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Non-Primordial Contribution to the Cosmic Microwave Background

This paper demonstrates that a non-primordial contribution to the Cosmic Microwave Background from dust-enshrouded starbursts at high redshift, as proposed by Gjergo & Kroupa, is fully consistent with Planck 2018 CMB anisotropy data, allowing for a contribution of up to ~3–5% without significantly altering standard cosmological parameter inference.

Original authors: Akshith Asundi, Vikram Khaire

Published 2026-07-17

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Akshith Asundi, Vikram Khaire

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, expanding balloon. If you blow it up, the air inside gets cooler and the light waves stretching across it get longer. About 380,000 years after the Big Bang, the universe cooled down enough for the first atoms to form, and the "fog" of hot plasma cleared. Suddenly, light could travel freely. This ancient light, stretched by the expansion of the universe over 13 billion years, is what we see today as the Cosmic Microwave Background (CMB). Think of it as the universe's oldest photograph, a faint, uniform glow of static that fills every corner of the sky.

Scientists have spent decades studying this "static" because it holds the secret recipe for how the universe was built. They treat it like a perfect, pristine echo from the beginning of time. However, just as a modern radio picks up static from lightning storms or distant cell towers, the universe might have its own version of "noise." Could some of the light we think is from the Big Bang actually be something else? Specifically, could it be the leftover heat from massive, dusty galaxies that formed very early in the universe's history? This is the question that keeps cosmologists up at night: is the CMB a pure relic of the Big Bang, or is it a mix of ancient light and more recent, dusty starlight?

The Cosmic Mix-Up: Is the Universe's Glow Pure or Polluted?

Two researchers, Akshith Asundi and Vikram Khaire, decided to play detective with the universe's oldest light. They were investigating a wild idea proposed by other scientists (Gjergo & Kroupa) suggesting that the Cosmic Microwave Background (CMB) might not be 100% "primordial."

Here's the theory they were testing: Imagine the early universe as a construction site. Massive galaxies were being built, and they were covered in thick, cosmic dust. These galaxies were so dusty that they swallowed the light from their newborn stars and re-radiated it as heat. As the universe expanded, this heat stretched out, turning into microwave radiation. The idea is that this "dusty heat" from ancient galaxies might be mixing with the original Big Bang light, making up a small percentage of the glow we see today.

The authors asked a simple but tricky question: If this dusty mix is real, would we be able to spot it in the patterns of the CMB?

The Sound of the Universe

To find the answer, the scientists looked at the CMB not just as a uniform glow, but as a musical instrument. The early universe was a hot soup of particles that rippled like sound waves. When the universe cooled, these ripples froze in place, creating a specific pattern of peaks and valleys in the CMB map. These are called "acoustic peaks."

Think of the CMB like a drum. If you hit a drum, the sound it makes depends on how much air is inside and how tight the skin is. In the universe, the "tightness" and the "air" are determined by how much energy is in the form of light (photons). If you add extra energy—like the heat from those dusty ancient galaxies—it changes the "sound" of the drum. It shifts the position of the peaks and changes how tall they are.

The researchers built a new computer model (using a tool called camb) that allowed them to turn a "dial" to see what would happen if the CMB had a dusty component. They introduced a parameter called $\epsilon_{CMB}$ .

If $\epsilon_{CMB} = 1$ , the CMB is 100% pure Big Bang light (the standard view).
If $\epsilon_{CMB} < 1$ , it means some of the light is "missing" from the Big Bang and replaced by the dusty glow.

The Verdict: A Tiny Bit of Dust is Okay, But Not a Lot

The team ran their model against the most precise data we have, collected by the Planck satellite in 2018. Here is what they found:

1. The "Perfect" Mix is Still the Favorite
When they let the data speak freely, the most likely answer was that the CMB is almost entirely pure Big Bang light. The best fit for the data was a value of $\epsilon_{CMB} = 1.0104$ . This is so close to 1 that it's practically the same thing. The data strongly suggests that the universe's glow is exactly what we thought it was: a pristine relic from the beginning.

2. A Little Dust is Allowed
However, the data isn't perfect. It allows for a tiny bit of "pollution." The researchers found that a dusty contribution of up to about 3% to 5% of the total CMB energy is still consistent with the observations.

The specific theory they were testing (GK25) predicted a dusty contribution of about 1.4%.
This 1.4% prediction fits comfortably inside the "allowed zone" (the 68% confidence interval).
Translation: The universe could have a little bit of dusty starlight mixed in, and our current telescopes wouldn't be able to tell the difference from the pure Big Bang light.

