Acoustic/Sound waves in early universe Friends,
Sound waves, being mechanical in nature cannot travel in empty space. But I came across an article " Magnetic Fields in early Universe" by Dario Grasso and Hector R. Rubinstein in which it was mentioned that 

"primordial density fluctuations, which are necessary to explain the observed structures in the Universe, gives rise to acoustic oscillations of primordial plasma...". 

What are these oscillations and if these are in the nature of sound waves, how could they expand in early universe. It is also to mention that in his famous book " Before Time Began" Helmut Satz also mentioned that small density irregularities created by quantum fluctuations cause the early Universe to sing. he called it as The Sound of Big bang. 
So my point is what is the nature of these waves and how could they travel in early universe without any medium.
 A: There is a medium: the plasma which filled space in those early times.
And on a large enough distance scale, space is 'full of stuff' even now, though at a low average density.
A: In the early universe the density would be so large that space would be effectively filled with a medium.  Today the universe is so sparse you need to travel for millions of miles to get from one chunk on matter to another.  
On a smaller distance scale where molecular bonding occurs one can have acoustic vibrations, on the smallest scale lattice vibrations in an individual complex molecule.  On a larger, but still not cosmic, scale one can have disturbances travel through a fluid like air or water.  You are correct that in space one cannot have sound.  Also, to have sound one need coupling between the elements of a medium.  Acoustics is typically viewed as a small perturbation of some other underlying equations.  For example acoustics in air, water, and other fluids can be derived from the equations of fluid mechanics.  
The concept of acoustics loses meaning without some interaction.  For example, as the earth moves its gravitational field affects nearby planets.  Are these perturbations "acoustics".  Most physicists would say not.  We typically associate the acoustic phenomenon as something supported by continuum.  But these concepts do get challenged when we think in terms of cosmology and astrophysics.
In the early universe one would have some exotic form of matter filling all space and interacting.  To figure out exactly how sound or acoustics would behave in this medium one would have to get a working model of the bulk medium and look at small amplitude oscillations in this bulk, i.e. do a perturbation analysis on these equations.
But long story short, there would be a medium filling space-time in the early universe.  
