A Down to Earth Guide to the Cosmos

As we saw in Chapter 1, our understanding of the creation of the Universe dates back to the 1920s, when Edwin Hubble spotted something peculiar while he was looking at the spectra of distant galaxies. Among the first things he noticed were the tell-tale signs of the existence of several types of gas, through the presence of absorption lines against the spectrum. But there was something different about them: instead of being in their usual positions, they were all slightly shifted to the red end of the spectrum. No matter which galaxy he looked at, all of the lines were shifted towards the red, an effect very similar to the Doppler effect, in which approaching sound waves, e.g. of a police-car siren, are squashed together and go up in frequency. When the light left the galaxies, the Universe was at a certain size, but in the intervening period of time the Universe and indeed space itself expanded and stretched out. This stretching is seen in the spectrum as it is stretched and the absorption lines appear to move position. If space had been contracting rather than expanding, the lines would have been shifted towards the blue end of the spectrum, as we can see in a small number of galaxies in our Local Group, like the Andromeda Galaxy.

It is important to note that on a local scale within galaxy clusters the individual galaxies are often found to move towards each other, but that is because their own motion is greater than the expansion of the Universe at a local level.

The discovery of the expanding Universe is in itself an intriguing concept but it leads to an even more amazing idea. If the Universe is expanding now, then if we follow it backwards in time, at some point it must have all started from the same position and some kind of explosion occurred to drive the expansion we see today. This is the Big Bang theory and, as we saw in Chapter 4, further investigation suggests the initial explosion happened between 13 and 15 billion years ago.

All theory? Not quite, as it is possible to see evidence of the Big Bang. In 1964 two radio technicians, Arno Penzias and Robert Wilson discovered the faint echoes of the Big Bang. You can actually tune in to the signal from the Big Bang yourself if you turn on your TV and select a channel that is not tuned to a particular station so that you see the ‘snow’ all over the screen. The majority of the interference causing this is terrestrial but a very small proportion comes from the Big Bang.

This leaves a rather obvious yet uncomfortable question: if the Big Bang caused the Universe to expand, and given the observations from Hubble, Penzias and Wilson that looks a very strong theory, then what happened before and what actually caused the Big Bang? I once heard another scientist being asked this question; his reply was a rather cunning one. He simply explained how space and time were supposed to be part of the same thing and, while it was commonly believed space was spontaneously created at the Big Bang, it was also thought that time was created then too. So he reasoned that it was meaningless to ask what happened before since there actually was no before! This idea is not particularly new – back in the fifth century, Augustine of Hippo claimed the Universe was created ‘not in time, but simultaneously with time’.

It is now a widely accepted idea that space and time are two sides to the same coin. They both form part of something called the space–time continuum. To understand how they are linked, imagine you are meeting a friend at a new bar you have found. You first need to communicate to your friend where the bar is. If the bar is on the first floor of a building you could simply specify its address and that it is on the first floor, and by doing this you are describing its position in relation to the three spatial dimensions. So far, then, you both know how to meet up ‘in space’ but without further information you can still very easily miss each other by being there at the wrong time. For this meeting to be a success, you also need to know about the location in the fourth dimension, time; in other words, the agreed time of your meeting. So, you see, if you only know the spatial coordinates (the address) you could both turn up at the wrong time, and if you only know the time coordinate (time of day) then you could turn up at completely the wrong location. One is not a lot of good without the other; they are both dimensions that describe position in the Universe.

When considering the creation of the Universe, there are implications of space and time being linked. This leads us back to that rather nasty concept that we must not really talk about – what happened before the Big Bang – because time did not exist if there was no before! If all this fanciful talk is wrong and something did actually cause the Universe to pop into existence, then what was it? There are a number of popular theories and one of them stems from a young branch of science known as string theory. In its very simplest form, string theory explains how everything in the Universe is made up of tiny strings. The strings vibrate in many different ways and the way they vibrate determines what particles we see. Like the strings on a guitar which vibrate to give us different notes, the strings in string theory vibrate in such a way as to give us the different particles which make up all the matter we see in the Universe.

String theory also suggests there may be many more dimensions than the four that we recognize (three space and one time). When this is applied to the Universe at large, the three spatial dimensions actually exist on a multi-dimensional sheet called a membrane, or ‘brane’ for short. It is thought that there are more than one of these branes, maybe even an infinite number, and they exist a tiny distance away from each other. The theory hints that every few trillions of years two of the branes collide with each other, sparking a new Big Bang and perhaps the creation of a new parallel Universe. It goes on to suggest that these brane collisions may have happened for an infinite length of time in the past and may well continue to happen for all eternity. Unfortunately, there is no solid evidence for any theory like this or any others that attempt to explain the cause of the Big Bang, so this question will, I fear, have to remain unanswered.

