Understanding Redshift in Physics

Our universe is one of the most mysterious and interesting thing existing out there in space. We know that our universe is constantly expanding like a hot air balloon. The universe is expanding and inflating regularly that we can barely notice. This expansion and inflation of the universe imply that the distance between the celestial objects is also increasing over the period of time!! I.e., the distant stars and galaxies are moving further away from the earth. This further results in the stretching of their light as they travel through space towards the earth. This stretching makes visible light look redder in shade, which is familiarly known as the cosmological redshift or redshift. 

Now let us have a look at these interesting concepts of redshift, redshift meaning, and redshift light with fascinating facts and concepts.

Redshift Meaning

The famous astronomer Edwin Hubble used the 2.5m which is about a 100-inch long telescope on Mount Wilson, California, to study observe and study the night sky. He found that some of the nebulae which were found to be quite fuzzy, luminous specks in the dark night sky were actually in fact galaxies, just like our milky way, even though every galaxy will be different and could be of widely varying sizes, each containing billions of stars. 

Very hot celestial objects (objects that are present in the outer atmosphere of the earth or space), such as stars are capable of generating visible light, which may travel a very long way before it strikes something. When we gaze at the stars at night, the light from the stars may have been travelling in a composed manner through space for more than hundreds of years. The light from the star strikes your eye and jiggling electrons in your retina, turns into electricity, which is sent along the optic nerve to your brain and hence we can see the star!! 

For thousands of years, human beings have been aspiring to understand the structure and nature of the Universe and seeking to determine its true extent. But, whereas ancient philosophers have believed that the world consisted of a disk, a ziggurat or a cube surrounded by many celestial oceans or some kind of ether (an organic substance), the turtle holding the universe and many more. Later the advancement of modern astronomy opened their eyes to new frontiers. By the 20th century, scientists and cosmologists have begun to understand just how vast (and maybe even unending) the Universe really is.

The universe is constantly expanding, inflating like a hot air balloon. This implies that the distant stars and galaxies are moving away from the earth. As a result of these transitions, it will stretch its light (light from the celestial bodies) as it travels through space towards us, the further it travels, the more it gets stretched. This stretching will make the objects appear red in colour, the more they travelled away they appear redder, and this effect is known as the redshift. 

Redshift is a key concept for astronomers. The term can be understood literally - the wavelength of the light is stretched, so the light is seen as 'shifted' towards the red part of the spectrum or redshift between the two spectral lines. 

Laboratory experiments that are performed here on the Earth have determined that each element in the periodic table emits photons only at certain wavelengths which are determined by the excitation state of the atoms. These emitted photons are manifest as either emission or absorption lines in the electromagnetic spectrum of an astronomical object, and by measuring the position of these spectral lines, we can determine which elements are present in the object itself or along the line of sight.

However, when astronomers perform this analysis, they note that for most astronomical objects, the observed spectral lines are all shifted to longer wavelengths, which is usually the red region of the spectrum. This is known as cosmological redshift or just redshift. 

The light or the photons emitted from the distant stars and more distant galaxies is not featureless but has distinct spectral features characteristic of the atoms in the gases around the stars. When these spectra are considered and examined, they are appeared to be shifted toward the red end of the electromagnetic spectrum. This shift is apparently a Doppler shift or Doppler effect and indicates that essentially all of the galaxies are moving further away from the earth over the period of time. 

Using the results from the nearest celestial objects, it becomes evident that the more distant galaxies and the stars are moving away from the earth at the highest accelerated rate. This is the kind of result one would expect for an expanding and inflating universe. The red line of the electromagnetic spectrum below is the transition from n=3 to n=2 of hydrogen and it is familiarly known as the H-alpha line seen throughout all the universe.

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Of course, making these measurements is a bit tedious and trickier than just saying that the star looks redder than it should be. Instead, astronomers and astrophysicists make use of markers in the electromagnetic spectrum of starlight. This is the actual study of spectroscopy. If you shine a flashlight beam through a clear prism, as a result of dispersion, we witness a rainbow emerging out the other side. But if you place a clear container filled with highly concentrated hydrogen gas between the flashlight and the clear prism, gaps appear in the smooth rainbow of colours, places where the light literally goes missing.

Redshift Light

When the universe was just a few minutes old, the surviving protons and neutrons recombined to form an atomic nucleus, mainly of what would become hydrogen and helium. The hydrogen and the helium that formed at a very early time in the universe are still charged, so fog remains impossible to see through. At this point, the foggy material is not unlike what we find inside a star, but of course, it fills the entire universe.

After the heavy uncontrollable action of the few minutes of existence, the universe stays much the same for the few hundred thousand years, continuing to expand and cool down, the hot fog becoming steadily thinner, dimmer, and redded as the wavelengths of light are stretched by the expansion of the universe.

After 380,000 years, when the part of the universe that we will eventually observe from the earth has grown to be millions of light-years across the fog finally clears. Due to the presence of electric charges of the electrons and the nuclei cancel each other out, the complete atoms are not charged so that now the photons can travel uninterrupted, which implies that the universe has slightly got transparent.

After this long wait for the fog clearance, what do we get to see or witness? Only fading red wavelengths scattered in all directions, which becomes redder and dimmer as the expansion of space continues to stretch the wavelengths of photons. Finally, light radiation ceases to be visible in all directions. The photons from that last glow have been travelling and stretched into space and even steadily appeared to be redder and these photons are detected now as Cosmic Background radiation and they are still arriving on earth from every direction in the sky.

Redshift is often compared to the high-pitched whine of an ambulance siren coming at you, which drops in pitch as the ambulance moves past you and then further away from you. That variation or change in the sound of an ambulance is due to what’s called the Doppler effect. It’s a good comparison because both sound and light travel in the form of waves, which are affected by their movement through air and space.

Did You Know?

• The earth’s atmosphere has not always been as it is today. If we were able to travel 3.5 billion years (to the time when the earth was about one billion years old) we will not be able to breathe, due to the absence of oxygen and the presence of many toxic gases such as hydrogen, helium, etc…

• The atmosphere around 3.5 billion years ago contained no oxygen. It was mostly made of nitrogen, hydrogen, carbon dioxide and methane that are extremely poisonous gases present in the atmosphere, but the exact composition is still unknown. What is known, however, is the huge volcanic eruptions that occurred around the period of time, releasing extremely hot steam, carbon dioxide, ammonia and hydrogen sulphide into the atmosphere. We know that hydrogen sulphide odour is extremely strong and it is poisonous when encountered or subjected to large amounts.

• Now, the earth is the only planet in our solar system that is capable of holding living organisms. The earth is having an ideal atmosphere with appropriate composition of gases. Today, the atmosphere of the earth is made of approximately 78% of nitrogen, 21% of oxygen and around 0.93% of argon. The remaining 0.07% is mostly carbon dioxide which is approximately 0/04% and the mixture of neon, helium, methane, krypton and hydrogen.