Question

In 2009, NASA and the European Space Agency (ESA) used more advanced equipment that allowed scientists to measure the exact temperature changes within the CMB. They collected data that indicates all CMB is roughly the same temperature. It changed by only 1/100,000 of a degree Celsius.

Why do you think all CMB throughout the Universe is so close in temperature?

Answer 1

Short answer: because the early universe was an almost-perfect thermal bath and (most important) an inflationary expansion stretched that uniform patch to cover the whole observable universe. The tiny differences you see are the small primordial ripples that later grew into galaxies.

A bit more detail, in plain terms:

- Before “recombination” (about 380,000 years after the Big Bang, when the CMB was released) the universe was hot, dense and opaque: photons, electrons and baryons were tightly coupled by frequent Thomson scattering. That coupling kept the photon-baryon fluid in thermal equilibrium, smoothing out temperature differences very efficiently.

- Without inflation this would leave a horizon problem: widely separated regions of the sky could never have come into causal contact to equilibrate. Inflation solves that: a very small, causally connected region that had a uniform temperature was blown up enormously, so the whole observable universe inherited that uniform temperature.

- The remaining temperature variations are tiny (ΔT/T ~ 10^-5, i.e. tens of microkelvin on a 2.7 K background). Those small anisotropies are just the magnified quantum fluctuations from the inflationary era plus later effects (acoustic oscillations in the photon-baryon fluid, gravitational redshifts — Sachs–Wolfe effect — and Silk damping). They are exactly the “seeds” that grew into the large-scale structure we see today.

So: thermal equilibration early on + inflationary stretching explains why the CMB is so nearly the same temperature everywhere, and the tiny measured deviations are the important imprints of early-universe physics.