By Kumar David –
You will remember that before religion took centre stage people whispered that phenomena they could not explain were the influence of dark forces and evil spirits. These mysterious goblins were hidden in a nether metauniverse, a multiverse of dark energy and dark matter. Religion came along and Christianity and Islam substituted God for the good and the devil (Lucifer, hell) for the dark side. The latter derived power from dark energy, a manifestation of poltergeist like dark matter. If you take astrophysicists at their word it seems we have made little progress in the last five thousand years. Ok, ok this is tongue-in-cheek; some of my best friends are outstanding physicists, but let’s take the micky out of these clever people just for fun this corona dampened festive season. But first everyone: Season’s Greetings and take good care of your Health.
Today’s column is for simple folk not nerds and buffs, but I need to introduce what PhD, FRS, ABCDEF astrophysicists say about the uncertainties at the forefront of their trade. In the spectrum from learned papers to magazine articles, videos and popular TV shows the refrain is the same: “We don’t have answers to the two biggest mysteries”. The two crucial concepts about which they frankly declare themselves clueless have names reminiscent of medieval witchcraft; ‘dark matter’ (DM) and ‘dark energy’ (DE). Surely these great men of science were awake to the esoteric associations when they zeroed in on these labels.
Dark Matter (DM)
The way cosmologists say it, the stuff we see and all the world’s optical and radio telescopes explore in the night sky make up less than 5% of the matter and energy in the universe. All the visible stars and galaxies to the outer bounds of the universe 13.7 billion lightyears away make up less than one-twentieth of the universe. What the devil is the rest? “We don’t know!” they declare. That’s not the worst, most of them also say science intrinsically will never be able find out! It’s a fine how-do-you-do isn’t it when genius science tells you “The truth is unknowable”! I am not in the field but my innate sense of scientific epistemology is averse to this.
Two precise observations have brought cosmology to the conclusion that DM exists. When one examines the structure of a galaxy (Milky Way, Andromeda, billions more) and takes account of the visible mass each contains, no way can the galaxy not swirl apart. The visible mass is too small to hold its spiral arms in place. Galaxies should display a tendency to fly apart like whirling stones at the end of a broken string because there is not enough gravity to bind it together in perpetuity, but the spiral arms are in no great hurry to do so. A great deal more gravity than is visible is needed to explain why galaxies don’t spin apart. Cosmology has no idea where the extra gravity needed to hold things in place comes from, so by reductio ad absurdum it must be an unobservable poltergeist – call it dark matter.
The second reason is the behaviour of light reaching us from a galaxy faraway in the universe. General Relativity (GR) says that light bends as it passes massive objects, so light from this galaxy will be bent a bit if it passes close to an intermediate galaxy or a massive black hole en route. Light rays passing around all sides of the intermediate galaxy are bent inwards, therefore an earthly observer will see a ring of light termed an Einstein Ring. So far so good. But voila, the observed rings are much bigger than they should be; light is bent more than it should be. The intermediate galaxy seems to contain greater mass than it should from considerations of its visible size. A huge amount of DM entwined within the galaxy is declared to be the culprit.
In ancient days the boom-and-flash of lightning was declared to be the grind of thunderbolts flung by Zeus; in medieval times women were burnt at the stake as witches if they bore an “evil sign”; hence gods and devils. Today DM is convicted for want of a discernible culprit. The analogy goes deeper. DM has never been detected though it is said to penetrate all space and go through everything even your bones and body. It is deemed omnipresent but no lab has detected or isolated it. What is it made of, electrons, protons, leptons, muons or what type of elementary particles? No clue. Humans have yet to overcome the custom of calling anything not yet explained God, angels or transcendent. According to the aforementioned school of physics DM bears a singular resemblance to my grandmother’s conception of spirits in that she says we will never ever corner them. Five millennia of civilisation has done the rounds and decided that the old lady was right after all.
Dark Energy (DE)
DE is in some ways stranger than its accomplice DM. Like DM it too is from precise and reliable observations. There is no dispute about that; the confounding observations are rock solid. This time the mystifying observation is the accelerating expansion of the universe. The universe is expanding like an inflating balloon; its parts moving away from each other. That was expected from non-fudged General Relativity (GR), more on that in a moment. But the crazy thing is that it is expanding at an accelerating rate. Billions of years ago it was not expanding as fast as it is now. DE theory claims to dissolve the paradox. As new space is created the new empty space is born with intrinsic additional energy, DE, it claims. Expansion creates energy in its emptiness, so expansion implies positive feedback. More energy pushes more expansion.
The gravitational attraction of ordinary plus dark matter which make galaxies fall inwards is defeated and the universe is ever more forcibly pushed outwards by DE. A crude analogy is the explosive energy of a detonation which makes things fly away; the release of energy pushes things out. However the explosion analogy does not explain why the bits are flying away faster and faster. Matter and DM slow down expansion by gravitational pull, DE speeds it up. So hypothesising the positive feedback of DE being created all the time in newly emerging space, if you buy the story, explains everything.
Scientists, unlike politicians, medical quacks and witch doctors are honest chaps. They admit it when they are foxed. NASA frankly concedes “Theorists don’t know what the correct explanation is, but they have given it a name. It is called dark energy. More is unknown than is known. We know how much dark energy there is because we know how it affects the universe’s expansion. Other than that, it is a complete mystery”.
That’s enough poking fun at avant grade astrophysics. What are the explanations for all these funny goings on? Let us recall Einstein’s great fudge. In 1915 he derived general relativity (GR) to describe a gravity-dominated universe but the maths displayed a puzzling expanding universe. So he fudged and included an arbitrary factor called the cosmological constant lambda () whose value was carefully contrived to stop the universe of his equations from expanding and stay put. Horror of horrors! In the late 1920s Hubble’s observations proved that the universe wasn’t static, it wasn’t staying put, it was expanding. A sheepish Einstein abandoned lambda calling the episode the “biggest blunder of his life”.
Now comes the next twist. Avant grade cosmology now abruptly wants lambda back! By picking a certain value for lambda it can make the acceleration of expansion in the equations fit the observed acceleration of real expansion. This is a modern day counter-fudge because there is no physical reason why it should have this value or even why lambda should exist at all. This to my mind however is not a problem. Why does the attraction or repulsion between charges depend (inversely) on the square of the distance between them, or why does Newtonian gravitation do the same with the distance between masses? That’s just the way the laws of physics are, the way the physical universe functions; full stop.
A more radical view of some cosmologists is that the time has come, after a century, to go beyond GR and formulate an all-new theory of gravitation. This will be a huge upset because GR has worked to perfection in every domain such as the solar system and in all cosmology except inside a black-hole and this bothersome DM-DE distraction. Some restructuring of GR equations may eliminate the need for DM/DE concepts entirely. And there is no alternative candidate visible that will preserve all that GR has achieved, let alone do more.
A more comic heresy (Alice in Wonderland cosmology) is that there are more universes, maybe millions floating around – the multiverse hypothesis. Poor Steven Hawking’s mathematics dragged him in this counterintuitive direction. Our universe is said to be one among many and the others are pulling and pushing. To keep the figurative description, DM is stuff from parallel universes spilling over into ours and the accelerating expansion of our universe is the outward pull of another universe. This is their substitute for DM and DE. Ah well, I think my grandmother’s version is more charming and convincing.