We should not forget that this is an incredibly hot and dense plasma state of the universe in the first few hundreds of thousands of years of its creation or after the Big Bang. After around 380 000 years since the Big Bang, when the universe cooled down to around 3000 K, the speed of electrons was reduced enough to be captured by a nucleus of hydrogen and helium to form the first atoms. The universe was not in a plasma state any more. Matter has organised in atom structures of mostly hydrogen and helium. The universe was not any more opaque but became transparent. That was a crucial moment as then photons suddenly got free and escaped, moving at the speed of light in all directions. That was uniform radiation of photons with frequency in spectrum of visible light. The remnants of this uniform radiation are today detected as cosmic microwave background radiation. The reason that this is not detected in spectrum of the light is the fact that the universe has stretched enormously since then, and with it waves within photons going from short wavelengths within the spectrum of visible light to long wavelengths within the spectrum of microwave radiation as we perceive it today.
In relation to baryonic acoustic oscillation, we have to go back to the moment when photons decoupled from matter around 380 000 years after the Big Bang. Up to that moment the pressure between baryon and photons was pushing both of them outwards. However, when photons decoupled from matter, this relieved the pressure. Photons then escaped while baryons remained fixed at this radius. This radius is often referred to as the sound horizon or the zone of the last photon scattering. The pressure created by photon and baryon interaction does not any more drive baryons outwards as this pressure has now gone with the escape of photons.
To understand this better we can imagine boiling water in a pot with a lid on top of it. When water starts to boil and the lid is still on, the pressure of molecule of water interaction will push the lid, making it dance and creating sound due to the increased pressure driving the molecules of water outwards. Once we remove the lid, the pressure will decrease and bubbles of boiling water will for a moment goes down or inwards.
Similarly, when photons escape, then the only remaining force for baryons is gravitational force pulling the matter of baryons towards the centre where there is dark matter which did not move from there all the time. This creates an area of more dense matter at the original centre and at the sound horizon.
We know that gravitation depends on mass density. As we now have zones where the number of baryons is more per unit of volume than in other places, then these will attract more mass than others and would eventually create stars and galaxies around these areas. At the same time, these places will have a higher temperature than others, as the number of baryons is higher with more interactions among these particles. They will therefore radiate more energy, which will cause radiation of energy not to be so uniform. This will be imprinted in cosmic microwave background radiation, which is detected as anisotropy of CMB or slight changes in temperature at some places on a map of CMB. Well, this increase in temperature corresponds to the area where baryonic mass density was higher than in other places. It corresponds to the ripple of baryonic acoustic wave oscillation, which was the seed for galaxies’ formation. These distances between slightly higher temperatures are constant and not changeable. It is close to half a billion light years and corresponds to the distance of the first galaxies being formed around a billion years after the Big Bang. The reason this distance is constant is that they are perceived from CMB, the signals which we receive now about something which took place around 10 billion years ago. It is therefore a snapshot of the time around 10 billion years ago.
When we look at our photographs from childhood we see how we looked at that time. The photograph is a snapshot of the time when we were children. The particular time of our age is arrested by the photograph made at that time. In other words, the picture in the photograph does not get older but remains a reflection of how we looked as children at that time.
Now we will go back to our universe and imagine one hypothetical scenario where the universe is a constant size, not expanding and not contracting. We looked at distances between galaxies, which are at different distances from us. For example, we looked at galaxies A and B, which are at a distance of 8 billion light years from us and A1 and B1, which are at a distance of 5 billion years from us. A1 and B1 are galaxies closer to us as it took 5 billion years for light from them to reach us. We can make a comparison of the distance between A1 and B1 galaxies with the distance between A and B galaxies, which are 8 billion light years away. We should not forget that the light of these galaxies we receive now is the snapshot of the time of how they looked at 5 or 8 billion years ago. It means that the universe has moved forward in time 3 billion years from the snapshot received from galaxies A and B to the snapshot received from the galaxies A1 and B1.
As we consider the case that the universe is not expanding or contracting, then baryonic acoustic oscillations, or the distance between them at the time when the first galaxies were formed, should correspond or exactly fit the distance between A and B galaxies as well as A1 and B1 galaxies.
Picture 5.01
Now we will consider the universe, which is expanding, as it is the case in reality. In this case, the distance between galaxies A and B is larger them BAO (baryonic caustic oscillation) as the time of the universe moved forward 2 billion years. CBM is a snapshot of the time 10 billion years ago while A and B galaxies is a snapshot of the time 8 billion years ago. 10 – 8 =2. During this time of 2 billion years the universe has expanded, and with this expansion the distance between the two galaxies A and B has expanded so that BAO or distance, which was between galaxies first created 10 billion years ago, cannot fit exactly. The same applies to the distance between galaxies A1 and B1. The universe has expanded so much, and with it the distance between A1 and B1 galaxies, that we can now fit 3 or 3 and a half BAO within this distance.
Picture 5.02
With current technological progress, the discovery of baryonic acoustic oscillations, for instance, and the general improvement of devices which help us to observe the universe more accurately, we are now able to make a very confident conclusion that the universe up to 9 billion years of age was slowing down in its expansion. It was because matter dominated the universe. The universe is about 14 billion years old. In many sources of information, it is stated that the age of the universe is about 13.7 billion years. Roughly, it can be said that it is 14 billion years.
From the time when the universe was 9 billion years old, the expansion of the universe began to accelerate. This acceleration of universe expansion in the last 5 billion years is due to dark energy taking dominance over matter.
How do we know that this expansion is due to dark energy?
It is actually a logical conclusion that there must be something or some form of energy in the empty space, which increases the space between galaxies. The names dark matter and dark energy are given as it is not yet clear what dark matter is made of or what is the source of dark energy.
To understand it better we can imagine throwing a ball with a powerful force (a cannon ball, for instance, from a cannon) upwards against the gravity of our planet. The initial speed of the ball will be high but as it travels away from the Earth’s surface it will slow down. It will eventually reach the pick of the distance between itself and the Earth and will start travelling back to the Earth, being pulled by the gravitation of the Earth. We have therefore an expansion of the space between the ball and the Earth initially. This expansion would slow down due to gravity, which would eventually reverse the ball’s direction with the ball now moving towards Earth with a subsequent reduction of space between the ball and the Earth.
Similarly, it was expected that the universe expansion should be decreasing until the maximum expansion is reached, after which the universe will reverse back and start contracting, ending up in a ‘big crunch’.
For up to 9 billion years of its existence the u
niverse was slowing down but then it started to accelerate significantly which is the opposite of what is expected when we take into account gravitational force as the most important fundamental interaction between particles at great distances.
Therefore, the recent discovery of the fact that the expansion of the universe is accelerating could be due to two possible mutually exclusive reasons, which are:
a.Either the concept we have about gravitation is wrong or we are missing something there, or
b.There must be some energy of empty space which is doing it. This means introducing a new ingredient to make sense of the current theory.
In history we had situations where we had to introduce new ingredients to make sense of an existing theory. An example is the Ptolemaic geocentric system. Claudius Ptolemy was a Greco- Roman astronomer and mathematician who lived in the first half of the 2nd century AD. At that time, the logical conclusion for the observation of the universe was that Earth is the centre of the universe and that all celestial bodies go around Earth. However, planet movements (those planets which were known at that time) were unusual and could not be explained by a geocentric model. In order to do so, there was a need to introduced a new ingredient or throw away the geocentric theory as not plausible. Ptolemy introduced a new ingredient instead. It was called epicycle, which explained unusual movements of the planets and fitted into an existing geocentric theory so well that the theory remained a logical and plausible explanation of astronomical phenomena until the Copernicus heliocentric system.
Similarly, we can reject the theory of gravitation or introduce a new ingredient, dark energy, to fit the current model and understanding of the universe.
At the present time of understanding of phenomena in the universe, the nature of gravitational force remains very logical, plausible and cannot be rejected.
We therefore have to introduce a new ingredient or dark energy to make a sense of the current observation.
Currently, we do not know what exactly the dark energy is. Like there are many speculations as to what dark matter is composed of, we have speculations as to what dark energy is made of. Two main candidates are:
1.The cosmological constant
2.Quintessence
The cosmological constant, which is also written as the Greek letter lambda, was used by Albert Einstein to refer to repulsive force, which is the opposite to gravitational force.
The exact nature of dark energy or the source of this energy is not yet understood. There is a mathematical calculation which provides a logical explanation from a mathematical point of view and, to some extent, I might be able to understand it. I am, however, not a mathematician or physicist and I am trying to make sense of the existing theories in this field from the viewpoint of a layman such as I am. I will therefore avoid using mathematics and try to make sense of it in my own specific way, which was one of my motivations to write this book.
To go back to the main topic, Einstein’s theory of relativity states that gravitational energy does not only depend on mass density but also on pressure exerted outwards.
In other words, pressure exerted outwards increases gravitational force, which is pulling unwoods. So force of gravity is equal to mass density + pressure in all 3 directions as we have 3 dimensions of space. Therefore, pressure is multiplied with 3. As gravity opposes the expansion of the universe, it is negative in the relation of the expansion. It could therefore be expressed roughly with an equation such as:
– (mass density + pressure times 3)
The negative sign is due to the negative effect gravitation has on the rate of expansion, making it slow down.
