In our last three episodes, I’ve spoken about dark mysteries of astronomy, but astronomy
isn’t the only field of science with puzzling and unanswered questions. In fact, physics
has presented the world’s researchers with a couple of doozies. That sounds like a great
new path of fascinating things to talk about, in Subatomic Stories.
Of all of the amazing scientific minds in the pantheon of modern physics, Einstein stands
out as what the ancient Romans would have called “primus inter pares,” first among
equals. His ideas are probably the most influential over the last century and a half or so.
Two of the things Einstein is known for is his equation E equals m c squared, and the
theory of the Big Bang, which is tightly coupled to his theory of general relativity. Much
has been said about just how right these two ideas are, including by me in many episodes
of this series. However, there is a deep and dark secret about
the two of them. When you combine them and then compare the resultant prediction to measurement,
it’s totally and completely wrong. It’s really quite embarrassing when you get down
to it. Let me explain.
So first is the equation E equals m c squared. To start out, it’s an equation, with a right
side and a left side, connected by an equals sign. The c-squared is just a constant number
so, ignoring that, the equation says that energy equals mass and vice versa.
People routinely say that this means that energy can be converted into matter and back
again, and that’s true to a degree, but only partly so. Actually, when you convert
energy, you convert it into equal amounts of matter and antimatter. Convert a photon,
and you make an electron and antimatter electron, or quark and antiquark. That’s how it works,
and it’s true for all kinds of energy. It’s also true that the two are always equal.
The matter and antimatter are in identical amounts. That’s an experimental fact that
scientists have confirmed again and again. Equal. Remember that. So that’s the first
thing.
The second thing involves the Big Bang. The universe was once much smaller and hotter
and it was full of energy. As the universe expanded and cooled, that energy transformed
into the matter we see around us today. And that’s a good thing. If that didn’t happen,
we wouldn’t be here.
But now we get back to that equal thing. According to all observation in all of our accelerator
experiments, energy should have made matter and antimatter in exactly the same quantities.
Our universe should consist of equal amounts of matter and antimatter. And yet, when we
look around us at the stars and galaxies that make up the cosmos, we see only matter.
So where is the antimatter? Well, of course, the short answer is that
we don’t know. That’s the mystery. But here’s what we do know.
Some of you might ask, how we know that there is no antimatter in the universe. After all,
antimatter emits the same kind of light as ordinary matter. Maybe there are matter and
antimatter galaxies. And that’s a reasonable supposition.
However, galaxies aren’t just stars. They’re also surrounded by hydrogen gas. And, presumably,
antimatter galaxies would be surrounded by antimatter hydrogen. And gas clouds from adjacent
galaxies intermingle. Now when hydrogen and antihydrogen atoms combine, the electron and
antimatter electron annihilate one another. When they do that, they emit two gamma rays
with a single, unique, energy. Astronomers have looked into space and just don’t see
those kind of gamma rays. So, we know that there are no antimatter galaxies. So that’s
not the answer. Another idea proposed by people first hearing
about the problem is maybe our visible universe is made of matter, and the antimatter is relegated
to parts of the universe so far away that we can’t see them. While we can’t disprove
that conjecture, it’s very difficult to imagine how matter and antimatter could be
separated like that. So, this also is highly unlikely.
So, what do we know? Well, we can look at the cosmic microwave background radiation,
which contains the residual photons from the Big Bang. By figuring out how many photons
of energy are contained in the microwave background to the number of protons in the universe,
researchers have determined that something very weird happened in the first moments of
the Big Bang. What they’ve determined is that there was
a very, very, slight asymmetry in the matter and antimatter of the early universe. For
every three billion antimatter particles, there were three billion and one matter particles.
The three billions cancelled, eventually resulting in the cosmic microwave background. The remaining
one turned into the stars and galaxies we see. And, I hope it’s clear that I’m talking
about ratios here and not a single particle. So, what is the physical phenomena that generated
this asymmetry? We don’t know. Over the decades, we’ve found a few odd quantum mechanical
phenomena that generate some of the asymmetry, but not enough to explain what we observe.
Like I said before, it’s a mystery. And it’s one of the biggest mysteries of modern
physics. This is one of the reasons that Fermilab is
putting such an effort into building the Deep Underground Neutrino Experiment or DUNE. One
of its goals is to look for differences in matter and antimatter neutrinos. If we see
differences, then maybe a process called leptogenesis is the cause of the asymmetry. Leptogenesis
is a super complicated idea and I made a long form video about it which is worth your time.
