1. One or more computer-readable media storing computer-executable instructions that, when executed, cause one or more processors to perform acts comprising: modeling query behavior over time to create a time-aware query model the modeling of query behavior being based at least in part on timing and frequency that queries are submitted to a search service over time; modeling uniform resource locator (URL) behavior over time to create a time-aware URL model, the modeling of URL behavior being based at least in part on timing and frequency that URLs are selected in response to the queries over time; determining a time-aware query-URL relationship based at least in part on the time-aware query model and the time-aware URL model, wherein the determined time-aware query-URL relationship indicates, for a query, a future change that elevates a first URL to a popularity level greater than a second URL; performing a search in response to the query; and ranking results of the search according to the time-aware query-URL relationship, wherein the ranked results include the first URL ranked higher than the second URL.
7. One or more computer-readable media as recited in claim 1 , further comprising: learning rules governing model selection, wherein the rules change over time based on query behavior over time and URL behavior over time; switching between two time-aware query models based on operation of the rules; and switching between two the time-aware URL models based on operation of the rules.