It’s one of the biggest mysteries in science.
It's the elephant in the room that you can't see.
It makes up the majority of the universe - 70%,
the other 30% being ordinary and dark matter.
But we don’t really know what it is. It’s
as if we are boats sailing on an invisible
ocean, trying the figure out what the heck
we are sailing on.
The burning questions we want to ask are:
What is dark energy?
What could be causing it?
Why is it there?
And finally, does it really exist, or could
our measurements be completely wrong?
Ever since 1929, when Edwin Hubble had discovered
that the universe was expanding, scientists
had sought to find out what this expansion
rate was.
Most scientists believed that while the universe
was known to be expanding, this expansion
rate had to be slowing down over time, because
of the attractive effect of gravity pulling
matter together. So scientists set out to
measure this slowing down of the expansion
rate.
This was not easy to do because in order to
make these kinds of measurements, you have
to be able to view the brightness and redshifts
of galaxies that are hundreds of millions
and billions of light years away.
These are object in the sky that we can not
see with the naked eye.
There was no good way to measure this accurately
for most of the 20th century, until very large
telescopes and the Hubble telescopes were
built.
In addition, some kind of measurement technique
was needed to establish a standard luminosity
that these galaxies could be compared to,
because not all galaxies are equally bright
at the same distance. This standard luminosity
is called a standard candle.
In the late 1980’s and early 90’s two groups
of astronomers, one led by Saul Perlmutter
and Adam Riess in the US, and Brian Schmidt
in Australia, came up with a clever way to
measure the true distances and redshifts of
very distant galaxies by trying to find recent
Supernova explosions – very special ones
called type 1A supernovae.
These provide a standard candle because all
type 1A Supernovae explode with about the
same luminosity, because they grow to about
the same size. So by measuring the brightness
of these supernovae, scientists can ascertain the distance.
And my measuring the red shift of this same
light, they could determine how much space
had expanded from the time the light left
the supernova.
When these two pieces of data were combined,
from galaxies that were at various distances
from earth, they could get an idea how much
space had expanded over different time frames,
and thus how fast the galaxies were moving
away from earth.
What they found was shocking to most scientists
because not only was the universe not slowing
down, or remaining constant, it was speeding
up. The expansion was accelerating. And no
one could explain it.
Saul Perlmutter, who won a Nobel prize for
this discovery, described this as throwing
an apple in the air, waiting for it to come
down, only to find that it took off like a
rocket into outer space. So the question is
if it took off like a rocket, where did the
energy come from to fuel this rocket?
This apparent energy that is fueling the accelerating
expansion of the universe is called “dark
energy.”
So what could be causing it? There are three
possibilities.
The first possibility is that this energy
is a property of space itself. This was buried
in a version of Einstein’s equation of general
relativity that contains something called
the cosmological constant. It’s represented
by the Greek letter lambda.
Ironically, Einstein had introduced lambda
to counterbalance the effect of gravity to
keep the universe static. He abandoned it
after Hubble’s discovery of the expansion
of the universe, calling it his greatest blunder.
Lambda can be thought of as the energy density
the vacuum of space, what we perceive as nothing.
It comes from a non-perfect cancellation of
quantum fluctuations. What do I mean by this?
According to Heisenberg’s uncertainty principle, quantum fields from which all particles
come, cannot sit still. These fields are constantly creating virtual particles in empty space.
So for example, electrons and positrons, protons
and antiprotons, quarks and antiquarks, come
in and out of existence in empty space.
This can be seen from this version of Heisenberg’s
equation:
The change or uncertainty in energy, times
the uncertainty in time is greater than or
equal to planck’s constant divided by two
times pi.
This equation also tells you that you can
have virtual particles form out of nothing
if the change in energy and/or change in time is so small that it is less than Planck’s
constant over two pi.
These virtual particles are not something that you can see.
So is there any experimental evidence that can attest to the fact that this energy exists?
