Astrophysics

Question 1

Astronomical Objects

Answer 1

Key Astronomical Objects

Universe

• The universe is everything that exists, including all space, time, matter, dark matter, energy, and dark energy.
• It is the largest known structure and contains billions of galaxies.
• The observable universe is the portion from which electromagnetic radiation has reached Earth since the universe’s formation (~13.8 billion years ago).
• The universe is expanding, with galaxies moving away from each other. The further apart they are, the faster they move (Hubble’s Law).

===

Galaxy

• A galaxy is a cluster of billions of stars held together by gravity.
• Earth and the Solar System are in the Milky Way, a spiral galaxy.
• The Sun takes ~230 million years to complete one revolution around the Milky Way.

===

Stars

• Stars fuse hydrogen into helium via nuclear fusion, releasing vast amounts of energy as electromagnetic radiation.
• They form from gas and dust pulled together by gravity.
• The Sun is the star closest to Earth, an average-sized small-mass star with a mass of 2 × 10³⁰ kg.

===

Solar Systems

• A solar system consists of a star and all gravitationally bound objects orbiting it (e.g., planets, moons, comets, asteroids).
• Earth is the third of eight planets orbiting the Sun, taking 365.25 days to complete one orbit.

===

Planets

• A planet is a large, spherical object orbiting a star, held together by its own gravity.
• The eight planets orbiting the Sun are:
  
  • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

===

Planetary Satellites

• Planetary satellites are bodies orbiting a planet.
• The Moon is Earth’s natural satellite, taking 27.3 days to orbit Earth (equal to its rotation period).
• Artificial satellites, like GPS and communication satellites, also orbit Earth.

===

Comets

• Comets are made of ice and rock, traveling in elliptical orbits around the Sun.
• They originate from the Oort Cloud.
• Halley’s Comet completes one orbit every 76 years.

Question 2

Stellar Evolution: Early Stages of Star Formation

Answer 2

1. Nebula

• A giant cloud of hydrogen gas and dust.
• Gravitational collapse: Atoms clump together, increasing density.
• Potential energy → kinetic energy → temperature rise.

===

2. Protostar

• Collapsing gas heats up, emitting infrared radiation.
• Work done by particle collisions increases kinetic energy and temperature.
• Detected via infrared telescopes. .

===

3. Nuclear Fusion

• Core reaches ~15 million K → hydrogen fusion (proton-proton chain) begins:
  
  • 4H → He + 2γ + 2ν + 2e⁺ (gamma photons, neutrinos, positrons).
• The momentum of gamma-ray photons creates an outward pressure called radiation pressure, balancing the inward gravitational attraction.

===

4. Main Sequence

• Stable fusion phase: Hydrogen → helium in the core.
• Duration:
  
  • Sun-like stars: Billions of years.
  • Massive stars: Millions of years (faster fuel consumption).
• Equilibrium: Outward radiation pressure = inward gravitational force.

===

Key Notes on Stellar Evolution

• All stars begin their life cycle through the same four stages: nebula, protostar, nuclear fusion, and main sequence.
• Hydrogen fusion occurs when the kinetic energy of hydrogen atoms overcomes electrostatic repulsion, releasing vast amounts of energy.
• The main sequence is the longest phase of a star’s life, during which it remains stable due to the balance between radiation pressure and gravity.

Question 3

Stellar Plasma

Answer 3

• Stars are primarily composed of hydrogen and helium, existing in a phase of matter called plasma.
• Plasma consists of:
  
  • Ionised hydrogen nuclei (H⁺), also known as protons.
  • Doubly ionised helium nuclei (He²⁺).
  • Free electrons.
• A small fraction of atoms remain non-ionised, with electrons occupying excited energy states.

