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📘 What evidence still backs the Big Bang?

Expert analysis of observational evidence for the Big Bang

14
lessons
~30 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. Hubble Expansion and the Scale FactorDerive the scale-factor evolution from observed Hubble parameter and curvature constraints.Hubble's law establishes metric expansion. Distance-ladder recalibrations fix H0 near 70 km/s/Mpc. Integration of the Friedmann equation yields a singularity at finite past time, setting the stage for thermal history.
  2. Discovery and Spectrum of the CMBQuantify the thermal spectrum and its implications for the radiation-dominated era.The CMB spectrum confirms thermal equilibrium at z approximately 1100. Photon number density and temperature fix the entropy per baryon. Any late energy injection would distort the spectrum beyond observed limits.
  3. CMB Anisotropies and Acoustic PeaksExtract cosmological parameters from the angular power spectrum.Acoustic oscillations in the photon-baryon fluid produce a harmonic series of peaks. Peak ratios fix Omega_b h^2 near 0.022 and Omega_c h^2 near 0.12. Flatness is confirmed to 0.5 percent.
  4. Big Bang Nucleosynthesis PredictionsCompute light-element yields as functions of eta and expansion rate.Standard BBN predicts Y_p = 0.247 and D/H approximately 2.5 times 10^-5 for eta near 6 times 10^-10. These abundances are independent of post-recombination astrophysics.
  5. Observed Primordial AbundancesCompare spectroscopic measurements to BBN calculations and quantify tension.Concordance between helium, deuterium, and CMB baryon density supports thermal equilibrium through the MeV era. Lithium-7 remains discrepant at the 3-sigma level.
  6. Baryon Acoustic OscillationsUse BAO as a standard ruler to measure the angular-diameter distance.The BAO feature calibrates the expansion history at low redshift. Combined with CMB data it tightens constraints on dark-energy equation of state to w = -1.03 plus or minus 0.03.
  7. Large-Scale Structure GrowthLink the transfer function to observed clustering amplitude sigma_8.Suppression of small-scale power by radiation pressure produces the observed turnover. Late-time growth factor confirms Omega_m approximately 0.3 and rules out significant warm-dark-matter fractions.
  8. Type Ia Supernovae and Luminosity DistancesConstruct the Hubble diagram and extract q0 and w.Union and Pantheon compilations give Omega_Lambda approximately 0.7 at greater than 5 sigma. Systematic uncertainties from dust and evolution are now subdominant to statistical errors.
  9. Cosmic Acceleration and Dark EnergyTest the cosmological constant against dynamical dark-energy parametrizations.Flat Lambda-CDM remains the minimal model fitting all current data. Early dark energy or modified gravity scenarios are constrained to sub-percent contributions at recombination.
  10. Inflationary PerturbationsMap slow-roll parameters to observed n_s and r.Single-field slow-roll inflation with epsilon approximately 0.0005 reproduces the observed tilt. Running of the spectral index remains consistent with zero at current precision.
  11. CMB Polarization and Tensor ModesDistinguish primordial tensor modes from lensing and foregrounds.Absence of detectable primordial B-modes rules out high-scale inflation with V^{1/4} greater than 1.5 times 10^16 GeV. Future CMB-S4 targets r approximately 0.001 sensitivity.
  12. Cosmic Neutrino BackgroundExtract N_eff and neutrino mass sum from CMB and large-scale structure.Extra relativistic degrees of freedom are limited to Delta N_eff less than 0.3. Sum of neutrino masses is bounded below 0.12 eV, approaching the oscillation lower limit.
  13. Falsification Tests of Alternative ModelsIdentify observables that discriminate between paradigms.Current data exclude non-expanding and non-thermal models at high significance. Any viable alternative must replicate the CMB spectrum, BBN yields, and BAO scale simultaneously.
  14. Future Probes and Remaining TensionsAssess how next-generation surveys will resolve or exacerbate current discrepancies.Improved BAO and weak-lensing measurements will test whether new physics is required beyond Lambda-CDM. Detection of primordial gravitational waves or non-zero running would directly constrain inflation.

Questions this course answers

If a small positive curvature term Ωk = +0.01 were allowed while keeping all other parameters fixed, how would the inferred age change?

Positive curvature adds a term that increases H(a) at late times, shortening the time needed to reach a = 1 and therefore lowering the age.

If the baryon density were doubled while keeping all other parameters fixed, which change would appear in the temperature power spectrum?

Extra baryon inertia shifts the oscillation zero point, boosting compression phases (odd peaks) while suppressing rarefaction phases (even peaks).

A new-physics model raises the expansion rate by 10 % around T=1 MeV but leaves eta unchanged. Which abundance pattern is expected?

Faster expansion freezes a larger n/p (higher Y_p) and shortens the deuterium-burning window (higher residual D/H).

A new metal-poor absorber is found with D/H = 2.40 × 10^{-5}. If standard BBN holds, this implies what shift in η relative to the Planck value?

Lower deuterium requires higher baryon density; the scaling D/H ∝ η^{-1.6} converts the 5 % drop in D/H into an ~8 % rise in η.

A new survey at z = 1.2 measures the BAO peak at the same comoving scale but with θ_BAO = 3.1°. Relative to the z = 0.5 result, this implies the universe was

A smaller angle at higher redshift means D_A(z=1.2) is larger than expected in an Einstein-de Sitter universe, requiring an accelerated expansion phase that stretches the distance integral.

A hypothetical warm-dark-matter model lowers the transfer function by 20 percent at k=0.5 h Mpc^{-1}. What sigma_8 would be inferred from the same primordial amplitude?

Because sigma_8 integrates T(k)^2 weighted by the window, a 20 percent drop in T at the relevant scales reduces sigma_8 by roughly 20 percent to 0.65.

Grounded in trusted sources

  • NASA
  • ESA
  • NSF
  • Planck Collaboration, CMB cosmology results
  • NASA / WMAP / COBE historical CMB milestones
  • Cyburt et al., Big Bang nucleosynthesis reviews
  • Riess / Perlmutter / Schmidt Type Ia supernova cosmology papers
  • Open cosmology textbooks / PDG cosmology review

Every Wunder lesson is built from real, reputable sources — never invented.

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