It turns
out there is. And this is called the Casimir
effect. I talked a little bit about this in
my Warp Drive video, but essentially it is
a physical effect that you can see and feel
due to the fluctuation of quantum fields in
empty space.
So there is some experimental evidence that this actually exists, and can have positive energy.
But the biggest problem with this theory is
that when you use the known equations of quantum
mechanics to calculate what this energy should be,
the theoretical energy is almost
120 orders of magnitude higher than what is
actually measured from the type 1A supernovae
data and cosmic microwave background.
This is the greatest mismatch between theory
and observation in all of physics. The worst
prediction of all time!
In other words, our equations indicate that
this energy should be much larger than what
it actually is. But obviously our equations
are wrong because if the energy was as much
as theorized, the universe would have expanded
so fast that galaxies, solar systems, and
we wouldn’t exist.
So the vacuum energy is close to zero, but
not quite zero.
Why is this the case? Why is there just enough vacuum energy to cause the
expansion rate to be just what we observe it to be?
One scenario could be that maybe the vacuum energy really is zero due to some
overlying symmetry in the universe which we don't understand yet.
But the slight amount of remaining dark energy is explained by some other mechanism.
The most extensively investigated alternative
mechanism to quantum fluctuations is a set
of theories generically called quintessence.
This is similar to Aristotle’s fifth element
“essense” – the other four being fire,
earth, water, and air.
Quintessence would be like an extra energy
field the pervades the entire universe, which
happens to be repulsive. This field would
be almost same throughout space and throughout
time, but not exactly the same. It is something
that can change slightly in time, and can
fluctuate from place to place.
Unlike lambda or vacuum energy, which is the same throughout the universe,
It would be a dynamical dark energy. In other
words, it can change slightly from place to
place, and in time.
The interesting thing about dynamical dark
energy, or quintessence is that it can be
measured. How? because the differences in
dark energy in different parts of space would
be measurable.
And if it is found to be true, it would eliminate the idea of dark energy emanating from quantum fluctuations.
It 
can be thought of as a fifth fundamental force,
the other four being gravity, electromagnetism,
strong nuclear force, and weak nuclear force.
This would be a force mediated by a new boson
particle.
There is a fine tuning problem that I want
to mention here, which is called the coincidence
scandal.
If you look at the current measured energy
density of the universe, you see that dark
energy consists of 70%, and matter is the
other 30%. These are really almost the same
in the grand scheme of things. 30% is almost
70%. The difference between the two is only
about 2.3.
The reason this is considered coincidental
is because the dark energy density of the
universe is staying constant from moment to
moment. But the energy density of matter is
going down dramatically because as the universe
expands, the energy density of matter goes
down. Matter becomes less dense as space expands.
If you go back to the time of the cosmic microwave
background, 380,000 years after the big bang,
the matter density was about a billion times more.
So the ratio between the two was about one
billion.
In the future the difference between the two
densities will also be dramatic.
Why do we happen to live in a moment in the history of the universe where the difference is only about
2.3? Why should it be that way? It seems like a profound coincidence.
If we go with the dark energy model explained
by vacuum fluctuations, which predict that
that dark energy stays exactly the same throughout space and throughout time, then you have to
concede that there is something really special about now.
But if you believe quintessence is true, then
you can say, hey this is not really a coincidence
because dark energy is dynamical. It can change. And there is some underlying mechanism that has made
it so that the energy density ratio right now
is only about 2.
What would we see if we could detect this
quintessence field?
For one thing, if the field exists it would
be slowly going down in value over time. We
would see this as a decrease in dark energy
density over time.
But there’s another way that quintessence
could be detected. The excitations of the
quintessence field would manifest itself as
a particle, probably a boson, but unlike every
other boson, it would have negative energy.
So in empty space, you could create some positive
energy particles and some negative energy
particles without violating conservation of
energy laws. This would mean that lighter
particles could decay into heavier particles
by emitting quintessence particles with negative
energy.
