That was fast: Apple passes Alphabet again

That didn't take long.

One day after Google parent Alphabet passed Apple to become the world's most valuable public company, Apple regained the crown Wednesday.

With Apple mostly flat and Alphabet down 3 percent, Apple's market capitalization hit $524 billion versus $520 billion for the search giant as of late morning.

The two are still trailed by Microsoft, Facebook and Exxon Mobil to round out the list of the world's five biggest companies.

Market cap: Apple vs. Alphabet (USD, million)

Chart source: FactSet

The last time Google was more valuable than Apple was in February 2010, when both companies were worth less than $200 billion. At the time, Apple had yet to release its first iPad, the newest iPhone on the market was the 3GS, and the Mac was the company's biggest product line, accounting for one-third of revenue. Steve Jobs was still at the helm.

Google was being guided by Eric Schmidt, who would hand control back to co-founder Larry Page the following year. The company was a little more than half its current size.

Apple and Google actually flip-flopped multiple times between 2008 and early 2010, before Apple went on a historic tear, jumping from $180 billion in value to over $650 billion in September 2012. At that point, the two companies were separated by over $400 billion. In 2011, Apple passed Exxon to become the world's most valuable company.

It's hard to believe that Google was the more valuable company from the time of its IPO in 2004 until April 2008. Then iPhone madness began.

Google's latest rise versus Apple began in July. From that point through the end of 2015, its shares soared 44 percent, while Apple's sank 16 percent.

Apple's main problem is its reliance on the iPhone, which now accounts for two-thirds of revenue. It's a massive business, but sales in the fiscal first quarter increased only 1 percent from a year earlier, while iPad and Mac revenue dropped. Investors are concerned that unless Apple changes course and decides to compete with lower cost Android manufacturers on price, the iPhone's best days are in the past.

Meanwhile, Google is convincing investors that in the transition from Web to mobile it will maintain its dominance. According to eMarketer, Google is poised to capture 32 percent of the mobile ad market this year and next, staying well ahead of Facebook, which is around 20 percent. The company generates so much profit from its digital ad business that it can invest in all sorts of potential growth areas, namely autonomous driving and extending life.

—CNBC's Everett Rosenfeld and Chris Hayes contributed to this report.