There are a lot of recommendations to find on Google on what the best exercise is for low back pain. However, when it comes to ACUTE low back pain, simplicity is best. You don’t need a ton of exercises at this stage.
Simple is better, and MOST people respond well to extension (bending your spine backward). Notice that I said most–some people don’t have a preference for direction or have a preference for flexion. Research has shown that most respond to extension.
If you have experienced acute low back pain, you probably are cringing at the thought of purposely pushing into extension when you are in such pain. Stay with me.
As always, I would recommend seeing a PT first and foremost so that can give you a thorough movement assessment to get to the root of your problem.
Start by lying on your belly, legs fairly wide with toes pointed inward.
Place your arms by your armpits. Then, ONLY WITH YOUR ARMS push up as far as you can. DO NOT USE YOUR BACK MUSCLES OR YOUR GLUTES to lift you up. (I am a bad model here, because my glutes are firing, but yours should not be)
This is not a yoga pose like cobra or upward facing dog. This is completely passive movement on your spine–no back muscles firing here.
You can go up as far as you can. You can stop here.
Or here. But my guess is that you will not be able to come up as high as I am below. Sometimes it can take a week or two to actually be able to extend your spine where your elbows are straight (not shown).