Sorry, but it depends on which modern physical cosmology you are talking about.
The Big Bang cosmology is no longer categorized as theoretical hypothesis; it is ongoing “scientific theory”, meaning scientific theory that is still undergoing testing.
Observable and verifiable evidences have already provided useful data and increase our knowledge on the universe and cosmology, such as the redshift from the late 1929 to the present using the Hubble and other space telescopes, and the CMBR from the 1964 to the present with WMAP and Planck probe.
CMBR have validated the predictions made in 1948 (by Gamow, Alpher and Herman) with how atoms formed in the period known as the Big Bang Nucleosynthesis (BBN) and the Recombination Epoch.
BBN is a period when the atomic nuclei first bond the hadron particles, eg protons and neutrons, creating ionised hydrogen and ionized helium (as well as lithium).
The Recombination Epoch is a period when electrons bonded with the ionized elements, so the atoms become electrically stable and neutral for the first time. The coupling of the electrons to the ionized elements had two-fold effect on the universe:
- The universe before the Recombination Epoch was opaque, like the event horizon of blackholes, where the observer cannot view beyond the event horizon; the bonding of electrons to ionized atoms resulted in the universe becoming transparent, so that we can observe or detect photons for the first time.
- The bonding of electrons to the atomic nuclei, resulted in decoupling of photons, which have redshifted to microwave EM, which we know it as cosmic background radiation of microwave, or CMBR. These photons or microwave, is the earliest detectable light that we can observe/detect. CMBR is even earlier than the quasars.
The Recombination Epoch started 377,000 years after the Big Bang.
The soon to be launch James Webb Space Telescope (JWST) mission in 2021, will hopefully uncover more than the Hubble, WMAP and Planck have accomplished already. Hopefully, it will detect another, even earlier cosmic background radiation, the Cosmic Neutrino Background Radiation. If not, the mission will attempt to observe the formation of earliest population of stars.
As I said, the Big Bang theory is ongoing scientific theory, where scientists are attempting to test other BB predictions.
BB isn’t just a thought experiment.