14.4 Cyclic and ekpyrotic models
3 min read•july 22, 2024
Cyclic and ekpyrotic models offer alternative explanations for the universe's origins and evolution. These theories propose endless cycles of expansion and contraction or collisions between higher-dimensional , challenging the traditional and inflationary paradigms.
These models aim to solve cosmological puzzles like the horizon and flatness problems. They make distinct predictions about and , which can be tested through observations and studies.
Cyclic and Ekpyrotic Models
Principles of cyclic and ekpyrotic models
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- Cyclic models propose the universe undergoes endless cycles of expansion and contraction with each cycle beginning with a big bang and ending with a big crunch resulting in no true beginning or end to the universe
- Ekpyrotic models suggest the universe is the result of a collision between two 3-dimensional branes (thin, spatially extended objects) in a higher-dimensional space triggering a big bang and leading to the expansion of the universe with repeated collisions causing cycles of expansion and contraction
- Both models aim to address the same cosmological problems as inflation such as the (why the universe appears uniform on large scales), (why the universe appears geometrically flat), and monopole problem (the absence of magnetic monopoles)
Cyclic vs inflationary model predictions
- Inflationary paradigm predicts a nearly scale-invariant spectrum of primordial density fluctuations, Gaussian distribution of density perturbations, and the existence of primordial gravitational waves
- Cyclic models predict a scale-invariant spectrum of density fluctuations, non-Gaussian distribution of density perturbations, and the absence or suppression of primordial gravitational waves
- Ekpyrotic models predict a nearly scale-invariant spectrum of density fluctuations, non-Gaussian distribution of density perturbations, and strongly suppressed primordial gravitational waves
Observational tests for alternative models
- Cosmic microwave background (CMB) observations search for in the CMB temperature anisotropies and measure the to constrain primordial gravitational waves
- Large-scale structure observations study the distribution of galaxies and clusters (Sloan Digital Sky Survey) to test the predicted non-Gaussianity
- Gravitational wave detection experiments directly search for primordial gravitational waves using ground-based (, ) and space-based detectors ()
- Precision measurements of the cosmic expansion history test the predicted cycles of expansion and contraction using Type Ia supernovae, (sound waves in the early universe), and other probes
Strengths and weaknesses of alternative cosmologies
- Strengths:
- Provide alternative explanations for the observed flatness and of the universe
- Avoid the initial singularity (the beginning of the universe) and the need for a quantum theory of gravity
- Offer a potential resolution to the problem of initial conditions (why the universe started with the conditions it did)
- Weaknesses:
- Require extra dimensions or exotic physics beyond the standard model (, )
- Face challenges in producing a nearly scale-invariant spectrum of density fluctuations
- Difficulty in reconciling with observational constraints on non-Gaussianity and primordial gravitational waves
- Ongoing research focuses on refining the models to better match observations and exploring ways to test and distinguish between cyclic, ekpyrotic, and inflationary models using future cosmological data (CMB polarization, 21cm cosmology)
Key Terms to Review (21)
Baryon Acoustic Oscillations: Baryon acoustic oscillations refer to the regular, periodic fluctuations in the density of baryonic matter (normal matter) in the early universe, which arose from the interplay between gravity and pressure waves in the primordial plasma. These oscillations left an imprint on the large-scale structure of the universe, influencing galaxy formation and distribution.
Big bang: The big bang refers to the prevailing cosmological model that describes the early development of the universe, suggesting it began from an extremely hot and dense state and has been expanding ever since. This event marks the beginning of time, space, and the fundamental forces, leading to the formation of galaxies, stars, and other cosmic structures as the universe cooled.
Branes: Branes are multidimensional objects that arise in string theory and related models, where our universe can be envisioned as a three-dimensional brane embedded in a higher-dimensional space. They play a crucial role in cyclic and ekpyrotic models by providing a framework for understanding how the universe can undergo cycles of expansion and contraction, potentially explaining phenomena such as the Big Bang and cosmic evolution.
Cosmic microwave background: The cosmic microwave background (CMB) is the remnant radiation from the Big Bang, filling the universe and providing a snapshot of the early cosmos when it was just 380,000 years old. This faint glow, almost uniform across the sky, carries crucial information about the universe's formation, composition, and expansion, connecting various areas of cosmological research and theories.
Cyclic model: The cyclic model is a theoretical framework in cosmology that suggests the universe undergoes an infinite series of expansions and contractions, resulting in a continuous cycle of big bangs and big crunches. This model challenges the traditional view of a singular beginning and end to the universe, proposing instead that time and cosmic events repeat indefinitely.
Ekpyrotic Model: The ekpyrotic model is a cosmological theory that suggests the universe originated from the collision of two three-dimensional worlds (branes) in a higher-dimensional space. This model provides an alternative to the traditional Big Bang theory and incorporates ideas from string theory, proposing a cyclical universe that undergoes periodic collisions and bounces, thus generating new epochs of cosmic evolution.
