June 23, 2025 • 20 mins
As the NSF-DOE Vera C. Rubin Observatory, a joint project of the U.S. National Science Foundation and the U.S. Department of Energy Office of Science, begins its mission to unlock new understanding of cosmic phenomena, we visit an archival lecture from its namesake, Vera C. Rubin. In the lecture, she discussed how galaxies form, how you might measure the matter in them and her observations of dark matter.
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(00:03):
This is the Discovery Files podcastfrom the U.S.
National Science Foundation.
High in the Andes
Mountains of Chile, sits the NSF DOEVera C.
Rubin Observatory.
Jointly funded by the U.S.
National Science Foundation and the U.S.
Department of Energy Office of Science,
the Observatory is a brand new astronomyand astrophysics facility
(00:25):
with cutting edge technologies,including the LSST camera.
It is the largest digital cameraever built, and will take detailed
images of the southern hemisphere
sky for the next decade,covering the entire sky every few nights
and creating an ultra wide,ultra high definition time lapse record.
The largest astronomical movieof all time.
(00:46):
The data collected at the observatorywill allow researchers
a better understanding of our universe
and reveal answers to questionswe have yet to imagine.
The telescope's namesake, Doctor Vera C.
Rubin, was an American astronomerborn in Philadelphia in 1928.
She earned her bachelor'sdegree in astronomy at Vassar in 1948,
her master's degree from Cornell in 1951,
(01:09):
and her PhD from Georgetown Universityin 1954.
Beginning in the early 1960s, DoctorRubin studied motion in space.
Her work provided convincing evidencefor the existence
of unseen dark matter in the universe.
Prior to her work, dark matterwas a concept that had been introduced
but not taken seriously.
(01:29):
Since then, scientists have figured outthat dark matter
makes up more than 80%of all the matter in the universe.
Scientists have spent the past decadeson a quest to figure out what
dark matter is, and what role it plays insculpting the structure of the universe.
Physicists try to identify its natureusing labs here on Earth.
While astronomers continueto make observations of dark matter’s
(01:50):
gravitational influence on other objectsin space, essentially, Vera Rubin's
work created a whole new subfieldof astrophysics around a brand new idea.
We now present some of DoctorRubin's thoughts on the universe
in excerpts from her lecture entitledWhat's the Matter in the universe?
Recorded in November of 1994.
(02:11):
We live in a universewhich is amazingly beautiful,
enormously large, incredibly complex,and one of the remarkable features,
I think, is that at leastsome of it can be understood.
60 years ago,
Hubble, Shapley and others
understood that we live in a galaxy.
(02:33):
That space is expanding,carrying the galaxies with it.
And they understood this in terms
of what they called the perfectcosmological principle.
Everywhere, in all directions,space was homogeneous,
isotropic and smooth.
For the last 20 years or so,we have understood that
(02:55):
at least the universe which we observe
with our optical telescopes,radio telescopes at all wavelengths.
The universe is very lumpythat stars form into galaxies.
Galaxies clump into groups, into clusters.
Clusters form into super clusters
(03:16):
and over large regions of space.
The added gravity of these largeconglomeration means that large
streams of galaxies are moving toward
regions of higher density.
Another thing we've learned is thatmost of the universe is not radiating.
It's not radiating any wavelengththat we can see with our telescopes
(03:40):
undetectable, with all the instrumentsthat we have so far been able to build.
We're really seeing only perhaps
10% of the universe.
I think there are really verymajor features
of the universethat we have not yet detected.
All the observational evidencewe see is consistent
(04:01):
with a low density universethat will expand forever, even counting
the dark matter which we can detectby its gravitational attraction.
We cannot count enough matterto halt the expansion of the universe.
In the next section.
Doctor Rubin explainedhow stars arrange into galaxies.
(04:23):
I presume all of you know thatif you go outside on a very clear night,
all of the starsyou see are members of our own galaxy.
That means they're gravitationally bound.
They're all orbitingin mostly circular orbits,
about a very distant centerat the position of the sun
it takes us about 200 million yearsto go once around the galaxy.
(04:47):
So we've been around 10 or 20 timesin the age of our galaxy.
The stars are not distributed at random,but they're flattened to a plane
so that when we look through the plane,we see vast numbers of stars
along the line of sight,
and we see these in a bright bandthat we call the Milky Way.
(05:08):
It was a puzzle to the ancientswhat the Milky Way was.
It was not until Galileobuilt his telescope
in 16 nineand pointed it at the Milky Way.
