Hubble's Law

Introduction

• Hubble's Law is a fundamental principle in astronomy that describes the relationship between the velocity at which galaxies are moving away from Earth and their distance from us.
• Named after American astronomer Edwin Hubble, this law provides crucial evidence for the expanding nature of the universe.

Hubble's Discovery

• Edwin Hubble made groundbreaking measurements of the speeds of galaxies and their distances from Earth.
• He constructed a graph showing the relationship between the velocity of receding galaxies and their distance from Earth.

The Graph and Proportionality

• Hubble's graph is a straight line that passes through the origin, indicating a direct proportionality between the velocity of galaxies (v) and their distance (d).
• This relationship is expressed through Hubble's Law, described by the equation:
• v = H0 × d
  • v represents the velocity of a receding galaxy.
  • d stands for the distance to the galaxy.
  • H0 denotes Hubble's constant, which has an approximate value of 70 km/s/Mpc (kilometres per second per megaparsec).

Implications of Hubble's Law

• The key implication of Hubble's Law is that the further a galaxy is from Earth, the faster it is moving away from us.
• This suggests that the entire universe is expanding, and all galaxies were once concentrated at a single point in space, known as the "Big Bang."

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Hubble's Law

Expanding Universe

• Hubble's Law provides strong evidence for the expansion of the universe in all directions. While a few galaxies exhibit blueshift (indicating motion towards Earth), the vast majority display redshift, signifying their motion away from us.
• This expansion conceptually traces back to the moment of the Big Bang, when the universe began.

Age of the Universe

• Hubble's Law, combined with precise knowledge of Hubble's constant, allows astronomers to estimate the age of the universe.
• In 2012, NASA's Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft estimated the universe's age to be approximately 13.772 ± 0.059 billion years.
• This estimate was later confirmed by the European Space Agency's Planck spacecraft in 2013, which calculated an age of 13.82 billion years.

Conclusion

• Hubble's law, formulated by Edwin Hubble, provides a foundational understanding of the expanding universe. It establishes a direct relationship between the velocity of galaxies and their distance from Earth.
• The consistent redshift of distant galaxies, as observed through this law, supports the idea that the universe originated from a single point in the Big Bang.
• By using Hubble's constant and this law, astronomers have estimated the age of the universe, contributing significantly to our knowledge of cosmic evolution.

Summary

• Hubble's Law describes the relationship between a galaxy's velocity and its distance from Earth, indicating that galaxies move away from us faster the further they are.
• This law supports the idea of an expanding universe and is based on the work of American astronomer Edwin Hubble.
• Precise knowledge of Hubble's constant allows astronomers to estimate the age of the universe, revealing it to be approximately 13.8 billion years old.