The anthropic principle

In his 1988 book ‘A Brief History of Time’ the British theoretical physicist Stephen Hawkins (1942-2018) stated:

‘The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron. … The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life.’

In this post I will talk about the view that the laws of physics, and the properties of the Universe as a whole, are somehow finely tuned to allow our existence. The term ‘anthropic principle’ was coined in 1973 by the Australian physicist Brandon Carter (1942-) to describe this viewpoint. However, these ideas had been circulating for decades beforehand.

Brandon Carter – image from Wikimedia Commons

Since 1973 the ideas behind the anthropic principle have been reviewed in many books, both popular science and those aimed at the more specialised reader. They were developed in detail in a book written in 1986 by the theoretical physicists John Barrow and Frank Tipler called ‘The Anthropic Cosmological Principle’.


Background: atoms, the atomic nucleus and the four fundamental forces

Before we go into detail about the anthropic principle it is worth discussing, at a high level, the structure of matter and the fundamental forces which drive the way that everything in the Universe behaves.

All ordinary matter in objects like ourselves, planets and stars is made up of atoms. An atom consists of a central nucleus, which has a positive electric charge, surrounded by a cloud of negatively charged electrons. Atoms are very small, typically around 0.0001 microns in diameter (a micron is a millionth of a metre). However the nucleus, which contains nearly all the mass of the atom, is much, much smaller, typically around 0.000 000 001 microns in diameter.

The nucleus consists of a number of protons, which have a positive electric charge and neutrons which have no electric charge. Because the electrons have a negative charge, and the number of protons and electrons in an atom is always the same, atoms have a net charge of zero.

  • The number of protons in the nucleus is called the atomic number and determines the atom’s chemical properties. You may remember from high school chemistry that the atomic number gives the position in the periodic table.
  • The number of neutrons in the nucleus does not affect the chemical properties of the atoms. In fact, all elements have different versions of themselves called isotopes, which have a different numbers of neutrons but the same number of protons.

The simplest possible atomic nucleus is that of hydrogen, which consists of a single proton.  Ordinary hydrogen atoms have no neutrons, but a small fraction of naturally occurring hydrogen atoms are deuterium or heavy hydrogen which have one proton and one neutron. Atoms which have 2 protons (regardless of the number of neutrons) are helium atoms, those which have 3 protons are lithium atoms and so on.  The element with the highest atomic number which naturally occurs on Earth is uranium, which has 92 protons.




An atom of the most common isotope of carbon has 6 protons and 6 neutrons in the nucleus surrounded by 6 electrons. Other isotopes of carbon are found on Earth which have 7 and 8 neutrons in the nucleus.

The Universe is governed by four fundamental forces.  All other interactions, such as the combination of two hydrogen atoms and one oxygen atom to form a water molecule, are due to these fundamental forces.

  • Gravity – an attractive force which acts on all particles having mass.
  • Electromagnetic force – This only acts on electrically charged particles and can be an attractive or repulsive force. If two particles have the same charge, such as two protons, the force is repulsive. If two particles have different charges, such as a proton and an electron, the force is attractive.

  • Strong force – this force acts on fundamental particles called quarks.  There are six types of quark, which have the rather odd names of up, down, charm, strange, top, and bottom.
    • a proton consists of two up quarks and a down quark
    • a neutron consists of one up and two down quarks

Internal structure of a proton – image from Wikimedia Commons

Because protons and neutrons are made up of quarks they too are acted on by the strong force. The force between protons and neutrons which is sometimes called the ‘residual strong force’ or ‘nuclear force’ only works over extremely short ranges, of less than 2.5 femtometres, where 1 femtometre (fm) is one thousand trillionth of a metre. It is this nuclear force which binds protons and neutrons together into atomic nuclei. For more details on the strong force see the notes at the bottom of this post.

  • Weak force – this acts over a short distance, 0.01 fm to 0.1 fm. It is this force which is responsible for a particular type of radioactive decay called beta decay. Without the weak force there would be insufficient oxygen produced in stars to support life (Clavelli 2008).

In addition to these four forces there are other fundamental parameters such as the  mass of the proton and electron and the average densities of ordinary matter, dark matter and dark energy in the Universe. (See my previous posts for more information on dark matter and dark energy.)

Fine tuning

One thing that is apparent is that the relative strength of the four forces (and other  fundamental parameters such the density of matter in the Universe) appear to be finely tuned to enable the production of stars, planets and the eventual evolution of intelligent life. If they were only slightly different, the Universe would be a very different place and life would never emerge. To illustrate this I’ll give some examples below.

  • Hydrogen is the most common element in the Universe. In the early Universe 10 to 1000 seconds after the Big Bang, when it was at a temperature of billions of degrees, about 25% of the primordial hydrogen was converted to helium by nuclear fusion. If the strong interaction had been only 2% stronger than its current value then all its hydrogen would have been converted to helium in the first few minutes of the Universe’s existence. There would be no hydrogen compounds in the Universe, such as water which is, as far as we know, essential for life.
  • If the strong interaction were only a few percent weaker, then deuterium (heavy hydrogen) would not be stable. This would mean that certain elements essential for life such as nitrogen and phosphorous, which are made in the centre of stars by nuclear reactions which fuse deuterium with other nuclei, would not be formed to any appreciable degree.
  • If the electromagnetic interaction were three times stronger, and all other forces the same strength, then any element heavier than carbon (atomic number = 6) could not form. For these elements, the repulsive electromagnetic force between the protons in the nucleus would be stronger than the attractive nuclear force holding the nucleus together. Therefore elements such as nitrogen and oxygen on which life is based would not exist.

  • If gravity were a thousand times stronger, then stars would be much smaller and burn their nuclear fuel more quickly. Instead of living for ten billion years, a typical star would live for about 10 million years.  As readers of a previous post will recall, it took hundreds of millions of years from when the Earth was formed until the emergence of the first single-celled lifeforms. So, in a Universe with stronger gravity, these mini-suns and would have stopped shining before even the first steps in evolution had started.

  • If the strength of all the forces were the same, but there were much less matter in the Universe, then the way the Universe evolved would have been very different. In our Universe, initial unevenness in the distribution of matter in its early stages eventually became the structures such as stars and galaxies which we see today. If the density of ordinary matter were 10% of its current value, then structures such as stars and galaxies would not have formed.

The strong and weak anthropic principles

In his 1973 work Carter distinguished between the strong anthropic principle (SAP) and weak anthropic principle (WAP).  Since then there have been many slightly different definitions of the WAP. The one below is from Barrow and Tipler (1986):

‘the observed values of all physical  and cosmological constants are not equally probable but they take on values restricted by the requirement that carbon-based life can evolve..’

The WAP is generally accepted by most astronomers. In fact it has been criticised as a tautology – a statement which must be true. For if conditions were very different, so that life couldn’t evolve, humanity wouldn’t be around to observe them. Even so, it is remarkable how finely tuned the Universe is.

Many physicists believe in the multiverse –  a collection of possibly an infinite number of other universes.

In other universes different relative strengths of the fundamental forces might apply, so they would look very different from our Universe  In some of them there might be more or fewer than four fundamental forces, perhaps even more than three dimensions of space. It seems likely that in the vast majority of these other universes conditions are such that life can never evolve.

While the WAP is generally accepted, the SAP is more contentious. The definition given by Barrow and Tipler is as follows:

‘The Universe must have those properties which allow life to develop within it at some stage in its history.’