Question 4

Evolution of a Low-Mass Star (0.5–10 M_☉)

Answer 4

1. Red Giant Phase

• Hydrogen depletion: Core fusion slows, reducing radiation pressure.
• Core collapse: Gravitational force > gas/radiation pressure → core compresses, heating up.
• Shell hydrogen burning: Hydrogen in outer layers fuses due to core heat.
• Core helium burning: Collapsing core reaches temps to fuse He → C/O.
• Outer layers expand/cool → red giant.

===

2. Planetary Nebula

• Helium exhaustion: Core contracts again, igniting helium shell burning.
• Unstable core: C/O core cannot fuse further → star collapses.
• Ejected layers: Outer gases expelled as a planetary nebula.

===

3. White Dwarf

• Degenerate core: Collapses into a dense, hot, solid white dwarf (no fusion).
  
  • Electron degeneracy pressure: Prevents further collapse (Pauli exclusion principle).
  • Cools slowly: Emits stored energy → eventual black dwarf (theoretical).

===

Key Physics:
- Mass limit: White dwarfs ≤ 1.4 M_☉ (Chandrasekhar limit).

Question 5

Electron Degeneracy Pressure

Answer 5

Extreme Compression

• Core collapse squeezes matter into a tiny volume, stripping electrons of their normal energy transitions.

===

Energy Level Forcing

• Normally, electrons occupy the lowest available energy levels, with only excited electrons in higher states.
• Under extreme compression, electrons are forced into higher energy levels—not because they’re excited, but because all lower states are completely filled (Pauli exclusion principle).

===

Electron Degeneracy Pressure

• Outward Force: Generated when electrons resist further compression due to quantum mechanical crowding.
• Critical Role: Halts gravitational collapse in white dwarfs (supports against infinite shrinkage).

Question 6

Evolution of a Massive Star (>10 M_☉)

Answer 6

Key Difference:Shorter main-sequence lifetime due to rapid fuel consumption.

1. Red Supergiant Phase

• Core Collapse:
  
  • H-exhaustion → He→C fusion → successive shell/core burning up to iron (Fe).
  • Each stage: Heavier elements fuse at higher temps(up to ~3×10⁹ K)/pressures (e.g., Si→Fe).
• Structure:
  
  • Onion-layer core: Fe at center, surrounded by shells of lighter elements.
  • Outer envelope: Expands to ~1,000× Sun’s radius.

===

2. Supernova (Type II)

• Iron Core Collapse:
  
  • Fe cannot fuse → core collapses in milliseconds.
  • Neutronization: p + e⁻ → n + ν_e (releases neutrinos).
• Explosion:
  
  • Rebound shockwave ejects outer layers at ~10% c.
  • Synthesizes elements heavier than iron (e.g., Au, U) via rapid neutron capture (r-process).

===

3. Compact Remnants

Neutron Star

• Formation: Core mass 1.4–3 M_☉
• Properties:
  
  • Density: ~10¹⁷ kg/m³ (1 sugar cube = 1 billion tons!)
  • Size: ~12 km diameter
  • Rotation: Up to 600 rev/sec (pulsars emit lighthouse-like beams)

Black Hole

• Formation: Core mass >3 M_☉
• Properties:
  
  • Singularity: Infinitely dense point at center
  • Event Horizon: Boundary where escape velocity > c
    
    • Schwarzschild radius: R_S = 2GM/c²
  • No light escape: All paths curve inward (Einstein’s relativity)

===

Key Physics (OCR Focus)

• Energy Scales:
  
  • Supernova energy ~10⁴⁴ J (≈10²⁸ H-bombs!)
• Observed Phenomena:
  
  • Pulsars: Precise radio/X-ray pulses (used to test general relativity)
  • Black Hole Detection:
    
    • X-rays from accretion disks
    • Gravitational waves (LIGO)

===

Density Comparison

• Earth: 5×10³ kg/m³
• White Dwarf: 10⁹ kg/m³
• Neutron Star: 10¹⁷ kg/m³
• Black Hole: ∞ (singularity)

Question 7

The Chandrasekhar Limit (1.4 M_☉)

Answer 7

Definition
- Maximum mass for a stable white dwarf supported by electron degeneracy pressure.

