The “perfect cosmological principle” isn’t true for our Universe Generations ago, cosmologists asserted that the Universe might not just be the same in all directions, but at all times. This set of images shows four different "slices" of the Universe's most massive cosmic halo on three different distance scales. If the Universe obeyed the perfect cosmological principle, these structures would not be seen to evolve over time.
Observationally, the Universe itself must decide. . Boylan-Kolchin et al., MNRAS, 2009 Back in 1923, an unprecedented scientific revolution occurred. Hubble’s discovery of a Cepheid variable in the Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading us, in short order, to the discovery of the expanding Universe.
Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Illustration via NASA, ESA, and Z. Levay (STScI) Distances to individual stars were measured beyond the Milky Way. Edwin Hubble’s original plot of galaxy distances, from 1929, versus redshift (left), establishing the expanding Universe, versus a more modern counterpart from approximately 70 years later (right).
Many different classes of objects and measurements are used to determine the relationship between distance to an object and its apparent speed of recession that we infer from its light’s relative redshift with respect to us. As you can see, from the very nearby Universe (lower left) to distant locations over a billion light-years away (upper right), this very consistent redshift-distance relation continues to hold. Earlier versions of Hubble’s graph were composed by Georges Lemaître (1927) and Howard Robertson (1928), using Hubble’s preliminary data.
By combining measured extragalactic distances with observed recession speeds, we discovered the expanding Universe. There is a large suite of scientific evidence that supports the expanding Universe and the Big Bang. At every moment throughout our cosmic history for the first several billion years, the expansion rate and the total energy density balanced precisely, enabling our Universe to persist and form complex structures.
Today, dark energy dominates the Universe, while early on, prior to the onset of the hot Big Bang, a phase of cosmological inflation occurred, preceding it and setting it up. But was the Universe evolving temporally, as the Big Bang predicts? This image shows a slice of the matter distribution in the Universe as simulated by the GiggleZ complement to the WiggleZ survey.
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