What do we know about the evolution of the universe?
The Scientific Process: INTRODUCTION
“Cosmos” comes from. Greek meaning harmony or order “Cosmology” is the study of the evolution of the universe
COSMOLOGY – IS THE UNIVERSE EVOLVING?
The universe is a vast and fascinating place. Over the past few thousand years, our view on what makes up the universe has changed and continues to change today. As we improve our technology and build larger telescopes we are seeing new and exciting things that challenge our theories and expands our knowledge. It is an exciting time to be an astronomer looking up at the skies wondering how the universe came to be like it is. What the universe looks like, and how it got to be like it is, is important to us; it affects the way we, as human beings, see ourselves either as tiny specks on a small planet or, as in the past, with us at the centre of everything.
Today, scientists have a view of how the universe has evolved which is based on it beginning with a massive explosion causing the universe to expand; this is the Big Bang. The study of how the universe has evolved, and is evolving, is called cosmology. The current theory includes dark matter, that enabled galaxies to form, and dark energy, which is accelerating the rate at which the universe is expanding. These three things together, the Big Bang, dark matter and dark energy, form the ‘Standard Model of Cosmology’. There are three main problems with this model: one, we don’t know what caused the Big Bang; two, we don’t know what dark matter is; and three, we don’t know what dark energy is. So why do scientists have this model? Is it even science or have we just made it up? Well there are clues - observations of the universe - that have led us to this model. We have used the scientific process to develop our ideas, observe the sky to give us evidence, develop a model based on scientific laws to explain the evidence, and then test the model with more evidence. This has led us to our current understanding of how the universe works.
The Cosmological Clues that are explained in this book form the evidence that we have for how the universe works. There are various theories that scientists have developed to explain the Cosmological Clues but none of them, including the Standard Model of Cosmology, explain them all. Cosmology is a subject that requires new ideas to solve the Cosmological Problems making it an interesting and creative field to work in. The imagination required to come up with the theories shows how creative scientists can be. Cosmology really is a place where art and science meets. The concepts of multiverses, time travel, worm holes and black holes are scientific ideas used in films, art and books. Scientists really do create ideas that no-one had imagined before.
It can be hard for those outside of cosmology to know what is an accepted theory by the majority of scientists and what is an outlandish idea proposed by a few. To make it worse for you the reader, it is changing; new evidence is appearing and new ideas are being created. By reading this book you will be better equipped to understand the scientific process and know how to apply it to cosmology so that you can make your own ‘scientific’ assessment of new theories and observations on the Big Bang, dark matter and dark energy.
THE STANDARD MODEL OF COSMOLOGY
What is our current model of how the universe evolved?
The Standard Model of Cosmology is made up of three distinct parts that come together to make a unified picture of the way the universe evolved: the hot Big Bang, cold dark matter and dark energy. The standard model is also called the ‘Lambda Cold Dark Matter’ model represented by AC DM. The A represents the dark energy and is the symbol for the cosmological constant proposed by Einstein. The CDM stands for cold dark matter. The Big Bang caused the expansion of the universe and is fundamental to the model but has no symbol. There is much observational evidence for the existence of all three elements of the ACDM model. However, there is also much we don’t know. We don’t know what caused the Big Bang and what the universe was at the point of the Big Bang explosion, we don’t know what dark matter is, and we don’t know what dark energy is.
Not knowing these fundamental things about the model means that it is not complete, there is plenty still to understand. It is not like the Standard Model of particle physics and quantum theory which are highly accurate and well known and research is focused on looking for evidence of where they fail. The ACDM model has flaws and gaps, but we use it because it fits a lot of the observations - the Cosmological Clues - and at the present time it is the best model we have. For this reason, it is accepted by the majority of cosmologists as a good working model, one that we can prove or disprove as we get more clues and for this reason it is called the ‘concordance model'. An overview of the ACDM model is shown in the timeline of Figure 1.1. It shows the evolution of the universe from the Big Bang through to today.
The Big Bang
The Big Bang is an explosion that is considered to be the moment in space and time (shortened to spacetime) that the universe started and resulted in the universe expanding. We don’t know what the universe looked like then, or why it exploded, but our model says that it did and that it has been expanding ever since that moment. The Big Bang was extremely hot, it contained all the energy of the universe. In the first millionth of a second after the Big Bang the basic elements and forces that we now see in the universe were formed, producing the sub-atomic particles of electrons, neutrons and protons, along with the strong and weak nuclear forces, gravity, electromagnetism and light. The universe at this stage was a very hot, dense soup of particles and light.
