Not just your house number.
Or the name of your street.
Where actually are you?
In a town, or a city, which is in a country, on a continent, on Earth.
But where is Earth?
It’s in the solar system, between Venus and Mars - you might say.
But where is that?
The solar system is the main part of the Oort cloud, a vast collection of comets, asteroids,
and icy objects swirling at the fringes of the Sun’s sphere of influence.
The Oort cloud resides in the Local Interstellar Cloud, which is in the Local Cavity of the
Orion Arm of the Milky Way Galaxy.
The Milky Way is a spiral galaxy, with beautiful, sweeping arms of millions upon millions of
stars, all rotating around a bright galactic core with a dark supermassive black hole at
its centre.
At over 100,000 light years across, the Milky Way is vast - but it is still just a sliver
of what we can see.
Zoom out further and you’ll see a Local Group, with 30-50 small galaxies and a monster
on a collision course: the Andromeda Galaxy.
Twice the size of the Milky Way, Andromeda is speeding towards us.
We will collide in a few billion years, tearing each other apart before coalescing into one.
But when you zoom out even further, the impending collision of two galaxies seems inconsequential.
At the scale of millions of light-years, the structure of the Virgo Supercluster, a collection
of thousands of galaxies, becomes apparent.
And dwarfing even that is the Laniakea Supercluster, hundreds of millions of light-years across,
containing several other superclusters like our Virgo, which is itself part of the Pisces–Cetus
Supercluster Complex - a galactic filament almost a billion light years long.
And it is now that the cosmic web becomes visible.
There are more filaments like our own, as well as great strings of superclusters
There are also giant stretches of space with virtually nothing in them, like the Boötes
Void, an area 330 million light-years across in which we have discovered barely 60 galaxies.
An inkblot on the speckled sky.
Zooming out further, we finally reach the edge of the observable Universe, where primordial
light has been travelling since nearly the beginning of time — 13.8 billion years — to
reach our eyes.
But since then, the Universe has expanded further, meaning that the true distance to
the edge of the Universe is about 46 billion light years in one direction, meaning the full observable Universe is a sphere 93 billion lightyears in diameter.
Structures any larger than a few billion light-years are hard to define with our current technology,
partially because we are trying to map something we inhabit, and partially there may well be a limit - something scientists call The End Of Greatness.
But at every level up to this point
— Planets, solar systems, galaxies, clusters, superclusters,
— the Universe is full of structure.
It is not a random and chaotic jumble.
It seems organized.
Yet the Universe started as a hot, dense soup of particles.
Why should it be structured?
Why did that hot soup evolve into a Universe where some parts are filled with beautiful,
sweeping arms of stars, while others are barren deserts?
And more importantly - how do we know?
How did we start this channel?
To begin with - a lot of research and in depth exploration of the universe's first moments.
And a huge part of that was through Wondrium, the educational subscription service.
And I'm not exaggerating.
Wondrium used to be called Great Courses Plus, which made university level lecture series
- they have now expanded to contain even more content.
From travelogues and tutorials to long form documentaries, Wondrium is now the best place
for all your high quality, enjoyable content that will make you smarter.
One great example of this is Sean Carroll's peerless lecture series "Dark Matter, Dark
Energy" - I am currently on my third listen through and I learn something new each time.
24 half hour cosmology lectures given by the best science communicator out there- for example
it really helped me understand the reasoning behind the theory of inflation.
Just superb - i am not exaggerating when i say that without Wondrium this channel may
not have existed.
It is the best educational subscription service out there.
And they’re giving viewers a great offer of a free trial.
So, get the fantastic education I did by heading over to wondrium.com/historyoftheuniverse.
Thanks to Wondrium for supporting education on youtube.
The year is 1502, and Nicolaus Copernicus should be studying medicine in Padua.
It is the centre for medical education in Europe - all the best teachers and students gather there to learn how to heal the sick.
But Nicolaus is a polymath - he is learning medicine, but he’s also reading Aristotle
and Plutarch and Plato, finding holes in Ptolemy’s theories in his spare time.
Finishing his degree he returns home to Poland to work as a physician for his father.
