of the greatest mysteries of cosmology in one fell swoop. “We’re on the verge of a breakthrough,” he says.
Why anything at all exists
Inflation isn’t the only outstanding mystery of the early universe. The big bang created equal amounts of matter and antimatter. These two annihilate each other whenever they meet, which means they should have wiped each other out almost immediately, leaving the universe devoid of anything to form stars and planets and life. Needless to say, we’re here, so something tipped the balance.
According to at the University of California, Los Angeles, and others, that something could be the Higgs boson, the particle and associated field that gives other particles mass. Our best measurements of the mass of the Higgs itself suggest its field could have been much stronger during cosmological inflation, when the early universe ballooned (see main story) before settling down to the value we find today. As it relaxed after inflation, the Higgs’s changing value could distinguish between particles and antiparticles, creating favourable conditions for matter to win out.
But the Higgs would need a little help from a friend – a heavy “Majorana” neutrino that boasts the unusual quality of being its own antiparticle. Kusenko and his colleagues suggest that these two-faced particles might have provided a route through which antimatter particles could convert to matter once the shifting Higgs value favours the latter ().
It’s an appealing idea, but of course there’s a catch: no one has ever seen a heavy Majorana neutrino. Then again, observed masses of known neutrinos do seem to point to their existence, and the search is ongoing at several dedicated neutrino experiments.
Read more: Peter Higgs on knowledge, immortality and the future of physics
This article appeared in print under the headline “The Higgs bang”
