Advanced science, Astrophysics, Cultural, International, Life as it is, Religious, Technical

Dark Matter and Dark Energy – Part II

In the 1st part on this topic the essential attributes of dark matter had been described. Dark matter was necessary in order to hold the basic fabric of galaxies together; otherwise, billions of stars at the edges of the galaxies would experience weaker gravitational pull and could even fall away from the galactic orbits. So, dark matters were invoked to be present all over the galactic system.  In this part, the role of the dark energy will be considered. Dark matter may keep the individual galactic system intact and maintain higher orbital speeds to outlying stars, but then what is giving the Universe impetus to expand?   

The ‘Standard Model’ of the cosmological system predicted that the Universe simply could not exist in a quiescent steady state – it has to be dynamic in nature, meaning it either has to expand or contract. Indeed, in 1929 Edwin Hubble made an astronomical observation and that had become incontrovertible showing that the Universe was actually expanding. That made Einstein to admit that his cosmological constant, Ʌ (lambda) introduced in the general theory of relativity with a particular value to force a steady state condition for the Universe was flawed. For the next 70 years, until 1998, cosmologists implicitly took Ʌ to be zero and the Universe was described as per Einstein’s field equations. Nobody thought of discarding the cosmological constant that Einstein had introduced, albeit mistakenly.

Then in 1998, another even more astounding evidence was produced based on observation using Hubble telescope, when it was shown that light from very distant supernovae was fading away and showing red shifts indicating supernovae were receding and receding at faster rates further they were from the Earth. In other words, there was an accelerated expansion in the Universe. The Universe’s current expansion rate is known as the Hubble constant, H0 which is estimated to be approximately 73.5 km per second per megaparsec. A megaparsec is the distance of 3.26 million light years. As the speed of light is 3×108 m/s or 9.46×1012 km/year, 1 megaparsec then equals to 3.08×1019 km. A galaxy 1 megaparsec away (3.08×1019 km) would recede from Earth at 73.5 km/s; whereas another galaxy 10 times of 1 megaparsec from the Earth would recede at 10 times of 73.5km per sec = 735 km per sec.  That was a shocking result and the cosmologists were taken completely by surprise.  

What is providing this gigantic Universe enough energy to expand and expand at an accelerated rate? Further observations had demonstrated that this accelerated expansion is in fact taking place in the vast extra-galactic spaces. This came to be known as the ‘metric expansion’. There was no evidence or verifiable evidence of expansion within the individual territories of galaxies. It may indeed be argued that if there were any expansion within a galactic system, then stars would move away from each other and even the planets revolving round the stars would recede. For example, Earth would recede from the Sun and that recessive path would look like a spiral trajectory and eventually Earth would secede completely from the Heliosphere! This would be a recipe for a total disaster for the Earth-bound lives like ours and luckily there was no such evidence of recession. 

Expansion of the Universe as per Standard Model

Albert Einstein’s cosmological constant, Ʌ in the general theory of relativity came to the rescue of this paradox of cosmological expansion. Dark energy was invoked to solve this problem. Dark energy is perceived to be the intrinsic energy of the empty space or simply the vacuum energy. It may be pointed out that space is viewed in the general theory of relativity as the product of gravitational field. As there are limitless empty spaces in the cosmological scale, dark energy can also be limitless. Although the precise mechanism of generation of dark energy is unknown, but some of the essential characteristics may be drawn. Dark energy is repulsive in character. Thus, dark energy can be viewed as something that reacts with ordinary matter (baryonic matter) making up the celestial bodies, but in opposite direction to ordinary gravity. Some scientists speculate that dark energy may even be a form of a new type of force – the fifth force – which is as yet unknown. The known four forces are: electromagnetic force, weak nuclear force, strong nuclear force and the gravitational force and the properties of these forces are well known. If indeed the fifth force does come into play, it would offer a situation where gravity and anti-gravity may come to exist in the same Universe. It may be that the attractive gravity exists within the scale of galaxies, whereas repulsive gravity exists in the vast extra-galactic space!  

Taking material accounting of galaxies into consideration, it is estimated that on the basis of mass-energy composition, the Universe is only 4.5% of ordinary matter, 26.1% of dark matter and 69.4% of dark energy. However, this distribution of mass-energy composition in observable celestial bodies and unobservable black holes do not remain fixed or invariant. At the early part of the Universe’s formation, after about 380,000 years following the Big Bang (13.8 billion years ago), the distribution mass and energy was quite different. Ordinary matter was 12% and dark matter was 63% and there was no dark energy, as shown in the Table below. The situation is quite different now and this shows that the Universe is changing or one can say evolving.

