pro read_density

  filename = "density_1024_036.dat"
  
  npix = 0l ; number of cells per dimension in the grid. It should be 1024
  time = 0.0 ; the expansion factor
  openr, 1, filename
  readu,1, time, npix
  grid = fltarr(npix, npix, npix) ; number of particles per cell. Each particle has a mass of 6.17e9 Msun/h
  readu, 1, grid
  grid = grid * 8
  close, 1


  stop


end