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posted Jan 30, 2015, 12:04 AM by   [ updated Dec 3, 2015, 9:13 PM by Upali Salpadoru ]

We are bounded to our mother Earth. We walk on land, sail the oceans and breathe the air.  Our space probes cruise among the planets that orbit round the sun. NASA analyses pictures from the Hubble Ultra Deep Field pictures  revealing 13 billion years of history of the Universe.

Fig.1  Courtsey  NASA . Hubble.

Electro magnetic radiation, including light from the  Sun, travelling at a speed of 300,000 km per second, reaches the Earth in 8min.  Light to cross the solar system takes eleven hours.


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.

Milkyway galaxy, Andromeda galaxy and a few more than 50 other galaxies form a portion of space known as our ‘Local Group’. All these galaxies are held together by Force of gravity. The mass necessary to hold them together comes only from very recently discovered, an illusive form of matter named as ‘cool dark matter’.

In 1924 American astronomer Edwin Hubble discovered that there are Galaxies even beyond milky way.   In 1929 he also discovered that these far away galaxies are moving Away from each other. This led the scientists to calculate backwards and determine when where the beginning was. This is what we now know as the ‘Big Bang theory. Considering the way  the distant  Galaxies are repelling, all space matter and time would have had the beginning 13.7 billion years ago.

Outside the  local group of Galaxies,  Virgo Galaxy is the nearest. This belongs to the Super cluster of Galaxies.. Light from our Milky way takes over 60 million years to reach Virgo. There are about 1500 galaxies in this cluster. In the centre of this galaxy is a black hole seven million times as massive as our sun. The attractive force of gravity in a black hole.  is so powerful that even light gets trapped in and cannot escape.


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.