Instantly after the Big Bang, the Universe was a seething mass of energy with unimaginably high temperatures and pressures. There were none of the particles of the sort we see today and no sign of the building block of the Universe, the hydrogen atom. The four forces that dominate the world today (gravitational, electromagnetism, strong nuclear and weak nuclear) were all part of one unified force. At 10^-43 seconds (1 million trillion trillion trillionth of a second), an incredibly small period of time after the Big Bang, gravity became separated from the other three forces. Compared to the others, gravity is quite weak but so much more dominant in our lives simply because its effect can be felt over vast areas of space.

Shortly after, another of the four forces, the strong nuclear force, became separated. This is the force that holds the particles together in the nuclei of atoms. As its name suggests, it really is a strong force, much more powerful than gravity, although the distance over which its effects can be felt is much smaller.

Prior to the separation of these two forces the Universe was a tiny fraction of a millimetre across. The separation of the strong nuclear force from the electromagnetic and weak nuclear forces effectively made the force of gravity repulsive for just 10^-32 seconds. Do not worry about how this happened, it just did. During this ‘inflationary period’ the expansion rate of the Universe soared, causing a sudden increase in size by 10⁵⁰ times from incredibly small to roughly the size of a melon. That is in comparison to its current estimated size of about 93 billion light years across. This expansion that took place in the inflationary period of the evolution of the Universe happened at staggering speed. A ‘ray’ of light takes 30 billionths of a second to travel 1cm in a vacuum, but during inflation the Universe expanded by 1cm every 10 billion-trillionths of a second, a lot faster.

Up until this point, the Universe has been home to temperatures and pressures so extreme we simply can’t imagine what the conditions were like. After the brief but rather swift expansion, particles called quarks and electrons started to form out of the energy. Quarks and electrons are the building blocks of matter, of all the things we see in the world today; cars, houses, stars, galaxies, even you and me, all are made up of different collections and arrangements of quarks and electrons. At the same time there also arose the creation of their anti-particle equivalents: the anti-quark and the positron or anti-electron. An anti-particle is just like the normal particle but opposite in its properties. For example, the electron, one of the tiny balls whizzing around the nucleus of an atom, has a negative electric charge but its anti-matter equivalent, the positron, has a positive electric charge. Anti-quarks are just another type of anti-matter and, fortunately for us, at this stage of the development of the Universe quarks outnumbered anti-quarks by a ratio of 1,000,000,001 to 1!

You may be familiar with the fact that a meeting between matter and anti-matter causes the two to annihilate each other. As soon as the quarks and anti-quarks appear, they indeed start annihilating each other until there are just quarks left. This process floods the Universe with electromagnetic energy, or light. But it is the remaining quarks that are most important as they will form the real building blocks of our Universe.

Things are happening quite fast now so let us move on. We can now really start to appreciate how far things have gone given that it has only been 10 billionths of a second since it all started. If you remember, there are four forces in the Universe. So far, the gravitational and strong nuclear forces are separated from the others and trying to exert their hold on proceedings. The remaining two forces – electromagnetism (which effectively controls the attraction of negative and positive particles) and the weak nuclear force (which causes the sudden decay of one type of atom into another) – separate from each other, leaving all the forces free to shape the Universe we see today.

Just one thousandth of a second after the Big Bang the strong force finally makes its presence felt as it causes the quarks to combine to form the protons and neutrons which will finally become the central part of atoms. I’ve not said much about the temperature of the Universe during the early stages – frankly, it is enough to say it has been very hot. After all the changes we have just seen in the first few fractions of a second, the temperature finally starts to cool to a rather more pleasant shirt-sleeve temperature of 1 million million degrees. At these high temperatures, the Universe is a very unstable place with radiation much more dominant than matter. When we take a look at the behaviour of matter in the form of protons, neutrons or electrons at different temperatures, we find that ‘cold’ particles do not move around particularly fast; however, ‘hot’ particles are terribly hyperactive and whizz around at a rate of knots. So, if the Universe is very hot, then all the particles are extremely energetic and will not join up and bind with each other. The energy from the heat is much stronger than the energy that holds them together but, once it cools down, things start to get interesting.

Over the next few fractions of a second, the temperature decreases to around 100,000 million degrees and the conditions start to settle, eventually giving matter much more chance to get a foothold. With this change in conditions, protons and neutrons can stick together thanks to the strong nuclear force. Protons have a positive electric charge so they repel each other (remember that opposite charges attract); however, the neutrons have no charge, as their name suggests, and are said to be neutral. The Universe starts to fill with atomic nuclei composed of one proton and a number of neutrons (generally not more than three). This marks an important point in the journey: the nucleus of the hydrogen atom is formed. From around three minutes after the Big Bang, nucleosynthesis (the process that makes stars shine) starts to fuse neutrons and protons together, flooding the early Universe with a specific type of helium. This process only lasts for around eighteen minutes though, since the Universe cools to a temperature at which fusion cannot continue. At the end of this era, there are about three times as many hydrogen atoms as there are helium ones, and only small amounts of other atomic nuclei.

