Question

what is big bang theory is and what evidence scientists have that it occurred​

Answer 1

• The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang ...
• Three main pieces of evidence for the Big Bang theory are the red-shift of light, cosmic background radiation and types of elements

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