One day, the universe, as we know it, at
least is going to end. And nearly 30
years ago, scientists reached a general
consensus on our universe's most likely
fate. It is that over trillions of
years, its galaxies will drift farther
and farther apart. Stars will cool off
and die out. Black holes will slowly
evaporate until all that's left is an
infinite expanse of cold, dark space.
This is the so-called big freeze powered
by dark energy. It's a consequence of
the accelerating expansion of our
universe. By far the most likely outcome
for the end of everything. Or so we
thought.
New groundbreaking findings from the
dark energy spectroscopy instrument or
desi may be uncovering a different
story. It's found that dark energy's
grip on our universe could be in fact
weakening. But what does this mean? If
true and the cosmic engine that powers
our universe is running out of steam,
then the very fate of the universe hangs
in the balance. I'm Alex Mccoan and
you're watching Astramm. Join me as we
investigate this profound shift in our
understanding of the universe and
explore why dark energy may be running
out of power. Could it be that all our
cosmological models are wrong? Or is
there another explanation?
Let's travel back to 1998.
At that point, we already knew that the
universe was expanding thanks to Edwin
Hubble's observations almost 70 years
earlier. But scientists were still
attempting to pin down exactly how fast.
In fact, many were wondering if the
expansion would eventually slow down.
Logically, this seemed to make sense.
The tug of gravity should bring
everything closer and closer together
over time. Right now, all of this could
be solved by working out the Hubble
constant, a value that represents the
speed at which the universe is expanding
right now. It can be measured in two
ways. By studying patterns in the cosmic
microwave background or by looking at
distant red shifts.
By the way, scientists have found that
these two methods don't produce the same
results. But I've got another video on
that particular crisis that you can
watch here to find out more. In this
video, however, we're going to
concentrate on the red shift method.
Historically, the red shift of seafared
variable stars has been used for this,
but type 1A supernova also work well.
And without knowing it, the astronomers
studying these immense explosions were
on the brink of a jawdropping discovery.
We know how bright type 1A supernova
should be because they all explode in
pretty much the same way. Thermon
nuclear detonations of white dwarfs in
binary systems. By comparing their true
brightness to how dim they look from
here on Earth, we can measure their
distances. Then by using the supernova's
red shift, the amount of light that has
been stretched along its journey, we can
determine how fast they're moving away
from us. and therefore how fast the
universe was expanding when their light
was emitted. Easy enough, right? Well,
when two independent teams of
astronomers observed more than 50 type
1A supernova, they found that the light
appeared fainter than expected, which
although perhaps sounds trivial, turned
out to be a pretty big deal.
Teams led by Sal Pelmuta, Brian Schmidt,
and Adam Ree discovered that distant
supernova were dimmer because they were
farther away than we expected. This was
a huge revelation as it came with a
rather shocking consequence. It meant
that the expansion of the universe was
not slowing but actually speeding up.
some unforeseen force was seemingly
pushing entire galaxies apart.
To visualize this, imagine if you took a
marker pen and drew dots on the outside
of an uninflated balloon. As you blow it
up with air, the dots get farther and
farther away from each other because the
balloon itself, the space the dots
reside in, is expanding in all
directions. This unexpected result
created more questions than it answered.
A big one being what is the force
driving this acceleration?
Unfortunately, the answer is still we
don't know. But it's been given the name
dark energy and most scientists think
it's a property of the vacuum of space
itself, the energy of empty space. Other
scientists think it may be a type of
energy fluid or field that fills space
or even some kind of defect in the
fabric of spaceime like one-dimensional
wrinkles socalled cosmic strings. And
others think it may just be a flaw in
our understanding of general relativity
on the scale of the observable universe
and that the matter could be resolved
without the need for dark energy at all.
But whatever it is or isn't, dark energy
presents a mystery that is intertwined
with the accelerating expansion of the
universe. This was such a monumental
discovery that following the 1998
finding, Ree Hmut, and Schmidt, the
three astronomers who led the
investigations, were awarded the Nobel
Prize in physics for their work.
This discovery of dark energy gave rise
to a new model of cosmology. The lambda
cold dark matter or lambda CDM model.
Lambda is a Greek letter that represents
the cosmological constant. First
postulated by Einstein. Today it
represents the mathematical parameters
of dark energy. The other three letters
CDM stand for cold dark matter. It's
become what we today refer to as the
standard model of cosmology and it gives
mathematical context to everything from
the big bang to the future fate of our
universe and of course all that happens
in between.
For decades, it's been the most widely
accepted and most accurate model we have
for the evolution of our cosmos.
