Cosmological Models and Modern Cosmology

`Λ` = cosmological constant / dark-energy component. `CDM` = Cold Dark Matter. The model also includes ordinary matter and radiation. It successfully accounts for many observations including: cosmic expansion; CMB properties; large-scale structure; galaxy clustering; * broad cosmic composition.

Cosmological models are tested by comparing predictions with observations. If observations disagree reliably with a model, the model or its assumptions must be reconsidered.

Important unresolved questions include: What exactly is dark matter? What is the physical nature of dark energy? What happened in the earliest physical stages of the universe? How should gravity and quantum physics be unified at extreme conditions?

Important framework relations include: **H=ȧ/a and observationally: v≈H₀d** at sufficiently low redshift.