Hubble Constant, Doppler Effect, Blue and Red Shift in Galaxies

The expansion rate of the universe is called the Hubble Constant in honor of Edwin Hubble (1889–1953). Currently the best measured value of the Hubble Constant is about 73 kilometers per second per megaparsec.

This implies that if a location in space is one million parsecs from another location, then in the absence of any other forces or effects the two locations will be moving apart from one another at the speed of 263,000 kilometers per hour!

Hubble measured the galaxies’ Doppler effect—the shift in the observed color of objects moving toward or away from an observer—by mounting a machine called a spectrograph on a telescope. He split the light from distant galaxies into its component parts and measured how far the wavelengths of emitted light shifted toward longer wavelengths.

Doppler Effect Definition

Doppler Effect is named after Christian Johann Doppler (1803–1853). It occurs when a source of sound (or any other wave) is moving toward or away from a listener. If the source is moving toward the listener, the sound wave’s wavelength decreases, and the frequency increases, making the sound higher-pitched.

Conversely, if the source is moving away from the listener, the sound wave’s wavelength increases, and the frequency decreases, making the sound lower-pitched. The next time a car or train passes by you on the street, listen to the sound it is making as it approaches and then moves away.

Doppler effect for light: Blue shift and Red Shift

When an object emitting light—or any kind of electromagnetic radiation, for that matter— moves toward someone, the wavelength of its emitted light is decreased.

Conversely, when the object moves away, the wavelength of its emitted light is increased.

For visible light, the bluer part of the spectrum has shorter wavelengths, and the redder part of the spectrum has longer wavelengths. Thus, the Doppler effect for light is called a “blue shift” if the light source is coming toward an observer, and a “red shift” if it is moving away. The faster the object moves, the greater the blue shift or red shift.