A Brief Explanation on Redshift

Introduction: 

The displacement of an astronomical object’s spectrum toward longer (red) wavelengths is known as redshift. The Doppler effect, a shift in wavelength that occurs when a source of waves (e.gLight or radio waves) and an observer are moving in relation to one another is the basis of this theory.

In 1929, American astronomer Edwin Powell Hubble discovered that distant galaxies were retreating from the Milky Way system, in which Earth is located, and that their redshifts increased proportionally with their distance. This generalisation served as the foundation for Hubble’s law.

Doppler effect: 

If the source moves away from the viewer, the frequency (pitch for sound) and wavelength of both sound and light vary. This is known as the Doppler effect. When it comes to astronomy, the Doppler effect is the sole way to tell if a celestial object is moving toward or away from us along our line of sight. Light from a moving object is blue-shifted, while light from a moving object is redshifted. The amount of blue or redshift indicates the speed at which the object is travelling.

Redshift: 

When a light-emitting device moves away from an observer, the light that reaches the observer shifts from blue to red, increasing the wavelength and reducing the frequency. This is referred to as redshift. The wavelength of the light changes from its rest wavelength to these bluer or redder wavelengths. The velocity of the galaxy divided by the speed of light equals the redshift, z. The redshift of all galaxies, i.e. their movement away from us, is referred to as recession velocity.

Irrespective of whether the observations were made in our own Milky Way galaxy or elsewhere in the Universe, all galaxies appear to be travelling away, and the further the galaxy is from us, the quicker it appears to be moving away.

Three types of the Redshift: 

• The velocity of galaxies relative to their neighbouring galaxies causes type I redshift. For example, our Milky Way is colliding with the very nearby Andromeda galaxy which will show the ensuing decrease in Andromeda’s wavelength from our perspective as the galaxy’s approach shows blueshift but a galaxy travelling away from ours will show a redshift at the same time.

• The most common type of redshift is Type II, which is caused by the expansion of space between two stationary bodies.

• Type III, gravitational redshift, is the most modest of all redshift types. Because gravitational forces from huge masses cause light to bend, even slightly, it distorts the entering light’s course, creating a shift in the wavelength of the approaching, visible light.

Evidence of Expanding Universe: 

In general, the more away a galaxy is, the more red-shifted its light is, according to astronomers. This suggests that the galaxies are travelling faster the further away they are.  If everything in the universe is moving apart from everything else, those light waves should be getting longer and further apart. The redshift is named by the fact that longer wavelengths of visible light appear red. 

This is analogous to an explosion in which the fastest moving bits travel the furthest away from the blast. The Universe, including space itself, is expanding, according to red-shift statistics. In reality, Hubble’s law, or the recession velocity of galaxies, is a trick of the eye because the galaxies are not moving farther or closer, the space between them is expanding.

Redshift measure by Astronomers: 

Spectroscopy is the most accurate way to quantify redshift. When a white light beam contacts a triangular prism, it is broken down into VIBGYOR. This is referred to as a spectrum. Astronomers can compare the spectra produced by various elements to the spectra produced by stars. They know the object is travelling towards us or away from us if the absorption or emission lines in the star’s spectra are shifted. 

The redshift parameter z is used by astronomers to describe redshift. This is determined using the following equation, where observed is the wavelength of a spectral line that has been observed, and emitted is the wavelength that line would have if its source was not moving:

Example: 

The wavelength of a calcium absorption characteristic is normally , however it has been observed in a galaxy to be . What exactly is a redshift?

Solution:

Given:

To find:

First, calculate the value of ,

Since the change in wavelength is positive, the object is moving away from our galaxy

⸫Exact redshift is 0.017.

Conclusion: 

We can conclude by stating that closer objects provide a more compressed wavelength of light, resulting in a higher frequency giving a blue appearance. The wavelength becomes less compressed and the frequency decreases as it moves further away. That would make it appear redder or simply lower frequency is red and higher frequency is blue. So, if things are turning redder and less blue, it’s moving away from us, and if everything is moving away from us, it’s indicating that the universe is expanding.