Question

When astronomers observe the spectra of distant galaxies,they notice that the hydrogen emission lines are shifted noticeably toward the red end of the visual spectrum,a phenomenon called red shift. Red is lowest frequency of visible light. What does red shift indicate about the movement of the distant galaxies?

Answer 1

We interpret that to mean that the object is moving away from us. So far, that's the only way we know of that light can get red-shifted.

The other question you should think about is;. How do we know what the wavelength of that light WAS when it started out toward us ?

Answer 2

Answer:

Option 1

They are moving away from us at high speed.

Explanation:

Spectral lines will be shifted to the blue part of the spectrum¹ if the source of the observed light is moving toward the observer, or to the red part of the spectrum when is moving away from the observer (that is known as the Doppler effect).

For example, a person can hear more intensely the sound coming from the siren of an ambulance when is approaching. The reason of that, is that the frequency of the wave traveling in the air will be higher when is moving toward the person than when is moving away.

The same scenario is happening with the galaxies, so the redshift indicates that the galaxies are moving away from us at high speed.

Remember that the frequency in the red part of the spectrum are lower than in the blue part of the spectrum.

The speed of the galaxies using the shifted of the spectral lines can be known by using the Doppler speed:

$v = c\frac{\Delta \lambda}{\lambda_{0}}$ (1)

Where $\Delta \lambda$ is the wavelength shift, $\lambda_{0}$ is the wavelength at rest, v is the velocity of the source and c is the speed of light.

Therefore, from equation 1 the equation for the redshift can be deduced:

$v = c(\frac{\lambda_{measured}-\lambda_{0}}{\lambda_{0}})$

$\frac{v}{c} = \frac{\lambda_{measured}-\lambda_{0}}{\lambda_{0}}$

$z = \frac{\lambda_{measured}-\lambda_{0}}{\lambda_{0}}$ (2)

Where z is the redshift.

Hence, the redshift represents this shift of the spectral lines to red part in the spectrum of a galaxy or any object which is moving away. That is a direct confirmation of how the universe is in an expanding accelerated motion.

Key terms:

¹Spectrum: Decomposition of light in its characteristic colors (wavelengths).