A variation to this definition was proposed by the physicist John Archibald Wheeler (1911-2008) in 1977, which he called the participatory anthropic principle.

‘Observers are necessary to bring the Universe into being’

What  the SAP is saying is that we cannot have a universe which doesn’t have, or have the potential to have, any observers. In some way the purpose of the universe it to give rise to intelligent observers. The SAP is not generally accepted by most astronomers. One particular criticism of it is that we cannot falsify it by observing a  ‘dead universe’ in which observers cannot exist. This is because other universes are, by definition, unobservable.

Final Anthropic Principle

Even more controversial is the Final Anthropic Principle (FAP). This idea was developed by Barrow and Tipler. They could see little point in having a Universe which has as its purpose giving rise to intelligent observers and these intelligent observers then become extinct. The FAP is defined as follows:

Intelligent information-processing must come into existence in the universe, and, once it comes into existence, it will never die out.

In the final chapter of The Anthropic Cosmological Principle they outline a future of the Universe in which the FAP is true. In this universe billions of years after the Big Bang intelligent carbon-based life (i.e ourselves ) eventually emerges. Over a long period of time as the civilisation develops it evolves into different forms of life, which are more robust and better able to survive the harsh conditions of interstellar travel and the long timescales needed. These new lifeforms will not be carbon based but could be for example intelligent self-replicating robots. The civilisation then spreads to neighbouring stars and eventually spreads throughout the galaxy.

Eventually it spreads to neighbouring galaxies and through the entire Universe. If in the far future, the Universe’s expansion slows down and stops and it starts contracting,  it may end in what is termed a singularity of infinite density where the Universe will come to an end and space and time will cease to exist. Barrow and Tipler called this point in time the Omega Point.

At the Omega Point life will have gained complete control of the Universe and will be able to process an infinite amount of information. The final two sentences of The Anthropic Cosmological Principle state:

‘At the instant the Omega Point is reached, life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know.  And this is the end’

In general the scientific community reacted with scepticism to the FAP.  To many it smacks of pseudo-science. In a review of The Anthropic Cosmological Principle the American science writer Martin Gardner (1914-2010) called it a ‘Completely Ridiculous Anthropic Principle’ which he suggested should be abbreviated to CRAP. In addition, although in the 1980s many astronomers believed in a Universe which wold would eventually collapse into singularity, as discussed in a previous post, this is not supported by current observations. The general consensus is that we live in a Universe which will expand forever and so there will be no Omega Point.



Although there are six types of quarks up, down, charm, strange, top, and bottom, which are sometime called ‘flavours of quark’, only the up and down quarks are found in nature. The other four flavours rapidly decay into either an up or down quark. Interestingly quarks are never found free but always in combination with other quarks. The combinations are:

  • mesons which consist of one quark and one antiquark
  • baryons which consists of three quarks or three antiquarks

The strong force, which acts on quarks because they have a property called colour charge, is carried by particles called gluons.  Gluons also have colour charge and so are subject to the strong force too. For a non-technical overview of the strong force see the following:


Clavelli, L. (2008) Problems in a weakless universe, Available at: 24 May 2018).


In this post I’ll talk about Nicolas Copernicus (1473 – 1543) and the heliocentric theory.  The move away from the prevailing Earth-centred theory of the Universe to the heliocentric theory represents one of the greatest advances in astronomy ever made.

Nicolas Copernicus – Image from Wikimedia Commons

Background – the need for a better theory

As mentioned in my last post, the geocentric theory was the generally accepted theory of the cosmos until the early 16th century, having been developed by the second century Greek astronomer Claudius Ptolemy.  To make it fit the observations, Ptolemy needed to fine-tune his theory, making it rather complex. Rather than moving directly around the Earth, the Sun, the Moon and the planets moved around small circles called epicycles and the centre of each epicycle moved, at a varying speed, around a larger circle called a deferent. A further complexity was that the centre of the deferent wasn’t the Earth but a point nearby, which Ptolemy called the ‘eccentric’. This was at a different location for each planet.  This is shown in the diagram below (to simplify the diagram only a single planet, Mars, is shown). 

Ptolemy’s model – (Note the equant is another theoretical point described in my previous post.)

By the year 1500 observations had shown that the predictions of Ptolemy’s theory still didn’t quite match the actual positions of the planets. Because the geocentric model was generally accepted, the way astronomers made it fit the observations was to add additional epicycles, as shown in the diagram below.

In the revised geocentric model:

  • each planet revolved around a small epicycle – shown in green
  • the small epicycle revolved around a main epicycle – the blue dashed line
  • the main epicycle revolved at an uneven speed around the deferent
  • the deferent was centred at a point near the Earth called ‘the eccentric’.

This made the theory rather unwieldy and it appeared somewhat cobbled together. In fact the term ‘adding epicycles’ is used today in a derogatory way to mean making a bad theory over-complicated in order for it to fit the facts.

In 1473 Nicolas Copernicus was born, in the city of Torun in northern Poland into an influential and wealthy family. His father died when was 10 and the education of Nicolas, a bright child, was supervised by his uncle Lucas Watzenrode the Younger. His uncle was a influential prince-bishop (a bishop who was also a secular ruler over a region of Poland), in contact with many of the leading intellectual figures in the country.

After he left school, Copernicus went to the University of Krakrow from 1491 to 1495 where he studied mathematics and astronomy, under some of the leading intellectuals in Poland. Although he did not graduate, his studies at Krakow gave him the grounding for his later work. Mrs Geek and I were lucky enough to see the rooms occupied by Copernicus’s tutors during our recent visit to Poland.

Part of a dining room at the Jagiellonian University in Krakow, Poland, where Copernicus studied. Founded in 1364, it is one of the oldest universities in the world. This photo was taken by Mrs Geek during our recent visit to Krakow.

After leaving Krakow, Copernicus studied ecclesiastical law and medicine in Italy before moving back to Poland in 1503 to live in the town of Warmia, which was governed by his uncle, and he lived there for most of the rest of his life.  Rather than being a professional scientist, his main job was as a Catholic clergyman and local government official.

In around 1510 Copernicus started work on the heliocentric theory of the Universe. His aim was to provide a more accurate and a simpler explanation of the cosmos. The main points of Copernicus’s theory were as follows:

  1. The Sun (not the Earth) was the centre of the Universe. The Earth and all the planets moved in perfect circles around it.
  2. The Moon was the only astronomical object which orbited the Earth.
  3. The Sun is much nearer to the Earth than any of the other stars, by a vast measure.
  4. The daily rising and setting of the Sun, Moon, planets and stars are explained by the rotation of the Earth on its axis.

The third point was needed because if some stars were relatively close to the Sun, for example only 100 times the distance between the Earth and the Sun, then there would be a measurable ‘stellar parallax’ effect, where the nearby stars would appear to be in a different position at different times of year.



Stellar Parallax – at different times of year, the nearby star appears to be in different positions with respect to the background of fixed stars.

Because stellar parallax had never been observed, Copernicus concluded that all the stars must be at a vast distance from the Sun. The parallax is so small that it couldn’t be measured in Copernicus’s time. It wasn’t detected until the mid nineteenth century.

Mercury and Venus

Mercury and Venus differ from the other planets in that, to an observer on the Earth, they never stray too far away from the Sun and, to viewers at low latitudes, they can only be seen for a few hours after sunset or a few hours before sunrise. The reason why Venus and Mercury always appear in the same part of the sky as the Sun is neatly explained by Copernicus’s theory in that their orbits lie inside the Earth’s orbit.

