How Space Changes Light: The Forces That Shape a Photon’s Journey

Key Takeaways

• Dust and gas scatter and absorb light, making distant stars appear redder than they are
• Dark matter bends light paths through gravity without absorbing or scattering photons at all
• Cosmic expansion stretches light wavelengths, reducing photon energy over vast distances

A Surprisingly Hostile Journey

Light leaving a distant star faces a surprisingly hostile journey. Space isn’t the clean, empty void it appears to be from a dark hillside at night, and the photons that arrive at Earth’s telescopes have often been scattered, stretched, absorbed, and bent along the way. Understanding what happens to light in transit changes everything about how astronomers read the sky.

What Light Actually Is

A photon is a packet of electromagnetic energy. Its energy is tied directly to its wavelength, with shorter wavelengths carrying more energy than longer ones. Violet light carries more energy than red light. X-rays carry far more than either. When something in space interacts with a photon, the photon is either absorbed, scattered, or has its wavelength physically stretched by the expansion of space itself. These are distinct processes with different causes and different consequences for what astronomers actually observe.

It’s worth being precise here: a photon traveling through a vacuum doesn’t lose energy to friction or resistance because there’s no medium to resist it. What changes its energy is specific physical interactions, and space contains enough matter and geometry to produce several of them.

Interstellar Dust and the Reddening of Starlight

Interstellar dust isn’t like household dust. It’s composed of tiny solid grains, typically between 0.1 and 1 micrometers across, made of silicates, carbon compounds, and ice. These grains are scattered throughout the Milky Way and other galaxies, concentrated especially in the spiral arms and in dense molecular clouds where new stars are forming.

When light passes through a dusty region, shorter wavelengths, particularly blue and ultraviolet light, scatter more strongly than red and infrared wavelengths. This effect is called interstellar extinction . The photons aren’t annihilated. Some are absorbed by the grains and re-emitted as heat at much longer wavelengths, while others are simply redirected away from the original line of sight. What reaches an observer’s telescope is disproportionately red, making the source star appear both dimmer and redder than it actually is.

This effect has real consequences for observation. The Orion Nebula , one of the most studied star-forming regions in the sky, is visible in optical light because it sits relatively close to Earth at roughly 1,344 light-years away and the dust between us and it is manageable. Regions like the galactic center, about 26,000 light-years away and buried behind thick dust lanes, are almost entirely invisible in optical wavelengths. The James Webb Space Telescope was designed specifically to observe in infrared wavelengths that dust can’t block as effectively, allowing it to peer into star nurseries and distant galaxies that earlier optical telescopes simply couldn’t access.

Astronomers correct for dust reddening by comparing a star’s observed colors against what its spectrum should look like based on its temperature, then calculating how much reddening has occurred. Without this correction, distance estimates and luminosity measurements would be systematically wrong.

Gas, Spectral Fingerprints, and What Absorption Lines Reveal

Gas between stars, known as the interstellar medium, interacts with light in a more selective way than dust. Rather than scattering all short wavelengths broadly, gas atoms absorb photons at very specific wavelengths that correspond to the energy jumps between electron shells within those atoms. When those electrons drop back to lower energy states, they re-emit photons, but in random directions rather than forward along the original beam.

The result is a spectrum with dark lines at specific wavelengths. These absorption lines are extraordinarily useful. Hydrogen, the most abundant element in the universe, absorbs light at a set of wavelengths called the Lyman series in the ultraviolet and the Balmer series in the visible range. Sodium absorbs at 589 nanometers, producing a pair of dark lines that appear consistently wherever sodium gas is present between a star and an observer.

In 1814, Joseph von Fraunhofer catalogued hundreds of these dark lines in the Sun’s spectrum without fully understanding them. Today, astronomers use the same principle to identify elements in stellar atmospheres, interstellar clouds, and distant galaxies. When a gas cloud is moving toward or away from Earth, those lines shift slightly in wavelength due to the Doppler effect, revealing the velocity of the gas. It’s a remarkable amount of information extracted purely from missing light.

Dark Matter’s Invisible Hand

Dark matter doesn’t absorb photons. It doesn’t scatter them. Light passes straight through it without any electromagnetic interaction whatsoever, which is precisely why dark matter is dark. Despite that, it has a measurable effect on light through gravity.

