Planck Stars as Dark Matter: Resolving the Black Hole Singularity.

Research demonstrates Planck Star Remnants (PSR) – stable, non-radiating objects formed from gravitational collapse via a quantum bounce at Planck density – represent a viable dark matter candidate. These remnants, potentially originating from evaporated primordial black holes, align with cosmological constraints and offer a novel explanation for observed dark matter density.

The enduring mystery of dark matter may admit a solution originating from the final stages of stellar collapse, according to new research. The work proposes that remnants of ‘Planck Stars’ – hypothetical objects formed when gravitational collapse halts at an extremely high density – could constitute a significant portion of the universe’s missing mass. These Planck Star Remnants (PSRs) are theorised to arise from the evaporation of primordial black holes and remain stable, causally disconnected from the external universe. Oem Trivedi, from Vanderbilt University and Ahmedabad University, collaborated with Abraham Loeb of Harvard University to explore this possibility, detailing their findings in the article ‘Could Planck Star Remnants be Dark Matter?’. Their analysis, utilising Loop Quantum Cosmology and the Israel junction conditions, suggests a viable pathway for PSR formation and estimates their potential contribution to the observed dark matter density, while remaining consistent with current astrophysical observations.

Quantum Cosmology and the Dark Matter Puzzle: Exploring Planck Star Remnants and Primordial Black Holes

Current cosmological research investigates a potential resolution to the dark matter problem through the theoretical framework of Planck Star Remnants (PSRs), linking quantum gravity with observed cosmological phenomena. The standard model of cosmology requires approximately 85% of the universe’s mass-energy density to be in the form of dark matter, the nature of which remains elusive.

The PSR concept emerges from Loop Quantum Cosmology (LQC), a theoretical approach attempting to quantise gravity. LQC predicts that, unlike classical general relativity which predicts a singularity at the centre of black holes and at the beginning of the universe, gravitational collapse does not lead to a singularity. Instead, matter undergoes a ‘quantum bounce’ at Planck density ((10^{98} kg/m^3)), effectively preventing complete collapse. This bounce creates a remnant – the PSR – which could contribute to the observed dark matter density.

Models demonstrate that this quantum bounce alters the expected behaviour of collapsing matter distributions. Crucially, these models predict specific observational signatures, including primordial gravitational waves with a unique spectral index – a characteristic pattern in the frequency and amplitude of these waves. Detecting such a pattern would provide evidence supporting the LQC framework and the existence of PSRs. Furthermore, the quantum effects predicted by LQC may modify gravitational interactions at small scales, potentially mimicking the behaviour predicted by Modified Newtonian Dynamics (MOND). MOND proposes alterations to Newtonian gravity to explain galactic rotation curves without invoking dark matter.

Alongside PSRs, primordial black holes (PBHs) are receiving considerable attention as potential dark matter candidates. PBHs are hypothesised to have formed in the very early universe from the collapse of significant density fluctuations. Unlike stellar black holes formed from the collapse of massive stars, PBHs could have formed across a wide range of masses, offering a plausible mechanism for generating the observed dark matter density.

The formation of PBHs is sensitive to conditions in the early universe, including the inflationary epoch – a period of rapid expansion immediately after the Big Bang – and the equation of state governing the universe’s behaviour at that time. Cosmological simulations and analytical models are employed to explore the parameter space of PBHs – specifically, their mass and abundance – to determine whether they can account for the observed dark matter.

Detecting PBHs presents a significant challenge. However, several avenues are being pursued. PBHs emit gravitational waves through mechanisms such as Hawking radiation – the theoretical emission of particles due to quantum effects near the event horizon – and through mergers. Gravitational wave detectors, including the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, are actively searching for these signals. Detection would not only confirm the existence of PBHs but also provide information about their mass distribution and contribution to the dark matter density.

Furthermore, PBHs are theorised to have acted as seeds for the formation of supermassive black holes (SMBHs) found at the centres of most galaxies. Investigating the connection between PBHs and SMBHs could provide insights into the origin and evolution of these enigmatic objects. Current research also explores the possibility that PSRs may form from the late-stage evaporation of PBHs, linking these two potential dark matter candidates.