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.

 

Carbon

 

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.

 

Notes

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:

https://www.livescience.com/48575-strong-force.html

Reference

Clavelli, L. (2008) Problems in a weakless universe, Available at: https://www.researchgate.net/publication/2020939_Problems_in_a_weakless_universe(Accessed: 24 May 2018).

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.

Horn_Antenna-NJ

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.

Quasar

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.

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.

earth-around-sun

 

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).

hydra

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.

History_of_the_Universe2.

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).

cobe-map

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. (Nobelprize.org 2016)

Notes

  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.

References

Cain, F (2013) How Many Stars are There in the Universe?, Available at:http://www.universetoday.com/102630/how-many-stars-are-there-in-the-universe/(Accessed: 4 September 2016).

Levine, A. G. (2009) The Large Horn Antenna and the Discovery of Cosmic Microwave Background Radiation, Available at:https://www.aps.org/programs/outreach/history/historicsites/penziaswilson.cfm (Accessed: 21 August 2016).

Nobelprize.org (2006) Nobel prize in physics 2006, Available at:http://www.nobelprize.org/nobel_prizes/physics/laureates/2006/ (Accessed: 11 September 2016).

Scott, D. and Smoot G. F. (2015) Cosmic microwave background, Available at:http://pdg.lbl.gov/2015/reviews/rpp2015-rev-cosmic-microwave-background.pdf(Accessed: 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.

Horn_Antenna-NJ

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.

Termografia_kot

  • 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.

plasma

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.


Notes

  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

References

Levine, A. G. (2009) The Large Horn Antenna and the Discovery of Cosmic Microwave Background Radiation, Available at:https://www.aps.org/programs/outreach/history/historicsites/penziaswilson.cfm(Accessed: 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.

hubble

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.

Quasar

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.


Notes

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.

References

1 http://www.hawking.org.uk/the-beginning-of-time.html

 

Dark Energy Over Time

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

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

The influence  of dark energy in the early Universe

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

Universe Expanding

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

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

Data Matter Volume

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

Dark Energy with time

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

Dark energy minus 7 billion years

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

future_universe

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

Dark energy in the future 

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

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

Dark energy 10 billion years

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

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

Next Post

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

Related Posts

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

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

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

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

Notes

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

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

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.

 

Carbon

 

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.

plasma

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.

Stardust

Notes

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.

Andromeda_Galaxy

 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.

hubble

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.

FateOfUniverse

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.

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.