Where Did Dark Matter And Dark Energy Come From?

Summary

This script explores the mysteries of the universe, focusing on the discoveries of dark matter and dark energy. Astronomers' initial quest to measure cosmic expansion led to the shocking revelation that the universe is accelerating, not slowing down. The script delves into the history of these discoveries, from Einstein's theory of relativity to the Hubble telescope's observations, highlighting the Nobel-winning discovery of dark energy's existence. It discusses the impact of these findings on our understanding of the cosmos, the ongoing search for the nature of dark matter, and the theoretical implications for physics, leaving viewers with a sense of the vast unknowns that still dominate our universe.

Takeaways

• 🔭 In the early 1990s, two teams of astronomers, the High-Z Supernova Search Team and the Supernova Cosmology Project, aimed to measure the universe's composition and expansion rate.
• 🌌 Astronomers initially expected the universe's expansion to slow down due to the gravitational pull of matter and radiation, but observations revealed an unexpected acceleration.
• 🏆 The discovery of the universe's accelerating expansion, attributed to an unknown force called dark energy, earned Brian Schmidt, Saul Perlmutter, and Adam Riess the 2011 Nobel Prize in Physics.
• 🌟 Fritz Zwicky first proposed the concept of dark matter in the 1930s after observing that galaxies moved faster than expected within clusters, suggesting the presence of unseen mass.
• 🌀 Vera Rubin's research on the rotation speeds of galaxies provided further evidence for dark matter, as stars at the edges of galaxies moved at unexpected speeds, indicating additional gravitational forces.
• 🔬 The search for dark matter has involved various candidates, including neutrinos, black holes, and exotic particles like axions, but none have been definitively identified as the source.
• 🌌 The universe's structure, resembling a cosmic web with clusters, filaments, and voids, has been mapped through extensive surveys, revealing the influence of dark matter in shaping this structure.
• 🚀 The standard model of particle physics, while successful in explaining many aspects of the universe, does not account for gravity or dark matter, indicating it is incomplete.
• 🌌 Dark energy, which makes up about 70% of the universe, was revealed through observations of the accelerating expansion of the universe and is theorized to be related to Einstein's cosmological constant.
• 🔮 The nature of dark energy and dark matter remains a mystery, with various theories proposed, including the possibility of a connection between the two or that they are manifestations of unknown physics.
• 🔮 Future telescopes and experiments aim to uncover more about dark matter and dark energy, potentially leading to a deeper understanding of the universe's fundamental properties.

Q & A

What were the main goals of the High-Z Supernova Search Team and the Supernova Cosmology Project?

The main goals of these two teams were to measure the rate of cosmic expansion over the life of the universe and to reveal the total amount of matter and energy present by observing the deceleration of this expansion.

What surprising discovery did astronomers make about the universe's expansion?

Astronomers discovered that contrary to their expectations, the universe's expansion is not slowing down; it is actually accelerating.

What significant award did Brian Schmidt, Saul Perlmutter, and Adam Reese receive for their work on dark energy?

Brian Schmidt, Saul Perlmutter, and Adam Reese were awarded the 2011 Nobel Prize in Physics for their discovery of dark energy.

What is the significance of the 1919 solar eclipse observations for Einstein's theory of general relativity?

The 1919 solar eclipse observations provided evidence that supported Einstein's theory of general relativity by confirming the prediction that gravity from the sun would act like a lens and deflect starlight from its straight-line path.

What was Fritz Zwicky's contribution to the concept of dark matter?

Fritz Zwicky is credited with coining the term 'dark matter' and hypothesizing its existence to explain the additional gravity needed to hold galaxy clusters together, despite observing that the galaxies were moving faster than expected.

How did Vera Rubin's observations of spiral galaxies contribute to the understanding of dark matter?

Vera Rubin's observations showed that the rotation speeds of stars in spiral galaxies did not decrease towards the edges as expected, suggesting the presence of additional, unseen mass, which led to the idea of dark matter.

What is gravitational lensing and how has it been used to detect dark matter?

Gravitational lensing is a phenomenon where the gravity of a massive object bends the light from a more distant object, creating multiple or distorted images. It has been used to detect dark matter by observing the gravitational effects on light from distant galaxies, indicating the presence of unseen mass.

What is the 'cosmic web' and how is it related to the distribution of galaxies?

The 'cosmic web' is a large-scale structure of the universe consisting of galaxy clusters connected by filaments and surrounded by vast empty regions known as voids. It is related to the distribution of galaxies as it represents the pattern in which galaxies are organized in the universe.

What is the current understanding of the composition of the universe in terms of dark matter and dark energy?

Current observations suggest that about 5% of the universe is made up of ordinary matter, 25% is dark matter, and 70% is dark energy.

What are some of the theoretical candidates for dark matter particles?

Some theoretical candidates for dark matter particles include axions, supersymmetric particles (such as selectrons and squarks), sterile neutrinos, and other hypothetical particles beyond the standard model of particle physics.

What is the significance of the Large Hadron Collider in the search for dark matter?

The Large Hadron Collider is significant in the search for dark matter as it allows physicists to test theoretical predictions and look for new particles that could account for dark matter through high-energy collisions.

