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WHERE DOES MATTER COME FROM?
by
Marcelo Gleiser

It is said, and rightly so, that cosmology is the branch of physics that asks the grandest questions. After all, few questions within science can equal the impact of: “Where does the universe come from?” or “What is the fate of the universe” or “Where does the matter we are made of come from?”

       But perhaps even more exciting than asking these questions is the fairly recent power that we have of answering them, at least partially, through a rational study of nature.

Marcelo Gleiser
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Antimatter

Quarks

       Most of us learned in high school that matter is made of atoms and that atoms are made of protons, neutrons and electrons. What we don’t usually learn in high school is that to each particle of matter there is another particle, an “anti-particle,” which is essentially the same as the particle but with opposite electric charge.

       Thus, the negatively charged electron has its “anti-electron,” called a positron, which has positive electric charge; the proton has an anti-proton, and so on. Now comes the interesting part. According to the laws of particle physics, matter and antimatter should be present in the universe in equal amounts. And yet, we have ample observational evidence that, at least in a very large volume that surrounds us and extends far beyond our galaxy, there is much more matter than antimatter.

       When particles collide with their anti-particles, the effects are devastating; they both disintegrate into electromagnetic radiation, their energy carried away in neutral particles called photons.  In other words, if there were as much antimatter as matter in the universe, we wouldn’t be here to ask grand questions. The universe is somehow unbalanced, biased toward the existence of matter over antimatter. One of the greatest challenges in modern cosmology is to unveil the roots of this cosmic imperfection.

       As with any scientific explanation, we need a few “basic ingredients,” a minimum amount of knowledge from which to build our models. The first ingredient we need is the Big Bang model of cosmology. According to this model, a small fraction of a second after the “beginning,” many kinds of particles and their anti-particles, in equal amounts, roamed about and collided with each other immersed in tremendous heat, as in a cosmic minestrone soup.

       In this hot cosmic furnace, many different types of particles were being cooked, not necessarily the familiar quarks (the constituents of protons and neutrons) or electrons. As the universe expanded and cooled, a sort of selection mechanism not only biased the creation of quarks and electrons over other types of particles, but also generated the excess number of particles over anti-particles. Surviving the annihilation with their antimatter cousins, these excess particles organized themselves into more complex structures, until eventually atoms, mostly hydrogen, were formed when the universe was about 300,000 years old. The mystery, then, is to understand what kind of physics could generate this bias.

 

Dr. Marcelo Gleiser, Associate Professor of Physics and Astronomy at Dartmouth College, is the author of THE DANCING UNIVERSE: FROM CREATION MYTHS TO THE BIG BANG, already a best-seller in his native Brazil and published in October 1997 in the U.S.A.

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