Cosmology

Bruno believed (and praised Copernicus for establishing a scientific explanation for the fact) that the Earth revolves around the sun, and that the apparent diurnal rotation of the heavens is an illusion caused by the rotation of the Earth around its axis. Bruno also held (following Nicholas of Cusa) that because God is infinite the universe would reflect this fact in boundless immensity.

The universe is then one, infinite, immobile.... It is not capable of comprehension and therefore is endless and limitless, and to that extent infinite and indeterminable, and consequently immobile.

Bruno also asserted that the stars in the sky were really other suns like our own, around which orbited other planets. He indicated that support for such beliefs in no way contradicted scripture or true religion.

In 1584, Bruno published two important philosophical dialogues, in which he argued against the planetary spheres (two years later, Christoph Rothmann did the same, as did Tycho Brahe in 1587). Bruno's infinite universe was filled with a substance—a "pure air," aether, or spiritus—that offered no resistance to the heavenly bodies which, in Bruno's view, rather than being fixed, moved under their own impetus (momentum). Most dramatically, he completely abandoned the idea of a hierarchical universe. The Earth was just one more heavenly body, as was the Sun. God had no particular relation to one part of the infinite universe more than any other. God, according to Bruno, was as present on Earth as in the Heavens, an immanent God, the One subsuming in itself the multiplicity of existence, rather than a remote heavenly deity.

Bruno also affirmed that the universe was homogeneous, made up everywhere of the four elements (water, earth, fire, and air), rather than having the stars be composed of a separatequintessence. Essentially, the same physical laws would operate everywhere, although the use of that term is anachronistic. Space and time were both conceived as infinite. There was no room in his stable and permanent universe for the Christian notions of divine creation and Last Judgement.

Under this model, the Sun was simply one more star, and the stars all suns, each with its own planets. Bruno saw a solar system of a sun/star with planets as the fundamental unit of the universe. According to Bruno, an infinite God necessarily created an infinite universe, formed of an infinite number of solar systems, separated by vast regions full of aether, because empty space could not exist (Bruno did not arrive at the concept of a galaxy). Comets were part of a synodus ex mundis of stars, and not—as other authors maintained at the time—ephemeral creations, divine instruments, or heavenly messengers. Each comet was a world, a permanent celestial body, formed of the four elements. Bruno's cosmology is marked by infinitude, homogeneity, and isotropy, with planetary systems distributed evenly throughout. Matter follows an active animistic principle: it is intelligent and discontinuous in structure, made up of discrete atoms. This animism (and a corresponding disdain for mathematics as a means to understanding) is the most dramatic respect in which Bruno's cosmology differs from a modern scientific understanding of the universe.

During the later 16th century, and throughout the 17th century, Bruno's ideas were held up for ridicule, debate, or inspiration. Margaret Cavendish, for example, wrote an entire series of poems against "atoms" and "infinite worlds" in Poems and Fancies in 1664. Bruno's true, if partial, rehabilitation would have to wait for the implications of Newtonian cosmology.

Bruno's overall contribution to the birth of modern science is still controversial. Some scholars follow Frances Yates stressing the importance of Bruno's ideas about the universe being infinite and lacking geocentric structure as a crucial crosspoint between the old and the new. Others yet see in Bruno's idea of multiple worlds instantiating the infinite possibilities of a pristine, indivisible One a forerunner of Everett's many-worlds interpretation of quantum mechanics.