How much of the universe is already
dead?
At first glance, it's a difficult
question to answer. Except for the
occasional supernova blast or colliding
black hole, at the very largest scales,
the cosmos appears static and
unchanging, a frozen snapshot in a long
and unknowable history. But Swedish
physicist Eric Holberg decided he wanted
to know more.
Today his name is largely unknown even
to astrophysicists. But in 1940 in a
darkened gymnasium in Stockholm he
crafted perhaps the most ingenious
experiment since the Renaissance days of
Galileo. In the process becoming the
first scientist to watch millions of
years of cosmic history unfold.
The idea was simple.
[Music]
In the gymnasium, he and his students
set to work placing 74 stations, each
one equipped with a bright light bulb
and a photo receptor that could measure
the light being received by that
station. The stations represented
portions of a galaxy, an individual bit
of mass that could influence its
surroundings and be influenced by it in
return. The light stood in for the
gravitational force, each reducing as
the distance squared. He divided the
stations into two camps, drawing the
rough figures of two spiral galaxies
just before a merger.
Then slowly, methodically, he set the
lamps in motion. He measured the amount
and direction of light at each station
and used that to estimate where the
station should be placed as if that
portion had advanced forward a million
years in time. After moving all the
stations, he repeated the measurement
and movement again and again and again.
Holberg had created the world's first
simulation of the cosmos built with an
analog computer.
In watching the evolution of galaxies
unfold before his very eyes, Holberg saw
exquisite beauty, the stretching the
galaxies undergo and the graceful
arching of tidal tales that arise during
a merger. But he also saw something else
in the form of light bulbs and photo
receptors arranged carefully on a
gymnasium floor. He witnessed how
galaxies die.
Holberg witnessed a universe that
changes. And he saw that the cosmos we
have learned to know and love, a
universe filled with vibrant galaxies
arranged in a grand cosmic web, will not
last forever.
There was a time billions of years ago
when the universe was plunged into
darkness before the first stars awoke.
And by the harsh laws of cosmic
expansion, there will be a time in the
distant future when the cosmos returns
to that darkness.
In his shadowfilled gymnasium, Holberg
had witnessed that impermanence.
But what he didn't see, what he couldn't
see without more advanced tools was that
the wider universe was already well on
its way to oblivion, well past its peak.
For today, following in H Homeberg's
pioneering footsteps, we have combined
advanced computer simulations with
comprehensive observations to learn that
much of the universe is already dead. We
are already in the age of twilight. What
we face in the long future is nothing
but decline.
In a sense, then this is a story of
betrayal because in a changing universe,
the very forces that give rise to star
formation eventually turn their backs on
their creations and kill them from the
inside out.
[Music]
4 and a half billion years ago, just
after its birth, our Earth was a literal
hell.
There is an earthshaking rumble
permanently echoing across the young
globe as the very crust itself shifts
and cracks. Molten rock spurts and oozes
over the constantly shifting landscape.
Temperatures reaching a thousand° on the
surface. The atmosphere almost
non-existent, barely different in its
makeup to the inside of a solar nebula.
And in the sky above this molten chaos,
is the moon, but not the moon we know.
[Music]
15 times as large as we see it today and
glowing a dull red, this young satellite
would have filled the horizon like a
dreadful portent. And so what happened
between then and now to give us the moon
of today, placid and distant? Well, it
is very slowly moving away. We know this
from what is called lunar laser ranging,
a technique where scientists shine laser
pulses at the moon and measure the
length of time they take to arrive. The
accuracy of these measurements improved
by optical receptors placed on its
surface during the Apollo missions.
Scientists have calculated that our moon
is receding at a speed of 1 in per year
and would only finish its journey in
roughly 15 billion years were that not
well after the time at which the sun
will have consumed the earth. Our
night sky is far from static. And on
cosmic scales, our retreating moon is
just a tiny example. Our entire universe
is full of change at scales great and
small.
Indeed, the astronomers of the
Renaissance were astonished to discover
that the heavens were just as tumultuous
as our lives here on Earth. Once they
realized that the Earth itself moved,
the idea of the permanent, fixed, and
unchanging cosmos
crumbled,
comets were discovered not to be just
strange atmospheric phenomena. They were
temporary visitors from the outer solar
system. The bright guest stars that
flickered and flared in the night sky
were really the deaths of massive suns.
The nebula were not mere clouds. They
were either the birthplaces or the
cemeteries of stars.
The universe was revealed to be
constantly shifting. And because of that
change, there are things that have not
yet had enough time to be born and
things whose time has passed. Things
that we will never see again.
To start with, the extremely early
universe was very different to today.
Due to the immense energies present, it
may have fashioned exotic particles like
magnetic monopoles or even stranger
topological defects like cosmic strings.
