Astronomy Study Guide

In a circular orbit, objects like planets continuously experience a gravitational force directed towards the centre of the orbit. This force acts perpendicular to the planet's direction of motion. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. Although the gravitational force does not change the magnitude of the velocity (speed) of the planet, it constantly changes the direction of the planet's motion. As the direction is continuously altering, the velocity vector itself is continuously changing. Therefore, while the speed remains constant in a perfectly circular orbit, the velocity is always changing due to the continuous change in direction caused by the gravitational force.

The discovery of Cosmic Microwave Background (CMB) radiation provided crucial evidence that solidified the Big Bang theory as the most widely accepted model for the Universe's origin. Before its detection, the Big Bang theory predicted that the early Universe was extremely hot and dense, and as it expanded, it would cool down. This cooling process would leave behind a faint glow of electromagnetic radiation, stretched to microwave wavelengths due to the Universe's continued expansion. In 1964, Arno Penzias and Robert Wilson, while working with a new antenna, detected a persistent background noise that they could not eliminate. This noise was isotropic, meaning it came from all directions, and had a temperature of about 2.7 Kelvin, corresponding to microwave radiation. This accidental discovery perfectly matched the predicted properties of the leftover radiation from the Big Bang. The existence of CMB radiation, acting as a 'fossil' of the early Universe, strongly supported the idea of an initial hot, dense state, offering compelling observational proof that the Universe indeed originated from a singular, extremely hot event and has been expanding and cooling ever since. This discovery provided a more robust explanation for observations like redshift and the abundance of light elements, integrating them into a cohesive cosmological model.