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