Outlines

00:00

🌌 Discovery of Dark Energy and the Accelerating Universe

In the 1990s, two teams of astronomers, the High-Z Supernova Search Team and the Supernova Cosmology Project, aimed to measure the universe's composition and expansion. They expected to find a slowing universe due to the drag of matter and radiation. However, they were surprised to discover that the universe's expansion is accelerating. This unexpected finding suggested the existence of an unknown force, dubbed 'dark energy,' which remains invisible but has a significant impact on cosmic expansion. The leaders of the experiment, Brian Schmidt, Saul Perlmutter, and Adam Riess, were awarded the 2011 Nobel Prize in Physics for their groundbreaking discovery.

05:01

🔭 Gravitational Lensing and the Inference of Dark Matter

The script discusses the 1919 solar eclipse observations by Arthur Eddington and Frank Dyson, which confirmed Einstein's general theory of relativity. Einstein's theory was initially seen as an academic curiosity during World War I, but post-war, scientists tested its predictions, particularly the gravitational lensing effect. Fritz Zwicky, in the 1930s, discovered that galaxies in the Coma Cluster were moving too fast and concluded that there must be 'dark matter' providing additional gravitational pull. Vera Rubin's work in the 1970s on the rotation speeds of spiral galaxies further supported the existence of dark matter, as the outer stars did not slow down as expected. Gravitational lensing, initially dismissed by Einstein as impractical, became a key method for detecting dark matter.

10:02

🌠 The Quest for Understanding Dark Matter

The script outlines the history and development of the concept of dark matter. It describes how astronomers, through observations such as galaxy rotation speeds and gravitational lensing, deduced the presence of unseen mass influencing cosmic structures. The realization that dark matter is a significant component of the universe was a turning point in cosmology. Despite being a major focus for decades, dark matter's true nature remains elusive, with various theories and candidates proposed, such as axions and supersymmetry particles, but none confirmed.

15:03

🌌 The Role of Dark Matter in Cosmic Structure Formation

Dark matter is posited to have played a crucial role in the formation of cosmic structures. The script explains that the early universe's slight density variations allowed gravity to pull material together, leading to the formation of the first stars and galaxies within dark matter halos. Over billions of years, these structures grew more complex, with dark matter providing the framework necessary for galaxy evolution. The narrative highlights the importance of dark matter in shaping the universe's large-scale structure, including the cosmic web of clusters, filaments, and voids.

20:04

🔬 The Search for Dark Matter's Identity

The script delves into the various hypotheses about the composition of dark matter. It discusses the exclusion of ordinary atomic matter and black holes as candidates due to observational constraints. The narrative explores the possibility of new particles beyond the Standard Model of particle physics, such as axions, as potential dark matter constituents. Despite extensive searches and theoretical proposals, dark matter's identity remains unknown, and its detection eludes scientists.

25:05

🌌 Dark Energy and the Accelerating Universe

The script recounts the discovery of dark energy, a mysterious component causing the universe's expansion to accelerate. It revisits Einstein's cosmological constant, initially introduced to maintain a static universe model, which was later discarded by Einstein as his 'biggest blunder.' The narrative explains how observations of distant supernovae in the late 20th century revealed the universe's accelerated expansion, resurrecting the concept of the cosmological constant as a possible explanation. Dark energy is now considered to constitute approximately 70% of the universe's content, with dark matter making up about 25%, leaving ordinary matter a minor component at less than 5%.

30:06

🌌 The Theoretical Exploration of Dark Energy

The script discusses the theoretical aspects of dark energy, exploring various hypotheses about its nature. It mentions the possibility that dark energy could be a property of space itself, akin to the quantum vacuum's tension. The narrative also explores the idea that dark energy might be related to the inflaton field responsible for the early universe's inflationary epoch. However, the exact nature of dark energy remains unknown, and its discovery has led to a theoretical playground with many proposed fields and models, all attempting to explain its enigmatic properties.

35:08

🔮 Future Observations and Theoretical Implications

The script looks forward to future astronomical observations and the potential they hold for unraveling the mysteries of dark matter and dark energy. It highlights upcoming large telescopes, such as the Vera Rubin Observatory and the European Extremely Large Telescope, which aim to provide more data on these elusive components. The narrative also contemplates the possibility that our current physical theories may be incomplete or incorrect, suggesting that the observed phenomena might be due to unknown aspects of gravity or other fundamental forces. The search for understanding the dark side of the universe continues, with the potential for profound implications for our comprehension of the cosmos.

40:10

🌌 The Enduring Mysteries of the Universe's Dark Side

The script concludes by emphasizing the enduring mysteries of dark matter and dark energy, which dominate our universe but remain poorly understood. It suggests that these components hold the keys to unlocking the fundamental nature of the cosmos. The narrative contemplates the possibility that the dark side might be a mirage due to the limitations of our current theories, hinting at a potential revolution in our understanding of physics. Until these mysteries are solved, the dark side of the universe continues to challenge scientists and hold tightly to its secrets.

Keywords

💡Dark Energy

Dark Energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. It is a central theme in the video, as it is described as an unexpected discovery that the universe's expansion is accelerating rather than slowing down. The concept is integral to understanding the current and future state of the cosmos, as it is suggested to make up approximately 70% of the universe's total energy content.

💡Dark Matter

Dark Matter is a form of matter that does not emit or reflect light, making it invisible to telescopes, yet its existence is inferred from its gravitational effects on visible matter. The video discusses dark matter as a significant but unseen component of the universe, accounting for about 25% of the universe's mass-energy density, and is crucial for the formation and behavior of galaxies.

