Universe and the Solar System – Comprehensive Study Notes

Learning Objectives

• By the end of the lesson, you should be able to:
  • Describe the structure and composition of the Universe.
  • State pre–Big Bang hypotheses on the origin of the Universe.
  • Explain red-shift as evidence for an expanding Universe.
  • Explain the Big Bang Theory and enumerate its supporting evidences.

Sense of “A Billion”

• The Universe is at least $(13.8\,\text{billion\;years})$ old; the Solar System $\approx(4.5{-}4.6\,\text{billion\;years})$.
• Thought experiment: Spending $1\,\text{peso}$ per second:
  • $1\,\text{billion\;pesos}=10^{9}\,\text{pesos}$.
  • $10^{9}\,\text{seconds}\approx31.7\,\text{years}$ of nonstop spending.
  • Illustrates how immense “a billion” really is.

Structure & Composition of the Universe

• Definition: All of space–time plus all matter–energy.
• Composition (latest concordance model):
  • $4.6\%$ baryonic ("ordinary") matter — protons, neutrons, electrons → atoms, planets, stars, nebulae, galaxies.
  • $24\%$ cold dark matter — has gravity, emits no light; proposed to hold galaxies together because visible mass is insufficient for the observed orbital speeds.
  • $71.4\%$ dark energy — yields a repulsive (antigravity-like) effect causing accelerated cosmic expansion.
• Three most abundant chemical elements: hydrogen, helium, lithium.

Large-Scale Structure

• Stars → basic building blocks of galaxies; born from interstellar clouds of gas & dust.
• Remaining gas/dust can form planets, asteroids, comets, etc.
• Galaxy = billions of stars; galaxies cluster → superclusters with vast empty voids between.
• “Clumpy” on small/medium scales but on the largest scales Universe appears homogeneous (same composition everywhere) and isotropic (looks the same in every direction).
• Milky Way visual anatomy (keywords from diagram):
  • Bulge, disk, spiral arms, halo (with globular clusters embedded).
  • Solar System (Mercury→Neptune) resides in one spiral arm.

Age, Size & Density

• Age: $13.8\,\text{billion\;years}$ (Planck/WMAP data).
• Observable diameter: at least $91\,\text{billion\;light\text{-}years}$.
  • $1\,\text{ly}=9.4607\times10^{12}\,\text{km}$.
• Mean cosmic mass–energy density: $4.5\times10^{-31}\,\text{g/cm}^{3}$.

Expanding Universe & Red-Shift

• 1929 – Edwin Hubble measured galaxy spectra:
  • Observed systematic shift of spectral lines toward longer (red) wavelengths.
  • Interpreted via the Doppler Effect: \lambda{\text{observed}}>\lambda{\text{emitted}}\Rightarrow v_{\text{recession}}>0.
  • Linear relation $v=H_{0}d$ (Hubble’s Law) ⇒ space itself expands.
• Fits Einstein’s General Relativity, which allows non-static solutions (de Sitter, Friedmann models).

Doppler Effect Refresher

• Change in observed wave frequency when source & observer move relative to each other.
• Astronomy convention: Red-shift (z>0) = moving away; Blue-shift (z<0) = approaching.

Cosmic Microwave Background (CMB)

• Discovered accidentally (1964) by Arno Penzias & Robert Wilson → Nobel Prize 1978.
• Uniform black-body radiation at $T\approx2.7\,\text{K}$ above absolute zero.
• Fills all directions; considered “afterglow” of recombination era ($\approx300{,}000\,\text{yr}$ after Big Bang).
• Minute temperature anisotropies (measured by COBE, WMAP, Planck) encode density fluctuations that seeded galaxies.

Theories/Hypotheses for the Origin of the Universe

• Ancient Mythological Views
  • Egyptian: Cosmic ocean "Nun"; creation by god Atum.
  • Babylonian: Chaos; god Marduk fashions cosmos.
• Geocentric Model (Ptolemy, 2nd C CE): Earth-centered; epicycles to fit planetary retrograde motion.
• Heliocentric Model (Copernicus, 16th C): Sun-centered; challenged Church doctrine.
• Newtonian (17th C): Infinite static space, absolute time, gravity obeys inverse-square law.
• Einsteinian Relativity (1915–1917): Space–time is dynamic; gravity = curvature; allows expanding/contracting universes.
• Steady-State Theory (Hoyle, Bondi, Gold; 1948): Universe eternal; matter continuously created to keep density constant during expansion. Falsified by CMB discovery.
• Big Bang Theory (Lemaître 1927 ➔ Hubble 1929 ➔ present): Universe emerged from hot dense singularity $\sim13.8\,\text{Gyr}$ ago; expanding & cooling ever since.
• Inflationary Universe (Guth 1980; Linde): $10^{-36}{-}10^{-32}\,\text{s}$ after Big Bang, Universe expanded exponentially → solves horizon, flatness, monopole problems.

