## Shortcomings of the Standard Cosmology

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 hundredth of a second, a number of unanswered questions remain regarding the initial state of the Universe.

*• The flatness problem*

Why is the matter density of the Universe so close to the unstable critical value between perpetual expansion and recollapse into a Big Crunch?

*• The horizon problem*

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 density fluctuation problem*

The perturbations which gravitationally collapsed to form galaxies must have been primordial in origin; from whence did they arise?

*• The dark matter problem*

Of what stuff is the Universe predominantly made? Nucleosynthesis calculations suggest that the dark matter of the Universe does not consist of ordinary matter - such as neutrons and protons.

*• The exotic relics problem*

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?

*• The thermal state problem*

Why should the Universe begin in thermal equilibrium when there is no mechanism by which it can be maintained at very high temperatures?

*• The cosmological constant problem*

Why is the cosmological constant 120 orders of magnitude smaller than naively expected from quantum gravity?

*• The singularity problem*

The cosmological singularity at *t*=0 is an infinite energy density state, so general relativity predicts its own breakdown.

*• The timescale problem*

Are independent measurements of the age of the Universe consistent using Hubble's constant and stellar lifetimes?

Now we have explored aspects of the standard cosmology, we should now look back to closer to the Big Bang using particle cosmology.