💡Supernovae

Supernovae are the brilliant explosions that occur at the end of a star's life cycle and are among the most luminous events in the universe. In the video, they are used as 'beacons' to measure cosmic distances and expansion rates, leading to the discovery of the accelerating expansion of the universe. The script mentions the High-Z Supernova Search Team and the Supernova Cosmology Project, which were pivotal in this discovery.

💡Gravitational Lensing

Gravitational Lensing is a phenomenon where the gravitational field of a massive object bends light from a source behind it, creating multiple or distorted images of the source. The video explains how Fritz Zwicky first hypothesized this effect could be used to detect dark matter, and it has since become a tool for astronomers to measure the mass of distant galaxies and galaxy clusters.

💡Cepheid Variables

Cepheid Variables are a particular type of star that pulsate radially, varying in both diameter and temperature and producing changes in brightness with a well-established period–luminosity relationship. The video mentions Edwin Hubble's use of Cepheid variables as 'standard candles' to measure distances to galaxies, which led to the discovery of the expanding universe.

💡Hubble's Constant

Hubble's Constant is a measure of the rate of expansion of the universe, named after Edwin Hubble. The video discusses the efforts to refine the value of this constant through extensive galaxy surveys, which is essential for understanding the scale and age of the universe.

💡Quantum Vacuum

The Quantum Vacuum refers to the lowest energy state of a quantum mechanical system, which is not empty but filled with transient energy fluctuations. The video suggests that the dark energy might be related to the energy of the quantum vacuum, although the exact relationship remains a mystery due to the discrepancy between theoretical predictions and observed values.

💡Neutrinos

Neutrinos are elementary particles that are part of the lepton family, known for their minimal mass and lack of electric charge, which allows them to interact only weakly with other matter. The video mentions that neutrinos were considered as a potential candidate for dark matter but were later ruled out due to their small mass and high speed.

💡Axions

Axions are hypothetical particles that were proposed to solve the strong CP problem in quantum chromodynamics. The video describes axions as a candidate for dark matter due to their tiny mass and very weak interaction with ordinary matter, although they have not yet been detected.

💡Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is a remnant radiation from the early universe, approximately 380,000 years after the Big Bang. The video notes that the CMB contains information about the universe's composition and the presence of dark energy, serving as crucial evidence for the current understanding of the universe's expansion.

💡Inflation

Inflation is a theory in cosmology that describes a period of extremely rapid (exponential) expansion of the early universe. The video mentions the epoch of inflation and the hypothetical 'inflaton' field that drove this rapid expansion, suggesting a possible connection to dark energy.

Highlights

In the 1990s, two teams of astronomers, the High-Z Supernova Search Team and the Supernova Cosmology Project, aimed to measure the universe's composition by charting cosmic expansion.

Astronomers expected the universe's expansion to slow down due to the presence of matter and radiation, but observations revealed an unexpected acceleration.

The discovery of the universe's accelerating expansion led to the hypothesis of dark energy, an unseen force driving the cosmos faster.

The 2011 Nobel Prize in Physics was awarded to Brian Schmidt, Saul Perlmutter, and Adam Reese for their work on dark energy.

Astronomers' realization in 1998 that dark matter is significantly outweighed by dark energy marked a major shift in understanding the universe's composition.

The discovery of dark energy raised fundamental questions about the nature and origins of this mysterious component of the universe.

During the 1919 solar eclipse, observations supported Einstein's general theory of relativity, altering our understanding of gravity and space-time.

Fritz Zwicky's work on the Coma Cluster suggested the existence of dark matter, which gravitationally binds the cluster despite the visible matter being insufficient.

Vera Rubin's observations of spiral galaxies' rotation speeds indicated the presence of unseen mass, supporting the dark matter hypothesis.

Gravitational lensing, first theorized by Einstein and observed in the 1970s, provided a method to measure the mass of dark matter in galaxy clusters.

Modern surveys using galaxy redshift data have mapped the universe's structure, revealing clusters, filaments, and voids shaped by dark matter.

Dark matter's properties, such as its 'cold' nature, are crucial for the formation and evolution of galaxies and the cosmic web.

Theoretical extensions to the standard model of particle physics, like supersymmetry, offer potential candidates for dark matter particles.

Observations of high-energy gamma rays from the galactic center may provide clues to the nature of dark matter.

The discovery of dark energy's influence on the universe's accelerated expansion challenges our understanding of fundamental physics.

Current observations suggest that dark energy behaves as Einstein's cosmological constant, remaining constant throughout the universe's history.

The nature of dark energy and dark matter remains one of the most significant unsolved mysteries in cosmology, with implications for our understanding of the universe.

