A light-year is defined as the distance light travels in a year. Distant galaxies are billions of light-years away, so we are seeing the light from these galaxies from billions of years ago. Some of the most distant galaxies are over 10 billion light-years away, so the light that we receive from them comes from a time when our galaxy was just forming. The Size and Structure of the Universe 1. Progress in many scientific fields, such as studies of stars, plants, or animals, has come by classification of different types of specimens, followed by comparisons of the different classes.
Cosmology is the study of the size and structure of the universe.
This does not mean that every area of space is identical, just that matter and energy are uniformly distributed throughout the universe. For example, the redshifts of galaxies moving away from us has been observed to be uniform in all directions. The Big Bang theory states that all the matter and energy that exists in the universe today was once present in a nearly infinitely small region with nearly infinitely high temperature and density. An explosion occurred, which within minutes created the light elements like Helium through nuclear fusion, and caused all the energy and matter to disperse outward. For a few 100,000 years, matter and radiation interacted, and galaxies and stars began to form and die.