Though he doesn't forget about the cosmos - or our place in it.
And so it is a decade later that he writes a pamphlet outlining his now ubiquitous theory:
we are not the centre of the Universe.
The Earth orbits the Sun, just like all the other planets.
We are not special.
Nicolaus knows the uproar this will cause, so he doesn’t publish right away.
It’s only in his 70th year — some say it’s actually on his deathbed that he approves the first printed copy — that the book finally comes out.
And it shakes the very foundations of science, astronomy, philosophy, and religion - an idea
so dangerous and fantastical that its proponents would be prosecuted for decades after its
publication.
The heretical notion that our Earth is just the third planet among many.
At the time it was a shocking thought - but it marked only the beginning of a larger revelation.
For if we’re just some planet orbiting a random star, what does that make us?
Not special.
It’s a sad realisation at first, but when we think deeper it opens up a world of possibilities.
If we’re not special, what else is out there — who else is out there — to discover?
Known as the Copernican principle, this idea is now a bedrock of modern cosmology, and has two related tenets: 1.
The Earth is not special — we don’t occupy some privileged position in the Universe 2.
Therefore, observations from the Earth should be relatively representative of observations from anywhere.
But, is this true? One modern day discovery that backed this up was the detection of the Cosmic Microwave
Background Radiation, or CMBR.
A fuzzy background static, it permeates the sky, no matter which direction you look.
No matter how powerful your telescope, if you keep zooming in on a part of the sky with nothing in it, you will still detect faint microwave radiation - always with almost the
same wavelength.
Emitted at a time close to the beginning of the Universe, at the moment when matter cooled
down enough for protons to capture electrons and become clear gas, it is everywhere.
In this regard, our bit of the universe is clearly not special.
Outer space, no matter which direction you look, has a temperature of about 2.7 degrees
Kelvin.
It doesn't matter if you are floating in outer space near the Milky Way or past distant Methusaleh,
the average temperature of empty space is about the same.
But there is an obvious issue with this - one that does not require a high-powered telescope
to recognise.
Our cosmic address is full of huge structures: Galaxies, superclusters, great walls of stars
hundreds of millions of lightyears across, and great voids with nothing in them.
The Universe clearly isn’t totally uniform.
We aren’t special - but there are structures.
How can we reconcile these two ideas? All large-scale structure in the Universe can be explained by two opposing forces: expansion
and gravity.
The Universe started hot and dense, and then underwent three phases of expansion.
The first lasted a fraction of a second and was extreme.
This is called Inflation.
It then continued to expand in a second stage for several billion years at a more reasonable rate, cooling down, spreading out, and slowing its expansion.
And in the last few billion years, it has entered a third stage, in which expansion has accelerated again.
What is responsible for these shifting regimes of expansion? The initial inflation has been theorised to have been caused by a mysterious field called
the inflaton, the latter expansion a mysterious energy so baffling it is simply known as “dark
energy”.
In truth we don´t know for sure.
But what we are sure of is the existence of gravity, the tendency for mass to clump together,
which has spent billions of years battling against the quickening expansion.
When a part of the early Universe became a bit denser, that overdensity attracted more
matter, which consequently made it more massive and so gravity acted even stronger to bring
more matter in.
Clouds of dust and gas became stars, which clump into galaxies, and so on.
It was and is gravity doing what gravity does, at its most basic level.
Problem solved.
But not quite.
Because you can't start this process with a totally even universe.
Where did the first overdensities come from?
The best answers we have now started to be revealed with space telescopes in the late
1980s, and the race to map the entire universe to higher and higher resolution.
The Cosmic Background Explorer (COBE) was launched from Vandenberg Airforce Base in
southern California by Nasa in 1989.
The cold war was ending, and a new era of cooperation between global superpowers was
about to begin - and what better way to start this new era than with the first “baby picture”
of the Universe.
COBE had a bold mission: take detailed images of the entire celestial sphere, and tell us
how uniform the background radiation really is.
