Redshift: The Cosmic Stretch
What is Light and How Do We See It?
To understand redshift, we first need to understand light. Light is a form of energy that travels in waves, similar to waves on the surface of a pond. Two key properties define a light wave:
- Wavelength: The distance between two consecutive peaks (or troughs) of the wave. Think of it as the distance between two consecutive roller coaster hills.
- Frequency: The number of wave peaks that pass a certain point every second. A high-frequency wave has its peaks very close together.
Our eyes see different wavelengths of light as different colors. Visible light is just a small part of a much larger family of waves called the electromagnetic spectrum[1]. The colors of the rainbow—red, orange, yellow, green, blue, indigo, violet—represent light with decreasing wavelengths and increasing frequencies. Red light has the longest wavelength and lowest frequency we can see, while violet light has the shortest wavelength and highest frequency.
| Color | Approximate Wavelength | Approximate Frequency |
|---|---|---|
| Red | 700 nanometers (nm) | 430 terahertz (THz) |
| Green | 550 nm | 545 THz |
| Violet | 400 nm | 750 THz |
The Cosmic Fingerprint: Spectral Lines
When we pass light from a star or galaxy through a prism or a spectroscope, we don't just see a smooth rainbow. We see a spectrum with dark or bright lines at specific colors. These are called spectral lines[2].
Every chemical element, like hydrogen, helium, or oxygen, absorbs or emits light at very specific and unique wavelengths. It's like a unique barcode or fingerprint for that element. For example, hydrogen has a very strong and recognizable spectral line in the red part of the spectrum. When we look at the light from a star, we can identify which elements are present by matching the lines in the star's spectrum to the known fingerprints of the elements.
The Doppler Effect: The Sound of a Passing Ambulance
You have experienced the Doppler Effect many times. Think about a police car or ambulance with its siren on. As it approaches you, the siren's pitch is high. The moment it passes you and moves away, the pitch suddenly drops to a lower tone.
What's happening? As the vehicle moves toward you, the sound waves are compressed. The wavelength gets shorter, and the frequency (which we hear as pitch) gets higher. As the vehicle moves away, the sound waves are stretched out. The wavelength gets longer, and the frequency gets lower.
Light behaves in a very similar way! This is the core idea behind redshift.
- Blueshift: If a star or galaxy is moving toward us, its light waves are compressed. The wavelengths become shorter, shifting the light toward the blue end of the spectrum.
- Redshift: If a star or galaxy is moving away from us, its light waves are stretched. The wavelengths become longer, shifting the light toward the red end of the spectrum.
So, when astronomers say a galaxy is "redshifted," they mean its light has been stretched to longer wavelengths, indicating it is moving away from us.
A Universe on the Move: Hubble's Discovery
In the 1920s, astronomer Edwin Hubble made a revolutionary discovery. He was measuring the redshifts of many galaxies and also estimating their distances. He found a stunning pattern:
The farther away a galaxy is, the greater its redshift.
This means that more distant galaxies are moving away from us at faster speeds. This relationship is now known as Hubble's Law. Imagine dots on an inflating balloon. As you blow up the balloon, every dot moves away from every other dot. The dots that were farther apart to begin with move away from each other faster than the dots that were close together. Our universe is behaving in a similar way—it is expanding!
Cosmological Redshift: The Stretching of Space Itself
While the Doppler Effect explains redshift for objects moving through space, there is a second, more dominant cause for the redshift we see from extremely distant galaxies: the expansion of the universe itself.
Think of the light wave as a traveler on a rubber band. As the light travels through space for billions of years to reach us, the fabric of space itself is stretching. This stretches the light wave along with it, increasing its wavelength and causing the redshift. This is called cosmological redshift. For very distant objects, this is the primary cause of the observed redshift, not just movement through space.
Measuring the Universe's Speed and Age
Redshift is not just a curious observation; it's one of the most powerful tools in astronomy. Astronomers quantify redshift with the letter z.
The formula for redshift is: $z = \frac{\lambda_{observed} - \lambda_{rest}}{\lambda_{rest}}$
Where:
- $\lambda_{rest}$ is the wavelength we measure for an element in a laboratory on Earth.
- $\lambda_{observed}$ is the wavelength we measure for the same element in the light from a distant galaxy.
For example, if we know that a hydrogen line should be at 656.3 nm (its rest wavelength), but we observe it at 1312.6 nm, the redshift is calculated as:
$z = \frac{1312.6 - 656.3}{656.3} = \frac{656.3}{656.3} = 1$
A redshift of z = 1 means the wavelength of the light has doubled. By measuring z, we can determine the galaxy's velocity away from us and, using Hubble's Law, its distance. This allows us to create 3D maps of the universe. Furthermore, by tracing the expansion backwards, we can estimate the age of the universe since the Big Bang, which is currently calculated to be about 13.8 billion years.
A Practical Example: The Andromeda Galaxy
Let's look at a real-world example close to home. The Andromeda Galaxy is our closest major galactic neighbor. When astronomers analyze its light, they find that its spectral lines are shifted toward the blue end of the spectrum. This is a blueshift! This tells us that the Andromeda Galaxy is moving toward our Milky Way galaxy. In fact, it's on a collision course and is expected to merge with our galaxy in about 4 to 5 billion years. This local motion is due to the gravitational pull between the two galaxies, which is stronger than the general expansion of the universe at this small (in cosmic terms) distance.
In contrast, almost every other galaxy we observe shows a redshift, confirming the overall expansion of the universe on large scales.
Common Mistakes and Important Questions
Q: Does redshift mean the galaxy is physically turning the color red?
A: No, this is a common misunderstanding. The galaxy itself is not necessarily red. "Redshift" is a technical term meaning that the entire spectrum of light—including the blue and green parts—has been shifted to longer wavelengths. A blue star in a highly redshifted galaxy would still appear blue relative to the redder parts of its own spectrum, but its light would be redder overall than a similar star nearby.
Q: If the universe is expanding, are we at the center?
A: No. A key and mind-bending concept is that there is no center to the expansion. From the perspective of any galaxy, it would seem that all other galaxies are moving away from it. Just like every dot on the inflating balloon sees all other dots moving away. The expansion is happening everywhere at once.
Q: Can redshift be caused by anything other than the expansion of the universe?
A: Yes, there are two other minor types. Doppler redshift (or blueshift) is from motion through space, like a star orbiting within its galaxy. Gravitational redshift occurs when light loses energy escaping a very strong gravitational field, like that of a black hole. However, for distant galaxies, the cosmological redshift from the expansion of space is by far the most significant.
Footnote
[1] Electromagnetic Spectrum (EM Spectrum): The full range of all types of electromagnetic radiation, from radio waves (longest wavelength, lowest frequency) to gamma rays (shortest wavelength, highest frequency). Visible light is a tiny segment in the middle.
[2] Spectral Lines: Dark (absorption) or bright (emission) lines in a spectrum caused by atoms or molecules absorbing or emitting light at specific, unique wavelengths. They act as a fingerprint to identify chemical elements in astronomical objects.