At the beginning of the 20th century, many scientists started thinking that Physics was pretty much over. Surely we have a few unsolved mysteries left but it seemed like they could all be ironed out with better measurements or maybe with very slight tweaks to what was already known.
The problem was, those mysteries didn't go away with better measurements and slight tweaks. They led to fundamental revolutions in our understanding of nature. Huge, important things, like Relativity and Quantum Mechanics.
Now that we have discovered those things, though, It might sometimes feel like there are, once again, just a few small problems left for physicists to solve before we can say we know everything about how the universe works.
Here are 5 of those problems that are actually a really big deal and are not going to go down without a fight:
1.Neutrino Mass
2.Matter-Antimatter Asymmetry
3.Dark Matter
4.Lithium-7
5.Axis Of Evil
1.Neutrino Mass
Neutrinos are tiny sub-atomic particles. There are TRILLIONS of them flying through you every second, but they hardly ever hit one of your atoms. Like, even a 14,000 Metric Ton neutrino detector will only detect a few neutrinos a day.
As strange as that might be, Physicists mostly understand why Neutrinos don't often interact with ordinary matter. What they don't understand, is why Neutrinos have mass or why that mass is so small?
Particle Physicists use the standard model which uses maths to describe how every known particle interacts with every other known particle. It's one of the most successful models in history. The Standard Model correctly predicts the results of literally trillions of experiments.
Now the problem is, the Standard model also predicts that Neutrinos shouldn't have any mass. But in the 1990's, Physicists studying Neutrinos coming from the sun realized that Neutrinos had to have mass.
There are a few different kinds of Neutrinos and the researchers found that the Neutrinos coming from the sun were switching types. But they would need mass to be able to do that switching which means that the standard model, has a pretty big hole in it.
Now, there is a way of changing the equation used in the Standard model so that it includes Neutrinos with mass. But on its own, the fact that Neutrinos have mass doesn't necessarily have to be a dealbreaker. But Neutrinos masses are also incredibly tiny compared to every other fundamental particle out there. Electrons are the next lightest particles we have found0 and they are still somewhere between 126,000 and 600 million times heavier than the lightest Neutrinos.
That huge gap makes a lot of Physicists think that fitting Neutrinos with mass into the current Standard Model, is a little bit like shoving sugar packets under the leg of a wobbling table and saying you fixed it. There are a few other possible explanations out there that also fit with the standard model and so far, we have not found any solid evidence to support them.
Other Physicists think that we need to throw out the Standard model altogether and turn to new models to explain the mysterious mass. Another possible solution to the Neutrino mass problem could help solve a second mystery.
2. Matter-Antimatter Asymmetry
Why is there so much matter in the universe? See, Matter has a sort of twin called Antimatter. Antimatter particles are just like regular matter particles, except they have the opposite charge. So regular matter has electrons, for example, which have a negative charge. But antimatter has what are called positrons, which are just like electrons except with a positive charge and whenever a particle of matter meets its corresponding particle of antimatter, they annihilate each other in a big explosion.
The problem is, Matter and Anti-matter act the same in a lot of ways, as long as they are kept separate from each other. Like, when we do experiments in particle accelerators and produce particles of matter, we produce particles of antimatter too. Antimatter can even make atoms, just like normal matter can.
The laws of Physics just don't seem to prefer one over the other. But when we look out into the universe, all we see is ordinary matter, like the stuff down here on Earth, there are no Antimatter stars, no Antimatter galaxies and no Antimatter dust clouds. If there were, they would occasionally run into similar pieces of matter, and they would annihilate each other in a big flash. But we don't see those flashes.
But why didn't the universe start out with equal amounts of matter and antimatter that then annihilated each other, with nothing left over?
There are a lot of possibilities and some of them have to do with our old friends, Neutrinos: You remember how Neutrinos are so weirdly light? If there are also incredibly heavy Neutrinos, they would balance out the light Neutrinos by creating a whole family that kind of averages out at a more reasonable mass.
These heavy neutrinos would have been around just after the Big Bang when they would have decayed into smaller lighter particles and in the process produced slightly more matter particles than antimatter particles.
So, if heavy neutrinos did actually exist that could help solve two mysteries at once. First, it might explain why neutrinos have such tiny masses. Second, it would explain why this matter all over the universe instead of antimatter. It would be such a nice elegant solution. The only problem is none of our experiments has found evidence for it.
3.Dark Matter
Let's zoom way out now from subatomic particles to the whole galaxy. Since gravity comes from mass astronomers can use the amount of matter they detect in the galaxy to calculate how strong its gravity should be. But they have known for almost a century that they must be missing something.
Stars orbit the centre of galaxies so fast that the galaxies calculated gravity should not be strong enough to hold onto these. Stars should escape into intergalactic space, but they don't. There must be some extra source of gravity out there holding galaxies together.
