Recessive Universe: A 'recessive universe' refers to a universe where galaxies are generally moving away from each other. This is a direct consequence of the expansion of space, as described by Hubble's Law. The Hubble constant (H₀) quantifies the rate of this recession.

Relationship to Age of the Universe: The Hubble constant is directly related to the age of the universe. A larger value of H₀ implies a faster rate of expansion, suggesting a younger universe. Conversely, a smaller value of H₀ indicates a slower rate of expansion, implying an older universe. The age of the universe is approximately the inverse of the Hubble constant (1/H₀), although this is a simplified relationship and assumes a constant rate of expansion, which is not entirely accurate.

Two key properties of the CMBR are:

1. Nearly Uniform Temperature: The CMBR has a remarkably uniform temperature across the entire sky, with only very small variations. This uniformity suggests that the early universe was in thermal equilibrium, meaning it was very homogeneous and evenly distributed. This is consistent with the Big Bang theory's prediction of a uniform distribution of matter in the early universe.
2. Blackbody Spectrum: The CMBR exhibits a blackbody spectrum, meaning its intensity is distributed across all wavelengths in a way that is characteristic of thermal radiation. This blackbody spectrum is a direct consequence of the universe being in thermal equilibrium in its early stages. The Big Bang theory predicts that the early universe was extremely hot, and as it expanded and cooled, the radiation emitted would have cooled and redshifted to microwave wavelengths, resulting in the observed blackbody spectrum.

These properties – the uniformity and the blackbody spectrum – are strong pieces of evidence supporting the Big Bang theory because they are consistent with the theoretical predictions of a universe that originated from a hot, dense state and has been expanding and cooling ever since.

The CMBR is considered strong evidence for the Big Bang theory because it is interpreted as the residual radiation from the early, hot, and dense state of the universe. The Big Bang theory predicts that the universe began in a very hot state and has been expanding and cooling ever since. As the universe expanded, the radiation from this early stage has been stretched to longer wavelengths, resulting in the observed microwave spectrum.

Temperature: The CMBR has a very low temperature of approximately 2.7 Kelvin (-270.45°C). This low temperature is consistent with the cooling of the universe as it expanded. If the universe is still expanding, the CMBR continues to cool.

Spectrum: The spectrum of the CMBR is remarkably close to a perfect blackbody spectrum. This is a key prediction of the Big Bang theory, as a blackbody spectrum is what is expected from radiation in thermal equilibrium. The observed spectrum has been measured with great precision and matches the theoretical predictions very well. The slight deviations from a perfect blackbody spectrum are attributed to factors like the expansion of the universe and the presence of matter and radiation in the early universe.

The combination of the CMBR's temperature and spectrum provides compelling evidence that the universe originated from a hot, dense state and has been expanding and cooling ever since, supporting the Big Bang theory.