CHAPTER THREE THE BIG BANG & BEYOND The present understanding of the origin of our Universe is described in the Timeline of the Universe section of the Origins web page. The Universe had a singular event about 15 billion years ago that is commonly described as the Big Bang: All matter and energy of the Universe was momentarily concentrated in a small volume, which then exploded, and the Universe that we see today is made up of the remnants of that explosion. If you read through their discussion of how our Universe is understood to have evolved from that explosive event, you will see that shortly after the event the four known physical forces began to act to shape our Universe. Fundamental Physics contributes to our understanding of the evolution of the Universe by testing and extending our knowledge of the forces of Nature. Gravity Reaches Out The large bodies and collections of objects that exist in our Universe tend to consist of nearly equal quantities of positive and negative charges. Therefore, there are not large electric forces between these large bodies. We know of only one type of gravitational "charge", the mass of an object, and the gravitational force is always attractive. Therefore, gravity is seen as the dominant force that acts over long distances in the Universe and has done much to shape our Universe to what we know it to be today. Gravity has caused large clouds of dust to condense into stars and planet systems, and has caused many such systems to form larger collections that we call galaxies. Gravity acts over all length scales, but it is dominant only at the long scales where other forces are smaller. Despite its universality of action, gravity remains our most poorly understood force. The gravity force between two laboratory-sized objects is minuscule, so scientists have been unable to measure it very accurately. The science community now agrees on the value of that force to within 0.15 %, but no better knowledge has been obtained despite the efforts of many skilled and learned scientists. Further, certain aspects of the gravity force, such as its relationship to the amount of matter that makes up a body, are vital to our understanding of the gravity interaction. As described in the STEP experiment, the thoughts and experiments of many of the best scientists have been devoted to investigating these interactions, and more such investigations are planned. These studies of the gravity force can either reinforce our present understanding of it, or can cause us to change, either slightly or more significantly, our understanding of the way distant bodies interact. Thus, our understanding of the ways that gravity has acted to shape our Universe may change as our measurements of the gravitational interaction improve. When you read through the investigations that are being performed in the Gravitation and Relativity subdiscipline of Fundamental Physics, try to imagine how the result of the study might change our interpretation of the observed interactions and our understanding of the sequence of events that shaped our Universe. Phase Transitions in the Universe When we look into the skies today, we do not see a uniform distribution of matter spread across the heavens that one might expect from an explosion like a Big Bang. Instead, we see stars, galaxies, and groups of galaxies at points in the sky. How did this nonuniform distribution of mass come about? How did the spreading out of the matter from the Big Bang evolve into the present collection of local collections of objects that we observe today? Most explanations of the development of the Universe call upon some transitions from a less-dense phase to a new denser phase to explain these local concentrations as the matter expands and cools. Such phase transitions have been objects of study in physics from the earliest times, and today rather sophisticated theories are used to predict the onset of the formation of a new phase. Fundamental Physics explores phase transitions in many of its experiments and in its theoretical investigations. In the Laser Cooling and Atomic Physics subdiscipline of Fundamental Physics, the experiments study how the atoms interact when a cloud of them is brought to this very cold state. The transition to a new phase and the new properties that are seen in the new phase are of paramount interest to these investigations. In the Condensed Matter and Low Temperature Physics subdiscipline, many studies investigate how phase transitions occur. From the nucleation of a solid phase from a liquid, and the effects of gas on the interactions of levitated liquid drops, and the effects of a flow of heat to the properties very near a phase transition, to effects of size and shapes on the properties of the fluid, investigators are exploring the details of how two phases interact near a phase transition. Further understanding these details may either support what we imagine occurred in the evolution of the Universe, or the new results could alter significantly these interpretations. As you explore the descriptions of the studies in the pages below, recall that the results of these studies may relate to how we understand the experiment we perform in our laboratory, and they may relate to the way we interpret what we see in the heavens. The tests of fundamental laws of physics generally have bearing across disciplines and across broad ranges of our understanding of nature. Check back soon for the Complete Story! Last Updated: