The Dark Arts

Dark Matter and Dark Energy - Basics

In my last article the Building Blocks of Nature, I talked about various fundamental particles of the universe. How they make the four fundamental forces and the particles of matter. But at the last, if you remember, I mentioned that these particles only make for about 4% of our universe. What about the other 96%. We classify the other constituents of the universe as dark matter and dark energy. Dark matter constitutes for about 22% of our universe and dark energy makes 74% of our universe. For how big a part they form of our universe our knowledge of them is very limited. As we talk further about it, it is important to know where the discussion about them originates from.

For long, we have believed in a static (non-expanding) universe. The idea was so deep built into our thoughts that when The General Theory of Relativity proposed of its existence in 1915, Einstein went against his own finding and proposed a cosmological constant, that could be thought of as an antigravity force. He postulated that this was an intrinsic property of the fabric of space-time. He claimed that space-time had an inbuilt tendency to expand, and this could be made to balance exactly by the attraction of all the matter in the universe so that a static universe would result.

The idea was largely accepted, but there was a brave scientist, Alexander Friedmann, who decided to not go against the nature of the universe predicted by the General Theory of Relativity and made two very simple assumptions; the universe looks identical in every direction we look, and this would be true if we were observing the universe from anywhere else. Thus, in 1922 Friedmann had predicted exactly what Hubble found several years later in 1965. Friedmann’s work remained largely unknown in the west until similar models were discovered in 1935 by the American physicist Howard Robertson and the British mathematician Arthur Walker, in response to Hubble’s discovery of the uniform expansion of the universe.

There are three different kinds of models that obey Friedmann’s two fundamental assumptions.

1.   The Universe is expanding sufficiently slowly that the gravitational attraction between the different galaxies causes the expansion to slow down and eventually to stop. The galaxies start to move towards each other and the universe contracts. The universe would end in a Big Crunch similar to the Big Bang. It suggests an expanding universe that is not infinite in space, but neither does space have any boundary. Space could be thought of as the surface of Earth that closes upon itself but rather than the two-dimensional surface of Earth it is three-dimensional. The fourth dimension, time, is also finite in extent, but it is like a line with two ends or boundaries, a beginning, and an end.
   
2.    The Universe is expanding so rapidly that the gravitational attraction can never stop it, though it does slow it down a bit. The expansion would reach a steady speed. Space in this model is infinite bent like the surface of a saddle.
   
3. The Universe is expanding only just fast enough to avoid collapse. The speed at which the galaxies are moving apart gets smaller and smaller, although it never quite reaches zero. The space in this model is flat and infinite.
   

But which Friedmann model describes our universe? Will the Universe eventually stop expanding and start contracting, or will it expand forever? To answer this question we need to know the present rate of expansion of the universe and its present average density. If the density is less than a certain critical value, determined by the rate of expansion, the gravitational attraction will be too weak to halt the expansion. If the density is greater than the critical value, gravity will stop the expansion at some time in the future and cause the universe to recollapse.

However, the average mass density which comes from adding all the masses of the stars that we can see in our galaxy and other galaxies, the total is less than one-hundredth of the amount required to halt the expansion of the universe, even for the lowest estimate of the rate of expansion! Our galaxy and other galaxies, however, must contain a large amount of “dark matter” that we cannot see directly, but which we know must be there because of the influence of its gravitational attraction on the orbits of stars in the galaxies. Moreover, most galaxies are found in clusters, and we can similarly infer the presence of yet more dark matter in between the galaxies in these clusters by its effect on the motion of the galaxies. When we add up all this dark matter, we still get only about one-tenth of the amount required to halt the expansion. However, we cannot exclude the possibility that there might be some other form of matter, distributed almost uniformly throughout the universe, that we have not yet detected and that might still raise the average density of the universe up to the critical value needed to halt the expansion. The present evidence, therefore, suggests that the universe will probably expand forever.

An interesting term that I mentioned in the paragraph above and also in the first paragraph is the existence of dark matter. We have no clue on how they work and how to study them, but we know they exist. The matter present in our Universe is not sufficient enough to form galaxies and hold stars in clusters. Stars should be scattered and not hold structures, thus, there must be some undetectable mass holding these galaxies. We call this matter – Dark Matter.

They do not interact with light. They do not emit or reflect them. Places with a high concentration of dark matter bend light passing nearby. From this, we know that it interacts with gravity. This is both interesting and overbearing to know that we know more about what dark matter is not, more than what it is. We know that dark matter is not clouds of normal matter without stars, because it would emit detectable particles. We know it is not anti-matter because anti-matter in the presence of matter annihilates it and emits Gamma rays. We know they are not made of black holes as black holes are the vacuums of space and the particles of dark matter do not interact with light and matter in a manner we would expect from black holes.

The mystery intensifies when we talk about Dark Energy. We can’t detect, measure, or taste it. We know that universe is expanding and not at a constant rate, but it is accelerating! Space does not change its property when it expands, there is just more of it. New space is constantly created. Wherever there is empty space in the Universe, more is forming every second. Dark energy is a property intrinsic to empty space. It is energy stronger than anything we know and keeps getting stronger as time passes by. Empty space has more energy than everything in the Universe combined.

There is a popular theory that all particles, whether of matter or anti-matter or even dark matter are made of fundamental strings that vibrate in various dimensions. Different particles are only different harmonics of the vibrating spring. Just compare it to any string instrument you love. This forms the foundation of The String Theory and our next discussion. See you then. I hope you liked the article. Interact and support. Thank you.