On April 24, 1992, the results were plastered on the front page of the New York Times, big
enough for the world to read: “Scientists Report Profound Insight on How Time Began”
COBE had mapped tiny variations — anisotropies — in the cosmic microwave background radiation.
They were very small, just one part in 100,000, but they were there.
This blockbuster result won Nobel Prizes in 2006 for two of the lead astronomers on the
telescope — John C. Mather and George F. Smoot — with the Nobel committee marking
it as the birth of cosmology as a “precise science.”
The success of COBE led Nasa to launch another telescope in 2001, called the Wilkinson Microwave
Anisotropy Probe, or WMAP.
This further proved the blockbuster capabilities of space telescopes, and instead of a vague
smear of under- and over-dense regions, WMP was able to tell fine structure, and patterns
started to emerge.
So again, in 2009, the Planck Satellite was launched by the European Space Agency to get
an even better picture of this baby Universe.
Each successive experiment kept demonstrating the same principle: there were tiny fluctuations
in the very early Universe, which, after being stretched out by expansion and clumped by
gravity, led to the structures we see today.
And there are dozens of other experiments, telescopes, and satellites, each discovering
and mapping new parts of this structure.
Back on Earth, one of the best telescopes for mapping the Universe is the Sloan Digital Sky Survey.
Perched on a mountain in New Mexico, this telescope creates a map of the whole sky in
an unusual way.
First, the astronomers take a regular, optical image.
Then they create an aluminium plate, with a precise hole drilled for each star, galaxy, or other astronomical object in the image.
They then run a fibre optic cable from each of the holes in this plate to spectrographs,
so they can find the colours of the stars, and therefore find out how far away they are and how fast they’re moving.
These rainbow-collecting aluminium plates are each custom-drilled and capture a unique slice of the Universe.
So far, over 10,000 plates have been drilled.
By combining data from different surveys, astronomers managed to define Laniakea, our
local supercluster.
This is our home — a group of thousands of galaxies all bound together by gravity.
And our universe maps are filled with many more fantastical places: Of course stretching out from our own Local Group of galaxies is the Virgo supercluster,
whose tendrils connect into the Centaurus supercluster.
Beyond that is the Perseus–Pisces Supercluster, and the South Pole Wall, which stretches across
over a billion light years.
And the Sloan Great Wall is about one billion light years in the other direction, an equally
mammoth structure.
And some astronomers believe that the milky way and a large part of Laniakea actually
reside within another supermassive void, known as KBC - proposed to be almost 2 billion light
years in diameter - though its existence is still hotly debated.
But even these have been dwarfed by a recent discovery.
The Giant Arc was discovered in 2021 and is about 9 billion light years away.
It is thought to span 3.1 billion light years, 3.5 percent of the observable cosmos - and
is so huge it actually challenges our assumptions about the universe on the largest scales.
If you zoom all the way out, a pattern appears.
Filaments tie together into walls and clusters, with large voids hanging between them - the
cosmic web.
But mathematically, what is the structure of this web? Is it like meatballs of matter in a soup of emptiness, or pancakes of matter arranged
at jaunty angles? Or perhaps there’s a third, more accurate way to think about it? Surprisingly, answering this seemingly ridiculous question: “which kitchen metaphor does the
large-scale structure of our Universe most resemble?” would obsess the field of cosmology for decades.
Truth, pride, and Nobel prizes were at stake.
At the largest scales, is the Universe like a meatball soup, a honeycomb, or a sponge?
A grizzled veteran of the soviet atomic program sits in his office in Moscow, contemplating
the Universe.
“How did the large-scale structure of the Universe arise?”
He asks himself.
Perhaps his contemplations make him hungry for lunch.
On the other side of the world, a mild-mannered Canadian sits in his office in Princeton,
contemplating the Universe.
“How do tiny particles become huge galaxies?”
Similarly hunger-inducing thoughts.
So perhaps he wanders down to the cafeteria for a break.
The name of this astronomer in America is Jim Peebles.
He would eventually win the Nobel Prize in physics in 2019 for “theoretical discoveries
in physical cosmology.” He and others of the “American School” believed that matter in the Universe was organized
like meatballs in a low-density soup.
