Are dark energy and the cosmological constant the same thing?
I'm only learning this stuff myself, but as I understand it: dark energy is the name given to whatever is responsible for the accelerating expansion of the universe. One possible explanation of this expansion (the simplest one!) is given by a nonzero cosmological constant.
However, other possibilities exist: the early universe expanded extremely rapidly (a process called inflation), and if that were caused by a cosmological constant then the rapid acceleration would never* have slowed down, so other mechanisms that can cause an acceleration must exist.
*Technical caveat: unless you regard your cosmological "constant" as a running coupling in a quantum field theory- don't worry if that means nothing to you.
who me worry :)
No, there are not the same thing. the cosmological constant was firstly introduced by Albert Einstein in his theory general relativity to hold back gravity, so it is a negative force also know as lambda. After Hubble discovery that our universe is expanding, it imply that cosmological constant exist. Because if lambda don't exist, then due to the gravity, our universe should collapse not accelerate, by the way, according to the survey about 69% mass-energy in the universe are dark energy. and dark energy is a hypothetical form that explain whatever matter is responsible for the accelerating or expanding of the universe, in other words, cosmological constant is the expression of the dark energy
Here's a link to the Cosmological Constant wiki: Cosmological Constant
Hubble didn't discover the accelerated expansion of the Universe, that was done much much later by Saul Perlmutter and his group, for which they received the Nobel in 2011.
Hubble discovered only the expansion of the universe. There is no need for a cosmological constant to describe simple expansion, the expansion just comes from initial conditions. A lack of cosmological constant; however, would imply a decelerating universe, which the data back in Hubble's time was nowhere near accurate enough to rule out.
Some good material to look at. Those should fill in a few blanks, keep in mind dark energy is only one explanation as a possible contributor to the cosmological constant.
http://arxiv.org/abs/1304.4446 :"What we have leaned from Observational Cosmology." -A handy write up on observational cosmology in accordance with the LambdaCDM model.
http://arxiv.org/pdf/1103.5331v3.pdf "What do we really know about Dark Energy?" by Ruth Durrer (this article relates closest to the OP question)
http://arxiv.org/abs/gr-qc/0508052 "In an expanding universe, what doesn't expand? Richard H. Price, Joseph D. Romano
http://arxiv.org/abs/1002.3966 "why the prejudice against a constant"
http://arxiv.org/abs/astro-ph/0203330 "On the Cosmological Constant Problems and the Astronomical Evidence for a Homogeneous Energy Density with Negative Pressure"
My signature contains numerous other articles, at http://cosmology101.wikidot.com/main
Separate names with a comma.