That made it appear as though company profits were growing at a much faster clip, which encouraged punters to buy more stock.In the two decades to 2021, US corporations spent $US12 trillion ($17 trillion) on share buybacks,That level has been accelerating.According to S&P Global, US firms distributed almost $US1 trillion to shareholders via buybacks last year.With what appears to be rapid growth in earnings, investors have been convinced to plough ever more cash into stocks and expanding the market capitalisation of corporations.As a result, US stock valuations have blown out.In 2011, during the aftermath of the Global Financial Crisis, America's top 500 companies traded at an average of 13 times earnings Global stock markets are now being targeted."Global equity markets are undergoing a meaningful shift," a recent Morgan Stanley study has concluded. "After years of software, platforms, and intellectual property growth, there is now a renewed interest emerging in traditional 'old-industry' hard asset-based companies."But is there enough cash to go around?And the unanswered question is what impact this will have on the current boom.Pumping money into global stock markets while reducing the number of available investments has helped drive a boom unlike any seen before.Reversing that course may bring the boom to a halt, particularly if those overblown valuations ever look like descending back to earth.