The understanding that galaxies are the dominant form of structure in the universe is based on a combination of observational evidence and theoretical models. Here are some key points that support this conclusion:
Observations of Galaxies: Astronomers have conducted extensive observations of galaxies across different wavelengths of light, such as visible, infrared, and radio waves. These observations reveal a vast number of galaxies at various distances and stages of cosmic evolution. The distribution and clustering of galaxies observed in large-scale surveys provide evidence for their dominance on cosmic scales.
Redshift and Hubble's Law: Edwin Hubble's observations in the 1920s demonstrated a correlation between the distance of galaxies and their redshift, which is a shift of light toward longer wavelengths due to the expansion of the universe. This relationship, known as Hubble's Law, implies that galaxies are moving away from us, and the greater the distance, the faster the recession. The observation of redshifts in galaxies supports the idea that the universe is expanding and that galaxies are moving apart from each other.
Cosmic Microwave Background (CMB): The cosmic microwave background radiation, which is a remnant of the early universe, provides important evidence for the large-scale structure of the universe. Precise measurements of the CMB have revealed fluctuations in its temperature across the sky. These fluctuations correspond to density variations in the early universe, which later led to the formation of galaxies through gravitational collapse.
Large-Scale Structure: Studies of the large-scale distribution of galaxies through surveys and computer simulations have shown the presence of cosmic filaments, clusters, and superclusters. These structures span vast regions of space, indicating that galaxies are not randomly distributed but form interconnected networks and clusters, further supporting the idea that galaxies are the dominant form of structure in the universe.
Dark Matter: The observed dynamics of galaxies and galaxy clusters suggest the presence of significant amounts of unseen matter called dark matter. The gravitational influence of dark matter is necessary to explain the observed motions of galaxies and the formation of large-scale structures. The understanding of dark matter's role in galaxy formation and its prevalence supports the idea that galaxies are the primary building blocks of the universe's structure.
It's important to note that our understanding of the universe is continually evolving as new observations and theoretical models emerge. However, the current body of evidence strongly supports the notion that galaxies play a central role in shaping the large-scale structure of the universe.