Lecture Notes: The Chandra Lecture Series - Dr. Hansa Padmanaban on the Early Universe

Index

1. Introduction by Samid H.

   • 1.1 Chandra Lecture Series Overview
   • 1.2 Speaker Introduction: Dr. Hansa Padmanaban

2. Dr. Hansa Padmanaban’s Lecture on Early Universe

   • 2.1 Overview of the Universe’s Early History
   • 2.2 Understanding the Scale of the Universe
   • 2.3 Zooming Out: A Movie Visualization
   • 2.4 The Expanding Universe
   • 2.5 Time and Observations in Cosmology
   • 2.6 Evolution of the Universe: Key Milestones
   • 2.7 Key Insights
   • 2.8 Conclusion

3. Lecture Notes: Dr. Hansa Padmanaban on Hydrogen and the Universe’s Evolution

   • 3.1 Exploring the Composition of the Universe
   • 3.2 Hydrogen and Cosmic Evolution
   • 3.3 Observing Reionization
   • 3.4 21 cm Hydrogen Line
   • 3.5 The Square Kilometer Array (SKA)
   • 3.6 Conclusion

4. Lecture Notes: Advances in Understanding the Universe – Dr. Hansa Padmanaban

   • 4.1 The Immense Scale of Data in Modern Astronomy
   • 4.2 First Detection of the Hydrogen Signal
   • 4.3 New Techniques in Observational Astronomy
   • 4.4 Carbon Monoxide (CO) Intensity Mapping
   • 4.5 Insights from the James Webb Space Telescope (JWST)
   • 4.6 Cosmic Timeline and Current Understanding
   • 4.7 Conclusion

2. Dr. Hansa Padmanaban’s Lecture on Early Universe

2.1 Overview of the Universe’s Early History

• Age of the Universe: 13.8 billion years old.
• Focus Area: The first billion years, when the first stars and galaxies formed, known as the “final frontier of cosmology.”
• Challenge: This period is studied through invisible light (radio frequencies) since visible light was not yet present.

2.2 Understanding the Scale of the Universe

• Microphysics vs Cosmological Scales:
  • Microphysics: Deals with very small scales, like protons.
  • Cosmological scales: Involves vast distances, such as galaxies.
• Units of Measurement:
  • Light Year (ly): The distance light travels in a year.
  • Parsec (pc): Another unit of distance; 1 parsec ≈ 3.26 light years.
  • Scientific Notation:
    • (10^{-8}): Small scales (e.g., subatomic).
    • (10^{16}): One light year.
    • (10^{22}): Typical size of galaxies.

2.3 Zooming Out: A Movie Visualization

• Earth to Cosmic Scales:
  • Satellite orbits, solar system, and nearby stars.
  • Galactic structures like the Milky Way and Andromeda (1 million light years away).
  • Cosmic Horizon: The farthest observable light, the Cosmic Microwave Background (CMB), marking the universe’s earliest light.

2.4 The Expanding Universe

• Hubble’s Discovery: A relationship exists between a galaxy’s speed and distance, supporting the concept of an expanding universe.
• Redshift: As galaxies move away, their light is stretched (red-shifted), showing increased distance and speed.

2.5 Time and Observations in Cosmology

• Observing the Past: Light takes time to travel, so observing distant objects allows scientists to see their past states.
• James Webb Space Telescope (JWST): A tool for studying distant objects and exploring the early universe.

2.6 Evolution of the Universe: Key Milestones

1. The Big Bang: Universe’s origin; unification of quantum mechanics and gravity.
2. Formation of Atoms: The first atoms formed from particles.
3. Cosmic Dawn: The first stars and galaxies lit up, ending the “Dark Ages” and beginning the “Cosmic Renaissance.”
4. Galactic Evolution: Structure formation, including star clusters and galaxy clusters.
5. Today: Humans appear late in the cosmic timeline.
6. Future: Sun will evolve into a red giant in ~5 billion years.