Of course this has never been seen. But if
we were able to harness it,
we could realize Zefram Cochrane’s dream,
and the dream of Star Trek fans everywhere,
because we could finally build that warp engine
and start our 5 year mission to other worlds.
But so far attempts to explain or detect quintessence
have not worked.
The models that fit the current data indicate
that dark energy is not dynamical, but stays
constant throughout space and time.
Could dark energy be photons or radiation?
No, because the energy density of photons
and radiation goes down as space expands. Dark energy however stays the same. So as more
space is created, more dark energy is created.
Why is it that if the density doesn’t go
down, it still makes the universe accelerate?
Because what it means is that every cubic
centimeter of space is expanding at a constant
rate.
So over very large distances, the cumulative
effect is an acceleration of the universe.
As more space is added, more energy is added,
and the result is accelerating expansion
But you might say, if more energy is being created, doesn’t this violate energy conservation? It turns
out that this does not, because the positive
energy is perfectly balanced by an increase
in negative gravitational energy of the universe.
This can be thought like lifting a ball off
the floor. The energy that you are adding
to the ball is balanced by the negative gravitational
potential energy of the ball.
The third possibility is that maybe we are
wrong about dark energy, and maybe it doesn’t
exist at all. There is a small minority of
scientists who believe this. Most notably
among them is Subir Sarkar, Theoretical physicist
at the University of Oxford in the UK.
His group examined the Type 1A supernova data
after the raw data was made available to the
public in 2014.
He argues first that the original supernova
data were not compelling because they had
a correlation of only 3 sigma, instead of
the typical standard for cosmology, which
is 5 sigma.
In addition, he argues that the original calculations
did not take into account the uneven distribution
of matter in the universe. If the Milky Way
itself is being pulled in one direction, then
this would cause an error in the measurements.
In other words, maybe we are the ones decelerating
instead of the rest of the universe accelerating.
Given these potential errors, he says the
original data correlation is likely to be
even lower than 3 sigma, more like 2 sigma.
He therefore believes dark energy may not
exist.
Dr. Sakar is firmly in the minority on his
views, for several reasons. Scientists knew
from the very beginning that it was 3 Sigma
data. So this is not new information. By the way, 3 sigma
data still correlates at a reasonably high
99.7%. 2 sigma is 95%. Cosmologists prefer
5 sigma which is 99.99997%. But even at a
2 sigma correlation of 95%, there is significant
correlation.
Part of the reason cosmologists readily accepted
this data back in 1998 is because prior to
this, cosmological models predicted that the
universe had something called a critical energy
density, which means that it was not curved
in 4 dimensions. It was flat in 4 dimensions.
But the matter only accounted for 30% of this critical density. So the remaining 70%
of the energy density had remained a mystery.
So when the Supernova data indicated that
the amount of dark energy needed to make the
universe accelerate is precisely 70% of the critical density, the mystery of the missing 70% energy
was solved. And it was a Eureka moment, and everything fell into place. And physicists quickly adopted
the dark energy model.
The other fact is that the 1998 supernova
data is not the only data that provides evidence
of dark energy. Subsequent independent studies
have confirmed this, in particular the 2000
cosmic microwave backround experiments and
the sloan digital sky survey in 2003 showed
strong evidence that the matter density of
the universe is about 30% of critical density,
meaning 70% dark energy is needed to justify
what we observe.
So dark energy makes many of the puzzle pieces
fit together to tell a story that is consistent.
This is why attempts to discredit dark matter
are not generally accepted. You could still
say what if everybody is wrong?
Maybe the universe is not flat. Or Maybe there
is something wrong with Einstein’s theory
of gravity. You could be right. But when scientists attempt to modify general relativity, it causes problems
and the whole equation breaks down.
For now, what is generally accepted is that
dark energy exists, and fits with both our
observations and the best models of cosmology.
So unlike what Aristotle believed, we are
not only not the center of the universe, we
are not even made of mostly the same stuff.
We are made of only the miniscule 4% that
the universe is made of.
