Galactic Red-shift

Definition of Galactic Red-shift

Galactic red-shift refers to the observation that the wavelengths of light coming from distant galaxies are longer than expected. This means the light is shifted towards the red end of the visible spectrum, which has longer wavelengths compared to blue or violet light.

This shift towards red indicates that these galaxies are moving away from us. The greater the red-shift, the faster the galaxy is receding. Specifically, "galactic" red-shift refers to this phenomenon observed in light from distant galaxies, distinguishing it from other types of red-shift such as gravitational red-shift.

Cause of Galactic Red-shift

The main cause of galactic red-shift is the expansion of the universe. As space itself stretches, the light waves travelling through it also stretch, increasing their wavelength.

This effect is similar to the Doppler effect, which you may know from sound waves: when a source moves away from you, the waves are stretched, causing a lower frequency. For light, this means the observed wavelength increases, shifting the light towards red. However, it is important to note that galactic red-shift is primarily due to the expansion of space itself, rather than just the motion of galaxies through space.

More distant galaxies show a greater red-shift because they are moving away faster, as the universe expands uniformly.

Measuring Red-shift

Scientists measure red-shift by looking at the spectral lines in the light from galaxies. These lines are specific wavelengths of light emitted or absorbed by elements, which are well known from laboratory measurements on Earth.

If these spectral lines appear at longer wavelengths than expected, the difference can be used to calculate the red-shift.

The red-shift (z) is calculated using the formula:

Where:

• $\lambda_{\text{observed}}$ = wavelength measured from the galaxy
• $\lambda_{\text{rest}}$ = known wavelength of the spectral line from a stationary source

Once the red-shift is known, the velocity $v$ at which the galaxy is moving away can be estimated by:

where $c$ is the speed of light ($3.0 \times 10^8 \text{ m/s}$).

For instance, if a spectral line normally at 500 nm is observed at 505 nm, the red-shift is:

The velocity of the galaxy is then:

Implications of Galactic Red-shift

Galactic red-shift provides strong evidence that the universe is expanding. This supports the Big Bang theory, which states the universe began from a very hot, dense state and has been expanding ever since.

There is a direct relationship between the distance of a galaxy and its speed of recession, known as Hubble's Law:

Where:

• $v$ = velocity of the galaxy moving away
• $d$ = distance to the galaxy
• $H_0$ = Hubble constant (rate of expansion of the universe)

This means the further away a galaxy is, the faster it appears to be moving away from us.

Note that the Hubble constant is measured with some uncertainty, and red-shift provides an approximate velocity, especially at low speeds.

This observation is crucial in cosmology and helps scientists estimate the age and size of the universe.