The Universe is all existing matter and space, including all stars, planets, galaxies and the space between them.
This post, which is the first in a series about cosmology discusses the origins of the Universe and its ultimate future.
(All my posts on this topic can be found at https://explainingscience.org/tag/cosmology/)
How large is the Universe ?
Our Solar System belongs to a galaxy which we call the Milky Way (shown below). The Milky Way is enormous compared to the Solar System., around 100,000 light years in diameter and contains around 400 billion stars. The Sun lies at the edge of the Milky Way, around 30,000 light years away from the centre.
In the early part of the twentieth century, there was a great deal of uncertainty about how big the Universe was. Some astronomers believed the Universe did not extend beyond the Milky Way galaxy, whereas others believed the Universe was much, much larger. In particular, astronomers were unsure whether fuzzy patches of light called spiral nebula were part of our galaxy or not.
Eventually it was discovered that these were in fact other galaxies. In 1924 in one of the greatest astronomical discoveries of the twentieth century, the American astronomer Edwin Hubble (1889-1953) proved that the Andromeda Nebula, which to the naked eye appears as a dim fuzzy patch in the constellation Andromeda, was another galaxy like our own, lying millions of light years away.
Today we still don’t know whether or not the Universe is infinite, but we do know that there are over 200 billion galaxies in the part of the Universe that we can see. These galaxies come in all shapes and sizes. Each large galaxy, like our own Milky Way, contains hundreds of billions of stars.
The Andromeda galaxy
How and when did the Universe originate?
In the late 1920’s a Belgian Catholic priest, Monsignor Georges Lemaitre (1894-1966), came up with a remarkable theory of the origin of the Universe. Around this time, astronomers proved that the Universe was expanding and therefore must have been more dense and compact in the past. Lemaitre applied the known laws of physics and came up with the idea that, billions of years ago, a single incredibly violent ‘creation event’ took place. He called this the ‘Cosmic Egg’, but this event is now much more familiar to us as the ‘Big Bang’. At a single point in time, which we now believe to be around 13.8 billion years ago, the entire Universe came into existence. The Big Bang theory has therefore been around for around ninety years. However, it has only been accepted by most astronomers since the late 1960’s.
We don’t yet know what happened at the exact instant of the Big Bang. However, over the last few decades physicists have pushed our our understanding back further and further to very beginning of time and applied the known laws of physics to arrive at a rough timeline of what happened during the very early evolution of the Universe.
For example, physicists have worked out that from the time when the Universe was only 10 second old to when it was about 20 minutes old it was at a temperature of over 1 billion degrees centigrade. This is hot enough to carry out a two nuclear reactions, the first one created deuterium (also known as heavy hydrogen) and the second fused two atoms of heavy hydrogen to form a single atom of helium and this reaction gives out a huge amount of energy. Essentially this is the same reaction which occurs, much more slowly, in stars and in the hydrogen bomb. In effect the entire Universe was behaving like a nuclear bomb.
The Expansion of the Universe
In the early 1920’s, The general consensus was that the Universe was static, that is to say, on average roughly the same number of galaxies would be moving toward us as would be moving away from us.
In the late 1920’s, however, Hubble discovered that in fact all galaxies are moving away from us, other than a few nearby galaxies – although nearby is in this case two million light years away! He made use of other astronomers’ measurements of speed as well as his own measurements of distance and discovered to his surprise that the further away a galaxy was, the faster it was moving away from us. In effect the whole Universe was expanding.
In fact the speed a galaxy is moving away from us is directly proportional to 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
Where V is the speed a galaxy is moving away from us, D is its distance and Ho the Hubble constant measures how fast the Universe is expanding.
What does the expansion of the Universe mean ?
The expansion of the Universe does not mean that objects which are held together by gravity such as the Earth, the Sun, our Solar System or even the Milky Way galaxy get larger over time. What is does mean is that objects which are not tightly bound by gravity get further away from each other. So although individual galaxies don’t get any bigger the distance between them increases.
The diagram above show schematically the expansion of the Universe. The axis labelled t represents time and shows how starting with the Big Bang the Universe Expands and the empty space between galaxies increases.
What is the ultimate fate of the Universe ?
Until the late 1990s, the generally held view was that although the Universe was expanding, when astronomers applied Einstein’s Theory of General relativity it gave the results that the effect of the gravity due to the matter in the Universe would be to slow the expansion down. The more matter in the Universe the more the expansion would be slowed down. This gave two possibilities:
- If the average density of matter in the Universe was high enough, then the expansion would slow down, stop and the Universe would start contracting. The Universe would then contract at a faster and faster rate and would eventually collapse entirely in a “big crunch”. At a distant point in the future, many billions of years from now, the entire Universe would cease to exist. Astronomers call this scenario a closed universe.
- If the average density was not high enough then the rate of expansion would slow down but not stop. The universe would just expand forever at a slower and slower rate. Astronomers call this scenario an open universe.
The diagram above shows the how the average distance between galaxies changes with time for a closed Universe (A) and an open Universe (B). We now know that the expansion of the Universe is speeding up (C).
In general opinion in the 1980s and 1990s was evenly divided between the two scenarios with perhaps a slight majority in favour of a closed Universe.
Over the last 15 years or so that position has changed. In 1998 new results were published showing how fast very distant galaxies were moving away from us. These are shown in simplified form in the diagram below.
At very great distances, more than around 2 billion light years away, galaxies appear to be moving more slowly away from us (line B) than would be implied by Hubble’s law (line A).
When we look at very distant galaxies, because of the time it takes for light to reach us, we are seeing them as they were billions of light years ago. For these galaxies the Hubble constant, which is the gradient of the graph and gives the rate of expansion of the Universe is lower than it is for closer galaxies. Therefore billions of years ago the Universe was expanding more slowly than it is now.
So because the expansion of the Universe was lower in the past than it is now it must be speeding up and it will go on expanding forever. I shall say more about this in a future post.
I hope you have enjoyed this pos, if you want to read more from Explaining Science, click on the Home link at the top of this page.
Revised 19 November 2019