Could he say that the Milky Waywas composed of lots of stars?
As you see it, from Chile,the Milky Way goes overhead.
We can't see all the way to the center,because the Milky Way
(05:31):
is composed of more than just stars.
There's gas and dust, very low density,
but long enough path lengthso that it brightens.
But we're not lookingtotally toward the center.
Seen head on, we think our galaxywould look like this a central bulge.
Even in the disk,the stars are not arranged at random,
(05:55):
but tend to clump in these long,stringy arms, which we call spiral arms.
The brightest features are not individualstars,
but clusters in which generallynew stars are being born.
Stars are born from the gas in the dust.
They evolve and they die.
We are located somewhere on an outer arm.
(06:18):
As I said, it takes us 200 million years.
Some stars are born and evolve and die
before 200 million years has gone by.
And so they don't even make onerevolution.
The arms. They're not a fixed pattern.
The orbit of individual starscarries them into the arm where they spin,
(06:39):
due to the extra gravity,due to the higher mass.
Spend more timethere, move out and continue their orbit.
The arms are really the locus of pointsof regions of higher density,
much like a traffic jam on a highway.
If you look from an airplane viewdown in a region where
(07:01):
there was a bridge or a bottleneck,you would almost always see cars there,
but they would be differentcars at the same time.
An important feature of galaxies is thatstars are very,
very far apartrelative to their diameters.
You could put about 1%
of all the starsyou could put a billion stars
(07:24):
in our galaxybetween the average separation of two.
And that means the stars don'tgravitationally interfere.
In contrast to stars in the galaxy,which are very far apart,
galaxies relative to their diametersare very close together.
Virtually no galaxy is isolated.
(07:45):
We have these two small satellite galaxies
visible to the naked eyein the Southern hemisphere.
They are gravitationallybound to our galaxy.
They orbit in pathsthat take these stellar systems
through the disk of our galaxy.
And when the gas in these systems meetsthe gas in our disk, there are shocks.
(08:08):
There are gravitational tidal disruptions.
These small galaxieswill soon cease to exist.
Successive passes through our galaxywill force them to lose energy,
and they will ultimately become partof our galaxy.
And we now understandthat this merger process
is a very active driving forcein the evolution of galaxies.
(08:33):
The galaxies I've been talking abouthave been
principally spiral galaxies,disk galaxies.
When we look at clusters of galaxies,especially centers of clusters,
we see thatmost of the galaxies are spheroidal,
relatively featureless,and we now understand
these as the end productsof the merger of two disk galaxies.
(08:57):
Clusters are importantfor several other reasons.
It was in the early 1930s
that Fritz Zwicky first suggested
that there must exist in clustersmuch matter that was dark.
He called it missing matter.
He recognized as soon as he hadvelocities of the individual galaxies,
(09:19):
that the galaxies
were individually moving so rapidlythat the cluster should disband.
But it was not disbanding,and therefore he concluded
that there had to be more matterin the cluster than you could see,
and that that matter was in factholding the cluster together.
The gravity is sufficient to have haltedthe expansion of the universe,
(09:42):
so centers of clusters of galaxiesare not expanding.
We are expanding from the Virgo Cluster,
but we're expanding slowerthan if the cluster were not there.
Astronomers often carelessly saywe're falling to Virgo, but that's not so.
We are expanding, but slowly.
(10:03):
If the universe lives long enough,our expansion from Virgo
may ultimately be halted,and we might then truly start to fall in.
Astronomers can point to the regionaround the cluster
in which new galaxies are at present,starting to fall in.
(10:24):
We're living in an erawhen clusters of galaxies are forming.
Doctor Rubin next explainedhow you might measure how much matter
is to be found in the universe.
And I'd now like to turn to the discussion
of how you learn how much matteris involved in all of this,
(10:45):
especially if some of it is dark.
And the answer starts with Newton.
What Newton understood when
the apple hit him on the headwas not that apples fall,
because everybody knewthat apples fall to the ground.
But what he understood wasit was the same force of gravity
(11:09):
that pulled the apple to the ground,
that pulled the earth around the sun,pulled the moon around the Earth,
and we would now say, pulls the sunaround the center of the galaxy.
The forward motion of the earth aroundthe sun is so great
that although we are continuallyfalling to the sun, we can't hit it
(11:30):
because our forward motioncarries us in the forward direction
and the sun is curling under us,if you like.
So if you know the distancesand the velocities,
you can calculatehow much matter there is in the sun.
And so you are using each of the planetsas a test particle
(11:52):
in the gravitational field of the sun.
