The ingredients for a sponge cake couldn’t be any simpler: an equal proportion of flour, butter, sugar and eggs. This usually equates to 4 large eggs per 250g of flour. Mix it all up and bake for 20 min at 170 °C (fan oven) for a humble yet scrumptious pudding to be enjoyed with an afternoon tea.
The ingredients for the Universe, however, are more interesting. You’ll need 1 part ordinary matter, 5.5 parts dark matter and 14 parts dark energy. The proportion of ingredients can vary slightly but that is generally how much you will need to ‘bake’ the current Universe – that is, if you can find a hot enough furnace. I say current because the proportion has changed over the lifetime of the Universe. During the early period of its existence, the Universe didn’t contain any dark energy.
Given the preponderance of dark energy and the abundance of dark matter, how come that the two main ingredients of the Universe are still two of the biggest mysteries in cosmology? What is dark energy and dark matter?
Let’s start with ordinary matter. By ordinary I mean everything we know of: cakes, cars and mountains, every life form from viruses to acorns and whales, and the planets, stars and galaxies. Essentially, ordinary matter is anything that is composed of atoms and whose physical properties, like size, mass and configuration that are explained by the known laws of physics. It is fair to say that we have a good understanding of atoms.
Yet something about the Universe in the way it behaves does not quite tally with our theories about matter. It was as though having mixed 250g portions of flour, butter, sugar and eggs we end up with a sponge cake with a mass of more than 1000g. Where does the extra mass come from? Similarly, we have calculated the mass of galaxies based on what we know they are composed of (i.e. stars and dust) and somehow they appear to be more massive because of their motion and the gravitational effects they exert on other neighbouring galaxies. The stars and light-emitting dust are things we can clearly observe. There must be some other invisible stuff in the galaxies that contribute to the total mass.
What the invisible yet massive stuff really is no one knows but speculations abound. It has been a good 80 years since we have postulated its existence but dark matter remains a mystery. We know, at the very least, that it does not interact with electromagnetic radiation like ordinary matter does. The reason we can observe ordinary matter is simply because we can see them either with our eyes (which detect light, a form of electromagnetic radiation) or other instruments (which detect other forms of electromagnetic radiation such as X-rays and infrared.) Dark matter is aptly named – it is dark in all sense of the word. The quest for the nature of dark matter is a perfect example of how our study of the big (cosmology) overlaps with that of the small (particle physics). If not protons and electrons, then what is this elusive particle that dark matter is composed of?
Even if we do solve the problem of the missing mass, we would still have a remaining 68% or so to explain. The 68% represents the amount of dark energy the Universe is made up of. Whereas dark matter has to do with explaining the unobservable mass in the Universe, dark energy has to do with explaining the accelerating expansion of the Universe.
About 14 billion years ago, the Universe sprouted into existence from the Big Bang and it has been expanding ever since. Now, given that there is matter in the Universe, the gravitational attraction should gradually slow down the expansion. However, from our observations of exploded stars and far-flung galaxies, it appears that the Universe was expanding at a slower rate compared to today. We’ve inferred that the rate of expansion has therefore been increasing but what exactly is causing this is not clear. It’s a form of energy that is capable of overcoming gravity. Again, there are theories that attempt to explain the nature of dark energy but they remain hypothetical.
Why, then, should we bother with such uncertainties about the Universe? The answer is simple: our quest for the nature of life, the Universe and everything has a direct impact on how we better our lives here on planet Earth. The advancement of science bears its fruits in the type of technologies we invent and the wonders we discover about the natural world. It is our understanding of electricity that allowed us to come up with the very oven in which we bake the cakes we are so fond of. On this note, it’s time to put the kettle on…