Observational Cosmology

Recombination

Epoch when energy of photons dropped below ionisation energy of hydrogen so free electrons and protons combined to form neutral hydrogen atoms. The baryon-photon plasma became neutral (X=0.5)

Decoupling

Epoch when photons stopped interacting with free electrons via Thompson scattering - they streamed freely through the universe. The scattering rate drops below the Hubble rate)

Last scattering

The last time a typical CMB photon underwent its last electron scattering process. Spherical shell from which all CMB photons we observe were emitted

Standard Candle

An object of known intrinsic luminosity which allows its distance to be inferred purely from its observed flux. The distance can then be calculated without any geometric measurement

Examples of standard candles

Type 1A supernova, Cepheid variable stars, RR Lyrae stars

Distance ladder

A succession of distance measurement techniques, each valid over a limited range, where each method is calibrated using the one below it to build up to cosmological distance. This stems from the fact that no single method can measure distances across all scales

Standard Ruler

An object of known physical size which allows its distance to be inferred purely from its observed angular size via the angular diameter distance.

Examples of standard rulers

Baryon acoustic oscillations (BAOs), sound horizon at recombination, angular size of CMB cold/hot spots

The horizon problem

The universe is homogenous on very large scales with very little anisotropies (10^-5K) but most parts of the CMB have non-overlapping light cones and so were never in causal contact with each other

Flatness problem

The universe is observed to be spatially flat to a high precision which means the early universe must have been even closer to flatness than we are today. There is no reason within Big Bang cosmology for the universe to be so fine-tuned to such precision in its initial conditions.

Magnetic Monopoles Problem

Grand Unified Theories predict magnetic monopoles to form in the early universe when GUT symmetry breaks. Monopoles should vastly dominate the energy density of the universe but not a single one has been observed

Inflation

Inflation is a short period of rapid acceleration in the early universe

Inflation solves horizon problem

Prior to inflation, universe was radiation dominated and the horizon distance was a lot smaller before inflation than it was after. The period of inflation caused the horizon distance to grow exponentially meaning before inflation, the CMB photons were in causal contact with each other prior to inflation

Inflation solves flatness problem

During inflation, the scale factor grows exponentially meaning it forces aH to increase rapidly, deriving Omega-1 to 0 regardless of initial conditions. Inflation is very efficient at flattening the global curvature.

Inflation solved magnetic monopoles

Inflation expands the universe so rapidly after the GUT phase transition that the monopole density is diluted to essentially 0.

Problems of inflation

What caused it to start and stop? Why did it not reduce photons to undetectable levels? Why does it not flatten out local curvature?

Luminosity distance

The distance defined by the relationship between an objects observed luminosity and flux. Preserves the inverse square law.

Horizon distance

The maximum distance from which light could have reached us since the Big Bang - defines the boundary of the causally connected observable universe

Proper distance

The physical distance between two points measured at a fixed cosmic time, accounting for the expansion of the universe via the scale factor a.

Angular diameter distance

The ratio of an object’s physical size to its observed angular size

Comoving distance

The coordinate distance between two points, stripped of the effect of expansion so it remains constant as the universe expands