Every object humans have ever studied, such as stars, planets (including the Earth), asteroids, gas clouds and living creatures such as ourselves, is composed of ordinary matter, which is made up of atoms. Interestingly, however, ordinary matter only composes 5% of the mass of the Universe. The remaining 95% is made up of dark matter and dark energy. This post is about dark energy which, as shown in the diagram below, constitutes about 70% of the total mass of the Universe.
This article is the third in my series on cosmology, which is the study of the origin and evolution of the Universe as a whole.
The complete list of cosmology posts is at https://explainingscience.org/tag/cosmology/
Evidence for Dark Energy
As discussed in my previous post Universe past present and future, astronomers have known since the 1920’s that the Universe is expanding. By this we mean that the distance between galaxies increases over the course of time, although the individual galaxies don’t get any bigger.
Up until 1998 the generally held view was that the expansion of the Universe was slowing down. There was a good reason for believing this. When astronomers applied Einstein’s theory of general relativity to the Universe, it predicted that the gravity caused by all the matter in the Universe would slow down the expansion. This is shown in the graph below.
This diagram shows the expansion of the universe has slowed down over time. Along the x-axis is time and the y-axis shows the distance between galaxies. After the initial big bang the Universe expands very rapidly. As time progresses the Universe continues to expand, but at a slower and slower rate, so the distance between galaxies increases more gradually.
Einstein’s theory predicted that the more matter in the Universe, the greater the slowing in the rate of expansion. Before 1998, cosmologists believed that if there was enough matter in the Universe the expansion would eventually halt and it would actually start to contract. This would mean that the galaxies would eventually start getting closer together again. As time progressed the rate of contraction would get faster and faster, and the eventually the entire Universe would collapse in the so-called “big crunch”.
This diagram shows the “big crunch”. Before 1998 there was no consensus among cosmologists about whether a big crunch would occur; we just didn’t have accurate enough measurements of the amount of matter in the Universe.
In 1998, following years of observations to accurately measure both the distances of far away galaxies and how fast they were moving away from us, it was discovered that the expansion of the Universe was actually speeding up. The observations are shown in a simplified form in the diagram below.
The diagram above shows how nearby galaxies are moving away from us at a slower rate than galaxies further away.
As you will recall from my previous post, it has been known since the late 1920’s that the speed a galaxy is moving away from us is depends upon its distance from us, a relationship which we now know as Hubble’s Law. This is normally written as the simple equation:
V= Ho x D
- V is the speed a galaxy is moving away from us
- D is its distance
- Ho, the Hubble constant, measures how fast the Universe is expanding.
The diagram has been redrawn below to shows that for distant galaxies the Hubble constant, which is the slope of the graph marked below with the letter B is lower than it is for closer galaxies, marked below with the letter A.
When we look at very distant galaxies, we are seeing them as they were billions of light years ago because of the time it takes for light to reach us. Therefore billions of years ago the Universe was expanding more slowly than it is now and it must therefore be speeding up.
The 1998 results came a great surprise to most astronomers and was of such huge importance that Science magazine named it scientific breakthrough of the year for 1998. Three of the discovers later won the 2011 Nobel prize for physics.
Because, as mentioned before, Einstein’s theory of general relativity predicts that gravity will pull the galaxies together and slow down the expansion, there must something stronger than gravity acting to push the galaxies apart. This force is called dark energy and counteracts the forces of gravity. The reason for the name “dark” is that it is invisible and fills the whole of space.
How much dark energy is there?
Dark energy is completely evenly distributed throughout the Universe. Unlike ordinary matter it doesn’t clump together and there are no objects made out of dark energy.
Cosmologists have worked out that the actual amount of dark energy which is needed to counteract gravity and cause the observed speeding up in the rate of expansion of the Universe is actually very small. It works out at about 0. 0069 trillionths of a gramme per cubic kilometer of space. This is an almost unfathomably tiny amount – 150 billion times times smaller than a small grain of sand. However, most of the Universe is empty space and although ordinary matter is found at high densities in objects such as stars and planets, the spaces between these objects are so great that averaged over the entire Universe the density of ordinary matter works out at the incredibly low figure of about 0.0005 trillionths of a gramme per cubic kilometer of space – 2 trillion times smaller than a grain of sand. Dark matter has a density of roughly 0.0027 trillionths of a gramme per cubic kilometer of space averaged out throughout the Universe.
What form does dark energy take ?
Cosmologists aren’t in agreement about what dark energy is. However, the simplest explanation of dark energy is something called the cosmological constant, which physicist usually give the symbol Λ (lambda) first proposed by Albert Einstein in 1917.
The cosmological constant can be considered as the ‘vacuum energy’. The idea being that empty space isn’t truly empty but contains a small amount of residual energy. It is this small amount of residual energy which causes empty space to gradually expand.
Another point about the cosmological constant is that, because it is a constant, it has the same value throughout the entire Universe and doesn’t vary with time.
When Einstein applied his newly discovered theory of general relativity to the Universe it predicted that gravity would, over time, cause the objects in it to get closer and closer together, causing the whole Universe to collapse. Einstein, like most physicists at the time, believed the Universe must be static- neither contracting nor expanding. To make general relativity predict a static Universe, he had to introduce the cosmological constant to counteract the force of gravity.
After it was discovered in the late 1920s that the Universe was expanding, the cosmological constant was no longer needed to force a static Universe. Einstein, who didn’t like the idea of empty space having some energy, discarded it and said that introducing it was the “greatest blunder of my life”. For the next 70 years most cosmologists believed that the cosmological constant was zero. Interestingly, since 1998 many cosmologists have come to believe that the cosmological constant may be non-zero. Perhaps Einstein didn’t make a blunder after all!
The cosmological constant is not the only explanation of dark energy and is by no means accepted by all cosmologists. There are other more exotic explanations called scalar fields in which the average dark energy density does change over time. Perhaps in the next 20 years we will determine the nature of dark energy.