What Galileo knew when he dropped
the, feather and the lump of coalor something from the Tower of Pisa,
was that it doesn't matter what the testparticle is, it doesn't matter
whether this is a planet here,or a bit of dust,
(12:15):
or an elephant or another sun.
If you just have somethingwhose velocity and distance
you can measure,you can interpret from that
how much matterthere is interior to that position.
And this is what we dowhen we attempt to determine
(12:35):
how much matter there is in a galaxy.
After she explained
how Isaac Newton and Galileo's workwas still relevant to measuring matter.
Doctor Rubin moved on to how observationsshe and her colleagues
made provided evidencefor the existence of dark matter.
So some years ago,
with my colleague Kent Ford,who is here tonight, we embarked on
(12:59):
a long range program to measure the motions of stars in galaxies.
Much of the work was done at Kitt Peak.
Kitt Peak is also run by a consortium
of universitiesfunded by the National Science Foundation.
So what Kent and I did was pick
some galaxies that were moderatelyinclined.
(13:21):
Stars are orbiting in the sensethat the arms are trailing.
So we use this instrumentcalled a spectrograph with a grading,
which dispersesthe light into its constituent
frequencies like a rainbow.
And we use a long slitso that each point on the galaxy,
we just make a skinny cutacross the center.
(13:45):
Reproduces in the spectrum
this line of hydrogen alpha.
And by measuring the location of eachone of these knots on the spectrum,
we can determine how rapidlyeach point in the galaxy is moving.
So even though we go observing on nightsthat we call dark,
the radiation from the night skyis displaced this much from the night sky.
(14:09):
So that's the evidencethat the universe is expanding.
This galaxy is moving awayfrom us, and its hydrogen
alpha line is displaced toward the red.
But you can see on one sidethe galaxy velocities
are low with respectto the center, high on the other.
This is due to the rotation of the stars.
(14:31):
And that'swhat we're attempting to measure.
And what you see and what was unexpectedis that the velocities
as far out as we canmeasure do not, in fact, fall.
The light from this galaxy would predictthat the rotation velocity
should have gone up, turned over,and then fallen way down.
(14:54):
And the fact that they didn't,
we interpret as meaning that there is
more matter in the galaxy that we can see,
and the velocities of these starsare remaining high in response
to the gravitational attractionof this matter that we don't see.
The question, does it exist?
(15:16):
Is generally answered affirmatively.
The only way we could explainthe observations without dark
matter is to say that Newton's lawsdo not hold over distances this large.
And the scientific communityis really reluctant to say that Newton's
laws don't hold,although there are very capable scientists
(15:39):
attempting to make relativisticcosmologies which are altered.
And if they are successful,we may have to reconsider.
But barring ignoring Newton's laws,then we have evidence that it exists.
Newton's laws,
for those of youthat remember your high school physics,
tell you that the mass interior to any R
(16:01):
can be calculated if you just knowthe distance R and the velocity squared.
So if for any test particle at a distanceR from the center of a mass M,
the mass can be calculatedwith a constant.
We have a rotation curve.
The light in the galaxywould predict that.
And so this is the contributionto the velocity from the dark matter.
(16:25):
And thereforeif you want to study the dark matter
you want to go to the largest radiusthat you can possibly get.
And the other commentI wanted to remember to make
was the clearlythe contribution to the dark matter
gets more importantas you go to larger and larger distances.
(16:45):
We close our visitwith Doctor Rubin's 1994
What's the matter in the universe lectureby hearing how measuring matter
lets astronomers and cosmologistsanswer questions about the universe.
And the question that cosmologists
would like to answer, and I thinkmost people would like to answer is,
do we live in a universethat will expand forever?
(17:06):
Or will the gravitational attractionof all the matter
inside of the universe ultimatelyhalt the expansion
and start it recontracting,or is it just exactly flat?
Do we live in a universe in whichthe density is just the closure density,
so that it will sort of halt the expansionand just kind of sit there?
(17:27):
And the evidence that's been gatheredmost recently, a lot of it
from X-ray observations of clusters,
is, in fact, that it does turn over.
We can observe many of the very
large clusters in X-rays,
and it turns out that the dominant mass inthe clusters
(17:48):
is very hot gas,but they still have densities
relative to the closure densitythat just sit around here.
And it's based on that kind of evidence.
The density in luminous galaxiesfalls off like this.
Dark matter is more dominantthan the galaxies.
(18:09):
The X-ray gas becomes even more dominantas you go farther out
the total density implies a mass
relative to its luminosity of about 100.