===

Key Implications
- Core Mass < 1.4 M_☉:
- Remains a white dwarf (C/O core).
- Electron degeneracy pressure balances gravity.
- Core Mass ≥ 1.4 M_☉:
- Collapse continues → neutron star or black hole.
- Neutronisation: p + e⁻ → n + ν_e.

===

Stellar Fate by Mass

1. Low/Medium-Mass Stars (<8 M_☉):
   
   • Evolve into white dwarfs (if core < 1.4 M_☉).
   • Example: Sun → Red Giant → Planetary Nebula → White Dwarf.
2. High-Mass Stars (>8 M_☉):
   
   • Core exceeds Chandrasekhar limit → Type II Supernova.
   • Remnant:
     
     • Neutron star (1.4–3 M_☉ core).
     • Black hole (>3 M_☉ core).

Question 8

Hertzsprung-Russell (HR) Diagram

Answer 8

Axes:

• Y-axis (Luminosity): Relative to Sun (10⁻⁴ to 10⁶ L_☉).
• X-axis (Temperature):
  
  • Left: Hot (~30,000 K) → Right: Cool (~300 K).
  • Alternative: Spectral classes (OBAFGKM).

===

Key Regions

1. Main Sequence (Diagonal band):
   
   • 90% of stars (e.g., Sun).
   • Trend: Hotter → Brighter (fusion rate ∝ mass^3.5).
2. Red Giants/Supergiants (Top-right):
   
   • High L, Low T → Enormous size (10–1000× Sun’s radius).
   • Example: Betelgeuse.
3. White Dwarfs (Bottom-left):
   
   • High T, Low L → Tiny size (~Earth’s radius).
   • Degenerate electron cores.

===

Stellar Evolution on HR Diagram

• Sun’s Path:
  
  1. Main Sequence (H→He fusion).
  2. Red Giant (He→C/O; expands, cools).
  3. White Dwarf (C/O core; shrinks, heats initially).

OCR Exam Focus

• Interpretation:
  
  • Luminosity ≈ Size × T⁴ (Stefan-Boltzmann law).
  • Main Sequence Lifetime: ∝ Mass/Luminosity (high-mass stars die faster).
• Calculations:
  
  • Absolute magnitude: M = m − 5log(d/10).
  • Wien’s law: λ_max = 2.9×10⁻³/T (K).

===

Key Exclusions

• Transitory Phases: Too brief to plot (e.g., protostars, supernovae).
• Black Holes: Invisible (no EM emission).

===

Visual Clues
- Size Gradient:
- White dwarfs (●) < Main sequence (★) < Giants (⬤) < Supergiants (🔴).
- Spectral Classes:
- O (Blue, 30,000 K) → M (Red, 3,000 K).

Question 9

Energy Levels (Astrophysics)

Answer 9

1. Energy Level Transitions

• Excitation:
  
  • Electron absorbs energy → jumps to higher level.
  • Energy sources:
    
    • Photon of matching ΔE = hf.
    • Thermal energy (collisions).
    • Electric field.
• De-excitation:
  
  • Electron drops to lower level → emits photon (E = hf).

===

2. Negative Energy Values

• Reference: E = 0 at infinite distance (free electron).
• Bound electrons: Negative energy values (confined by nucleus).
  
  • Ground state (n = 1): Most negative (e.g., −13.6 eV for hydrogen).
  • Ionisation energy: Minimum energy to remove an electron (e.g., +13.6 eV for H).
• Negative Energy Convention:
  
  • Bound electrons have negative energy because work must be done against the electrostatic attraction to free them.

===

3. Hydrogen Atom Example

• Ground state: E = −13.6 eV.
• Photon absorption:
  
  • If E_photon > 13.6 eV → ionisation + kinetic energy.
• Spectral lines:
  
  • Lyman series: Transitions to n = 1 (UV).
  • Balmer series: Transitions to n = 2 (visible).

