All objects with a temperature above absolute zero (0 K, -273.15 oC) emit energy in the form of electromagnetic radiation.
A blackbody is a theoretical or model body which absorbs all radiation falling on it, reflecting or transmitting none. It is a hypothetical object which is a “perfect” absorber and a “perfect” emitter of radiation over all wavelengths.
The spectral distribution of the thermal energy radiated by a blackbody (i.e. the pattern of the intensity of the radiation over a range of wavelengths or frequencies) depends only on its temperature.
The characteristics of blackbody radiation can be described in terms of several laws:
2. Wien’s Displacement Law, which states that the frequency of the peak of the emission (fmax) increases linearly with absolute temperature (T). Conversely, as the temperature of the body increases, the wavelength at the emission peak decreases.
3. Stefan–Boltzmann Law, which relates the total energy emitted (E) to the absolute temperature (T).
In the image above, notice that:
In 1965, the cosmic microwave background radiation (CMBR) was discovered by Penzias and Wilson, who later won the Nobel Prize for their work. The radiation spectrum was measured by the COBE satellite and found to be a remarkable fit to a blackbody curve with a temperature of 2.725 K and is interpreted as evidence that the universe has been expanding and cooling for about 13.7 billion years. A more recent mission, WMAP, has measured the spectral details to much higher resolution, finding tiny temperature fluctuations in the early Universe which ultimately led to the large-scale structures we see today.