Matter would attract matter, which would attract more mass, clumping together in roughly spherical
assemblies, leaving vast areas of low density.
This was a bottom-up approach.
The large scale structure gets formed from clumping smaller-scale structures.
Meanwhile, behind the Iron Curtain, Soviet astronomers were busy developing their own
theory.
Yakov Zeldovich was the Russian musing on the cosmos.
By the 70s, he was a renowned physicist who had cut his teeth in the Soviet nuclear program,
before turning his attention to the large scale structure of the Universe.
He developed a theory where the density in the Universe was organized in 2D pancakes,
and where the pancakes overlapped, you would have galaxy formation.
In that way, the large-scale structure comes first, and galaxies are formed out of them.
In Zeldovich’s Universe, there are large empty voids, with filaments on the edges of pancakes and clusters where pancakes intersect.
Zeldovich was a brilliant scientist, with a lot of sway in the Soviet Union.
But because of his role, he was rarely if ever permitted to leave the USSR.
So his ideas took longer to disseminate than they might have otherwise.
And so at the height of the Cold War, you had Jim Peebles — a soft-spoken Canadian
living in the USA — advocating for a universe dominated by meatball clumps.
And the Russian school — led by a hardened veteran of the Soviet atomic program — believing
in the pancake model.
Meatballs vs pancakes.
There’s no evidence of animosity between the two, but without hard data, each stuck
to their own beliefs.
But luckily, between them, was an unlikely interloper:
Martin Rees — an English astronomer who would one day become the UK’s Astronomer
Royal.
He acted as a sort of go-between for the American and Russian schools.
The English were more permissive about which meetings they were allowed to attend than
either the Americans or the Russians.
And so Rees’s postdoc, a man from Kentucky named Richard Gott, then developed a third
theory that better matches the findings of later experiments like the Sloan Digital Sky
Survey.
Gott met Zeldovich at a conference in Estonia, then part of the USSR, in 1977.
And this meeting, along with developments in observation, would lead to a new understanding
of the Universe.
As recounted by Gott in his book The Cosmic Webb, Zeldovich was a character:
“Always use the median,” he said.
“In Russia the watches are not made very well, so when friends get together they compare
the times on their watches.
One says 5 minutes to 5, another says 5 o’clock, the other says 11 o’clock.
Use the median!” Inspired by the brilliant Soviet, Gott would develop a new theory: the Universe is like
a sponge.
The topology of Gott’s Universe lies conceptually between the American meatballs and the Russian
pancakes.
According to him, high-density and low-density regions are connected and complementary.
In other words, you could switch high-density for low-density and it would be the same.
As new maps like those from the Sloan Digital Sky Survey became available, this is the structure
that was observed.
Neither meatballs nor pancakes, but a sponge.
But this is not everything that our telescopes have seen.
As our methods of scanning the skies have improved and we have seen further back in
time with increasing detail, something strange has been observed.
A phenomena referred to by some as The End Of Greatness.
At around a billion light years the universe finally becomes homogeneous, the same everywhere.
Large scale structure disappears.
Why?
Imagine you are in Ecuador, standing on the equator.
Look up in the night’s sky with a powerful telescope and observe the Cosmic Microwave
Background Radiation in all of its majesty.
Take a careful measurement of its temperature.
Now fly to Indonesia, on the equator, on the exact opposite side of the Earth, and look
up.
If you take a careful measurement of the temperature of the CMBR, you’ll find it matches the
measurement you took in Ecuador.
The light from each point took about 14 billion years to reach you, meaning the points would
now be 28 billion light years apart, except that space has been expanding in the intervening
years, so they’re really much further apart than that.
And yet, the Universe is only about 13.8 billion years old.
To put it simply - there hadn’t been enough time for the light that you saw in Indonesia
to have reached the light you saw in Ecuador.
So how could they possibly be the same temperature?
How could they “know” what temperature to agree on if they couldn’t thermodynamically talk to each other?
Coincidence you might think? So you pick two other random points in the sky.
They will also match temperature to within one part in 100,000.
This is the Horizon problem.
