Dark Energy – What is it?

 Based on the gravitational laws that affect matter, one could logically come to the conclusion that the universe is slowly decelerating in its expansion. The gravitational pull that large masses in our universe exert on each other should lead to all the matter in the universe to re-draw itself to its centre and compress into a single, infinitely dense point, in accordance to the Big Crunch theory. However, recent observations and studies have shown that the expansion of the universe is actually accelerating. The most widely accepted explanations for this behaviour, that prevents the collapse of all matter, is referred to as dark energy.

Dark energy is an unknown form of energy that affects the universe on the largest scale, seemingly accelerating its expansion. It was first proposed in 1998 by two independent teams of astrophysicists, who discovered the universe’s accelerating expansion while studying type Ia supernovae. It was concluded that gravitational attraction could not be the cause of this, so a new form of energy was suggested, later known as dark energy, which led them to win the Nobel prize in physics in 2011.

Dark energy is thought to be a repulsive force, opposing gravity. It is believed by many scientists that the vacuum energy of empty space could be the source of dark energy, however the amount of vacuum energy predicted by quantum field theory extremely larger than the observable value of dark energy. It is estimated that it accounts for approximately 70% of the universe’s total energy and matter while still being unobservable.

But it only seems to act on the largest scales of the universe, as gravity still beats dark energy at small scales. This means that its effects are hardly seen, having to observe extremely far away galaxies and other cosmic objects separated by massive “empty” distances in space. Doing so it was also seen that the distance between the observed cosmic bodies is proportional to the speed at which they repel. Scientists estimate that every million years galaxies are getting 0.007% further away from each other. This may not seem like a lot, but when the right calculations are done it can be taken that a cosmic object 1 billion lightyears away from us is receding at 21500 km/s, which is approximately 7% of the speed of light. It came to many physicist’s surprise that when one of the oldest galaxies ever discovered, Galaxy GN-z11, 32 billion lightyears away, it was observed that dark energy is propelling this galaxy at an estimated speed of 687,000 km/s, or approximately 229% the speed of light, demonstrating that dark energy can expand the fabric of space-time to unimaginable degree, unbound to our earthly limits.

So how do galaxies retain their shape since travelling at unimaginable velocities? The answer is that the one thing preventing galaxies from spreading apart internally and morphing abnormally is dark matter, but that we will explore in our next blog.