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We've been failing to detect dark matter
for decades but finally the latest
failure to detect Dark Matter may have
actually proved its existence one of
these is true either most of the matter
in the universe is invisible and formed
by something not explained by modern
particle physics or our understanding of
gravity is completely broken the debate
over which is true has raged for decades
but may finally have been resolved in an
unlikely way the proof that Dark Matter
exists and really is an exotic unknown
substance may have come from the
discovery of two galaxies that appear to
have no dark matter at all
today on spacetime journal Club we'll
look at the papers that reveal this
discovery the effect of dark matter who
was first noticed in 1933 when the
legendary Swiss astrophysicists Fritz
Zwicky noticed that the galaxies in the
Coma Cluster were moving too quickly to
remain gravitationally bound within that
cluster he guessed the presence of
Dunkel materia a dark matter invisible
to the telescope but whose gravitational
effect held the cluster together as with
many of his wiki's predictions like the
existence of neutron stars and
gravitational lensing this wasn't taken
seriously until decades later the hunt
for dark matter began in earnest after
the early 70s when Vera Rubin discovered
that the rotation rates of spiral
galaxies were too fast given the mass of
their visible stars alone they should
throw themselves to pieces spinning that
quickly that is unless a gravitational
force of unseen origin was holding them
together since the 1970s generations of
astronomers have searched for the origin
of dark matter
gradually ruling out the possibilities
if it were dust or gas we'd see it's
obscuring effect on light passing
through it if it were dark compact
bodies like failed stars or stellar
corpses or primordial black holes then
we'd see the warping of
of more distant stars in their
gravitational fields in other words we'd
see their gravitational lensing all
known particles of the standard model of
particle physics have likewise been
ruled out if dark matter really is a
type of matter then it must be an
unknown type generations of experiments
to find this particle have also failed
to locate the exotic dark matter
particle so far this troubled quest has
inspired theorists to explore another
possibility what if the discrepancies in
the orbits within galaxies and clusters
is not due to an unknown source of
gravity but due to a failure of
gravitational theory itself the first
such proposal was Mons modified
Newtonian dynamics proposed by Mordecai
Milgram in the early 80s the idea is the
Newton's law of universal gravitation
might change between short distances
like within the solar system and large
distances like entire galaxies and
clusters such a proposal doesn't just
modify Newton's gravity but also
requires modification of Einstein's
general relativity over the years
various efforts have been made to refine
and justify such a modification and new
proposals keep popping up like the
quantized inertia and dark fluid that
last one is something we covered
recently but no consistent theory has
appeared a few points have come out in
favor of the dark matter as mater
hypothesis versus the modified gravity
schemes
there's gravitational lensing the
warping of light from more distant
objects and due to the gravitational
fields of galaxies and clusters this has
given us an independent measure of the
mass of those objects and those masses
were the same as measured from the
orbits the Cosmic Microwave Background
fluctuations also give us a measurement
of the relative amount of dark matter in
the early universe compared to atoms
once again roughly consistent with what
we measure from other methods modified
gravity theories had to
hard to keep up with the mounting
evidence that dark matter looks like
actual physical stuff but still we have
an impasse astronomers and particle
physicists can't find the matter that
dark matter is made of while those who
would like to overthrow the underlying
theory of gravity struggle to make their
models fit all the observational
evidence but there's one feature common
to almost all modified gravity models
that could falsify all of them they pin
gravity on the baryons
they say that most of the gravity in the
universe comes from normal stuff like
stars and gas in particular the protons
and neutrons they contain their baryons
that gravity then follows different
rules at different distances giving the
illusion of there being more baryonic
matter than there really is in this
scheme baryons on galactic size scales
should always produce the effect of dark
matter dark matter and baryons should
never be observed separately the first
piece of evidence that baryons and dark
matter can actually be separated came
with the bullet cluster this is a pair
of clusters that recently collided
because galaxies are relatively small
compared to the space between the
galaxies in these clusters mostly he
passed right through each other on the
other hand the gas in these clusters
collided and ended up left at the
location of the collision while the
galaxies moved on so where is the dark
matter if modified gravity models are
correct you'd expect it to be wherever
most of the baryons are in this case
that's the gas
there's about twice as much mass in that
trainwreck of gas in the middle compared
to the stars in the galaxy clusters on
either side the true mass distribution
of the bullet cluster was mapped using
gravitational lensing this time the
warping of life of galaxies behind that
cluster it turns out that most of the
mass here's a invisible it's not stars
or gas but rather dark matter
and be it hanging out with the stars and
not the gas the Dark Matter also appears
to have passed straight through the
collision site which is exactly what
you'd expect if dark matter is what we
think it is some exotic weakly
interacting particle but not what you'd
expect if dark matter were just a
modification of gravity produced by the
baryons then the mass should be exactly
where we see it hanging out with the gas
some modified gravity proposals have
claimed they are still consistent with
the observation of the bullet cluster so
apparently we need more evidence cue the
new discovery a galaxy that may have no
dark matter whatsoever this weird object
