Creating Darkness: From the Big Bang to the Lab

Author: Denis Avetisyan

New research explores how the universe may have generated dark matter during its earliest moments, and seeks to replicate the process in controlled laboratory settings.

This review examines cosmological particle production within Quantum Field Theory in Curved Spacetime, alongside experimental investigations using Bose-Einstein condensates as analog gravity systems, to understand dark matter origins.

The enduring mystery of dark matter necessitates exploration of unconventional production mechanisms beyond standard particle physics. This thesis, ‘Cosmological production of dark matter in the Universe and in the laboratory’, investigates particle creation within expanding spacetime, both in the early Universe and through analog simulations using Bose-Einstein condensates. We demonstrate that cosmological particle production, driven by inflationary dynamics and potentially involving tachyonic instabilities, offers a viable pathway to account for observed dark matter abundance. Could carefully designed analog experiments, mapping phonon behavior to cosmological scenarios, provide novel insights into quantum effects in curved spacetime and refine our understanding of dark matter’s origins?

The Universe’s Genesis: Unraveling the Initial Conditions

Despite its remarkable success in describing the large-scale structure and evolution of the universe, the prevailing ΛCDM model doesn’t fully address the fundamental question of its initial conditions and the ultimate source of its matter and energy. While the model accurately predicts many observed phenomena – like the cosmic microwave background and the expansion rate – it relies on assumptions about the very beginning that remain largely unexplained. The nature of dark energy, represented by the cosmological constant Λ, and the composition of dark matter, which together constitute approximately 95% of the universe’s total energy density, are prime examples of these open questions. Furthermore, the model doesn’t inherently explain how this matter and energy initially came into existence, leaving a gap in understanding the very genesis of the cosmos and prompting investigation into processes that could have populated the early universe with the observed constituents.

The very fabric of spacetime underwent a period of extraordinarily rapid expansion in the universe’s first moments, known as inflation. This dynamic era presents a significant challenge to understanding particle creation, as the conventional mechanisms for generating matter are drastically altered by the expanding universe. Instead of particles arising from collisions or decays in a static background, the inflationary spacetime itself can directly contribute to their creation. The intense gravitational fields and time-varying nature of this epoch suggest that particles – potentially including those comprising dark matter – could have been ‘squeezed’ out of the vacuum due to quantum fluctuations. Determining the precise processes by which these particles emerged, and their subsequent abundance, requires a sophisticated understanding of quantum field theory in curved spacetime, and remains a central puzzle in cosmological research.

Cosmological particle production represents a vital, yet largely unexplored, avenue for refining the standard model of cosmic evolution. The prevailing ΛCDM model, while remarkably successful, doesn’t fully account for the origins of the universe’s matter and energy; particle production offers a mechanism by which the extreme conditions of the early universe – particularly during inflation – could have directly generated the particles that constitute today’s cosmos. This process isn’t simply about creating particles, but about their generation from the expanding spacetime itself, effectively converting gravitational energy into matter. Current research suggests this mechanism could offer a compelling explanation for the observed abundance of dark matter, a significant component of the universe whose composition remains a mystery. Investigating the specific pathways and rates of particle production during this epoch is therefore crucial, potentially bridging the gap between theoretical cosmology and observational evidence and offering a more complete understanding of the universe’s fundamental building blocks.

Quantum Fields in Curved Spacetime: A Dynamic Arena

Quantum Field Theory in Curved Spacetime (QFTCS) extends the principles of quantum field theory to scenarios where spacetime is not flat, specifically addressing the expanding universe. Unlike standard quantum field theory which assumes a fixed, Minkowski spacetime background, QFTCS accounts for the dynamic geometry described by general relativity. This is crucial because the expansion of the universe itself can provide the energy necessary to create particles from the vacuum, a process not predicted in flat spacetime. Consequently, QFTCS is utilized to model phenomena like Hawking radiation from black holes and the generation of primordial density perturbations that seeded large-scale structure formation. The framework treats gravity as a classical background, influencing the quantum fields defined upon it, and allows calculations of particle number densities and energy spectra arising from the evolving spacetime geometry.

