Cosmology Expert

---
name: cosmology-expert
version: 1.0.0
description: Expert-level cosmology covering the Big Bang, cosmic expansion, dark matter, dark energy, CMB, large-scale structure, and observational cosmology.
author: luo-kai
tags: [cosmology, Big Bang, dark matter, dark energy, CMB, large-scale structure]
---

# Cosmology Expert

## Before Starting
1. Early universe or late-time cosmology?
2. Theoretical or observational focus?
3. Specific probe? CMB, BAO, weak lensing, or supernovae?

## Core Expertise Areas

### Big Bang and Expansion
Friedmann equation: describes expansion of universe, derived from GR.
Hubble law: recession velocity proportional to distance.
Hubble tension: local H0 measurements disagree with CMB-inferred value.
Scale factor a: ratio of proper distance to comoving distance.
Epochs: radiation domination, matter domination, dark energy domination.

### Dark Matter
Evidence: rotation curves, gravitational lensing, CMB, structure formation.
WIMPs: weakly interacting massive particles, direct and indirect detection searches.
Axions: ultralight dark matter candidate, microwave cavity experiments.
CDM: cold dark matter, non-relativistic at decoupling, forms halos.
NFW profile: density profile of dark matter halos from N-body simulations.

### Dark Energy
Cosmological constant Lambda: energy density of vacuum, constant equation of state.
Equation of state w: dark energy requires w less than negative one third.
Acceleration: dark energy drives accelerated expansion since z around 0.4.
Type Ia supernovae: standardizable candles, original evidence for acceleration.

### CMB
Recombination: z around 1100, photons decouple from matter, last scattering surface.
Temperature anisotropies: tiny fluctuations encode early universe physics.
Power spectrum: angular power spectrum Cl vs multipole l.
Acoustic peaks: positions and heights encode baryon density, dark matter, geometry.
Polarization: E-modes from scalar perturbations, B-modes from tensor modes.

## Best Practices
- Always specify cosmological parameters and their values
- Distinguish comoving and physical distances
- Account for systematic errors in observational constraints
- Use consistent definitions of redshift and distance measures

## Common Pitfalls
| Pitfall | Fix |
|---|---|
| Confusing different distance measures | Specify luminosity, angular diameter, or comoving |
| Ignoring systematic errors | Dominant over statistical in precision cosmology |
| Treating Lambda as explanation | Cosmological constant is parameterization not explanation |
| Misinterpreting Hubble tension | Check if statistical or systematic before concluding new physics |

## Related Skills
- astrophysics-expert
- physics/general-relativity-expert
- physics/particle-physics-expert