University of Virginia history professor Philip Zelikow has taught the course, "The Modern World: Global History Since 1760" for 16 years -- but this semester is different. Instead of delivering it to 120 students on campus, he'll be teaching 42,000 students around the world.

While online learning is not new, access to top-notch professors at some of the world's most prestigious universities is. Along with the University of Virginia, Harvard University, Stanford University, Brown University, Johns Hopkins University, Princeton University, the Massachusetts Institute of Technology and Columbia University are among dozens of elite institutions partnering with start-up companies such as Coursera, Udacity and edX, to deliver so-called massive open online courses or MOOCs.

Now, Greek history at Wesleyan University, poetry at the University of Pennsylvania, astronomy at Duke University, and "Introduction to Music Production" at the Berkley College of Music are all just a click away. And they're absolutely free.

Since 2011, more than 2.5 million students from around the world have enrolled in MOOCs. Even though they are not offered for college credit and completion rates are low, some educators see the potential to revolutionize higher learning.

"Thanks to these free online courses, you can shop a range of disciplines and do it all from the comfort at your own home," Coursera co-founder Daphne Koller said.

Zelikow was apprehensive at first. 

"I'm not a techie guy who's interested in experimenting with all this computer stuff," he said. "In fact, I was kind of a skeptic about all this online stuff. I thought it was fad-ish."

But after spending hundreds of hours preparing for this semester's course, Zelikow now sees the potential in expanding online.

"Bruce Springsteen is involved in selling recorded music to people all over the world. And he also sells tickets to live concerts," Zelikow said. "Nobody thinks the recorded music is just as good as the live concert. But he wants to be in both those lines of business."

He says there's not only value in reaching thousands of students worldwide, but believes the move online has actually improved his course for those taking it on campus.

"I thought of ways to use this [online course] to actually re-invent the ways I teach my ordinary class at the university and make it a better class than it used to be, to solve certain problems that are kind of structural problems in the way we teach our residential courses," Zelikow said.

Dawn Smith, 38, has taken "Fundamentals of Pharmacology" through the University of Pennsylvania and a public health class through Johns Hopkins after deciding to change careers.  

"I needed some textbook knowledge," Smith said. "I felt in order to be taken seriously as a candidate I needed to show I was doing something proactive."

Critics of MOOCs complain about their size, saying it leads to minimal student-professor interaction.

"I've met students from Germany that I've spoken to quite frequently - Australia, Japan, China, and then some in Africa," Smith said.

Although she would "absolutely recommend" this online platform, Smith acknowledges the limitations, too.

"There isn't that immediacy of being able to ask a question and then have an answer," Smith said. "There's no one standing in front of you showing you how to do something."

Other concerns include measuring student progress and the sustainability of these courses over a long period of time.

Siva Vaidyanathan, University of Virginia media studies professor, says he thinks MOOCs are an "interesting experiment," but that they're just that - an experiment.

He doesn't believe they can replace a traditional college education.

"Imagine taking a university and removing all the really fun stuff," Vaidyanathan said. "And all you're left with is me talking to you through a camera. That's not that good for anybody."

As professors, students and investors navigate this new terrain, there are questions about the potential for profit in the future and the place MOOCs may have in higher education.

For some, however, these online classes bring about hope.

"Education is such an equalizer. It raises people's abilities … lets people build a better life," Coursera co-founder Andrew Ng said.

Even Vaidyanathan sees the silver lining.

"I hope somewhere in some corner of the world … some child discovers calculus, discovers physics, or discovers poetry through a MOOC and gets … inspired to change the world," he said.