Question 10

Spectra Types

Answer 10

Continuous Spectra

• Produced by: Hot, dense objects like stellar cores or incandescent solids
• Characteristics:
  
  • Contains all visible wavelengths without gaps
  • Appears as a smooth rainbow of colours
• Example: The Sun’s core emission (before atmospheric absorption)

===

2. Emission Line Spectra

• How formed:
  
  • Excited electrons drop to lower energy levels
  • Emit photons with precise energies (ΔE = hf)
• Key features:
  
  • Shows bright coloured lines on dark background
  • Acts as elemental “fingerprint” (each element unique)
  • E.g., Hydrogen emits at 656.5 nm (red), 486.3 nm (blue-green)

===

3. Absorption Line Spectra

• How formed:
  
  • Cool gas absorbs specific wavelengths from continuous light
  • Creates dark lines at exact emission frequencies
• Stellar application:
  
  • Reveals star composition (Sun’s spectrum shows H/He lines)
  • Dark lines match emission lines of the elements

===

Key Physics Concepts

• Photon energy: E = hc/λ (where h = Planck’s constant)
• Hydrogen energy levels: Eₙ = −13.6/n² eV
• Energy conservation: Absorbed photon energy = emitted photon energy

Question 11

Transmission Diffraction Gratings

Answer 11

Definition: A glass/plastic slide with regularly spaced parallel slits used to disperse light into its spectrum.

===

Key Advantages Over Prisms

• Greater angular dispersion (sharper colour separation).
• Higher resolution (distinguishes closer wavelengths).
• Sharper fringes vs. double-slit setups.

===

2. Angular Separation:
   • Between orders: Δθ = θ₂ - θ₁.
   • Example: Red (656 nm) vs. blue (486 nm) in hydrogen spectrum.

===

3. Maximum Visible Order:
   • When θ = 90° → n_max = d/λ (round down to integer).

===

Astronomical Applications

• Stellar Composition: Analyse absorption/emission lines.
• Doppler Shifts: Measure star velocities from wavelength changes.

Question 12

Black Body Radiator

Answer 12

• Definition: A theoretical object that absorbs all incident radiation and emits a continuous spectrum determined solely by its temperature

===

• Key Property: Perfect emitter and absorber at all wavelengths.

Question 13

Wien’s Displacement Law

Answer 13

• Definition: Relates a black body’s peak emission wavelength (λ_max) to its temperature (T).
• Equation:
  λ_max T = 2.9 × 10⁻³ m·K

====

Key Concepts

1. Temperature Dependence:
   • Higher T → Shorter λ_max (inverse relationship).
   • Hot stars: Peak in blue/UV (λ ≈ 300 nm for T ~10,000 K).
   • Cool stars: Peak in red/IR (λ ≈ 700 nm for T ~4,000 K).

====

2. Intensity:
   
   • Higher T → Greater radiation intensity at all wavelengths.

===

3. Stellar Applications:
   
   • Measure star temperatures from observed λ_max.
   • Classify stars (e.g., O-type = hot, M-type = cool).

Question 14

Stefan’s Law

Answer 14

**Stefan-Boltzmann Law

• Definition: Relates a star’s luminosity (L) to its surface temperature (T) and radius (r).
• Equation:
  L = 4πr²σT⁴
  
  • σ = Stefan-Boltzmann constant (5.67×10⁻⁸ W m⁻² K⁻⁴).

===

Key Relationships

1. Temperature Dependence:
   
   • L ∝ T⁴ → Small T changes hugely affect luminosity.
   • Example: If T doubles, L increases by 16×.
2. Size Dependence:
   
   • L ∝ r² → Larger stars emit more light at the same T.
3. Combined Effect:
   
   • A star can be luminous because it’s:
     
     • Very hot (e.g., blue giants).
     • Very large (e.g., red supergiants).

