When we gaze into the night sky, we are not just seeing stars as they are now, we are seeing their light as it was years, centuries, or even billions of years ago. That light has traveled unimaginable distances, weaving its way through the galaxy. Along the way, it can change, not in brightness or shape, but in colour. This change is known as redshift.
What Is Redshift
Light is made up of waves, just like ripples on water. The colour we perceive depends on the wavelength of those waves:
- Short wavelengths = bluer light
- Long wavelengths = redder light
When light stretches to longer wavelengths, it shifts toward the red end of the spectrum. This is called redshift.
Why Does Redshift Happen?
There are three main causes:
- Doppler Effect – Similar to how an ambulance siren sounds lower as it moves away, light from objects moving away from us gets stretched, making it redder.
- Cosmic Expansion – The universe itself is expanding, stretching space and the light traveling through it. This is what we see in distant galaxies.
- Gravitational Redshift – Massive objects like black holes can pull on light, sapping some of its energy and lengthening its wavelength.
Reading the Cosmic Barcode, Spectral Lines
Every chemical element absorbs and emits light at very specific wavelengths, creating patterns known as spectral lines. For example, hydrogen, oxygen, and carbon each leave behind unique fingerprints in the spectrum of a star or galaxy. One of the most important sets is the “Balmer series” of hydrogen, which includes lines like H-alpha, H-beta, and H-gamma. These appear in the visible part of the spectrum and are among the easiest for astronomers to measure in distant galaxies.
These spectral lines are also the key to measuring redshift. If the hydrogen H-alpha line is normally seen at 656 nanometers but is observed in a galaxy at 720 nanometers, scientists can calculate how much the light has been stretched. The amount of shift divided by the original wavelength gives the redshift value z:
\[z = \frac{\lambda_{\text{observed}} - \lambda_{\text{rest}}}{\lambda_{\text{rest}}}\]This dual role, identifying elements and measuring redshift, allows astronomers to understand both the composition of distant objects and their motion or distance in the expanding universe.
Redshift in Our Galaxy
While the most famous examples of redshift involve distant galaxies and the expanding universe, redshift also happens within the Milky Way. Stars on the far side of our galaxy moving away from us, or light passing near massive stars and black holes, can be subtly shifted toward the red.
Astronomers use this effect to:
- Measure the motion of stars
- Detect planets through the wobble of their host star’s light
- Map out our galaxy’s structure and rotation
Why It Matters
Redshift is not just an interesting quirk, it is a cosmic measuring tool. It helps us:
- Measure speed: How fast stars or galaxies are moving away from us
- Estimate distance: More redshift often means farther away
- Understand the universe’s history: Observing redshift lets us rewind the clock and see how things looked billions of years ago
JADES-GS-z14-0
One galaxy stands out as a record breaker, JADES-GS-z14-0. Discovered by the James Webb Space Telescope in 2024 as part of the JADES survey, this galaxy has a redshift of \(z≈14.1793±0.0007\), making it the most distant confirmed galaxy known to date. We see it as it was only about 290 to 300 million years after the Big Bang, when the universe was just 2 percent of its current age.
Despite its youth, JADES-GS-z14-0 is impressively bright and massive, measuring around 1,600 to 1,700 light-years across with a stellar mass of about 500 million Suns. It even contains oxygen, detected by ALMA in the most distant oxygen observation ever made. This means that multiple generations of stars had already lived and died in this galaxy, enriching it with heavier elements far earlier than expected.
A Universe Written in Red
The next time you look at the night sky, remember, those twinkling points are telling a story in colour, one that astronomers have learned to read. Each subtle shift toward red is like a fingerprint, revealing motion, gravity, and the very stretching of space itself.
Light does not just travel, it changes. In those changes, we find the keys to understanding our cosmic home, from the stars in our own galaxy to record-breakers like JADES-GS-z14-0, whose stretched light carries whispers from the dawn of time.