A recent astrophysical result has cosmologists scratching their heads. Apparently the microwave shadows that galaxy clusters ought to be casting aren’t showing up as predicted by our current understanding of Big Bang theory, and nobody knows why.
One of the most significant discoveries in cosmology in the 20th century was the serendipitous observation in 1965, By Arno Penzias and Robert Wilson, of the microwave background radiation, a frozen relic of the Big Bang. This radiation permeates all of observable space; it consists of photons that date back to the decoupling of matter from energy in the early years of the Universe, and its discovery dramatically confirmed one of the cornerstone predictions of Big Bang theory. The Universe has been expanding for the 13 billion years or so since its cataclysmic birth, and the photon wavelengths have been correspondingly stretched as well, lowering the radiation’s temperature to a nippy 2.75° Kelvin.
The farther we look out into space, the farther we look back in time, so the background radiation, born as it was in the Universe’s infancy, should appear to emanate from the furthest observable edges of space – a spherical shell at a distance, roughly, of 13 billion light years from any observer. This has an important predictive consequence, namely that large objects closer in should cast a discernable shadow.
Well, some results are in, and they are troubling. The predicted shadows are not appearing as expected. Learn more here.