Dark Matter Stays Cold: What the Webb Telescope Saw ✦

Original: "The free-streaming length of dark matter from JWST observations of 28 strong gravitational lenses"

arXiv:2606.05277v1 · 2026-06-03 · CC BY 4.0 · ⏱ 1 min · Cosmology Galaxies

The James Webb Space Telescope confirmed that dark matter is cold.

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Metaphor: Galaxies work like uneven panes of glass, distorting the light of distant lamps — quasars. Astronomers pointed James Webb at 28 such panes — massive galaxies. Tiny dark matter clumps around them act like scratches, adding a barely perceptible tremor to the picture.

To make out this tremor, the telescope recorded the heat of cosmic dust around the quasars—this eliminated interference from ordinary stars.

Measurements with unprecedented precision showed: the distortions are too small to have been created by fast, "warm" dark matter. So the invisible matter particles are slow and heavy — at least 10,000 times more massive than an electron. They didn't blur the expanding universe in its early youth, allowing galaxies to be born.

Measuring brightness and analyzing the light composition of each image required sifting through 176 million possible pictures — for one of the lenses there were 22 million variations. This result carries forward the ideas of pioneers like Fritz Zwicky and Vera Rubin, shutting down many hypotheses about light dark matter.

🎯 To reach their final conclusions, scientists sifted through 176 million possible arrangements of dark matter clumps — for each lens, from half a million to 22 million variations.

m_{\mathrm{hm}} = M_0 \left(\frac{m_{\mathrm{therm}}}{3\ \mathrm{keV}}\right)^\zeta

For spin-1/2 fermions, M₀ = 4.0×10⁸ M⊙, ζ = –3.564; for spin-3/2, 2.1×10⁸ M⊙, ζ = –3.585. Shows how quickly the formation of small halos is suppressed as the particle mass decreases.

\lambda_{\mathrm{FS}} = 5.2 \left(\frac{m_{\mathrm{hm}}}{10^7 \, M_\odot}\right)^{1/3} \ \mathrm{кпк}

Approximate distance particles can travel before fluctuation growth begins at the epoch of matter-radiation equality. The lighter the particles, the larger λ_FS and the more smeared the small-scale structure.

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Dark Matter Stays Cold: What the Webb Telescope Saw ✦