Protons, Neutrons and Electrons: The Staff of Life

You, this computer, the air we breathe, and the distant stars are all made up of protons, neutrons and electrons. Protons and neutrons are bound together into nuclei and atoms are nuclei surrounded by a full complement of electrons. Hydrogen is composed of one proton and one electron. Helium is composed of two protons, two neutrons and two electrons. Carbon is composed of six protons, six neutrons and six electrons. Heavier elements, such as iron, lead and uranium, contain even larger numbers of protons, neutrons and electrons. Astronomers like to call all material made up of protons, neutrons and electrons ``baryonic matter''.

Until about ten years ago, astronomers thought that the universe was composed almost entirely of this ``baryonic matter'', ordinary stuff. However, in the past decade, there has been ever more evidence accumulating that suggests there is something in the universe that we can not see, perhaps some new form of matter.

The Dark Matter Mystery

By measuring the motions of stars and gas, astronomers can ``weigh'' galaxies. In our own solar system, we can use the velocity of the Earth around the Sun to measure the Sun's mass. The Earth moves around the Sun at 30 kilometers per second (roughly sixty thousand miles per hour). If the Sun were four times more massive, then the Earth would need to move around the Sun at 60 kilometers per second in order for it to stay on its orbit. The Sun moves around the Milky Way at 225 kilometers per second. We can use this velocity (and the velocity of other stars) to measure the mass of our Galaxy. Similarly, radio and optical observations of gas and stars in distant galaxies enable astronomers to determine the distribution of mass in these systems.

The mass that astronomers infer for galaxies including our own is roughly ten times larger than the mass that can be associated with stars, gas and dust in a Galaxy. This mass discrepancy has been confirmed by observations of gravitational lensing, the bending of light predicted by Einstein and his theory of general relativity.

Candidates for the Dark Matter

What is the nature of the ``dark matter'', this mysterious material that exerts a gravitational pull, but does not emit nor absorb light? Astronomers do not know.

There are a number of plausible speculations on the nature of the dark matter:

• Brown Dwarfs: if a star's mass is less than one twentieth of our Sun, its core is not hot enough to burn either hydrogen or deuterium, so it shines only by virtue of its gravitational contraction. These dim objects, intermediate between stars and planets, are not luminous enough to be directly detectable by our telescopes. Brown Dwarfs and similar objects have been nicknamed MACHOs (MAssive Compact Halo Objects) by astronomers. These MACHOs are potentially detectable by gravitational lensing experiments. If the dark matter is made mostly of MACHOs, then it is likely that baryonic matter does make up most of the mass of the universe.
• Supermassive Black Holes: these are thought to power distant quasars. Some astronomers speculate that there may be copious numbers of black holes comprising the dark matter. These black holes are also potentially detectable through their lensing effects.
• New forms of matter: particle physicists, scientists who work to understand the fundamental forces of nature and the composition of matter, have speculated that there are new forces and new types of particles. One of the primary motivations for building ``supercolliders'' is to try to produce this matter in the laboratory. Since the universe was very dense and hot in the early moments following the Big Bang, the universe itself was a wonderful particle accelerator. Cosmologists speculate that the dark matter may be made of particles produced shortly after the Big Bang. These particles would be very different from ordinary ``baryonic matter''. Cosmologists call these hypothetical particles WIMPs (for Weakly Interacting Massive Particles) or ``non-baryonic matter''.

MAP and Dark Matter

By making accurate measurements of the cosmic microwave background fluctuations, MAP will be able to measure the basic parameters of the Big Bang model including the density and composition of the universe. If our ideas about the origin and evolution of galaxies and large-scale structure are correct, then MAP should be able to measure the density of baryonic and non-baryonic matter to an accuracy of better than 5%. It will also be able to determine some of the properties of the non-baryonic matter: the interactions of the non-baryonic matter with itself, its mass and its interactions with ordinary matter all affect the details of the cosmic microwave background fluctuation spectrum.

Other Interesting Sites and Further Reading:

• Visit the dark matter home page at the Center for Particle Astrophysics at Berkeley.
• A list of popular books on dark matter and the Big Bang.
• A recent introductory html article by David Spergel on searching for dark matter. This article is geared towards physics undergraduates and will appear in "Some Outstanding Problems in Astrophysics", edited by J.N. Bahcall and J.P. Ostriker.

• OGLE home page: one of the experiments searching for MACHOs. A faster site for Europeans.
• MACHO home page: The Berkeley/Livermore/Australia search for MACHOs.

• HST Gravitational Lensing Home Page.

Back to the Introduction to Cosmology Page

Back to the MAP Home Page

Last updated: March 20, 1996