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The Theories

What are the ideas that can explain the Cosmological Clues?

The Scientific Process: INTERPRETATION


A theory is an in-depth explanation of why something behaves as it does that has been extensive^ tested. Any theory of how the universe works has to be able to explain the observations that make up the six Cosmological Clues: galaxies moving away from us, galaxies uniformly rotating to their edges, uniform microwave light across the sky, helium being 25% of the elements, galaxies forming a cosmic web, and supernova seeming more distant than they should. We have seen the evidence for the Cosmological Clues in Chapter 3, now we have to explain them.

What theories do we have that could explain these Cosmological Clues? There seem to be many ideas around. Some are accepted ideas with evidence, some are more speculative ideas that require some evidence. This chapter explains some of the key ideas and identifies which are which. I will not discuss all the ideas because there are quite a few but mostly they are variations on the themes that will be discussed here.

The main theory is the generally accepted ‘Standard Model of Cosmology’ called the ACDM model, read as lambda cold dark matter. Historically, the main rival theory to ACDM is the not so accepted ‘Steady State Theory’. In science a model is not the same as a theory. A model is a tool to help aid our thinking about how the World works and is used in the daily work of scientists so that over time and with more evidence the model can be built up into a theory that is fully tested. Our cosmology theories are really scientific models because they cannot explain all the evidence and have not been fully tested. You will see in this chapter that the models are detailed and scientifically rigorous and do fit large parts of the evidence, but there are some gaps and unknowns which I will highlight .

The main addition to the ACDM model is inflation - the idea that the universe expanded rapidly just after the Big Bang. It has not been accepted as part of the ACDM because there has been no evidence for it, but the working assumption of astronomers is that it happened because the theory gets rid of some important Cosmological Problems: the Horizon Problem and the Flatness Problem. It also explains why the universe is expanding and solves a problem from particle physics called the monopole problem. Inflation modifies ACDM by adding a massive expansion in the very early universe.

A way of altering the ACDM model is to change the laws of gravity so that it is not necessary to introduce unknown dark matter. Although this can explain some of the Cosmological Clues, changing the tried and tested fundamental laws of gravity is not something that is very popular with most scientists. There are many ways the laws can be changed, so there exist a variety of different modified gravity models, I will discuss the basic ideas here.

Then there are the more imaginative ideas: the multiverses, string theory and quantum gravity. This is when the number of ideas really increases. These are speculative ideas that so far have no observational evidence, although there are mathematical frameworks for them. It is the beauty of the equations that spur scientists on in this field, as well as the potential big prize of being able to explain how all of physics and the universe fit together: the ‘Life, Universe and Everything’ prize (to paraphrase Douglas Adams). I will give a brief overview of these alternative universes, enough so you can see how they fit into cosmology and relate to the six Cosmological Clues.


The Lambda Cold Dark Matter Model (ACDM)

The Standard Model of Cosmology is not just about the Big Bang, dark matter and dark energy. It is much more. It can explain how matter forms into stars, galaxies and the cosmic web, it has a set of measurable factors that define the model, it is based on the mathematics of the laws of physics, and it can explain our observations of the Cosmic Microwave Background, galaxies moving away from us, and the elements in the universe. It is a powerful model but it also has it’s problems; there are things it does not explain and it has introduced dark secrets.

Overview of CDM: From 0 to 14 Billion Years

The Standard Model of Cosmology is called the ACDM. The A is the Greek symbol for Lambda and represents dark energy and the CDM stands for Cold Dark Matter, two key ingredients of the model. The basic assumption of the model is that it started with an extremely hot, Big Bang explosion and from then on the universe expanded and cooled. This section will give an overview of the model, describing the important events, starting from the Big Bang and moving forwards in time up to the present day. (Figure 1.1 provides a pictorial overview.)

The universe started as a single point. All the energy that exists in the universe today existed at this point. We do not know how this can happen, our laws of physics break down for such a point, but conceptually that is how we think of it. In physics we call such a point a singularity: a point that has an infinite amount of energy and is infinitely small. As scientists we do not like singularities, and normally avoid them, but we do not know how else to describe the start of the universe so we put up with it until we find something better.

About fourteen billion years ago, the singularity changed and created spacetime. The energy released was so massive that it made space and time expand and it continues to expand today. As it expanded the universe started to cool: a universe double the size has half the temperature. This process of cooling is essential to ACDM, it is the cooling process that allows different physical processes to happen at different temperatures which manifests itself in the universe changing and evolving.