They are all gone now. There was a time
when the universe itself was capable of
creating new elements. Primordial
hydrogen and helium condensed out of the
thick nuclear soup within minutes after
the big bang. Generation after
generation of stars lived and died,
forming new elements and enriching the
cosmos, eventually giving rise to
planets, oceans, and the complex
molecules that would make life possible.
But those that form in a star are
heavier elements. Except in a few rare
cases, all the hydrogen the cosmos will
ever have formed in roughly a dozen
tumultuous minutes just after the
universe began.
There was a time when the cosmos was
completely unrecognizable. Roughly
380,000 years after the Big Bang, the
universe was so different that it was a
completely distinct state of matter. The
gas of the universe had a temperature of
over 10,000 Kelvin that was hot enough
to convert it into a plasma. When the
universe changed from that state to
something more similar to the cosmos of
today, it released a flood of radiation,
a one-time only event that created the
cosmic microwave background that we can
just barely still see today. That will
never happen again. There was a time
before stars, a time before galaxies, a
time before planets. There was a time
when the first stars poured their light
out, a super massive type of sun that
seems to have totally vanished in the
modern universe. There was a time when
the universe was flooded with new stars
and chaotic colliding galaxies.
Now that time is done.
[Music]
This story of cosmological change began
with Edwin Hubble's discovery that the
galaxies around us were moving and in
fact most of them moved in only one
direction away. The universe was
expanding.
And this was key for an astic universe.
Everything would be the same. No matter
where we look, no matter how deep our
observations, we would always see
roughly the same things. the same kinds
of galaxies, the same kind of activity,
the same kinds of cosmic structures.
And that is not what we found.
But as well as this, Hubble was also
able to increase the size of the
universe by some distance. Before
Hubble, many had believed that the Milky
Way was all there was. But his
measurements of distant galaxies
ballooned the universe from 300,000
lightyear to a breadth of 280 million
lightyear. Over the first half of the
20th century, estimates continued to
grow as astronomers used more powerful
telescopes and more effective methods of
judging distance, reaching over 4
billion lightyears across by 1955.
But then in the late 1950s, the first
radio surveys were done of the sky and
all previous estimates were blown out of
the water.
Physicists
[Music]
found strange sources of radio emission,
pinpoints of almost impossible
intensity, dubbing them quasi stellar
objects or QSOs. The name would
eventually be shortened to quazars, and
they would turn out to be stupendously,
almost impossibly bright, compact
sources of electromagnetic energy that
could outshine a million galaxies at
once. And that was not all. For the key
discovery was that there were no nearby
quazars. They were all billions of light
years away. And so once again, the
universe ballooned in size. The most
distant quazar found in the 1960s being
12 billion lightyears away. The furthest
we have seen in modern times, closer to
13 billion, existing barely 600 million
years after the Big Bang.
Quazars were primarily creatures of the
distant universe, which meant they were
only active in the young universe,
existing at a time billions of years
before the modern epoch.
In a static universe, we would expect to
have some quazars as nearby neighbors,
but there were none. We could only
conclude that the universe was different
in the past with just the right
conditions to give rise to quaazars. a
set of conditions that no longer exists.
And this is a change in the universe
driven by its expansion. But that is not
all. For as well as this, we've
discovered that it's not just the
quazars that are different, but the
galaxies themselves.
Observations of the distant cosmos with
the James Webb Space Telescope reveal a
youthful universe inhabited by smaller,
dimmer galaxies than the grand spirals
and massive ellipticals that we see
today. The earliest yet found Z14 is
from roughly 282 million years after the
Big Bang and is full of new stars being
born. Although it isn't actually that
small, still roughly the size of the
small melanic cloud, giving it the
potential over the intervening 13 1/2
billion years to our time to grow into a
galactic leviathan.
Indeed, deep cosmic surveys find that
the cosmic web, the grand arrangement of
galaxies on the very largest of scales,
took time to evolve from a diffuse
collection of gas into the intricate
lattis work of the modern era.
[Music]
And so a changing cosmos tells us that
the universe has a finite age. But the
neverending expansion of the cosmos also
means something else.
That the universe will die. And in
actual fact, it is well on its way.
We put the relevant laws of physics into
a computer, set up some initial
conditions at an early cosmic epoch, add
in all the ingredients we know of, and
let it all churn.
Eric Holberg was the first to recreate
our cosmos in a laboratory, but he would
be far from the last. For today, 80
years later, all over the world,
gymnasiums have been replaced by
supercomputers and light bulbs by N
bodies. One of the largest such
simulations created by Vulkarus Springer
and Lars Hearnquist and dubbed
Illustrous TNG or Illustrous, the next
generation, requires the use of nearly a
100,000 computer cores working in tandem
to recreate just a tiny portion of the
visible universe. Their code can follow
the evolution of gas as it converts into
stars and trace the formation of
structure across the cosmos.