A link is in the description. Leptogenesis might not be the right explanation
for the matter/antimatter asymmetry. But we won’t know until we test it. So that’s
what we’re doing. Okay, let’s take a look at the questions
our viewers have posted this week. Talking about the matter/antimatter asymmetry
was fun, but questions are funner. So let’s get started. Jacek Piterow notes that the
expansion speed of the universe has changed and asks if it will again. Hi Jacek. That’s
a good question. Obviously, since we don’t understand everything, the answer to your
question could be yes. After all, the laws of physics could spontaneously change tomorrow.
It seems unlikely, but I’m in the business of finding the unexpected.
But the changing speed of the expansion of the universe wasn’t some weird and arbitrary
change. It has a clear explanation that should calm any fears of a spontaneous changing of
physical laws. It comes down to a couple of things that are woven together. First, gravity
by matter – ordinary or dark – is attractive. Second, gravity by dark energy is repulsive.
Third, the density of dark energy is constant. And four, the universe is expanding. Those
are the four important points.
Shortly after the Big Bang, there was a fixed amount of total regular and dark matter in
the universe. Dark energy was the same constant density it is now. The visible universe had
a smaller volume. Multiplying density by volume gives total dark energy which is, of course,
small due to the small volume of the universe. But, remember, the universe was expanding.
The total amount of matter was constant, but the dark energy was increasing.
At about five billion years ago, the amount of dark energy equaled the amount of matter.
And, of course, over the last five billion years, the volume has increased more, leading
to more dark energy. Since dark energy is a repulsive form of gravity, it was five billion
years ago that repulsion overcame attraction and the expansion of the universe sped up.
We see that five billion years ago wasn’t a time when dark energy “turned on,” so
to speak. It just became dominant then.
Sapele Steve asks how I can say that there are galaxies without dark matter if we don’t
know that dark matter even exists. Hi Steve. I suppose you’re right. I should have said
that these galaxies rotate at speeds expected for the known laws of physics and observed
ordinary matter. It’s more accurate to say that these galaxies don’t need any dark
matter to explain their behavior. But that’s kind of the same as saying they don’t have
dark matter, so..... Beppi Menozzi asks if the cosmological constant
could be slowly changing. Hi Beppi. Sure. That idea is called quintessence. And what
you suggest is possible. However, we’ve looked, and the data is consistent with dark
energy being a constant density. If it’s changing, it’s changing very slowly. Improved
measurements in the next few years will tell us more.
Einstein X admonishes me for having a misleading animation of the Big Bang. Hi Einstein. Any
relationship to >>the<< Einstein? Yep. You’re right. I don’t like that animation either.
It conveys the idea that the Big Bang was like a firecracker and that’s wrong. On
the other hand, a more accurate animation would just flash the entire screen to white
and then fade to expanding galaxies and such. It’s more accurate, but not as visually
interesting. Plus buying animations is expensive. If viewers know of a better one, post the
link below and I’ll see what I can do. OP N Namdev asks how constant energy density
leads to repulsive gravity. Hi OP. That’s simple. It doesn’t. It’s just that dark
energy exerts the opposite type of pressure compared to ordinary matter. If you’re interested
in a deeper explanation, I posted a link to a blog post by Sean Carroll in the description
that goes into it in more detail.
Scott Powers asks how I know that dark matter isn’t just gas surrounding and within galaxies.
Hi Scott. Luckily, that’s an easy one. While gas is invisible to our eyes, it’s not to
other wavelengths of light, be it infrared, radio, ultraviolet, etc. And we have telescopes
that can see all of those wavelengths and more. We know precisely how much gas is out
there, and it’s ten times more than the stars we see. Indeed, gas is the dominant
form of ordinary matter.
Amazingly, if you removed the stars from the universe, it would change the amount of ordinary
matter by only ten percent or so. But dark matter is five times more than the gas we
know is out there. So your question is reasonable, but we know that gas isn’t the answer.
Line Noise asks if the universe is expanding, why are not galaxies and the Earth and such
expanding as well? Hi Line. Your question is a common and reasonable one. The answer
is simple. Dark energy is low density and works only if big volumes are involved. On
small volumes like the solar system, the Milky Way, or even the Virgo cluster of galaxies,
ordinary matter is concentrated and the attractive force of ordinary gravity is strong enough
to overcome dark energy. I put a link in the description for additional reading.
Samuel Machado asks me if I can extract dark energy and use it for something. Hi Samuel.
Well, I can’t. But then I have sworn to use my powers only for good. But someone like
this guy can. I wouldn’t cross him if I were you.
Okay, so that’s all the time we have for questions today. I’m having a blast doing
this. I hope you are too. If you are, please like, subscribe and share. This series is
great fun. It allows me to talk about all sorts of interesting bits of physics from
my home. And that’s a great way to spend my time because, even at home, physics is
everything…