In cosmology, the Infinite Universe theory is a model developed in 1948 by Fred Hoyle along with Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory.
According to this model, as the Universe expands, new matter is constantly created between expanding galaxies, so believed the supporters of the Infinite Universe theory (also known as Steady State theory), which was popular in the 1950-60s. Thus, though the Universe has always existed in an unchanged state, the cosmological principle is respected.
This theory has lost its relevance when the cosmic microwave background (relic radiation) was discovered. Relic radiation is considered radiation from the Big Bang. Astronomers study it to find out how a known part of the Universe looked like in the early era of its existence or even in the first fraction of a second.
Proponents of the stationary universe theory do not offer an alternative explanation for the CMB.
Although Hoyle’s theory failed, he conducted a series of studies that showed how atoms heavier than helium appeared in the Universe. (They appeared during the life cycle of the first stars at high temperatures and pressures). Ironically, he was also one of the creators of the “Big bang” term.