Teddy Rosenthal. 12/25/2020
Intergalactic Repulsion
First, let’s take a look at how we know that the universe is expanding. In 1925, American astronomer Edwin Hubble saw that galaxies that were farther away from Earth emitted light with spectral lines that had shifted towards the red end of the electromagnetic spectrum.
This phenomenon is now known as redshift, where light waves experience the Doppler effect. Light takes longer to reach the Earth when the object emitting the light is moving away from the Earth. Since the light takes a longer time to reach a certain point, the waves appear longer because they are stretched out. If these galaxies appeared to be redshifted from Earth, that meant that they were moving away from Earth.
In 1929, Hubble was able to study the rate at which galaxies travel by observing a direct proportionality between a galaxy’s distance and the velocity at which it travels away from us (as shown in the image down below). This is summarized in Hubble’s Law, v=Hd, where ‘v’ is velocity, ‘H’ is the Hubble constant and ‘d’ is the distance of a galaxy. This law is used to calculate the velocity of any galaxy given its distance, or vice versa.
An Expanding Universe
A similar pattern to the repulse between galaxies is shown in atoms or molecules filling a vacuum. The law that governs this principle is known as entropy and it causes particles to fill in a space until the system is homogeneous.
It can then be deduced that galaxies are moving away from each other to fill some sort of empty space. The fact that galaxies are moving away from one another must mean there is a background force acting as some repulsive form of gravity that causes space itself to expand.
The term ‘dark energy’ was conceived in 1998, as a way to refer to this phenomenon. In 1915, Albert Einstein published his paper on his general theory of relativity, and with it, his field equations. His equations were supposed to find a link between the geometry of spacetime and the mass density of the universe.
In 1917, Einstein decided to add a value known as the cosmological constant, ‘Λ,’ to his equation. It would assume that the universe is static and not expanding. Its value was set to 0 by Einstein but later repurposed with a new value that would represent the newly observed expansion and acceleration of the universe. A simple rearrangement of the constant (moving it from one side of the equation to the other) turned it from a geometric value to a contribution to the total mass density of the universe. This means that the cosmological constant represents something that exists and is now understood as the simplest and most coherent realization of dark energy. It is used in Einstein’s field equation to predict its own effect on the universe.
Energy in Empty Space
How does dark energy affect our understanding of the universe?
Since it is understood that there is energy residing in empty space between particles and in outer space, the cosmological constant has a positive value. The energy in empty space is also recognized by quantum theory as vacuum energy and is represented by the cosmological constant.
Vacuum or dark energy takes up 70% of the composition of the universe, meaning that its value is significant alongside the density of the matter, dark matter and radiation in the universe. Since it is also used in Einstein’s field equation (which is used to predict the geometry of the universe given the total energy density and the cosmological constant), there must have been dramatic changes in the accepted geometry as the cosmological constant gained positive value.
While this is true, predictions were made involving the needed mass and energy for the universe to be flat (in 3-dimensions where space can be understood as vast amounts of adjacent cubes of space and where parallel lines never cross), which requires the total energy to equate to the integer ‘1,’ and the predicted energy constant of dark energy allows it to equal one.
Currently, many predictions have been made to explain the shape of the universe, and the existence of dark energy presently supports the theory of a flat universe, though nothing is certain.
Substantial mysteries involving dark energy are still yet to be answered. One such example is the density of dark energy in any given region remaining constant over time, even though its density should be decreasing similar to matter and radiation.
Despite scientists working day and night to solve the obscurity of this “anti-gravity,” all that is known is the existence of dark energy, which will determine the future of the universe and humanity.
Cover Photo: (NBC News)