3. The "All Dust" Idea is Dead
The researchers also tested the most extreme version of the theory: What if the CMB is entirely made of dusty starlight, with zero Big Bang light?

The answer was a hard NO.
The data ruled this out with incredible certainty—42 standard deviations (42 $\sigma$ ). In science, 5 $\sigma$ is usually enough to claim a discovery; 42 $\sigma$ is like finding a needle in a haystack and then proving the haystack was never there to begin with. If the CMB were 100% dust, the "sound" of the universe (the acoustic peaks) would look completely different, and the Planck data would have screamed that something was wrong.

4. When Did the Dust Happen?
The team also wondered when this dust might have formed. They let the model pick any time between redshift 5 and 50 (a huge range of cosmic history). The result? The data didn't care. The CMB patterns couldn't tell the difference between dust forming early or late. The only thing the data could measure was how much dust there was, not when it happened.

The Bottom Line

This paper is a reality check for a fascinating but controversial idea. It confirms that while the universe might have a tiny, invisible layer of "cosmic dust" mixed into its background glow (up to about 5%), the Big Bang is still the main star of the show.

The standard model of cosmology remains robust. The "Hubble Tension" (a disagreement about how fast the universe is expanding) and other cosmic mysteries are not solved by blaming the CMB on dusty galaxies. The data shows that if there is any dust, it's a very small guest at the party, not the host.

For now, the Cosmic Microwave Background remains the most reliable time capsule we have, and the idea that it's mostly just the echo of the Big Bang stands firm. But the door is left slightly ajar: a future, more sensitive telescope might one day catch that tiny 1.4% whisper of ancient dust, proving that even the oldest light in the universe has a little bit of a modern story to tell.

Technical Summary: Non-Primordial Contribution to the Cosmic Microwave Background

Problem Statement
The standard cosmological model ( $\Lambda$ CDM) interprets the Cosmic Microwave Background (CMB) as purely primordial thermal radiation released at recombination ( $z \approx 1090$ ). However, recent work by Gjergo & Kroupa (GK25) challenges this assumption, proposing that dust-enshrouded starbursts in the progenitors of massive early-type galaxies (ETGs) at $z \sim 15\text{--}20$ contribute a non-negligible fraction (conservatively $\sim 1.4\%$ ) to the present-day CMB energy density. While previous tests of this scenario relied on the CMB monopole spectrum (specifically COBE-FIRAS data), these tests face a degeneracy: if the dust emission is spectrally indistinguishable from a blackbody (Planckian), the monopole cannot exclude a fully non-primordial CMB. This paper addresses the gap by performing the first observational test of the GK25 scenario against the CMB anisotropy power spectrum, which probes the dynamical imprint of radiation content on the sound horizon and acoustic peaks independently of absolute spectral calibration.

Methodology
The authors introduce a dimensionless emissivity parameter, $\varepsilon_{\text{CMB}}$ , to quantify the non-primordial contribution. This parameter rescales the primordial photon energy density ( $\rho_\gamma$ ) above a transition redshift $z_t$ (motivated by the GK25 peak star formation epoch at $z \approx 15\text{--}20$ ). The effective photon energy density is defined as:
$\rho^{\text{eff}}_\gamma(z) = \begin{cases} \varepsilon_{\text{CMB}} \rho_\gamma(z) & z > z_t \\ \rho_\gamma(z) & z \leq z_t \end{cases}$
where $\varepsilon_{\text{CMB}} < 1$ represents a deficit of primordial photons (grey-body CMB) and $\varepsilon_{\text{CMB}} > 1$ represents an excess (super-blackbody).

To ensure self-consistency, the authors modified the Fortran source code of the Boltzmann solver CAMB. The modification propagates $\varepsilon_{\text{CMB}}$ through:

Background expansion: Rescaling $\rho_\gamma$ alters the Hubble parameter $H(z)$ and the comoving distance to recombination.
Perturbation evolution: The rescaled density affects the Boltzmann hierarchy for scalar, tensor, and vector perturbations.
Sound horizon and acoustic peaks: The photon-baryon ratio and sound speed are updated using $\rho^{\text{eff}}_\gamma$ , directly impacting the acoustic peak structure.
Derived parameters: The conversion from the sampled acoustic scale $\theta_{\text{MC}}$ to the Hubble constant $H_0$ is updated to reflect the modified radiation content.

