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.

the largest number of relevance to anything in the universe?
I thought that was gogoplex or something.
I believe that number was designed as an estimate as the largest number ever needed to quantify the total number of elements of a set of the single most numerous 'thing' in the universe.
what they decided that 'thing' is, i don't know.