Brixton Academy, brim-full and beer-clogged, is unprecedentedly still, as swollen reverberations throb through several thousand feet. An arc of light looms ahead, and these unearthly vibrations surround you like a conquering army. As it reaches its climax, there are no clear shapes onstage, just a vast shadowy canvas soaking up globulous projections, and all you can see and hear emanating from it is a sensual golden glow with vague silhouettes lurking behind it, given form by the glorious flood of Jónsi Birgisson's stratospheric vocals.

Tonight Sigur Rós sound like discovery - space, internal and outer; four dimensionality and deep sea diving; supersonic flight and cells dividing under a microscopic lens. Unassuming as the band are, the turquoise flashes of light that accompany their crescendoes obscure the band behind a screen of bright fog and sweep the crowd instead. They're like scientists in awe of the perfect artistry in the natural design that their work reveals. It's a humility reflected in the music; Sigur Rós are not about awkward, revolutionary, painful invention but the uncovering of organic, harmonious melodies that spring up fully formed, existing exactly as they are fated to be. They sound like a natural reaction, so obvious that no-one else has thought of it.

Unearthly, perhaps, but it would be too trite to package this up as wondrous music by emotionally sophisticated Martians, extra-terrestrials floating among us with their string sections and their funny made-up lexicon. Ragged human breath fills the void betweeen Birgisson's weird, homeless vowel-strings and rolling purrrrs; if you pay fanatical attention, occasionally also an off note, just slightly too shrill. If Sigur Rós' music describes outer space, it's the sound of human wonder at the mysteries above one's head, just within reach; not alien civilisations, but moon landings, alphabetic formulae that paint supernovas, our own Earth seen from space for the first time, men in glass helmets hurtling recklessly at the vertical unknown in the name of wonder and hope. It's a human, childlike expression of everything truly eyepopping, set beyond the disappointing confines of communicative language, conveyed instead via twinkles and crashes and symphonic swells.

When the yellow glow onstage gives way to shattering drum crashes and electric smashes of blue strobe, though, it's clear that this is no benign, detached display of intellectual virtuosity or simpleminded ooh-ing and aah-ing at the great unknown, but a rock show - as melodies soak each other up and trumpeters cross the stage like a line of penguins, amid glockenspiels and brass flutters, Sigur Rós' humanity is anything but uncertain; they are but one inspired example of the best that we have to offer.

Pictures: Laura Chapman