Tech’s $18T market cap faces AI bubble test as debt soars

For investors, AI’s massive capital expenditure cycle is raising the stakes; the recent selloff could be the first tremor of a leverage-driven unwind. The $18 trillion concentrated in five tech giants represents a market cap almost as large as China’s economy, creating an unprecedented concentration risk.

A sharp selloff in major tech stocks during late June 2026 has reignited fears that the artificial intelligence sector is caught in a massive bubble, with some experts warning that a bust could eclipse any previous market dislocation. The immediate catalyst—a shift in capital allocation by the largest tech firms from share buybacks to debt-funded AI infrastructure—highlights how the industry’s growth is increasingly underpinned by leverage, leaving it vulnerable to rising interest rates and cyclical shifts.

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The five biggest companies on Wall Street, all technology giants deeply invested in AI, now command a combined market capitalization of $18 trillion, nearly equivalent to the entire economic output of China.

The sheer scale of the valuation at risk is staggering. The five biggest companies on Wall Street, all technology giants deeply invested in AI, now command a combined market capitalization of $18 trillion, nearly equivalent to the entire economic output of China. This concentration of wealth in a handful of names has drawn comparisons to the dot-com era, but with an important difference: today’s tech behemoths are not speculative startups but established cash machines that, until recently, were returning capital to shareholders. The pivot to debt—taking on new borrowings to finance server farms, model training, and acquisitions—signals both the enormous capital requirements of the AI buildout and a potential lengthening of the timeline before these investments generate returns. As Brent Fredberg of Brandes Investment Partners notes, the debt taken on so far is “relatively modest,” but the vulnerability is clear: the US Federal Reserve has recently floated the possibility of higher interest rates, which would sharply increase the cost of servicing that debt and could force a painful retrenchment.

Adding to the unease is the dramatic case of SpaceX. Fresh off a blockbuster IPO, the space and AI venture announced plans to issue $25 billion in bonds this week, sending its stock price lower and raising fresh questions about its financial health. The move illustrates the growing dependence on capital markets even for companies that were recently darlings of the new space economy. More broadly, it underscores a worrying trend: AI-related companies are racing to secure funding in a manner reminiscent of pre-crash patterns.

A subtler but equally dangerous dynamic is the phenomenon of “circular financing.” Big tech firms invest heavily in AI startups, which then use that capital to purchase big tech’s own cloud computing services and AI tools. Fredberg describes this as a potential “house of cards,” where reported growth metrics are inflated by a closed loop of spending that does not reflect genuine end-customer demand. If this cycle were to reverse—if funding tightened or startups failed—the unwinding could ripple through earnings and valuations at both the large providers and their portfolio companies.