Mogwai returns with a fourth album that exchanges its trademark post-rock traversals between so loud and so soft for a sweeter, subtler sound without dropping any intensity. It’s an enormous close-up of a heartbeat: for all of its steadiness, the secret thump and pump of blood through Mogwai’s innermost chambers nevertheless fascinates with each swell. “Killing All the Flies” expands in a molten aortic flow, heavy and thick with ephemeral, glowing heat; and “Ratts of the Capital” explodes in a furious, full-body guitar flush. Wrap your arms around Mogwai’s chest and press your ear close.