Because the universe did not originated in a Big Bang, that's just a theory that is becoming more and more disproved. The universe is infinite and eternal, in constant change? sure, but in no way it started 13500 million years ago.
The timeline is certainly up for grabs, there are uncertainties in the various ways we measure or infer distances and the age of objects, but so far nothing that invalidates the overall scheme. Roger Penrose's idea of conformal cyclic cosmology is a plausible alternative, but even that still has an event in our past that looks an awful lot like a big bang. There are just too many observations any alternative theory needs to explain, like galactic red shift and the cosmic background radiation. If stars are infinitely old, how come they still have any hydrogen left?
It's always a good idea to keep an open mind though. What are the alternatives you think have legs?
Also, the Big Bang "theory" is a theory in the technical sense, in that the overwhelming majority of evidence ever collected is at least neutral towards the Big Bang, to say nothing of the virtually incontrovertible evidence in support (esp. the cosmic microwave background radiation, redshift correlated with distance to virtually all extra-galactic objects, low metalicity in ultra-distant (read early) objects, etc etc etc). It could still be wrong, but we'd need some other theory that adequately explains all the available evidence, and makes several new, testable predictions that are also observed to be correct. The term "theory" is not used colloquially here as a fancy way to say "guess" or "idea" (those are, in the same technical sense, best called "conjectures" or (generously) "hypotheses"). It has a very narrow meaning here, and dismissing the Big Bang theory as "just" a theory really reveals your ignorance on the subject here.
I still lean towards the Big Bang as the most likely model, but it’s not as well established as, say, germ theory.
I actually had an idea in undergrad (20+ years ago) to probe Hubble Constant variation using quasar reverberation mapping and very-long-baseline-interferometry. Then the professor I was working with pointed out that the baseline I needed was something like 100,000 times earth's orbital diameter.
Back then, one of the sexier ideas was that the universe might have locally different fundamental constants, and that variation could reveal some information about the higher-dimensional "space" that the universe existed inside of. I've been out of that field for a long time though, so I've no idea what the cutting edge is. I just know that the Big Bang theory is still pretty safe.
There are plenty of things wrong in the theory, but none of the evidence suggests that "expansion from a singularity" is wrong, let alone "infinite and eternal" is right.
A singularity means that all timelines goes through it, but it is impossible to make continues extension of the timelines past the singularity in the current physical models. That literally means that anything is possible with the timeline prior that. The timeline can jump, be replaced by a set of random points, became a multidimensional surface, go back etc. It may even indeed disappear, but we do not know.
But as a non-physicist I recognize that my "theories" are just for funsies and have less value than bellybutton lint.
> Twenty-thousand years in the future, Cass, a humanoid physicist from Earth, travels to an orbital station in the vicinity of the star Mimosa, and begins a series of experiments to test the extremities of the fictitious Sarumpaet rules – a set of fundamental equations in "Quantum Graph Theory", which holds that physical existence is a manifestation of complex constructions of mathematical graphs. However, the experiments unexpectedly create a bubble of something more stable than ordinary vacuum, dubbed "novo-vacuum", that expands outward at half the speed of light as ordinary vacuum collapses to this new state at the border, hinting at more general laws beyond the Sarumpaet rules. The local population is forced to flee to ever more distant star systems to escape the steadily approaching border, but since the expansion never slows, it is just a matter of time before the novo-vacuum encompasses any given region within the Local Group.
Olber's Paradox[2] and the inability to reconcile it with our current astronomical observations seems to disprove that our current universe is itself infinite.
[0]: https://www.scientificamerican.com/article/why-is-the-night-...
Why word it like this instead of using 13.5 billion? Is this a common thing to do in astronomy? Or are you trying to make the claim of 13.5 billion years sounds more ridiculous? Genuine question.