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1. Revisiting primordial black holes formation from preheating instabilities: the case of Starobinsky inflation

   https://doi.org/10.1088/1475-7516/2025/02/009  

   del-Corral, Daniel; Gondolo, Paolo; Kumar, K Sravan; Marto, João (February 2025, Journal of Cosmology and Astroparticle Physics)

   In recent years, the formation of primordial black holes (PBH) in the early universe inflationary cosmology has garnered significant attention. One plausible scenario for primordial black hole (PBH) formation arises during the preheating stage following inflation. Notably, this scenario does not necessitate any ad-hoc fine-tuning of the scalar field potential. This paper focuses on the growth of primordial density perturbation and the consequent possibility of PBH formation in the preheating stage of the Starobinsky model for inflation. The typical mechanism for PBH formation during preheating is based on the collapse of primordial fluctuations that become super-horizon during inflation (type I) and re-enter the particle horizon in the different phases of cosmic expansion. In this work, we show that there exists a certain range of modes that remain in the sub-horizon (not exited) during inflation (type II modes) but evolve identically to type I modes if they fall into the instability band, leading to large density perturbation above the threshold and can potentially also contribute to the PBH formation. We outline the conditions that govern the potential collapse of typeI and type II modes with wavelengths exceeding the Jeans length,which we derive based on the effective sound speed of scalar fieldfluctuations. Since the preheating stage is an `inflaton' (approximately) matter-dominated phase, we follow the framework of the critical collapse of fluctuations and compute the mass fraction using the well-known Press-Schechter and the Khlopov-Polnarev formalisms, and compare the two. Finally, we comment on the implications of our study for the investigations concerned with primordial accretion and consequent PBH contribution to the dark matter. 

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2. Observation of the effect of gravity on the motion of antimatter

   https://doi.org/10.1038/s41586-023-06527-1  

   Anderson, E. K.; Baker, C. J.; Bertsche, W.; Bhatt, N. M.; Bonomi, G.; Capra, A.; Carli, I.; Cesar, C. L.; Charlton, M.; Christensen, A.; et al (September 2023, Nature)

   Abstract Einstein’s general theory of relativity from 1915¹remains the most successful description of gravitation. From the 1919 solar eclipse²to the observation of gravitational waves³, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac’s theory⁴appeared in 1928; the positron was observed⁵in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted⁶by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter^7–10. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP. 

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3. Gravitational Waves from Black Hole Emission

   https://doi.org/10.3390/universe11010021  

   Islam, Tousif; Khanna, Gaurav; Liebling, Steven L (January 2025, Universe)

   Using adiabatic point-particle black hole perturbation theory, we simulate plausible gravitational wave (GW) signatures in two exotic scenarios (i) where a small black hole is emitted by a larger one (‘black hole emission’) and (ii) where a small black hole is emitted by a larger one and subsequently absorbed back (‘black hole absorption’). While such scenarios are forbidden in general relativity (GR), alternative theories (such as certain quantum gravity scenarios obeying the weak gravity conjecture, white holes, and Hawking radiation) may allow them. By leveraging the phenomenology of black hole emission and absorption signals, we introduce straightforward modifications to existing gravitational waveform models to mimic gravitational radiation associated with these exotic events. We anticipate that these (incomplete but) initial simulations, coupled with the adjusted waveform models, will aid in the development of null tests for GR using GWs. 

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4. Discretely Charged Dark Matter in Inflation Models Based on Holographic Space-Time

   https://doi.org/10.3390/universe8110600  

   Banks, Tom; Fischler, Willy (November 2022, Universe)

   The holographic space-time (HST) model of inflation has a potential explanation for dark matter as tiny primordial black holes. Motivated by a recent paper of Barrau, we propose a version of this model where some of the inflationary black holes (IBHs), whose decay gives rise to the Hot Big Bang, carry the smallest value of a discrete symmetry charge. The fraction f of IBHs carrying this charge is difficult to estimate from first principles, but we determine it by requiring that the crossover between radiation and matter domination occurs at the correct temperature Teq∼1eV=10−28MP. The fraction is small, f∼2×10−9, so we believe this gives an extremely plausible model of dark matter. 

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5. Observational Constraints on Dynamical Dark Energy Models

   https://doi.org/10.3390/universe10030122  

   Avsajanishvili, Olga; Chitov, Gennady Y; Kahniashvili, Tina; Mandal, Sayan; Samushia, Lado (March 2024, Universe)

   Scalar field ϕCDM models provide an alternative to the standard ΛCDM paradigm, while being physically better motivated. Dynamical scalar field ϕCDM models are divided into two classes: the quintessence (minimally and non-minimally interacting with gravity) and phantom models. These models explain the phenomenology of late-time dark energy. In these models, energy density and pressure are time-dependent functions under the assumption that the scalar field is described by the ideal barotropic fluid model. As a consequence of this, the equation of state parameter of the ϕCDM models is also a time-dependent function. The interaction between dark energy and dark matter, namely their transformation into each other, is considered in the interacting dark energy models. The evolution of the universe from the inflationary epoch to the present dark energy epoch is investigated in quintessential inflation models, in which a single scalar field plays a role of both the inflaton field at the inflationary epoch and of the quintessence scalar field at the present epoch. We start with an overview of the motivation behind these classes of models, the basic mathematical formalism, and the different classes of models. We then present a compilation of recent results of applying different observational probes to constraining ϕCDM model parameters. Over the last two decades, the precision of observational data has increased immensely, leading to ever tighter constraints. A combination of the recent measurements favors the spatially flat ΛCDM model but a large class of ϕCDM models is still not ruled out. 

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