There is an intimate relationship between the fields of particle physics and cosmology, which been exemplified by a long line of physicists working in both simultaneously: Albert Einstein, Stephen Hawking, Kip Thorne, and many others. Cosmology is the study of the universe and its structure, whereas particle physics is the study of fundamental particles such as quarks and photons, the smallest known objects. Although at first they may seem as unconnected as anything can be, cosmology and particle physics are in fact closely linked.
Unlike the complex systems on Earth, which much be described using higher-level explanations rather than properties emerging from the lowest levels, intergalactic and cosmological phenomena are comparatively simpler. For instance, in the vast distances of space, only one of the four forces of nature has any real influence: gravity. Although stars and galaxies are very far away and many times larger than ourselves, we have a remarkably accurate picture of how they work, derived from fundamental physical laws which direct their constituent particles.
The domain of cosmology most closely connected to particle physics is the study of the Big Bang, the gigantic explosion that created all the matter in the universe and the spacetime of which the universe itself is composed. The Big Bang started off as a point of near-infinite density and zero volume: a singularity. Then, it quickly expanded to the size of an atomic nucleus, which is where particle physics comes into play. To understand how the earliest moments of the Big Bang influenced the universe as it is today, we must use what we know about particle physics to create plausible cosmological models.
One of the motivations for creating ever more powerful particle accelerators is to conduct experiments which simulate the physical circumstances as early as possible in the history of the universe, when everything was very compact and hot. Cosmologists must be well-versed in particle physics in order to make significant contributions to the field.
Another key to understanding the relationship between particle physics and cosmology is to look at the study of black holes. The physical properties of black holes are relevant to the long-term future of the cosmos. Black holes are collapsed stars with such immense gravity that not even light can escape their grasp. For a while, it was thought that black holes emitted no radiation, and would have been eternal, a paradox to physicists. But Stephen Hawking theorized, based on insights from particle physics, that black holes do indeed emit radiation, which was thereafter dubbed Hawking radiation.
Particle physics is also highly relevant into investigations of dark matter, invisible matter whose existence is known due to its gravitational influence on visible matter, and dark energy, a mysterious force that pervades the universe and causes its expansion to accelerate. These are central questions in modern cosmology.