The expansion of the universe is not solely based on the increasing visibility of objects. Instead, it is supported by multiple lines of evidence from various branches of science, including cosmology, astrophysics, and observational data. Here are a few key pieces of evidence that demonstrate the expansion of the universe:
Hubble's Law: In the 1920s, the astronomer Edwin Hubble observed that distant galaxies were moving away from us, and the speed of their recession was directly proportional to their distance. This relationship is known as Hubble's Law and is consistent with the expansion of the universe.
Cosmic Microwave Background (CMB): The CMB is the residual radiation from the early universe, emitted about 380,000 years after the Big Bang. It provides crucial evidence for the expansion. The CMB exhibits a nearly uniform temperature in all directions, but it also has small fluctuations in temperature. These temperature fluctuations were mapped by satellites such as the Planck spacecraft, revealing a pattern consistent with an expanding universe.
Redshift of Spectral Lines: When astronomers observe the light coming from distant galaxies, they observe a redshift in the spectral lines. This redshift indicates that the wavelengths of the light have been stretched as the galaxies move away from us. The greater the redshift, the faster the galaxy is moving away. This effect is consistent with the expansion of space.
Large-Scale Structure of the Universe: The distribution of galaxies and galaxy clusters on the largest scales also supports the idea of an expanding universe. Observations show a "clumpy" structure, with galaxy clusters forming filaments and sheets separated by vast voids. This cosmic web of structure is a result of the initial density fluctuations that have been amplified by the expansion of the universe over billions of years.
These pieces of evidence, among others, strongly indicate that the universe is expanding. It's important to note that the expansion of the universe does not simply imply that more objects are becoming visible over time. Instead, it suggests that the space itself is expanding, causing galaxies and other celestial objects to move away from each other.