The exploration of dark energy is very au courant. And it is truly an odyssey, one that pries layers back to the very core of what spacetime is. It asks us to explore what the universe is composed of and how its behaviour has evolved. There are necessary sidebars to this story, many of which present additional mysteries. Therefore, dark energy has been written as a miniseries of articles.
It's very well established in science that the universe came into existence with a Big Bang. Since then, the universe expanded and matter clustered into gas clouds, stars, and galaxies, shown below.
The universe can be extrapolated back to a single point of origin about 13.8 billion years ago ? an infinitely tiny space filled with an almost infinite density of energy. The abundance of light elements in the universe, along with the existence of cosmic microwave background (CMB), as well as the Hubble's law (more about this later on) and what scientists know about the nature of large-scale structures, such as galaxies, all point to a single point of origin filled with infinite density. This tiny ultra-dense point immediately and rapidly expanded, cooling as it did so. After an initial brief phase of extremely rapid expansion, called cosmic inflation, lasting only a tiny fraction of a second, the universe was seeded with electrons and quarks - the first particles of matter that would later combine to form every structure in the universe today from gas clouds to stars to planets, trees and us. 380,000 years later, photons began to decouple from electrons and stream outward in all directions. These photons are what the CMB is made of. This fascinating "baby period" of the universe is explored in the articles, Our Universe Parts 2 through 11, starting here.
The universe experienced massive shifts in its energy composition as it evolved from a tiny space filled with pure dense undefined energy into an incredibly large space filled with an astonishing assortment of energy and matter particles - those which underlie all the forces and kinds of matter that are observed today.
First Glimpse of Something Amiss: Accelerating Expansion?
Our present universe is brimming with matter, and we know that matter is attracted to matter through the force of gravity. Until the late 1990's, scientists therefore assumed that the universe's rate of expansion must be decreasing as gravity pulls all matter together.
Then, observations of extremely distant galaxies by the Hubble Space Telescope began to question that basic assumption, as evidenced in this 1999 NASA press release. Every galaxy observed appeared to be moving 260,000 km/h faster for every 3.3 million light-years away from Earth.
Around this time, observations of distant Type 1a supernovae blew the assumption right out of the water. Hubble observations proved that the velocity at which distant regions of space are moving is increasing with time.
This means that the rate of expansion of space is increasing rather than decreasing, as indicated by an outward curve in the NASA diagram below, one that has not been repeated since cosmic inflation.
|(Credit: NASA: Ann Field (STScl)|