Venus cannot appear more than 46 degrees away from the Sun.  The green line shows the limits of Venus’s apparent position from the Sun.  

Copernicus refined his theory over the next 20 years to match accurate observations that he and other astronomers had taken. As he did so, his ideas began to circulate among the educated elite within Europe. He had finalised his theory by 1530, but he was extremely reluctant to publish it.  He was well aware that it would cause a massive controversy, for at the time the Ptolemaic system was generally accepted by virtually all astronomers. With his ecclesiastical background, Copernicus also knew there would be religious objections, for certain verses in the bible could be interpreted as saying that Earth was stationary and rest of the Universe was in motion around it.

Under pressure from colleagues to make his ideas more widely known, in 1543 he finally agreed to publish them in a book called ‘De revolutionibus orbium coelestium‘ (On the Revolutions of the Heavenly Spheres). By this time Copernicus was ill and near the end of his life. His book, like most scientific literature of time, was written in Latin and, perhaps to ward off the religious objections he knew would arise, he dedicated the book to the pope. The printing of the book was supervised by the German theologian Andreas Osiander, as Copernicus was too ill to do it himself.  Unbeknown to Copernicus, Osiander added a preface saying that the heliocentric theory should only be considered as another model of the Universe, which could be used to predict the positions of the stars and planets, and should not be taken as true, the truth being known only to God.

Although the book escaped initial censure, over the 50 years following its publication the Catholic church became more and more hostile to heliocentrism and eventually regarded anyone holding these views as a heretic. The Italian astronomer and philosopher Giodarno Bruno was burned at the stake in 1600; one of the main charges against him was promoting a heliocentric view of the Universe.

In 1616 De revolutionibus was placed on the list of banned books by the Catholic church, where it would remain for the next 200 years.

The Protestant churches too were extremely critical of Copernicus’s ideas. About ten years before the publication of of De revolutionibus, when Corpernicus’s ideas were becoming known, the German theologian Martin Luther said:

‘There is talk of a new astrologer who wants to prove that the Earth moves and goes around instead of the sky, the Sun, the Moon, just as if somebody were moving in a carriage or ship might hold that he was sitting still and at rest while the Earth and the trees walked and moved. But that is how things are nowadays: when a man wishes to be clever he must needs invent something special, and the way he does it must needs be the best! The fool wants to turn the whole art of astronomy upside-down. However, as Holy Scripture tells us, so did Joshua bid the sun to stand still and not the Earth’ (Pogge 2005).


Refining the heliocentric theory.

Like all earlier astronomers, Copernicus still believed, probably for philosophical reasons, that the planets must move in perfect circles.  So in order to make his theory  fit the facts he needed to retain the concept of epicycles.  The heliocentric theory was refined in the early seventeenth century by Johannes Kepler (1571-1630), who formulated a set of rules which became known as Kepler’s laws of planetary motion. These state that the planets move in elliptical orbits around the Sun and that they move at varying speeds around these orbits, moving faster when they are closer to the Sun.  In this theory, Kepler  removed the need for epicycles altogether and produced a simpler model which accurately fitted the observations. Kepler’s theory, which in turn led to Issac Newton developing his theory of gravity, is such a large topic that I will discuss it in more detail in a future post.

Legacy of Copernicus

Copernicus removed the Earth from the centre of the Universe and his theory provided the foundation for the later work of Kepler. In cosmology, there is an important concept called the ‘Copernican principle’. It states that the Earth, the Solar System and even the Milky Way galaxy are not in a special place in the universe. We belong to an average planet, orbiting an average star, on the edge of an average galaxy.

Copernicus’s contribution to science has been acknowledged in many ways. He is one of the few people to have an element in the period table named after him, copernicium, element 112.  There are numerous statues and monuments to him including this one below in the Jagiellonian University in Krakow.

Photo taken by Mrs Geek

In Torun, Copernicus’s birthplace, the Nicolaus Copernicus University has over forty thousand students and is one of the largest universities in Poland, and the airport in Poland’s fourth largest city Wrocław is named after him.




Pogge, R (2005) A brief note on religious objections to Copernicus, Available at: (Accessed: 20 November 2017).

Geocentric Cosmology

Today it is generally accepted as a scientific fact that the Earth is one of eight planets which revolve around the Sun, that the Sun is one of 400 billion or so stars in our Milky Way galaxy and that the Milky Way is one of hundreds of billions of galaxies in the observable Universe.

Location of our solar system in the Milky Way galaxy

However, for most of human history a geocentric model was the standard explanation of the cosmos. In this model the Earth is the the centre of the Universe and all the planets and stars revolve around it. Although it has been long superseded, this model could actually still be used to predict the motion of the Sun, Moon and the planets to a reasonable accuracy.

The basis of the geocentric model

Anyone watching the sky over a period of time will notice that the Sun rises in the east, is at its highest in the sky at midday and the sets in the west. Like the Sun, the Moon and the stars also rise in the east, get higher in the sky and set in the west. It  was perfectly natural for ancient astronomers to assume a geocentric model in which the Earth doesn’t itself rotate but that the Sun, Moon and stars all rotate around it. After all, people on the Earth do not actually feel that it is rotating.

Ancient astronomers noticed that the time at which a given star rises wasn’t the same but was roughly 4 minutes earlier each day and that virtually all of the thousands of stars in the sky didn’t move with respect to each other but were fixed. So, they put them together in structures  which became known as constellations. However, five of the brightest stars weren’t fixed but moved around a narrow band of sky which became known as the zodiac. These five objects: Mercury, Venus, Mars, Jupiter and Saturn became known as the wandering stars or planets. (The outermost two planets Uranus and Neptune were only discovered in the last 250 years)

The geocentric model

The name most associated with the geocentric model  is the ancient Greek astronomer Claudius Ptolemy. He lived in Alexandra, Egypt in the second century CE, although no one knows the exact dates of his birth and death. Ptolemy didn’t invent the geocentric model – no single astronomer did.  Ptolemy’s contribution was to refine the model which had been built up by Greek astronomers over the previous centuries,  so that it could accurately predict the positions of the stars and planets.

The geocentric model 

An early version of the geocentric model is shown in the diagram above. At the centre of the Universe was the Earth. The Earth did not rotate and was surrounded by a set of eight invisible spheres to which the Sun, Moon, planets and stars were attached. The outermost of these spheres was a sphere of fixed stars. All the stars were attached to this sphere. Moving inward from the sphere of fixed stars were Saturn’s sphere, Jupiter’s sphere, Mars’s sphere, the Sun’s sphere, Venus’s sphere, Mercury’s sphere and finally, closest to the Earth, the Moon’s sphere. To simplify the diagram, only the sphere of fixed stars, the Sun’s and Mars’s spheres are shown.

To a viewer in the northern hemisphere, the sphere of fixed stars rotated anticlockwise around a point in the sky directly above the North pole, known as the North celestial pole, once every 23 hours 56 minutes (see note 1).

As viewed from Earth,  the Sun’s, Moon’s and the planets’ spheres rotate more slowly than the sphere of fixed stars. For example, the Sun’s sphere takes 24 hours to complete a single rotation ,compared to the 23 hours 56 minutes for the sphere of fixed stars. So, compared to the sphere of fixed stars, the Sun’s sphere slipped back by around 4 minutes giving it a slow backwards rotation.  The diagram above also shows that axes about which the Sun’s, Moon’s and planets’ spheres rotated didn’t go through the North pole but was inclined at an angle to it.

