Physicists recently confirmed that the Large Hadron Collider (LHC) at CERN, the particle physics laboratory in Geneva, had indeed found a Higgs boson last July, marking a culmination of one of the longest and most expensive searches in science.
The finding also means that our universe could be doomed to fall apart. "If you use all the physics that we know now and you do what you think is a straightforward calculation, it is bad news," says Joseph Lykken, a theorist who works at the Fermilab National Accelerator Laboratory in Illinois. "It may be that the universe we live in is inherently unstable."
A reconstructed particle collision in the CMS detector of the LHC.Image: CMS/CERN
The Higgs boson helps explain why particles have the mass they do. The Higgs particle that the LHC has found possesses a mass of approximately 126 giga-electron volts (GeV)—roughly the combined mass of 126 protons (hydrogen nuclei). (One GeV equals a billion electron volts.)
Based on the data analysis so far, the discovered particle is consistent with the Standard Model of particle physics, the highly successful theory that describes the subatomic world, although other models cannot be ruled out. "It is looking very much like the Standard Model Higgs boson—although there may be a very massive Higgs particle that also exists, and which our experiment is not sensitive enough to detect," says Joseph Incandela, the spokesman for the CMS (Compact Muon Solenoid) experiment at the LHC, one of the two experiments that detected the current Higgs particle,
And that very nature of being a Standard Model Higgs may be the reason our universe is ultimately unstable. It has to do with the so-called vacuum stability in the Standard Model.
According to the description currently favored by physicists, a vacuum is not completely devoid of matter but instead teems with particles and antiparticles that pop into existence and then run into one another and annihilate themselves, all in very short times. The inherent uncertainty embodied in quantum mechanics permits these spontaneous fluctuations—as long as the particles don't live for more than a fleeting instant, the process violates no laws of physics.
The Standard Model also says, as Lykken puts it, that "for the vacuum of empty space to be stable, we should be living at a minimum of potential energy." In other words, most things end up resting in a place of lowest energy. A ball rolls downhill and settles in a low point; getting it to move away from this point requires a kick of energy. In the case of the universe it would be like living at the bottom of a valley bordered by hills: the value of the Higgs potential would be lowest point of the valley.
Our universe might end if our valley really isn't the lowest one around. Physicist Benjamin Allanach of the University of Cambridge explains: "The shape of the Higgs potential is determined precisely by the Higgs mass." The observed 126 GeV mass seems to imply the universe does not exist in the lowest possible energy state but is in fact positioned in a slightly unusual place. "It turns out that for a Higgs boson of 126 GeV, we might be in the gray area where the universe is at a local minimum that is not the global minimum," says physicist Matthew Strassler of Rutgers University.
It is sort of like being in a valley whose floor is higher than that of an adjoining valley. If you didn't know that a deep valley was on the other side of the hill, you would think you were at the lowest level you could be. If you somehow managed to get to the other side, however, you could fall much lower.
This situation would normally not pose a problem, as you couldn't travel between valleys—except in quantum mechanics, which allows particles to tunnel through hills unpredictably. As a result, "in the future our universe could spontaneously and randomly tunnel through to the deeper one, with potentially catastrophic consequences," Allanach says.
Such a metastable universe is not a new idea. As far back as 1979, physicists were trying to calculate the implications of the mass of the Higgs boson on cosmology. In 2001 theoretical physicists Paul Steinhardt of Princeton University and Neil Turok of the Perimeter Institute for Theoretical Physics in Canada described a cyclic universe, which alternates between expansion and contraction, and is consistent with the sort of metastability implied by the observed mass of the Higgs boson. More recently, Giuseppe Degrassi of the University of Rome and Jose Espinosa of the Autonomous University of Barcelona and their collaborators have calculated the broad implications of the Higgs mass.
"We now know with a large degree of confidence that our vacuum is on the unstable side and we were able to calculate its decay lifetime," Espinosa says. "This lifetime turns out to be way larger than the [present] age of the universe."
Most theorists don't seem to be too worried about the destruction of our universe, because metastability would not manifest itself anytime soon—if ever. Also, they expect that the LHC will find other particles in due course. Then, new calculations could indicate that the universe has more stability. Specifically, the fate of the universe depends quite sensitively not only on the Higgs but also on the mass of the top quark, another fundamental particle whose mass hovers at about 180 GeV. "The top quark strongly affects the vacuum by its quantum fluctuations because it is so heavy," Allanach says. "If the Higgs mass were really 127 GeV and the top mass were a little lower than its most likely value, then actually the universe would be completely stable and the vacuum would be in the true minimum."
Steinhardt says, "There is a tiny sliver of metastability. Why is the universe just at this point? Is this actually a profound thing we have to understand?"
But assuming that everything is known about the Standard Model and no new particles and forces will be found in the future, then the universe might be in the gray region where it is long-lived but somewhat unstable and therefore might disappear a few billions of eons from now. "And maybe not even billions of years, but billions of eons or billions of billions" of eons, Strassler stresses. "This is not something that keeps me awake."