A Five-Dimensional Trace in the Cosmic Microwave Background

Original: "First Search for Kaluza-Klein Gravitons and Radion Using Planck Data"

arXiv:2607.02651v1 · 2026-07-02 · CC BY 4.0 · ⏱ 1 min · Cosmology HEP Theory

Data from the Planck space observatory have for the first time allowed constraints on non-Gaussianity from particles from extra dimensions—Kaluza–Klein gravitons and the radion.

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Every second, billions of relic photons pass through your body—messages from an era when the Universe was 380,000 years old. But encoded in this ancient light are even earlier events—the sighs of inflation, capable of birthing particles from curled-up dimensions like inaudible overtones. The sensitivity of future telescopes is growing, and in ten years, we may decipher these spectral lines, unlocking physics inaccessible to colliders.

🎯 The idea of using cosmology to test particle physics dates back to the 1970s, but only the fantastic precision of Planck data turned it into a working tool.

🎬 In Greg Egan's novel 'Schild's Ladder,' a universe with extra dimensions is described, where physicists discover a way to travel through the 'bulk.' Although our reality is more modest, the search for messenger particles from hidden dimensions in the CMB is a step towards 'hearing' the echo of a multidimensional cosmos.

\mu = \sqrt{\frac{M^2}{H^2} - \frac{9}{4}}

If μ ≲ 1, the particle is produced efficiently; for large μ, exponential suppression occurs—this is how the quantum-mechanical filter works.

f_{\rm NL} \sim -\frac{10 M}{3 H^4} \frac{c_g^2 \dot{\sigma}_0}{2 \Lambda_c^2} S_{\rm max}

Here M is the UV physics scale, c_g is the coupling constant, Λc is the confinement scale, Smax is the maximum form factor. The formula links the fundamental model parameters to the observable quantity.

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A Five-Dimensional Trace in the Cosmic Microwave Background