Entropy cycle: The entropy cycle is a theoretical framework in cosmology that describes the recurring process of entropy increase within a cyclic universe model. In this context, the universe undergoes a series of expansions and contractions, each leading to a rise in entropy, which plays a crucial role in determining the structure and fate of the universe. This cycle suggests that even as the universe resets itself through contraction, the accumulated entropy from previous cycles influences the subsequent expansion phase, shaping cosmic evolution over time.
Flatness Problem: The flatness problem refers to the question of why the universe appears to be so geometrically flat, given that any slight deviation from flatness would have led to a universe that either collapses or expands too quickly for galaxies and cosmic structures to form. This issue is especially significant when considering the observed density of matter in the universe, which is very close to the critical density required for flatness. The flatness problem is crucial in understanding the inflationary paradigm, alternative theories of cosmology, and models such as cyclic and ekpyrotic scenarios.
General Relativity: General relativity is Einstein's theory that describes gravity not as a force but as a curvature of spacetime caused by mass. This revolutionary concept redefined our understanding of gravity, allowing for profound implications on the nature of the universe, including structure formation, cosmic evolution, and the behavior of light in strong gravitational fields.
Gravitational Waves: Gravitational waves are ripples in spacetime caused by the acceleration of massive objects, such as merging black holes or neutron stars. These waves carry energy away from their sources and can be detected by sensitive instruments, providing valuable insights into cosmic events and the nature of gravity itself.
Homogeneity: Homogeneity refers to the property of being uniform or similar in composition and structure throughout a given region. In cosmology, this concept implies that the universe is approximately the same in all locations on a large scale, which is essential for understanding the distribution of matter and energy in space. This uniformity provides a foundation for many cosmological models and theories, linking concepts such as cosmic microwave background radiation, the formation of structures, and alternative models of the universe's evolution.
Horizon problem: The horizon problem refers to the question of why regions of the universe, that are far apart and seemingly unconnected, have similar physical properties, like temperature and density. This issue arises because these regions have not had enough time to influence each other since the Big Bang, due to the finite speed of light. It connects to various concepts, including how quantum fluctuations during inflation could seed structure formation and why alternative theories struggle to address this coherence across vast distances.
Large-scale structure: Large-scale structure refers to the organization of matter in the universe on scales larger than galaxies, encompassing galaxy clusters, superclusters, and the vast cosmic web of filaments and voids that form the overall architecture of the cosmos. Understanding large-scale structure is essential for comprehending how the universe evolved and the distribution of galaxies over time.
LIGO: LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment designed to detect gravitational waves—ripples in spacetime caused by massive accelerating objects, such as merging black holes or neutron stars. Its groundbreaking discoveries have revolutionized our understanding of the universe and provided new insights into events predicted by cyclic and ekpyrotic models, while also establishing gravitational wave astronomy as a crucial tool for multi-messenger observations.
LISA: LISA, or the Laser Interferometer Space Antenna, is a proposed space mission designed to detect and measure gravitational waves in the universe. By utilizing three spacecraft positioned in a triangular formation, LISA aims to observe low-frequency gravitational waves, providing crucial insights into various cosmic phenomena like merging black holes and the early universe's dynamics.
M-theory: M-theory is a theoretical framework in physics that unifies all consistent versions of superstring theory and proposes an 11-dimensional universe. It serves as a potential solution to the puzzles of string theory, such as the existence of multiple dimensions and the nature of fundamental particles, connecting deeply with ideas about the cosmos, cosmic cycles, and the nature of reality.
Non-gaussianity: Non-gaussianity refers to the statistical properties of a random variable that deviate from a normal (Gaussian) distribution. In cosmology, it is particularly significant when analyzing the cosmic microwave background (CMB) fluctuations and primordial density perturbations, as it can provide insights into the underlying physics of the early universe, including models like cyclic and ekpyrotic scenarios.
Primordial density fluctuations: Primordial density fluctuations refer to small variations in the density of matter in the early universe, which are thought to be the seeds for the large-scale structure we observe today. These fluctuations are crucial for understanding how matter clumped together to form galaxies, stars, and other cosmic structures, and they play a significant role in models explaining cosmic evolution, the cosmic web, and baryon acoustic oscillations.
String Theory: String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. This theory aims to reconcile quantum mechanics and general relativity, suggesting that the fundamental building blocks of the universe are not particles but rather tiny vibrating strings that give rise to all forces and matter.
Tensor-to-scalar ratio: The tensor-to-scalar ratio, often denoted as $r$, quantifies the relative contributions of tensor perturbations (gravitational waves) and scalar perturbations (density fluctuations) in the early universe's inflationary phase. This ratio is crucial for understanding the dynamics of inflation, as a higher value indicates a greater presence of gravitational waves compared to density fluctuations, which can help distinguish between various inflationary models and their predictions.
Virgo: Virgo is a prominent constellation in the night sky, recognized as one of the twelve zodiac signs. It represents the maiden and is significant for its association with various cosmic phenomena, especially in the context of galactic structures and gravitational wave astronomy. Virgo is also home to the Virgo Cluster, a massive cluster of galaxies that plays a critical role in understanding cosmic evolution and the dynamics of galaxy formation.