Transcripts

in the early 1990s two teams of

astronomers had set out to measure just

what the universe was made of

these two teams known as the high zed

supernova search team and the supernova

cosmology project brought together

astronomers from around the globe with

the goal of charting cosmic expansion

over the life of the universe

the astronomers expected the presence of

all the matter and radiation in the

universe to act like a drag on the

expansion steadily slowing it as the

cosmos aged by measuring this

deceleration all the matter and energy

that make up the universe would be

revealed

these international collaborations

cooled on the world's mightiest

telescopes such as the eight-meter keck

telescope in hawaii and orbiting hubble

space telescope to scour the sky

their target was supernovae acting as

beacons of the distant cosmos whose

brightness as measured through our

telescopes revealed how the universe had

expanded as their light traveled for

billions of years

but the astronomers

were in

for a shock

our universe

is not slowing down

it

is speeding up

after repeatedly searching for flaws in

their observations and checking their

calculations they presented their

conclusions to a startled world

and to account for this accelerated

expansion there was something else in

the universe

something unexpected driving the cosmos

faster and faster

and whilst there must be immense

quantities of this stuff for the effect

to take place it remained and remains

totally invisible to our telescopes

whatever is out there between the stars

and galaxies accelerating the expansion

it is truly

dark

this result netted the leaders of the

experiment brian schmidt saul perlmutter

and adam reese the 2011 nobel prize in

physics and this discovery of dark

energy came at the end of a tumultuous

century

astronomers had discovered that our sun

was an ordinary star in an ordinary

galaxy nowhere in particular in an

expanding universe

shocking was the revelation that in

terms of mata all the atoms in all the

stars in all the galaxies

a little more than cosmic

bit players

all the matter we can see appeared to be

dwarfed by huge quantities of unseen

matter revealed only by its

gravitational pull

and as 1998 dawned we realized that even

this dark matter was significantly

outweighed by the presence of the newly

discovered dark energy

two vast components of the cosmos

unseeable and for millennia

undetectable

it is currently the biggest question in

cosmology

what are these new components of our

universe

how

do they work

and where did they come from

[Music]

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thanks

on the 29th of may 1919 a total eclipse

of the sun passed over the southern

atlantic

two expeditions headed by british

astronomers arthur eddington and frank

dyson set off to observe this phenomenon

one team headed to brazil the other to

west africa

the expeditions were beset by problems

from faulty instruments to weather that

refused to cooperate

on the island of principe off the coast

of africa the eclipse was due at two in

the afternoon

but throughout the morning eddington had

sat in the pouring rain the sun not

peaking from behind the grey until half

an hour before the eclipse

clouds continued to cross the sky even

as it darkened eddington busied himself

changing photographic plates in the

telescope only glancing momentarily at

one of nature's most spectacular sights

would the observation be a lost cause

would all this effort be a waste of time

a few days later eddington had his

answer

[Music]

and our picture of the universe

changed

forever

the headline in the new york times

proclaimed lights or the skew in the

heavens and the little-known albert

einstein became the most famous

scientist in the world

four years earlier during the turbulent

years of the first world war einstein

had published his general theory of

relativity the culmination of a decade's

hard work relativity gave us a new

picture of gravity replacing newton's

forces with malleable space-time

during the war years relativity was an

academic curiosity but with peace

restored scientists had set out to test

the predictions of this new scientific

theory

their goal had been to measure the

locations of background stars whose

light passed close to the edge of the

sun stars that would be visible when the

immense glare of the sun was dimmed

during an eclipse

if einstein was correct the gravity of

the sun should act like a lens gently

deflecting the starlight from its

straight-line path

and on the photographic plates obtained

in brazil and africa the stars had

shifted by the predicted amount

einstein's theory

had triumphed

whilst the result made him a star

einstein thought that this gravitational

lensing had little practical consequence

stars were simply not massive enough to

use as natural telescopes to magnify the

distant universe

nothing could be there was nothing in

the universe with enough mass to have

such an effect

[Music]

or at least

as far

as he knew

in the 1930s astronomer fritz zviki was

working at the california institute of

technology

[Music]

zviki was a brilliant scientist with a

prickly personality combining geometry

and profanity he referred to his enemies

as spherical bastards implying they were

a bastard no matter which way you looked

at them

but as an original thinker zviki's mind

roamed through the mysteries of the

universe

he coined the word supernova to describe

immense stellar explosions and realized

a super dense remnant known as a neutron

star would be left in the ashes

and in 1933

fritz vicki was focusing on the coma

cluster located 300 million light years

away coma is immense and home to

thousands of individual galaxies

this was barely a decade after the

realization that the milky way was not

alone in the universe and numerous other

galaxies exist scattered throughout the

cosmos

zviki reasoned that the individual

galaxies in coma were held in their

orbits by the gravitational pull of all

the other galaxies around it

he realized that by measuring the

galactic speeds he could determine just

how much mass was in the coma cluster

providing a new way of weighing the

galaxies

but instead

what vicky found was a big problem

the galaxies in the coma cluster were

moving far too fast with only the

gravitational effect of the galaxies in

play each galaxy in coma should have

been travelling sedately in its orbit

with immense speeds of almost a thousand

kilometers per second

coma should have been tearing itself

apart

it was clear significantly more gravity

was needed to hold coma together

what was the source

of this extra gravity

with nothing obvious zviki concluded

that there must be

duncal materi

dark matter gravitationally binding the

coma cluster and there had to be a lot

more dark matter than the stars we can

see

zviki's realization that much of the

mass in the universe is dark

was startling

questions immediately sprung to mind

how much is there what is it made from

and how can we measure

zwicky's discovery should have set the

astronomy world on fire

but instead

there was silence

possibly because of his brusque style

this notion of dark matter lay buried in

the pages of scientific journals and for

the next few decades astronomers had a

new focus

measuring the expansion speed of the

universe

by the 1970s there were new

technological advances large telescopes

and sensitive instruments for observing

the heavens

and with these a new generation of

astronomers was asking new questions

about the universe

vera rubin the target was large spiral

galaxies similar to our own milky way

working with her collaborator kent forte

rubin wanted to understand how these

galaxies spin

[Music]

