I have a strategy i'd like to implement that has been backtested using the SPX and NDX indexes. Is there a way to buy/sell the "cash" indexes? If not, how closely correlated are the futures and the indexes? I would expect the futures to have a premium/discount that varies throughout the day, which could potentially make the strategy not worthwhile. As I'm new to futures... what would be the best way to implement a strategy that works well based upon the cash index? Or am i better off trading the SPY and QQQ instead as they would more closely mimic the corresponding indexes? Thank you.