The diagram below shows a two dimensional slice through the model with the Sun, Moon and all the planets shown.

The uneven motions of the planets

If you observe how a planet moves over a period of time you will notice that it does not move at a steady rate. At times it will move more slowly against the background of stars, stop altogether,  move in the opposite direction, stop again and then continue moving in the original  direction.  The reason for this is shown for one of the planets, Mars, in the diagram below.


As you can see from the diagram, the Earth is closer to the Sun than Mars, and moves faster in its orbit. At point 3, the Earth approaches Mars; to an observer on the Earth, between points 3 and 5 Mars will appear to change direction and move in a reverse direction against the background of stars. When it reaches point 5 the Earth is receding from Mars and so Mars will continue in its normal direction.

In the geocentric model the apparent uneven speed and change of direction of the planets’ motion was explained by introducing a concept known as an epicycle.

Epicycles and deferents – for simplicity only Mars is shown

Rather than than revolve directly round the Earth, a planet moves at constant speed in a small circle called an epicycle. The centre of the planet’s epicycle, marked with an ‘x in the diagram above, moves in a circular path around a larger circle called a deferent. For each planet the Earth lies at the centre of its deferent. At times the planet will be moving in the same direction as the orbital motion of the deferent and other times it will moving in the opposite direction. This will make the motion of the planet appear to be uneven – in keeping with the observations taken by the ancient astronomers.

The inferior planets Mercury and Venus

Mercury and Venus differ from the other planets in that, to an observer on the Earth, they never stray too far away from the Sun and, to viewers at low latitudes  they can only be seen for a few hours after sunset or a few hours before sunrise. For this reason they were known by the Greek astronomers as the the inferior planets. The superior planets (Mars, Jupiter and Saturn) are not tied to the position of the Sun and at certain times were visible in the middle of the night.

The reason why Venus and Mercury always appear in the same part of the sky as the Sun is that their orbits lie inside the Earth’s orbit. This is shown in the diagram below.

Venus cannot appear more than 46 degrees away from the Sun.  The green line shows the limits of Venus’s apparent position from the Sun.  

In the geocentric model this was explained by locking the Mercury and Venus’s motion to that of the Sun. The centres of their epicycles took exactly 1 year to orbit the Earth and were always in a direct line between the Earth and the Sun. This is shown in the diagram below.

As Venus moves around the Earth the centre of its epicycle always lies on the red line between the Earth and the centre of the Sun’s epicycle. This ‘locks’ the position of Venus to the Sun so that, to an observer on Earth. it cannot appear too far from the Sun. Mercury is locked in a similar manner.

Fine tuning the geocentric model.

The geocentric model which I’ve described so far had been developed by astronomers in the centuries before Ptolemy, although very little of the early astronomers’ original writings survive. Ptolemy’s contribution was to make two further modifications to make it more accurately fit observations of stars and planets. The first one I’ll describe is to deal with a phenomenon known as the ‘precession of the equinoxes’.  Even though the telescope hadn’t been invented, Greek astronomers had been taking precise observations of stars and measuring their positions for hundreds of years. Although the stars appeared fixed with respect to each other, over hundreds of years the alignment of the entire celestial sphere was slowly moving with respect to the Earth.

An effect of the precession of the equinoxes is that the pole star gradually changes. Today the bright star Polaris in the constellation Ursa Minor (the Little Bear) lies near the celestial North pole and all the other stars appear to rotate around it. In Ptolemy’s time another star in Ursa Minor, β Ursae Minoris, was the pole star and in about 13,000 years time the bright star Vega will lie close to the celestial North pole.

Ptolemy explained procession of the equinoxes by giving the celestial sphere an additional very slow rotation once every 26,000 years about a different axis in addition to its daily rotation around the Earth’s North-South axis.

Ptolemy had to make one more adjustment to the model to allow it to fit historic observations of the stars and planets and thus be able to accurately predict their future positions. Even when epicycles were added, the position of the planets was not where the model predicted they would be. The motion of the planets was still uneven compared to the background stars. The reason for this is that the planets move around the Sun  in elliptical (oval-shaped) orbits and the speed of a planet is fastest when it is closer to the Sun (see note 2). This is shown in an exaggerated form in the diagram below. In reality none of the planets’ orbits are actually this elliptical.

However Ptolemy, like all other Greek astronomers before him, believed that the all the planets’ deferents and epicycles must be perfect circles. This was for philosophical reasons, as the heavens were the epitome of perfection and the circle was seen as a perfect shape. So what he had to do was to modify the geocentric theory in a way summarised in the diagram below.

Firstly, the Earth didn’t lie at the centre of the deferent. He introduced a new point which he called the ‘eccentric’ for the centre of the deferent. The eccentric was some distance away from the Earth. Secondly, the centre of the epicycle didn’t move at a constant speed around the deferent but an uneven speed:

  • when it was closer to the Earth it moved faster – the dashed line
  • when it was further away it moved more slowly – the solid line.

The uneven speed was such that for each planet there was a point called the equant, at which the deferent would appear to move with a constant speed. A further complexity was that distance between the Earth, the equant and the eccentric varied for each planet.

So, strictly speaking, Ptolemy’s theory wasn’t truly geocentric, because the planets didn’t orbit the Earth, but each of their deferents orbited a hypothetical point called its eccentric which was offset from from Earth and was different for each planet. However, the theory fitted the observations in that it was possible to work out the past and the future position of the stars and planets using it. In around 150 CE Ptolemy published his theory in a book called ‘The Almagest’, which over the succeeding centuries was translated into many languages including Arabic and Latin and became the most influential astronomy textbooks for the next 1500 years.

A sixteenth century Latin translation of The Almagest -Image from Wikimedia Commons 

Next post

I hope you enjoyed this rather long post. In my next post I’ll talk about how Ptolemy’s theory was swept away by the scientific revolution


(1) To a viewer In the southern hemisphere, the sphere of stars wold appear to to rotate clockwise around a point in the sky directly above the South pole.

(2) For the more technically-minded reader the motions of the planets around the Sun are governed by Kepler’s laws of planetary motion, which state:

  • The orbit of a planet is an ellipse with the Sun at one of the two foci.
  • A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  • The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.


The cosmic microwave background part II

As discussed in my previous post, the accidental discovery of the cosmic microwave background (CMB) in 1964 by Arno Penzias and Robert Wilson would prove to be one of the greatest scientific discoveries of the early twentieth century. One of the first things it achieved was to provide confirmation of the big bang theory.


The telescope where Penzias and Wilson made their accidental discovery – Image from NASA 

Although the big bang theory is generally accepted today, in the mid 1960s many astronomers still believed in the rival steady state theory according to which the Universe is infinitely old and will exist for an infinite time in the future and, taken as a whole, doesn’t evolve or change over time.  This is described in more detail in my post the steady state theory. However, in the 1960s evidence was beginning to mount against the steady state theory, as observations of distant objects were beginning to show that the Universe had been changing over time. In 1963 a new class of astronomical objects called quasars were discovered. These are incredibly bright objects which can be up to 1,000 times the brightness of the Milky Way, but are very small when compared to size of a galaxy. Quasars are only found at great distances from us, meaning that the light from them was emitted billions of light years ago.


