What is Redshift?

It is an astronomical phenomenon. In this phenomenon, increase in wavelength with corresponding decrease in photon energy and frequency of radiation of light. It is the displacement of spectrum of any kind of astronomical object to the longer wavelengths (red) side.

In other words, we can say, the change in wavelength occurs when light or radio waves and the person who is observing, are in motion with respect to each other.

The three main causes of electromagnetic redshifts are:

• The radiation travels between objects which are moving away from each other. This is known as relativistic redshift and this is the example of relativistic Doppler Effect.
• The radiation moves towards an object in a weaker gravitational potential.  This is known as gravitational redshift.
• The radiation travels through expanding space. This is known as cosmological redshift. And this can also be observed by the Hubble’s Law.

The gravitational waves which travel with the speed of light, also shows the redshift phenomena. The value of a redshift is generally denoted by z, corresponding to fractional change in wavelength (always positive for redshift) and, by the ratio of wavelength i.e., 1+z (always greater than 1 for redshift).

The opposite change i.e., decreases in wavelength and with respect to that, the increase in frequency and energy, is known as blueshift or can be called as negative redshift. The value corresponding to fractional change in wavelength will be negative for the blue shift and the value by the ratio of wavelength will always be less than 1.

Some Examples of Strong Red Shifting Include:

A gamma ray perceived as an X-ray, visible light perceived as radio waves are seen in spectroscopic observations of astronomical objects and are therefore used in terrestrial technologies such as Doppler radar and Radar Guns. Some other physical processes that lead to shift in frequency of electromagnetic radiation includes scattering and optical effects. Currently, the highest known redshifts are the galaxies and the objects which are producing gamma ray bursts. The most reliable redshifts are known from the spectroscopic data, and the highest known redshift of galaxy is GN-z11 whose redshift was calculated as 11.1, this was corresponding to 400 million years after the big bang. The previous know highest redshift was UDFy- 38135539 which had the redshift of z=7.5.

How to Measure, Characterize and Interpret the Redshift?

To measure the redshift first of all we have to search for the features like absorption lines, emission lines or any other variations in the light intensity in the spectrum. After finding, these features can be compared with known features in the spectrum of various chemical compounds which is found in experiments where that compound is located in earth. The most common atomic element present in space is Hydrogen. The spectrum of featureless light through hydrogen will show a signature spectrum which is quite specific to the hydrogen which has certain features at regular intervals. If the same pattern is observed in the spectrum from a distant source but at shifted wavelengths, then it is identified as hydrogen. And if in case same spectral line is identified in both spectra, but at varying wavelengths then the redshift can be calculated as-

z= λ_obsv -λ_emit/ λ_emit

1+z= λ_obsv / λ_emit

Where λ_obsv represents observed wavelength, λ_emit represents emitted wavelengths.

To calculate the redshift of an object, we have to know the wavelength range of the emitted light in the rest frame of source. And based on frequency it can be calculated as-

z= f_emit – f_obsv/ f_obv

1+z= f_emit/ f_obsv

Where f_emit represents emitted frequency, f_obsv represents observed frequency.

This has to be keep in mind that the redshift cannot be calculated by looking at unidentified features for which rest frame is unknown.

Redshift can be characterized by the relative difference between observed and emitted wavelength or frequency of an object. It is a dimensionless quantity.

Types of Redshift

• Doppler Redshift: If a source of light is moving away from the person observing, the redshift occurs. And if the source of light moves closer to the person observing, then the blueshift occurs. If the source of light is moving away from the person observing, with a velocity (v),  which is very much less than the speed of light, then the redshift is given by-

z=v/c

     where c is the speed of light.

• Cosmological Redshift: It is the consequence of relative expansion of space. Space expansion is considered to be its one of the major interpretations. As the space is expanding, the distance between the two remote galaxies is also increasing. As a result, the wavelength of photons propagating through this stretch i.e., the distance between the objects is creating the cosmological redshift.
• Gravitational Redshift: This theory states about the time dilation within a gravitational well and that time dilation is considered as the gravitational redshift or is also known as Einstein redshift.

Observation of Redshift

Generally, the redshift observed in astronomy is due to the reason that the absorption and emission spectra are distinctive and are calibrated from the spectroscopic experiments in the laboratories on Earth. We generally take two types of observations:

• Local Observations: This observation is taken within our Milky Way Galaxy and is related to the line-of-sight velocities corresponding to the objects which are being observed. This local observation and detection of redshifts has helped the astronomers to diagnose as well as to measure the characteristics of planetary systems
• Extragalactic Observations: These are the observation which are taken for every distant objects which are corresponding to the Hubble flow of universe. The largest observed redshift till date is cosmic microwave background radiation whose value was about z=1089.

Applications of Redshifts

Redshifts were initially being used to measure the rotation rate of planets, they were used for measuring the velocities of interstellar clouds, they were also used to measure the rotation of galaxies, the temperature of different emitting and absorbing objects were also being measured using Doppler broadening and as diagnostic tool. Redshift measurements are one of the most important spectroscopic measurements which was ever made in astronomy.

Context and Applications:

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for      

• Bachelors in Science (Physics)
• Masters in Science (Physics)