Signals of Bursts from the Very Early Universe

TL;DR

This work investigates the signatures of explosive events in the very early universe, exploring how bursts—potentially tied to horizon-scale phenomena like baby universes or massive black hole formation—could manifest today. It develops a framework centered on neutrino propagation and the critical $t_{free}$ that separates escape from containment, and it derives observational channels including localized CMB nonthermal features, a soft X-ray bump from positron production, and a nonthermal relic neutrino component. By coupling propagation physics, redshift, and emission models, the study estimates possible radiative and thermal signatures on the CMB and in the present neutrino/photonic backgrounds, while highlighting gauge conditions, uncertainties, and pathways for future refinement. If realized, these signatures would offer a novel probe of the very early universe and may connect to gravitational-wave backgrounds and the broader landscape of high-energy cosmology.

Abstract

We consider possible observable signals from explosive events in the very early universe, ``bursts". These could be expected in connection with massive black hole or ``baby universe'' formation. We anticipate that such major disruptions of spacetime would be associated with neutrino and perhaps other pulses. While these seem to be not detectable directly, we discuss how they could lead to potentially observable signals. We analyze how the pulses from very early times may ``escape'', that is propagate to the last scattering epoch at the time $t_{cmb}$ and later, or alternatively be absorbed earlier, ``contained''. The possibly detectable signals include effects on small regions of the CMB, a soft x-ray resulting from positron production, or a nonthermal addition to the relic neutrino background.

Figures (2)

• Figure 1: The ( energy, time) plane for the free flight of neutrinos. $E_{em}$ is the energy and $x$ is the scaled time . The plot shows which regions can or cannot contribute to a given $E^\nu_{cmb}$ at scaled time $x=1$. The inclined line rising to the right, $x=s\,E_{em}/GeV$, separates the plane into the regions of "escape" (above the line) and "contained" (below the line). In the well-defined ${\bar{E}^\nu}$ model, the potentially contributing regions would correspond to a vertical band at some $E_{em}={\bar{E}^\nu}$ (not shown). For the spread spectrum model, where all energies up to a certain maximum $E_{max}$ can occur, the region to the left of $E_{max}$ potentially contributes. Points that can contribute to a given $E^\nu_{cmb}$ are found by following a curve down from the point where it starts at $x=1$. The two curves are examples for particles with different energies at $x=1$. The curve to the right, Curve I, is for a relatively higher energy and the curve to the left, Curve II, is for a lower energy. Curve I encounters the $E_{max}$ limitation before encountering the 'escape' limitation. For curve II it is the other way around. These two possibilities lead to the different lower limits of integration in Eq \ref{['nudi']}.
• Figure 2: Symbolic, in only two spatial dimensions and not to scale, representation of how bursts would appear on the last scattering surface (lss) at $t_{cmb}$. The large thick circle stands for the intersection of our backward light cone (BLC) with the lss. Its radius is indicated by the arrow and its thickness is meant to represent the finite size of the observable region. The radius increases by $10^{-3}$ ly in one earth year. The larger of the smaller circles represent"escaping" neutrinos, which reach $t_{cmb}$ or later times. According to Eq \ref{['trava']}, these are all of about the same size. The smallest circles represent "absorped" bursts, whose radius is governed either by $t_{free}$, or the "diffusion distance".