Neutrinos, Dark Matter, and the Early-Universe Particle Physics Interface
Cosmology as Particle Physics Laboratory
Relic Species and Cosmological Sensitivity
Cosmology constrains neutrino masses, relativistic species, dark matter microphysics, primordial black holes, and other relics through their effects on expansion, free streaming, structure suppression, and the thermal history. The Universe is both an accelerator and a calorimeter, integrating particle interactions over cosmic time.
Particle-Cosmology Connections
| Component | Key cosmological effect | Primary observables |
|---|---|---|
| Massive neutrinos | Suppress small-scale growth | CMB, LSS, lensing |
| Warm dark matter | Erase small-scale power | Subhalo counts, Lyman- |
| Axion-like particles | Alter dark sector and radiation content | CMB, laboratory constraints |
| Primordial black holes | Extra structure and lensing signatures | Microlensing, CMB, GW |
Neutrino Free Streaming
Relativistic neutrinos wash out clustering below a characteristic free-streaming scale, leaving a detectable suppression in the matter power spectrum.
Massive neutrinos most directly affect cosmological structure by
Neutrino thermal velocities prevent clustering on small scales and suppress power there.
Correct answer: suppressing growth below the free-streaming scale
Dark Matter Microphysics
Cold dark matter remains the simplest working model, but small-scale anomalies motivate exploration of warm, self-interacting, fuzzy, and interacting dark matter. Each model modifies halo structure, subhalo abundance, and astrophysical signatures in distinct ways.
What is one reason cosmology is powerful for constraining dark matter?
Cosmic surveys can probe the gravitational and thermal consequences of particle properties inaccessible to laboratories.
Correct answer: Because dark matter affects structure formation over vast scales and times, leaving cumulative observational signatures.
Cold, Warm, and Self-Interacting Dark Matter
Cold dark matter
- Very small primordial velocity dispersion.
- Excellent fit to large-scale structure.
- May face small-scale phenomenology issues.
Alternative dark matter
- Can alleviate small-scale tensions.
- Introduces new parameters and constraints.
- Must preserve large-scale successes of CDM.
Which observable is particularly useful for testing small-scale dark matter suppression?
The Lyman- forest probes matter power on small, high-redshift scales.
Correct answer: Lyman- forest
Testing a New Relic Species
-
1
Compute its contribution to the energy density and expansion rate.
-
2
Determine its free-streaming or interaction scale.
-
3
Propagate the effect into CMB and matter power spectra.
-
4
Fit joint data to constrain the allowed parameter space.