The Features of the Observable Universe and Appropriate Units of Distance

Observable Universe Features

Observable Universe: The part of the universe that can be seen from Earth. It includes all objects that can be detected, either directly or indirectly.
Limitations: The observable universe is limited by the speed of light. Light from distant objects takes time to reach us, so we see the objects as they were in the past.
Galaxies and galaxy clusters: The universe consists of billions of galaxies, each containing billions of stars. Galaxy clusters are large groupings of galaxies.
Cosmic microwave background radiation (CMB): A faint cosmic ‘background noise’ permeates the universe. It is an afterglow from the Big Bang and offers direct evidence for the Big Bang theory.

Units of Distance

Astronomical Unit (AU): The distance from the Earth to the Sun. It’s approximately equal to 149.6 million kilometres.
Light Year: The distance that light travels in one year, approximately 9.46 trillion kilometres.
Parsec: A unit of distance used in astronomy, equal to about 3.26 light years.
Redshift: Not a unit of distance, but a method for determining distance. The shift in light towards the red end of the spectrum implies the object is moving away from the observer, and the amount of shift can be used to calculate the distance.

Remember that these units are used because of the vastness of the universe and the unimaginable distances between celestial bodies. Conventionally used units like kilometres or miles are not practical for these kinds of measurements.