Is There Cosmological Redshift Within the Milky Way?

A star’s light can reach you shifted toward the red or blue end of the spectrum, but that does not automatically mean the universe expanded between the star and you. The key question is whether the source and observer are participating in the large-scale expansion of the universe.

The Milky Way is a gravitationally bound system. Its stars orbit within the Galaxy rather than moving apart with the Hubble flow, so there is no ordinary cosmological redshift between two points inside it. Local spectral shifts instead come chiefly from relative motion and differences in gravitational potential.

What cosmological redshift means

On sufficiently large scales, the universe is described approximately by an expanding spacetime. The distances between galaxies that follow the overall cosmic expansion increase as the universe’s scale factor grows. Light traveling through that expanding background has its wavelength stretched. For a comoving source and observer in an expanding-universe model, if the light was emitted when the scale factor was $a_{\mathrm{emit}}$ and observed when it is $a_{\mathrm{obs}}$, the cosmological redshift is

$$1+z=\frac{a_{\mathrm{obs}}}{a_{\mathrm{emit}}}.$$

This description applies to the large-scale, approximately homogeneous universe. A galaxy is not simply a small patch of that smooth background: its matter is concentrated by gravity, and its stars and gas have motions governed by the Galaxy’s gravitational field. They are not just comoving markers being carried apart by cosmic expansion.

That is why cosmologists do not apply the Hubble law, $v\approx H_0d$, to stars within the Milky Way. The law describes the large-scale recession of distant galaxies, not the orbital motion of objects in a bound galaxy.

What shifts light inside the Milky Way?

Two local effects are especially relevant:

• Doppler shift: If a star moves toward you, its light is blueshifted; if it moves away, its light is redshifted. The relevant velocity is the component along the line of sight.
• Gravitational redshift or blueshift: In a static or nearly static gravitational field, light changes frequency as it moves between locations at different gravitational potentials. The net effect depends on the emission and observation locations, not on a simple Hubble expansion along the path.

Astronomers also account for the motion of Earth, the Sun, and the observer’s chosen reference frame. For example, a star’s measured velocity may be reported relative to the Sun or to the local standard of rest. Those corrections are distinct from cosmological redshift.

A numerical check: the misleading Hubble-law estimate

Suppose a star is about $8\,\mathrm{kpc}$ away, a typical scale for distances within the Milky Way. If you insert that distance into the Hubble law using $H_0=70\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, you get

$$8\,\mathrm{kpc} = 0.008\,\mathrm{Mpc}$$ $$v_{\mathrm{Hubble}} = H_0d$$ $$= (70\,\mathrm{km\,s^{-1}\,Mpc^{-1}})(0.008\,\mathrm{Mpc})$$ $$= 0.56\,\mathrm{km\,s^{-1}}.$$

The arithmetic is correct, but interpreting $0.56\,\mathrm{km\,s^{-1}}$ as the star’s actual cosmological recession speed is not. The star is bound to the Milky Way, and its motion is set by its orbit and other local dynamics. Orbital speeds in the Galaxy are typically on the order of hundreds of kilometres per second. The Hubble-law result is only what the large-scale formula would give if it were extrapolated into a regime where it does not describe the motion.

What about light passing through the Galaxy?

A photon from a distant galaxy can be cosmologically redshifted because it traveled through expanding intergalactic space. When it enters the Milky Way, the Galaxy does not add an ordinary Hubble-flow redshift to that journey. Local motion and gravity can affect the measured signal, but those are separate contributions.

There is no perfectly unique way to divide every observed redshift into “cosmological,” “Doppler,” and “gravitational” pieces in all circumstances, especially across large distances or in strong gravitational fields. The total observed wavelength shift is well-defined; the breakdown depends on the physical model and reference frames. For a star within the Milky Way, however, the practical interpretation is straightforward: its measured shift is not a Hubble-expansion redshift.

Common mistakes

• Using $H_0d$ at any distance: The Hubble law is a large-scale relation, not a rule for every pair of objects.
• Treating every redshift as cosmological: A receding star can be Doppler-redshifted without cosmic expansion causing the shift.
• Saying expansion has no effect anywhere locally: The useful point is that expansion does not make the Galaxy’s bound contents follow a simple Hubble flow. It is not necessary to imagine a sharp boundary where expansion suddenly switches off.
• Confusing a distant source with the path through the Galaxy: A galaxy’s cosmological redshift is mainly associated with propagation through the expanding universe, not with the final leg through the Milky Way.