The model was constrained using Markov Chain Monte Carlo (MCMC) sampling via Cobaya, combining:

Planck 2018 high- $\ell$ and low- $\ell$ TT, TE, and EE likelihoods.
Planck 2018 CMB lensing likelihood.
Baryon Acoustic Oscillation (BAO) measurements from 6dF, SDSS DR7, and SDSS-III BOSS DR12.

Three distinct runs were performed:

Run 1: $\varepsilon_{\text{CMB}} \in [0.5, 1.0]$ (testing the physical deficit scenario) with fixed $z_t = 17$ .
Run 2: $\varepsilon_{\text{CMB}} \in [0.5, 1.5]$ (allowing for super-blackbody excess) with fixed $z_t = 17$ .
Run 3: Both $\varepsilon_{\text{CMB}}$ and $z_t$ (sampled over $[5, 50]$ ) as free parameters.

Key Results

Consistency with Data: The Planck 2018 anisotropy data are fully consistent with a non-primordial contribution at the level predicted by GK25.
- In Run 1, the posterior for $\varepsilon_{\text{CMB}}$ rises monotonically toward the prior boundary at $\varepsilon_{\text{CMB}} = 1$ , yielding a 68% lower bound of $\varepsilon_{\text{CMB}} \gtrsim 0.971$ (allowing up to $\sim 2.9\%$ dust contribution) and a 95% lower bound of $\varepsilon_{\text{CMB}} \gtrsim 0.953$ (allowing up to $\sim 4.7\%$ ). The GK25 conservative estimate of $1.4\%$ ( $\varepsilon_{\text{CMB}} \approx 0.986$ ) falls comfortably within the 68% credible interval.
- In Run 2, the posterior resolves into a well-defined Gaussian centered at $\varepsilon_{\text{CMB}} = 1.0104^{+0.025}_{-0.024}$ , confirming that the data prefer $\varepsilon_{\text{CMB}} \approx 1$ and that the Run 1 result was not an artifact of the prior boundary.
Exclusion of Extreme Scenarios: A fully non-primordial CMB ( $\varepsilon_{\text{CMB}} \to 0$ ), where the dust contribution approaches 100% of the energy density, is excluded at high significance ( $42\sigma$ ). This demonstrates that the anisotropy spectrum can rule out a completely dust-origin CMB even if the monopole spectrum cannot (due to spectral degeneracy).
Insensitivity to Transition Epoch: When $z_t$ is treated as a free parameter (Run 3), the posterior for $z_t$ is flat across the range $5 \lesssim z_t \lesssim 50$ . The data constrain the amplitude of the non-primordial contribution but are insensitive to when it occurred, provided the transition happens well before recombination ( $z \approx 1090$ ).
Impact on Cosmological Parameters: The inclusion of $\varepsilon_{\text{CMB}}$ does not significantly shift the standard $\Lambda$ CDM parameters. The derived Hubble constant ( $H_0$ ) and matter clustering amplitude ( $\sigma_8$ ) remain consistent with the baseline $\Lambda$ CDM values (shifts are $< 1\sigma$ ). Consequently, the GK25 scenario neither alleviates nor exacerbates the existing Hubble or $\sigma_8$ tensions.

Significance and Claims
The paper claims to provide the first quantitative test of the GK25 dust-enshrouded starburst scenario against the CMB anisotropy power spectrum. Its primary significance lies in establishing that:

Current CMB precision cosmology has sufficient "room" to accommodate a percent-level dust contribution without detectable consequences for parameter inference.
The anisotropy spectrum provides a qualitatively independent probe to the monopole spectrum; while the monopole is limited by spectral degeneracy (cannot distinguish a perfect blackbody dust from primordial radiation), the anisotropy spectrum breaks this degeneracy by constraining the dynamical effect of the radiation density on the sound horizon.
The proposed $\varepsilon_{\text{CMB}}-z_t$ framework is a general, reusable tool for testing any model where a fraction of the CMB is non-primordial, not limited to the specific GK25 dust scenario.

The authors conclude that while the conservative GK25 prediction ( $1.4\%$ ) is fully consistent with current data, the optimistic extreme (near 100% contribution) is decisively ruled out. Distinguishing a $\sim 1\text{--}2\%$ dust contribution from standard $\Lambda$ CDM will require next-generation experiments (e.g., Simons Observatory, CMB-S4) with improved sensitivity to the Silk damping tail or direct observations of the ETG formation epoch.

The Cosmic Mix-Up: Is the Universe's Glow Pure or Polluted?

The Sound of the Universe

The Verdict: A Tiny Bit of Dust is Okay, But Not a Lot

The Bottom Line

Technical Summary: Non-Primordial Contribution to the Cosmic Microwave Background

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