A quasar.  Image from ESO

The fact that quasars are only found in the early Universe provided more evidence that the Universe has changed over time, further calling into question the steady state theory. The existence of the cosmic microwave background proved to be the final nail in its coffin. Although Fred Hoyle, one of the originators of the steady state theory, did come up with a convoluted theory to account for the CMB, virtually no other astronomers were convinced by his explanation.  The big bang theory, on the other hand, predicted the existence of the CMB, and therefore its discovery provided further confirmation of its validity.  It is almost impossible to explain by the steady state theory, whereas the big bang theory predicted its existence.

Prediction of the microwave background

By a strange coincidence, at the same time as Penzias and Wilson were making their discovery, the astronomers Robert Dicke (1916-1997) and Jim Peebles (1935-) had performed some detailed calculations on the conditions in the early Universe. These calculations also predicted the existence of the CMB and were about to start a search for it using a sensitive radio-telescope (see note 1).

Before they could start their search, they were made aware that Penzias and Wilson had detected a weak microwave signal which was the same strength in all directions. This turned out to be exactly what they were about to start looking for. According to some accounts, Dicke said to his colleagues on hearing of the discovery, “well boys we’ve been scooped” (Levine 2009).

Penzias and Wilson had not predicted the CMB and, when they published their results, did not explain their accidental discovery.  Nevertheless, it was they who were awarded the Nobel prize, rather than Dicke and Peebles, who had done the calculations which actually explained Penzias’ and Wilson’s observations.

CMB astronomy

One fascinating thing that measurements of the microwave background allow us to determine is the speed and direction in which our immediate cosmic neighbourhood is moving with respect to the rest of the Universe. As readers of a previous post may recall, the Sun is one of around 400 billion stars in the Milky Way galaxy, and the Milky Way is one of over 200 billion galaxies in the observable Universe (Cain 2013). Our galaxy, together with the large spiral galaxy in the constellation Andromeda and around 50 smaller galaxies form a collection of galaxies bound together by gravity, called the Local Group.


What our Local Group might look like from a distance of millions of light years (image from NASA)

Precise observations show that when we look at the CMB in more detail, it is not exactly the same in all directions but rather has some fluctuations in its strength. One cause of these is the fact that as the Earth moves around the Sun at a speed around 110,000 km/h.  This causes the CMB to be fractionally stronger if we observe it in the direction in which the Earth is moving around the Sun, and fractionally weaker if we look in the opposite direction, due to an effect known as a Doppler shift. This is shown in the diagram below.



If we subtract the fluctuations in the microwave background due to the Earth’s motion around the Sun, then there is still a variation in the microwave background caused by the Sun’s motion around the centre of our Milky Way galaxy. The Sun orbits the centre of the galaxy at a speed of nearly 800,000 km/h, taking about 250 million years to do a complete orbit. In addition, all the galaxies in the Local Group are moving with respect to each other (see note 2).

Sun in Milky Way

If we subtract the unevenness in the CMB due to the motion of the Sun around the galactic centre, and also our Milky Way’s motion within the Local Group, we are still left with an unevenness in the microwave background. It appears strongest towards a point which lies in the constellation Hydra and weaker at the opposite point in the sky. This because the Local Group is moving at 2 million km/h towards this point (Scott and Smoot 2015) (see note 3).


The digram above is part of a sky chart showing the constellation Hydra. The blue dot marks the point towards which our Local Group of galaxies is moving.

A window on the very early Universe

The radiation we observe today as the CMB was emitted when the Universe was around 400,000 years old.  Before this time the Universe consisted of a plasma through which radiation cannot pass, so this marks a limit as to how far back in time we can see, as no radiation emitted before this time can ever reach us.


What we are seeing when we look at the CMB is the oldest light in the Universe. It is an imprint of how the Universe looked like when it was 400,000 years old. In the 1970s and 1980s calculations on the early Universe predicted that there should be a slight unevenness in the way matter was distributed. Over billions of years matter condensed around these clumps of slightly higher density to form the structures  we observe today.

It was also predicted that this initial unevenness should leave an imprint we could observe today. The background radiation should be slightly stronger in the regions where the density of matter was fractionally greater than the average value, and fractionally weaker where it was lower. These tiny fluctuations proved very elusive to find and were only discovered in 1992 by the Cosmic Background Explorer (COBE) satellite which was launched to study the microwave background (see note 4).


A map of the microwave background from COBE. The red areas show regions where the radiation is slightly stronger  than the average level (shown as green) and the blue areas where the radiation is slightly weaker

This proved to be such an important finding in our understanding of the early Universe that it won its discoverers George Smoot and John Mather the Nobel Prize for physics. ( 2016)


  1. Interestingly, the cosmic microwave background was first predicted by George Gamow, Ralph Alpher and Robert Herman in 1948. However this prediction was ignored by most of the astronomical community and there was no effort made before Penzias and Wilson’s discovery to look for it. Dicke and Peebles claimed to be unaware of this early prediction when they did their calculations.
  2. In fact our the Milky Way and Andromeda are on a collision course and will collide in about 4 billion years’ time. The is described in more detail in my previous post: The Ultimate Fate of the Universe.
  3. According to Scott and Smoot (2015) the coordinates of the point in space towards which the Local Group is moving is in galactic coordinates (l, b) = (276◦ ± 3 ◦ , 30◦ ± 3 ◦ ). This is equivalent to Right Ascension= 11h 6m 37.6s  Declination=  -27° 20′ 1″ in the coordinate system used in most star maps.
  4. There are causes of unevenness other than the motion of the Earth, Sun and Milky Way and the initial unevenness confirmed by COBE. One of them is due to an effect called the Sunyaev–Zel’dovich effect, which the light waves or photons from the CMB receive an energy boost when passing through clouds of hot gas. This is also known as the inverse Compton effect.


Cain, F (2013) How Many Stars are There in the Universe?, Available at: 4 September 2016).

Levine, A. G. (2009) The Large Horn Antenna and the Discovery of Cosmic Microwave Background Radiation, Available at: (Accessed: 21 August 2016). (2006) Nobel prize in physics 2006, Available at: (Accessed: 11 September 2016).

Scott, D. and Smoot G. F. (2015) Cosmic microwave background, Available at: 28 August 2016).

The cosmic microwave background: part I

In 1964 two young American radio astronomers, Arno Penzias and Robert Wilson, made an accidental finding which would win them both the Nobel prize and turned out to be one of the greatest scientific discoveries of the twentieth century.

The story started when Penzias and Wilson were given observing time on a large radio telescope at Bell Labs in New Jersey. The telescope had originally been designed for satellite communications, but with the advances in satellites in the 1960s it had become surplus to requirements.


Penzias and Wilson at the telescope where they made their discovery (image from NASA)

They had been intending only to map radio signals from objects in our Milky Way galaxy, but their telescope was sensitive enough to pick up a faint background signal. This signal was unusual in that it had the same strength in all directions and at all times of day. Initially they thought that it was a man made signal but this was quickly ruled out. However, if the signal was originating from space then it should be emitted by an object (such as a galaxy, or a cloud of gas or dust) and be much stronger when the telescope was pointing in the the direction of the object, much weaker when the telescope was pointing away, and would disappear when the object had set below the horizon – but this was clearly not the case. As they continued their investigations they discovered “a white dielectric substance” – pigeon droppings – inside the their telescope. It turned out that some pigeons had decided to roost there. It was possible that this could have produced a false signal, so to remove this possibility they cleaned out the mess and tried removing the pigeons and discouraging them from roosting, but they kept flying back. Unfortunately for the pigeons, they found a more permanent solution to the problem.