Mass curves spacetime. When a photon travels near a massive concentration of matter, including matter that emits no light at all, its path bends. This is gravitational lensing , predicted by Albert Einstein’s general theory of relativity and first confirmed during the solar eclipse of May 29, 1919. Modern telescopes observe it constantly. Clusters of galaxies act as enormous lenses, magnifying and distorting the images of galaxies behind them. The arcs and rings visible around galaxy clusters in Hubble Space Telescope images are background galaxies whose light has been bent and stretched by the foreground cluster’s gravity.

The Bullet Cluster , observed in 2006 using X-ray data from the Chandra observatory combined with gravitational lensing maps, provided some of the clearest evidence for dark matter’s existence. Two galaxy clusters had passed through each other, and the hot gas in each cluster, visible in X-rays, had slowed and lagged behind due to electromagnetic drag. The gravitational lensing signal, indicating where the mass was concentrated, showed the mass had passed straight through without slowing, tracking the galaxies rather than the gas. Something massive that doesn’t interact electromagnetically had moved differently from the ordinary matter. That something is almost certainly dark matter, though what it actually consists of at the particle level remains genuinely unsettled.

Cosmological Redshift and the Expanding Universe

The most sweeping effect on traveling light comes from the universe’s expansion itself. Space isn’t static. The distance between galaxy clusters increases over time, not because galaxies are flying through space away from each other, but because the space between them is growing. A photon traveling across that expanding space has its wavelength stretched along with the space it occupies.

This stretching is called cosmological redshift . A photon emitted as ultraviolet radiation by a galaxy 13 billion years ago might arrive at Earth today as infrared or even microwave radiation. The energy of that photon has decreased substantially, not because it lost energy to resistance, but because its wavelength grew. The cosmic microwave background radiation, the afterglow of the early universe detected everywhere in the sky, was originally emitted as high-energy radiation when the universe was about 380,000 years old. Today it peaks at a temperature of 2.725 Kelvin, stretched by a factor of roughly 1,100 from its original wavelength.

Redshift is also how astronomers measure the distances to galaxies too far away for any other technique. When a galaxy’s spectrum shows familiar absorption or emission lines shifted systematically toward longer wavelengths, the amount of that shift tells you how much the universe has expanded since the light was emitted. Edwin Hubble’s 1929 observation that more distant galaxies are more redshifted than closer ones was the first direct evidence that the universe is expanding.

Dark Energy and Accelerating Expansion

Dark energy doesn’t interact with light directly any more than dark matter does. Its role is to drive the accelerating expansion of the universe, and that acceleration has a cumulative effect on photons traveling across cosmic distances. The expansion of space isn’t constant. Since roughly 5 billion years ago, it’s been speeding up, which means photons emitted from very distant sources experience more stretching per unit of distance traveled than photons from nearby sources. This makes distant supernovae appear dimmer than they would in a non-accelerating universe, which is exactly what two independent research teams found in 1998 when they compared Type Ia supernovae at different distances. That discovery, which led to the 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess, confirmed that dark energy is real and that it’s winning its competition against gravity on the largest scales.

What dark energy actually is remains the most significant open question in cosmology. The simplest explanation is a cosmological constant, a fixed energy density inherent to space itself, but that explanation raises its own deep theoretical problems about why the constant has the value it does. It’s the kind of question where the honest answer is that nobody knows, and the evidence currently available isn’t enough to choose between competing models.

How Astronomers Untangle All of This

Every observation of a distant object arrives as a mix of these effects layered on top of each other. A galaxy’s spectrum might show absorption lines shifted by cosmological redshift, dimmed by dust reddening in both the host galaxy and the Milky Way, and with its position on the sky distorted by gravitational lensing from an intervening mass. Separating these effects requires knowing something about each one independently.

NASA and the European Space Agency operate observatories specifically designed to collect data across the full electromagnetic spectrum, from radio waves to gamma rays, because different wavelengths carry different information and are affected differently by the intervening medium. The combined picture from multiple wavelengths is far more reliable than any single view.