Question 15

Estimating the Radius of Stars

Answer 15

Method: Combine Wien’s Law and Stefan-Boltzmann Law via a 3-step process:

1. Find Surface Temperature (T):
   
   • Measure peak wavelength (λ_max) from star’s spectrum.
   • Apply Wien’s Law:
     T = 2.9 × 10⁻³ / λ_max
     (λ_max in metres, T in kelvin).
2. Find Luminosity (L):
   • Use inverse square law if flux (F) and distance (d) are known:
     L = 4πd²F
3. Calculate Radius (r):
   • Rearrange Stefan-Boltzmann Law:
     r = √(L / 4πσT⁴)
     (σ = 5.67×10⁻⁸ W m⁻² K⁻⁴).

Question 16

Units for Astronomical Distances

Answer 16

Astronomical Unit (AU)

• Definition:
  
  • Mean distance from Earth to Sun = 1.496 × 10¹¹ m (≈1.5 × 10¹¹ m).
• Key Points:
  
  • Based on Earth’s elliptical orbit (varies from 1.471–1.521 × 10¹¹ m).
  • Used for solar system distances (e.g., Mars at 1.5 AU).

===

Light-Year (ly)

• Definition:
  
  • Distance light travels in 1 year = 9.5 × 10¹⁵ m.

===

Parsec (pc)

• Definition:
  
  • Distance at which 1 AU subtends 1 arcsecond = 3.09 × 10¹⁶ m.
• Key Equations:
  
  • 1 arcsecond = 1/3600 degree.
  • Trigonometry:
    1 pc = 1 AU / tan(1 arcsecond) ≈ 1 AU / (1/3600 degree.)
• Uses:
  
  • Interstellar distances (e.g., Proxima Centauri = 1.3 pc).

===

Why It Matters

• AU: Measures planetary orbits.
• pc/ly: Quantifies stellar/galactic distances.
• Conversions:
  
  • 1 pc ≈ 3.26 ly.
  • 1 ly ≈ 63,241 AU.

Question 17

Stellar Parallax

Answer 17

Stellar Parallax

• Definition:
  The apparent shift in position of a nearby star against a fixed background of distant stars when observed from different points in Earth’s orbit.

Key Principles

• The angle subtended by 1 AU (Earth-Sun distance) at the star’s distance (d).

2. Parallax Angle (p):
   
   • Measured in arcseconds (1” = 1/3600°).
   • Smaller p → larger distance.

===

Parallax Equation
For small angles (p < 1°):

d (pc) = 1 / p (“)
- d = distance to star in parsecs (pc).
- p = parallax angle in arcseconds

• Limitation: Accurate for distances ≤ 100 pc (angles become too small beyond this).

===

Units:

• 1 arcminute (‘) = 60 arcseconds (“).
• 1 arcsecond (“) = 1/3600 degree.
• 1 radian ≈ 206,265 arcseconds.

Question 18

Cosmological Principle

Answer 18

Isotropic

• The universe appears the same in all directions to every observer.
• Key point: While matter clumps locally (e.g., galaxies, voids), the large-scale structure is uniform when averaged over vast volumes.

===

Homogeneous

• Matter is uniformly distributed with uniform density at large scales.
• Exam tip: Avoid vague phrasing like “looks the same in all directions”—this is not accepted.

===

Universal Laws of Physics

• The same physical laws apply everywhere in the universe (e.g., gravity, electromagnetism).
• Verified by observations (e.g., spectral lines of distant galaxies match Earth-based experiments).

Question 19

The Doppler Effect

Answer 19

• Definition: The apparent change in wavelength/frequency of waves when the source and observer are in relative motion.

===

Key Observations

1. Direction Matters:
   • Source moving TOWARDS observer:
     
     • Wavefronts squashed → λ decreases (λ – Δλ), f increases.
   • Source moving AWAY from observer:
     
     • Wavefronts stretched → λ increases (λ + Δλ), f decreases.
2. Electromagnetic Waves:
   • Observed in light spectra (e.g., redshift of distant galaxies).
   • Redshift (λ increases): Evidence of universe expansion.