is a so-called ultra diffuse galaxy or
udg these can be the size of the Milky
Way but maybe with 1% of the number of
stars sister Milky Way Peter van dokkum
and Shani Danieli of Yale University and
their team were studying one of these UD
G's in the neighborhood of the
elliptical galaxy NGC 1052 and this udg
is charmingly named and you see 1052 / d
f2 but today we're gonna call it fritz
to understand how Fritz formed the team
measured the motions of stars within
that galaxy first some bright lots of
stars globular clusters then did a few
stars of the rest of the galaxy
remember that Ricky and Rubin discovered
dark matter in the first place by
observing that the velocities were too
fast in clusters and in galaxies
indicating an unseen source of mass to
hold those things together well this
team found the velocities were extremely
small at least compared to similar you
DG's discovered in the past from the
stellar orbits they inferred a mass for
Fritz of 130 million times that of our
Sun although with an error bar of 18
million solar masses the comparison our
galaxy has a mess around ten thousand
times greater with the vast majority of
which is dark matter
this was intriguing
to determine the ratio of stars to dark
matter the team calculates in Fritz's
still a mess based on the amount of
light received from the galaxy this is
actually pretty straightforward because
we have an excellent understanding of
how much and what type of light stars of
different masses produce they got a
stellar mass of a hundred million suns
that's basically the same mass from the
stellar orbits that told the researchers
that we don't even need dark matter to
account for the mass of this galaxy and
most Fritz should have about the same
mass in dark matter as it doesn't stars
but even that would be bizarre because
normally in a galaxy like this the dark
matter would outweigh the stars by a
factor of 400 the team has since found a
second galaxy in the same group that
also appears to be missing dark matter
and they have several more candidates
for which they still need to measure
velocities it seems it is possible for a
galaxy to be missing dark matter which
is an ironically powerful argument in
favor of the existence of dark matter if
dark matter isn't a substance but rather
a modification of the laws of gravity
then it shouldn't be possible to
separate it from the galaxy yet in this
case that seems to be exactly what
happened the authors have some ideas of
how this could have happened but they
emphasize that these are just guesses
for the moment perhaps these UD G's form
from gas and stars ejected in a past
collision with the nearby elliptical
galaxies while perhaps that elliptical
galaxy once harbored a quasar its
central black hole underwent a feeding
frenzy that blasted enough gas out of
the galaxy to form a new mini galaxy but
without the dark matter or perhaps those
same quasar winds landed some gas
exterior to the elliptical galaxy
triggering it to collapse into stars
even without the dark matter that would
normally be needed to start that process
obviously more work is needed to
understand these dark metal light
galaxies then
document team are honored and no doubt
many other astronomers will be racing to
investigate this weird new class of
object hopefully if we can understand
how a galaxy can form without dark
matter we can learn more about the
nature of dark matter itself which be
nice I mean it only makes up 80% of all
matter in space-time thank you to
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dark matter works and what it may
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so you all seem to have completely
understood the holographic universe
video on first watch so there's nothing
to say here
well you fan did request a new video the
explanation of the holographic universe
explained point taken any further
explanations will get their own entire
videos today we're just going to use the
response section to address your
humorous comments and also to show some
of the incredible artwork you came up
with depicting Roger Penrose dressed as
Gandalf with a lightsaber and a TARDIS
okay maybe just one science one from
Tony face who muses the whole
information stored on the surface of a
volume idea had got me thinking pick a
sphere that encompasses whatever you
define of as me so everything about me
can be encoded on the outside of that
surface could you say that everything I
could possibly measure about the outside
universe is also encoded on the inside
of that same sphere and thus every
closed 2d surface in 3d space is
essentially a limit of the know ability
of one region about the other Tony face
consider a career switch this is indeed
a reasonable interpretation of the
universe beyond our cosmic horizon as a
hologram living in the light bill
quite reach us trapped at the expanding
horizon and that sounds like another
entire episode okay to the face mollify
shares with us they're tuning in to that
video randomly was like Oh bleep I just
walked into the final exam without ever
going to class so you're saying our
videos are like a nightmare to you well
at least they're free cook for 20
informs is that while we may feel that
we live in a three spatial dimension
universe serve up down left right
forward back we're actually in a higher
dimensional universe of up up down down
left right left right B a B a select
start that's right you already have all
the power-ups you just don't know it
bailed agust points out that Roger
Penrose is a Time Lord in
four-dimensional space but looks like a
Jedi in a three-dimensional surface
infinitely far away fleam notes that
this is a form of Jedi Time Lord duality
try RT guys try allottee he's also a
wizard as our Roger Penrose fan out
reveals here we have a brilliant one
from over decks on Twitter I see a
soldier of ministereth in the background
there but wasn't he Gandalf the White
not the gray by the time the
fellowshipped reached the tower of watch
oh yeah TARDIS he time traveled and also
from Twitter we have Steven Halliday's
brilliant drawing in which the
lightsaber blade is the matrix
mathematics of the more penrose
conditions truly an elegant weapon for a
more civilized age and finally Roger
Penrose as dr. Gandalf the White on
Shadowfax by David surely no fun is here
just wow David I'm gonna just go ahead
and replace Ian McKellen with Roger
Penrose in my mental image of Gandalf
[Music]