GeographyThe Universe & The Big Bang Theory

The Universe & The Big Bang Theory

Notes by ~ SE7EN

Source: Chapter 01 @ PMF IAS (Physical Geography)

Basic Terms

  • Cosmos = Universe.
  • Cosmic = Relating to the universe/cosmos.
  • Cosmic rays = Highly energetic atomic nuclei/particles traveling through space at near-light speed.
    • Direct exposure → Gene mutations → Cancer.
  • Cosmology = Scientific study of large-scale properties of the universe.
  • Cosmological = Relating to the origin & development of the universe.
  • Astronomy = Scientific study of celestial objects (stars, planets, comets) & phenomena originating outside Earth's atmosphere (solar wind, gravitational waves).
  • The Universe = All existing matter & space.
    • Components: Physical (subatomic particles → galactic super-clusters) + Non-physical (light, gravitation, space).
  • Scale: ~100 billion galaxies (avg. 100 billion stars each).
    • Milky Way = 100 billion to 400 billion stars.

The Big Bang Theory

The Expanding Universe

  • Big Bang Theory = Prevailing cosmological model for universe's birth.
    • Event: 13.8 billion years ago → All space contained in a single point of very high density & temperature → Expands in all directions ever since.

The Evolution of The Universe Since The Big Bang

  • Time T = 10⁻⁴³ Sec | 10³² °C → "Inflation" phase (cosmos expands from size of atom to grapefruit in a fraction of a second).
  • Time T = 10⁻³² Sec | 10²⁷ °C → Seething, hot soup of electrons, quarks, and other particles.
  • Time T = 10⁻⁶ Sec | 10¹³ °C → Cooling cosmos permits quarks to clump → protons & neutrons.
  • Time T = 3 min | 10⁸ °C → Too hot to form atoms → charged electrons & protons block light (universe opaque).
  • Time T = 3,00,000 years | 10³ °C → Electrons combine with protons/neutrons → atoms (mostly hydrogen & helium; trace lithium & beryllium) → Light shines (universe transparent).
  • Time T = 1 billion years | -200 °C → Gravity makes primordial H & He coalesce → giant clouds (galaxies); smaller gas clumps collapse → first stars.
  • Time T = 15 billion years | -270 °C → Gravity clusters galaxies → first stars die & spew heavy elements into space → Turn into new stars & planets.

Big Crunch (The Death of The Universe)

  • Causal Chain: Universe reaches max size → begins collapsing under gravity → becomes denser & hotter → ends in a single point of ultra-high density (reversing the Big Bang).

Accelerating Expansion of The Universe & Dark Energy

  • Hubble’s law = Recession velocity of a galaxy is continuously increasing with time.
    • Outcome: Universe gets increasingly colder as matter spreads across space.
  • Timeline: Accelerated expansion began ~5 billion years ago (transition to dark-energy-dominated era).
  • Dark energy = Unknown form of energy hypothesized to permeate all space → drives accelerating expansion of universe.

Evidence for Big Bang Theory

  • Physical Phenomena: Cosmological redshift + Cosmic Microwave Background (CMB/CMD) + Gravitational waves.

Doppler-Shift or Redshift and Blueshift

  • Doppler-Shift = Relative motion changes observed wavelength of light:
    • Moving away (Redshift) = Light shifts to lower frequencies / longer wavelengths (red end).
    • Moving closer (Blueshift) = Light shifts to higher frequencies / shorter wavelengths (blue end).
  • Edwin Hubble = Linked galactic redshift to an expanding universe (galaxies drifting apart).
    • Hubble’s law = Farther galaxies → Recede faster from Earth.

Cosmic Microwave Background (CMD)

  • CMB/CMD (Relic Radiation) = Faint background radio glow strongest in the microwave spectrum.
    • Origin: Oldest light in the Universe (thermal remnants left over from the Big Bang).
    • Significance: Landmark proof of the Big Bang & accelerating expansion of the universe.

Gravitational Waves

  • Prediction: Albert Einstein (1916) in General Theory of Relativity.
  • Concept: Ripples in the fabric of spacetime caused by violent and energetic cosmic processes.
  • Generation: Massive accelerating objects (e.g., orbiting neutron stars or black holes) disrupt spacetime → radiate waves of distorted space at speed of light.
  • First Detection (2015): LIGO (USA) physically sensed spacetime distortions from two colliding black holes nearly 1.3 billion light-years away.

Einstein’s Theory of General Relativity

  • Special Relativity (1905) = Physics laws same for all non-accelerating observers; speed of light in a vacuum is constant; space & time interwoven into spacetime.
  • General Relativity (1915) = Massive objects distort spacetime → Experienced as gravity.
    • Evidence: Gravitational lensing & Gravitational waves.

Gravitational Lensing

  • Concept: Light around a massive object (e.g., black hole) is bent → Acts as a lens for background objects.

