Even as bubble concerns grew, Nvidia became the first company to be worth $5 trillion dollars. The bubble framing has serious advocates. Jeremy Grantham, in GMO’s January 2026 quarterly letter, applies the firm’s two-standard-deviation test and concludes that US equities have been in bubble territory for a prolonged period, noting that AI capital expenditure as a share of GDP is exceeding even the railroad cycle. Some headlines have shifted to worrying about why it is so hard to spot a bubble in real time.