Investors have been throwing off their fears about the state of the global economy and piling into high-flying US shares.

The benchmark S&P 500 index rallied on Thursday to close at 4,293.93, up 20 percent since its lows of early October, driving it into official bull market territory.

Performance has been driven by buzz around artificial intelligence (AI) and machine-learning technology, which accountancy group PWC reckons will add more than $15.7trillion (£12.5trillion) to the global economy by 2030.

This has triggered a clamour for the shares of mega-cap US technology companies that are likely to reap the benefits, notably chipmaker Nvidia.

Its shares have soared an incredible 170 percent this year, as the vast majority of generative AI programmes train using its graphic processing units (GPUs).

Microsoft’s OpenGPT AI natural language processing chatbot is the fastest growing app of all time, described by Tesla's Elon Musk as "scary good". Users love it and fear it in equal measure.

Microsoft's shares are soaring too, as are Apple, Amazon and Google-owner Alphabet, which has its own chatbot called Bard. Facebook-owner Meta Platforms and Tesla are rebounding after crashing in 2022.

Yet without these seven market-thrashing wonder shares, the S&P 500 would actually have fallen slightly this year.

Investors don't care, though. They are desperate to buy shares and make money again, with CNN’s Fear and Greed Index hitting the “Extreme Greed” level on Thursday.

This terrifies market analysts, who are urgently warning that today is actually a rotten time to buy shares, as many are now overvalued in the frenzy.

Today's bull market is built on a very thin platform, and it could come crashing down at any time.

If it does, investors who buy at today's pricey levels could be plunged into a world of pain.

Despite the tech frenzy, the US economy is now heading towards a recession, with Cristian Gattiker, chief commercial officer at Julius Baer, warning: “The US will likely cool off more than other countries after having been a growth stronghold.”

Axel Rudolph, senior market analyst at online trading platform IG, warned that this is risky time to buy shares as greed hits extreme levels based on just a handful of stocks.

Victoria Scholar, head of investment at Interactive Investor warned: "Throwing money into the market right at the top is rarely a good idea.”

Investor sentiment has been boosted by successful extension of the US debt ceiling and hopes that the US Federal Reserve will freeze interest rates at its next meeting on June 14.

The Fed has increased lending rates every single month since March last year to 5.25 percent today.

Investors are desperate to see the Fed "pivot", the point at which it start cutting interest rates rather than hiking them. When that happens, today's bull market will really take off.

READ MORE: Next big stock market rally needs three things. Yesterday it got two of them

US inflation is falling but stood at 4.4 percent in April, more than double the Fed’s two percent target. That's almost half the UK's figure of 8.7 percent, though.

There is zero hope of the Bank of England freezing rates at its next meeting on June 22. Instead, it is expected to increase bank rate for the 13th time in a row to 4.75 percent.

Market consensus suggests UK base rates could soon hit 5.5 percent, which partly explains why the FTSE 100 isn't in a bull market.

High interest rates are bad for the economy because they drive up borrowing costs for consumers and businesses, and now risk triggering a UK house price crash.

It means savers can get up to 5.35 percent a year on cash without risking their capital.

The FTSE 100 has grown steadily in recent months, climbing 10.78% since the lows of October 12, which is when the US bull market began.

Yet many analysts believe the US rally will be short-lived.

Matt Britzman, equity analyst at Hargreaves Lansdown, says it has grown too fast with growth concentrated in just a few stocks. “A pullback wouldn’t be too much of a surprise as markets take a breather.”

While the stock market rally is good news for pension and stocks and shares Isa investors, today's extreme greed should be met with extreme caution.