Observations, and the Standard Model of Cosmology
The standard model by which we understand the global properties of the Universe has emerged over the past 90 years based on the equations of General Relativity with the assumption that our Universe is homogeneous and isotropic. The relevant constants in this model are the Hubble constant (or current rate of cosmic expansion), the relative fractions of the species of matter that contribute to the energy density of the Universe, and these species’ equation of state, and the total density of all species of matter (which is equivalent to knowing the geometrical topology of the Universe).
To understand the formation of structures in the Universe, it has been necessary to have initial conditions where the Universe starts out nearly uniform in temperature and density, but with small approximately scale-free Gaussian perturbations. These initial conditions are thought to arise from a period of inflation – where the universe exponentially expanded – soon after the Big Bang. It has also been necessary to divide pressureless matter into two components. One of baryons, and the other of Cold Dark Matter, a so far invisible form of matter whose principal form of interaction is via gravity.
This standard model of the Universe is currently constrained by a number of observations. These include the primordial abundance of H, 2H,He, and Li, as predicted by Big Bang Nucleosynthesis, the power spectrum of baryon acoustic oscillations (BAOs) as seen in the Cosmic Microwave Background, the size of the BAOs delineated in the large scale structure of galaxies as well as the shape of the galaxies’ power spectrum, distances measured to supernovae using their apparent brightness as a function of redshift, and the locally measured value of the Hubble Constant. These observations are consistent with a 13.7 Billion year old Universe which is geometrically flat, composed of 4.5% of baryons, 22.5% Cold Dark Matter, and 73% a form of Dark Energy which has negative gravitational pressure.
I will briefly overview the standard model, and discuss the current set of observations that provides the strongest constraints on the Cosmological parameters. I will also highlight future observations that might provide improved insights into understanding the cosmological parameters which govern the evolution of the cosmos on global scales.