Constraints on Primordial Black Holes and Small-Scale Primordial Curvature Perturbations from the Cosmic Microwave Background Measurement of the Effective Number of Relativistic Species

Kernekoncepter

The existence of scalar-induced gravitational waves, generated alongside primordial black holes, could affect the effective number of relativistic species (Neff) in the early universe, providing a novel method to constrain the abundance of primordial black holes and the small-scale primordial curvature power spectrum.

Resumé

• Bibliographic Information: Zhou, J.-Z., Kuang, Y.-T., Chang, Z., & L¨u, H. (2024). Constraints on primordial black holes from Neff: scalar-induced gravitational waves as an extra radiation component. arXiv preprint arXiv:2410.10111v1.
• Research Objective: This paper investigates the constraints on the abundance of primordial black holes (PBHs) and the small-scale primordial power spectrum imposed by the cosmic microwave background (CMB) measurement of the effective number of relativistic species (Neff).
• Methodology: The authors calculate the energy density spectrum of scalar-induced gravitational waves (SIGWs) up to the third order, considering primordial non-Gaussianity. They then use the current observational data of Neff to constrain the energy density of SIGWs, which in turn limits the parameter space of the small-scale primordial power spectrum and the abundance of PBHs.
• Key Findings: The study finds that the presence of SIGWs, generated during the formation of PBHs, can significantly impact the value of Neff. By comparing theoretical predictions with the observed Neff values from Planck + BAO + BBN data, the authors derive constraints on the amplitude of the primordial power spectrum (Aζ) for different values of the non-Gaussian parameter (fNL).
• Main Conclusions: The research concludes that current Neff observations can effectively constrain the amplitude of the small-scale primordial power spectrum. However, the constraints on the abundance of PBHs are sensitive to the value of fNL. For fNL less than zero and near zero, Neff observations can effectively limit the abundance of PBHs with a mass around 10^-12 solar masses. For fNL > 0, current Neff observations cannot effectively constrain the abundance of PBHs.
• Significance: This study highlights a novel method to probe the early universe and constrain the properties of PBHs and the primordial power spectrum using cosmological observations of Neff.
• Limitations and Future Research: The authors acknowledge that their calculations consider only up to the third-order contributions to the energy density spectrum of SIGWs. Higher-order contributions, especially for large Aζ, could further tighten the constraints. Additionally, future research could incorporate the influence of other potential extra radiation components beyond SIGWs.

Statistik

The Planck + BAO + BBN data provides a 95% confidence level upper limit of ΔNeff (Neff - 3.046) < 0.175.
The total energy density spectrum of SIGWs is constrained to be less than 1.3 x 10^-6 ΔNeff/0.234.

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Constraints on primordial black holes from $N_{\text{eff}}$ : scalar induced gravitational waves as an extra radiation component

by Jing... kl. arxiv.org 10-15-2024

https://arxiv.org/pdf/2410.10111.pdf

Dybere Forespørgsler

How might future advancements in gravitational wave astronomy, such as the development of space-based detectors, further enhance our ability to constrain the properties of primordial black holes?

Answer: Future advancements in gravitational wave astronomy, particularly the development of space-based detectors like LISA (Laser Interferometer Space Antenna), promise to revolutionize our understanding of the early universe and potentially provide compelling evidence for the existence of primordial black holes (PBHs). Here's how:

Wider Frequency Range: Ground-based detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory) are sensitive to gravitational waves in the high-frequency regime (∼ 10-1000 Hz), primarily originating from mergers of stellar-mass black holes and neutron stars. Space-based detectors like LISA will be sensitive to lower frequencies (∼ 0.1 mHz to 1 Hz), a range where PBH binaries are expected to emit gravitational waves. This broader frequency coverage will allow us to probe a much wider mass range of PBHs, potentially down to masses inaccessible to current detectors.

Increased Sensitivity: Space-based detectors will operate in the pristine environment of space, free from seismic noise and other terrestrial disturbances that limit the sensitivity of ground-based detectors. This enhanced sensitivity will enable the detection of fainter gravitational wave signals, potentially revealing mergers of smaller PBHs or those located at greater distances.

Continuous Wave Observation: Unlike ground-based detectors, which are limited by Earth's rotation, space-based detectors can observe continuously. This continuous observation is crucial for detecting stochastic gravitational wave backgrounds, such as those potentially generated by the early universe or a population of PBHs. By analyzing the properties of these backgrounds, we can glean valuable information about the early universe and the formation mechanisms of PBHs.