We can even make some analogy between building of the confidence of a person and expansion of the universe. The building of confidence will be like expanding or increasing the strength of personality where positive thoughts about himself or herself would make the confidence grow bigger. Opposing thoughts (like gravity) or comments on someone’s capability have a negative effect on his or her confidence and is therefore negative in relation to the building of their confidence. An example is a negative comment about someone’s capability like: “You are not able to do it” or “It is not possible for you to do it in such a short time.” However, if these persons are receiving a negative comment to a negative comment then the final comment becomes positive which will boost a person’s confidence,making it become bigger. For example: “It is not that you are not capable of doing it” or “It is not true that you are not capable of doing it” or “It is not true that you cannot do it in such a short time.”
Similarly, if pressure in the equation outlined above turns out to be negative, then negative in brackets with negative outside brackets gives a final positive value, making the universe speed up in its expansion instead of slowing down.
A possible source of negative pressure is looks to be in quantum fluctuation where a virtual particle is constantly created from an empty space and where there is temporary violation of the law of conservation of energy but quickly comes to zero net of energy following annihilation of matter and antimatter particles.
The energy for the creation of a virtual particle is borrowed from a vacuum. That consequently creates negative pressure in the vacuum. If particles do not annihilate each other to establish zero net energy of vacuum again, then the negative pressure in the universe will continue to exist which will be the driving force of universe expansion. This will happen if, for some reason, created partials and antiparticles do not meet each other so they continue to exist instead of being annihilated.
Let me try to explain this in a slightly different way. We can imagine an empty room. The space of this room is occupied with plenty of invisible boxes. Every now and then, two of the invisible boxes come out of the room, becoming visible. One of these would be matter and another, antimatter or virtual particles. Their place is now empty. In other words, we have negative pressure created as boxes are out, becoming visible or virtual particles. As boxes quickly fuse together, annihilating their existence, they go back to where they came from, becoming again invisible. In that way, negative pressure created is restored as the boxes are back. However, if the boxes continue to exist and do not annihilate each other, then their place in the room remains empty. We now have negative pressure which gives negative value when summarised with mass density in brackets. Negative value multiplied with negative value outside the brackets of formula gives positive value. It means that gravitation is now positive and enhances the extension of the universe.
An example:
– (mass density is say number 10 + pressure 10 x 3)
which is:
– (10 + 30)
which is:
– (40) which is finally – 40.
It is because if we multiply negative value (-) outside the brackets with positive value (+) in the brackets, the final result we get will be negative value (-)
If, however, we multiply negative value outside the bracket (–) with negative value inside the brackets then we finally get positive value.
– (mass density 10 + negative pressure of –10 x 3)
which is:
– (10-30)
which is:
– (–20) which is finally 20 and not –20
Quintessence is another theory or attempt to explain the reason behind negative energy. Unlike the cosmological constant, which does not change in space and time, quintessence, as a form of dark energy, will vary in space and time. In the Encyclopedia Britannica it is defined as a transient vacuum energy resulting from potential energy of a dynamic field.
Quintessence is referred to as a fifth element. In the time of Aristotle it was believed that all matter on Earth was composed of 4 elements: earth, water, fire and air. Aristotle believed that all celestial bodies and the space above Earth, heaven, were composed of ether or from the fifth essence, which comes to be quintessence from the Latin.
&
nbsp; Some scientists today regard dark energy as a fifth fundamental force.
Unlike the cosmological constant which is based on the logic of negative pressure in empty space, quintessence tried to explain the existence of dark energy in different ways. Professor Alex Filippenko was giving an example about broken symmetry in one of the documentaries about dark energy, which is easily accessible on YouTube. Instead of me trying to paraphrase what Professor Filippenko was teaching us, I would recommend watching this documentary.
What we know today is that the expansion of the universe is accelerating. The reason behind it is still part of speculation with as yet no theory emerging to be proven or confirmed by experiment. Therefore, there remain still a number of possibilities to explain the current observation of the universe, including one where the concept we have today about gravitation based on the theory of relativity can be changed.
6
SPACE-TIME
Curved space-time tells mass-energy how to move;
mass-energy tells space-time how to curve.
John Wheeler
John Wheeler, physicist, was the first American involved in the theoretical development of the atomic bomb. He worked with Niels Bohr in explaining the principle in relation to nuclear fission. He also gave ideas for names such as black hole and wormhole.
His quote about gravity and relation to space-time contains basically all that is needed to understand or have a rough idea of this concept.
Journey Through Time Page 10
