Xanthan gum is a great alternative to yeast in baked goods. Yeast works with gluten to make breads and cakes rise, but since their low-carb counterparts don't usually contain gluten (when homemade), they can't rise without help. This is where xanthan gum comes into play.

Xanthan gum is a natural thickener derived from the fermentation of glucose or sucrose by a certain bacterium, which is also responsible for those little black spots on broccoli, and it's harmless to humans (yay!). This informative website gives more details.

Again, Bob's Red Mill carries xanthan gum. It's a bit expensive, but when you only use about a teaspoon at a time in any given recipe, it goes a long way.  La Branche d'olivier, as well as other stores, carries the brand.