Dark Energy Over Time

This post continues the subject of dark energy, which, as discussed in my previous post is a mysterious form of energy which makes up about 68 per cent of the mass of the Universe and is the reason why the Universe is expanding at an ever-faster speed. This post will discuss how the percentage of dark energy changes over time and how this has influenced and will influence the evolution of the entire Universe.

This post is the fourth in my series on cosmology, the study of the origin and evolution of the Universe as a whole. To view the others please click on the category “Cosmology” at the end of this post.

The influence  of dark energy in the early Universe

As mentioned in my previous post, cosmologists have estimated that the amount of dark energy in the Universe works out at about 0.0069 trillionths of a gramme per cubic kilometer of space, about 150 billion times smaller than a small grain of sand. Although this amount is incredibly small, it is still considerably greater than the average density of matter. The most widely accepted explanation of dark energy, the cosmological constant, means that the amount of dark energy in a given volume of space remains constant over time.

The expansion of the Universe is shown in the diagram above, in which the x-axis shows time. It illustrates that, in the distant past, galaxies were much closer together than they are now.

In fact, if we go back in time roughly 7 billion years then, on average, the distance between galaxies was roughly half what it is today.

As shown in the diagram above, when galaxies were two times closer together, the same number of galaxies took up a volume eight times smaller. the density of matter (which is equal to mass divided by the volume) was therefore eight times higher. However, the density of dark energy was exactly the same as it is now. This is shown in the first and third columns of the table and in the pie-chart below.

The percentages of dark matter, dark energy and ordinary matter in the Universe when it was seven billion years old.

If we travel back further and further in time,  the density of ordinary matter and dark matter gets progressively higher and higher, because the same amount of matter is in a smaller and smaller space. However, because the density of dark energy is constant, its percentage contribution to the total mass of the Universe gets smaller and smaller.  When the Universe was one tenth its current size then dark energy made up only 0.2% of the total mass of the Universe and at times earlier than this its contribution was insignificant. It is therefore clear that, although it dominates the Universe now, dark energy was completely unimportant in the early Universe and played no part in its initial expansion. The current most widely accepted explanation of the initial expansion of the Universe is that, in the first microscopic fraction of a second after the Universe came into existence, it underwent an incredibly rapid expansion called inflation. After this first minute fraction of a second had passed, inflation switched off and the Universe has been expanding ever since. The exact details of this process are still not fully understood.

The diagram above shows how the Universe expands over time. The Universe expands rapidly after the big bang, but all the time its expansion is slowed by gravity (A). About 7 billion years ago, it is still expanding but its rate of expansion is at a minimum (B). After this time dark energy dominates the Universe and its expansion speeds up (C).

Dark energy in the future 

As the Universe expands, and the galaxies get further and further away from each other, the density of ordinary matter will continue to fall. In roughly 10 billion years the Universe will have expanded so that the distance between galaxies will be roughly twice what it is today. This means that because we will have the same amount of matter in a space eight times bigger than it is today, the density of matter will be roughly one eighth.

Because the amount of dark energy in a given volume of space remains the same as the Universe expands, it will make up an even greater proportion of the total mass of the Universe than it does now. This is shown in the pie-chart below.

As the Universe continues to evolve and expand, the contribution of dark energy which accelerates its expansion will continue to get greater and and greater. In fact, in around 30 billion years time when the average distance between galaxies is roughly 10 times its current value, it will consist of 99.95% dark energy.

I will say more about the ultimate fate of the Universe in a future post.  As we go further forward in time to around 100 billion years from now, about 7 times longer that the age of the Universe, the distance between our galaxy and other galaxies will be so great that the light from them won’t be able to reach us (see notes).  If there are any astronomers in our galaxy at this time then when they look out with their telescopes then rather than seeing hundreds of billions of galaxies in the observable universe that they do today they will only see a single large galaxy – our own.

Next Post

My next post will be about dark matter, the mysterious substance which makes up about a quarter of the Universe, but about which we know very little indeed.

Related Posts

This post is the fourth my series about cosmology. The other posts are:

(1) The Universe Past, Present and Future. This describes what is meant by the Universe and gives an overview of its origins, evidence for its expansion and discusses its ultimate fate. To view this post click here.

(2) A brief history of the Universe.  This gives a history of the Universe from just after the big bang until the current date. To view this post click here.

3) Dark Energy. This post gives the reasons why cosmologist believe dark energy exists and why it makes up nearly 70% of the mass of the Universe. To view this post click here.

Notes

Strictly speaking not all galaxies are getting away from us. Our own galaxy the Milky Way and its neighbour, the Andromeda galaxy, together with a number of small satellite galaxies form a group of galaxies called the Local Group. The galaxies in the local group are bound together by the force of gravity so they won’t get further away from each other as the Universe expands.

However, the Milky Way and the  Andromeda galaxy are on a collision course. The  Andromeda galaxy is approaching the Milky way at 400,000 km/h and they will collide in roughly 5 billion years time to form a giant galaxy. So in in around one hundred billion years time our observable Universe will consist of a single galaxy.