And one of the primary goals of
illustrous TNG is to understand the
detailed mechanisms behind the rise and
slow decline of galaxies.
We know from observations with the James
Webb Space Telescope that galaxies first
appeared within a few hundred million
years after the Big Bang. At first, they
were small, less than 1% of the
modern-day mass of the Milky Way and
relatively dim. But those first galaxies
grew quickly, feasting on their
surrounding material. Simulations like
illustrious TNG show us what only
observations cannot as they reveal the
hidden flows of gas onto the galaxies
and how they intermix and mingle to form
stars.
And it is clear that things move slowly,
not with a crash nor a bang, but a
whisper, secret movements in the dark,
the conspiracy of gravity, and slim
slivers of matter.
The universe was young then, only 100
million years old, a fraction of its
modern size, and all was darkness.
The fires of nucleiosynthesis that gave
rise to the first elements, all of it
was a dim and distant memory. But over
time, the dim, diffuse clouds found each
other. And with their stronger gravity,
they pulled on their surroundings,
drawing them closer. And then in some
unknown region of the vast and cooling
cosmos, the first spark of revolution
was lit.
The primordial elements had through the
inevitable crushing of their own
gravitational weight ignited into
fusion. The first release of energy in a
100 million years. The first new light
to shine upon the universe.
Change had come. The cosmic dawn had
broken.
[Music]
But those first stars were not yet done.
Clumps of gas and newborn stars gathered
together, huddling their warmth against
the cold and dark universe beyond. More
gas piled onto these safe havens, these
refuges, these cities. Beginning as
simple spheres, then flattened discs.
Then, as pertubations did their work,
ripples of gravitational influence
working their way through their volumes,
spiral arms of dazzling starlight
emerged.
The first galaxies were born, and they
would rise to dominate the visible
universe.
One of the best places to see the night
sky is in the deep desert. There, far
away from any source of light pollution
or interference, you can enjoy a crisp,
clear view of the heavens, and the
Atakama Desert of Northern Chile is one
such special place.
Spanning hundreds of kilometers along
the western spine of the Andes
Mountains, the rugged elevated desert
makes for worldclass observing. There,
once you allow your vision to adjust to
the darkness, your naked eye can discern
up to 3,000 individual stars. And so, as
you can imagine, this makes the ideal
place for a telescope array.
[Music]
The Atakama Large Millm Subm Array, or
ALMA, is situated 5,000 m above this
bone dry desert.
It is an intererometer, an assemblage of
66 individual dishes, all working in
tandem to pierce deep into the sky. And
in 2015, Japanese astronomer Hideki
Umahhata used it to find a set of proto
galaxies.
Using ALMA, he was able to map out the
SSA22 protocluster of galaxies. A
monster being born nestled in a cradle
of filamentary matter nearly 12 billion
lightyears away. As galactic wounds go,
SSA22 is gigantic, reaching over 200
million lightyears on a side. Individual
gas bubbles carved out by the infant and
temperamental galaxies are 400,000
lightyears across each. Indeed, it was
one of the first such protoclusters of
galaxies to emerge on the cosmic scene,
and Elma brought us right into the
delivery room.
And yet within a mere 3 billion years,
these galaxies would start to collapse.
But for a time, they were truly
glorious.
[Music]
As galaxies first began to grow, the gas
pervading the universe fractured and
fragmented, collapsing itself into a
series of filaments and sheets. At the
nodes of these filaments, their points
of intersection, sat the newly
constructed galactic cities. Each
growing galaxy was attached to the wider
universe through numerous umbilical
cords, drawing from the resources of
their surroundings to grow themselves
and in the process produced stars at an
astounding rate. Indeed, at their peak,
roughly 3 to 3 and 1/2 billion years
after the Big Bang, galaxies produce
stars at a rate over 10 times greater
than they do today.
Indeed, if any intelligent creatures
lived on the surface of any planets in
that young universe, they wouldn't need
a telescope like Elma to witness stellar
miracles. They could simply look up.
Our own sky is dominated by the light of
only the largest stars. A stars like our
sun are too dim and feeble to be seen
over interstellar distances with the
naked eye. But if you were to take this
same view back to the era of peak star
formation, you would instead see over
30,000 stars, perhaps more. This era is
known as cosmic known, the peak of the
universe. And it was glorious,
but it wouldn't last long.
[Music]
Our modern universe is different. Star
formation takes place in only a few
selected sites, special places where the
right conditions are met, like the
nebula NGC 346, a star forming cluster
in the large melanic cloud. Otherwise,
the vast majority of galaxies are quiet,
sleepy places.
Even our own Milky Way galaxy is only
able to create about seven stars a year.
