In 1998, astronomers found that the expansion of the universe was not slowing down due to
gravitational attraction as expected, but something astonishing was happening. The
expansion was actually getting faster and faster. It was accelerating.
Something must be powering this acceleration of the universe.
But since we don’t know the source of this energy, it is called Dark energy.
It’s one of the most mysterious aspects of the nature, and appears to be the most
dominant energy form in our universe making up more than two thirds, or 68.5% of all the
energy in it. The rest is regular matter that we can see and dark matter that we can’t see.
Now, dark energy should not be confused with dark matter. They are both labeled dark because
we don’t know what they are. But these are two different things. Dark matter does not absorb,
reflect or emit any light, so it is undetected. It is inferred to exist only because of its
gravitational effect on galaxies and galaxy clusters. We know that it’s not just energy
but matter in the form of particles, or at least that is what the best theories propose.
Dark energy, on the other hand, appears to be some kind of energy that is powering the
accelerating expansion of the universe. But wait, is it possible that dark energy could
also be due to some kind of particle? And could this particle be related
to the Higgs boson and dark matter? That fascinating story is coming up right now…
Before we get started, I want to give a big shoutout to MagellanTV without whom
videos like this couldn’t have been made. One fascinating documentary that made me
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thank you for that. Now back to the show. The story of dark energy goes back to
none other than Albert Einstein trying to solve his new field equations of general
relativity which he presented in 1915. His solution, published in 1917, required
a certain addition to the field equation. The infamous cosmological constant, Lambda.
The reason he added this term is because in those days, it was reasonable to think that the universe
was static and finite. It was thought that the universe had a specific size which did not change.
It was the solution for a static universe. Without this lambda term,
the equations modeled a dynamic universe which was either expanding or collapsing, but not staying static. The constant
would cancel out any expansion or collapse and show that the universe was static. But in the 1920s, American astronomer Edwin
Hubble showed that most galaxies were moving away from our own Milky Way galaxy,
and the farther away they were, the faster they were receding. The universe had to be expanding.
Although Einstein initially did not believe the universe could be expanding, he eventually
admitted defeat 1931, and said that adding lambda to his equation was his “greatest blunder.”
But it turns out that even when Einstein thought he was wrong,
he was more or less, right… about lambda! In an astonishing turn of events due to the
discovery of the accelerating universe, it turned out that inserting lambda back
into Einstein’s equation, could account for the accelerating expansion of the universe.
The positive value for lambda just means that it works as a repulsive force, a bit
like anti-gravity. We can illustrate the force of such a potential energy with gravity and the
cosmological constant like this, F=-GM/R^2 +Λ/3 R
Where F is the overall force, G is Newton’s gravitational constant, M is the mass of the
object, Lambda is the cosmological constant, and R can be understood as distance from another object.
In this form, the negative sign of the gravitational potential
means attraction, and the plus sign on the cosmological term means repulsion.
If the first term is larger, gravity dominates and objects would tend to clump up.
If the second term is larger, repulsion would dominate and objects would tend to fly apart.
It’s important to note that for gravity, the potential gets weaker and weaker at
longer and longer distances because the R term is in the denominator,
and it’s squared. But note that for the lambda expansion portion,
the R term is in the numerator and is not squared. This means that things nearby will be attracted
to each other to form solar systems and galaxies. But over very large distances,
the gravitational term rapidly diminishes due to R squared getting larger. And because the R
is in the numerator for the cosmological constant term, expansion forces dominate, and things repel at large distances.
So what we see is in the universe is that on smaller distance scales such as, for example
within our solar system, or even within our Milky Way galaxy, things continue to stick together. But
stretched out over very large scales, millions and billions of light years, galaxies in general
move away from each other. This is because the cosmological constant dominates at these scales.
Today the most widely accepted model in cosmology has this cosmological constant,
but unlike the days of Einstein, where he put it in to make the universe static, we now put
it in to make it expand at an accelerated rate. And today that cosmological constant represents
dark energy. So this constant is thus, a possible candidate for dark energy.
But this is just a number, a mathematical solution to our observations. What does it actually mean?
Physically it can be thought of as the energy of empty space, also known as the vacuum energy.
Quantum mechanics shows that empty space contains energy.
