dabubuda wrote:"No less a visionary than Albert Einstein theorized about a force that would add energy to the expansion of the universe; a force which he referred to as the "cosmological constant."

Could anyone tell me whether this visionary here is referred to Einstein? or another visionary?

What is the exact meaning of "no less X1 than X2?"

I appreciate any discussion here.

The idiom 'no less than' highlights the person/noun....it goes to show the greatness of the person.

For example: Almost two dozen representatives flew in for meetings with top officials, including no less than the country's president.

In your example, it means that the theory was propounded by Einstien (the visionary) and the usage of the idiom reflects the importance of the theory...i.e. it carries weight because it was theorized by Einstein.

"No less than" can also be used to express surprise at a fact - example: the convention was attended by no less than five hundred people. Here the idiom is used to express surprise on the number 'five hundred' and to enforce it....this number is large as compared to the estimates for possible attendance.