she had entered astronomy when it was

essentially a male-only activity and she

encountered resistance throughout her

career including a lack of female

bathrooms in the telescopes she used

but this didn't stop her as she

undertook meticulous observations of the

rotation speeds of spiral galaxies the

expectation was that these galaxies were

like the solar system with most of the

mass concentrated towards the center

just like the planets the speeds of

stars in the spiral disk were expected

to fall as gravity gets weaker towards

the edge

but this

was not what rubin found

the rotation speed of stars did not

diminish but remained roughly the same

all the way to the edge of the galaxies

just like vicki rubin was left with a

gravitational puzzle

what was the source of gravity needed to

keep the stars moving

at their unexpected speeds

zviki's dark matter was back in the

spotlight

it was becoming clear that what we could

see with our telescopes the myriads of

stars and clouds of gas

were nothing but minor cosmic players

astronomers were faced with a stark

question

is there an entire shadow universe of

dark matter out there

the evidence was growing

and it was now they turned to einstein's

seemingly inconsequential theoretical

prediction made more than half a century

before

they turned to gravitational lensing

indeed zviki had suspected this also

years before he had reasoned that with

the extra dark matter galaxies and

clusters of galaxies would be powerful

gravitational lenses

but it took until the 1960s for

scientists to really come to grips with

this concept and find a point of energy

staggeringly huge enough to properly

observe

quasars

appearing as little more than points of

light through the most powerful

telescopes

they shine with the brightness of more

than a hundred trillion stars

it was realized that quasars were

powered by supermassive black holes the

gravitational pull of these black holes

accelerates in falling material to close

to light speed as it swirls into the

inner regions mata crashes together

heating it to immense temperatures

and it is this accreting disk of matter

that glows so brightly

and with this immense brightness

astronomers could spot quasars at

incredible distances many billions of

light years from earth by the 1970s with

their observational approaches honed

finding quasars was routine

then in 1979 juan quasar

uninspiringly labeled

[Music]

q0957 plus 561

provided a puzzle

astronomers realized that this appeared

to be two quasars very close to each

other in the sky this is not too unusual

but it was found that these two quasars

appeared to be

identical

the astronomers realized that they were

seeing double a gravitational mirage

between us and the quasar sits a massive

galaxy and its gravity distorts our view

producing two separate images of the

single object

zviki was right again and more than 80

years since it was first observed under

the clear skies of africa and brazil

gravitational lensing had found its

purpose

in the decades that followed thousands

of gravitational lenses have been

discovered many of these are due to

individual galaxies but the most

spectacular are produced by the immense

gravity of gravity clusters

distant galaxies that would be too faint

to see in our telescopes are magnified

by these cluster lenses appearing as

immense arcs

every instance of gravitational lensing

provides an opportunity to measure how

much mass is present

by measuring the distortion

and the conclusion

is always

the same

there is a lot more mass

than that visible as stars and gas

in fact

most of the matter in our universe

is completely

dark

but it wasn't just gravitational lenses

that revealed the ubiquitous presence of

dark matter

everywhere astronomers looked dark

matter was needed to explain their

observations

from immense flows of galaxies through

the universe to the presence of gas in

galaxy clusters that had been

gravitationally squeezed by dark matter

dark matter was everywhere

and it dominated the gravitational pulls

of the cosmos

finally we came to realize

that it

had shaped

our universe

[Music]