“To get rid of them, we finally found the most humane thing was to get a shot gun…and at very close range [we] just killed them instantly. It’s not something I’m happy about, but that seemed like the only way out of our dilemma,” said Penzias. (Levine 2009)

However, even when the pigeon droppings were eliminated, the background noise was still there, so their investigations continued. The cause of this background noise was finally traced to what is known as the cosmic microwave background (CMB) – electromagnetic radiation created when the Universe was only 400,000 years old, long before the first stars and galaxies formed.

Electromagnetic Radiation

You may remember from your high school science lessons that light is a form of electromagnetic (often abbreviated to EM) wave.

EM radiation

The diagram above shows a wave of visible light. The blue lines show the strength of the invisible field called the electric field (see note 1).  The wavelength is the distance between successive peaks, marked as A in the diagram, or successive troughs, marked as B in the diagram. The wavelengths of light waves are so small that they are usually measured in nanometres (normally abbreviated to nm). 1 nm is equal to one billionth of a metre. Visible light which can be detected by the human eye consists of electromagnetic waves with wavelengths in the range 380 nm to 750 nm. The eye sees different wavelengths as different colours.

There are electromagnetic waves which have wavelengths much shorter and much longer than visible light. This can be seen in the diagram below, which shows all the different types of electromagnetic waves in order of their wavelengths and, if you’ve studied physics at high school, you’ll know is called the electromagnetic spectrum

EM spectrum

Diagram from NASA

  • The waves with the shortest wavelengths, less than 0.01 nm, are gamma rays, which on Earth are normally produced by radioactive materials. Gamma rays are extremely dangerous to living organisms and exposure to gamma rays even at low levels increases the risk of getting cancer.
  • X-rays have wavelengths from 0.01 nm to 10 nm. They can easily pass through soft tissue but less easily through bones so are commonly used for medical imaging
  • The wavelengths of ultraviolet light (usually abbreviated to UV) lies between X-rays and visible light. Most UV, particularly at the shorter range of wavelengths, is absorbed by the Earth’s upper atmosphere before it hits the ground. However, some UV reaches the Earth’s surface where overexposure can cause sunburn leading to a increased risk of skin cancer.
  • Infrared radiation (sometimes abbreviated to IR) is radiation with a wavelength longer than visible light. up to about 1 mm. Objects at room temperature emit IR and the warmer they are the more IR they emit. So IR is used in thermal imaging.


  • Microwaves have wavelength in the range 1 mm up to 1 metre. Microwaves have many applications including communications, radar, and microwave ovens. Most radio astronomy is carried out at microwave wavelengths.
  • Radio waves haves wavelengths over 1 metre. Their main use is to transmit and receive TV and radio signals (see note 2).

There are many devices such as radio transmitters, microwave ovens, suntan lamps, lasers and x-ray machines which are used because they emit EM radiation at one particular wavelength. For example a laser will only emit light of a particular colour. In addition to this, all objects also emit EM radiation called thermal radiation or blackbody radiation over a wide range of wavelengths. When we plot the amount of this thermal radiation emitted by a hot object such as star against the wavelength of the radiation we get a curve with a similar shape to that shown below (see note 3).

BB Sun


Although a star such as the Sun emits a significant amount of radiation over a wide range of wavelengths, you can see from the diagram that there is a clear peak in the visible light part of the spectrum. The maximum radiation is emitted at a wavelength of just over 500 nm, the wavelength of green light.

For all objects the wavelength where the most thermal radiation is emitted depends on its temperature: the hotter the object, the shorter the wavelength. This is shown in the digram below. This curve is known as the Planck spectrum.

BB different temperatures

The amount of radiation emitted by objects at different temperatures

You will have noticed this for yourself if you have studied a coal fire. Initially when the coals start to get hot they glow a dull red, because most of the radiation is emitted in the infrared and there is only a little visible light. As they get hotter they emit more light at the shorter wavelengths. So the coals glow a brighter red and then finally a bright orange/red when the fire gets really hot.

The table below shows the peak wavelength for a selection of objects at different temperatures and whereabouts in the EM spectrum it is.

Black Body Temperatures

Explanation of the cosmic microwave background from the early Universe

As readers of my previous post will know, the generally accepted theory of the origin of the Universe is that it was created in an event known as the Big Bang 13.8 billion years ago. The early Universe was incredibly hot and for the first few hundred thousand years consisted of a special state of matter normally only found at very high temperatures, called a plasma. Rather than consisting of atoms, a plasma consists of a sea of electrically charged particles, called ions and electrons. Light cannot pass through a plasma, and the Universe at this time would have been a hot glowing fog.


A plasma

The whole Universe would have been full of radiation emitted by the ions and electrons. Cosmologists are able to prove that the the amount of radiation emitted by the plasma would have followed a Planck spectrum curve such as those shown below.

BB cooling

As shown in the diagram, as the Universe cooled over time the peak wavelength increased.

  • 10 seconds after the Big Bang the temperature of the Universe was 100 billion degrees. The peak wavelength was around 0.00003 nm, well into the gamma ray region of the spectrum. This is shown as A in the diagram.
  • 10 hours after the Big Bang the temperature of the Universe had fallen to 100 million degrees. The peak wavelength was around 0.03 nm, in the X-ray region of the spectrum. This is shown as B in the diagram.
  • 1000 years after the Big Bang the temperature of the Universe was around 100,000 degrees. The peak wavelength was around 30 nm, in the UV region of the spectrum. This is shown as C in the diagram.
  • 400,000 years after the Big Bang, the Universe had cooled to aound 3,000 degrees. The peak wavelength was around 1,000 nm in the infrared region of the spectrum. This is shown as D in the diagram.

At D, the Universe had reached a temperature low enough so that ordinary atoms could exist. At this point in time, which astronomers call the recombination time, the Universe became transparent to radiation, as EM radiation could pass unhindered straight through the hydrogen and helium gases which filled the Universe.

Over the billions of years since that time much of the hydrogen and helium gas which was initially fairly evenly spread out formed into clumps which condensed to form the stars and galaxies we see in the Universe today. Cosmologists have shown that all the EM radiation which filled the early Universe has been stretched out to longer wavelengths by its expansion so that its peak wavelength where the radiation is strongest is now is around 1.1 millimetres in the microwave range, corresponding to a temperature of only  2.7 degrees above the coldest possible temperature absolute zero.  This is what we observe today as the cosmic microwave background.

In my next post I talk about the significance of the cosmic microwave background and why it has been important to astronomers in the 50 years since its discovery.


  1. Light also has a magnetic field which is always at right angles to the electric field. To simplify the diagram this is not included.
  2. Some TV transmissions are in the microwave region of the spectrum.
  3. This diagram is an oversimplification. The curve shown is that of a perfect black body at a temperature of 5,500 degrees Celsius, which is the temperature of the Sun’s surface. In reality, like nearly all objects, the curve of the amount of the Sun’s radiation emitted at each wavelength, shown in yellow in the diagram , doesn’t exactly fit the Planck spectrum, although it is a good approximation  If we look at the amount of the Sun’s radiation at different wavelengths which actually reaches the Earth’s surface, then this differs from the Planck spectrum curve even more because different wavelengths are absorbed differently by the Earth’s atmosphere.  This is shown in red in the diagram below.
  4. Solar radiation


Levine, A. G. (2009) The Large Horn Antenna and the Discovery of Cosmic Microwave Background Radiation, Available at: 21 August 2016).


