A redshifted photon reaches its receiver with less energy than it had at emission, when those energies are measured by observers following the cosmic expansion. In general relativity, that difference does not have to arrive in another object or turn into dark energy. An expanding universe generally lacks the single, conserved total-energy account that works so well for an isolated experiment in an unchanging background.
Local energy and momentum still obey precise conservation laws. The difficulty comes from comparing measurements made far apart, at different times, in an evolving spacetime—not from a detector finding that energy has disappeared inside it.
What changes between emission and detection
For light, energy per photon is inversely proportional to wavelength:
E = hc / λ
Here, h is Planck’s constant, c is the speed of light, and λ is wavelength. A longer wavelength means a lower photon energy. The Chandra science team’s redshift explanation describes that relationship in the context of cosmic expansion.
Consider a calculated example, not an observed galaxy. Suppose a spectral feature was emitted at 500 nanometers and arrives at 1,000 nanometers. Using hc ≈ 1,240 electronvolt-nanometers gives about 2.48 electronvolts at emission and 1.24 electronvolts at reception. Doubling the wavelength halves the energy per photon.
Astronomers express the wavelength change as redshift, z:
1 + z = observed wavelength / emitted wavelength
In this example, z = 1. Ned Wright’s cosmology tutorial gives this definition. It does not mean the light became half as fast: a nearby observer still measures light traveling at c in a vacuum.
A detector that absorbs the arriving photon responds to its arriving energy. It does not receive 2.48 electronvolts and secretly discard half. The comparison is between the emission and reception measurements.
Energy belongs to a measurement frame
A photon does not carry an observer-independent energy label. Even in special relativity, observers moving relative to one another can measure different frequencies for the same light. A Doppler shift does not require the photon to break into smaller photons along the way.
Cosmological redshift adds an evolving spacetime to the problem. In the usual large-scale model, freely traveling light has its wavelength increased in proportion to the universe’s scale factor between emission and reception. The scale factor describes changes in separation among observers participating in the smooth expansion.
The relation concerns that specified family of observers. It is not a claim that every observer, moving in any direction, must assign the same energy. Nor does it require galaxies, atoms, or measuring instruments to stretch along with the cosmic distances. The distinction between local measurements and cosmological comparisons also underlies why galaxies can recede faster than light.
Why the familiar energy ledger does not extend automatically
In an ordinary mechanics problem, you can often define one total energy and show that it stays constant. A key ingredient is a suitable symmetry under shifts in time: the background relevant to the experiment does not change merely because the experiment starts later.
An expanding cosmological spacetime generally does not have that symmetry. Sean Carroll explains the connection between time symmetry and conservation, including why both radiation and vacuum energy challenge an everyday global ledger.
General relativity retains a local energy-momentum conservation equation. But extending that equation into a unique total for the entire universe requires additional conditions and definitions. There is no general rule that adds up matter, light, and a uniquely defined local gravitational-energy density everywhere and produces the familiar constant total.
The general-relativity discussion by Michael Weiss and John Baez distinguishes these issues. Special spacetimes and appropriately isolated systems can have useful conserved energies. The statement is not that energy accounting is impossible in every relativistic setting.
An expanding box of radiation illustrates the difference
Imagine an ideal region that follows the cosmic expansion, filled with freely traveling radiation, with no absorption, emission, or net loss of photons across its boundaries. This is a calculation about a homogeneous radiation field, not a box with physical walls.
If the scale factor doubles, the region’s volume grows by a factor of eight. Its photons each have half their former energy. The total radiation energy in that region therefore halves, and its energy density becomes one-sixteenth of its starting value.
One factor of eight comes from dilution; the additional factor of two comes from redshift. Simply counting fewer photons per unit volume misses the second effect.
The cosmological fluid equations can express this change through a pressure-and-volume relation. That bookkeeping is useful, but it does not identify a material container outside the universe that receives the difference. Treating the analogy as a literal piston introduces something the model never specified.
What the missing energy does not establish
Redshift alone is not evidence that photons deposit their lost energy into dark energy. Nor is it evidence that the light must collide with invisible particles or steadily emit weaker photons. Those would be additional physical mechanisms requiring their own evidence.
A spectrum establishes a wavelength shift; its interpretation depends on the source, observer, and physical setting. Cosmic distance measurements add further model choices rather than turning redshift into a universal speed or distance reading.
For a cosmologically redshifted photon, the useful questions are therefore: which observers measured the energies, and does the spacetime admit the global conservation rule being assumed? Once those are specified, the smaller received energy is a consistent prediction of the expanding-universe model, rather than an unexplained loss in a laboratory experiment.
AI-generated illustration for Galileo Whispers of a generic distant galaxy field. This is not an observation, a redshift measurement or a depiction of energy flowing to another object.
