The wavelength of light does not decrease over time. The wavelength of light is a fundamental property of electromagnetic radiation and remains constant as long as the light propagates through a vacuum or a transparent medium.
The concept you may be referring to is the expansion of the universe, which leads to a phenomenon known as cosmological redshift. According to the Big Bang theory, the universe has been expanding since its inception. As space itself expands, the wavelength of light traveling through that expanding space also stretches or "redshifts."
This means that the light waves from distant celestial objects, such as galaxies, are stretched as they travel through expanding space, resulting in an increase in their wavelength. This effect is known as cosmological redshift because the light is shifted towards the red end of the spectrum.
However, it's important to note that this redshift is not a result of the light itself changing its wavelength over time, but rather the stretching of space through which the light travels. The wavelength of light remains constant from the moment it is emitted by a source.
The concept of cosmological redshift and the expanding universe was first proposed by Belgian astronomer Georges Lemaître in the 1920s and later supported by observational evidence, such as Edwin Hubble's observations of galaxies. The discovery of the cosmic microwave background radiation in 1964 further confirmed the Big Bang theory and the expansion of the universe.