The weight of darkness Dark matter and Universe

through the history of humanity we have

been fascinated by our skies

across different civilizations we have

followed the motion of the stars

the planets their moons and much much

more

all of these inquiries have revealed a

wealth of information about the workings

of our universe

and our place in it as cosmologists

we study the whole history of the

universe right from the big bang to the

way we see it today

this is an image of the night sky taken

by the hubble space telescope

on the one hand simply by looking at the

fraction of this image that is black

it may appear that much of our universe

is in fact

empty with vast expansive voids full of

nothingness

on the other hand we also realize that

we as humans

are living in a very special place in

the universe a place where there is an

excessive material structure

and over abundance of stuff compared to

the rest of the universe

almost every bright point in this image

is a galaxy

or even a cluster of galaxies and you

and i

we live on a planet in one particular

galaxy in this multitude

in this unique location of the universe

that is not in fact

empty and has gathered mata into this

relatively localized region of a small

galaxy

some of the main questions that i study

aims to answer how these structures came

into being

from this map of the universe as we see

it today i try to

understand its evolution through time

to delve deeper we need to take a step

back and consider

that while every bright point in that

previous image was a galaxy

illuminated by millions of stars in gas

over the last century or so

we have actually come to understand that

the sum of

all the luminous matter in the universe

that is all the matter that can

emit light or absorb light is actually a

very small

fraction of the total magic matter

budget of the universe

in fact luminous material contributes a

mere 15 percent

of the total mass in the universe the

rest of all the matter

is actually dark as far as we know it

does not emit

or absorb any light at all not invisible

spectra through which we see

we cannot see it in x-rays or infrared

or any other part of the electromagnetic

spectrum that we know and love

this invisible matter is what we now

know as dark matter

so we cannot see dark matter how the

ended scientists come to it for its

existence in the first place

the story of dark matter is much like

finding footsteps in the sand

dark matter like everything else in the

universe which has mass and you can

weigh

exerts a gravitational force gravity is

also the only known force that can

act over large astronomical distances

between stars that are

separated by light tears or galaxies

that are even further apart

physicists actually inferred the

presence of dark matter

through its gravitational signature on

the stuff that we can

see a good analogy actually comes if you

do a simple

thought experiment by thinking what

happens if we could not see the moon

if he turned it off one day say it

absorbed all the light of the sun and

did not

reflect any and we could not see it

would we still be able to infer that

it’s there

the answer of course is yes we’d study

the rising and ebbing of the tides

in our oceans and eventually over time

we’d come to infer that we have a

massive neighbor

orbiting around us exerting a

gravitational pull

on our vast malleable seals in a similar

way

the earliest hints of dark matter came

through its gravitational signature on

the motion of galaxies in a cluster

as early as 1930s scientists were trying

to explain the high velocities of the

galaxies in the coma cluster

the coma cluster is a nearby system of

bound galaxies

this means that the galaxies in the

cluster move

in a choreographed fashion orbiting each

other bound together

by some overarching force since gravity

is the only force that can act over

such large distances scientists knew

that the motion of the galaxies in the

cluster had to be explained by the total

mass

total gravitating mass in the system but

despite

adding up all the mass that was there in

the luminance galaxies

they could not explain the fast

velocities of these galaxies

in fact it appeared that you needed a

lot more mass to actually hold the

system together

in the first place to bind the system

together

it seemed as if there needed to be an

overarching spherical distribution of

mass

that was not present only in the

luminous galaxies

the most definitive or final evidence

for the existence of dark matter

actually came much later

around the 1970s from the work of vera

rubin

she was a pioneering american scientist

working

with her colleague kent ford and they

were trying to study

the motion of the stars and the disc of

the andromeda galaxy

they were trying to learn how fast

different parts of the disc was rotating

what they expected to see was what we

see on the left of this image

the discs should slow down as they went

far out from the center

but what they found instead but it was

more like the image on the right

the stars in the outer parts of the disc

were actually moving way too fast

as if their motion was dictated by a

much more massive overarching

gravitational field

of a spherical halo of matter around it

this mass this gravitational potential

could not be associated with any bright

visible matter

and yet it had a strong influence on the

rotation of the discs

almost around the same time two other

scientists or strikering peoples

found that one could not even form these

disk like galaxies in the first place

if it wasn’t for a spherical halo of

mars around it

these disks would disintegrate away if

if the only mass in the system was that

associated with the light in the

galaxies

therefore the very existence of thindus

galaxies

challenged the picture of the universe

as we knew it at that time

pointing to the necessity of invoking

some form of matter that was visible

gravitationally

but invisible in light these

revolutionary results

urged scientists to start thinking out

of blocks and eventually accept

the mounting evidence for a new kind of

matter

that was visible only through its

gravity

so by the late 1970s early 1980s dark

matter detectives were out there trying

to investigate the nature and behavior

of this very mysterious component of the

universe

all galaxies from the dimmest ones to

the brightest ones we now began to

understand

were actually formed inside of dark

matter halos

these massive dark matter halos provided

a sort of a gravitational

well which drew in other mass becoming

fertile regions of the universe where

everything formed stars craters galaxies

every light thing that you can think of

not only did dark matter

bend and pave the movement of galaxies

scientists also knew that by virtue of

its gravity a clump of dark matter could

also bend the path of light itself

this prediction comes from einstein’s

theory of general relativity

for example in this image a large

cluster of galaxies that is embedded in

a massive dark matter halo

acts like a cosmic lens bending the

light from galaxies

that are behind it making them look like

distorted arcs

this phenomenon which is known as

gravitational lensing

along with measuring the motion of