Astronomers call this source dark matter and unlike antimatter, we have no idea what dark matter is made of. All they really know is that dark matter interacts with regular matter through the gravitational force and it is invisible to telescopes. Also, it makes up about 85% of the matter in the universe.
Now there is a much simpler possibility, what if astronomers are just wrong about the laws of gravity. Maybe if they found the right laws they could explain everything without needing dark matter.
But dark matter just explains too many things too well from the way that galaxies are distributed in a large-scale to the way that matter clump together just after the Big Bang. Plus astronomers have actually found pockets of dark matter that are completely separated from any visible matter. In other words, they have seen gravitational effects that should be caused by matter in places where there is no detectable matter even changing the laws of gravity would not explain that so dark matter definitely exists we just don't know what it is?
But we do know what it is not. For example, lots of people used to think that dark matter was probably just a lot of really dim ordinary matter, like small, failed stars called brown dwarfs or even neutrino. But experiments have ruled out a lot of those sorts of options.
There are still plenty of other ideas out there waiting in the wings for upcoming experiments. But for now, 85% of the matter of the universe remains completely unexplained.
4.Lithium-7
There's also something weird about matter itself. Starting about a second after the Big Bang and lasting for about three minutes protons and neutrons came together in the first-ever atomic nuclei.
Physicists can use what they know about particle physics in the early universe to predict how much of each element should have formed this way. Hydrogen, for example, has just a single proton in its nucleus and because it is so simple, about 70% of the atoms in the universe should be hydrogen and that's exactly what astronomers see when they look at old stars.
That same model also predicts that protons and neutrons should have come together to form helium about 27% of the time. So, 27 percent of the atoms should have been helium. Again exactly what astronomers see when they check and just about every element they look at matches in the same way and then there is Lithium.
One form of lithium called lithium-7 has three protons and four neutron and astronomers see four times less of it than the model predicted. This huge difference makes them think there must be something wrong with either with the model or the measurements or both.
Astronomers make a few assumptions about the early universe in order to predict how much of each element was produced. Then, to measure how much of each of those elements is actually out there, they use the light from stars where again, they have to make some assumptions about things like the star's temperature and stability. They could try to change some of those assumptions to fit lithium, but there's a problem these assumptions work so well for the other elements that tweaking them to fit lithium screws everything else up.
So, a lot of physicists think the lithium problem means that there is some part of physics that we are missing. Like the idea of supersymmetry, which says that every particle has a kind of twin sibling with a much larger mass and there's another idea that the things we think are constants of nature and basically set in stone aren't actually constant.
If supersymmetry is real that would mean there were more particles in the early universe. And if the things we think are set in stone actually aren't that would change how the particles interacted. So, both could help explain the weird lithium numbers if we ever find evidence for them but so far we haven't found it.
5.Axis Of Evil
The Cosmic Microwave Background or CMB is the oldest light in the universe. It's often represented as a pattern of reds and blues which show the different densities of matter that eventually led to big structures like our galaxy. The CMB was a really important discovery in the 1960s, because it helped confirm the Big Bang Theory. But it also hides an axis of evil and yes that's actually what scientists call it the axis of evil.
See researchers expect that matter in the early universe shouldn't have been bunched up too much in any one place or direction. But that's not what they see in the CMB. Instead, they see a kind of split between a more dense half and a less dense half with an axis of evil' between the two. And when they try to divide up the CMB in other more complicated ways than just seeing which half is denser the axis of evil is still there.
At first, astronomers thought there must have been something wrong with the measurements or maybe that there was something like a nearby dust cloud that was messing things up. But they have checked and checked and checked and they can't get rid of this. Axis make things even weirder the axis of evil lines up with the plane of our solar system. We point right at it and that's just bizarre.
Astronomy is guided by something called a Copernican principle which says that there's no reason our place in the universe should be special. But lining up with a cosmological axis that formed billions of years before earth did seems like it puts us in a pretty special place.
Now it's completely possible that there's nothing weird about the alignment at all. There is probably about a 1 in 1,000 chance that the conventional Big Bang model would produce a universe with matter bunched up like it is in the CMB. Those odds aren't too bad and with trillions of planets orbiting trillions of stars throughout the universe someone was bound to line up with the axis of evil.
So, maybe we just got lucky and besides the alignment isn't perfect, it's just surprisingly good. But scientists still want to know why this axis exists and whether there's a reason our solar system lines up with it? Unfortunately, they haven't come up with Much
None of these mysteries will be easy to solve. But there are lots of smart people working on all of them and sometimes even on two or more at once. So, maybe someday soon we will be telling you about the solutions to some of these problems. But in the meantime, they will keep reminding us that there is still a lot we don't know about the universe.
Source: NASA, CERN, Wikipedia, Nature