Confusingly, dark matter doesn't have
anything to do with dark energy. We just
use the word dark to describe two
different things that we don't
understand. Well, while dark energy is
the force that's causing the universe to
expand at an accelerating rate, dark
matter is what we call the stuff in our
universe that doesn't interact with the
electromagnetic field. As in, it doesn't
absorb or emit or interact at all with
light. However, we believe it exists
because we can observe its gravitational
interaction within the universe. And the
word cold in the model's name just
refers to this dark matter having very
little energy since temperature can be
thought of as a way to measure the
energy of atoms. But if those two things
make up the entire name of the model,
the lambda cold dark matter model, what
about everything we can see and do know
about? It may not be reflected in the
name but it is captured within the model
itself. Visible matter and radiation
including light all belong to the
category known as barionic matter. Using
independent measurements and
observations, the scientific community
has come to a consensus that under this
standard model, about 68% of our
universe is made up of dark energy or
this fundamental property of spaceime
that we know very little about. with
visible matter making up only about 5%
and dark matter accounting for 27% of
our universe. Let that sink in. 68% of
our universe is made up of something we
don't understand and can't measure
directly.
However, a crucial aspect of our
standard model is that lambda or dark
energy is assumed to be a constant.
Almost every cosmological calculation
relies on this being fact. It tells us
that our universe has an ever
accelerating rate of expansion and that
it will therefore face a cold lonely
fate. Galaxies will continue to drift
further apart as the space between
everything expands. Stars will burn out,
die, and cool off. All of the heat and
energy in our universe will be spread
impossibly thin over incomprehensible
distances until the universe's final
temperature hovers somewhere barely
above absolute zero, ending in what
astronomers call the big freeze.
But what if we're wrong? For the first
time since the discovery of dark energy,
we may have uncovered one of its key
properties, and it's not what
astronomers expected.
In 2024, the Dark Energy Survey released
the largest ever sample of supernova
results from a single survey, having
studied the deaths of more than 1,500
stars.
Initially, the data seemed in line with
the idea that dark energy is constant.
But when they combined the data with
other results, like measurements from
the cosmic microwave background and
earlier galaxy surveys, something didn't
quite add up. It was starting to look
like dark energy might in fact change
over time.
giving you unlimited access to all Brilliant has to offer. Now, a changing
dark energy is not what scientists
expected to find, and it caused them a
big problem. Although it was a promising
result, there were uncertainties
involved. So, it was nowhere near being
decisive proof. To find out the truth,
astronomers needed a far bigger, more
detailed map of the universe to test
whether this hint was real. And for
that, they turned to Desessie.
Installed on the 4 male telescope in
Arizona, the dark energy spectroscopic
instrument is charting the optical
spectra of tens of millions of galaxies
and quazars across billions of years of
cosmic history with one mission to test
whether or not dark energy is constant.
Able to capture light from 5,000
galaxies simultaneously, Desi is
measuring the position and receding
velocity of some 40 million galaxies in
order to accurately measure the
expansion history of the universe over
the past 11 billion years. This is a
truly international effort involving
more than 900 researchers from more than
70 institutions around the world. And in
2025,
the first major data sets were released.
They included results from the first
three years of the survey with two more
to go. And what they show is nothing
short of extraordinary.
Desi used data from nearly 15 million
galaxies to build the largest
three-dimensional map of the universe
ever created. This 3D map lets us fly
through millions of galaxies, each
containing 200 to 300 billion stars.
Earth is at the center of this
animation, and every dot is an entire
galaxy. The blue represents farther
galaxies from us, while the white shows
closer objects. In other words, this map
is almost unfathomably dense with
galaxies.
Using this data, a team of astronomers
have measured barrier acoustic
oscillations or BAOs from more than 14
million galaxies and quaazars. BAOs are
basically the fossilized echoes of sound
waves from the early universe. By
tracking how they change in size across
cosmic time, researchers can work out
exactly how fast the universe was
expanding at any given moment. Here, the
lighter blue color represents sound
waves rippling through the primordial
cosmic sea of our early universe. Over
time, those slight variations froze in
place, carrying with them a slightly
more dense collection of matter. Over
hundreds of millions of years, those
waves would gather more and more
material, eventually becoming stars and
galaxies. These waves are still visible
today in the form of the cosmic web.
The researchers then combined this desi
data with other measurements like the
microwave radiation left over from the
dawn of the universe known as the cosmic
microwave background, supernova red
shifts, and weak gravitational lensing
or how the light from distant galaxies
warps due to gravity. And what they
found added more weight to the 2024
discovery. Not only does it look like
dark energy's influence is changing, but
it's potentially decreasing over time.
