An Alternative Explanation for an Expanding Universe
In the early 20th century, astronomers made an observation that would forever change our understanding of the cosmos. When they looked at distant galaxies, they noticed that the light from these galaxies was consistently shifted toward the red end of the electromagnetic spectrum. This phenomenon, known as redshift, suggested that these galaxies were moving away from us. The farther a galaxy was, the greater its redshift, and the faster it appeared to be receding. This foundational discovery led to the dominant theory of modern cosmology: the idea of an expanding universe that began with a Big Bang.
Yet, in the world of science, a single explanation is rarely accepted without challenge. As the concept of an expanding universe took hold, a competing idea emerged. What if the universe wasn’t expanding at all? What if, instead, light simply lost energy as it traveled across the immense voids of space? This alternative hypothesis became known as “tired light.” It proposed a static universe where the observed redshift wasn’t a sign of cosmic expansion but a symptom of photons growing weary on their epic journeys. This article explores the tired light hypothesis, its proposed mechanisms, the evidence used to test it, and why it’s not supported by the scientific community today.
The Redshift Riddle
To understand tired light, one must first grasp the observation it attempts to explain: cosmological redshift. Light travels in waves, and the color we perceive depends on the length of these waves. Blue light has shorter wavelengths, while red light has longer wavelengths. In the 1910s, astronomer Vesto Slipher began studying the light from what were then called “spiral nebulae.” He used a technique called spectroscopy, which splits light into its constituent colors, revealing a pattern of bright or dark lines. These lines correspond to specific chemical elements and act as a cosmic barcode.
Slipher discovered that the spectral lines from most of these nebulae were shifted toward the red end of the spectrum compared to the same elements here on Earth. This is analogous to the Doppler effect for sound. We’re all familiar with how the pitch of an ambulance siren sounds higher as it approaches and lower as it moves away. The change in pitch is due to the sound waves being compressed or stretched. A similar principle applies to light. A redshift indicates that the source of the light is moving away from the observer.
In the 1920s, astronomer Edwin Hubble, using the powerful Hooker telescope at Mount Wilson Observatory, built upon Slipher’s work. He established that these nebulae were actually entire galaxies, just like our own Milky Way. More importantly, he meticulously measured their distances and their redshifts. He found a clear relationship: the farther away a galaxy is, the greater its redshift. This observation, now known as the Hubble-Lemaître law, became the cornerstone of the expanding universe model. In this model, space itself is stretching, carrying galaxies along with it and stretching the wavelength of the light that travels through it.
The Tired Light Proposition
The idea of an expanding universe, one that had a beginning and was growing larger every moment, was revolutionary. It was also, for some, unsettling. It replaced the classical view of an eternal and static cosmos with a dynamic and evolving one. It was in this environment that the tired light hypothesis was born, proposed in 1929 by Swiss astronomer Fritz Zwicky, among others.
Zwicky questioned the expanding universe interpretation of Hubble’s findings. He suggested an alternative: perhaps the universe is static, and the redshift is caused by photons of light losing energy as they travel through space. A photon’s energy is directly related to its frequency and inversely related to its wavelength. If a photon were to lose energy for any reason, its frequency would decrease, and its wavelength would increase. This would shift its light toward the red end of the spectrum without requiring any motion between the source and the observer.
The appeal of this idea was its simplicity. It explained the redshift-distance relationship in a static universe, avoiding the complexities of a cosmic origin event like the Big Bang. Under this model, the farther away a galaxy is, the longer its light has been traveling to reach us, and the more energy it would have lost along the way, resulting in a greater redshift. It was a neat and tidy explanation that preserved a more traditional view of the cosmos. The central question was what physical mechanism could possibly cause photons to get “tired.”
Several ideas were put forward. One possibility was that photons interact with the sparse matter and dust in the intergalactic medium. Each tiny collision could sap an infinitesimal amount of energy. Over billions of light-years, these tiny losses could accumulate to produce the observed redshift. Another, more exotic proposal was that photons have some unknown intrinsic property that causes them to lose energy over time, a sort of natural decay. This would mean our understanding of the nature of light was incomplete. Zwicky himself speculated that gravitational drag could be responsible, with photons losing energy as they interact with the gravitational fields of all the matter they pass on their journey. For the hypothesis to be viable, one of these mechanisms would have to be real.
Confronting the Evidence
Science advances by testing hypotheses against observation. A good scientific model doesn’t just explain what we already know; it makes specific, testable predictions about things we haven’t yet measured. Over the decades, as astronomical technology improved, astronomers were able to perform several key tests that pitted the expanding universe model against the tired light hypothesis. In every case, the universe’s behavior matched the predictions of expansion and contradicted those of tired light.
Test 1: The Surface Brightness of Galaxies
One of the first major tests involves the surface brightness of distant galaxies – that is, how bright a galaxy appears spread out over its apparent area in the sky. Both models predict that distant galaxies should look dimmer simply because they are farther away. However, they make very different predictions about how their surface brightness should change.
In a static tired light universe, a distant galaxy appears dim because its photons have lost energy (making them redder) and because they have spread out over a larger area on our detector, an effect of simple geometry. The number of photons arriving per second is not affected.
In an expanding universe, there are two additional effects. First, the expansion of space itself stretches the wavelength of the light, which is the cause of the redshift and represents a loss of energy. Second, because space is stretching, the rate at which photons arrive from the distant source is also reduced, an effect known as time dilation. This means that a distant galaxy in an expanding universe should appear significantly less bright on its surface than a similar galaxy at the same distance in a static universe. The predictions are mathematically precise.
