The Dark Matter Universe Part 1: The Case for Dark Matter
Dark Matter: The theoretical substance said by many to account for four fifths of the matter in the universe. Proof of dark matter could answer countless questions about cosmology and the origin of the universe. Until recently physicists considered dark matter to be inert or unreactive. However new theories about the origin of dark matter suggest the universe may be much more interesting than previously thought. Much more. Some scientists now suggest a highly reactive so called “Dark Universe” which coexists with our own. Scientists are now asking the question: Does an invisible world exist as the mirror image of our own?
The Case for Dark Matter
Fritz Zwicky
First suggested by Fritz Zwicky in 1934 Dark Matter solved the problem of “Missing Mass” in his observations of the COMA cluster, a cluster of galaxies 316 light years from our own. The “Missing Mass” problem arose when Zwicky applied the virial theorem to the cluster and found the total kinetic energy far exceeded its gravitational binding energy. The virial theorem calculates the total kinetic energy of a complex system. Kinetic energy is energy resulting from motion. The Gravitational Binding Energy is the energy needed to rip apart a loose system held together only by gravity, such as a galaxy cluster. In short, the galaxies were spinning far too fast to be held together by the gravitational mass, or mass which creates a gravitational field, he could see with his telescope. The overall mass of galaxies was being calculated by the “mass to light” ratio, or an average of the amount of observable light and the mass required to produce that amount of light. Zwicky, faced with this problem, concluded unobservable lightless matter, or “dark” matter, may account for the observed discrepancies.
Further suggestions for “dark” matter came with the discovery of gravitational lensing. Gravitational lensing, a concept first introduced by Orest Chwolsen in 1924 and mathematically quantified by Einstein in 1936, occurs when gravity from a super massive object (like a galaxy cluster or black hole) warps space-time which bends light around it. By this process a cluster of galaxies will bend the light radiating from more distant bodies and consequently magnify and distort it. Using Einstein's equations the distortion caused by a super massive object can be calculated and the overall mass inferred. When the distortion caused by galaxy clusters is measured scientists find the observable mass insufficient to account for the degree of lensing. Again the conclusions drawn by scientists suggest the existence of mass immune to electromagnetism.
The World of WIMPs
Invisible or not dark matter must be made of something quantifiable. The latest theories come not from the infinite heavens studied by cosmologist, but instead from the sub atomic worlds explored by particle physicists. The story begins with the discovery of Radioactive Beta Decay by Henri Becquerel in 1896. Beta Decay happens when W and Z bosons, or force carrying particles, are transmitted and cause the emission of beta particles such as electrons or positrons. W and Z bosons are very massive, compared to electrons and positrons, and work through the weak atomic force (one of the four fundamental forces of nature: weak force, strong force, electromagnetism and gravity).
Due to their large mass and weak force W and Z bosons are able to knock a positron or electron from an atom, but can't compete with the strong atomic force which holds atomic nuclei together. However nothing in the standard model of physics accounts for the unique mass of W and Z bosons. As always, scientists have a theory: novel particles are causing W and Z bosons to take on mass. These particles however do not cause neutrons (the basic unit of atomic nuclei along with protons) or photons (light particles) to take on mass and therefore must only interact through the weak atomic and gravity (as all mass interacts with gravity). Scientists consequently dubbed these unique particles Weakly Interacting Massive Particles, or WIMPs. Invisible particles of gravitational mass- do WIMPs make a good candidate for dark matter?
The number of WIMPs in the universe is crucial to their candidacy for dark matter. At the moment of the “Big Bang” particles were created in a dynamic equilibrium. As all the matter in the universe burst forth from a singularity, massive amounts of collisions destroyed particles such as WIMPS. About ten nanoseconds after the creation of the universe it had already expanded to the point where particles weren't colliding and cooled enough that new particles could not be made. The number of particles in the universe thus became locked in time. Scientists, given the mass and energy of WIMPs, can estimate the amount of WIMPS left over from creation. In what is called the WIMP Coincidence scientific estimates for the amount of WIMPs still floating around today matches exactly the amount of mass in the universe accounted for by Dark Matter.
So we have evidence which suggests the universe is much more massive than previously thought and a theoretical particle which could explain it all. However, WIMPs are just the tip of the iceberg. If current theories hold true and the team at CERN discovers WIMPs with the Large Hadron Collider then a world of possibilities opens up. Literally. Next Week: superWIMPs and the Dark Matter Universe.