Timeline of the Universe (Key Events)

• $t=0$: Big Bang; space & time come into existence.
• $10^{-36}{-}10^{-32}\,\text{s}$: Inflation; observable Universe inflates from sub-atomic size to grapefruit-scale.
• $10^{-6}\,\text{s}$: Quark–gluon plasma cools; protons/neutrons “freeze out.”
• $1\text{–}3\,\text{min}$: Big-Bang nucleosynthesis → $\text{H},\,\text{He},\,\text{Li}$ nuclei form.
• $380{,}000\,\text{yr}$: Recombination; first neutral atoms; CMB released.
• $(10^{8}\,\text{yr})$: First stars & protogalaxies ignite.
• $(\text{few}\times10^{9}\,\text{yr})$: Expansion starts accelerating (dark energy era).
• $9\,\text{Gyr}$: Solar System forms from supernova-enriched nebula.
• $10\,\text{Gyr}$: Earliest simple life on Earth.
• $13.7\,\text{Gyr}$: Present day; billions of galaxies & exoplanets observed.
• $20\,\text{Gyr (future)}$: Sun becomes red giant → life on Earth ends.
• $10^{100}\,\text{yr}$ ("Heat death"): No new stars; black holes evaporate; Universe cold & dark.

Technology ↔ Cosmology Interplay

• Refracting telescope (Galileo, $1609$): Moons of Jupiter, Venus phases → heliocentrism.
• Spectroscopy (19th C onward): Identified chemical signatures; measured galaxy red-shifts.
• Radio telescopes (1930s–1960s): CMB detection.
• Satellites: COBE (1992), WMAP (2003), Planck (2013) — high-precision CMB maps.
• Space Telescope (Hubble, 1990–): Deep-field imaging; Type Ia supernovae → accelerated expansion ⇒ dark energy.
• Particle accelerators (LHC): Recreate energies <10^{-12}\,\text{s} after Big Bang; test inflationary-era physics.

Ethical, Philosophical & Practical Implications

• Shift from mythological to empirical explanations embodies scientific revolution & secularization of cosmology.
• Questions of ultimate origin, purpose, and fate remain open; Big Bang doesn’t answer “why,” only “how.”
• Dark matter & dark energy illustrate that >95\% of the Universe is still mysterious → ongoing research funding, detector development, AI-assisted data analysis.
• Space exploration (JWST, future ELT class telescopes) may uncover biosignatures on exoplanets, altering philosophical outlook on life’s uniqueness.

Connections to Previous Concepts

• Relates to Newton’s laws (gravity binds galaxies), thermodynamics (expansion = cooling), quantum mechanics (early-Universe particle physics), and chemistry (stellar nucleosynthesis → periodic table origin).
• Demonstrates scientific method: hypotheses → predictions → observations (e.g., CMB as smoking gun for Big Bang).

Open Questions & Future Directions

• What is the particle nature of dark matter? WIMPs vs. axions vs. modified gravity.
• What generates dark energy? Cosmological constant $\Lambda$, quintessence, or new physics?
• What occurred "before" inflation or Big Bang? Bounce models, multiverse hypotheses.
• Can quantum gravity (e.g., string theory, loop quantum gravity) remove the initial singularity?
• How will future instruments (e.g., Euclid, LSST, LISA) refine cosmic expansion parameters or detect primordial gravitational waves?

Study Tips

• Memorize key percentages (baryonic vs. dark components) and chronological benchmarks.
• Practice deriving Hubble’s Law plots from red-shift data sets.
• Compare & contrast Steady-State vs. Big Bang to better grasp evidentiary reasoning.
• Use mnemonic: "HHeLi" for primordial nuclei timeline.
• Relate cosmological eras to corresponding temperature scales and particle interactions.