When you look at the microwave background radiation, it is extremely uniform.
Everything is the same temperature, no matter where you look.
When you look at colourful images from experiments like WMAP or Planck, the colour differences
are deceptive.
The differences between red and blue are actually tiny.
They are differences of one part in 100,000.
For almost all intents and purposes, that’s the same temperature.
But this poses a fundamental physical challenge.
And it’s one of the reasons for the development of the idea of inflation.
The theory, proposed by Alan Guth in the 1980s, is that there must have been some very rapid
expansion in the early Universe.
With inflation, parts of the Universe that are now separated by unimaginable distances
could have been in very close proximity in the distant past.
But in order for inflation to solve the horizon problem, the expansion would have had to take
place much faster than the speed of light.
This may seem counterintuitive because of the “universal speed limit” of light,
but that only applies to matter and energy within the Universe.
Space itself doesn’t need to obey the speed limit.
In fact, due to the rate of the expansion of the universe, space today is still expanding
faster than the speed of light - two galaxies more than 15-20 billion light years away would
be moving away from each other faster than light.
This means that the part of the Universe you saw in Indonesia, and the part you saw in
Ecuador, used to be much closer together.
So close, in fact, that they could equilibrate to basically the same temperature and cause
the haze of uniformity that we see when we look out.
But of course Inflation is an extraordinary claim, for which extraordinary evidence is
required - and definitive observational proof for this theory remains elusive.
Despite this, indirect proof continues to pile up, which makes inflation an attractive
theory.
As we know, a Universe that underwent inflation would be homogeneous, which, on large scales,
ours is.
An inflationary Universe would also be flat, which ours appears to be.
This “flatness” refers not to the number of dimensions, but instead to the effect of the density of matter and energy on the Universe.
Cosmologists can measure the shape of the Universe in two independent ways: First, they can count up all the mass and energy in the Universe and divide it by the “critical energy density”
— the density at which the Universe would be flat.
And they can also measure it geometrically, by measuring angles.
On a positively curved sphere, like the Earth, a triangle drawn on the surface will have angles that add up to more than 180 degrees.
The 3D geometry of the Universe would be “closed” like this if it had more stuff in it.
Eventually, gravity would pull everything together into a Big Crunch.
In a Universe with less stuff in it, expansion would continue forever and space would be
hyperbolic: it would be like the inside surface of a donut.
In a flat Universe, straight lines and angles make sense: triangles have angles that add
up to 180 degrees.
And results from both types of experiment show that what we have is a flat Universe,
which is another point for inflation.
And so this is strong evidence for the theory - but how then did inflation work to make
the Universe we see? One of almost perfect homogeneity - but not exactly perfect?
So much can be gleaned from the “baby picture,” the Cosmic Microwave Background Radiation,
which shows us the after-effects of very early fluctuations in density in the early universe.
But anything before the CMBR (which was emitted about 380,000 years after the Big Bang) is obscured by the fact that everything was too hot for light to travel in straight lines:
excited electrons kept getting in the way - and inflation is supposed to have happened within a tiny fraction of a second after the birth of the universe.
And so there are nearly 380,000 years of physics that are hidden by the haze of 3-degree radiation.
Where did the early fluctuations seen in the CMB come from?
It's easy to point at a tree and explain that it grew from a seed, but where did that seed
come from?
Imagine a box with nothing in it.
Close the lid so no light can get in.
Take all the air out with a pump and seal it.
Cool it down to absolute zero.
Insulate it with lead so no radiation can penetrate.
Is it empty?
No.
The Universe is a fundamentally noisy place.
There is no such thing as truly empty space.
Even if there were no matter inside the box, space is always permeated by quantum fields
with different values at different positions.
Some of those fields, like the Higgs field, give things mass.
Others, like the electromagnetic field, give things charge and carry light.
One of the big discoveries of early modern physics — by the likes of Heisenberg, Einstein,
and Planck — was the inherent noisiness of the Universe.
There is uncertainty at the smallest scales, which shows up as a faint buzz in the quantum
fields.
At the quantum level, even “empty” space is filled with quantum fields whose values
are varying randomly.