In fact, they found that dark energy's
push may have weakened by roughly 10% in
the last 4.5 billion years. You can
think of it kind of like a foot still on
the gas pedal, but gently lifting, which
means that the car is still
accelerating, but at a slower rate than
before.
Likewise, the expansion of our universe
is still accelerating, but not quite as
rapidly as it was a few billion years
ago. And if dark energy is evolving
rather than remaining constant, then our
standard model of cosmology is missing
something and something big.
Assuming this model shattering discovery
is correct, it could mean a very
different future for our cosmos. Instead
of a big freeze where our universe
expands forever, leaving us with a cold,
dark, lonely end, we may end up
experiencing an alternative reality. If
dark energy's influence continues to
weaken over time, decreasing until it
becomes negative, then eventually
expansion of our universe could stop
altogether.
Instead of a big freeze, it would end in
a much more violent cataclysm.
Imagine the raw power of the Big Bang
exploding and expanding out with enough
energy, heat, and matter to form all the
galaxies and stars across our universe.
And now imagine the exact opposite of
that. A reverse big bang where
everything changes direction and rushes
in toward a single point with enough
intensity to undo everything that ever
existed in one cosmically colossal big
crunch.
I've spoken before about the possibility
of a big crunch as well as other
theories about how physics gets weird at
the end of the universe. If you want to
know more, I recommend watching the
video linked here. Now, if dark energy
is an evolving variable, there is one
other perhaps even more important to us.
Impact. It means our current model of
cosmology is incomplete. It could mean
that the theory of general relativity
needs modification, too, and that
there's more to learn about particle
physics, possibly involving new fields
or forces.
But before we rewrite any textbooks, we
have to be sure. And right now, we're
close, but we're not quite there. Sigma,
or standard deviation, is the way
scientists measure confidence. The
higher the sigma, the less likely a
result is due to random chance. And in
the chase for evolving dark energy,
those sigma levels have been climbing.
On its own, desi data lands us just
above two sigma, which is about 95%
confidence. Interesting, but nothing
you'd bet your career on. Combine Desi
with other instruments, and the number
rises past 3 sigma, even brushing the
4.2 two sigma mark in some combinations.
Roughly 99.997%
confidence.
While this is considered to be strong
evidence, meaning that the result was
unlikely due to random chance, it's
still not quite at the level physicists
require for it to be considered a solid
discovery. For example, other evidence
with this confidence level has turned
out to be false before. So physicists
typically wait for even clearer results
before claiming a new discovery. The
gold standard in physics comes when
confidence reaches 99.99994%
which is sigma 5 and even then things
can still be wrong. For example in 2011
physicists at the Grand Sasso laboratory
in Italy measured some nutrinos arriving
a few nanconds early. a feat that would
require the particles to have traveled
faster than the speed of light, breaking
the cosmic speed limit. They reported a
confidence level of six sigma, even more
impressive than the gold standard for
new discoveries. To put that into
perspective, barring any unexpected
mistakes in the experiment, 5 sigma
represents about a 1 in 1.7 million
chance of the result being a fluke. and
six sigma is more like a one in 500
million chance of the result being
wrong. So this was quite an astonishing
observation and it made headlines around
the globe. Unfortunately, it was later
revealed that the physics-breaking
observation had actually been due to a
faulty fiber optic cable and clock
synchronization error rather than
nutrinos breaking the laws of physics.
Needless to say, there's still a little
way to go with the concept of evolving
dark energy.
Now in its fourth year of the 5-year
survey, Desi is well on its way to
having measured 50 million galaxies and
quazars, continuing to offer valuable
data in the quest to understand dark
energy. But it's not our only tool. Over
the next few years, new experiments will
be conducted. Other missions will
further the exploration of the nature of
dark energy and complimentary data sets
will come out for analysis. European
space ay's Uklid and NASA's Nancy Grace
Roman Space Telescope will extend
measurements across even larger volumes
of space and at higher red shifts. And
the Very Rubin Observatory will scan the
sky for millions of supernova, providing
the enormous data set needed to
hopefully bring us to the five sigma
mark.
I personally can't wait to see what they
find. But in the meantime, the new Desi
findings are tantalizing and bring us to
the precipice of a whole new view of
both our standard model of cosmic
evolution and the ultimate fate of our
universe. Let me know in the comments
what you think. Will the universe end in
a big freeze or is a big crunch on its
way? What ending or rebirth do you
imagine for our universe?
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