Just seven across its entire expanse of
over a 100,000 lightyear. Indeed, in
your entire lifetime, there'll only be
around 500 new stars awakening within
our galaxy. And astronomers consider the
Milky Way to be relatively active for
our time. And so, what happened in the
10 or so billion years that separates us
from the cosmic known? Why is the
universe so much less fruitful today
than it was then?
[Music]
Hastily built, the galaxies of the early
universe were small and inelegant.
They'd not yet formed grand spiral arms
or settled into stately equilibrium.
Within them, titanic forces competed for
dominance, like rival street gangs,
waging wars for control of their
neighborhoods.
Some material drifted down from all
directions, but most of it came rushing
along the filaments that connected each
galaxy to the broader cosmos. As the new
gas rushed in, gravitational
pertubations triggered collapse of
nearby gas clouds. Those clouds, once
destabilized, would fragment into
hundreds or even thousands of star
forming clumps at once. And those first
rounds of star formation were the
subject of the J ADES
or James Webb Space Telescope Advanced
Deep Extragalactic Survey. The team
co-led by Marsha Reek of the University
of Arizona in Tucson searched for what
amounted to tiny smudges of light. And
yet those smudges represented millions
or billions of newborn stars lighting up
all at once.
When those stars died, they exploded in
massive supernova blasts, which in turn
sent new shock waves racing through
their neighborhoods. And round and
around the feedback cycle went as new
material led to new star formation.
Each galaxy was a picture in turbulence.
And not just within themselves.
For many early galaxies didn't just
gather new material quietly from their
surroundings.
Sometimes they stole it from each other.
The candle survey was designed to watch
this cosmic drama unfold. The largest
project the Hubble Space Telescope ever
took part in, the survey used about 60
days worth of observing time between
2010 and 2013 and provided deep imaging
of over 250,000
galaxies.
The young universe was smaller with the
average distance between galaxies far
shorter than it is today. And so as
these galaxies grew, they battled over
resources. When galaxies interact with
each other, it's not always slow waltzes
and gentle dances. Sometimes it's
complete consumption.
Our own Milky Way hides many such
remnants of its violent history. It's
not obvious to the casual observer with
one stellar population looking largely
like any other. But thanks to the
enormous cataloges of billions of stars
we've been able to gather over the
years, careful astronomers have been
able to tease apart the populations,
deducing which stars were born natively
in the Milky Way and which are later
arrivals.
These investigations have revealed
dozens of merger remnants. Some more
recent ones are still partially intact
with small cores of high density
remaining. What we call the globular
clusters. Some are stretched and
elongated like the Sagittarius stream.
The mighty gravity of the Milky Way
distorting them beyond recognition. And
others are completely assimilated. The
leftover populations of stars scattered
far and wide within the galaxy. Like the
entertainingly named Gaia Sausage. Each
of these encounters, merges, and
collisions led to fresh rounds of star
formation. When a larger galaxy would
consume a smaller one, it would lead to
cannibalism. But when a galaxy
encountered another one of an equal
size, it led to a period of star
formation unlike anything either galaxy
had seen before.
In the most intense cases, the
gravitational interactions between
merging galaxies was so strong that
their newly merged form could create
stars at a rate over a thousand times
greater than the Milky Way of today.
But perhaps the most important
ingredient of all, the central player
that regulated the rate of star
formation, the key component that
determined how efficiently early
galaxies could convert gas into stars
was dust.
Humble simple dust.
Today, dust is everywhere in the
universe. These are small, usually
microscopic grains of various chemical
compounds. Indeed, it's so abundant that
you can even see it. The diffused dust
in the solar system is responsible for
the phenomenon known as zodiacal light.
If you look to the western sky shortly
after sunset on a clear dark night, you
might be able to see a faint glow
stretching from the horizon and
narrowing to a point higher in the sky.
That is the sunlight scattering of the
dust contained in the space between the
planets. Indeed, there's so much dust
that every year the Earth accumulates
several tons of new mass simply by
swimming through it.
And dust plays a crucial role in star
formation.
To make a star, you can't simply bring
together a reservoir of gas. Indeed, it
will just sit there existing as a nebula
for millions of years. That's because
that gas is hot and its temperature
prevents it from collapsing into the
small sizes needed to become a star. But
dust can cool off a nebula by
efficiently emitting radiation. By
cooling through dust, a gas cloud can
continually contract until its central
densities reach the critical thresholds
needed to ignite nuclear fusion and
become a star.
But the extremely young universe was
completely devoid of dust as only
hydrogen, helium, and trace amounts of
lithium and burillium were made in the
primordial big bang. Indeed, this is one
of the reasons why we don't quite
understand how early stars formed. But
once those stars lived and died, they
enriched their surrounding environments
with heavier elements. And those
elements would quickly mix together to
create the first dust.
