Investors were encouraged by the latest vaccine progress and news that the transition of power in the U.S. will finally begin.
Bellwether market index also adds Honeywell and Amgen and expels Exxon and others. Move reflects tech's expanding role in the economy.
The S&P 500 index dropped nearly 3% as the market erased all of its gains from a rally the day before.
Companies that have been big winners, including tech firms, suffered steep declines.
But the bigger context is volatility in the markets has actually been falling over the last 30 years.
Algorithms don’t generally create steep declines, but they do amplify trends, one expert says.
Let's break down today's wild ride in the stock market. Should you be concerned?
There's a reason equities are doing so well while bond yields crater.