Dark matter: Gordian knot of cosmology
THERE is an adage, "What you see is what you get." In the case of our Universe, it is au contraire – what you see is not what you get. Just when we thought that all the mysteries of the Universe have been unmasked, cosmologists were abashed to find out that almost everything around us cannot be seen or detected. They were further embarrassed to note that this "shadowy substance," the so-called dark matter or missing mass, determines the shape, age, size, and destiny of the Universe.
The theory of the missing mass was first proposed in the 1930s by Fritz Zwicky, an eccentric astrophysicist at Caltech, from observations that some galaxies travel at unexpectedly high speeds in a cluster of other galaxies. His work followed by several anomalous cosmological observations led cosmologists to conclude that the Universe is made up of two kinds of matter – ordinary matter that is luminous and can be seen, and dark matter that is non-luminous and cannot be seen because it neither emits detectable radiation nor interact with light.
Why does the Universe need dark matter? Stars moving in nearly circular orbits about their galactic center are bound by the gravitational force. According to laws of physics, their orbital speeds should decrease with increasing distance from the center. But observations indicate otherwise; constant orbital speeds beyond the visible edge of the galactic disk. This is quite unlike the orbits of our planets where larger is the orbital radius, slower is the speed. To explain a speed that is independent of the radius demands that the mass of all the matter - stars, gas, and interstellar dust, that lies inside the star's orbit, increase linearly with the radius. Visual observations, however, show that the mass is constant and most of it is concentrated in the central region.
To resolve this dilemma, astronomers concluded that there is as much, if not more, of a galaxy unseen as there is seen. Galaxies must contain a large quantity of matter to supply the necessary gravitational force for stars to maintain a constant orbital speed. Calculations indicate that indeed galaxies should contain huge amounts of dark matter, some as much as 10 times their visible mass. As with individual galaxies, galaxy clusters also exhibit similar effect, suggesting there is more matter in the clusters than we can account for. Dark matter is the invisible gravitational glue that holds together galaxies and clusters of galaxies.
Whatever dark matter is, it cannot hide from us. We know of its existence from a phenomenon known as "gravitational lensing." As light from distant galaxies pass by a cluster of galaxies on its way to Earth, it is deflected by the gravitational field of the cluster, just like bending of light by a lens. The amount of deflection light undergoes cannot be explained by luminous matter alone, suggesting the presence of relatively large amounts of dark matter.
There is now irrefutable evidence that visible matter accounts for only 4% of the Universe's mass. The remaining 96% is invisible of which approximately 21% is believed to be dark matter, also known as cosmological Spam. The remaining 75% is attributed to a pervasive "dark energy."
What can this dark matter be? Dark matter is a catch-all term that is used to describe a variety of candidates, although none is proven. They are divided loosely into two categories: MACHOs (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). Possible MACHOs are massive black holes, neutron stars, burnt-out white dwarf stars, or brown dwarf stars. The first suspect of WIMP is neutrino. Others include particles predicted by theory but not discovered as yet, such as magnetic monopoles, photinos, and several exotic elementary particles produced during the Big Bang. Another contender for dark matter is the hypothetical axion, believed to be lighter than the WIMPs. Current theories favor WIMPs over the MACHOs.
The nature of the dark matter is still an enigma, a Gordian knot in cosmology. Finding this elusive matter would be a major step toward understanding the structure and evolution of the Universe. It will also enable us to ascertain whether the Universe is destined to eternal expansion or eventual collapse into its point of origin.
The writer is a Professor of Physics at Fordham University, New York.
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