The background radiation is associated with a temperature of about 2.73 Kelvins (not Celsius). There is no particular reason for it to be at this specific value. This temperature is a snapshot of an ongoing process taken at our point in time. This temperature continues to change from where it used to be - many orders of magnitude higher in the early stages of our universe model - to today's measured value. The expanding universe view implies this temperature continues to decline as the universe expands, as you pointed out. It is just happening on an extremely slow time scale that we consider this value to be a constant.
The Cosmic Microwave Background (CMB) has a temperature of 2.7K due to the cooling of the universe as it expanded since the Big Bang. Initially hot and dense, the universe became transparent around 380,000 years after the Big Bang, allowing this radiation to travel through space. The CMB is a key piece of evidence for the Big Bang theory.
Cosmic microwave background (CMB)
The reason the background radiation in the universe has a temperature of about 2.7K, which is roughly 3ºC above absolute zero, is deeply connected to the events following the Big Bang. Cosmic Microwave Background (CMB) radiation is the afterglow of the Big Bang, produced when the universe was still young, hot, and dense. Over the last 13.8 billion years, as the universe expanded, this radiation cooled down.
Initially, the universe was opaque because photons were constantly colliding with electrons. However, about 380,000 years after the Big Bang, the temperature dropped to approximately 3000K, allowing electrons to combine with protons to form neutral hydrogen atoms, making the universe transparent to radiation. This period is called the recombination era. The CMB we observe today is the residual electromagnetic radiation from this era.
The universe has expanded significantly since then, stretching the wavelengths of these photons and decreasing their energy, which has cooled the radiation to its current temperature of about 2.7K. This cooling is expected in an expanding universe, as predicted by the Big Bang theory. Thus, the observed temperature of the CMB provides strong evidence supporting the Big Bang theory.