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 ago||Present day (2000 CE)|
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