Observable universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time. While the spatial size of the entire universe is unknown, it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day. To idealize what this number represents, and therefore the size of the universe, we need to understand the light-time measurement system well.
Light Year
Light-year is the distance light travels in one year.
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances. We use light-time to measure the vast distances of space. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications.
The speed of light in vacuum is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second or 186,000 miles per second).
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