Universe’s mass-energy composition

 13.8 billion years agoPresent day (2000 CE)
Dark energy69.4%
Ordinary matter12%4.5%
Dark matter63.0%26.1%

In the Universe, the amount of ordinary matter (baryonic matter) is fixed and as the Universe expands, the average density of ordinary matter in the Universe is continuously diminishing; as density is the amount of material divided by the volume. Similarly, the dark matter density of the Universe is also decreasing as Universe expands. But the dark energy density had been found to remain constant, no matter how much or how fast the Universe expands. It is due to fact that vacuum energy is constantly added (as space has intrinsic vacuum energy) to the pool of dark energy as Universe expands and hence the dark energy density remains constant.

In the metric expansion, the space or more appropriately, spacetime fabric is created extra-galactic. Space is not something which is devoid of other things. Space is the gravitational field. Like electromagnetic field, gravitational field generating space is granular in character. The quantum of space is so incredibly small that we cannot sense them, similar to solid granular atoms we cannot feel. Space granules are literally trillions of times smaller than atoms. Space granules or space quanta are not within the space, space quanta are the space. A new branch of physics, called ‘loop quantum gravity’ shows how space quanta make up the space. When Universe expands, space is produced with spacetime quanta and the intrinsic dark energies increase.

Although the evidence of accelerated expansion of the Universe was baffling, but it was not unexpected. The Universe had undergone very rapid expansion at the early phase of its existence, some 13.8 billion years ago, after that it slowed down for billions of years and then the expansion phase started about four or five billion years ago. When this expansion will stop or even reverse, nobody knows. But it is definite that the Universe as a whole is not static, it is very much dynamic, vibrant and evolving. If anybody says that the Earth, Sun and Moon and even the whole Universe were created by some unknown Creator and then he left the whole thing in a quiescent state, then there is every reason to question such unfounded claims and discard them as totally baseless.  

  • Dr A Rahman MSRP CRadP FNucI
Advanced science, Astrophysics, Cultural, International, Life as it is, Religious

Dark Matter and Dark Energy – Part I

Until about 100 years ago, the prevailing scientific perception was that our universe was eternal, invariant and in quiescent state. But science has progressed tremendously since then and the very perception of Universe had changed significantly. Albert Einstein’s general theory of relativity in 1916 had revolutionised our view of spacetime of the Universe. Following the general theory of relativity, the Russian physicist Alexander Friedmann in 1922 as well as Belgian astronomer Georges Lemaitre in 1927 independently produced solutions to Einstein’s field equations to show that Universe is actually expanding. 

The planet Earth is one of the eight planets orbiting the Sun. The Sun has curbed out a region of space in the sky where its influence is most dominant and that is called the Heliosphere, as shown in the diagram below. This Sun provides us on this planet, Earth with all the energy we need to live and flourish. The distance between the Sun and the Earth is known as one Astronomical Unit (au) and it is estimated that the gravitational field of the Solar system fades away at about 100,000 au (~1.58 light years).

The Sun may seem overpowering to us and indeed it is, but in the wider perspective the Sun is just an average or below average star in our galaxy, the Milky Way. It is estimated that there are around 300 billion stars, yes, 300,000,000,000 stars in an average galaxy and our galaxy is no more than an average galaxy. In a galaxy there are lots of other celestial bodies such as white dwarfs, neutron stars, supernovae, pulsars (pulsating stars), black holes and many more. Our spiral galaxy, the Milky Way, is about 100,000 light years (ly) across, which means that travelling at the speed of light (300,000 km per second) it will take 100,000 years to go from one end of this galaxy to the other end. One may consider that the speed of light is such that it would go round the Earth seven and half times every second! Our nearest galaxy is Andromeda, which is roughly 2.5 million light years away from us and that galaxy is about 220,000 ly across. It is estimated that there are over 100 billion galaxies in the Universe! So, altogether there would be 30 billion trillion stars (like our Sun) in the Universe (=300 billion stars per galaxy x 100 billion galaxies). The extent of the observable Universe is estimated to be about 93 billion light years across following the Wilkinson Microwave Anisotropy Probe (WMAP)! Now you have a feel of the enormity of the Universe! An image of the Universe is shown below.