In Quantum Field Theory in Curved Spacetime (QFTCS), the definition of the vacuum state – representing the lowest energy state of a quantum field – is not uniquely determined by the spacetime geometry and significantly influences predictions for particle production. Unlike flat spacetime QFT where a single vacuum is generally sufficient, in curved spacetime multiple, mathematically consistent vacuum choices exist – such as the Bunch-Davies, Adiabatic, and Instantaneous vacua – each corresponding to different boundary conditions imposed on the quantum fields. The chosen vacuum dictates the mode functions used to quantize the field, and consequently affects the calculation of $\langle 0 | \hat{\phi}(x) \hat{\phi}(y) | 0 \rangle$ , the two-point correlation function. Differences in these correlation functions directly translate to varying predicted rates for particle creation, highlighting the importance of carefully selecting a physically motivated vacuum state appropriate for the specific cosmological context.

Calculations within Quantum Field Theory in Curved Spacetime (QFTCS) rely heavily on Bogoliubov coefficients, which establish a transformation between particle creation and annihilation operators in different vacuum states. These coefficients quantify the mixing between positive and negative frequency modes, directly yielding the particle production rate. The WKB approximation simplifies the calculation of these coefficients, particularly for slowly varying backgrounds. Crucially, the choice of vacuum state – including the Bunch-Davies vacuum (often considered the ‘standard’ choice mimicking the Minkowski vacuum), the Adiabatic vacuum (appropriate for nearly-flat spacetimes), and the Instantaneous vacuum (useful for specific limiting cases) – significantly influences the calculated particle production rates, as each defines a different initial state for the quantum fields; the resulting particle fluxes are therefore observer-dependent and intrinsically linked to the chosen vacuum.

Bose-Einstein Condensates: Cosmic Emulation in the Lab

Bose-Einstein Condensates (BECs) function as analog systems for cosmological particle production by replicating the physical conditions relevant to the early universe. Specifically, the collective excitation of a BEC – phonons – behave analogously to massive fields in curved spacetime. This allows researchers to experimentally investigate phenomena such as parametric resonance and particle creation, which are difficult or impossible to directly observe in astrophysical settings. By manipulating the BEC’s parameters – typically through control of interatomic interactions and external potentials – the effective spacetime geometry experienced by the phonons can be tuned, enabling the emulation of expanding universe scenarios and the observation of associated particle production rates. These laboratory-based experiments offer a complementary approach to numerical relativity and provide a means to validate theoretical predictions regarding the origin of particles in the early universe.

The creation of an analog cosmology using Bose-Einstein Condensates (BECs) relies on the acoustic metric, a mathematical transformation that maps the BEC’s effective spacetime to that described by Friedmann-Lemaître-Robertson-Walker (FLRW) metrics, which govern the expansion of the universe. Specifically, the speed of sound in the BEC corresponds to the speed of light in the curved spacetime being modeled, and density fluctuations within the BEC represent gravitational perturbations. This allows researchers to recreate, on a laboratory scale, the conditions of the early universe, such as rapid expansion and particle production, by manipulating the BEC’s properties. The resulting analog spacetime permits the investigation of cosmological phenomena inaccessible through direct observation, offering a controlled environment to test theoretical predictions regarding the universe’s evolution.

Bose-Einstein Condensates (BECs) provide a unique platform for quantifying entanglement generated during analog cosmological particle production. Entanglement, considered a key signature of quantum field theory in curved spacetime (QFTCS), can be directly measured in BEC systems using Entanglement Measures. Specifically, the Logarithmic Negativity, a quantifier of entanglement for mixed states, is employed to characterize the correlations between created particles. This allows for empirical verification of theoretical predictions from QFTCS regarding particle creation rates and entanglement spectra in the early universe, offering a testable link between laboratory experiments and cosmological models. The ability to directly measure entanglement in a controllable setting addresses challenges inherent in observing these effects in astrophysical scenarios.