Observations
[Music]
with ALMA reveal a dusty universe with
small galaxies obscured in relatively
thick hazes. All this frenetic activity
led to more stars being produced in the
first 3 billion years after the big bang
than in the 10 billion years since.
Cosmic noon was the peak, the point when
star formation reached its zenith.
And then it stopped.
Cosmic noon ticked over to cosmic
afternoon. And today in the modern
universe, we are in cosmic evening.
Galaxies continued to form. They grew
larger and more massive. The modern
Milky Way is 10 times bigger than it was
at cosmic noon. And the building of
galaxies continues even to the present
day with dim clumps of gas still
emerging into small galaxies. But the
pace is much more sedate, much more
measured. Except for the occasional rare
exception, the galaxies of our time are
much calmer, quieter places,
and some
are completely dead.
[Music]
80 years ago, if Eric Holberg had been
able to tune his crude simulation to a
specific scenario with a particular pair
of galaxies with just the right masses
merging together in just the right way,
he might have seen something familiar.
This is because his studies laid the
groundwork for a new understanding of a
certain type of galaxy with farreaching
consequences for how the universe
evolves and the ultimate fate of
everything within it. If Eric Holberg
had just the right setup in his
gymnasium laboratory, he could have
recreated
M87.
When astronomer Charles Messier
published his famous catalog of deep sky
objects in 1781, he had no idea that
number 87 was different from any of the
others. And in fact, he wasn't
particularly bothered whether it was or
not. Forsier had only one thing on his
mind. He really really liked comets.
To quote French astronomer Francois
Argo, "At a time when he was in
expectation of discovering a comet, his
wife took ill and died. While attending
upon her, being withdrawn from his
observatory, Montana demoj anticipated
him by discovering the comet. Messier
was in despair. A friend visiting him
began to offer some consolation."
Messier, thinking only of his comet,
exclaimed, "I had discovered 12. Alas,
that I should be robbed of the 13th by
Montana." Then remembering that it was
necessary to mourn for his wife whose
remains were still in the house, he
exclaimed, "H this poor woman." And
again wept for his comet. Messier was an
avid comet hunter and not wanting to
fool himself or his fellow astronomers
with false observations, he had created
his catalog of objects to help clarify
fuzzy things in the sky. Fuzzy things
that were definitely not comets. And M87
was definitely not a comet. M87 was and
still is an elliptical galaxy.
[Music]
And as well as this, M87 was not just
any elliptical galaxy, but a true
Leviathan containing several trillion
stars. And even with backyard
telescopes, you can tell that it looks
different.
There is no flattened disc and no
majestic swirl of spiral arms. This is
because M87 is different to the Milky
Way. Different even to most of the
galaxies surrounding the Milky Way. M87
is dead.
A graveyard of galactic proportions.
Billions of years ago, it was ablaze
with activity with new stars made every
year by the hundreds. But not anymore.
And among astronomers, elliptical
galaxies like M87 have a nickname, the
red and dead galaxies.
There are still stars, but not like
there once were. They mostly small
dwarfs, dim and red, only a fraction of
the luminosity of the sun. And among
those still clinging to some feeble form
of life, elited the bones of their
ancestors, the black holes, places of
infinite gravity and no escape. The
neutron stars and white dwarves,
leftover cores, once capable of fusion
energies now laying in slumber as they
slowly, slowly cool off. And finally,
the oblivious planets and asteroids and
other debris of once magnificent solar
systems.
There are hundreds of billions of red
and dead elliptical galaxies just like
it scattered around the cosmos like the
rotting corpses left from a great
battle. Indeed, somewhere between 20 to
30% of all galaxies are ellipticals. And
yet, because they can grow to such
massive sizes, roughly 40% of all stars
in the universe call an elliptical home.
And soon enough, their numbers will
grow. It's the fate of every galaxy in
the universe. Even our own Milky Way, as
calm and steady and sedate as it's been
for billions of years, will someday
become one of these red and dead
galactic graveyards.
Just as astronomers do not fully
understand what led to the great rise in
star formation rates in the early
universe, they do not yet understand
exactly how galaxies die. But
observations of dead galaxies reveal
that it's not usually one single
culprit, but a series of events that
strangle a galaxy of newborn stars. And
these culprits are not any new process
or event that a galaxy undergoes, but
the very same ones that gave rise to
rapid star formation in the first place.
In other words, it seems that galaxies
are a victim of their own success.
The technical term that astronomers use
to describe the death of a galaxy is
quenching. Some series of processes that
extinguish the fire of stars, preventing
new ones from forming and relegating the
galaxy to the rest of the Red and Dead
collection. There are any number of
mechanisms that can participate in the
quenching process, and any of them might
be more or less important in the
evolution of any particular galaxy, but
in the end, all galaxies end up sharing
the same fate.
