The third and strongest piece of support for the beginning of the universe comes from the impossibility of an infinite past. This is because an actual infinite number of anything cannot exist in the real world. We might think that since we use the concept of infinity in mathematics there would be no problem here. But mathematicians who work with the concept of infinity, do so by adopting some arbitrary rules to avoid the absurdities and contradictions that come with an infinite number of anything. And these rules don't apply to the real world.
Infinity only works in the abstract realm and only with some special rules.
To see the absurdity and contradictions of an actual infinite number of things in the real world, imagine a library having an infinite number of black books and an infinite number of green books alternating colours on the shelves and numbered consecutively on the spines. Not really, but that is what you would have to say if you want to claim the infinite is possible in the real world. Suppose we withdrew all the green books. Since our hypothesis leads to a contradiction, the hypothesis must be false-a library with an infinite number of books cannot exist. While we can avoid these contradictions in the mathematical realm by making up rules like not allowing ourselves to subtract or divide when using infinity, we cannot in the real world prevent people from taking books out of libraries.