In the second article of the series (EM Spectrum and Astrophysics), we saw how the spectrum is the most important tool for an Astrophysicist to decode the Universe. There is a lot of information we can achieve from the spectrum of a celestial body. For example, if we study the spectrum of a star, we can gain the knowledge of its temperature, surface gravity, elemental abundance, density, evolutionary stage and much more. Spectroscopic techniques also allow us to study the galactic motion. Today, in the sixth article of our series, let us understand the three types of redshifts and their importance in Astrophysics.
Wavelength And Frequency
Many of us are familiar with the concept of Electromagnetic (EM) spectrum. Consider any source of light. It can be the Sun or you study lamp. That light is of specific color. With every color of light, we associate three terminologies: Wavelength, Frequency and Energy. First let us understand what is wavelength and frequency. Consider light as a wave shown below.
The regions above the dashed line are called crests and those below the dashed line are called troughs. We define the horizontal distance between two consecutive crests or troughs as the wavelength. The maximum vertical distance corresponding to a crest or a trough is called the amplitude of the wave.
A crest and the consecutive trough comprise a single wave-cycle. Frequency of a wave is defined as the number of cycles that a wave may traverse in one second. For instance, if we see the pictorial representations of two different waves (one at the top, followed by one waveforms at the bottom) in the above figure, we may infer the following:
- The first wave has the highest frequency as there is a maximum number of wave cycles that cross a particular spatial point in one sec.
- The second wave has the maximum wavelength, as there is a maximum distance between two consecutive troughs or crests of this wave.
- The more is the wavelength, the lesser is the frequency of that particular wave, i.e., wavelength and frequency follow an inverse relationship.
We shall deal with the visible part of the EM spectrum. The wavelength of the visible light lies between 400 nm (1 nm = 10^-9 m) to 800 nm. At 400 nm lies the violet part of VIBGYOR and at 800 nm lies the red part.
What Is Redshift?
Suppose that a source emits light of a particular color and at particular wavelength (such a source of light is monochromatic source). Now it is not always necessary that the observed wavelength of light is exactly equal to the emitted wavelength. If the observed wavelength is more, then the phenomenon is known as redshift. Else, it is known as blueshift.
As shown in the image above, the middle spectrum is the one that is emitted by the source originally. If we observe the dark absorption lines in the upper spectrum, we can see that the corresponding lines are shifted to the red end. They are shifted to the blue end in the lower spectrum and hence the name blueshift. Even if it doesn’t correspond to visible light, an increase/decrease in wavelength is always known as redshift/blueshift.
Redshift and blueshift is caused by various astrophysical phenomenon. In Astrophysics, redshift is denoted by a dimensionless quantity z. The general expression is: 1+z = Observed wavelength/Actual wavelength. A positive value of z corresponds to redshift and negative value corresponds to blueshift.
Interpreting The Redshift
You must be thinking what is the standard reference here? With what spectrum are we comparing to calculate the redshift? Well, it is indeed a good question. Every element has its own signature spectrum. The most common element in space is hydrogen. So if we have the spectrum of hydrogen from a distant galaxy, and if the lines of that spectrum overlap the one that we have in our laboratory, then there is no redshift. But if the lines are at a fixed distance towards the red end, then it is redshifted and the source is moving away from us.
Hydrogen is not the only reference spectral line. Spectral lines of other elements such as nitrogen, oxygen, calcium, magnesium etc can also be used. We know the wavelengths of the spectral lines of these elements in lab. This is the beauty of astrophysics. So many other fields of physics come together in astrophysics to decode the universe.
Let us learn about the three types of redshifts and their importance in Astrophysics.
Types of Redshifts
1. Relativistic Redshift
We all are familiar with the Doppler effect in sound waves. If a source of sound is approaching us, its frequency will increase and vice versa. The Doppler effect in astrophysics is similar but here it corresponds to light. Everything in the Universe is in relative motion. Stars and galaxies are moving with respect to each other. If the spectrum of a star/galaxy shows redshift, it means that it is moving away from us.
Using the formula for relativistic Doppler effect, we can even determine the velocity of the star with which it is moving away from us. When we study the spectra of our galactic neighbor, the Andromeda Galaxy, we find that it is blueshifted. It is approaching the Milky Way at 140 Km/hr. In fact, the two galaxies will merge in another 5 billion years.
2. Gravitational Redshift
This type of redshift is a consequence of general theory of relativity. According to gravitational redshift, when photons travel from lower gravitational potential to higher, they lose energy. So suppose a star emits light of particular wavelength from its surface. If we wish to study the spectrum of that star away from its surface, it will be redshifted. The energy of the photons decreases (and hence the wavelength decreases) while escaping the gravitational field of the star.
Didn’t understand? Here is a simple analogy: It is like a baby getting tired while climbing a flight of stairs. Remember, if the energy of photons decreases, its frequency also decreases while the wavelength increases.
The amount of gravitational redshift depends on the density of the object. So the compact objects such as white dwarfs and neutron stars show more gravitational redshift than normal stars such as the Sun. Black holes have infinite gravitational redshift. This type of redshift is the proof that photons have mass: not the rest mass, but gravitational mass. Also this is one of the classical experimental proof of Einstein’s general relativity.
3. Cosmological Redshift
The cosmological redshift is an outcome of the fact that the space is expanding. In 1920s, cosmologist Edwin Hubble found that farther a galaxy is in deep space, faster it is moving away from us. This is Hubble’s law. This observational law is the proof that the Universe is expanding. The space itself is expanding. In fact, it was the spectrum of these far away galaxies that gave us this clue. The spectrum was highly redshifted.
However, it should be noted that there is a distinction between the redshift due to local Doppler effect and the cosmological shift. We cannot attribute the cosmological redshift to the relative velocity between two galaxies. The photons redshift because of the global feature of the space-time metric through which they are traveling. Because of this expansion, two remote galaxies can be receding away from each other at the speed greater than that of light. It, however, does not mean that special relativity is violated.
I hope this article has made clear the three different types of redshifts. Redshift is a very important parameter for an astrophysicist, especially in studying the extragalactic astronomy. While I was at Indian Institute of Astrophysics, I was working on the Arp catalog of peculiar galaxies. That was the first time I realized the importance of redshift. For many galaxies, redshift was the only way to find the distance to the galaxy. Now, I am working on interacting galaxies and again this parameter is a key in my research project.
With today’s article, we have begun astrophysics. So far, we had discussed some crucial aspects of astronomy. The next articles of the series will be very detailed and specific. So stay tuned. There will be something for everyone in ahead. If you want to ask any questions, feel free to contact me. Your feedback is very important.