Hubble's law is a fundamental principle in cosmology that describes the relationship between the distance and the recessional velocity of galaxies in the expanding universe. It states that the farther a galaxy is from the observer, the faster it appears to be moving away from the observer.
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Hubble's law states that the recessional velocity of a galaxy is proportional to its distance from the observer, with the constant of proportionality known as the Hubble constant.
The Hubble constant is currently estimated to be around 70 kilometers per second per megaparsec, meaning that for every megaparsec (about 3.26 million light-years) of distance, a galaxy appears to be moving away at an additional 70 kilometers per second.
Hubble's law provides evidence for the expansion of the universe, as predicted by the Big Bang theory, and is a key piece of evidence supporting the cosmological model of an expanding universe.
The redshift observed in the light from distant galaxies is a direct consequence of Hubble's law, as the recessional velocity of the galaxy causes the light to be shifted toward longer, redder wavelengths.
Hubble's law and the observed expansion of the universe have important implications for the age and evolution of the universe, as well as the distribution of matter and energy on the largest scales.
Review Questions
Explain how Hubble's law relates to the expansion of the universe.
Hubble's law, which describes the relationship between the distance and recessional velocity of galaxies, provides strong evidence for the expansion of the universe. As the universe expands, the distance between galaxies increases, and the farther a galaxy is from the observer, the faster it appears to be moving away. This observed relationship between distance and velocity is a direct consequence of the expansion of the universe, as predicted by the Big Bang theory.
Describe the connection between Hubble's law and the Cosmic Microwave Background (CMB).
The Cosmic Microwave Background (CMB) is a key piece of evidence supporting the Big Bang theory, which is closely tied to Hubble's law. The CMB is the oldest light in the universe, dating back to the time when the universe became transparent to radiation, about 380,000 years after the Big Bang. The uniform nature of the CMB across the sky provides strong support for the idea that the universe was once in a hot, dense state and has been expanding and cooling ever since, as described by Hubble's law.
Analyze how Hubble's law and the observed redshift of distant galaxies contribute to our understanding of the age and evolution of the universe.
Hubble's law, combined with the observed redshift of distant galaxies, allows us to infer the age and evolution of the universe. By measuring the redshift of galaxies and applying Hubble's law, we can estimate the distance to those galaxies and their recessional velocity. This information, in turn, can be used to extrapolate backwards in time to the point where all the matter and energy in the universe was concentrated in a single, infinitely dense point - the Big Bang. The rate of expansion implied by Hubble's law, along with other cosmological observations, enables scientists to calculate the age of the universe, which is currently estimated to be around 13.8 billion years old. This understanding of the universe's age and evolution is a crucial outcome of Hubble's groundbreaking work.
The shift of the observed wavelength of light from a distant object toward longer, redder wavelengths, caused by the Doppler effect resulting from the object's motion away from the observer.
Cosmic Microwave Background (CMB): The thermal radiation left over from the early stages of the universe, which is observed to be almost perfectly uniform in all directions, providing strong evidence for the Big Bang theory.
The prevailing cosmological model for the universe, which describes how the universe expanded from an initial state of extremely high density and temperature, and continues to expand to this day.