The biggest number

I have given you all an index card. You have one minute to write down the biggest number that you can (barring infinity) using any mathematical function that you know (addition, subtraction, exponentation, ackermann, Knuth's up arrow, etc). Go.

>>15
wtf? that's total nonsense. the total number of atoms in the universe is only about 10^120 or so. A googolplex is far larger than that.