Light as a Record of What It Passed Through

There’s an underappreciated point buried in all of this. Every photon that arrives at a telescope carries a record of everything it encountered on the way. The dust that reddened it, the gas that punched absorption lines into its spectrum, the gravitational wells that bent its path, and the expanding space that stretched its wavelength are all readable in the data, if you know what to look for.

This makes light not just a signal but a form of evidence about the universe between the source and the observer. Astronomers studying the early universe are, in a real sense, doing archaeology with photons. The light from a quasar 12 billion light-years away passed through billions of years of cosmic evolution before arriving at the detector, and the marks left on it by hydrogen gas clouds, dust, and gravitational lensing along the way contain information about the structure of the universe at multiple epochs.

Summary

The forces acting on light in space range from the practical, like dust scattering that requires infrared telescopes to overcome, to the cosmic, like the stretching of wavelengths by expanding space. Gas leaves chemical fingerprints. Dark matter leaves gravitational ones. Dark energy makes the universe’s expansion accelerate, adding more stretch to every photon’s journey. None of these effects destroys information. They transform and encode it, turning every photon into a record of its own history and of the space it crossed.

What makes this genuinely interesting is that the universe is legible. Every distortion in a photon’s journey can, in principle, be read and reversed to reveal what the source actually looked like. The skill is in learning the language.

Appendix: Top 10 Questions Answered in This Article

Does light lose energy as it travels through empty space?

Light traveling through a true vacuum doesn’t lose energy to friction or resistance. Energy changes come from specific physical interactions such as absorption by gas atoms, scattering by dust, or the stretching of wavelengths by the expansion of space itself.

What does interstellar dust do to light?

Interstellar dust grains scatter and absorb shorter wavelengths of light more strongly than longer ones, making distant stars appear both dimmer and redder than they actually are. Photons absorbed by dust grains are re-emitted as heat at infrared wavelengths rather than continuing in their original direction.

Why are some parts of the sky invisible in optical light?

Dense concentrations of interstellar dust, particularly toward the galactic center, block optical light so effectively that those regions can only be studied using infrared or radio wavelengths. Telescopes like the James Webb Space Telescope were built to observe in infrared specifically to see through these dusty regions.

What are absorption lines and why do they matter?

Absorption lines are dark gaps in a spectrum caused by gas atoms absorbing photons at very specific wavelengths. They act as chemical fingerprints, revealing which elements are present along the line of sight between a star and the observer, and their slight shifts in position reveal the velocity of the intervening gas.

Does dark matter affect light?

Dark matter doesn’t absorb or scatter light, but it does bend light’s path through gravity, a phenomenon called gravitational lensing. Observations of the Bullet Cluster in 2006 confirmed that the gravitational lensing signal from a galaxy cluster collision tracked the dark matter component, not the visible gas.

What is cosmological redshift?

Cosmological redshift occurs when the wavelength of a photon is stretched by the expansion of space during its journey across the universe. The longer the photon travels, the more space expands around it, and the more its wavelength increases and its energy decreases by the time it reaches an observer.

How was the expansion of the universe discovered?

Edwin Hubble’s 1929 observations showed that more distant galaxies have larger redshifts than closer ones, meaning they are receding faster. This relationship, now called Hubble’s Law, was the first observational evidence that the universe is expanding rather than static.

What is dark energy and how does it affect light?

Dark energy is a form of energy that drives the accelerating expansion of the universe, a property confirmed by observations of distant Type Ia supernovae in 1998. It affects light indirectly by increasing the rate at which space expands, causing photons to be stretched more per unit of distance traveled compared to a non-accelerating universe.

How do astronomers correct for dust reddening when measuring stars?

Astronomers compare a star’s observed colors to what its spectrum should look like given its temperature class, then calculate how much extra reddening has occurred due to intervening dust. This correction, called extinction correction, is applied to distance and luminosity measurements to recover accurate values.

What is gravitational lensing and how is it observed?

Gravitational lensing is the bending of light around massive objects such as galaxy clusters, caused by the curvature of spacetime described by general relativity. Hubble Space Telescope images of galaxy clusters regularly show arcs and rings that are background galaxies whose light has been distorted by the foreground cluster’s gravity.