The Steady State Theory

This post, the latest in my series about cosmology, the study of the origin and evolution of the Universe as a whole, talks about the Steady State theory. This is an elegant alternative theory to the Big Bang, which was very popular among astronomers in the 1950s, although it has now been discarded.

What is the Steady State Theory?

The Big Bang theory states that the Universe originated from an incredibly hot and dense state 13.7 billion years ago and has been expanding and cooling ever since. It is now generally accepted by most cosmologists. However, this hasn’t always been the case and for a while the Steady State theory was very popular. This theory was developed in 1948 by Fred Hoyle (1915-2001), Herman Bondi (1919-2005) and Thomas Gold (1920-2004) as an alternative to the Big Bang to explain the origin and expansion of the Universe. At the heart of the Steady State theory is something called the Perfect Cosmological Principle which states that the Universe is infinite in extent, infinitely old and, taken as a whole, it is the same in all directions and at all times in the past and at all times in the future.  In other words, the Universe doesn’t evolve or change over time.

The theory does, however, acknowledge that change takes place on a smaller scale.  If we take a small region of the Universe, such as the neighbourhood of the Sun, it does change over time because individual stars burn up their fuel and die, eventually becoming objects such as black dwarfs, neutrons stars and black holes.  The Steady State state theory proposes that new stars are continually created all the time at the rate needed to replace the stars which have used up their fuel and have stopped shining. So, if we take a large enough region of space, and by large we mean tens of millions of light years across, the average amount of light emitted doesn’t change over time. See Note 1.

The Sun

The Sun will last for about 5-6 billion years before it runs out of fuel. Image from NASA

How does does this work with the expanding Universe ?

The Universe is composed of galaxies, each of which contains many billions of stars. Our Milky Way is a large galaxy and is believed to contain over 400 billion stars.

Milky Way from outside

What the Milky Way would look like from a great distance. Image from NASA

As discussed in my previous post, it has been known since 1929 that the Universe is expanding, which means that when we look at distant galaxies they appear to be moving away from us. The further away a galaxy is from us, the faster it appears to be moving away. This relationship, which is known as Hubble’s law, is shown in simplified form in the diagram below.


The horizontal x-axis gives the distance from Earth, and the vertical y-axis gives the speed.  The astronomer Hubble (who discovered the expansion of the universe) plotted a sample of galaxies on the graph accordingly.

Hubble proved that the galaxies are all moving away from each other, which implied that the average distance between galaxies in increasing and so the Universe must be changing over time.

The Steady State theory gets round this by assuming that new matter is continuously created out of nothing at the incredibly small rate of 1 atom of hydrogen per 6 cubic kilometers of space per year. See Note 2. This new matter eventually forms new stars and new galaxies and, if we take a large enough region of the Universe, its density, which is the amount of matter in a given volume of space, doesn’t change over time. If we take two individual galaxies then their relative distance will get further and further apart due to to the expansion of the Universe. However, because new galaxies are being formed all the time, the average distance between galaxies doesn’t change. This is shown in a simplified form in the diagram below.

Steady State Theory

In the diagram above I have taken a small region of space and marked two galaxies with a red dot and a green dot to allow them to be identified. All the other galaxies are marked with a white dot. Then the upper part of the diagram shows the Big Bang theory where the distance between all the galaxies increases as the Universe expands. In the Steady State theory, shown in the lower part of the diagram, the distance between  the red and the green galaxies  increases but extra galaxies are created so the average distance between galaxies doesn’t change. Indeed if the Steady State theory were true then an observer would measure the same values of:

  • the average density of the Universe,
  • average distance between galaxies,
  • average brightness of galaxies
  • how the speed that galaxies are moving away varies with their distance

at all points in the Universe whether the observations refer to 10 trillion years in the past, now, or 10 trillion years in the future.

The origin of the Universe 

One of the elegant features of the Steady State theory is that because the Universe is infinitely old the question of its origin doesn’t arise. It has always existed. Unlike the Big Bang theory, the Steady State theory has no point far back in time  when a ‘creation event’ occurred causing the Universe to come into existence. To Fred Hoyle, who was a committed atheist, this was an attractive point of the theory.

Evidence against the Steady State theory

The Steady State theory was very popular in the 1950s. However, evidence against the theory began to emerge during the early 1960s. Firstly, observations  taken with radio telescopes showed that there were more radio sources a long distance away from us than would be predicted by the theory.  By a long distance, I mean billions of light years. Because of the times it takes light to reach us then, when we look at objects billions of light years from us, we are looking back billions of years in time.  So what these observations were saying is that there were more cosmic radio sources billions of years ago that there are now. This would suggest that the Universe is changing over time which contradicted the Steady State theory

Another piece of evidence which emerged to discredit the theory emerged in 1963, when a new class of astronomical objects called quasars was discovered. These are incredibly bright objects which can be up to 1,000 times the brightness of the Milky Way, but are very small when compared to size of a galaxy. Quasars are only found at great distances from us, meaning that the light from them was emitted billions of light years ago. The fact that quasars are only found in the early Universe provides strong evidence that the Universe has changed over time.


A quasar.  Image from ESO

However the real the nail in the coffin of the Steady State theory was the discovery in 1965 of the cosmic microwave background radiation. This is a weak background radiation which fills the whole of space and is the same in all directions. In the Big Bang theory this radiation is a relic or snapshot from the time the Universe was young and hot and was predicted long before it was discovered. However, in the Steady State theory it is almost impossible to explain the origin of this radiation.

Is the Steady State theory a good theory?

For the reasons given above, by the early 1970s the Steady State theory was no longer accepted by the vast majority of  cosmologists. The Big Bang theory is now generally believed to explain the origin of the Universe. However, despite this it can still be argued that the Steady State theory is a good theory.

In the words of Stephen Hawking:

‘the Steady State theory was what Karl Popper would call a good scientific theory: it made definite predictions, which could be tested by observation, and possibly falsified. Unfortunately for the theory, they were falsified’ (Ref 1).


Stephen Hawkins NASA

Image from NASA


Further reading and related posts

This post is the sixth in my series about cosmology. A few other posts in this series 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 briefly 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.

(4) Dark Energy over Time. This post  discusses how the amount of dark energy in the Universe has varied over time and its implications on its future evolution. To view this post click here.

(5) Dark Matter. This post discusses evidence for dark matter, the mysterious substance which makes up around 25% of the mass of the Universe. To view this post click here.

(6) The ultimate fate of the Universe. To view this post click here.

Science Geek Publications

I have written a short e-book on extraterrestrial intelligent life and how humans have tried to  make contact with it.  The book is available to download from Amazon in Kindle format by clicking here.

I have also written a short e-book on the Moon. This can be downloaded by clicking here.

I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


1 A light year is the distance that light travels in a year. 1 light years is equal to 9.46 trillion km . The nearest to the Earth, other than the Sun, lies 4.2 light years away.

2  To continuously create matter and to drive the expansion of the Universe. Fred Hoyle introduced into the Steady State model something he called the C-field, where C stands for creation.




Ultimate Fate of the Universe

This post, which is the latest in my series on cosmology, is about the ultimate fate of the Universe, a subject which has engaged scientists, philosophers and religious leaders for centuries. There are many possible endings to the Universe, but the outline I will present here, which is sometimes called “The Big Freeze”, is the most commonly held view based upon the known laws of science.

