Question

The Universe was once (up to when it was about 400,000 years old) filled with a hot, dense, opaque, ionized gas. When the gas "recombined" at 400,000 years, when it was about 3000 K, it became transparent. We see this today as an opaque spherical wall of gas surrounding us in all directions, with a redshift of around a factor 1000. If the peak of the Planck spectrum of this gas was at 1 micron (0.000001 meters) when it was emitted, what wavelength does this "cosmic microwave background" peak at when we observe it from Earth today

Answer 1