WMAP – 2010 image of the observable Universe

In 1915-16 when Albert Einstein produced the general theory of relativity, his field equations predicted that the Universe was expanding. But the prevailing scientific perception (as well as theological dictum) was that the Universe was static and in Steady State. So, he introduced arbitrarily (against the grain of the field equations) in 1917 a quantity called the cosmological constant, Ʌ, with a particular value which would block out the expansion of the Universe. The cosmological constant is the energy density of space or vacuum energy. But in 1929 American astronomer, Edwin Hubble made astronomical observations of distant galaxies that showed red shifts, which was an evidence that the Universe was actually expanding, not static. That red shift was shown to be proportional to the distance of that galaxy from Earth (linear redshift-distance relationship). It did turn the whole of prevailing wisdom on its head and Einstein was left deeply embarrassed. He humbly admitted that the introduction of the cosmological constant was the ‘biggest blunder’ of his life. Without this constraining factor, the equations would naturally lead to predictions of an expanding Universe.

The general theory of relativity produced the spacetime continuum. There is no gravitational force of attraction in the conventional sense. The gravitational field is the space. The gravity creates a curvature in space, more like a heavy body when placed in a trampoline would create a dent, which other lighter bodies would roll down in particular trajectories and that is the analogy of gravitational attraction. Within about two months of publication of the general theory of relativity, the German physicist Karl Schwarzschild provided the proof of existence of gravitational sinkholes, now called the black holes, in the Universe. By solving the field equations, he produced a radius, now called the Schwarzschild radius, that defines the boundary of a black hole. A black hole curves the space towards itself so sharply that nothing, not even the light, can escape it once it is within the grip of the black hole and that is why this body is termed as the black hole.

As mentioned above, Universe is truly unimaginably large. The visible part of the Universe contains celestial bodies that are made up of ordinary baryonic matter such as protons and neutrons, and non-baryonic matter such as electrons, neutrinos etc. For each one of these ordinary matters, there are corresponding anti-matters. For example, there are anti-protons, anti-neutrons, anti-electrons etc. The sinister attribute of these ordinary matters and anti-matters is that when they happen to come in contact with each other, they annihilate each other in a flash and an equivalent amount of energy is created as per Einstein’s mass-energy equivalence equation. Since we are in this ordinary world, there may be anti-world somewhere, made up of anti-matter. But we must never meet each other. If we do, we will end up in a flash into an enormous bundle of energy – creating billions and trillions of times more energy than the Sun.  

In our visible Universe containing billions of galaxies and each galaxy containing billions of stars, it is estimated that there are also a large number of black holes hidden in each galaxy. Black holes exert tremendous amount of gravitational pull to keep billions of stars within the galaxy together. But there is a physical dilemma. If black holes are situated nearer the central core of the galaxy where most of the turbulent celestial activities are taking place, then what is keeping the outlying stars in place where the gravitational pull is much weaker? Still, it had been found that even the remotest of the stars have the same orbital motion as the ones nearer the centre. How do those stars get sufficient gravitational pull to have same orbital motions? To resolve this dilemma, astrophysicists and cosmologists came up with the solution that there must be large amounts of unseen matter dotted all over the galaxies which exert gravitational pull to the stars to have similar orbital motion! This unseen matter is called the dark matter.

There are similarities and dissimilarities between ordinary matter and dark matter. Whereas ordinary matter interacts with light, or generally speaking with electromagnetic energies, dark matter does not. Light goes straight through the dark matter. But it has gravitational pull exactly like the ordinary matter. Although unseen by modern scientific devices, dark matter can be detected by its gravitational fingerprint. Dark matter keeps the fabric of the galaxy intact.

What is this dark matter and what are their constituents, the modern physics has no clue. It cannot be made up of baryonic matters, meaning ordinary protons and neutrons. If they were, they would react to light energy, but they do not. It is speculated that it could be made up of esoteric constituents such as axions, Weakly Interacting Massive Particles (WIMPs), Gravitationally Interacting Massive Particles (GIMPs), supersymmetric particles etc. These are pure speculations. Also, as dark matter and dark energy together comprise 95.5 per cent of Universe’ all mass-energy composition, they may be coupled or tangled quantum mechanically! 

The next article will deal with the dark energy and why dark energy is needed to have the expansion and accelerated expansion of the Universe that is taking place at the moment. In fact, without the dark energy the Universe might have collapsed under its own gravitational pull or might not even have come into existence in the first place.

Dr A Rahman MSRP CRadP FNucI