Importance of Gravitational Waves

  • Cosmic Sirens: Measure universe's expansion rate (Hubble constant) to understand origin & future.
  • Hubble Constant Calculation Method:
    • Light Flash → Establishes system's velocity (recession speed).
    • Gravitational Waves → Establishes precise system distance from Earth.
    • Velocity + Distance → Calculates exact Hubble constant (expansion rate).

Singularity

  • Singularity = Spacetime condition where gravity is so intense that spacetime ceases to exist & known laws of physics become invalid.
    • Theoretical core of black holes predicted by General Relativity.

Schwarzschild Radius

  • Schwarzschild Radius = Threshold radius where star mass collapse exerts gravitational force so intense that nothing, including light, can escape.

Chandrasekhar Limit

  • Chandrasekhar Limit (1930) = Proposed by Indian astrophysicist Subrahmanyan Chandrasekhar.
    • Definition: The maximum mass (~1.44 solar masses) for a dying star to remain a stable white dwarf.
    • Over Limit: Mass > 1.44 solar masses → Gravitational collapse into a neutron star or black hole.

Galaxy

  • Definition = System of millions/billions of stars, gas, and dust held together by gravitational attraction.
  • Scale: Smallest (~1,00,000 stars) to largest (~3,000 billion stars).
  • Classifications:
    • Regular Galaxies (Spiral & Elliptical).
    • Irregular Galaxies (1/10th of all galaxies; stars are very old).

Regular Galaxies: Spiral vs. Elliptical

  • Spiral Galaxies (E.g., Milky Way):
    • Disc-shaped with greater concentration of stars near the center (bulge).
    • Star distribution is nonuniform.
    • Center = Old stars; Outer Arms = Youngest stars.
    • Rich in interstellar gas → Continuous formation of new, bright stars.
  • Elliptical Galaxies:
    • Member stars are mostly very old; no new star formation.
    • They are the brightest galaxies in the universe.

Dark matter

  • Anomalous Rotation: Outer arms of Milky Way rotate too fast for the amount of visible matter inside them.
    • Cause: Extra mass provided by dark matter.
  • Dark Matter = Hypothetical form of matter accounting for ~85% of total matter in the universe.
    • Does not interact with electromagnetic radiation (light) → invisible & hard to detect.
    • Interacts with the rest of the universe only through gravity.
  • Universe Content: Dark Energy + Dark Matter = 95.1% (Normal matter = 4.9%).

Our Galaxy (The Milky Way)

  • Shape: Flat disc with a central bulge.
  • Dimensions:
    • Diameter = 1,50,000 to 2,00,000 light-years.
    • Nucleus thickness = 10,000 light-years.
    • Disc thickness = 500 - 2,000 light-years.
  • Light-year = Distance light travels in one year at 3,00,000 km/s.
    • Sun-Earth Distance = 14,95,98,000 km = 8.311 light-minutes.
  • Milky Way Features:
    • Contains 100–400 billion stars.
    • Inner stars travel faster than outer stars.
    • Sagittarius A* = Supermassive black hole at the galactic center.
    • Solar System Location = Orion Arm, located 26,000 light-years from the center (~1/3rd out).
    • Star Demographics: Sun-like stars are rare; dim and cooler red dwarfs are highly common.
    • Sun’s Rotation: Speed = 285 km/s; completes one orbit in 220 million years.
    • Andromeda = Closest large galaxy to Milky Way (2 million light-years away).

Star Formation (Stellar Evolution or Life Cycle of a Star)

  • Overview: Nebular collapse → Protostar stage → Main sequence fusion → Red Giant expansion → Core collapse remnants.

Stellar Evolution Pathway

  1. Nebula = Gas & dust cloud (primarily H & He); birthplaces of stars.
  2. Protostar = Early star; nuclear fusion not yet active (luminosity solely from gravitational contraction heating).
    • Surrounding dust blocks visible light, making observation difficult.
    • Nuclear fusion definition: 2 Hydrogen atoms fuse → 1 Helium atom, liberating massive energy (requires initial temp of millions of °C).
  3. T Tauri Star = Young, gravity-contracting star; intermediate stage between Protostar and low-mass main sequence star.
  4. Main Sequence Star = Stable star with core nuclear fusion in full swing (e.g., Sun).
  5. Branching Life-Cycle Paths:
    • Low-mass star (Sun-like): Main Sequence → Red Giant → Planetary Nebula → White Dwarf → Black Dwarf.
    • High-mass star: Main Sequence → Red Supergiant → Supernova → Neutron Star or Black Hole.

Red Dwarf

  • Properties: Faintest main sequence stars (brightness <1/1000th of the Sun); surface temp ~4000 °C; invisible to naked eye.
  • Abundance: Compose 3/4ths of stars in the Milky Way (e.g., Proxima Centauri).

