Final answer:
The Big Bang theory states the universe began as an extremely small, hot, and dense volume 13.8 billion years ago, which then rapidly expanded. The Inflationary Hypothesis suggests a period of intense exponential growth shortly after the Big Bang. Cosmic background radiation is a crucial evidence for this model.
Step-by-step explanation:
The Big Bang theory is a well-established scientific theory within the field of cosmology that describes the early development and shape of the universe. Contrary to the suggestion that the Universe was initially the size of Earth, astronomers and physicists believe that all of the matter within the observable universe was once concentrated into an extremely small, hot, and dense volume much smaller than Earth, nearly 13.8 billion years ago. This initial state led to the Big Bang, where space itself rapidly expanded, not like a traditional explosion through space, but as a rapid expansion of space itself. Theorists also explore the Inflationary Hypothesis, which posits that there was a brief period of extraordinarily rapid expansion or inflation immediately after the Big Bang, during which the scale of the universe increased exponentially. The cosmic background radiation that we observe today is one of the key pieces of evidence supporting the Big Bang model and its subsequent periods of expansion and evolution. The vast portion of mass and energy in the universe that remains unexplained is still a topic of active research in modern astronomy and physics.