Sooner or later, some mechanism, some
process goes out of control within the
galaxy and snuffs out star formation.
[Music]
Sometimes it's just a period of too
intense star formation that does the
trick. During these periods, thousands
of giant bright stars can appear at
once. Those stars pump out enormous
amounts of particles and radiation,
creating stellar winds to accompany
their high energy starlight. These
winds, stronger than any hurricane, but
so thin that you can feel them rushing
by you, can combine together and sweep
through the galaxies, heating up the
rest of the interstellar gas. And when
the gas is hot, it can't cool off to
collapse, condense, and form stars.
Sometimes the stellar winds can be so
strong that they drive material out of
the galaxies altogether, stopping
accretion in its tracks. Loosened from
the gravitational grip of the galaxy
like dandelion seeds in the wind, that
material can no longer participate in
the game of star formation. And this
process of galactic expulsion goes
double when supernovi go off. Since
clusters of massive stars tend to be
born together, they have roughly the
same age and life cycle, meaning that
they also tend to die as a group. When
hundreds or thousands of supernovi
explode nearly simultaneously, they
launch massive plumes of material that
blow out holes in interstellar gas like
the local bubble that the solar system
is currently sailing through. And in
2016, a team of curious astronomers led
by Arti Camps Ferina went out in search
of these explosive events. After weeks
of searching, they found a beautiful
site. A series of three bubbles
surrounding M33, the triangulum galaxy,
nested together like Russian dolls.
Even the very mechanisms that lead to
accretion can quench a galaxy,
preventing it from forming further
stars.
As galaxies drift through the
intergalactic medium, they enjoy a
steady supply of infalling material that
gently rains down, mixing with its own
interstellar medium and refilling its
reservoirs of star forming fuel. The
galaxies aren't moving around randomly
in the universe. They are drawn towards
each other and drawn towards larger
clusters. In the modern universe, the
clusters are home to a thousand galaxies
or more. Bound together with their
mutual gravitational attraction.
Threading and winding its way through
those galaxies is a hot plasma. That
plasma is thin enough to register as a
vacuum in Earth laboratories, but can
still play an important role. When a new
galaxy enters into a cluster, it
encounters the hot gas. The movement of
the galaxy through the hot gas heats up
the material within the galaxy itself,
which shuts off the formation of new
stars. As well as this, the simple act
of friction between the gas in the
cluster and the gas in the galaxy rips
away material. Astronomers can even see
this process affecting galaxies right
now. When they do, they observe long
tentacles of gas streaming behind the
galaxies.
And of course, the astronomers call them
jellyfish galaxies.
Once again, the twin effects of heating
of gas and removing it from the galaxy
couldn't quench it, turning it from blue
and bright to red and dead. But arguably
the greatest galaxy killer of all isn't
the thunderous winds from supernovi
explosions or the friction and heat
generated from mergers. Instead, it is
perhaps the black heart that sits in the
center of almost every galaxy.
When we look at red and dead galaxies in
radio wavelengths, we can see a long
thin jet of intense radiation piercing
nearly 5,000 light years through the
body of the galaxy. And when the
astronomers of the event horizon
telescope trained their globe spanning
instrument on the center of M87, they
captured the shadow of an enormous super
massive black hole.
A super massive black hole surrounded by
a disc of intense radiation. A jet of
energetic particles capable of reaching
thousands of light years and an empty
ghost town of a gigantic galaxy.
It looked like the scene of a crime with
the knife still thrust into the victim
and the sinister villain hiding in the
shadows.
Super massive black holes, those with
more than a million suns worth of mass,
appear very early in cosmic history. By
some observations, just a few hundred
million years after the Big Bang. And
this means that the story of galaxy
evolution is twinned with the story of
black hole evolution. Like a dark secret
that they try to hide their whole lives,
a dark secret that ultimately destroys
them. For the most part, the rest of the
galaxy do not really notice the black
hole at the center. While each black
hole is individually massive, they still
weigh less than 1% of the entire mass of
a galaxy.
Except that is when black holes awaken.
And these are the quazars. The same
radiointense objects that astronomers
first noticed in the 1950s and cemented
our understanding of a changing evolving
universe.
[Music]
Indeed, soon after quazars were first
discovered, Fred Hy, the same astronomer
that railed against the idea of an
expanding universe until his death,
suggested that super massive stars might
power them with their intense gravity.
But then Richard Fineman pointed out
that such gigantic stars couldn't
support themselves. Eventually,
astronomers realized that it must be a
black hole at the center. But as with
the big bang theory, despite his idea
not gaining wider acceptance, Hy's
choice of name stuck and so they became
known as super massive black holes.
