When the term 'expansion' is mentioned, the increase in size comes to our mind. However, in this case, the expansion of the universe is different. Let’s dig in and find out more.
LEARNING OBJECTIVES
The expansion of the universe refers to the increase in the distance between any two gravitationally unbound parts of the universe that can be observed with time. This process is an intrinsic expansion whereby there is change in the scale of space. The universe doesn’t expand into anything and it doesn’t require space to exist outside it. Generally, space and objects in space do not move. The metric that governs the geometry and size of spacetime is the one that changes in scale. Despite objects and light in spacetime being unable to travel in speeds faster than light, the metric itself is not restricted by this limitation. It appears to an observer as if space is expanding and all apart from the nearest galaxies are receding into the distance.
During the inflationary epoch which was about 10-32 of a second after the Big Bang, there was a sudden expansion of the universe. The volume of the universe increased by a factor of approximately 1078 (distance expansion by a factor of about 1026 in all the three dimensions). A slower and gradual expansion of space continued later, until 9.8 billion years after the Big Bang when it began to gradually expand more quickly. It is still expanding up to today.
The metric expansion of space is completely different from the explosions and expansions that are seen in daily life. It seems to be a property of the whole universe rather than a phenomenon that just applies to one part of the universe. It can be better observed from “outside” the universe.
Metric expansion is a major feature of the Big Bang cosmology. However, this model is only valid on large scales (the scale of galaxy clusters and above), since gravitational attraction joins matter together strongly enough that at this time, metric expansion cannot be observed on a smaller scale.
Physicists have proven the existence of dark energy which appears as a cosmological constant in the simplest gravitational models, as a way to explain the acceleration.
METRICS AND COMOVING COORDINATES
To understand the metric expansion of the universe, it is important to discuss what a metric is and how metric expansion works.
A metric refers to a concept of distance. It states in mathematical terms the manner in which distances between two points that are close in space are measured, in terms of the coordinate system. Coordinate systems are used to locate points in space by assigning certain positions on a grid called the coordinates to every point. X-y graphs and latitude and longitude are common examples of coordinates. A metric is a formula describing how a number called “distance” is to be measured between two points. The metric expansion of space is described using the mathematics of metric tensors. The coordinate system that is used is known as commoving coordinates. This is a type of coordinate system that takes into account both time and space and the speed of light.