Red Giant

  • Scale: Diameter 10 to 100 times the Sun; high brightness with lower surface temp.
  • Formation Causal Chain: Core runs out of hydrogen fuel → Core contracts under gravity & heats up → Hydrogen fuses to helium in a shell surrounding the hot, dense degenerate helium core → Shell volume increases → Shell fusion releases more energy and pushes harder against gravity → Outer volume expands enormously.
  • Fusion: Core hot enough to fuse helium into heavy elements like carbon.

Degenerate Matter

  • Definition: Extreme high-density state where core fuel runs out → fusion slows → gravity collapses star → electrons are displaced from regular shells & compressed close to nuclei, minimizing atomic volume.

Red Supergiant

  • Process: Massive red giant condenses further → Core heats up → Burns remaining hydrogen → Outer layers expand enormously.

Planetary Nebula

  • Definition: Shell of gas & dust ejected into space when a dying red giant collapses into a white dwarf.
  • Solar Fate: Sun expands to red giant (past Venus orbit) → gravity collapse → ejects outer layers as planetary nebula (lasts tens of thousands of years) → shrinks to white dwarf.

White Dwarf

  • Properties: Very small, hot star remnants; spent nuclear energy; high-density degenerate matter (one spoonful = several tonnes).
  • Significance: Last active stage of low-to-medium mass stars like the Sun.

Nova

  • Nova = Occurs on a white dwarf in a binary system.
    • Mechanism: White dwarf pulls hydrogen from companion star → Gas builds up on white dwarf surface → Triggers surface runaway nuclear fusion → Sudden stellar brightening.
    • Repetition: Recurs at regular intervals as long as gas accretion continues.

Black dwarf

  • Remnant: A white dwarf that has cooled sufficiently and no longer emits significant heat or light.
  • Existence: None exist yet (cooling time required is longer than the current age of the universe (13.8 billion years)).

Similar Term: Brown Dwarfs

  • Definition: Objects too large to be planets but too small to be stars.
  • Causal Factor: Formed from collapsing gas clouds, but core density is insufficient to trigger nuclear fusion.

Supernova

  • Definition: Explosive death of a star; briefly achieves brightness of 100 million Suns.
  • Consequences:
    • Primary source of cosmic rays.
    • Shock waves compress nearby nebular clouds → triggers new star birth.
  • Triggers: Type I (Ia) and Type II.

Type I Supernova or Type Ia Supernova (read as one-a)

  • Mechanism: White dwarf in binary system accumulates companion star material → Core temp rises → Ignites carbon fusion → Runaway nuclear fusion completely blasts the star to bits.
  • Cosmic Beacons: All Type Ia supernovae have nearly the same maximum brightness.
    • Used to measure recession distance (weaker light = farther) & verify universe's expansion rate.

The Difference Between Nova and Type I Supernova

  • Nova: Brightness increases up to 1 million times; white dwarf survives; outbursts repeat regularly.
  • Type I Supernova: Shines as bright as an entire galaxy of billions of stars; white dwarf is completely destroyed.

Type II Supernova

  • Mechanism: Gravitational collapse of the iron core of a massive star (e.g., supernova of a red supergiant).

Importance of Supernova: Creating and Dispersing New Elements

  • Elements Oxygen to Iron: Manufactured via successive nuclear fusion shells in dying massive stars.
  • Elements Heavier than Iron (e.g., Uranium, Gold): Synthesized during a supernova explosion due to massive energy & free neutron release.
  • Dispersion: Explosion expels all synthesized elements into space → Recycled into new star and planet formation.

Neutron Stars

  • Properties: Extremely dense stars composed mainly of neutrons (e.g., mass of 3 Suns fit into a 20 km diameter sphere).
  • Causal Origin: Gravitational collapse forces electrons and protons to combine into neutrons.
  • Limit: If mass is any greater, gravity overcomes neutron degeneracy pressure → further collapse into a black hole.

Black Holes

  • Properties: Infinite density; gravitational pull so strong that nothing, not even light, can escape.
  • Mechanism: Ultimate core collapse of massive stars at end of lifetime → Distorts spacetime & accretes surrounding matter.

Constellations

  • Definition = Group of stars forming a recognizable shape in the night sky.
  • Famous Constellations:
    • Ursa Major (Great Bear, Big Dipper, Saptarshi): Moves around the Pole Star; northern sky (not visible from parts of the southern hemisphere).
    • Orion (The Hunter): Visible in winter late evenings. Has three middle belt stars.
    • Cassiopeia: Visible in winter early nights; looks like a distorted W or M.
    • Leo Major.
  • Sirius Location: Imagine a straight line passing eastwards through the three middle stars of Orion.

Pole Star

  • Definition: Star situated along Earth's rotational axis.
  • Polaris (North Star): Current pole star; a system of 3 stars.
  • Properties: Stationary reference point; visible only from the northern hemisphere.
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