# Physics - What We Do and Don't Know

Cosmology and elementary particle physics span a range from the largest to the smallest distances about which we have any reliable knowledge. The cosmologist looks out to a cosmic horizon, the farthest distance light could have traveled since the universe became transparent to light over ten billion years ago, while the elementary particle physicist explores distances much smaller than an atomic nucleus. Yet our standard models really work--they allow us to make numerical predictions of high precision, which turn out to agree with observation. Up to a point the stories of cosmology and particle physics can be told separately. In the end, though, they will come together.