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  • Writer's pictureolivershearman

The coming and going of light: Redshift and Blueshift

In the vast expanse of the cosmos, light does more than just illuminate; it tells stories of distant worlds, ancient stars, and the very expansion of the universe itself. Two phenomena at the heart of these cosmic tales are redshift and blueshift, concepts that reveal the motion of celestial bodies and the dynamic fabric of space-time. These phenomena are intertwined with the Doppler effect, a principle most are familiar with through the changing pitch of a passing siren. In the realm of astronomy, however, redshift and blueshift serve as powerful tools to decipher the universe's secrets.


This image shows the core principle idea behind Doppler redshift, the most commonly discussed form of redshift. Image credit: link here


At its core, redshift occurs when the light from an object in space stretches to longer wavelengths, which we perceive as moving toward the red end of the spectrum. Conversely, blueshift happens when the light compresses into shorter wavelengths, shifting toward the blue end. These shifts are not about the actual color change of the celestial objects but rather a metaphorical representation of their motion relative to us.


To grasp redshift and blueshift, one must first understand the Doppler effect. Imagine the sound of an ambulance siren: as it approaches, the sound waves compress, increasing the pitch, and as it moves away, the waves stretch, decreasing the pitch. Light behaves similarly, with its "pitch" (frequency) increasing (blueshift) as objects come closer and decreasing (redshift) as they recede from us.


Edwin Hubble's observations in the 1920s revolutionized our understanding of the universe. By studying distant galaxies, Hubble discovered that their light was redshifted, indicating they were moving away from us. More importantly, the farther away a galaxy was, the more redshifted its light appeared. This observation was pivotal in concluding that the universe is expanding, a cornerstone of the Big Bang theory.


Imagine observing a distant galaxy. If its light shifts to the red end of the spectrum, it's moving away from Earth. This redshift has been observed in nearly all distant galaxies, providing direct evidence of the universe's expansion. On a smaller scale, blueshift occurs in objects moving toward us. For instance, the Andromeda Galaxy exhibits a blueshift, indicating it's on a collision course with the Milky Way, set to merge billions of years from now.


We can even use redshift and blueshift to tell if a planet is near a star and pulling it back and forth which causes blueshift and redshift from our perspective. That is shown in an artists impression below.



Image credit: ESO - link here


Redshift manifests in several forms, each revealing different aspects of the cosmos:

1. Cosmological Redshift: This type is due to the expansion of the universe. As space itself expands, it stretches the light traveling through it, increasing the wavelength.

2. Doppler Redshift: Caused by the relative motion of an object away from the observer, this type mirrors the Doppler effect's principles.

3. Gravitational Redshift: A prediction of Einstein's general relativity, gravitational redshift occurs when light moves away from a massive object, losing energy and shifting to longer wavelengths.


Redshift and blueshift do more than just track the movement of celestial bodies; they are essential in measuring the universe's scale, understanding its structure, and deciphering the laws that govern its dynamics. By analyzing these shifts, astronomers can determine the speed and direction of an object's motion, estimate distances across vast cosmic scales, and explore the universe's accelerating expansion.


Cosmological redshift can be shown in concept by the image below regarding the expansion of the universe also causing the lengthening of the wavelengths of light travelling through space.



Image Credit: Thoughtco. - link here


Finally of the three, gravitational redshift represents when light needs to 'climb out' of a massive bodies gravity well and that requires energy, since redder light is lower energy, the wavelength will 'use energy' and become redder. This is visualized below.



Image credit - Vortex youtube video - link here


Redshift and blueshift represent the universe's rhythm, a cosmic dance of light that narrates the ongoing expansion of space, the motion of galaxies, and the dynamics of celestial objects. Through these phenomena, we gain not only insights into the universe's vastness but also a profound understanding of the fundamental forces that shape our reality. As we continue to explore the cosmos, redshift and blueshift will remain invaluable in our quest to unravel the mysteries of the universe, reminding us of the ever-changing, dynamic nature of the cosmos.


If you would like a teaching resource that can help with resource, then please check out my online and offline versions of the relevant science article on this topic.


Thanks for reading

Cheers and stay curious

Oliver - The Teaching Astrophysicist

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