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).

Short post of the month February 2016 – gravitational waves

The subject of February’s short post of the month is gravitational waves. These were predicted by Albert Einstein back in 1916, and after decades of searching have finally been detected. In an announcement made on 11 February at a Washington DC press conference David Reitze, the executive director of the LIGO Laboratory, said:

Ladies and gentlemen, we have detected gravitational waves. We did it!”

(Castelvecchi and Witze 2016). Later that day president Obama tweeted his congratulations to the team:

Obama LIGO Tweet

What are gravitational waves?

Gravitational waves are ripples in space time. As gravitational waves pass through an object they cause it to move slightly. The Universe is believed to be awash with gravitational waves, because when massive objects move, such as the Earth orbiting around the Sun, they emit gravitational radiation. However compared to other forms of radiation such as light and radio waves, gravitational waves are very very weak, which is why they have proved so difficult to detect.

Gravitational waves

Image from NASA

The gravitational waves detected were due to the one of most violent events in the Universe, the merger of two black holes, which were rapidly rotating around each other.  Even so, the signal was still incredibly weak

How were they detected?

In the LIGO facility a laser beam is split into two and travels down two 4km tunnels which are at right angles to each other. The two beams then reflect back and forth many times  between two mirrors before they are eventually recombined at an electronic light detector.


The apparatus is very finely tuned so that the waves from the two light beams, shown as  A and B in the diagram below, are completely out of phase with each other and as a result cancel each other out completely when they recombine giving no net signal at the light detector C.  Some of you may remember from your high school science lessons this is known as “destructive interference”.

Destructive Interference

When gravitational waves pass through the LIGO facility, the waves cause the tunnels to change their shape by a minute amount.  As a result of this, the distance travelled by each beam of light also changes very slightly, so that they are not completely out of phase and when the beams recombine they no longer completely cancel out. This produces a small signal at the detector.

On 14 September the same signal was found at the two separate LIGO detectors: Livingston in Louisiana first and Hanford in Washington State 7 milliseconds later. The fact that the patterns of the signals were the same and that there was a time delay between the two detections provided the proof that the signals were due to gravitational waves.


What does this mean for astronomy?

Up until now astronomers have only been able to see the Universe by detecting electromagnetic radiation through telescopes which work at different wavelengths, for example those of visible light, radio waves and x-rays. The instruments at LIGO allow astronomers to observe the Universe in a whole new dimension, in effect to “feel” the Universe vibrating.

That view was reinforced by Stephen Hawking, who in an interview for BBC News (2016) said

“Gravitational waves provide a completely new way at looking at the Universe. The ability to detect them has the potential to revolutionise astronomy. This discovery is the first detection of a black hole binary system and the first observation of black holes merging.”


BBC (2016) Einstein’s gravitational waves ‘seen’ from black holes, Available at: (Accessed: 23rd February 2016).

Castelvecchi, D. and Witze, A. (2016) Einstein’s gravitational waves found at last,Available at: (Accessed: 21st February 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.

A Brief History of the Universe

This post covers a brief history of the Universe from the Big Bang until the the present day. This, as I am sure you’ll agree, is a pretty big topic so I can only give a outline of some of the key events and when we believed they happened. This post is the second in my series about cosmology, which is the study of the origin and evolution of the Universe as a whole. To view my previous post click here.

Before I start talking about the history of the Universe I first need to give a brief overview of atoms, which are the building blocks of all matter – with a notable exception which I will explain below.

What are atoms ? 

An atom consists of a nucleus, which has a positive electric charge, surrounded by 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.  This means that the nucleus is one hundredth thousandth of the size of the whole atom.

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

  • The number of protons is called the atomic number and  determines which element it is. You may remember from high school chemistry that this is its 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.  Atoms which have 2 protons (regardless of the number of neutrons) are helium atoms, 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.




A 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 first second after the Big Bang

There is no generally agreed account of what happened at the exact instant of the Big Bang. If we work backward in time, using our existing physical theories, then at the exact instant of creation the Universe would have had an infinite density and an infinite temperature. What this really means is that we don’t have a physical theory which explains what happened at this time.

What is generally accepted in that in the first microscopic fraction of a second, after the Universe first came into existence,  it underwent an incredibly rapid expansion and has been expanding and cooling ever since (see notes 1 below).  For the first part of the first second, the conditions in the Universe were so hot and dense that it consisted only of some special particles.  These cannot be detected under normal conditions, but you may have heard about the recently discovered Higgs Boson, which can be very briefly seen in the extreme conditions created in particle accelerators, like the Large Hadron Collider in Switzerland.  The Higgs Boson is just one example of the particles which would have been present during the first minute fraction of a second.

When it was one second old the Universe was expanding and cooling rapidly. However it was still at a temperature of around 1 trillion degrees Centigrade.  At this point the ordinary matter in the Universe consisted of a sea of protons, neutrons and electrons. None of these particles were bound together into atoms, because atoms cannot exist at such high temperatures (see notes 2 below).

Universe Temperature

This graph shows how different aspects of the Universe came into existence as the temperature cooled.