From Inflation’s Echo to Cosmic Structure: A Connected Narrative

The prevailing cosmological model, known as ΛCDM, robustly supports the theory of inflation – a fleeting but momentous epoch of exponential expansion in the universe’s earliest moments. This inflation wasn’t simply a stretching of space; it fundamentally shaped the conditions for structure formation. Quantum fluctuations, magnified to cosmic scales during inflation, became the seeds of all the galaxies and cosmic voids observed today; these are known as primordial fluctuations. These weren’t random, but rather followed statistical properties described by a power spectrum, effectively imprinting a specific pattern on the early universe. The existence and characteristics of these fluctuations, detectable in the cosmic microwave background, provide compelling evidence for inflation and allow cosmologists to probe the physical conditions of the universe fractions of a second after the Big Bang.

The universe’s large-scale structure – the cosmic web of galaxies and voids – didn’t arise spontaneously, but rather grew from minuscule quantum fluctuations present in the very early universe. These weren’t random; they possessed a specific statistical signature, described by what scientists call Scalar and Tensor Power Spectra. Essentially, these spectra detail the amplitude of fluctuations at different wavelengths, revealing that some wavelengths were initially preferred over others. Scalar perturbations, relating to density variations, dominated the formation of galaxies and clusters, while Tensor perturbations, manifesting as gravitational waves, offer a complementary window into the inflationary epoch. By precisely mapping these power spectra – through observations of the Cosmic Microwave Background and galaxy distributions – cosmologists can reconstruct the conditions of the early universe and trace how these initial, incredibly subtle ripples of energy ultimately evolved into the vast and complex cosmic structures observed today.

Cosmological particle production, the creation of matter during the universe’s earliest moments, serves as a crucial refinement tool for inflationary models. Investigating this process allows researchers to better constrain the parameters governing inflation and, consequently, its downstream effects on cosmic evolution. This work doesn’t simply address the origins of matter; it delves into the potential genesis of dark matter, a substance comprising a significant portion of the universe’s mass-energy density. By understanding how particles were created in the extreme conditions of the early universe, scientists gain insight into the mechanisms that could have produced these elusive particles. This refined understanding of particle creation not only strengthens the connection between inflation and the large-scale structure observed today but also provides a pathway toward resolving fundamental questions about the universe’s composition and its ultimate fate.

The research delves into the creation of dark matter through cosmological particle production, a process where quantum fluctuations in the early universe give rise to observable particles. This mirrors a fundamental tenet of emergent order – that large-scale phenomena aren’t dictated from above, but arise from countless local interactions. As David Hume observed, “It is not possible to find a more simple and regular order than that which prevails in the operations of nature.” The thesis demonstrates this principle by showing how the vacuum state, subject to the expanding spacetime of inflation, spontaneously generates particles – a global effect stemming from the local rules of quantum field theory in curved spacetime. Control, in the traditional sense, is absent; influence, through the underlying physics, is paramount.

The Road Ahead

The pursuit of dark matter’s origin, as explored through the lens of cosmological particle production, reveals less a quest for definitive answers and more an unveiling of deeper, more intricate questions. The system is a living organism where every local connection matters; the theoretical framework, while elegant, inevitably relies on approximations within the extreme conditions of the early universe. Future work must address the sensitivity of these results to deviations from those simplifying assumptions, perhaps by incorporating more complete backreaction effects or exploring alternative models of inflation.

Analog gravity, utilizing Bose-Einstein condensates, provides a compelling, albeit limited, testing ground. The challenge lies in bridging the gap between these condensed matter systems and the truly cosmological scales. Refinements in analog simulation techniques, coupled with a more nuanced understanding of the correspondence between effective spacetimes, could reveal subtle signatures of particle creation previously obscured by experimental noise.

Ultimately, the search isn’t about imposing a preordained structure onto the universe. Top-down control often suppresses creative adaptation. Instead, the focus should remain on identifying the fundamental local rules that govern quantum fields in curved spacetime, trusting that complex phenomena, like dark matter, will emerge as natural consequences of their interplay. The universe doesn’t need a blueprint; it evolves.

Original article: https://arxiv.org/pdf/2602.10331.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Universe’s Genesis: Unraveling the Initial Conditions

Quantum Fields in Curved Spacetime: A Dynamic Arena

Bose-Einstein Condensates: Cosmic Emulation in the Lab

From Inflation’s Echo to Cosmic Structure: A Connected Narrative

The Road Ahead

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