But why is this important for the large scale structure of the universe?
To answer that question we have to go back to the very start.
In the beginning, the observable universe was hot, dense, and tiny.
The also tiny quantum fluctuations were buzzing along, with very small differences in the
roiling patchwork of energy fields - as they do today on such small scales.
And then, suddenly and dramatically - inflation: The observable universe rapidly expanded by
26 orders of magnitude in a tiny fraction of a second.
Changes this extreme are hard to describe.
We don’t have the words to properly conjure the scale of this explosion in our heads.
26 orders of magnitude is a grain of sand 1 millimetre across expanding to a septillion
metres across.
That’s one hundred million lightyears — something on the scale of Laniakea.
And the violence of this detonation was staggering: in 10^-33 seconds the observable Universe
went from microscopic to huge.
For all intents and purposes, the expansion was instantaneous.
And so this inflation amplified the quantum noise of the vacuum into genuine differences
in density - thus giving gravity all it needed to work its attractive magic, gathering mass
together into stars, galaxies, superclusters, and filaments.
These variations would have been the first seeds - inconceivably tiny quantum fluctuations
the forefathers of vast intergalactic filaments many millions of light years in length, tiny
acorns growing into a vast forest.
But that is not the end of the story.
After inflation was complete, the Universe continued to expand and cool down, but it
remained extremely hot and dense.
If the large-scale structure in the Universe were purely shaped by the expansion of quantum
fluctuations, you would expect everything to be distributed fairly randomly.
Indeed - if you look at a population of galaxies and ask how far each galaxy is from each other
galaxy in the population, you should see a simple relationship and regular clustering.
Galaxies would tend to be fairly close together because of the clumping effect of gravity.
And so you should be less and less likely to find two galaxies separated by larger distances.
But this is not what astronomers see.
When they do this kind of analysis they observe one peak for galaxies that are close together
— as expected — and another one for a separation of about 150 megaparsecs.
What explains this overabundance at what seems like an arbitrary distance? The answer lies in what’s known as Baryon Acoustic Oscillations.
Galaxy dwarfing sound waves.
For hundreds of thousands of years after the Big Bang, it was too hot for atomic nuclei to capture electrons.
The universe was a sea of charged particles, and photons of light couldn’t travel in straight lines.
They would get bounced around, scattering in every direction until they hit the next stray electron.
As it cooled, some of the regions that were more dense would pull more and more stuff
in.
But photons exert a pressure when they get too close together, and this pressure would push out, carrying some matter with it.
And this is where Dark Matter entered the picture.
Dark Matter outweighs regular matter in the Universe by five times.
It was, and remains, the dominant gravitational force in our Universe.
The cosmic web of stars, galaxies, and filaments is really a web of Dark Matter, with regular
matter tagging along.
It accounts for something like 85% of the gravity in the Universe, so everything from
galaxy formation to star clustering and gravitational lensing is dictated by its properties.
And yet, we still don’t know what it really is.
What we know is that Dark Matter interacts gravitationally, but doesn’t seem to be
affected by electromagnetism (such as light), or the other fundamental forces.
So as clumps of density began to form in the early Universe, the photons would eventually get squeezed so much that they would burst out in a ferocious wave of plasma, carrying
some regular matter — also known as baryons — but leaving the Dark matter behind in
the centre.
Imagine a pebble dropped in a pool.
There is a central splash, and then concentric rings of ripples.
Now imagine millions of pebbles dropped at slightly different times.
It would create a complex jumble of splashes, ripples, and regions where the rings interact.
This was the state of the Universe after inflation.
But the Universe was cooling.
And once it reached 3000 Kelvin, protons were able to capture electrons and the phase changed.
Instead of a bubbling, opaque plasma, it became a transparent gas.
The moment the Universe became transparent, the Cosmic Microwave Background Radiation
escaped.
And the expanding rings suddenly stopped, because the photons that were carrying the
matter were no longer coupled with that matter.
The photons rushed off in a straight line, leaving the ring frozen at the distance it
had reached.
