Basics

Abstract

Telescopes of all kinds look deep into the sky and find planets, stars, the Milky Way, other galaxies, clusters of galaxies, the large-scale structure in the distribution of galaxies and quasars and, finally, a distant region: the fireball. Its radiation is that of a black body, with a temperature of 2.73 K now. The fireball is apparently opaque; we cannot see behind it. With consideration and care, we call the space within this fireball and the objects that we can identify with our telescopes the metagalaxy Is this all? Is this the universe? The universe is defined as the system (or object) that is not part of any metasystem. There may be many things outside or behind the metagalaxy, of course. There can be nothing outside, behind or in addition to the universe: the universe contains it all, by definition. Nobody observes the universe from the outside. The universe, is it a subject of physics?