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.
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