As the universe expanded it cooled down. It is the process of cooling that is the key to the universe evolving and caused the changes that make the universe look like it does today. Between 10 seconds and 20 minutes after the Big Bang, the neutrons and protons cooled enough to combine and form the nuclei of helium which, along with hydrogen, are the lightest elements in the periodic table. In fact, over 98% of visible matter in the universe today is hydrogen and helium. It took a further 380,000 years before the universe had cooled enough for the electrons to combine with the nuclei to form atoms. At this point light stopped hitting the particles and they could move freely through the universe, travelling vast distances, and we can see that light today in what is called the Cosmic Microwave Background (CMB). Further expansion and cooling allowed gravity to pull the atoms together until they were dense enough for nuclear reactions to take place and stars were born giving off heat and light. Over time the stars have been pulled together by gravity to form galaxies (large groupings of stars).
There are three key pieces of evidence that have led to the view that the Big Bang happened:
- 1. Nearly all galaxies are moving away from us, explained by the universe expanding.
- 2. The Cosmic Microwave Background has been detected and measured. The best explanation for the CMB is as a consequence of the Big Bang.
- 3. The measured ratio of hydrogen to helium in the universe is 3:1. The Big Bang, and the particles it produced in the first few minutes, can explain this ratio.
Dark matter is matter that we cannot see, does not interact with any other types of matter other than by the pull of gravity and we don’t know what it
Figure 1.1: Timeline of the universe. The ACDM model of the evolution of the universe starting from the Big Bang through to today. Inflation has been added at the beginning although it is not part of the Standard Model of Cosmology. Credit: C. Devereux and P. Farrell.
is. Dark matter makes up 85% of all the matter in the universe and is essential for the formation of stars and galaxies. Dark matter cannot be seen directly. It was first observed, indirectly, by Fritz Zwicky in 1930, when he found that clusters of galaxies were rotating faster on the outside than was expected. In order to explain this, it was necessary to add more matter into the cluster than could be seen from the stars in the cluster - much more matter, in fact about 85% more. As happens sometimes in science, this observation was ignored for 50 years because it was hard to accept that there was that much unknown matter in the universe. But in 1980, Vera Rubin made a similar observation on galaxies and found that the stars on the outside of the galaxies were rotating faster than the}' should be if the}' followed the laws of gravity. Once again the existence of dark matter was proposed as the solution to this problem. Now the evidence was getting stronger for dark matter to be there and scientists could not just ignore it, even though there is no theory as to what this dark matter could be.
In the late 20th century, scientists carried out a detailed survey of the observable galaxies across the night sky. When the positions of these galaxies were plotted into a map it was found that there was a structure to where the galaxies lay, it formed filaments and blobs that look like a spider’s web. This is called the cosmic web - it is the large-scale structure of the universe. We can use computers to make simulations of this large-scale structure using our knowledge of cosmology and from this see whether we can reproduce the way the cosmic web looks. This has shown us that we can only reproduce what it looks like if we include an extra 85% of matter compared to the matter we see in stars. We have called this unknown matter ‘dark matter’ and we still do not know what it is. We can’t see it and it doesn’t interact with ‘normal matter’ other than by gravity. Reproducing the observed cosmic web using computers is a further piece of evidence that dark matter exists.
W 'e can consider dark matter to be cold or hot. Hot means that it is moving at speeds close to the speed of light, it has high energies, which in physics terms means hot. Cold dark matter moves at slower speeds, not close to the speed of light (although it can still be really fast), it has less energy and we call it cold. By putting cold or hot dark matter into our computer simulations of the cosmic web we find that hot dark matter does not reproduce what we observe but that cold dark matter does. Cold dark matter has become part of the Standard Model of Cosmology.
Dark energy causes the acceleration of the expansion of the universe. It is not known what dark energy is but the accelerating effect was observed in 1998 when supernovae were found to be moving away from us faster than expected. The evidence for dark energy is sufficient to include it in the Standard Model even though we don’t know what it is.
The Cosmological Principle
All physics theories that describe nature must obey the laws of physics, for example, gravity, sub-atomic particle physics, quantum theory. Cosmologists have an extra important principle that any model of cosmology must obey - the ‘cosmological principle’. This states that the universe does not have any unique, special place. Wherever you are in the universe, whichever galaxy or star you are looking from, the universe looks the same.