===

Doppler Equations

1. Relative Speed Condition (when Δv &laquo_space;c):Δλ/λ = Δf/f = Δv/c
   - Δλ: Observed wavelength shift (m)
   - Δf: Observed frequency shift (Hz)
   - Δv: Relative speed between source & observer (m/s)
   - c: Wave speed (e.g., 3×10⁸ m/s for light)
2. Velocity Components:Δv = vₛ - vₒ
   - vₛ: Source velocity (m/s)
   - vₒ: Observer velocity (often 0 for stationary observer)

Question 20

Hubble’s Law

Answer 20

Hubble’s Law Equation

• Recessional velocity of a galaxy ∝ distance from Earth
  v = H₀ × d
• v: Recessional velocity (km s⁻¹)
• H₀: Hubble constant
  
  • 67.8 kms⁻¹ Mpc⁻¹ or 2.197 x 10⁻¹⁸s⁻¹
• d: Distance to galaxy (Mpc)

===

Key Relationships

• Direct proportionality:
  
  • Distance ↑ → Recessional velocity ↑
  • Example:
    
    • Galaxy at 10 Mpc: v ≈ 674 km/s
    • Galaxy at 100 Mpc: v ≈ 6,740 km/s
  • 1 Mpc = 3.086 × 10¹⁹ km
  • Unit consistency is critical (watch Mpc vs. km)
• Uncertainties: Random/systematic errors in distance measurements affect H₀ precision

Question 21

Red-Shift

Answer 21

Key Observations from Galactic Redshift:

• All galaxies show redshift → moving away from Earth
• Distant galaxies exhibit:
  
  • Greater redshift than nearby galaxies
  • Higher recessional velocities (v ∝ distance)

===

1. Mechanism:
   
   • Light waves stretched by universe expansion → λ↑ (redshift), f↓
   • Spectral lines shift toward red end of spectrum
2. Hubble’s Law Relationship:
   v = H₀ × d
   - demonstrates linear expansion relationship
   - Rewinding time shows galaxies converging to a singularity

===

Common Pitfalls:

• Confusing redshift (λ↑) with blueshift (λ↓)
• Forgetting redshift indicates relative motion, not light “aging”

Question 22

The Big Bang Theory

Answer 22

• Origin: ~13.7 billion years ago from an infinitely dense, hot singularity.
• Key Event:
  
  • Rapid expansion of space-time began.
  • Universe has been cooling ever since.

===

Evidence for the Big Bang

1. Hubble’s Law:
   
   • Galactic redshift shows universe is expanding.
   • Distant galaxies recede faster (v = H₀d).
2. Cosmic Microwave Background (CMB):
   • Uniform radiation at 2.73 K (microwave wavelength ~1 mm).
   • Key Properties:
     
     • Comes from all directions.
     • Matches thermal profile of a cooling hot body.
   • Redshifted from gamma rays → microwaves over 13.7 billion years.

===

Why CMB Supports the Big Bang

• Only a hot, dense origin explains:
  
  • The uniformity of CMB.
  • Its blackbody spectrum.
• Ruled Out Alternatives:
  
  • Steady State Theory cannot explain CMB.
• Temperature Evidence:
  
  • Current 2.73 K matches expected cooling from a Big Bang.
  • Younger universe would be hotter (>2.73 K).

Question 23

Estimating the Age of the Universe

Answer 23

Assumption:

• Recessional velocity (v) of galaxies has remained constant since the Big Bang.

