Knocking on Heaven's Door - Lisa Randall [180]
In addition to puzzles about energy, we also have a further cosmological mystery about matter: Why is there matter in the universe at all? Our equations treat matter and antimatter on the same footing. They annihilate when they find each other, and both disappear. Neither matter nor antimatter should remain when the universe has cooled.
Whereas dark matter doesn’t interact very much and therefore sticks around, ordinary matter interacts quite a lot through the strong nuclear force. Without an exotic addition to the Standard Model, almost all of our usual matter would have disappeared by the time the universe had cooled to its current temperature. The only reason matter can be left is that there is a predominance of matter over antimatter. This isn’t built into the simplest versions of our theories. We need to find reasons that protons exist but can’t find antiprotons with which they can annihilate. Somewhere a matter-antimatter asymmetry must be built in.
The amount of leftover matter is smaller than the amount of dark matter, but it is still a sizable chunk of the universe—not to mention the source of everything we know and love. How and when this matter-antimatter asymmetry was created is another big question that particle physicists and cosmologists very much want to tackle.
The question of what constitutes the dark matter of course remains critical as well. Perhaps eventually we will find that the underlying model connects the dark matter density to that of matter, as recent research suggests. In any case, we hope to soon learn a lot more about the dark matter question from experiments—a sampling of which we’ll now explore.
CHAPTER TWENTY-ONE
VISITORS FROM THE DARK SIDE
When the LHC’s chief engineer, Lyn Evans, spoke at the California LHC/Dark Matter conference in January 2010, he closed by teasing the audience about how for the last couple of decades, “You theorists have been thrashing around in the dark (sector).” He added the caveat, “Now I understand why I spent the last fifteen years building the LHC.” Lyn’s comments referred to the paucity of high-energy data over the previous years. But they were also hints about the possibility that LHC discoveries might shed light on dark matter.
Many connections exist between particle physics and cosmology, but one of the most intriguing is that dark matter might actually be made at the energies explored by the LHC. The remarkable fact is that if a stable particle species with weak scale mass exists, the amount of energy carried by particles of this type that survived from the early universe to today would be about right to account for dark matter. The result of calculations about the amount of dark matter that is left over by an initially hot—but cooling—universe demonstrate that this might be the case. That means that not only is dark matter literally right under our noses, its identity might prove to be too. If dark matter is indeed composed of such a weak mass particle, the LHC might not only give us insight into particle physics questions, it might also provide clues to what is out there in the universe and how it all began—questions that are incorporated into the science of cosmology.
But LHC experiments are not the only way to search for dark matter. The fact is that physics has now entered a potentially exciting era of data, not just for particle physics, but also for astronomy and cosmology. This chapter explains how experiments in the upcoming decade will search for dark matter using a three-pronged approach.