Dark Matter and Gravity Waves from a Dark Big Bang (2302.11579v1)

Abstract: The Hot Big Bang is often considered as the origin of all matter and radiation in the Universe. Primordial nucleosynthesis (BBN) provides strong evidence that the early Universe contained a hot plasma of photons and baryons with a temperature $T>\text{MeV}$. However, the earliest probes of dark matter originate from much later times around the epoch of structure formation. In this work we describe a scenario in which dark matter (and possibly dark radiation) can be formed around or even after BBN in a second Big Bang which we dub the Dark Big Bang''. The latter occurs through a phase transition in the dark sector which transforms dark vacuum energy into a hot dark plasma of particles; in this paper we focus on a first-order phase transition for the Dark Big Bang. The correct dark matter abundance can be set by dark matter cannibalism or by pair-annihilation within the dark sector followed by a thermal freeze-out. Alternatively ultra-heavydark-zilla'' dark matter can originate directly from bubble collisions during the Dark Big Bang. We will show that the Dark Big Bang is consistent with constraints from structure formation and the Cosmic Microwave Background (CMB) if it occurred when the Universe was less than one month old, corresponding to a temperature in the visible sector above $\mathcal{O}$(keV). While the dark matter evades direct and indirect detection, the Dark Big Bang gives rise to striking gravity wave signatures to be tested at pulsar timing array experiments. Furthermore, the Dark Big Bang allows for realizations of self-interacting and/or warm dark matter which suggest exciting discovery potential in future small-scale structure observations.

• The paper presents a novel Dark Big Bang model where a first-order phase transition in a decoupled dark sector produces both dark matter and gravitational waves.
• It utilizes theoretical analysis and numerical predictions to link dark matter production, including ultra-heavy 'dark-zilla' particles, with observable nHz gravitational wave signals.
• The findings challenge standard cosmological models by providing testable predictions for dark matter densities and gravitational wave spectra in upcoming pulsar timing array experiments.

An Examination of Dark Matter and Gravitational Waves from a Dark Big Bang

The paper "Dark Matter and Gravity Waves from a Dark Big Bang" explores an alternative cosmological scenario that diverges from the standard model of cosmology, wherein dark matter (DM) originates from a Dark Big Bang (DBB) rather than the Hot Big Bang responsible for visible matter and radiation. This paper presents a novel exploration into how DM and potentially dark radiation can emerge from a first-order phase transition within a decoupled dark sector. It explores the theoretical implications, limitations, and observational potential related to this hypothesis, with an emphasis on gravitational wave (GW) emissions resulting from such a phase transition.

The paper integrates particle physics and cosmology by predicting and analyzing the astrophysical consequences of a DBB occurring shortly after or even alongside the epochs of Big Bang Nucleosynthesis (BBN). The authors suggest that a phase transition transforms latent dark vacuum energy into a hot dark plasma, resulting in a rich phenomenology of DM generation mechanisms, potential GW signals, and an enriched understanding of structure formation.

Dark Sector and Dark Big Bang Hypothesis

Central to the hypothesis is a decoupled dark sector containing a scalar field that initially resides in a metastable state. It undergoes a phase transition into its true vacuum, which produces dark particles potentially via mechanisms like dark matter cannibalism or annihilation, yielding thermal freeze-out scenarios. Notably, the paper explores ultra-heavy "dark-zilla" particles emerging from bubble collisions. The phase transition should occur within an early stage of the universe, specifically when the universe is less than a month old, corresponding to temperatures above $\mathcal{O}(\text{keV})$, ensuring structure formation constraints are maintained.

Gravitational Waves from Dark Big Bang

A significant focus of the paper is the gravitational wave spectrum predicted to result from bubble collisions during the DBB. The gravitational waves propose a potentially detectable signature in ongoing and future pulsar timing array experiments such as NANOGrav and the Square Kilometre Array (SKA). Specifically, these experiments are sensitive to nHz-frequency ranges—the expected frequencies for GW signatures from a DBB happening around or after BBN.

Theoretical Implications and Observational Prospects

The analysis presents detailed theoretical predictions regarding energetics and nucleation rates, with numerical results indicating possible DM densities spanning from keV-scale WIMPlike dark matter to dark-zillas with masses up to $10^{12} \, \text{GeV}$. Such wide-ranging implications suggest comprehensive potential avenues for direct and indirect confirmation via observational astrophysics.

Given the void in direct detection of DM, proposing alternative origins and characteristics like those predicted by a DBB offers innovative paradigms for both theoretical and observational research. Importantly, the connections drawn between GW signatures and DM properties imply that forthcoming developments in GW astronomy will provide critical tests for this model, with implications that extend across cosmology and particle physics.

Conclusion

The paper serves as a pivotal entry point into assessing decoupled dark sector dynamics within an alternative cosmological framework, emphasizing the interplay between theoretical constructs and potential empirical validation through gravitational wave astronomy. Future experiments and observational advancements in small-scale structures and GWs will be crucial in either validating or refuting this intriguing Dark Big Bang scenario, which stands as a potential cornerstone for understanding the elusive nature of dark matter in our universe.