 Fig.1 Courtsey NASA . Hubble.  Fig.2 Terry lectures by Nancy Abrams and Joel primack. The sun, at the centre of the solar system has a mass of over 300,000 times that of the Earth. Every second it loses several billions of kilograms due to the conversion of matter into energy and the emanation of solar wind. (Solar energy consists of electro magnetic radiations while solar wind contains protons, electrons and a few lesser known particles such as neutrinos etc.) Yet our sun is a mediocre in the cluster of stars named Milky way.  Fig.3 Milky way Galaxy has more than a hundred billion stars. For the light to travel from one end of this galaxy to the other end it takes more than ‘one hundred thousand years’. It is just a dot in the known Universe.
 Fig.2. All matter and Energy in the Universe. It may be even a shock to many people to hear that we can see, even with the space telescopes, less than a 0.5% of the Known Universe. It is not because of the distance but the strange matter that fill the space is completely immune to all our senses. We cannot see it , hear it or feel it even with the most sophisticated instruments we have developed. The invisible Universe consists of Dark matter and Dark energy. While Dark matter is responsible for keeping the galaxies in a cluster attracted to each other Dark energy is responsible for the expansion of the Inter galactic space. Double Dark Theory The question that now arises is . then how do we know that dark matter exist. In the early days of scientific investigations , a hypothetical substance called ‘Ether’ {not the alcohol) was assumed to be present This medium was introduced to explain the propagation of heat and light waves in space. This had to be abandoned when Newton considered to be particles which called as ‘corpuscles. Yet the wave theory could not be discarded. Maxwell introduced the ‘Electro Magnetic Theory’ for the propagation of waves in space. While the two theories were amalgamated as a dual theory for the propagation of light the idea of Ether fell by the wayside. Albert Einstein while accepting the dual theory proposed that light travelled in extremely small packets called quanta’. Later these came to be known as Photons. Just as the light was categorised as a wave at one time and a wave as at another Physicists and Astronomers viewed space as empty at a time and filled with some medium at another. According to the data available now , considering the gravity that pulls the galaxies ogether in a star cluster, there has to be a certain mass. This far above the mass of matter in the stars and in the cluster. So we have to conclude that there has to some mass of an unknown substance in space. This is what has been termed as ‘Dark MatterJust as Electro magnetic and nuclear forces Gravity does not repel. Now what makes the Universe expand pushing the Galaxies away from each other at phenomenal speeds? The scientists have attributed this phenomenon to Dark Energy. As the Universe was formed there was no ‘dark energy], only Dark matter may have been present so the scientists think Dark Energy is a phenomenon developed much later as a property of space itself. |
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