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At the beginning of the 20th century, many scientists thought that we had learned all there was to know about physics. However the better we got at physics, the more mysteries we realized we had left to solve! Join Michael Aranda for a new episode of SciShow and learn about some of the universe's baffling secrets!
Max Tegmark, Our Mathematical Universe: My Quest for the Ultimate Nature of Reality (
Lisa Randall, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe (pages 12-23) (
Dark Matter and the Dinosaurs pages 16-17,
Dark Matter and the Dinosaurs; Endless Universe: Beyond the Big Bang, by Paul Steinhardt and Neil Turok (
Our Mathematical Universe, pages 59-62

Michael: At the beginning of the 20th century, many scientists started thinking that physics was pretty much over. Surely there were 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've 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 aren't going to go down without a fight.

 Neutrino Masses (0:47)

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 math 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. 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 it's 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've found, and they're 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 wobble 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 haven't 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.

 Matter/Antimatter Asymmetry (2:32)

Another possible solution to the Neutrino mass problem could help solve a second mystery 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're 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'd occasionally run into similar pieces of matter, and they'd annihilate each other in a big flash... But we don't see those flashes.

So 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 exists, that could help solve two mysteries at once: first, it might explain why neutrinos have such tiny masses, and second, it would explain why there's matter all over the universe instead of antimatter. It would be such a nice, elegant solution. The only problem is: none of our experiments have found evidence for it.

 Dark Matter (4:19)

Let's zoom way out now from subatomic particles to whole galaxies. Since gravity comes from mass, astronomers can use the amount matter they detect in the galaxy to calculate how strong its gravity should be. But they've known for almost a century that they must be missing... something.

Stars orbit the centers of galaxies so fast that the galaxy's calculated gravity shouldn't be strong enough to hold onto them. These stars should escape into intergalactic space... but they don't. So 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. Aall they really know is that dark matter interacts with regular matter through the gravitational force, and then it seems invisible to telescopes. Also it makes up about eighty-five percent 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 to 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've seen gravitational effects that should be caused by matter in places where there's no detectable matter. Even changing the laws of gravity wouldn't explain that. So dark matter definitely exists, we just don't know what it is.

But we do know what it isn't. 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 neutrinos, 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, eighty-five percent of the matter of the universe remains completely unexplained.

 Lithium-7 (5:59)

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 nucleum. Physicists can use what they know about particle physics and the early universe to predict how much of each element should have formed this way. Hydrogen, for example, has just a single proton its nucleus, and because it's so simple about seventy percent 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 twenty-seven percent of the time, so twenty-seven percent of the atoms should have been hlium. Again, exactly what astronomers see when they check. And just about every element they look at matches in the same way... and then there's lithium.

One form of lithium called lithium-7 has three protons and for neutrons, and astronomers see four times less of it than the model predicts. This huge difference makes them think there must be something wrong with either 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 stars' temperature and stability. They could try to change some of those assumptions to fit lithium, but there's a problem. These assumptions works 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's some part of physics that we're missing. Like the idea of supersymmetry, which says that every particle has a kind of twin sibling with a much larger mass. Then 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.

 Axis of Evil (7:47)

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 and 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've checked, and checked, and checked, and they can't get rid of this axis.

To 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 the Copernican principle, which says there's no reason our place in the universe should be special. Bbut 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's 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. Sso 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 one.

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 I'll be telling you about the solutions to some of these problems, but in the meantime, they'll keep reminding us that there's still a lot we don't know about the universe.

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