Does our universe have an origin? If so, where does it come from?
As early as 1927, Belgian astronomer Georges Lemaître pointed out that the universe must have been in a very dense state in its early stages. In 1932, Lemaître further proposed that the universe originated from an explosion known as the 'primeval atom.' In 1948, American scientists George Gamow, Ralph Alpher, and Robert Herman introduced the concept of 'hot big bang cosmology,' suggesting that the universe began with a massive explosion of a 'primeval fireball,' during which fragments continuously expanded in space.
Hot big bang cosmology posits that the universe expands in an 'adiabatic' manner, evolving from hot to cold. In the early universe, the density of radiation and matter was very high, and photons would be absorbed or scattered by matter after traveling only a short distance. Subsequently, matter would re-emit photons, leading to frequent interactions between radiation and matter. The universe was opaque to radiation, reaching a state of thermal equilibrium where radiation followed the laws of black body radiation. When the temperature of the universe dropped to about 3000K, protons combined with electrons to form hydrogen atoms, significantly reducing the continuous absorption of radiation. Matter and radiation interacted almost not at all, making the universe transparent to radiation, allowing photons to travel freely through space. The thermal radiation of the universe primarily consists of visible light and infrared. To this day, due to the redshift caused by the expansion of the universe, the peak intensity of cosmic radiation at 3000K has shifted to the microwave range, known as the cosmic microwave background radiation. Alpher and others calculated that the temperature corresponding to the microwave background radiation is around 5K. In 1965, American scientists Arno Penzias and Robert Wilson detected microwave noise from the universe in all directions at a wavelength of 7.35 centimeters, with a signal strength equivalent to black body radiation at a temperature of 3.5K. The discovery of microwave background radiation strongly supported the hot big bang cosmological model, leading to widespread acceptance of big bang cosmology among scientists.
