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It is now common knowledge that the universe is expanding. However that was not always the case. Before the 1920s astronomers and scientists were not even sure if some the objects in space were nearby star formations or were actually more distant groupings of stars. The standing belief was that the Universe was static and that it was not prone to much change. This all changed with the work of astronomer Edwin Hubble. Hubble was the first to confirm the difference between nebulae and galaxies and find evidence of universe expansion. He also discovered the concept of red shift. These concepts would lead to revolutionary discoveries about the formation and composition of the universe.
So what are the concepts that explain the expansion of the universe? The first is the big bang. One thing that scientists first had to determine was what caused the expansion of the universe. The widely accepted theory for the universe’s expansion is the big bang theory. The theory states that at the beginning all matter and energy was concentrated in single point. Eventually this concentration of matter exploded, expanded, and eventually started condensing under the influence of gravity into the stars, planets, and galaxies that now exist. Due to this original expansion the universe has continued to spread out. With the concentration of matter that make up celestial objects being held together by individual and collective fields of gravitation. In essence you can say the momentum of the Big Bang keeps the universe expanding.
The second important concept is the Doppler Effect. Hubble and fellow scientists called it red or blue shift. After it was discovered that light had wave properties the conclusion was that it would functions with similar wave mechanics to sound. The Doppler theory states that the further a wave is from an observer the longer its wavelength. The closer the wave is to the observer the short the wavelengths. When using light the longer wavelengths make a shift towards the red end of the electromagnetic spectrum. When it is closer it makes a shift toward the blue end. This is how we know that stars and galaxies are moving away from each other and Earth. Scientist noticed that most galaxies exhibited a red shift in the light they emit.
The nature of the universe is something astrophysicists are still trying to understand. One what is through coming up with a unified field theory that explains the interactions of energy and matter in the universe? This research holds clues to the nature of universe expansion.
We’ve also recorded an entire episode of Astronomy Cast all about the Universe Expansion. Listen here, Episode 28: What is the Universe Expanding Into.