To repeat, things like relativity and quantum ARE A PART OF OUR UNIVERSE.
As far as we know, yes.
And the current model Big Bang theory (ΛCDM) and the earlier models (1920s models of Friedmann, Robertson & Lemaître, eg Friedmann equations and the relationship between Redshift and distance; the BB Nucleosynthesis & CMBR of 1948; and the Inflationary model of 1980s) implement both General Relativity and Quantum Mechanics into understanding the physical cosmology of our Observable Universe.
The Static Universe model of Albert Einstein (1917) and the Steady-State model of Fred Hoyle, Hermann Bondi & Thomas Gold (1948) have also relied on General Relativity, but they used slightly differently to that of the Big Bang model.
In the Cosmological Considerations in the General Theory of Relativity (1917), Einstein wrote that the universe is eternally unchanging in appearance, so space neither expand, nor contract. And Einstein used a
Cosmological Constant on his
field equations that formulated for General Relativity (1915).
The purposes of the Cosmological Constant are - to provide exact solution to his field equations, that prevent the universe from spatially expanding or contracting, which in turns the universe would have a closed topology and positive spatial curvature.
Alexander Friedmann used the Friedmann Metric (1922) that was applied to Einstein’s field equations to give negative curvature and open shape of the universe. The negative curvature allow for the universe to expand or to contract, plus it allowed for energy density of the universe to change over time.
Georges Lemaître (1927) and Howard Percy Robertson with Geoffrey Walker (1931), have both independently came up the same metric as that of Friedmann. So the metric is known since then as the FLRW metric (Friedmann-Lemaître-Robertson-Walker metric). Einstein’s field equations became the Friedmann equations, when FLRW metric is used.
But Robertson (1924) and Lemaître (1927) - again independently proposed using Vesto Slipher’s Redshift model on distance, where objects (eg galaxies) that are shifted to the red in EM spectrum, would indicate the objects moving away from the observer, which in turn, are indications of the universe is expanding.
Edwin Hubble discoveries of redshift in 1929 not only show the universe is expanding, but also prove Friedmann metric is correct, and Einstein’s Cosmological Constant to be incorrect.
Einstein’s Static Universe was refuted, and Einstein supposedly said his Cosmological Constant was his greatest blunder.
As to the 1948’s Steady-State model of Bondi, Gold & Hoyle. They also proposed the universe to expanding too, the same as that of another team in 1948 (George Gamow, Ralph Alpher & Robert Herman). However, the Steady-State universe proposed that energy density is constant, unchanging and eternal.
The only way this perfectly constant energy density can happen, if the universe continued to create new matters.
Gamow, a former student of Alexander Friedmann, worked together with Alpher, where they proposed matters (atoms) were formed through the process of Primordial Nucleosynthesis or the Big Bang Nucleosynthesis (BBN) as it would later be known. Gamow also proposed the Hot Bang Bang (HBB), where the universe have hot and dense beginning.
Alpher and Herman proposed that when the electrons bonded with ionized atoms for the first time, the universe cooled down considerably cooled became transparent...but the bonding have caused the photons to decouple from the earliest elements (at that time, only hydrogen, helium and lithium exist in the universe).
These photons shifted to the red end of the spectrum, over time, becoming the microwave photons, hence the Cosmic Microwave Background Radiation (CMBR).
The CMBR is the oldest light detected, and not only leave residual heat signatures of that period (the Recombination Epoch, that started about 378,000 years after the Big Bang), it also tell us the energy density have also changed.
The discovery of CMBR in 1964 by Arno Penzias & Robert Wilson, have changed the status of the two competing hypotheses. The Big Bang hypothesis have been verified, hence it became a scientific theory, while the Steady-State hypothesis have been debunked.
The earliest periods of the universe changed from hot and dense, to much cooler and less dense universe AFTER the Recombination Epoch.
That’s what debunked Hoyle’s Steady-State model, the unchanging density of expanding universe doesn’t work. That’s why Hoyle is wrong.
Of course, Hoyle didn’t give up his cosmological model, a revised version of the Steady-State model was proposed before he died, but the newer version was even less popular than the original 1948 version.