Do Photons Weaken When Passing Through Cosmic Voids?

Short answer

• A "void" is just a region of the universe with unusually low matter density — not a vacuum.
• Photons traveling through this sparse environment neither weaken from "having nothing to be absorbed by" nor slow down from "empty space."
• In fact, photons encounter *less* absorption/scattering in a void than in a dense "wall" (clusters, filaments). Strictly speaking, the claim "photons weaken in voids" is itself incorrect.
• If someone tells you "photons weaken in voids because matter is sparse," it's almost certainly a misunderstanding — either conflating "cosmological redshift from expansion" with a void-specific effect, or mistaking gravitational effects (ISW/Rees-Sciama) for "weakening."

Clearing up two intuitive misconceptions

Misconception 1: "Sparser means more absorption, so light weakens"

• The intuition goes: "In a void, photons fly along, get 'rubbed' by thinly scattered gas, and gradually lose energy."
• Reality: gas density in voids is far below the cosmic average, so the probability of scattering/absorption is *lower*. Voids are more transparent to visible light than walls, not less.

Misconception 2: "Voids are vacuum — do photons get 'tired' in vacuum?"

• Photons need no medium to propagate; they don't slow down or weaken from "having nothing to push them along."
• Light speed in local vacuum is always c; photons don't "spend energy" to keep traveling.
• What genuinely reduces photon energy over long journeys is cosmological redshift from universal expansion — not the sparseness of voids.

The real cause of photon energy loss: cosmic expansion

As the universe expands, space itself stretches, lengthening light's wavelength → lowering frequency → reducing per-photon energy (E = hν). This happens everywhere in the universe, regardless of whether the path passes through a void or a cluster. It depends only on how long the light has traveled and how much the universe expanded in that time.

What actually happens when photons cross a void

Voids do have gravitational effects, but they are small energy/temperature perturbations, not continuous weakening.

1) Linear ISW effect (Integrated Sachs-Wolfe)

In a dark-energy-dominated, accelerating universe, large-scale gravitational potential hills (voids) and wells (superclusters) gradually become shallower. A photon climbing a potential hill loses energy (redshifts) and regains it coming down — but because the hill is shallowing, it exits with slightly less energy than it entered (slightly colder). This leaves a "cold spot" imprint in the CMB along void directions, at only a few microkelvin.

2) Rees-Sciama effect (nonlinear ISW)

On small, strongly nonlinear scales (e.g., near void centers), potential evolution is more complex than the linear approximation. Simulations show the RS effect enhances the cooling at void centers and can even produce a small heating ring at the edges — a "cold center, warm ring" pattern. The magnitude remains microkelvin-scale.

3) Shapiro delay and secondary effects

Gravitational time delay slightly alters photon travel times. For large voids/superclusters, the redshift and time-delay parts largely cancel, leaving only a small residual — nothing like "photon energy being drained along the way."

Why do people claim "voids dim light"?

Several contexts get confused:

1. CMB cold spot: An unusually cold spot in the microwave background (seen by WMAP/Planck) was once attributed to a huge void along the line of sight. The slight cooling comes from ISW/RS effects, not absorption inside the void. 2. TeV gamma rays and the extragalactic background light (EBL): Very high energy gamma photons are absorbed via pair production (γ+γ → e⁺+e⁻) on the EBL. Since the EBL is weaker in voids, TeV gamma rays suffer slightly less absorption along void directions — they appear *brighter/harder*, not dimmer. Some unusually hard TeV spectra may be explained this way. This concerns gamma rays only, not visible light. 3. Popular-science "light dims as it travels": This mixes geometric dilution (inverse-square spreading), cosmological redshift (lower per-photon energy), and dust/gas absorption inside galaxies and clusters — all universal effects, not void-specific.

What does low density actually do?

• Visible/microwave light: sparser matter → less absorption/scattering → voids are more transparent than walls.
• Very high energy gamma rays: lower EBL in voids → slightly less absorption → void directions appear slightly brighter.
• Gravity (ISW/RS): the shallowing potential of underdense regions causes a small one-time energy adjustment, not continuous decay.
• Cosmic expansion: lowers photon energy universally; almost unrelated to whether a void is on the path.

Conclusion

"Photons weaken in giant voids because matter is sparse" does not hold up physically. It likely stems from misreading cosmic expansion, gravitational potential effects (ISW/RS), or EBL absorption of high-energy gamma rays. From standard cosmology: absorption/scattering in voids is *less* than in dense structures; ISW/RS is a tiny one-off perturbation; and the real cause of long-distance energy loss is universal expansion, essentially independent of voids.