galaxies is today one of the

primary ways in which we infer the total

mass of dark matter

in dark matter halos and also generally

in the universe

the evolution of the universe is

actually a very delicate balance between

two fundamental components

dark matter and dark energy along with

learning about dark matter

over the last decade we have also

learned that our universe is expanding

at an average

increasing speed today we think that

this expansion is driven

largely by an unknown form of energy

that we call

dark energy together dark matter and

dark

energy are responsible for the observed

state of the university

this movie is a computer simulation of

the volume of a volume of the universe

the color traces the dark matter density

at early times we begin

with a largely uniform distribution of

dark matter everywhere

but we have small fluctuations of

density from place to place

with time as the overall universe

expands because of dark energy

the regions that have slightly more dark

matter than average

slow down detached from the overall

expansion

become unstable and collapse

gravitationally

forming filaments and sheets and dark

matter halos

and this web and network of dark matter

throughout the universe

just how these structures form just how

fast they form and how much of it forms

depends depends intimately on the exact

amount of dark matter and dark energy

that there is in the universe

in regions where dark matter clumps up

and comes together

galaxies are born the galaxies that we

see are therefore embedded

in an underlying web of dark matter the

number of galaxies

its abundance and clustering therefore

tells us

about the underlying dark matter that

it’s lying on

and knowing and inherently knows about

the energy that was there in different

components of the universe at the

beginning

the web of galaxies therefore holds a

light to the web of dark matter behind

it apart from knowing how much dark

matter there is in the universe we also

want to know what it is

is it a particle is it a fluid does it

interact in any what other ways apart

from gravity

what for example is the mass of the dark

matter particle

one way i try to understand the

properties of dark matter

is by studying the dark matter halos

very closely

halos form at the knots of the cosmic

web these are regions with the highest

density of dark matter in the universe

where it has come together

gravitationally

forming a clump of matter by

understanding the properties of the

of the halo like its shape its size or

where its boundaries are

we can start to prove the detailed

microphysical properties

of dark matter by and large

we currently think that most of the

observations in the universe are well

explained by dark matter

being heavy particles that only interact

gravitationally

and are largely slow moving with little

or no

random or dispersed velocities at all

we call this basic picture the cold dark

matter paradigm

cold because of its heavy and calmed

impairment or kinetic properties

this coldness has very important

implications for the structure of dark

matter heroes

in fact it has deep implications for the

kind of halos that we see in the

universe

if that matter is massive cold and slow

moving

even the smallest perturbations or the

weakest gravitational

waves will make dark matter collapse and

form

tiny small halos in fact in the cold

dark matter scenario

we expect to find halos that are as

small as the earth mass

but if dark matter had random velocity

velocities if it was a light particle

these random velocities would help dark

matter escape out of the weakest

gravitational potentials

forming only the more massive dark

matter halos but not the tiny ones

in the cold dark matter scenario we

think halo formation is hierarchical

the low mass halos form first merging

together

to form larger masses halos

so within any one massive halo in the

cold dark matter picture

we expect to see many more smaller

substructures

like the halo and the top panel shown

over here on the other hand in the

warmer scenario if dark matter is

light one particle we see a much

smoother distribution of matter without

the small

clamps of matter within a larger halo

one of the ways to understand the nature

of dark matter

particles is to therefore look for these

small clumps of the eulers

to go after the signatures of the

smallest of halos

powerful telescopes are now looking for

these low mass objects

we look for them in clusters of galaxies

and in the halo of our own milky way

hoping to find not only the dimmest

galaxies that live in these halos

but also the gravitational signatures of

the tiniest of halos that never light up

through its impact for example on the

tidal streams of the milky way

this today is an exciting avenue to

search for the properties of dark matter

our body of knowledge about the physical

universe

increases by leaps and bounds hand in

hand

with technology at every single stage

right from the very first telescope that

galileo made

revealing the moons of jupiter and

heretically suggesting that we are not

in fact the center of the universe

to the gravitational wave detector which

was made for the ligo experiment

that can detect the distortions in the

very fabric of space-time from the

merging of black holes

the experiments that are planned for the

next decade

will allow us to learn about dark matter

and dark energy

with great precision the vera rubin

observatory for example

plans to map almost half of the entire

observed

sky this large experiment which actually

brings together scientists from all over

the world

will not only provide us with a huge

sample of galaxies that will give us

extremely good statistics it will also

probe ever fainter galaxies galaxies

that are much fainter that have been

looked at with experiments such as these

before

this these faint galaxies will help us

see further out in space

more distance in space and also find a

very low mass halos

in nearby galaxies like our own milky

way

at the same time along with

the vro experiment we also have the

james webb telescope

which will open a window to the time in

the universe when the first stars and

galaxies were forming revealing an

exciting

and so far observationally relatively

unexplored epoch in the universe

we have come a long way to understand

the composition

of the universe astrophysical probes of

dark matter

are some of the most promising

directions to learn about the material

that makes up most of our universe while

dark matter

may seem abstract sciences replete with

stories of paradigm shifting discoveries

such as these

just like the discovery of neutrinos or

the discovery of genes dark matter was

in fact conspicuous in its apparent

absence

today while there is obviously a lot

left to learn

we also do think that we already know a

lot about dark matter

we already know a lot about our universe

and we know it quite precisely with the

help of wonderful physicists

powerful computers and path breaking

technology making telescopes

and yet as always at this exciting

juncture for cosmology

the more we learn the more questions we

uncover and we look at ourselves and

hope to look at ourselves and our

universe

from ever changing new perspectives

you