4 billion years in the future – Milky Way and Andromeda merge 

As readers of my previous posts will recall, the Sun is one of around 400 billion stars in the Milky Way galaxy and the Milky Way is one of over 200 billion galaxies in the observable Universe (ref 1). Our galaxy, together with the large spiral galaxy in the constellation Andromeda and around 50 smaller galaxies form a collection of galaxies called the the Local Group.


The Andromeda galaxy – the nearest large galaxy to the Milky Way. It lies 2.5 million light years, roughly 25 million trillion km, from Earth.

As readers of my previous post The Universe Past, Present and Future will recall, the Universe is expanding and, in general, the further away a galaxy is from us the faster it appears to be moving away from us. This does not apply to the galaxies in the Local Group. In fact, the Andromeda galaxy is moving towards the Milky Way at about 400,000 km/h and the Milky Way and Andromeda are expected to collide in about 4 billion years time. When this happens, a large new single galaxy will be formed.  The new galaxy which will be formed by the merger is sometimes called Milkomeda (ref 2) and, over billions of years, it will gradually absorb the other Local Group members.


When this merger occurs it is unlikely than any stars within either galaxy will collide directly because the distances between stars is so great, but the clouds of gas found between stars will collide and merge.

 120 billion years in the future – all galaxies too far away to detect

Outside the Local Group the expansion of the Universe will continue. As discussed in a previous post, this will be driven by dark energy. The  speed that a galaxy is moving away from us depends upon its distance. As a galaxy moves a greater distance away from us, then the speed at which it moves increases. When it moves so far away that it is travelling faster than the speed of light we can no longer see it. This is shown in the table below.

recession with time

This table shows how the distance of  a nearby galaxy which is now 10 million light years away from us (about four times the current distance of Andromeda and thus well beyond the Local Group) increases with time. The last column shows how fast this galaxy is moving in units of kilometres per second. 1 kilometre per second is 3,600 km/h.

In about 105 billion years time, this galaxy would have moved out to a distance of 14.7 billion light years and would be moving away from us at a speed 301,700 km/s, which is more than 1 billion km/h. However, the speed of light is around 300,000 km/s and, because this galaxy would be moving faster than the speed of light, its light would not be able to reach us and it would no longer be visible. See Note 1. In fact, in 120 billion years time, all galaxies outside the Local Group will have moved so far away that the light from them will be unable to reach us.

Hubble Sphere

In the diagram above, the furthest distance than a galaxy can be away from us but still remain observable (in principle) is marked with the red circle.

  • The left hand side of the diagram shows the situation today with all galaxies outside the Local Group moving away from us. but plenty of galaxies still visible. The diagram is greatly simplified because there are actually 200 billion galaxies in our observable Universe.
  • The right hand side of the diagram shows the situation in 120 billion years time when all galaxies have moved outside our observable Universe.  By this time Milkomeda will have absorbed all the Local Group galaxies.  So if there are any astronomers around, at this distant point in the future, the entire observable Universe will consist of a single galaxy, Milkomeda.

 1-10 trillion years in the future – the dark era.

As described in a previous post, stars consume hydrogen to produce helium and later on towards the end of their lives they make heavier elements. New stars are being created all the time from clouds of gas and dust. Many of the beautiful glowing nebulae seen through telescopes consist of glowing gas and dust clouds lit up by newly formed stars.


The Orion Nebula, a region of star formation

However the supply of hydrogen is finite and in about 1 trillion years time there won’t be enough hydrogen left to forms any more new stars (Ref 3). In about 10 trillion years the longest lived stars which are small faint stars called red dwarfs will have come to the end of their lives.  All stars which started life with an original mass less than 10 times the mass of the Sun, which is over 97 % of all stars, will have become very dense cold objects called black dwarfs. Very massive stars more than 10 times the mass of the Sun will have become neutron stars or black holes:

  • Neutron stars are super dense objects in which the mass of a star is concentrated in a object about 10 km in diameter. A neutron star is so dense that 1 litre of its material would weigh 500 billion tons.
  • Black holes are objects in which the gravity is so strong that once an object enters a region around the black hole called the event horizon it cannot escape. Much has been written about black holes by popular science and science fiction writers. For a good overview I would recommended the article by Stephen Hawking article which can be found at

Black Hole

A black hole

None of these object emits any light so the observable Universe in 10 trillion years will be very dark and very cold, having a temperature a fraction above the lowest possible temperature which physicists call absolute zero. It will consist of black dwarfs, neutron stars and black holes, planets and other smaller bodies associated with them. At the centre of Milkomeda will be a large black hole many millions times the mass of the Sun.

10-100 quintillion years in the future – Milkomeda shrinks

(1 quintillion is a million trillion or 1,000,000,000,000,000,000.)
Milkomeda will consist of  objects which are all in motion with respect to each other. Occasionally these objects will get close enough to each other so that their trajectories change slightly. When this happens the speed of one object may speed up and the other may slow down. This is essentially the same effect that spacecraft which visit the outer planets use.  The Voyager space probes, for example, took energy from Jupiter’s orbit to slingshot them into the outer solar system and beyond.

After a number of such collisions an object may get enough energy to escape from Milkomeda. Over a vast period of time, around 100 quintillion years, this will cause the galaxy to gradually shrink as dead stars and possibly their attached planets escape, and the remaining objects would be more tightly bound.

100,000-1,000,000 quintillion years in the future -Gravitational radiation causes objects to fall into a massive black hole

In 1916 Albert Einstein predicted the existence of gravitational waves. One object orbiting another will emit something called “gravitational radiation” causing it to lose energy and spiral slowly inwards towards the more massive object. It is generally accepted by astrophysicists that gravitational waves do exist, although they are incredibly difficult to detect and, despite astronomers looking for them for decades, they have never been observed.

Albert Einstein

Assuming Einstein’s theory is correct, then also assuming that the Earth has survived the Sun’s red giant phase (which is unlikely) and that it not been detached from the Solar System by a near collision with a passing star (which almost certainly will happen if we wait long enough), in 100,000 quintillion years time it will spiral down to the surface of the Sun (see note 2). It also means that, over an even longer timescale of about 1,000,000 quintillion years, the eventual fate of all the massive objects which have not escaped from the galaxy is to fall into the super massive black hole which lies at its centre.

Next Post

In my next post in this series I will talk about even longer timescales. What will eventually happen to the black hole at the centre of Milkomeda?  And what is the eventual fate of the objects which have not fallen into this super massive black hole?

Related Posts

This post is the sixth in my series about cosmology. The other posts in this series 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 briefly 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.

(4) Dark Energy over Time. This post  discusses how the amount of dark energy in the Universe has varied over time and its implications on its future evolution. To view this post click here.

(5) Dark Matter. This post discusses evidence for dark matter, the mysterious substance which makes up around 25% of the mass of the Universe. To view this post click here.


1 This distances in this table should be considered as approximate only and it assumes that the rate of expansion of the universe will not vary with time in the future, which may not be the case.

2 In fact the Earth would be broken apart by tidal forces due to the remnant Sun’s gravity before it hit the remnant Sun’s surface.


1 Cain, F (2013) How Many Stars are There in the Universe?, Available at: 19 February 2015).


3 Barrow, J D and Tipler F J. The Anthropic Cosmological Principle 1996 pp641. ISBN 0-19-282147-4.