When a clump of matter comes too close
to the central super massive, either in
the form of a wandering gas cloud or an
errant star, the extreme gravity of the
black hole rips it to shreds, forming
its material into a thin disc. The disc
of accreting material reaches
temperatures of trillions of Kelvin. And
at that temperature, it emits a flood of
high energy radiation, ultraviolet, and
X-rays that rush through the rest of the
galaxy. Indeed, the quazars powered by
super massive black holes don't just
heat a patch of neighborhood
interstellar gas. They can literally
inflate an entire galaxy, puffing it up
like a hot air balloon. The hot gas
can't collect, can't condense, can't
fragment, can't form stars. It can only
drift aimlessly and uselessly in a
superheated ionized state.
Of course, as with all things galactic,
in moderation, an active quazar can be a
good thing. It can slow down star
formation, preventing a galaxy from
using up all its gas at once. In this
way, the central black hole, despite its
relative smallness compared to the
galaxy at large, plays an outsized role
in galactic evolution, serving as a
critical regulator to keep a galaxy
running at its most efficient level. But
when a quazar gets too bright, it can
destroy a galaxy from the inside out.
This typically occurs after a major
merger event. When a galaxy meets
another of equal size, their twin black
holes quickly find each other and merge.
And that unified black hole makes its
way to the center of the new galaxy.
Once planted there, it engorges on the
feast of material crashing around,
taking advantage of this short-lived
surplus.
Once this quazar activates, it drives so
much gas to a high temperature that it
doesn't just slow down star formation,
but stops it completely. So much
material leaves the galactic environment
that only a feeble trickle remains. And
if that weren't enough, the quazar has
one more trick up its sleeve. The jet.
The ferocious energies of the accretion
disc whip up enormously strong magnetic
fields. But these magnetic fields are so
strong that some of the accreting gas
follows their influence instead,
following twisting corkcrew paths around
the black hole and eventually emerging
in the form of powerful jets racing out
of the poles of the system. The
particles in the jet are accelerated to
nearly the speed of light and a cocoon
of magnetic fields keep the jet straight
and narrow for up to tens of thousands
of light years. Any loose material in
the path of the jet is simply shoved out
of the galaxy wholesale, never to
participate in the grand dance of star
formation ever again.
And so this was the smoking gun that
killed M87.
Simply too many mergers and a too eager
super massive black hole.
Though not all galaxies will die through
merger, some will be lucky, distant
enough from their nearest neighbors that
they will escape that fate. But even if
they manage to keep producing stars for
eons to come, eventually their lights
will dim too. The universe is still
changing, still expanding, still on
average pulling galaxies away from each
other. There is only so much material
available in the cosmos. Slowly, ever so
slowly, all galaxies will simply run out
of gas.
In this sense, galaxies are temporary
creations. They will live for much
longer than the current age of the
universe, but they still only have a
finite lifespan. And so while cosmic
noon, the peak of the universe, came and
went in the blink of a cosmic eye, a
mere 3 billion years after the big bang,
cosmic evening and cosmic twilight will
last much longer.
[Music]
The automated system awoke the
astronomer.
She roused herself from deep slumber
methodically, but did not rush. Checking
the chronometer, she'd been asleep for
over 140,000 years. The analysis could
wait until after coffee.
Her station was a remote one, as were
all such stations. It was on the surface
of the moon once known as Titan. But
long ago, it had cooled and solidified,
losing its oceans and its atmosphere in
the process. It still orbited what
remained of Saturn, which in turn
orbited the black dwarf sun, which now
had a temperature of just under minus
170° C. The cache of antimatter powered
her station and would continue to do so
for a long time to come, more than
enough to chart the end of the universe.
There was no larger purpose to this
mission. She knew the recordings she
kept, the notes she diligently took,
would ultimately be read by nobody and
would in time dissolve along with all
other microscopic objects.
And so the astronomer worked, passing
through the meager data collected by her
observatory outpost. And there it was,
exactly what she had hoped for. After
several million years of false
positives, this was something. A new
star had been born. It was small, a
fraction of what the sun once was, but
it was still a star. Its formation had
been triggered by the random collision
of two brown dwarfs, giving the new
entity just enough mass to reach the
critical fusion densities in its core.
After completing an all sky scan, she
added the new object to her log book. In
total, there were now 78 stars shining
in the Milky Way galaxy, the highest
number in over a trillion years.
Content, the astronomer returned to her
stasis chamber to await the next signal.
The world of the faristant future of the
universe would be utterly alien to us.
Accustomed as we are to planets, oceans,
living suns and skies full of stars.
But in the end, darkness will come for
us all.
As we know, the universe is expanding.
And that expansion has driven a cosmos
of change. A change from a hot, dense
plasma into a cool gas. A change from a
thin soup of hydrogen into pockets of
star formation. A change from the dark
ages to the greatness of cosmic known.
