Abstract
This study applies the generalized sub ADF (GSADF) tests to investigate whether bubbles exist in the United States markets over the period of 2015-2019, focusing on the COVID-19 period. We use daily Dow-Jones stock price indexes for the first time during the time period of 2015/1/7-2020/3/17. Empirical results demonstrate the existence of bubbles in the US stock market during some sub-sample time periods. Especially important, we find the third bubble begins from 2020/2/26 and grows gradually, not bursting until recently, a situation that is ongoing in the US market. Our results have important policy implications for investors who attempt to invest in the US stock market