EP,
because of the necessary change in status quo of the gravity to start the universe. Either a millisecond or billions of years, there was a status quo before that moment, that got changed. Like you said, it's trivial for math, and a problem for physics. But something that physics presently cannot explain doesn't mean it's an impossibility. See, for example, the recent developments around the "dark matter" - science doesn't understand what holds this matter together, and how its elements interact with each other. Electromagnetism and gravity don't seem to apply. Does it mean it's an impossibility? No. it's just awaiting further research.
I'll give you an example of impossibility. Are you familiar with the problem of the allmightiness of God?
It goes like this: If God is all-powerful, then he could create a rock so heavy that even God cannot lift it. However, if God cannot lift it, then he's not all-powerful. But if God cannot creat a rock so heavy that even God cannot lift it, then God isn't all-powerful.
Now, that's an impossibility. Infinity isn't an imposibility. It's just a problem that presently physics aren't prepared to understand.
Eden