And so that distance is 150 megaparsecs, exactly where we see the second bump in the graph
of galaxy separations.
This is one of the ways that cosmologists can see past the CMBR, by deciphering the
patterns in its seemingly random distribution.
Dark Matter is central to this story - and indeed the structuring of the universe that
followed.
In fact in a lot of ways Dark Matter is the true main character in the narrative of the
universe - and yet lay almost completely undetected until the mid 20th century.
Back in 1933, a young and ambitious Swiss astronomer named Fritz Zwicky noticed something
odd about the speed of stars in a galaxy.
He observed that two independent measures of the mass of a cluster of galaxies weren’t
lining up.
If he counted up all the stars and added their mass, he got a number ten times less than
if he instead calculated the mass by looking at the speeds of the galaxies in the cluster.
He concluded that there must be an enormous amount of unseen mass.
It would be more than 30 years later before further evidence would come to light, as pioneering
astronomer Vera Rubin set her sights on spiral galaxies.
In predicting the motions of stars around the centre of galaxies, astronomers had always made three assumptions: One: Gravity depends on distance and mass.
The closer and more massive two objects are, the stronger gravity acts on them.
Two: Spiral galaxies have most of their stars in their very bright central regions.
Three: Stars on the edges of galaxies would have less gravity acting on them and would
travel more slowly.
This is analogous to our own solar system: most of the mass is contained in the Sun,
so the close planets like Mercury, Venus, and Earth travel much faster than the gas
giants Saturn, Uranus, and Neptune.
So imagine Vera Rubin’s surprise when she observed the speeds of stars in nearby spiral
galaxies.
She found the relationship between speed and distance traced a straight line on her graph.
Instead of acting like the solar system, the stars on their edges of spiral galaxies travelled
just as fast as those in their cores.
And so there was a fourth, hidden assumption that astronomers were making, which Rubin
made explicit: While it was true that “Spiral galaxies have most of their stars in their central regions,” stars don’t account for most
of the mass.
In order to account for this increased speed, normal matter — stars, dust, nebulae — could
only account for 15% of the total matter in the Universe.
So what was this dark matter?
The leading theory is WIMPs — Weakly Interacting Massive Particles.
These would be elementary particles like protons or neutrons, but which would have peculiar properties.
Unlike protons, which have a charge and interact with light, these WIMPs would be invisible, and only interact by gravity.
They would also be much either much heavier or much more numerous in order to make up for the missing mass in the Universe.
Today we know that all galaxies except some dwarf galaxies have larger halos of dark matter surrounding them.
Scientists estimate that the Milky Way´s halo could extend up to 15 times further than the matter we can see.
Searches for these particles — at particle colliders like the Large Hadron Collider in Switzerland, at giant vats of noble gases in Antarctica, and other places, have so far
come up without definitive proof for the WIMP theory.
But scientists are hopeful, as dark matter explains so much about the large-scale structure
of the Universe.
It makes everything make sense: the echoes of baryonic shockwaves, the speeds of galaxies,
and the patterns of clustering.
In the growth of structure across the universe, normal matter, what we see and are - is only
really a bit-player.
The main role in building the universe goes to this mysterious invisible stuff.
And so, from miniscule quantum fluctuations, galaxy dwarfing sound waves and finally vast
tides of dark matter - our webbed universe was born.
Problem solved?
Not quite.
There are further structural mysteries out there.
Mysteries we haven't even begun to unravel.
We are all moving in the same direction.
We’re spinning on the Earth, which is revolving around the sun, which is spiralling around
the galaxy, which itself has motion.
But zooming much further out, all the galaxies around us in Laniakea are pulled towards the
same point in space.
We are all moving in the same direction.
Where are we going?
In the 1970s as scientists studied the CMB for the first time it became clear that our
local group of galaxies was not stationary - it was moving relative to the CMB.
In fact, we are rushing at around 1000 km a second, towards a point 250 million lightyears
away.
This point was dubbed The Great Attractor.
But what was there?
Why this particular point, and not somewhere else?
If, according to the Copernican principle, no point in space is more special than any
other, why should so much mass move towards this one spot?
