Cosmological and particle physics aspects of axions

The present talk is divided in two parts. In the first part, the strong CP problem in QCD will be briefly reviewed, and it will be shown how axion models can overcome this problem. The chiral anomaly and the so called Peccei-Quinn symmetry play a fundamental role in this discussion, and these aspects will be also reviewed. Hidden axion sectors will be exemplified, in particular the Kim-Shifman-Vainshtein-Zakahrov or Dine-Fischer-Srednicki-Zhinitsky scenarios. In the second part, some cosmological problems will be discussed. One is the isocurvature problem. Essentially, the axion constant is bounded to $f_a<10^{12}$GeV by cosmological observation. However, such axion will give rise to enormous isocurvature fluctuations from the early universe if the inflationary model is of high energy scale. This apparent inconsistency has generated a large discussion in the scientific community, called the axion-isocurvature tension. The other application is that hidden cosmological axion like models may be an interesting arena for dealing with problems such as the generation of cosmic rays with large energy or even the cosmological constant problem.