Modern Physics

Hubble’s Law

Hubble’s law determines the speed with which a galaxy is moving away from the Milky Way from the estimated distance of that galaxy.

Edwin Hubble (1889-1953) was an important American astronomer and responsible for determining the conditions for the separation of galaxies and the consequent expansion of the universe. The so-called Hubble’s law , determined in 1923, shows the speed of separation between the galaxies that make up the universe.

After the Big Bang , galaxies were being formed at the same time they were moving away from each other, making the universe something bigger and bigger, as in the following illustration:

Splipher Observation

In the year 1912, astronomer Vesto Melvin Splipher noticed a shift in the spectral lines of the Andromeda Galaxy towards the wavelengths that indicated the blue color for light. This observation was possible thanks to the Doppler effect applied to light. In the mutual distance between the light source and the observer, there is a decrease in the frequencies perceived by the observer; if there is approximation, the perceived frequencies will become higher and higher.

By observing that Andromeda’s spectral lines were shifting towards the blue , Splipher understood that this galaxy was approaching us. By analyzing, for two decades, the spectral lines of 40 different galaxies, the astronomer realized that the vast majority of them had spectral lines with a red shift , which indicated that these celestial bodies were moving away from the Milky Way .

Hubble’s Law

After analyzing the behavior of stars called Cepheids and the Andromeda galaxy using images captured by the Mount Wilson telescope , Edwin Hubble and Milton Humason determined the estimated distance between Andromeda and other galaxies. By comparing the distances between galaxies and their speeds away from each other, astronomers noticed that the more distant galaxies were moving away at a faster speed.

Hubble’s law determines the speed of departure of a galaxy as a function of its distance.

  • v = Speed ​​away from a galaxy (km/s);
  • 0 = Hubble constant (71 km/s.Mpc);
  • d = Distance from the galaxy (Mpc).

Interpreting Hubble’s Law

The constant H 0 has a value of 71 km/s.Mps, which means that at each distance of 1 Mpc (read megaparsec ), the speed of a galaxy increases by 71 km/s. The megaparsec unit corresponds to 3.09 x 10 19 km, that is, every 3.09 x 10 19 km, the speed of departure of a galaxy increases.

Vesto Melvin Splipher’s observation and the law defined by Edwin Hubble reveal that the universe is constantly expanding. Any observer, at any position in the universe, would perceive the same expansion, so it cannot be said that there is a center of the universe.

age of the universe

According to Hubble’s law, a galaxy at a distance of 10 Mpc (30.9 x 10 19 km) from the Milky Way has a speed of departure of 710 km/s. If we think that everything in the universe originated at the same point, we can determine the time required for the galaxies to move away and thus estimate the age of the universe.

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