A universe defined as today's S&P 500 members, applied to ten years of history, embeds survivorship-bias and membership hindsight: you are trading companies that turned out to be big enough to be in the index. Results improve by several percent a year for free.
The correct construction rebuilds membership daily from point-in-time data: listed and trading on that date, above a liquidity threshold measured over a trailing window, above a price floor, with no pending delisting. Instruments enter and leave as they actually did.
This also sets capacity honestly. If the universe on an average day in 2012 contained 180 names with median dollar volume of $4m, then a rule allocating 2% of daily volume supports roughly $14m of position per name, which is the real constraint on the strategy's capacity.
Related: survivorship-bias, point-in-time-data, capacity, selection-bias