Despite the self-consistency and remarkable success of the standard Hot Big Bang model in describing the evolution of the universe back to only one hundreth of a second, a number of unanswered questions remain regarding the initial state of the universe.
Why is the matter density of the universe so close to the unstable critical value between perpetual expansion and recollapse into a Big Crunch?
Why does the universe look the same in all directions when it arises out of causally disconnected regions? This problem is most acute for the very smooth cosmic microwave background radiation.
The perturbations which gravitationally collapsed to form galaxies must have been primordial in origin; from whence did they arise?
Of what stuff is the Universe predominantly made? Nucleosynthesis calculations suggest that the darrk matter of the Universe does not consist of ordinary matter - neutrons and protons?
Phase transitions in the early universe inevitably give rise to topological defects, such as monopoles, and exotic particles. Why don't we see them today?
Why should the universe begin in thermal equilibrium when there is no mechanism by which it can be maintained at very high temperatures.
Why is the cosmological constant 120 orders of magnitude smaller than naively expected from quantum gravity?
The cosmological singularity at t=0 is an infinite energy density state, so general relativity predicts its own breakdown.
Are independent measurements of the age of the Universe consistent using Hubble's constant and stellar lifetimes?
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