Big Bang Theory Lecture Notes

Introduction

• The Big Bang is a physical theory explaining the universe's expansion from an initial high-density, high-temperature state.
• It is supported by various cosmological models that account for phenomena such as light element abundance, cosmic microwave background (CMB) radiation, and large-scale structure.
• The theory is essentially universally accepted, backed by empirical evidence.

Key Concepts

Cosmic Inflation

• Solves horizon and flatness problems through a phase of accelerated expansion.
• Explains the uniformity of the universe.

Age of the Universe

• Estimated at approximately 13.787 billion years based on expansion rate measurements.

Expansion Timeline

• Inflationary Epoch: Rapid expansion phase.
• Observable Universe: Expansion continues, accelerating due to dark energy.

Fundamental Observations

Empirical Evidence

• CMB radiation discovered in 1964, found to be uniform and matching predictions of Big Bang models.
• Redshift observations support expanding universe theory.

Cosmic Microwave Background

• Discovered by Penzias and Wilson, providing substantial confirmation of Big Bang predictions.
• COBE satellite confirmed CMB spectrum as a nearly perfect blackbody.

Light Element Abundance

• Predictions of isotopes like Helium-4 and Deuterium agree with Big Bang nucleosynthesis models.

Historical Milestones

Key Figures

• Alexander Friedmann: Proposed expanding universe via Friedmann equations.
• Edwin Hubble: Empirically confirmed universe expansion with Hubble's law.
• Georges Lemaître: Proposed the universe emerged from a "primeval atom."

Development of the Theory

• Initial resistance due to preference for steady-state universe models.
• Big Bang became favored after CMB discovery and observational evidence.

Current Challenges and Areas of Research

Unresolved Questions

• Nature of dark matter and dark energy.
• Baryon asymmetry: Matter-antimatter imbalance.

Theoretical Considerations

• Need for a theory of quantum gravity to describe the Big Bang singularity.
• Inflationary models and their consequences are under exploration.

Cosmological Models

Lambda-CDM Model

• Accounts for dark matter and dark energy in universe evolution.
• Widely accepted framework for understanding cosmic history post-inflation.

Alternative Theories

• Steady-State Model: Dismissed due to lack of evidence.
• Cyclic Universe & Multiverse: Speculative theories addressing the universe's pre-origin state.

Implications and Future Directions

Hubble's Law

• Universe's expansion rate and its implications for cosmic structure.

Accelerating Universe

• Dark energy's role in accelerating universe expansion.
• Continuing investigations into the nature of dark energy.

Future Observations

• Gravitational-wave observatories may provide insight into early universe conditions.
• Ongoing efforts to resolve mysteries like the horizon and flatness problems.

Philosophical and Religious Context

• Big Bang's implications for creation theories and the universe's origin.
• Ongoing discourse between science and religion regarding cosmology.

Etymology

• Term "Big Bang" coined by Fred Hoyle, though intended as a striking image rather than a pejorative.