The idea of an expanding universe has captured the imagination of both scientists and the general public. The first clue to this cosmic expansion came from observing light from distant galaxies, which showed a phenomenon called redshift. Let’s explore what redshift is, its different types, and what it tells us about the universe.
When we look at the spectrum of light from the sun, we see dark lines that match specific wavelengths. Similarly, when we examine the spectra from faraway galaxies, these lines are shifted towards longer wavelengths, or the red end of the spectrum. This shift is known as redshift. The main reason for this is that as light travels through expanding space, the photons are “stretched,” leading to longer wavelengths. This is called cosmological redshift.
Physicists identify three main types of redshift:
Although these types seem different and are described by different equations, they share a common principle.
An important experiment by Pound and Rebka in 1959 involved sending gamma rays up and down a 22-meter tower at Harvard. They found that photons detected at the top of the tower were redshifted compared to the source, as predicted by General Relativity. This redshift happens continuously as the photon climbs, losing energy with each millimeter.
According to Einstein’s equivalence principle, being at rest on Earth is the same as being in a rocket accelerating in space. So, if blue photons are emitted from the back of a rocket, they will appear redshifted when they reach the front due to the rocket’s acceleration.
In the case of the accelerating rocket, observers inside will see the photon as green, while stationary observers outside will still see it as blue. This difference occurs because the photon’s wavelength and energy depend on the observer’s frame of reference. As the rocket accelerates, the photon appears redshifted to those inside, showing the Doppler effect.
To understand cosmological redshift, we need to think on a cosmic scale where galaxies act like molecules in a fluid. At this level, the universe looks the same in every direction, or homogeneous and isotropic. As this cosmic fluid expands, the density decreases over time, leading to the redshift of light from distant galaxies.
When two co-moving observers exchange a photon, its wavelength stretches due to the universe’s expansion during its journey. This can also be seen as a series of Doppler shifts, where each observer measures a slightly longer wavelength than the previous one, attributing the shift to their relative motion.
The key point is that redshift is not an intrinsic property of photons but depends on the circumstances of the observers at the points of emission and absorption. Thus, the three types of redshift can be unified under a single mathematical framework, despite appearing different based on the observer’s frame of reference.
A common question is: do we expand with the universe? The answer is no. On the scale of everyday life, the universe is not homogeneous or isotropic. Matter is concentrated in objects like planets and stars, and local spacetime curvature is dominated by these masses.
Even in deep space, where one might expect to be unaffected by local gravitational forces, the electromagnetic forces holding our bodies together prevent any expansion. Only under extraordinary circumstances, such as the hypothetical scenario where electromagnetic forces are turned off, would one experience expansion due to the influence of dark energy.
In summary, while redshift shows that the universe is expanding, it does not mean that everything within it, including atoms, molecules, stars, galaxies, and even ourselves, is stretching apart. Understanding redshift helps clarify the dynamics of our universe without leading to misconceptions about the nature of space and matter.
Explore an online simulation that demonstrates redshift. Adjust parameters like the speed of a galaxy or the gravitational field strength to see how the redshift changes. Take notes on how each type of redshift (Doppler, gravitational, cosmological) affects the light spectrum. Discuss your findings with classmates.
Using the formula for redshift, $z = frac{lambda_{text{observed}} – lambda_{text{emitted}}}{lambda_{text{emitted}}}$, calculate the redshift for various galaxies given their observed and emitted wavelengths. Present your calculations and explain what the redshift tells you about the motion and distance of these galaxies.
Participate in a debate on the implications of the expanding universe. Divide into groups to argue different perspectives, such as the impact on future space exploration or the philosophical implications of an ever-expanding cosmos. Use evidence from the article to support your arguments.
Design a simple experiment to simulate gravitational redshift using available classroom materials. Consider how you might use light sources and detectors to mimic the conditions described in the Pound and Rebka experiment. Present your experimental setup and predicted outcomes to the class.
Create a visual or multimedia project that illustrates the concept of the expanding universe and redshift. Use drawings, animations, or videos to show how light from distant galaxies shifts over time. Share your project with the class and explain the science behind your visualization.
Redshift – Redshift is the phenomenon where the wavelength of light or other electromagnetic radiation from an object is increased, or shifted to the red end of the spectrum, typically due to the object moving away from the observer. – Example sentence: The redshift observed in the light from distant galaxies provides evidence for the expansion of the universe.
Universe – The universe is the totality of all space, time, matter, and energy that exists, including galaxies, stars, and all forms of matter and energy. – Example sentence: The study of the universe’s origin and evolution is a central focus of cosmology.
Photons – Photons are elementary particles that are the quantum of the electromagnetic field, including electromagnetic radiation such as light, and are the force carrier for the electromagnetic force. – Example sentence: When electrons transition between energy levels in an atom, they emit or absorb photons.
Wavelengths – Wavelengths are the distances between successive crests of a wave, especially points in a sound wave or electromagnetic wave. – Example sentence: Different wavelengths of light are perceived as different colors by the human eye.
Gravitational – Gravitational refers to the force of attraction between all masses in the universe, particularly the attraction of the earth’s mass for bodies near its surface. – Example sentence: The gravitational pull of the moon causes the ocean tides on Earth.
Cosmological – Cosmological pertains to the science of the origin and development of the universe, including the study of its large-scale structures and dynamics. – Example sentence: The cosmological principle assumes that the universe is homogeneous and isotropic on a large scale.
Doppler – The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source. – Example sentence: Astronomers use the Doppler shift to determine whether a star or galaxy is moving toward or away from Earth.
Expansion – Expansion in cosmology refers to the increase in distance between any two given gravitationally unbound parts of the observable universe with time. – Example sentence: The expansion of the universe was first observed by Edwin Hubble through the redshift of distant galaxies.
Galaxies – Galaxies are massive systems consisting of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. – Example sentence: The Milky Way is the galaxy that contains our solar system.
Energy – Energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object, and is conserved in isolated systems. – Example sentence: According to Einstein’s equation $E=mc^2$, energy and mass are interchangeable.
