T equals zero. By singularity, we mean physical quantities become infinitely big. Somehow we don't know what happens at t equals zero. So if you wrap your brain into knots, trying to understand not just the very, very early universe, but literally t equals zero, you're going to come across paradoxes. These things are just not defined at the big bang. One tiny moment after the big bang, ten to the minus some seconds, then you can talk about the whole universe,. and it might already be infinitely big then. And there's some finite number of particles per cubic centimeter, et cetera. O that's one but the other thing is more straightforward. You