We didn´t know then - and we still aren´t sure.
One challenge was that whatever we were heading for was very hard to observe because we had to look through the centre of our own galaxy - an area known as the Zone of Avoidance,
and one through which our view is obscured by interstellar dust.
But we could see through it with x-rays - and this method revealed a group of galaxies known
as the Noma cluster.
But this alone wasn´t enough to cover the flow of galaxies that was seen.
And so, scientists watched the Great Attractor, and came to a remarkable conclusion.
It is itself in motion, towards a greater source of mass.
650 million light years away lingered a vast supercluster - the largest for a billion light
years.
Known as the Shapley supercluster, this leviathan contains the mass of nearly 10,000 milky ways.
But that wasn´t the end.
In 2017, researchers proposed a further element.
Dubbed the “Dipole Repeller”, it is a large void in almost the exact opposite direction
to the Shapley supercluster.
Whereas Shapley pulls, this area of space doesn´t literally repel, but it´s under-density
means it can't overcome more powerful gravitational forces.
And we are sandwiched in the middle of this massive stellar flow, almost a billion light
years across.
Though this may seem unbeatably large - this is only the beginning of the structural oddities
that appear in the large-scale mapping of the cosmos.
One long standing mystery that came to light in the early 2000s was related to the assumed homogeneity of our universe.
The issue is fairly simple: If you divide the sphere of the CMBR into
four or eight, the top right is on average hotter than the bottom left.
This effect disappears if you divide the sky into more and more pieces.
But for the quadrupole and and the octopole, the angle of the line between the hot part
and the cold part matches almost exactly with the plane of our solar system.
This is a result so odd that astronomers have named it the “Axis of Evil,” because it
threatens to break our most fundamental ideas about the Universe.
Why would the background of the Universe be aligned with our solar system?
We don’t know.
Nobody does.
The chief scientist of the WMAP telescope, Charles Bennett, thinks it must be a coincidence.
He’s quoted in New Scientist saying, "I do think there is a bit of a psychological effect, people want to find unusual things."
But even with new data from the Planck Telescope, the effect is reproduced.
In the words of Dominik Schwarz from the University of Bielefeld in Germany: “For a long time, part of the community was hoping that this would go away, but it
hasn’t," It is still totally unexplained.
We understand so much about our physical world.
We can shoot satellites into the sky to take baby pictures of the Universe and use these
images to rewind the cosmos back to its first moments.
Between General Relativity and the Standard Model of particle physics, we think we have
it all figured out.
But there is much we do not know.
In order to have structure in the Universe, first the Universe must begin.
So how did we get our start?
What set off inflation?
The answer to this question may be hidden in the patterns of the Universe’s largest
structures.
One idea is that, in the primordial Universe, there is a roiling sea of dense vacuum energy,
and it just takes something to set off an inflationary bubble, which then grows into
a universe.
But there’s nothing saying ours was the only inflationary bubble.
Perhaps another Universe formed nearby, just on the other side of the CMBR.
And if it went through inflation, it might collide with us.
So this is one big mystery that the CMB may yet hold the answer to - great spots in the
sky, resulting from collisions with other bubble universes.
We have seen nothing yet, but even the possibility of evidence for multiverse theories is so
sparse that merely a method to find a glimmer of proof is enough to excite scientists.
Where are you?
The Observable Universe, Laniakea, Virgo Supercluster, Local Group, Milky Way, Orion Arm, Local Cavity,
Local Interstellar Cloud, Oort Cloud, Solar System, Earth.
At every scale, there is structure, there is order.
This structure is governed by simple principles — gravity, which is dominated by Dark Matter,
and expansion, which is now driven by Dark energy but which at one stage was the result
of inflation.
And yet these principles turn matter and energy into huge, beautiful structures, bathed in
the warm glow of the Cosmic Microwave Background Radiation.
Copernicus, Shapley, Zwicky, Rubin, Zeldovich, Peebles, Gott — all of these scientists
and the teams that supported them have brought us this far in uncovering its secrets.
And yet, despite all that we know, there are still mysteries, out there,
written in the stars.
