Why didn't the universe always exist?
You are referring to the Big Bang theory.
The Big Bang theory only focused on the evolution of the “Observable Universe” only, attempting to explain the following origins:
- The origin of all planets, stars, galaxies and other astronomical objects known to us today.
- The origins of fields & forces and of matters (eg atoms, molecules) and the known elementary particles as they are defined in the Standard Model in particle physics...plus the fundamentals connections & interactions between fields/forces and particles.
- I need to add that the origins of atoms can be explained in the 1948’s model of the Big Bang in 3 stages:
- The Big Bang Nucleosynthesis (BBN) where the lightest elements formed (eg hydrogen, deuterium, helium & lithium atoms) without electrons bonding to these atoms.
- The Recombination Epoch explained how the universe became transparent for the first time, when the atoms became stable and electrical neutral with their bonding of electrons, and the energy released during decoupling of photons are the light observed as the Cosmic Microwave Background Radiation (CMBR).
- Elements heavier than lithium were formed by the stars, eg Stellar Nucleosynthesis & Supernova Nucleosynthesis.
- The universe has been expanding since the earliest stage (the Planck Epoch) when the universe was hotter and denser and more compact than it is, today.
The explanations of the current model of the Big Bang theory - the ΛCDM model (Lambda-CDM model) - formulated in the late 1990s, is the accumulation of the 3 previous models (models 1920s (eg gravitational redshift, the Hubble law), 1948 (eg BBN, CMBR), 1980s (the inflationary model)).
ΛCDM model included additional information extrapolated from observations and data (eg observations of the CMBR from WMAP & Planck missions) that the universe have been accelerating in expansion since about 4 billion years ago.
All models of the Big Bang theory, is based on the Observable Universe, and this universe as we know it began 13.798 billion years ago. The Big Bang models don’t go beyond the 13.798 billion years, but if you are interested in highly theoretical models of cosmology, than there are two models that speculate the universe being infinite in age:
- The cyclical universe model, where the universe undergone a series of birth, death, rebirth, eg a series of expansion (Bang) and contraction (Crunch), where all the matters and particles get recycled;
- the Multiverse model, where there are infinite numbers of universes.
There are lot more other models, but these two are the ones that proposed infinite or eternal universe.
The Multiverse model grew from the BB’s inflationary model (earlier 1980s).
But regardless of which alternative theoretical models you look at, they are speculative, and currently untestable.