10 to 1000 seconds after the Big Bang

At around 10 seconds after the Big Bang the Universe had cooled to 100 billion degrees. This is cool enough for atomic nuclei to exist. In fact, the whole Universe acted as a giant nuclear reactor. The reactions are shown in the diagram below. Firstly protons (symbol H) and neutrons (symbol n)  fused together to form the nuclei of deuterium (also known as heavy hydrogen) atoms (D) and then two deuterium nuclei fused together to form helium nuclei (He). This is essentially the same reaction as that which occurs in a hydrogen bomb.


Big Bang fusion

These two reactions generate a huge amount of energy, but the cooling produced by the expansion of the Universe was so rapid that when the Universe was around 1000 seconds old it had cooled to around 1 billion degrees and was no longer hot enough and dense enough for any further nuclear reactions to take place. At this time the matter in the Universe consisted of 73% hydrogen and 27% helium with trace amounts of deuterium, lithium and beryllium. None of the heavier elements existed. There were all created later by nuclear reactions inside stars.


Atoms form

The early Universe was so hot that the matter was in a special state which we call a plasma. In a plasma in the electrons are not bound to the atomic nucleus to form atoms but can move around freely. Light cannot pass through the plasma, which would have been like a hot dense glowing fog.


A plasma

However, as the Universe continued to expand and cool it reached a temperature where helium atoms could exist. Later, when it was roughly 400, 000 years old, and at a temperature of around 3,000 degrees, ordinary hydrogen atoms could exist and the Universe became transparent to light. The faint radiation which we can observe today called the cosmic microwave background was created at this time.

First Stars Form

As the Universe continued to expand and cool, matter began to clump together. When it was about 100-150 million years old, about 1% of its current age, large clumps of matter existed which were around 100 to 300 times the mass of of Sun. These clumps of matter contracted, getting hotter and hotter as they did so. Eventually they were so hot that nuclear reactions could start, and thus the first stars were born. These early stars, which astronomers call population III stars, were super massive compared to the Sun and shone extremely  brightly for about 10 million years. (This is a very short lifetime for a star, as the Sun will last for about 10 billion years.) They ended  their lives in massive explosions called supernovae in which the star was completely destroyed. These supernovae spread the elements made in the star – like carbon, nitrogen, oxygen, silicon, magnesium, iron and uranium – throughout the Universe.

Galaxies Form

It is still not fully understood how galaxies , which contain hundreds of billions of stars, form. One theory which has gained strength in recent years is sometimes called the  “bottom up” theory.  According to this theory the first galaxies began to form when the Universe was around 1 billion years old  from lumps of matter, including stars and gas clouds which had coalesced.

As our universe continues to evolve, small galaxies are frequently gobbled up by larger ones. The Milky Way contains the remains of several smaller galaxies that it has swallowed during its long lifetime. In fact, the Milky Way is “digesting” at least two small galaxies even now, and may pull in others over the next few billion years.


Milky way structure Population II and Population I Stars 

Our own Milky Way galaxy, shown below, is believed to date from just over 1 billion years from the creation of the Universe.

Sun in Milky Way

Around the Milky Way is a halo containing old stars, called Population ll stars, which do not have many elements other than hydrogen and helium.  The traces of the  heavier elements will have come from the earlier Population III  stars which exploded, as discussed above. When stars are formed, the materials left over combine to form planets.  Because elements such as iron, oxygen and silicon are only present in tiny quantities in these Population II stars, they cannot be orbited by rocky planets like the Earth, Mars and Venus.   The disk of the galaxy and the central bulge contain younger population I stars, such the Sun. These are much richer in heavier elements and are likely to have planets.

The Ultimate in Recycling ?

Much of the material in the Population I stars and their planets has been re-cycled. It will  have been created in earlier Population II (and Population III) stars which exploded as supernovae, scattering the debris throughout the Universe.  As mentioned above, some of the debris later clumps together to form stars and planets, such as the Earth.



1 This is called inflation and is accepted by most cosmologists. In this theory the part of the Universe we inhabit expanded from a minute fraction of the size of an atomic nucleus to a diameter of about 1 metre in a period of around 0.00000000000000000000000000000001 seconds after the instant of creation.

2. Most cosmologists now believe that 85% of the matter in the Universe is another mysterious form of matter called dark matter. No one knows what dark matter consists of, but is clear is that it not made up of atoms in the same way as ordinary matter. Dark matter cannot form stars and does not clump together to form structures like gas clouds in the same way that ordinary matter does. It is completely invisible to telescopes because it is transparent to light. However its existence is inferred because of its gravitational effects on visible matter. Dark matter is such a huge and interesting topic that I shall cover it is a future post.


The Universe Past, Present and Future

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, discusses the origins of the Universe and its ultimate future.

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. It is around 100,000 light years in diameter and contains over 200 billion stars. The Sun lies at the edge of the Milky Way, around 30,000 light years away from the centre.Sun in Milky Way

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 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 small 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 1920s 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 nearly ninety years. However, it has only been accepted by most astronomers since the late 1960s.

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.

I will come back to the Big Bang in a later post.

The Expansion of the Universe

Astronomers used to believe that other galaxies might be static, moving towards us or moving away from us. 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 1920s, 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.

big bang

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.

Distant Galaxies

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.

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