Now, for a much greater leap in time. Around 70,000 years after the Big Bang, tiny irregularities in the distribution of matter slowly start to get amplified and it is thought that dark matter plays a part in this process. The temperature and density of the Universe continue to drop and around 380,000 years after the Big Bang they reach a level that finally allows electrons to attach to the hydrogen and helium atoms. There is another long wait, as much as 150 million years, before the first stars start to form out of the gravitational collapse of hydrogen and helium clouds. This first generation of stars, called Population III stars, are just theoretical at the moment since none have been positively identified. It should be possible to identify them using spectroscopes, which study the light from them to reveal a lack of heavy elements and a particular quantity of hydrogen and helium in their core. This first generation of stars would have been responsible for converting the lighter elements, hydrogen and helium, into the heavy elements. A process that went on over billions of years to produce other stars, planets and even living creatures like you and me.

Some theories suggest that the Population III stars may well have existed outside the confines of the gravitational bonds of a galaxy. At the same time as their creation during the first billion years of the existence of the Universe, the collapse of larger gas clouds led to the formation of the first galaxies. One of the earliest we can see is called UDFj-39546284 and is found in the constellation Fornax in the southern hemisphere sky. It is around 13.2 billion light years away and at this great distance we are looking back in time at an object that formed just 480 million years after the Big Bang. Being able to look backwards in time like this, because of the vast distances in space, is an incredibly useful tool for us in understanding how the Universe has evolved. It is almost like an archeologist digging up layers of the ground to find out how history has changed.

Unfortunately there is a limit to how far back we can go. During the inflationary period the Universe expanded faster than the speed of light so that every part of space was rushing away from every other part of space faster than light can travel. This means if a star gave off a photon of light during the inflation phase (although no stars actually existed so soon after the Big Bang), an observer in another part of the Universe would effectively be carried away by the expansion of space at a speed faster than the light that was heading towards them. The light would never actually catch up with the observer regardless of the fact that the expansion slowed down fractions of a second later. For astronomers, the inflationary period acts as a barrier to attempts to study the early Universe as any radiation being emitted by events prior to this would never reach us here on Earth. We can only ever hope to see as far back as the inflationary period and even that may well be beyond the limits of our technology. Instead, we are left with making assumptions and intelligent guesses based on observation and possible scenarios.

As we saw in Chapter 5, the first galaxies to form, like UDFj-39546284, are thought to have been spiral in shape and, over time, gravity caused those nearest to each other to collect into clusters and superclusters. Studies of the distribution of galaxies in the Universe today still reveal the clusters and superclusters whose general structure was laid out in the Universe almost 13 billion years ago. In those same studies we can see signs of galactic collisions which caused spiral galaxies to merge, forming larger elliptical galaxies.

In the galaxies Population II stars formed and then, as they died, they seeded the galaxy with heavier elements that eventually formed the majority of the stars we see today, the Population I stars. Not only are the heavy elements found inside the stars to a lesser degree but they are also found forming huge discs around the hot young stars called proto-planetary discs, which will eventually form planets just like ours. From this point on, stars continue to evolve and die, planets come and go, galaxies merge and maybe even life will evolve in some other remote corner of the Universe. The cosmos will continue to expand at a rate that seems to be slowly getting faster and faster until, many billions of years in the future, it meets its fate.

It is amazing to think that all of this, our knowledge of how the Universe has evolved, even its very beginnings, has come from simple study of the night sky. Observations have led to theories, and theories have been tested against new observations. The process of science and its investigative nature is subjected to its most rigorous test when applied to studies of deep space and yet, while there are some mysteries still to be solved, it is slowly unpeeling the layers and unlocking the story of how our Universe evolved.

November: Northern Hemisphere Sky

A few prominent stars dominate the November northern hemisphere sky in the east, such as the bright red Betelgeuse in Orion just north of the celestial equator, Aldebaran in Taurus to the north-west and Capella in Auriga even further north. Aldebaran along with its companion stars in the Hyades star cluster form a celestial ‘V’ shape which conveniently points towards another red star, called Menkar, the second-brightest star in Cetus and the starting point for our November northern sky guide.

Menkar lies just north of the celestial equator and is said to have a declination of just over 4 degrees. Menkar’s distinct colour suggests it is nearing the end of its life having completed the hydrogen-burning phase and possibly even the helium phase too. It is a star which has a mass about three times that of the Sun and a diameter about 90 times greater, but eventually its outer layers will be lost to space, forming a planetary nebula, a fate that also awaits our Sun.

