Big Bang
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The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a br
Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. Galaxies are color-coded by redshift.
Abell 2744 galaxy cluster – Hubble Frontier Fields view
XDF size compared to the size of the Moon (XDF is the small box to the left of, and nearly below, the Moon) – several thousand galaxies, each consisting of billions of stars, are in this small view.
XDF (2012) view – each light speck is a galaxy – some of these are as old as 13.2 billion years – the universe is estimated to contain 200 billion galaxies.
Flatness problem
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The flatness problem is a cosmological fine-tuning problem within the Big Bang model of the universe. Measurements find the current universe close to perfectly flat and expansion of the universe incre
The local geometry of the universe is determined by whether the relative density Ω is less than, equal to or greater than 1. From top to bottom: a spherical universe with greater than critical density (Ω>1, k>0); a hyperbolic, underdense universe (Ω<1, k<0); and a flat universe with exactly the critical density (Ω=1, k=0). The spacetime of the universe is, unlike the diagrams, four-dimensional.