looking up at the night sky the black

is filled with stars

some

form patterns

for millennia humanity has interpreted

shapes from these arrangements creating

meaning from their placement

from the ancient egyptians to the inca

these constellations have captured our

imaginations

but behind these simple patterns

lurk complex

incomprehensibly huge

clusters

many billions of light years away many

billions of light years across with many

billions of stars and galaxies within

them

the great wall

the giant ark

and indeed where we live

laniakea

and so it was in the 1970s that

astronomers started to chart the

locations of galaxies

they began to create an atlas

of the universe

the idea was simple a measurement of

cosmic speed due to the expansion of the

universe reveals a galaxy's distance

but this meant collecting and analyzing

the light from many individual galaxies

a laborious task dedicated programs were

established to measure galaxy light with

one of the first being the cfa redshift

survey based at harvard's university

center for astrophysics and as

measurements were gathered the structure

of the universe was revealed

it was clear galaxies were part of a

cosmic web with individual clusters

linked with filaments embedded in this

structure were huge empty regions known

as voids giving the universe a

sponge-like appearance

by the mid-1990s two astronomers john

hucra and margaret geller had analyzed

the light of eighteen thousand galaxies

they labeled immense structures such as

the great wall and the stickman each

composed of thousands

more modern surveys of galaxies

including the 2df galaxy redshift survey

and sloan digital skies survey have

mapped more than a million revealing

clusters filaments and voids throughout

the universe

but this raised the question

where did all this cosmic structure

come from

astronomers knew that the fiery birth of

the universe in the big bang left it

featureless the hydrogen and helium

forged in the first few minutes were

smoothly spread through the cosmos

but underlying this

was dark matter

4. in the time before the first stars

when the cosmos was truly dark

dark matter was starting to stir

in the beginning the matter in the

universe wasn't perfectly smooth

a burst of expansion known as inflation

in the earliest moments had written very

slight ripples in the universe

some places in the early universe were

slightly more dense than others other

places were less dense

and even though these density

differences were tiny they were enough

for gravity to do its work

and this gravity from very slight over

densities started pulling material

in

matter began to flow and hydrogen and

helium gas came along for the ride

after 100 million years densities grew

and gas pooled

chunks of hydrogen and helium fragmented

out of the clouds from this cooling and

collapsing gas the first stars were born

and in the universe again there was

light

forming in the dense knots of dark

matter these infant stars lit up the

sights of the first galaxies

over the billions of years that followed

dark matter drew in more mass with

growing quantities of dark matter

accompanied by more stars and gas

and so 14 billion years after the birth

of the universe

these fully formed galaxies sit at the

hearts of immense halos of dark matter

unravelling the process of galaxy

evolution is complex astronomers need to

understand the ebb and flow of gas the

life cycles of stars and the formation

of black holes

combining all this physics is difficult

so astronomers turn to supercomputers to

solve the highly coupled non-linear

equations

it is now routine for astronomers to

generate synthetic universes watching

the interplay of the numerous processes

that have shaped our cosmos

the resultant insights are astounding

and beautiful watching galaxies emerge

from the early cosmic fog

and quite clearly the dominant factor is

dark matter

providing the framework against which

galaxies can evolve

without it it would have been extremely

difficult for stars and galaxies to

begin to form

astronomers realized that dark matter

had to have some particular properties

to build cosmic structure as well as its

gravitational influence they realized

that mata must move relatively

sluggishly through the universe as too

much speed would make it disperse rather

than clump

they called it cold dark matter with the

cold referring to its slow speed

and so dark matter clearly played a huge

role in the development of complexity in

the universe

but one

huge question remained

just

what

is it

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in 1930 wolfgang poorley was faced with