To the north-east of Menkar is a faint cluster of stars called the Pleiades Cluster, which looks like a tiny version of the Plough in Ursa Major. The cluster is easily visible to the naked eye and is often used as a test for eyesight and atmospheric conditions. There are at least 1000 stars in this cluster; although between six and twelve have been recorded with the naked eye by observers, binoculars will reveal up to a hundred stars and telescopes many more. It measures 15 light years across, is 440 light years from the Earth and, at this distance, appears in our sky to be about four times the diameter of the full moon. On moonless nights, it is possible to detect a hint of fuzziness around some of the brighter stars, especially Merope, the southernmost of the these. Studies of the direction of travel of the cluster and its nebulosity show that the two are unrelated, that the cluster is simply moving through a dusty part of space and it is the light from the stars which is reflecting off the gas and dust molecules, much like car headlights lighting up fog.

Due west from the Pleiades Cluster is a modest 2nd magnitude orange star called Hamal, the brightest star in the constellation of Aries. About 2000 years ago one of the two points where the path of the Sun crosses the celestial equator was in Aries but it has now moved to Pisces as we saw here. The rest of Aries looks like a curved line with Beta and Gamma Arietis to Hamal’s west pointing south to the celestial equator. To the north-east of Hamal is the most westerly star in Aries, known as 41 Arietis and it points to the most southerly stars in Perseus.

Perseus represents another hunter, or more accurately a Greek hero sent to slay the snake-haired monster Medusa. The constellation depicts the triumphant hero grasping Medusa’s severed head. The most southerly star in Perseus, which Hamal seems to point towards, is Zeta Persei and it marks one of his feet, the other being to its north-east without any prominent stars. To the north-west of Zeta is a star by the name of Algol, which is perhaps the best-known of the eclipsing binary stars. These binary star systems orbit each other with the orbital plane along our line of sight, which means we see them alternately passing in front of each other, making the combined amount of light momentarily dip. In the case of Algol there are three stars in the system but only the two brighter ones eclipse each other every 2 days, 20 hours and 49 minutes, when the star system dips by just over 1 magnitude in brightness from 2.1 to 3.4. Moving north from Algol is a line formed by Misam and Iota Persei. North of Zeta is a rather more jagged line of stars made up of Menkib, Adid Australis, Nu Persei and Adid Borealis.

Between Menkib and Adid Australis is a large emission nebula known as NGC1499, or the California Nebula. Emission nebulae, as their name suggests, are visible because they emit their own light with a characteristic red glow that is the result of energized hydrogen atoms. This fine example in Perseus gets its name from a striking similarity to the shape of the state of California but is somewhat longer, measuring 100 light years. Even though it is a 5th magnitude object, it is visually challenging to observe without optical aids; in fact, exceptionally dark and clear skies are needed to see it. Using a wide-field telescope and an h-beta (hydrogen-beta) filter will help significantly.

To the north of Algol and Zeta Persei is the brightest star in the constellation, called Mirfak, or Alpha Persei. It is also the brightest star in a cluster of stars known as the Alpha Persei Moving Group, which is a collection of about a hundred stars that all broadly share the same speed and direction of movement. They lie at a distance of about 600 light years and cover a volume of space measuring just over 30 light years in diameter. The other stars in the cluster can be seen scattered around Mirfak from a nice dark site.

Mirfak is also at the centre of a curved line of stars that points to the north-west and towards the constellation of Cassiopeia, which is seen as a giant ‘W’ in the sky (or ‘M’ from low southern hemisphere latitudes just skirting the horizon). Take a glance at the area of sky between the two and you will see what seems to be a couple of hazy patches. This is the famous Perseus Double Cluster of stars and from a dark site they can easily be seen with the naked eye, with each cluster covering an area of sky about the same size as the full moon. The clusters, which have the catalogue numbers NGC869 and NGC884, are just a couple of hundred light years apart and nearly 7000 light years away from us. Spectral studies show that their light is shifted towards the blue end of the spectrum so they are heading towards us at a speed of roughly 20km per second. Open clusters like these are usually young but these two are among the youngest, with an average age between them of just 4.3 million years. Through even the smallest of telescopes with a low magnification, this pair of clusters is visible in the same field of view and looks mesmerizing. Even modest binoculars offer an impressive sight.

Returning to Perseus and looking to its north-east is the large, faint constellation called Camelopardalis, whose most westerly stars form a line which points to Perseus in the south and Polaris, the north Pole Star, in the north. This line is not the most prominent, however, and is marked at its most southerly point by the variable star CS Camelopardalis, which appears as a faint yellow-white star. To the south-west of this star is Miram, an orange star which sits at the northern tip of Perseus and is a stunning binary system with contrasting gold and blue stars. Between CS Camelopardalis and Miram is the radiant, or the point in the sky, that the Perseid meteors all seem to come from during the annual meteor shower seen each August.