Beginning to this universe, yes, we can follow the physics.
Beginning to the rounds of rebirth, no.
The link below shows how to reconcile this, by having cyclic multiverse models of the universe. Anyway, in DN 27, DN 1, the Buddha did mentioned the cosmos expands and contract.
Given the cyclic multiverse models, there’s a few possibility to the beginning of rebirth question, given the careful wordings of the Buddha saying no known beginning.
-
There’s a beginning, but it lies in the infinite past, or effectively infinite past time ago. Like a number line, 0 is the beginning, we are at infinity+3. That’s what some of the discussion in physical cosmology goes to.
-
There’s no way to know whether there’s a beginning or not via recollection of past lives. This is supported by the suttas in SN 15 where even recollecting for a long time, very many lifetimes, very fast, there’s no beginning to be found. Another theoretical reasoning is that past life memories are considered information, and information is physical, the smallest bit of information requires the smallest unit of area to exist. Given that the observable universe is finite (due to finite speed of light and finite beginning to the universe), there’s limited information that can ever be packed in this universe, so even if the mind can access to infinite information, it cannot manifest those into the physical universe (in speech).
-
There’s no beginning. This is the philosophically most neat of solutions. For if there’s any beginning point, we have to answer, how did sentient beings start in the first place to get reborn? Who was the first Buddha? How can there be a first caused which is itself uncaused? Etc. Also, if we haven’t lived infinite lifetimes, might not it justifiable to want to stay in samsara longer instead of developing dispassion and disenchantment with beginningless rebirth?
I am all for 3 and for all practical purposes, it’s best to just take it as no beginning.