Last Scattering Surface

The Last Scattering Surface marks the epoch when the early universe changed from an opaque plasma into a mostly transparent gas. Before this transition, photons repeatedly scattered from free electrons, chiefly through Thomson scattering, so their paths did not preserve information from earlier times. As the universe expanded and cooled, electrons combined with nuclei to form predominantly neutral hydrogen; the sharp fall in free-electron density greatly increased the photons’ mean free path.

The surface is therefore a probability-weighted region rather than an infinitely thin boundary. Some photons last scattered slightly earlier or later than the peak epoch, producing a finite thickness in redshift and distance. “Last scattering” describes the final important interaction, not necessarily the final microscopic collision of every photon with every particle.

The CMB records conditions in the universe when it was roughly 380,000 years old, long before galaxies and stars formed. Tiny temperature and polarization fluctuations on the Last Scattering Surface preserve information about primordial density variations, acoustic oscillations in the photon–baryon plasma, and the geometry and contents of the universe.

Hot and cold regions in CMB maps are not simply photographs of matter at one distance. Their observed temperature also reflects gravitational redshift, Doppler motion, scattering along the later line of sight, and integrated effects caused by evolving gravitational potentials. The characteristic angular scale of the acoustic peaks supplies a powerful measurement of cosmic geometry, while the peak pattern constrains baryons, dark matter, and the primordial perturbation spectrum.¹

Observers see the Last Scattering Surface in every direction because their past light cone intersects the recombination-era universe over an almost spherical shell centered on the observer. Its present-day comoving distance is approximately 14 billion parsecs, although the exact value depends on the cosmological model and the distinction between comoving and proper distance.²

Satellite missions made this surface measurable with increasing precision. NASA’s Wilkinson Microwave Anisotropy Probe mapped temperature and polarization anisotropies, while ESA’s Planck mission refined measurements of the acoustic spectrum and polarization.

The surface is not directly visible in ordinary optical light: photons from that epoch have been redshifted into microwave wavelengths. CMB maps are consequently reconstructions of an ancient radiation field, corrected for foreground emission from the Milky Way and other astrophysical sources.

The Last Scattering Surface has a related but distinct polarization structure. Thomson scattering produces linear polarization when the radiation field has a quadrupole anisotropy, allowing CMB polarization to probe velocity fields, density perturbations, and possible primordial gravitational waves.¹

Reionization did not erase the surface, but it added a later, much thinner scattering screen when the first luminous objects reionized intergalactic hydrogen. A small fraction of CMB photons scattered during that era, suppressing some primary anisotropies and generating large-angle polarization that constrains the timing and integrated optical depth of reionization.

There is also a subtle observational limit: photons emitted from the opaque period before recombination cannot reach us directly as electromagnetic radiation. Information from still earlier times may instead be sought through relic neutrinos, gravitational waves, or other non-photonic signals, whose interaction histories differ from those of CMB photons.

Recombination

The early-universe transition during which free electrons combined with atomic nuclei, reducing the density of charged particles that scattered CMB photons.

Thomson scattering

The low-energy scattering of electromagnetic radiation by free charged particles, especially electrons.

Photon–baryon plasma

The tightly coupled early mixture of radiation, electrons, and baryonic matter that supported acoustic waves before recombination.

Optical depth

A measure of the cumulative probability that radiation is scattered or absorbed along a path.