Dark Matter and Dark Energy | Nobel Prize 2019 (Physics)

Dark Matter and Dark Energy | Nobel Prize 2019 (Physics)

On October 8th 2019, the Royal Swedish Academy
of Sciences has decided to award the Nobel Prize in Physics “for contributions to our understanding
of the evolution of the universe and Earth’s place in the cosmos” with one half to James Peebles “for theoretical discoveries in physical
cosmology” and the other half jointly to Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting
a solar-type star” This was the announcement of the Nobel Prize
in physics of 2019. In this video, we will explore what kind of
theoretical discoveries were made and what an exoplanet orbiting a solar-type star tells
us about our universe. Part 1, James Peebles. Since 1916, Albert Einstein’s general theory
of relativity is the basis for all calculations concerning our universe. But when he solved his own equations, he wasn’t
so sure whether they were right. His so-called field equations led to the conclusion
that our universe is expanding! Since this contradicted the common knowledge
at that time, he manually added a constant term to his equations, the famous cosmological
constant, which acted as a counterbalance. After solving the “new” equations, the conclusion
was that the universe stood still. Over 10 years later, experimental observations
showed that, indeed, our universe is expanding! So there was no need for his counter-acting
constant anymore and it was discarded. Einstein called this his “biggest blunder”. Little did he know that his constant would
return over half a century later… But first, let’s talk about dark stuff. Dark matter and dark energy. First off, why are they called “dark”? Well, that’s because we cannot see them. The way our vision works is that photons interact
with the things around us and they reflect them into our eyes. This makes us “see” the objects. However, if there is no, or almost no interaction
with photons, none of them can be reflected into our eyes and therefore we don’t see anything. That’s the thing with dark matter and dark
energy. We know that they are there, but no-one has
ever seen them. The difference between dark matter and dark
energy is: one pulls, one pushes. In the 1930’s, experiments measured the rotational
speed of some far away galaxies. From this speed we can calculate the mass
of the galaxy, but after adding the masses of all orbiting objects, the result was too
small! The conclusion was that there had to be some
additional mass that holds the galaxy together, but for some reason, we cannot see it. That’s dark matter. We now estimate that 26% of our universe is
dark matter and in 1982, James Peebles was one of the physicists who proposed a mechanism
how we could explain this situation, called cold dark matter. In order to understand dark energy, we have
to talk a little about curvature. Einstein’s general theory of relativity tells
us that mass and energy have an effect on how our space is curved. There are basically three possibilities for
this curvature. One is that the universe is flat. This happens when the amount of mass and energy
in our universe is just right, at a very precise, so-called critical, value. In such a universe, two parallel lines would
go on forever, without touching. The other option, when we have too much matter
and energy, results in a so-called closed universe, where two parallel lines will eventually
meet somewhere. An extreme example of this would be the surface
of a sphere. Even if you start with two parallel lines
somewhere, they will eventually meet at the top of the sphere. The final option, when we have too little
mass and energy, is called open universe, where two parallel lines will eventually drift
further and further apart. Well, experimental measurements as well as
theoretical considerations give us a clear answer: the universe is flat. This means that we apparently have just the
right amount of matter and energy in our universe. However, when we look at our universe, even
if we count the stuff we cannot see like dark matter, we only reach 31% of this critical
value. This means 69% are missing! Here again, James Peebles was one of the leading
scientists who proposed a solution in 1984: re-introducing Einstein’s cosmological constant,
which was now seen as the energy of free space, energy of the vacuum. This, together with ordinary matter and cold
dark matter is enough to support the calculations of a flat universe. However, this was still just a theory on where
the missing 69% of our universe were. In 1998, experiments showed that our universe
is not only growing, but doing so in an accelerated manner. Calculations showed that ordinary matter was
not able to cause this, so something else was responsible for pushing our universe apart. By assuming that dark energy is responsible
for the accelerated expansion of our universe, it quickly goes from just-a-theory to something
that could be investigated in experiments. Part 2, Michel Mayor and Didier Queloz. On October 6th 1995, Mayor and Queloz announced
that they had found a planet orbiting a star similar to our sun. 51 Pegasi b orbits its star 51 Pegasi, which
is around 50 light years away from us, with an orbital period of about four days. This implies that it moves very closely to
its star, only around eight million kilometers, which leads to a surface temperature of more
than 1000°C. For comparison: we orbit our sun at a distance
of about 150 million kilometers. The surprise regarding 51 Pegasi b was that
it is very large. Like Jupiter, which has a volume of around
1300 times the Earth’s volume and is 300 times as heavy. According to previous knowledge how solar
systems are formed, a planet like Jupiter should form very far away from its orbiting
star and thus take a long time to orbit. For instance, Jupiter takes almost 12 years
to orbit our sun. But 51 Pegasi b, which can be compared to
Jupiter, orbits its sun in just 4 days! This means that the previous knowledge of
how solar systems form had to be revised. So how exactly did they find the new planet?
Planets don’t send out light, so it’s almost impossible to see them directly. Therefore Mayor and Queloz moved to an indirect
method, using the Doppler effect. Similar to a car that moves towards you, or
away from you, produces different sounds, so can the wavelength of light coming from
a moving source stretch or compress. But why does the sun move? If we just consider one star and one planet
for simplicity, it’s easy to see why the star should not be at a fixed place. Gravity pulls the planet towards the sun,
but also the sun towards the planet. However, since the star is usually much heavier,
the resulting effect is much smaller. Still, the star will also move. The best way to illustrate this situation
is using the center of mass frame. If we keep the center of mass (which is NOT
the center of the star) fixed, both the star and the planet move around it. And that’s why light coming from this star
undergoes the influence of the Doppler effect. 2019, the year of cosmology. First, James Peebles, helping us understand
that the stuff around us is actually more that we can actually see, and second, Michel
Mayor and Didier Queloz, turning our previous knowledge about the formation of planetary
systems on its head. And that’s pretty much it for this video,
thanks for watching!


    Wonderful job. Thank you for bringing quality content to the community.
    P/s: I was wondering how did you draw all the things? Was it with an iPad? I'm doing online tutor and really inspired by the way you are doing, looking awesome!

    Great presentation mate !
    If you do any sort of follow ups, I'd love to hear a bit about the latest theories in solar system formation.

    So if there is a geodesic deviation then how is the universe not open? Why say there is dark energy at all and that the universe is flat if you could just say that the universe is not flat?

Leave a Reply

Your email address will not be published. Required fields are marked *