Can / Could

Both “can” and “could” are best used to express factual possibility and scientific likelihood. As opposed to “may” and “might,” which imply permission or human interpretion, “can” and “could” emphasize capacity and likelihood:

Experiments show that polluted water can be purified by slow percolation through rocks and sediments. (“Can” emphasizes the capacity for purification.)

The results suggested that low-energy photons could have been causing the problem. (“Could” suggests scientific likelihood.)