This is where one needed the work of Abdus Salam. I mentioned to you that his contributions have been one of the motivations for this line of research. So let me now move to the particle physics side of the whole problem where Salam had contributed. The work by Abdus Salam, Steven Weinberg and Sheldon Glashow on gauge theories in the late sixties and early seventies led to the discovery of the link between what is called the weak interactions and the electromagnetic interactions. To the uninitiated here, I may add that physicists have broadly divided all the basic interactions into four compartments. One is gravity, which of course was the first one to be discovered with Newton's Law of Gravitation as the starting point. Then there is the electromagnetic interaction which we are very familiar with even in daily life, with the electric power that we use for lighting being part of electromagnetic interactions. Then there are the weak and strong interactions which are confined to nuclear distances, with very short range. The work which Salam, Weinberg and Glashow had initiated and completed was to establish a link between weak force and the electromagnetic force. It is called the electroweak interaction.
In science, it is not enough to have a theory which formally or theoretically tells how to
combine electromagnetism with weak interaction. Physicists would like
to have evidence that it really works. For this they would like to test it
in the laboratory. To test the Salam Weinberg hypothesis required
experiments of high energy particles which could go upto something like
100
or Giga electron-Volts. The electron-Volt is a unit for
particle energy and a giga electron-Volt is a billion electron-Volts. So
these are very high energy particles. One can ask how and where can you produce such
particles? The answer is that you can produce them in particle
accelerators. The gigantic accelerator at CERN in Europe was put to use to verify that this theory was indeed correct.
The success of this approach raised hope that having combined two
interactions into one, the next step would be to bring the third (the strong) interaction
into the same fold with the electroweak interaction, so as to arrive at what people call the Grand Unified Theory. Several different
theoretical approaches were there because people did not have a unique
Grand Unified Theory. The general expectation was that any such theory
would require particles with energies as high as 
. Can one produce energies of this
order in any laboratory? The answer is NO. The accelerators at
CERN or Fermilab cannot reach this level at all. They fall short by
more than twelve orders of magnitude. Therefore if the physicists want to
test a grand unified theory they have no way of doing it. And if a
physical theory is not testable by any practical method, it then becomes pure
speculation. It does not get the status of an
established scientific theory. So physicists while following the line set up
by Salam and Weinberg, were in a quandary as to how they
would test any grand unified theory. That is where the big bang cosmologists
came to their help.
You may recall that cosmologists needed help from particle physicists to understand the early universe. Now particle physicists went to cosmologists for help. In the big bang cosmology you had very fast moving particles close to the big bang. Therefore they felt that by combining their ideas with the cosmological model they could explore whether the grand unified theories or any other high energy theory would work. Out of this mutual need was born `Astroparticle physics'. I can give you another example where joining of forces was motivated in astronomy. When quasars were discovered in the early nineteen sixties - 1963 was when the first two quasars were discovered and it was realized that they were somewhat unusual objects - astrophysicists felt that these quasars were so compact that their gravitational field was very strong. So strong that you cannot rely on Newton's law of gravitation, you have to use the General Theory of Relativity. Till then astrophysicists had not felt the need to use general relativity but now as a result of the discovery of quasars they felt that inputs from relativity were needed. So the subject of `relativistic astrophysics' was created, that brought astrophysicists in touch with general relativists. I recall attending the first Texas Symposium which was held in Dallas, Texas, in 1963, where the issue was first addressed.
When it comes to joining forces you can ask what is it going to do for both subjects? I will now make a checklist of problems which one can try to understand and solve.