be both a dark-matter candidate and a solution to the strong CP problem. If inflation occurs at a low scale and lasts sufficiently long, the axion mass can be as low as 1 peV/². There are two distinct scenarios in which the axion field begins its evolution, depending on the following two conditions: Broadly speaking, one of the two possible scenarios outlined in the two following subsections occurs: Pre-inflationary scenario If both (a) and (b) are satisfied, cosmic inflation selects one patch of the Universe within which the spontaneous breaking of the PQ symmetry leads to a homogeneous value of the initial value of the axion field. In this "pre-inflationary" scenario, topological defects are inflated away and do not contribute to the axion energy density. However, other bounds that come from isocurvature modes severely constrain this scenario, which require a relatively low-energy scale of inflation to be viable. Post-inflationary scenario If at least one of the conditions (a) or (b) is violated, the axion field takes different values within patches that are initially out of causal contact, but that today populate the volume enclosed by our Hubble horizon. In this scenario, isocurvature fluctuations in the PQ field randomise the axion field, with no preferred value in the power spectrum. The proper treatment in this scenario is to solve numerically the equation of motion of the PQ field in an expanding Universe, in order to capture all features coming from the misalignment mechanism, including the contribution from topological defects like "axionic" strings and domain walls. An axion mass estimate between 0.05–1.50 meV was reported by Borsanyi et al. (2016). The result was calculated by simulating the formation of axions during the post-inflation period on a supercomputer. Recent progresses in determining the present abundance of a axion using numerical simulations lead to values between 0.02 and 0.1 meV, although these results have been challenged by the details on the power spectrum of emitted axions from strings. Phenomenology of the axion field Searches Axion models carefully choose coupling strengths that are too weak to have been detected in prior experiments. It had been thought that these "invisible axions" solved the strong CP problem while still being too small to have been observed before. Current literature discusses "invisible axion" mechanisms in two forms, called (Kim–Shifman–Vainshtein– and (Dine–Fischler–– The very weakly coupled axion is also very light, because axion couplings and mass are proportional. Satisfaction