by Prasad Ganti
Just when we thought that things are falling into place and that we know enough about the Universe, came the revelation that the Universe is dominated by dark matter and dark energy; which between them account for 96% of whatever is present in the Universe. It is only the 4% of constituents which are under our cross hairs and get termed as “spectacular discoveries”. Although the effects of the 96% constituents has been studied and understood well, its nature is not known.
Dark matter is said to constitute 23% of the Universe. It is not a recent discovery. First thought of during the 1930s by the Swiss Physicist Fritz Zwicky at Caltech. He found that dark matter has influence at Galactic levels, not at the level of stars or their planets. He discovered that the outer reaches of Galaxies rotate at the same speed as the inner ones, unlike in our Solar system where the planet Mercury, the planet closest to Sun, travels a lot faster than the distant Neptune. This led to the speculation that there is some dark unknown matter on the fringes of the galaxies which makes this happen. Not visible to any telescope or any detector mankind has ever devised.
Decades later, the American astronomer Vera Rubin did studies on spiral galaxies and came to the same conclusion as Zwicky did. Firstly that the galaxies rotate around their centers. Secondly, the outer reaches of the galaxies rotate much faster than what the theory of gravitation predicts. Subsequent discoveries like Cosmic Background Microwave Radiation and Gravitational Lensing supported the dark matter theory. Zwicky and Rubin could not be ignored anymore.
The matter, that we know of, which we, the Earth and our Solar system are made up of, is now known as Baryonic matter. It consists of the electrons, protons, neutrons etc. What does the dark matter consists of. There are two theories, neither of them have been proved. The first theory is that there are particles other than known ones like protons, called WIMPs (Weakly Interacting Massive Particles) from which the dark matter could be made of. Labeled Axions, Neutralinos, these are postulated to be much heavier than protons, but do not get influenced by electromagnetic radiation like light or X-rays. But they are subject to the forces of gravity.
Second theory specifies the existence of MACHO (Massive Astrophysical Compact Halo Objects). They are made of regular Baryonic matter that does not emit any kind of radiation but drifts through interstellar space, not affiliated with any solar system. I read an article in a recent issue of Scientific American that these could be primordial black holes. Meaning that the black holes formed during the early life of our Universe.
There are detectors for WIMPs. One example being the CDMS (Cold Dark Matter Search) in Soudan mines in Minnesota. Neither a WIMP nor any MACHO have been discovered to date. There is no room for pessimism though. It took a while to detect neutrinos, the Higgs Boson, and the Gravity waves. All of them were predicted by theory and much later confirmed experimentally.
Nature seems to be a moving target. Just as we think we get getting to know most of the things, the more ignorant we find ourselves to be. The quest to know is a journey and not a destination. Maybe some day, we will know the mystery of the dark matter. And then, some other unknown pops up.