Galaxies are found in the “walls” of huge voids; very few galaxies are found in the voids themselves. the text says that the structure of filaments and voids has been present in the universe since shortly after the expansion began 13.8 billion years ago. in science, we always have to check to see whether some conclusion is contradicted by any other information we have. in this case, we can ask whether the voids would have filled up with galaxies in roughly 14 billion years. observations show that in addition to the motion associated with the expansion of the universe, the galaxies in the walls of the voids are moving in random directions at typical speeds of 300 km/s. at least some of them will be moving into the voids. how far into the void will a galaxy move in 14 billion years? is it a reasonable hypothesis that the voids have existed for 14 billion years?

a coal burning power plant

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.