Tech sector plunges as Wall Street’s fears over stretched stock valuations intensify - MarketWatch

1. DeepSeek's emergence raises fears over Big Tech's profitability and valuations. 2. S&P 500 dropped 1.9%, reflecting investor concerns over large-cap stock concentration. 3. Nvidia's 15% decline signals worries over AI sector competition and valuation risks. 4. Upcoming earnings reports from Big Tech may lead to further market volatility. 5. Historically high valuations suggest potential for a significant correction in the S&P 500.