Plenary Lectures and VIP Guests

Dr. Ethan Siegel

Abstract:

The Big Bang has been typically identified as the beginning of the Universe: an event occurring at the “t = 0” moment of time, arrived at by extrapolating back our expanding Universe to a state of arbitrarily high temperatures and densities. Several lines of evidence converge on this picture, namely the major cornerstones of the Big Bang: the expanding Universe, the leftover bath of radiation observable as the cosmic microwave background, the abundance of the light elements forged during an early period of nucleosynthesis, and the growth and evolution of cosmic structure in our Universe. While the extrapolation back to a singularity, naively, is justifiable in an expanding Universe under the rules of General Relativity, those conditions would have led to observable effects that run contrary to what we see. Instead, the Universe is better described by cutting off the matter-and-radiation dominated Universe at some early time, patching on an inflationary epoch where space is expanding exponentially and dominated by some sort of vacuum energy. This is not mere theory more than 40 years on, but is supported by a vast suite of observable evidence. The case for this conclusion, even in the absence of B-mode polarization in the CMB, is laid out here, in the context of the full suite of cosmic evidence that supports, but is not limited by, the Big Bang picture alone.