I. Ciufolini & E. Coccia 
Gravitation [PDF ebook] 
From the Hubble Length to the Planck Length

Although gravity is the dominant force of nature at large distances (from intermediate scales to the Hubble length), it is the weakest of forces in particle physics, though it is believed to become important again at very short scales (the Planck length). The conditions created in particle accelerators are similar to those at the time of the early