The ACDM story started when the universe was one second old and the temperature was ten billion degrees, about 1,000 times the temperature at the centre of the Sun. It’s size was a few light-years across which is a bit less than the distance to our nearest star (Proxima Centuari). Most of the energy in the universe was in the form of light, with the rest of the energy being in small, fundamental particles mainly protons, neutrons, electrons and positrons (the electron’s antimatter). They were colliding, annihilating and reforming with each other. It was a cosmic soup of charged particles and light. The universe was a dense, thick fog caused by the light being randomly scattered by the collisions with the charged particles. It is at one second old that one of our Cosmological Clues becomes relevant; Big Bang Nucleosynthesis. This is when protons and neutrons started to combine to form the nuclei of simple chemical elements: helium, deuterium, lithium and beryllium.

By 3 minutes old, the universe had cooled to 1 billion degrees and had grown to a few hundred light-years across. The positrons had virtually all been annihilated leaving mainly electrons, and the nuclei had finished forming since all the neutrons had combined with the protons. There were more protons than neutrons so the universe was left with 75% protons (which is hydrogen), 25% helium (two protons and two neutrons) and traces of the remaining elements. This matches the proportion of the elements that we see in the universe today.

The universe continued to cool but it was another 380,000 years before our next interesting event happened when the universe was at 3,000 degrees. This was when electrons combined with the nuclei of the elements to form atoms, a process called recombination. Atoms have no charge, the negatively charged electrons cancel out the charge of the positively charged protons. Light is not scattered by neutral atoms so it continued in a straight line in whatever direction it was going when recombination happened. The fog in the universe had now cleared. This is now our next Cosmological Clue because this light is what we see today as the Cosmic Microwave Background. The time it took for the CMB to form is called the ‘Epoch of Recombination’.

What followed next is the called the Dark Ages. Nothing eventful happened. Just slow gravitational attraction of particles gently moving towards each other. There were no sources of light. The CMB light continued to travel through the universe. This went on for several hundred million years.

Then the first light of the first stars started to appear. It had taken several hundred million years for enough particles to collapse together into a dense ball so that they heated up to over ten million degrees and started reacting with each other to produce nuclear fusion. The result of the fusion was that light, heat and energy was given off by the stars. The dark ages were over. Light was being created in the universe again in a cosmic dawn.

The first stars are likely to have been very big, a few hundred times more massive than the Sun. The biggest stars have the shortest life and after a few million years some of the largest stars will have started to die in supernova explosions leaving behind neutron stars and black holes. In these explosions heavy elements were produced and spread back out into the universe. These heavier elements were themselves pulled together by gravity, along with the lighter elements, to form new second generation stars. The heavier elements allowed smaller stars to form that can live much longer. The Sun is a second generation star and has a lifetime of 12 billion years. The smallest stars, a tenth of the mass of the Sun, can live 1,000 billion years, much longer than the age of the universe today.

An import ant consequence of stars appearing in the universe was that they gave out ultra-violet (UV) light and the massive first stars would have given out a lot of strong UV light. The UV light affected the atoms surrounding the stars by stripping off electrons in what is called ‘ionisation’. The atoms became charged. Now the high energy light was no longer absorbed and all wavelengths of light could travel through space unimpeded. The universe became visible as we see it today. This process is called ‘reionisation’. The reionisation happened slowly, appearing first around the stars in bubbles, with more and more bubbles appearing until all the universe was reionised. It took about one billion years for all the hydrogen to be ionised and is called the ‘Epoch of Reionisation’. The galaxies we see today contain significant amounts of ionised hydrogen, there is 1 neutral hydrogen to every 10,000 ionised hydrogen atoms.

As the stars were forming, gravity continued to pull them together to form galaxies. The earliest galaxies formed at about a billion years and were irregular shaped and small. As they merged, they grew until they became the smooth, large galaxies of today. At the centre of galaxies black holes formed (or already existed), they attracted more matter and the energy from matter falling into the black holes produced ‘active galactic nuclei’ that ejected vast amounts of energy, heating up the galaxies and stopping stars from forming. The matter thrown out into the space between the galaxies (the interstellar medium) would fall back to be used again. The space between stars is not empty, it contains the gas, dust and molecules that form future stars.

The newly formed galaxies very slowly started to gather into groups and clusters. This process has taken so long that even the first clusters are still in the process of forming today.

At about 3.5 billion years old star formation was at it’s peak, the cosmic noon.

At 9 billion years the Solar System formed, with the Sun and the planets forming at the same time. Earth began.

At 10 billion years life started on Earth.

At 11.5 billion years the effects of dark energy started to dominate and expansion started to accelerate.

Today we are at 13.8 billion years from the Big Bang. The ancestors of humans appeared on Earth about 6 million years ago and it was a mere 200,000 years ago that humans appeared. The telescope was invented 400 years ago and the modern computer 50 years ago and today satellite telescopes and supercomputers allow us to look at the skies and work out how the universe formed.

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