With so many amazing entries this year, I couldn’t possibly let you miss out on some of the brilliant science writing I received. Here is our third runner-up, Neeraj who is 16 years old and from the UK, who has written about how we work out how big the universe really is.
Uncovering the Unseen Universe
Under a clear night sky, you look up towards the heavens and the first thing you do is start counting the stars until you realise there are too many of them, maybe infinite. That’s when you start to think like a scientist and ask questions. How big is the universe? What is it made of? You look up and see stars, galaxies, nebulae, planets and all kinds of mesmerising things the universe has to offer. Many astronomers and astrophysicists do observations of the night sky, taking data of light coming from the galaxies and much more, using big telescopes which can see objects in the sky which are millions of light-years away.
Astronomers observed several galaxies and used the spin of the galaxies to predict the amount of matter that should be present to cause that spin. When the calculations were done, the results shocked astronomers. The galaxies should have had more matter in them than the matter we could see to move as they did. Those galaxies would fly apart, or they would not have formed, or would not move as they do if they did not contain large amounts of unseen matter. Scientists named that unseen matter dark matter.
Well, dark matter isn’t really “dark” because it does not absorb any electromagnetic waves. In fact, what we refer to as dark matter does not even interact with the electromagnetic force. The only reason we see objects is because matter interacts electromagnetically, and photons which are absorbed and released reach our eyes, enabling us to see the matter. This is something which is always there but you can’t see it happening. What surprises scientists is that there is much more dark matter in the universe than normal matter. The ratio of dark matter to ordinary matter is about 6 to 1. The universe is made up of 5% normal matter which is the all of the atoms we can see (basically the whole periodic table), 27% dark matter and 68% dark energy.
There have been many possible candidates for dark matter such as Primordial Black holes (i.e. black holes which were present at the beginning of time), MACHOs (Massive Compact Halo Particles), Axions and WIMPs (Weakly Interacting Massive Particles). Scientists have been experimenting, trying their best to theorise and detect these possible dark matter candidates. Some experiments have come to a dead-end whereas some have yet to find good data. The dead-ends do not mean failure as these will tell us which candidates to rule out so we know where not to look. For example, neutrinos were once thought to be candidates of dark matter but when the research was done, it was found that the detection did not suggest the kind of halo distribution (i.e. region that has decoupled from cosmic expansion and contains gravitationally bound matter) required to do the job of dark matter, and that matter of neutrinos was too hot. This helped to change the standard model of dark matter into “cold” dark matter and one possible candidate was ruled out.
Astronomers did more observations of galaxies and found some observations did not fit the model of dark matter theory. They explained their observations with ‘MOND’ (Modified Newtonian dynamics). This involves a modification to Newton’s inverse square law whereby gravity falls off more slowly at a greater distance. Why would a change to Newton’s law be more appealing? The next generation of galaxy surveys, including the James Webb Telescope, will be able to measure galaxy dynamics to a greater depth, age and accuracy which might resolve this. The uncertainty in the calculations of dark matter is huge. The data might have been wrong. The dark matter mystery still remains unsolved and this means more challenges and opportunities for the younger generation of scientists to face. The are many more discoveries yet to be made which might change Physics. We should never stop exploring.