The idea that the universe is expanding, which is a fundamental aspect of the Big Bang Theory, is supported by several key pieces of evidence:
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Redshift of Galaxies: One of the most compelling pieces of evidence for the expansion of the universe comes from the observation of redshifts in the light from distant galaxies. Edwin Hubble discovered that the light from galaxies is shifted toward the red end of the spectrum, which indicates that they are moving away from us. The greater the distance to a galaxy, the faster it appears to be receding, which is encapsulated in Hubble's Law. This relationship suggests an expanding universe.
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Cosmic Microwave Background Radiation (CMB): The CMB is the remnant radiation from the hot, dense state of the early universe, formed about 380,000 years after the Big Bang. It fills the universe and is remarkably uniform in all directions, but slight fluctuations in temperature provide critical information about the early universe's conditions. The existence of this radiation supports the idea of an expanding universe, as it aligns with the predictions of the Big Bang model.
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Abundance of Light Elements: The Big Bang nucleosynthesis predicts that the universe's earliest moments produced specific amounts of light elements such as hydrogen, helium, and lithium. Observations of the current universe's elemental composition match these predictions closely, indicating that the universe has indeed evolved from a hot, dense state and has expanded over time.
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Large Scale Structure of the Universe: The distribution and structure of galaxies in clusters and superclusters also provide evidence for the expanding universe. Observations indicate that galaxies are not randomly distributed but form a complex web-like structure that has evolved over billions of years due to gravitational forces acting within the expanding universe.
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General Relativity: The theoretical framework of general relativity, formulated by Albert Einstein, describes how gravity interacts with the fabric of space-time. The solutions to the equations of general relativity include models that predict an expanding universe. Since Hubble's observations were made, further research has refined these models and has shown that the expansion rate can be affected by factors such as dark energy.
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Accelerating Expansion: Observations of distant supernovae, particularly Type Ia supernovae, have revealed that the rate of the universe's expansion is actually accelerating. This discovery, which has led to the concept of dark energy, provides additional evidence that the dynamics of the universe's expansion are more complex than a simple linear trend, reinforcing the notion of an expanding universe.
Together, these points form a robust body of evidence supporting the conclusion that the universe is expanding, as informed by the Big Bang Theory.