And now we stand in a universe 13.8
billion years old in the twilight years
of the cosmos.
Star formation peaked long ago. And
while galaxies have continued to
assemble and grow, their total
brightness has already begun the long
slow decline into oblivion. Advancing
this decline like the rapid onset of a
terminal disease is dark energy. the
mysterious substance that is causing the
expansion of the universe to accelerate.
We don't know for sure its true nature,
its cause, or its ultimate evolution.
But if it continues to operate as it's
done for over 5 billion years, then the
fate of our cosmos is sealed.
Accelerated expansion drives everything
away from everything else. In doing so,
it starves the galaxies of the vital
material they need to build new stars.
As well as this, in enough time, it will
pluck any stars, planets, and remnants
away from the galaxies one by one,
dissolving them. But the grimness of
this fate is offset by its longevity.
When we discuss the timeline of the
universe, we find that we are still in
its infant stages. And what we perceive
as the decline from the cosmic noon is
really the brief initial flash that set
the stage in motion that put all the
pieces in place for the cosmos to live a
long life.
And in those distant time scales when
the universe is orders of magnitude
older than its present age, new wonders
will appear.
Strange creatures that we can't possibly
fathom in the youthful time of the
present.
[Music]
In 100 billion years, the observable
universe will disappear from view. Any
galaxy not already bound by gravity to
our local group will be caught up in the
accelerated expansion of dark energy and
pulled away from us faster than the
speed of light until they disappear.
And in that time, a new kind of star
will emerge in the Milky Way. One that
we do not see in the present epoch
simply because the universe is not yet
old enough. The largest stars will be
long gone, as will almost all sunlike
stars. Instead, our galaxy will be
dominated by the red dwarfs. But as they
evolve, they will gain temperature,
becoming blue dwarfs, small, dim, but
bright blue, like jewels set within the
galaxy.
After a trillion years, Mecomda, the
nickname given to the future galaxy,
composed of the mergers of the Milky
Way, Andromeda, and all the other local
group members, will begin to dissolve.
Random chance encounters will scatter
individual stars, planets, and remnants
out of the galaxy where they will
immediately get caught up in the
accelerated expansion and fly away. At
the same time, the cosmic microwave
background, our backbone observation of
the birth of the universe will become so
redshifted and cold that it will be
impossible to observe. And it is in this
strange new reality that another kind of
star would appear for the first time.
[Music]
In the present day, stars have a
fundamental lower mass limit set by the
need to achieve fusion reactions in
their cores. But far into the future,
the galaxy will be so enriched with
heavy metals that it can lower the
temperatures and masses of possible
stars. Indeed, the smallest stars will
be so dimminative and cool that their
outer surfaces could have temperatures
below zero. Encased in clouds of ice,
these frozen stars will barely be
detectable, but they will exist.
Sometime between 10 and 100 trillion
years from now, the last star will be
born. By that time, the universe will be
more than 10 to the power of 2,000 times
larger than it is today. In what remains
of the galaxy, brown dwarfs will
occasionally collide, reaching the
thresholds necessary to convert into red
dwarfs. But that will be the end of
stars as we know it. Some 10 to 20
trillion years after that, the last star
will go out.
In the ensuing eons, the galaxy will
continue to dissolve. By 10 the power of
15 years, our sun, long since cooled to
become a black dwarf, will have a
temperature of only 5° above absolute
zero. Through gravitational
interactions, all planets would have
detached from their parent stars and
likely been flung into the deep abyss.
And by 10 the^ of 30 years, all
macroscopic objects would have either
completely detached from the galaxy or
fallen into the central super massive
black hole.
[Music]
By this time, all macroscopic objects
will be solitary, living in pockets of
the universe all of their own.
In 10 the^ of 100 years, all black holes
will disappear through Hawking
radiation. And starting around 10 the
thousand years from now, even the black
dwarfs will die, exploding in supernova
detonations as they lose their
electrons, lowering the limit needed for
catastrophic collapse.
And finally, in 10 to the power of 1,500
years, perhaps the last kind of star
appears on the cosmic scene.
Although due to the accelerated
expansion of dark energy, there will be
no observers to witness it. A form of
cold fusion moderated by muons will
convert all planetary mass objects into
solid spheres of pure iron.
These are the iron stars. They will not
glow except through the feeble heat due
to their temperature a shred above
absolute zero.
But they will persist.
For how long is anybody's guess?
Indeed, one estimate suggests that some
10 to the^ of 10 to the^ of 76 years
from now, all objects will dissolve
through quantum tunneling, even the iron
stars. And beyond this point, all
molecules, atoms, and nucleons will
disappear.
The universe will be composed of nothing
more than a thin soup of subatomic
particles and radiation separated from
each other by vast expanses of
nothingness.
And the decay that will lead to this
cold empty cosmos
has already begun.