Building blocks of Nature

Particles

For long people have gazed upon stars and wondered what makes this universe? How do stars twinkle? What makes things move? Is matter continuous like the water appears to be or is it coarse and divisible like particles of a heap of sand? When do we stop dividing and expect that what we are looking at is the fundamental particle that has made it? Does every matter in this world however different they may seem come from some basic elementary particle? What makes everything? What drives them? How do they combine? What is force or energy? How are these fundamental particles glued together? Are energy and matter the same, even though when one can only be felt and the other perceived? And when would we know that we have ultimately found the building block of everything?

There was a time when these questions were very far-fetched. We did not have the knowledge nor the means to probe into them. All we had were opinions. Opinions from different philosophers and scholars of the time.

Given the decades of pondering, theorising and experimenting, we do have answers to a few of these questions today.

Particles - The building blocks of nature

Particles have different characteristics; mass, spin, charge, decay, etc. But today, while we look upon forces and matter, we talk about the special characteristic which makes this distinction possible – spin. What did I say you ask? Particles of force? Yes, you heard it right. Particles of force! Using the wave/particle duality, everything in the universe, including light and gravity, can be described in terms of particles.

The broad distinction between particles by the Standard Model of Particle Physics

Based on the spin of a particle, they can be divided into two types – Fermions and Bosons.

Fermions are particles with a spin number in the form of n/2 where n ϵ I – {0}. And,

Bosons are particles with a spin number in the form of n, where n ϵ I.

Calling them spin can be misleading because quantum mechanics tells us that the particles do not have any well-defined axis. What it really tells us is how particles look from different directions.

A particle of spin 0 is like a dot. It looks the same from every direction. A particle of spin 1 is like an arrow: it looks different from different directions. A rotation of 360^𝜊 gives the same look. A particle of spin 2 is like a double-headed arrow: it looks the same when turned around half a revolution, i.e., 180^𝜊. Higher spin particles look the same if one turns them through small fractions of a complete revolution.

One really interesting fact is that there are particles that do not look the same if one turns them through just one revolution. You have to turn them through two complete revolutions! Such particles have a spin of ½.

Fermions form the matter in the universe and bosons give rise to the force between the matter particles.

One interesting thing that distinguishes matter particles from force particles is that the matter particles follow Pauli’s Exclusion Principle. It says that two similar particles cannot exist in the same state; that is, they cannot have both the same position and the same velocity, within the limits given by the uncertainty principle. If the matter particles have very nearly the same positions, they must have different velocities, which means that they will not stay in the same position for long. This explains why matter does not collapse over each other under the influence of forces with spin numbers 0, 1 or 2. If it were the case, all particles would collapse into each other and what we will have would be a uniform dense “soup”. Even an atom could not be formed let alone us, or life.

Fermions can be further classified into quarks and leptons.

Quarks come in six “flavours” – up, down, charmed, strange, bottom and top. These “flavours” come in three “colours”. It is important to keep in mind that when I write colour, I don’t actually mean colour as we know it in our worldly sense but these are just for some fun naming purposes.

A proton or neutron is made up of three quarks, one of each colour. A proton contains two up quarks and one down quark; a neutron contains two down and one up. We can create particles made up of the other quarks (strange, charmed, bottom, and top), but these all have a much greater mass and decay very rapidly into protons and neutrons.

Up and down quarks make protons and neutrons

Leptons are further of six types – electron, electron neutrino, muon, muon neutrino, tau and tau neutrino. The most famous one is, obviously, electron.

The six leptons

The particles have a matter-antimatter pair too. Antimatter is the same as matter and differs only in electric charge. When matter and anti-matter come in contact, they annihilate each other and only energy is left. Anti-matters make for an interesting topic for discussion but this would be beyond this article's word limit.

Matter - Anti-matter annihilation

So, some other day.

Coming to bosons, bosons are characterized by Bose-Einstein statistics and all have integer spins. Bosons may be either elementary, like photons and gluons, or composite, like mesons. They make what we call force particles.

A matter particle, such as an electron or a quark, emits a force-carrying particle. The recoil from this emission changes the velocity of the matter particle. The force-carrying particle then collides with another matter particle and then is absorbed. This collision changes the velocity of the second particle, just as if there had been a force between the two matter particles. Force carrying particles do not obey the exclusion principle. This means that there is no limit to the number that can be exchanged, and so they can give rise to stronger forces. If the force-carrying particles have a high mass, it will be difficult to produce and exchange them over a large distance. So, the forces that they carry will have only a short range. On the other hand, if the force-carrying particles have no mass of their own, the forces will be long-ranged. 

The force-carrying particles exchanged between matter particles are said to be virtual particles, because, unlike “real” particles, they cannot be directly detected by a particle detector. We know they exist, however, because they do have a measurable effect. They give rise to forces between matter particles.

We group force-carrying particles into four categories according to the strength of the force that they carry and the particles with which they interact. They are gravitational force, electromagnetic force, weak nuclear force and strong nuclear force.

The force particles

However deeply I wish to discuss these forces in detail, the word limit won’t allow me to do so, so, maybe some other time.

I’ll only name the particles associated with each force. Gravitational forces are carried by gravitons. It is a long-range force with mass 0 and spin 2. Electromagnetic forces are carried by photons. It is a long-range force with mass 0 and spin ±1. Weak nuclear forces are carried by W and Z bosons with mass 80.4 GeV/c² and 91.2 GeV/c² respectively and a spin number 0. It is a short-range force. Lastly, strong nuclear forces are carried by gluons whose mass is zero and the spin number is 1. 

So, this was about the 4.6% constituents of our universe. Say again? Where's the rest 95.4%? Well, for starters it is undetectable and for the second, we would talk about it on my next blog. So, see you again. 

Auf Wiedersehen!