===

Step-by-Step Derivation:
1. Start with Hubble’s Law:
v = H₀ × d
- v = recessional velocity (km/s)
- H₀ = Hubble constant (km/s/Mpc)
- d = distance to galaxy (Mpc)

2. Time = Distance / Velocity:
   t = d / v
   Substitute Hubble’s Law:
   t = d / (H₀ × d) = 1 / H₀
3. Hubble Time (Age of Universe):
   t = 1 / H₀

===

• For H₀ = 67.4 km/s/Mpc:
  t ≈ 1 / 67.4 ≈ 0.0148 billion years/Mpc
  Convert units:
  1 Mpc = 3.086 × 10¹⁹ km
H₀ = 67.4 / 3.086 × 10¹⁹ ≈ 2.18 × 10⁻¹⁸ s⁻¹
t = 1 / (2.18 × 10⁻¹⁸) ≈ 14.5 billion years

===

Why the Discrepancy?

• Simple calculation gives 14.5 Gyr, but:
  
  • Early universe expansion was faster (inflation).
  • Dark energy later accelerated expansion.
• Best estimate: 13.7 ± 0.02 Gyr (CMB data).

Question 24

Evolution of the Universe

Answer 24

Stage 0: The Big Bang (t = 0)
- Conditions:
- Infinitely hot, dense singularity
- Creation of space and time

Stage 1: Inflation (0 → 10⁻³⁵ s)
- Key Events:
- Rapid expansion (inflation)
- Only gamma photons and EM radiation exist

Stage 2: Particle Era (10⁻³⁵ s → 10⁻⁶ s)
- Particle Formation:
- Quarks, leptons, photons, and antiparticles appear
- More matters than antimatter: Leaves a matter-dominated universe

Stage 3: Hadron Era (10⁻⁶ s → 225 s)
- Key Developments:
- Quarks combine due to cooler K→ protons/neutrons form
- Matter-antimatter annihilation → high-energy photons

Stage 4: Nucleosynthesis (225 s → 1,000 yr)
- Nuclear Fusion (cools more):
- Light nuclei form (deuterium, helium, lithium)
- Matter is in plasma form (protons and electrons are not bound to one)
- Rapid expansion of the universe until 25% of matter is helium nuclei

Stage 5: Electron Formation (1,000 → 3,000 yr)
- Key Change:
- Free electrons emerge
- Fusion ends

Stage 6: Recombination (3,000 yr → 300,000 yr)
- Critical Transition:
- Electrons + nuclei → neutral atoms (H, He)
- Photon decoupling: Universe becomes transparent
- CMB photons released (now detectable as microwaves)

Stage 7: Structure Formation (300,000 yr → Present)
- Timeline:
- 30 Myr: First stars ignite
- 1-2 Gyr: Galaxies form via gravitational collapse
- 9 Gyr: Solar system forms from supernova remnant
- 11 Gyr: Primitive life on Earth
- 13.7 Gyr: Modern humans evolve

Question 25

Dark Energy & Dark Matter

Answer 25

Dark Energy

• Purpose: Explains the accelerating expansion of the universe.
• Key Properties:
  
  • Hypothetical negative pressure opposing gravity.
  • Permeates all space uniformly.
  • Cannot be detected directly.
• Current Estimates:
  
  • 68% of the universe’s total energy content.
• Critical Evidence:
  
  • Supernova observations show increasing recession speeds of distant galaxies.
• Unsolved Mystery:
  
  • Nature of dark energy remains unknown (linked to cosmological constant/vacuum energy).

===

Dark Matter

• Purpose: Explains anomalous galactic rotation curves and gravitational lensing.
• Key Properties:
  
  • Does not interact with EM radiation (invisible).
  • Detected only via gravitational effects:
    
    • Galaxy rotation curves: Outer stars orbit too fast for visible mass.
    • Gravitational lensing: Bends light from distant objects.
• Current Estimates:
  
  • 27% of the universe’s mass-energy.
• Observational Evidence:
  
  • Rotation Curves:
    
    • Expected: Orbital velocity ↓ with distance (like solar system).
    • Observed: Velocity remains constant → implies invisible mass.
  • Bullet Cluster: Direct proof of dark matter via separation of visible and gravitational mass.
• Unsolved Mystery:
  
  • Particle candidates (WIMPs, axions) remain undetected.