2.7 Key Insights

• The early universe holds the key to understanding cosmic origins.
• Technological advances, such as the James Webb Space Telescope, enable deeper exploration of the universe’s past.

2.8 Conclusion

• The lecture provided an introduction to the universe’s early history and explored key aspects of cosmic evolution, with a focus on the first billion years.

3. Lecture Notes: Dr. Hansa Padmanaban on Hydrogen and the Universe’s Evolution

3.1 Exploring the Composition of the Universe

• Dark Universe:
  • Dark Matter: Indirectly inferred, not yet directly detected.
  • Dark Energy: Dominates ~70% of the universe, theorized to have negative pressure.
• Ordinary Matter: Comprises only 5% of the universe, mostly in the form of hydrogen and helium gases.
  • Hydrogen: Forms 92% of baryonic matter.

3.2 Hydrogen and Cosmic Evolution

• Reionization:
  • Hydrogen was initially neutral.
  • Ultraviolet (UV) light from early stars ionized hydrogen, splitting protons and electrons.
  • By 1 billion years after the Big Bang, overlapping ionized bubbles marked the completion of reionization.

3.3 Observing Reionization

• Quasars: Bright, distant galaxy nuclei powered by black holes. Their light interacts with hydrogen, creating an absorption spectrum.
• Challenges: Observing faint galaxies responsible for reionization is difficult, and the JWST may miss crucial faint galaxies.

3.4 21 cm Hydrogen Line

• Quantum Mechanics: Hydrogen atoms have two states of proton and electron spins (aligned or opposite).
• 21 cm Radiation: Emission occurs when hydrogen transitions between these states.
• Mapping Hydrogen: 21 cm radiation is used to map hydrogen during the Cosmic Dawn and reionization.

3.5 The Square Kilometer Array (SKA)

• Overview: A global radio telescope project in South Africa and Australia, covering an area of 1 square kilometer.
• Goals: Observing the early universe and Cosmic Dawn through 21 cm emissions and mapping hydrogen evolution.
• Data Scale: SKA will process 11 exabytes daily, a scale that surpasses the entire global internet’s data (2020).

3.6 Conclusion

• Understanding hydrogen’s evolution through techniques like 21 cm mapping is crucial for studying the universe’s early phases.

4. Lecture Notes: Advances in Understanding the Universe – Dr. Hansa Padmanaban

4.1 The Immense Scale of Data in Modern Astronomy

• Exabyte Scale: The SKA will process data at the scale of 11 exabytes daily.
• Context: 1 exabyte equals (10^{18}) bytes—an immense amount of data, surpassing global internet traffic.

4.2 First Detection of the Hydrogen Signal

• 2018 Experiment: Initial detection of hydrogen signal at a redshift of 17 (around 300 million years after the Big Bang).
• Indian-Led Counter-Experiment: The detection was not confirmed due to possible instrumental effects.

4.3 New Techniques in Observational Astronomy

• Cross-Correlation: Combining signals from multiple telescopes (radio + optical) to enhance reliability and reduce noise.
• Example: MeerKAT (radio) + Optical Telescopes.

4.4 Carbon Monoxide (CO) Intensity Mapping

• CO as a Star Formation Tracer: CO molecules emit radiation that can trace star formation.
• CO Mapping: COMAP project measures CO emissions from redshifts (z \sim 2–7).

4.5 Insights from the James Webb Space Telescope (JWST)

• JWST’s Findings: Detection of unexpectedly bright and massive galaxies at early cosmic times.
• Implications: JWST observations challenge existing models of

early galaxy formation.

4.6 Cosmic Timeline and Current Understanding

• Main Questions:
  • How did the universe evolve into its present state?
  • When did the first stars and galaxies form?

4.7 Conclusion

• Dr. Padmanaban emphasized that with emerging technologies and collaborations (like the SKA and JWST), our understanding of the early universe is poised for significant advancement.

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