a problem

radioactivity had been studied for

several decades but one kind of particle

decay was behaving strangely

this beta decay appeared to be

relatively simple with an atomic nucleus

spitting out an electron or its

antimatter cousin the positron but for a

collection of identical atomic nuclei

the particles that got spat out had

different properties some sped away at

high speed others moved more slowly this

meant that some had high energies whilst

others had lower energies

how could this be the conservation of

energy demanded that each of these

ejected particles should have the same

speed poorly wondered if the

conservation of energy was being broken

a distressing conclusion he couldn't

stomach

instead he suggested another thing is

also spat out during beta decay

another particle that carried away the

seemingly missing energy

named the neutrino this purely

hypothetical particle had some peculiar

properties the fact that it did not

collide with atoms told us it wasn't a

photon the particle of light

it was something else something

different

it wasn't until the 1950s that paulie's

hypothetical particle was finally

detected in sensitive experiments set up

next to nuclear reactors a prestigious

source of neutrinos

astronomers realized that the universe

was full of neutrinos most produced in

the frenetic nuclear reactions in the

earliest epoch of the universe

given that they barely interact with

mata it was suggested that these

neutrinos could be the dark matter

but as experiments improved it was

realized that neutrinos are just too

minuscule and too fast to account for

dark matter's gravitational dominance

physicists would have to continue the

search

what else could be dark enough to be

dark matter

what else

could be massive enough

one possibility was quickly ruled out

dark matter could not be made of atoms

it is possible to hide atomic matter

making it dark by cooling it so that it

doesn't emit any energy we could imagine

dark matter in the form of chilled

clouds of hydrogen between the stars but

the cooking of elements during the big

bang puts strong limits on how much

atomic matter there can be

and it's simply not enough to account

for all that is needed

another obvious candidate is black holes

these are clearly dark being the

ultimate trap for light and we will only

feel their presence through their

gravitational attraction

possibly these black holes were born in

cracks in space-time defects that

existed in the first moments of the big

bang

huge numbers of these primordial black

holes could course between the stars but

as einstein realized the gravitational

pull of each black hole would make it

behave like a gravitational lens

distorting and focusing light of the

more distant universe

as a black hole passes directly in front

of a more distant star the resulting

magnification would for a period of

around a month make the star appear

brighter

even if all dark matter were in the form

of black holes the chances of such a

passage is exceedingly small however in

the 1990s astronomers were searching for

these rare brightening events

but after more than a decade of hunting

for the gravitational microlensing the

result was disappointment

dark matter cannot be composed

of black holes

whilst astronomers were gazing skyward

particle physics were hunting for dark

matter in the world of the subatomic

through the course of the 20th century

they had unraveled the fundamental

nature of matter finding the particles

from which mass and forces are

constructed

comprised of quarks and leptons photons

and gluons as well as the more esoteric

w and z bosons this zoo of particles was

rounded off in 2012 with the discovery

of the higgs boson

this standard model of particle physics

has proved to be a great success

it accurately accounts for the tumult of

particles pouring out of the immense

collisions at the large hadron collider

and other particle physics experiments

despite this

we know

it must be

incomplete

missing from the standard model of

particle physics are at least two key

aspects of the universe

it can account for the strong and weak

nuclear forces and electromagnetism but

gravity is still resisting being added

to the quantum world

and also missing

is dark matter

nowhere in the possible particles is

there an obvious place for dark matter

to be hiding knowing the mathematics of

the standard model must be incomplete is

a potential gold mine for theoretical

physicists they can explore how the

mathematics can be unfolded and extended

and every new addition to the standard

model adds room for candidates for dark

matter but just like there are many

directions you can wander off from a

single point

there are many potential paths you can

take when extending mathematical ideas

one of the promising ideas was proposed

in the 1970s just as the mathematics of

the standard model was crystallizing the

key problem was the strong force the

force responsible for binding quarks

into protons and neutrons and gluing

protons and neutrons together in atomic

nuclei as originally derived the

mathematics allowed the strong force to

break a possible symmetry in the

universe something known as charge

parity violation

but all experiments appeared to show the

strong force was well behaved refusing

to break the symmetry

the mathematics needed an extension

known as the pache queen mechanism to

shore up the symmetry

and it was the presence of this

mechanism that implied the existence of

a new member of the zoo of fundamental

particles

nobel prize winner frank vilcek dubbed

this new particle the axion taking the

name from a popular washing detergent as

it cleaned up several problems in

physics

whilst these axions hypothetically

individually possess a tiny amount of

mass they would be extremely numerous

and would only interact with atomic

matter very rarely

it appeared to be an ideal dark matter

candidate

but to date not a single axion has been

detected in the uncountable number of

high energy collisions at the large

hadron collider

it remains

a contender

other approaches to extending the

standard model of particle physics have

yielded other possibilities for dark

matter supersymmetry doubles the

particle zoo as well as electrons and

quarks there are selectrons and squawks

there are also photinos we knows and

xenos

perhaps hidden within this larger family

is the sought after dark matter

different mathematical ideas have

sterile neutrinos massive cousins of

those that we know or possibly hooper

ons or cue balls or the latent higgs

field or asymmetric dark matter the

problem remains that physicists have a

shopping list of possible candidates but

as yet no experimental result suggests

that we are even on the right path to

understanding what dark matter is

finally in their quest some particle

physicists have joined with the

astronomers to look back to the heavens

several of the mathematical ideas for

dark matter suggest that it might

self-annihilate in the same way that an

electron and positron can collide and

transform into two photons and if it

does self-annihilate then perhaps we can

detect the high-energy radiation that

would be emitted

telescopes are currently trained on the

locations where we think dark matter

density would be the highest

and there is a potential signal an

unexpected glow of high energy gamma

rays from the galactic centre

it is still not clear whether these are

from mundane astrophysical sources or

possibly could reveal the discovery of

dark matter

and only time will tell

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though the question still lingered on at

the frontier of cosmology at the end of

the 1990s clear progress was being made

with many possibilities opening up the

composition of the universe was getting

a little less blurry

and then

as sometimes happens in physics

a result came along that totally upended

what had come before

we opened this story with the discovery

of the other dark component

dark energy in the closing years of the

20th century

as the shock of this discovery subsided

questions began to grow

just what was this new dark mysterious

stuff of the universe

whilst dark energy sat at the bleeding

edge of modern physics

it was realized that its story

was much older

and so

we return

to einstein

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with his general theory of relativity in

hand he wanted to deduce the equations

that govern the dynamics of the universe

but something

wasn't right

einstein felt that the universe was

static and unchanging the same now as it

had always been

his equations however presented a

different picture that the universe

should be a dynamic place changing and

evolving

perplexed by this he explored his

equations wondering if he'd missed

something

he realized that there was a term which

could act like an anti-gravity and as

there seemed to be little evidence for

anti-gravity he assumed it was zero

but

he still wondered

what if it wasn't

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through this he found a remarkable