Multi-Messenger Astronomy: Combining gravitational wave observations from space-based detectors with electromagnetic observations from telescopes across the electromagnetic spectrum will usher in a new era of multi-messenger astronomy. This synergy will be particularly powerful in studying PBH mergers, as the electromagnetic counterparts to these events can provide independent confirmation of their nature and shed light on the environments in which they reside.
By leveraging these advancements, future gravitational wave astronomy holds immense potential to constrain the properties of PBHs, including their mass distribution, abundance, and formation mechanisms. These insights will be invaluable in determining whether PBHs contribute significantly to dark matter and unraveling the mysteries of the early universe.

Could alternative cosmological models, such as those involving modified gravity theories, potentially explain the observed Neff values without invoking the presence of primordial black holes or significant primordial non-Gaussianity?

Answer: Yes, alternative cosmological models, particularly those involving modified gravity theories, offer intriguing possibilities for explaining the observed Neff values without requiring the presence of primordial black holes (PBHs) or significant primordial non-Gaussianity. Here are some compelling examples:

Extra Relativistic Species: Some modified gravity theories predict the existence of additional relativistic particles or fields in the early universe beyond the Standard Model. These extra species would contribute to the effective number of relativistic degrees of freedom (Neff), potentially accounting for the observed values without invoking PBHs or non-Gaussianity. For instance, models with extra neutrinos or axions could alter the expansion rate of the early universe and affect Neff.

Modified Expansion History: Certain modified gravity theories, such as those involving scalar fields or extra dimensions, can lead to deviations from the standard cosmological expansion history. These deviations can impact the abundance of light elements produced during Big Bang Nucleosynthesis (BBN) and the temperature anisotropies in the Cosmic Microwave Background (CMB), both of which are sensitive to Neff. By adjusting the parameters of the modified gravity model, it might be possible to reproduce the observed Neff values without requiring PBHs or non-Gaussianity.

Interacting Dark Energy: Models where dark energy interacts with other components of the universe, such as neutrinos or dark matter, can also influence Neff. These interactions can modify the energy density and clustering properties of dark matter, potentially affecting the growth of structure and the CMB power spectrum. By carefully tuning the interaction parameters, it might be possible to explain the observed Neff values without resorting to PBHs or non-Gaussianity.
It's important to note that while these alternative models offer intriguing possibilities, they are not without their challenges. They often introduce new parameters and complexities, and their predictions must be carefully tested against a wide range of cosmological observations. Nevertheless, they highlight the fact that the observed Neff values, while suggestive of new physics, do not necessarily point uniquely to PBHs or non-Gaussianity. Further research and observations are crucial to disentangle these possibilities and gain a deeper understanding of the early universe.

What are the broader implications for our understanding of the early universe and fundamental physics if primordial black holes are indeed confirmed to be a significant constituent of dark matter?

Answer: The confirmation of primordial black holes (PBHs) as a significant constituent of dark matter would have profound implications for our understanding of the early universe and fundamental physics, revolutionizing several key areas:

Early Universe Physics: The formation of PBHs requires large density fluctuations in the very early universe, potentially probing energy scales far beyond the reach of current particle accelerators. Understanding the mechanisms that generated these fluctuations could provide invaluable insights into inflation, phase transitions, or other exotic phenomena that may have occurred in the first fraction of a second after the Big Bang.

Gravity and Black Hole Physics: PBHs could span an enormous mass range, from microscopic black holes with masses below a gram to supermassive black holes found at the centers of galaxies. Studying their properties, such as their mass distribution, clustering, and merger rates, would provide stringent tests of general relativity in extreme gravitational environments and potentially reveal clues about the nature of dark matter and dark energy.

Large-Scale Structure Formation: The presence of PBHs as dark matter would influence the formation and evolution of galaxies and large-scale structures in the universe. Their gravitational interactions with baryonic matter could leave distinct imprints on the cosmic web, potentially observable in galaxy surveys and the distribution of matter in the universe.

Baryogenesis and Matter-Antimatter Asymmetry: Some models propose that PBHs could have played a role in generating the observed matter-antimatter asymmetry in the universe. Their evaporation via Hawking radiation could have injected a slight excess of matter over antimatter in the early universe, potentially explaining why we observe a matter-dominated universe today.

New Physics Beyond the Standard Model: The existence of PBHs could hint at new particles or fields beyond the Standard Model of particle physics. These new particles could have interacted with PBHs in the early universe, leaving observable signatures in their properties or the cosmic microwave background radiation.
In essence, confirming PBHs as a significant component of dark matter would open up a new window into the early universe and fundamental physics, providing a unique laboratory to test our current understanding of gravity, particle physics, and cosmology. It would usher in a new era of exploration, potentially leading to groundbreaking discoveries that could reshape our understanding of the cosmos and our place within it.