Although the cosmological constant solution is a simple solution to the problem,
it’s something we have to add in by hand. We calculate what the number needs to be, based
on the observed rate of expansion of the universe, and then insert it into Einstein’s equation so
that it fits with our data. This hand calculated value of lambda is about 2 x 10^-35 s-2 .
But it’s important to note that this value is nowhere near the theoretical prediction of what
the energy of the vacuum should be, based on the ground state energy of all the quantum fields.
The theoretical value is about 10^120 times larger. This has been called the worst
prediction in the history of physics. So when it comes to lambda, something is not adding up
between our observations and our theories. Nevertheless, the cosmological constant is
the simplest solution to dark energy and works well…as long as its value constant.
But is it really constant? Some of the newest and most advanced
observations seem to indicate that it might not be a constant. If that's true, this would lead to a
solution with dynamical dark energy, or a theory where dark energy changes over time and space.
In this were the case, It could be evidence in favor of viewing dark energy as a field,
not as a constant of nature. This is because fields can take on different values in space
and time rather than staying constant. We could then imagine dark energy as
a scalar field with some potential. This is in some ways is like how the
Higgs field works. The Higgs field is also a scalar field, and it has a non zero potential.
The dark energy field, on the other hand, may or may not have a non-zero potential.
In the context of dark energy such a field is more commonly known as a quintessence field.
Now anytime you have a fields, you have particles. This is because excess energy
in a quantum field, or excitation in the field is what we observe as particles.
So this would point to a particle solution for dark energy. These particles must then interact
with matter particles of the standard model to act as a kind of an anti-gravity force.
This force would be like electromagnetism but instead of causing repulsion and
attraction with charged particles, it would only impart repulsion and to
all matter particles. This called the “quintessence” model of dark energy.
The term quintessence comes from the ancient Greeks who believed that the universe was made
of 5 elements, earth, air, water, fire, and a fifth element that all the objects
in the heavens were made of. Latin scholars later named this fifth element quintessence.
Today, this term in physics used to describe the hypothetical dark energy field.
Now the question we have to ask is, if such a quintessence field exists,
And its effects are so profound,
why didn’t we detect it? Well, it could be that
the interaction between the standard model particles and the dark energy field is so weak,
that we simply do not have good experiments, or sensitive enough instruments to detect it.
But it could also be that such a field does not exist.
A particle solution like quintessence would likely affect many things because particles interact with
the stuff of the universe. This has an impact. These interactions should be detectible.
Furthermore, depending on the nature of the field, it could possibly also be part of a
solution explaining dark matter in addition to dark energy. Since quintessence fields
give rise to particles, such a solution may show a connection between the dark matter particles as well as dark energy.
But we should still remember that theoretical dark matter particles have a different effect
in the universe than theoretical quintessence particles.
There are stark differences such as
Dark matter appears to be present in clumps in the universe, whereas dark
energy is smoothly distributed. As the universe expands,
dark matter gets less dense, but the density of dark energy stays constant.
Dark matter has an attractive effect to slow the expansion of the universe,
whereas dark energy has a repulsive effect accelerating the expansion of the universe.
Having said that, it is possible that both such particles are part of the same quantum
field theory, connected perhaps at a higher level in as yet an undiscovered grand unified theory.
If they are related, that relationship is hidden from our current understanding of
the universe, and is probably quite subtle. An interesting consequence of a dynamical
dark energy solution like quintessence is that the repulsion could get stronger and
stronger as the universe expands, meaning the universe would have a runaway acceleration.
It would become so strong, that it would eventually rip everything apart.
First, all galaxies would move away from each other, so that it would appear as if
our Milky Way galaxy is the only galaxy in the universe, because we would not be able
to see the light of any other galaxy. Then our galaxy would be ripped apart.
All the stars including our sun would then be ripped apart.
Eventually the repulsion would become so strong that cells and later molecules,
then atoms would be ripped apart until everything in the universe becomes
nothing but fundamental particles. This is called the "Big Rip." And of course in such a
universe, no life could likely exist. Finally, spacetime itself would rip
apart and all distances in the universe will be infinite. This might be scary,
but we can’t rule out the possibility. Fortunately for us, this would happen
hundreds of billions of years in the future, so no need to lose sleep over this tonight.
And if you have a question about this or any of other subject in my prior videos, please leave
them in the comment section. I’ll do my best to answer it. And if you enjoy my videos, please
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