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Alex Millar | University of Melbourne | Australia

View Blog | Read Bio

Dark Matters: Creation from Annihilation

Hanging around a pool table might seem like an odd place to learn physics, but a couple of hours on our department’s slanted table could teach you a few things about asymmetry. The third time a pool ball flew off the table and hit the far wall I knew something was broken. The pool table’s refusal to obey the laws of physics gives aspiring physicists a healthy distrust of the simplified mechanics they learnt in undergrad. Whether in explaining why pool balls bounce sideways off lumpy cushions or why galaxies exist, asymmetries are vital to understanding the world around us. Looking at dark matter theories that interact asymmetrically with visible matter can give us new clues as to why matter exists.

Alternatives to the classic WIMP (weakly interacting massive particles) dark matter scenario are becoming increasingly important. Natural supersymmetry is looking less and less likely, and could be ruled out in 2015 by the Large Hadron Collider. Asymmetric dark matter theories provide new avenues to search for dark matter and help explain where the material in our universe comes from -baryogenesis. Baryogenesis is in some ways a more important cosmological problem than dark matter. The Standard Model of particle physics describes all the matter that you are familiar with, from trees to stars, but fails to explain how this matter came to be. In fact, the Standard Model predicts a sparsely populated universe, where most of the matter and antimatter has long since annihilated each another. In particle colliders, whenever a particle of matter is created, an opposing particle of antimatter is also created. Antimatter is matter with all its charges reversed, like a photo negative. While it is often said that opposites attract, in the particle physics world opposites annihilate. But when we look at the universe around us, all we see is matter. There are no antistars and antiplanets, no antihumans living on some distant world. So if matter and antimatter are always created together, how did this happen? If there were equal amounts of matter and antimatter, each would annihilate the other in the first fractions of a second and our universe would be stillborn. The creation of this asymmetry between matter and antimatter is known as baryogenesis, and is one of the strongest cosmological confirmations of physics beyond the Standard Model. The exact amount of asymmetry determines how much matter, and consequently how many stars and galaxies, exists now.

And what about the other 85% of matter in the universe? This dark matter has only shown itself through gravitational interactions, but it has shaped the evolution of the universe. Dark matter keeps galaxies from tearing themselves apart, and outnumbers visible matter five to one. Five to one is a curious ratio. If dark and visible matter were entirely different substances with a completely independent history, you would not expect almost the same amount of dark and normal matter. This is like counting the number of trees in the world and finding that it’s the same as the number of pebbles. While we know that dark and visible matter are not the same substance (the Standard Model does not include any dark matter candidates), this similarity cannot be ignored. The similarity in abundances between dark and visible matter implies that they were caused by the same mechanism, created in the same way. As the abundance of matter is determined by the asymmetry between antimatter and matter, this leads us to a relationship between baryogenesis and dark matter.

Asymmetric dark matter theories have attracted significant attention in the last few years, and are now studied by physicists across the world. This has give us a cornucopia of asymmetric dark matter theories. Despite this, there are several common threads and predictions that allow us to test many of them at once. In asymmetric dark matter theories baryogenesis is caused by interactions between dark and normal matter. By having dark matter interact differently with matter and antimatter, we can get marginally more matter in the universe then antimatter. After the matter and antimatter annihilate each other, there is some minuscule amount of matter left standing. These leftovers go on to become the universe you know. Typically, a similar asymmetry in dark matter and its antiparticle is also made, so there is a similar amount of dark matter left over as well. This promotes dark matter from being a necessary, yet boring spectator in the cosmic tango to an active participant, saving our universe from desolation. Asymmetric dark matter also provides new ways to search for dark matter, such as neutrinos generated from dark matter in the sun. As asymmetric dark matter interacts with normal matter, large bodies like the sun and the earth can capture a reservoir of dark matter, sitting at their core. This can generate ghostlike neutrinos, or provide an obstacle for dark matter in direct detection experiments. Asymmetric dark matter theories can also tell us where we do not expect to see dark matter. A large effort has been made to see tell-tale signs of dark matter annihilating with its antiparticle throughout the universe, but it is yet to meet with success. While experiments like the Fermi space telescope have found potential signals (such as a 130 GeV line in 2012), these signals are ambiguous or fail to survive the test of time. The majority of asymmetric dark matter theories predict that there is no signal, as all the anti dark matter has long since been destroyed.

As on the pool table, even little asymmetries can have a profound effect on what we see. While much progress is made from finding new symmetries, we can’t forget the importance of imperfections in science. Asymmetric dark matter can explain where the matter in our universe came from, and gives dark and normal matter a common origin. Dark matter is no longer a passive observer in the evolution of our universe; it plays a pivotal role in the world around us.

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