effect that he could get this

anti-gravity to counter the dynamic

nature of his equations and make his

model universe static

and after introducing this cosmological

constant and being pleased with himself

he told the world

but not all the world agreed

especially the russian physicist

alexander friedman he held no favoritism

for a static and unchanging universe

pointing out to einstein that the notion

of a dynamic and expanding universe is

much more likely

initially einstein objected to

friedman's conclusion but eventually was

convinced and so he threw this

cosmological constant into the

scientific waste bin

he later supposedly called it his

biggest blunder

and this dismissal meant that no one

considered the notion of a cosmological

constant for many decades

and whilst these theorists were

grappling with the mathematics of an

expanding universe astronomers were

charting the heavens at the mount wilson

observatory edwin hubble was using the

world's largest telescope the 100 inch

hooker to measure the distance to

galaxies hubble's goal was to identify

particular stars known as cepheids as he

could calculate their true brightness

these standard candles revealed the

distance to their host galaxies

hubble combined his galactic distances

with galactic speeds measured with

doppler shift and found something quite

peculiar the more distant the galaxy the

faster it was moving

and other than the closest galaxies

all were moving away

whilst he didn't realize it hubble had

discovered conclusive proof

the universe

was expanding

and over the remainder of the 20th

century astronomers have refined

hubble's observations

huge observing programs were organized

to chart the universe measuring the

distances and velocities of many

thousands of galaxies

the goal was a single number known as

hubble's constant which was a measure of

the rate of universal expansion

and this brings us back to the very

beginning of our story

modern surveys of the heavens with

supernovae acting as standard candles

finally revealed that this universal

expansion

is

accelerating

and so again we are left to ask the

question

what is the dark element of the cosmos

causing

this effect

to begin with it cannot be mata or

radiation as these slow down expansion

to speed up expansion something else is

needed something that seems like an

anti-gravity

astronomers called this mysterious stuff

dark energy

but in reality einstein's cosmological

constant

was back

the existence of dark energy was not the

only surprise

when the cosmic accounting was completed

astronomers realized that all the atomic

matter the stars the planets the immense

gas clouds and galaxies comprised less

than five percent of the universe

dark matter the scaffolding of the

cosmos is about 25

dark energy made up the remainder

70

of the universe

many physicists were baffled

why is the universe completely dominated

by this mystery stuff

and in the two decades since its

discovery the evidence for the existence

of dark energy has become overwhelming

its presence is written into the

distributions of galaxies spread over

billions of light years it is also

written into the cosmic microwave

background the radiation left over from

the fiery start of the universe

and it has seemingly bizarre properties

as the universe expands matter and

radiation thin out

dark energy however appears to remain

constant

this means that in the early universe

matter and radiation dominated and the

presence of dark energy had no impact

but about five billion years ago the

tables were turned

mata had thinned out to the point where

dark energy became dominant and the

acceleration of cosmic expansion began

in earnest

this accelerated expansion dilutes mata

even faster meaning the future universe

will be completely dominated by dark

energy

in about 100 billion years this

expansion will have carried all galaxies

outside of our local group beyond our

cosmic horizon

eventually

the universe will again

return

to darkness

[Music]

in quantum mechanics empty space is not

truly

empty

heisenberg's uncertainty principle

allows energy to fluctuate in and out of

existence on tiny scales

this quantum nature of the vacuum

provides empty space with a tension

attention that can be measured in

experiments

and whilst our cosmological equations

offer few clues to the nature of dark

energy it can tell us something

unlike normal gas that possesses an

outward pushing pressure

dark energy has a tension

that pulls

and there is one thing in nature that

possesses this similar tension

the quantum

vacuum

but problems begin when physicists try

to calculate just how much energy there

is in such a fluctuating vacuum as their

answer is simply ridiculous

the mismatch between their mathematics

and the amount of energy observed in

dark energy differ by an immense factor

of 10 to the power of 120.

some have called this the worst

prediction in all the physics but it

signals that if the vacuum is

responsible for dark energy

we have a lot to learn about its true

properties

and that is just one suggestion

indeed with so little in terms of

observational evidence proposing ideas

for the nature of dark energy has become

a theoretical playground

[Music]

various fields have been proposed some

aloof sitting in the background of the

universe and doing their own thing

others have wondered whether dark energy

is related to the other bout of

accelerated expansion in the life of our

universe

the epoch of inflation is thought to

have taken place when the universe was

about 10 to the minus 35 seconds old the

driving force behind inflation was

another mysterious and hypothetical

field the inflaton

could dark energy be a resurgence of the

inflaton

the best we can say

is maybe

of course there is one other invisible

element controlling the cosmos

so the question

has been asked

are dark energy

and dark matter related

again physicists can derive mathematical

models where there are interactions in

the dark sector where dark matter and

dark energy possess a common origin in

fact the dark sector could potentially

host its own physics as rich as those in

the atomic world

but with the limited observational

evidence

we are again stuck in the world of

theory

clearly more observations and

measurements are required a whole host

of experiments are underway with many

more planned for the immense telescopes

that are currently making their way from

drawing board to reality in chile the

8.4 meter telescope at the vera rubin

observatory will turn its eye to the sky

in 2022 the 40-meter european extremely

large telescope also in chile will have

its first light in 2024

the key goals of these and other

telescopes is to uncover the mysteries

of dark matter and dark energy this

includes searching for the signs that

dark energy might not be exactly

einstein's cosmological constant perhaps

dark energy has evolved and changed over

the billions of years of the universe

maybe it has decayed into mata and

radiation these would provide new clues

to the true nature of dark energy and

open new avenues of research to unravel

its physics

however as of today all observations

suggest that dark energy appears to be

precisely einstein's cosmological

constant

unchanging over the life of the universe

but this sends a shiver down the spine

of cosmologists as it might mean that

the cosmological constant is uncrackable

just a part of the universe a

fundamental property of space

without an origin to understand

going even further for some failure to

fathom the nature of dark matter and

dark energy represents a failure of the

fundamental laws of physics

they argue instead of hunting the

heavens and particle accelerators for

physical substances perhaps we should

look closely at our cherished theories

for example what if gravity is only

approximated by einstein's theory of

relativity and the truth has yet to be

discovered this could mean that gravity

behaves strangely on large scales not

simply getting weaker

this too could account for the rotation

curves of galaxies the gravitational

lensing of light and the accelerated

expansion without the need of a physical

dark matter or dark energy

but as yet all contenders have failed to

match the success of einstein's

mathematics in explaining the universe

the search

continues

and so today our universe is completely

dominated by its dark side

and in truth

it remains

mysterious

if dark matter and dark energy are

physical things we need to determine

their true properties to understand

their place in the universe

without these questions like where they

arose in cosmic history and how they

impact the fundamental nature of the

cosmos will remain difficult

if not impossible to answer

but if the dark side is only a mirage

due to the limits of our physical

theories then an even bigger upheaval to

our understanding is on the cards

until then the dark side of the universe

holds the keys

and it is holding them tightly

you've been watching the entire history

of the universe don't forget to like and

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thanks for watching and we'll see you

next time

[Music]