Multi-Hop Open-Domain Question Answering over Structured and Unstructured Knowledge

Abstract

Within the emerging research efforts to combine structured and unstructured knowledge, many approaches incorporate factual knowledge, e.g., available in form of structured knowledge graphs (KGs), into pre-trained language models (PLMs) and then apply the knowledge-enhanced PLMs to downstream NLP tasks. However, (1) they typically only consider \textit{static} factual knowledge, whereas, e.g., knowledge graphs (KGs) also contain \textit{temporal facts} or \textit{events} indicating evolutionary relationships among entities at different timestamps. (2) PLMs cannot be directly applied to many KG tasks, such as temporal KG completion. In this paper, we focus on \textbf{e}nhancing temporal knowledge embeddings with \textbf{co}ntextualized \textbf{la}nguage representations (ECOLA). We align structured knowledge, contained in temporal knowledge graphs, with their textual descriptions extracted from news articles, and propose a novel knowledge-text prediction task to inject the abundant information from descriptions into temporal knowledge embeddings. ECOLA jointly optimizes the knowledge-text prediction objective and the temporal knowledge embeddings, which can simultaneously take full advantage of textual and knowledge information. The proposed fusion method is model-agnostic and can be combined with potentially any temporal KG model. For training ECOLA, we introduce three temporal KG datasets with aligned textual descriptions. Experimental results on the temporal knowledge graph completion task show that ECOLA outperforms state-of-the-art temporal KG models by a large margin. The proposed datasets can serve as new temporal KG benchmarks and facilitate future research on structured and unstructured knowledge integration.