When astronomers like Edwin Hubble and Richard Tolman first devised this test in the 1930s, telescopes weren’t powerful enough to perform it conclusively. By the late 20th century, with instruments like the Hubble Space Telescope, it became possible to measure the surface brightness of extremely distant galaxies with high precision. The results were unequivocal. The surface brightness of distant galaxies decreases exactly as predicted by the expanding universe model. This observation is in direct conflict with the predictions of the simplest tired light models.
Test 2: The Stretching of Time
Perhaps the most compelling piece of evidence against tired light comes from cosmic explosions known as Type Ia supernovae. These events occur when a white dwarf star in a binary system accumulates too much matter from its companion and explodes. They are incredibly bright, often outshining their entire host galaxy, which allows them to be seen across billions of light-years. Critically, they are also “standard candles.” They all reach a similar peak brightness and, more importantly, have a predictable light curve – the way their brightness increases and then fades over days and weeks.
The expanding universe model makes a specific prediction about these light curves. If a supernova explodes in a distant galaxy that is receding from us at high speed, then all physical processes in that galaxy – including the supernova explosion – should appear slowed down from our perspective. This is a direct consequence of cosmological expansion, an effect called time dilation. A supernova that takes 30 days to fade should appear to take 60 days to fade if it’s in a galaxy whose redshift indicates space is expanding at half the speed of light between it and us. The duration of the light curve should be stretched by a factor directly related to its redshift.
The tired light hypothesis makes a very different prediction. In a static universe, there is no expansion and no relative motion causing time dilation. Light gets redder as it travels, but the timing of events at the source should be unaffected. The number of photons emitted per second during the supernova explosion would be the same for a distant observer as for a local one. The supernova’s light curve should have the same duration regardless of its distance from us.
In the late 1990s, two independent teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search Team, measured the light curves of dozens of distant Type Ia supernovae. They found that the light curves were stretched – their duration was longer – by exactly the amount predicted by the expanding universe model. This observation of cosmological time dilation provided powerful confirmation of cosmic expansion and was a severe blow to the tired light hypothesis.
Test 3: The Cosmic Microwave Background
In 1965, two radio astronomers at Bell Labs, Arno Penzias and Robert Wilson, discovered a faint, uniform glow of microwave radiation coming from every direction in the sky. This glow, known as the Cosmic Microwave Background (CMB), is hailed as one of the greatest confirmations of the Big Bang theory.
According to the Big Bang model, the early universe was an incredibly hot, dense sea of plasma. As the universe expanded, it cooled. About 380,000 years after the Big Bang, it cooled enough for protons and electrons to combine into neutral hydrogen atoms. At this point, the universe became transparent, and the light that had been trapped within the plasma was free to travel through space. The CMB is this relic light, the afterglow of the universe’s fiery birth, redshifted by billions of years of cosmic expansion from visible light all the way down to the microwave part of the spectrum.
The Big Bang model makes a very specific prediction about the spectrum of this radiation – the intensity of the light at different frequencies. It should have a perfect black-body spectrum, which is the unique spectral signature of an object in thermal equilibrium.
Tired light models struggle to explain the CMB. If the universe is static and infinitely old, where did this uniform, thermal radiation come from? Some proponents suggested it could be the combined light of all ancient stars and galaxies, redshifted over eons. However, for this to work, the starlight would need to be “thermalized” – scattered and absorbed by intergalactic dust until it reached a uniform temperature and a perfect black-body spectrum. The problem is that any process that could do this so perfectly would also have other observable consequences. Specifically, it would severely blur the light from distant galaxies.
Observations from satellites like NASA‘s Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe (WMAP), and the European Space Agency‘s Planck satellite have confirmed that the CMB’s spectrum is the most perfect black-body spectrum ever measured in nature. It’s a stunningly precise match to the Big Bang prediction. At the same time, we see sharp, clear images of the most distant galaxies, showing no evidence of the blurring that would be required for starlight to be thermalized into the CMB in a tired light scenario. The existence, uniformity, and precise black-body spectrum of the CMB are all naturally explained by the hot Big Bang model and present insurmountable problems for the tired light hypothesis.
Summary
The tired light hypothesis was born from a scientifically sound impulse: to question a new and radical theory and explore simpler alternatives. It proposed that the redshift of distant galaxies was not due to an expanding universe but was instead caused by light losing energy on its journey to Earth. For a time, it was a plausible contender, an explanation that fit the initial observation of a redshift-distance relationship within the framework of a static, eternal universe.
Science does not rest on a single observation. It builds a case from multiple, independent lines of evidence. As astronomers developed new ways to probe the distant universe, the tired light hypothesis was put to the test, and it failed repeatedly. Observations of galactic surface brightness, the time dilation of supernova light curves, and the perfect black-body spectrum of the Cosmic Microwave Background all powerfully contradict the predictions of a static universe with tired light. These same observations provide overwhelming support for the standard cosmological model of an expanding universe that began with a Big Bang.
While the idea of tired light persists in some fringe discussions, it has been robustly falsified by decades of astronomical data. It remains an important historical example of the scientific process at work, demonstrating how alternative ideas are proposed, tested, and ultimately discarded when they no longer fit the evidence. The cosmos has spoken, and its message is clear: the redshift we see is the signature of expansion. We live not in a tired, static cosmos, but in a dynamic and evolving one.
