Astronomy Study Guide

Cosmology, the study of the Universe's origin and evolution, has been shaped by competing scientific models. For many years, two prominent theories, the Big Bang theory and the Steady State theory, offered explanations for the Universe's beginning. The Big Bang theory posits that the Universe originated from an extremely hot, dense point and has been expanding and cooling ever since. Conversely, the Steady State theory suggested that the Universe has always existed in a roughly similar state, with new matter continuously being created to maintain a constant density as it expands. Over time, scientific observations and discoveries, particularly the detection of cosmic microwave background radiation and the observed red-shift in distant galaxies, have accumulated significantly more evidence supporting the Big Bang theory. This overwhelming body of evidence has led to the widespread acceptance of the Big Bang model as the most accurate description of the Universe's origin and evolution among the scientific community.

The Doppler Effect describes the change in the observed frequency and wavelength of a wave when its source is moving relative to an observer. When a wave source moves towards an observer, the waves are 'compressed' in front of the source. This compression results in a shorter perceived wavelength and a higher observed frequency. Conversely, when a wave source moves away from an observer, the waves are 'stretched out' behind the source. This stretching leads to a longer perceived wavelength and a lower observed frequency.

A common example is the changing pitch of a siren on an ambulance as it approaches, passes, and then moves away from a stationary observer. As the ambulance approaches, the sound waves are compressed, leading to a higher pitch (higher frequency). As it moves away, the sound waves are stretched, resulting in a lower pitch (lower frequency). The speed of the wave in the medium (e.g., sound in air) remains constant, but the relative motion between the source and observer alters how frequently the wave crests arrive at the observer's location. This principle applies to all types of waves, including sound waves, light waves, and water waves.