And so we are left, I think, with evidence
that clearly indicatesthat at the present time,
there are no observations that implythat we live in a closed universe.
(18:34):
Now, the problem isthat based on the most accepted
cosmologies,theorists would like the amount
of matter in the universeto just match the closure density.
So if we live in a universewhich is closed,
if the density of the universeis high enough to hold the expansion,
(18:55):
then that matter must be unconventional.
It must be unlike the matterwe have so far.
Detected in the universe.
And that's the problem that cosmologistsand particle physicists are attempting
to seewhether they can build particle detectors
(19:17):
to detect non baryonic matter.
That could be the constituentthat closes the universe.
Doctor Rubin and her colleagueat the Carnegie Institute of Washington,
Kent Ford, studied more than 60 galaxies.
The work resulting from these observationseventually
convinced the science communitythat dark matter was real.
(19:37):
Doctor Rubin passed away in 2016,but her legacy lives on.
The NSF DOE Vera C.
Rubin Observatory will provide a vastastronomical dataset
for unprecedented discoveryof the deep and dynamic universe.
As the observatory embarks on its questto capture
the cosmos, we'll have to wait and see ifdark matter is revealed once and for all.
(19:59):
But there are sure to be countlessdiscoveries in the years to come.
What's the matter in the universe?
was the 29th annual Jansky Lecturefrom the National Radio
Astronomy Observatory,recorded in November of 1994.
For the Discovery Files, I'm Nate Pottker.
If you want to see the entire lectureor watch video versions of other podcasts,
head over to NSF's YouTubechannel @NSFscience.
(20:22):
Please subscribe wherever you get podcastsand if you like our program,
share with a friend and considerleaving a review.
Discover how the U.S.
National Science Foundationis advancing research at NSF.gov.
This is the Discovery Files podcastfrom the U.S.
National Science Foundation.
High in the Andes
Mountains of Chile, sits the NSF DOEVera C.
Rubin Observatory.
Jointly funded by the U.S.
National Science Foundation and the U.S.
Department of Energy Office of Science,
the Observatory is a brand new astronomyand astrophysics facility
(00:25):
with cutting edge technologies,including the LSST camera.
It is the largest digital cameraever built, and will take detailed
images of the southern hemisphere
sky for the next decade,covering the entire sky every few nights
and creating an ultra wide,ultra high definition time lapse record.
The largest astronomical movieof all time.
(00:46):
The data collected at the observatorywill allow researchers
a better understanding of our universe
and reveal answers to questionswe have yet to imagine.
The telescope's namesake, Doctor Vera C.
Rubin, was an American astronomerborn in Philadelphia in 1928.
She earned her bachelor'sdegree in astronomy at Vassar in 1948,
her master's degree from Cornell in 1951,
(01:09):
and her PhD from Georgetown Universityin 1954.
Beginning in the early 1960s, DoctorRubin studied motion in space.
Her work provided convincing evidencefor the existence
of unseen dark matter in the universe.
Prior to her work, dark matterwas a concept that had been introduced
but not taken seriously.
(01:29):
Since then, scientists have figured outthat dark matter
makes up more than 80%of all the matter in the universe.
Scientists have spent the past decadeson a quest to figure out what
dark matter is, and what role it plays insculpting the structure of the universe.
Physicists try to identify its natureusing labs here on Earth.
While astronomers continueto make observations of dark matter’s
(01:50):
gravitational influence on other objectsin space, essentially, Vera Rubin's
work created a whole new subfieldof astrophysics around a brand new idea.
We now present some of DoctorRubin's thoughts on the universe
in excerpts from her lecture entitledWhat's the Matter in the universe?
Recorded in November of 1994.
(02:11):
We live in a universewhich is amazingly beautiful,
enormously large, incredibly complex,and one of the remarkable features,
I think, is that at leastsome of it can be understood.
60 years ago,
Hubble, Shapley and others
understood that we live in a galaxy.
(02:33):
That space is expanding,carrying the galaxies with it.
And they understood this in terms
of what they called the perfectcosmological principle.
Everywhere, in all directions,space was homogeneous,
isotropic and smooth.
For the last 20 years or so,we have understood that
(02:55):
at least the universe which we observe
with our optical telescopes,radio telescopes at all wavelengths.
The universe is very lumpythat stars form into galaxies.
Galaxies clump into groups, into clusters.
Clusters form into super clusters
(03:16):
and over large regions of space.
The added gravity of these largeconglomeration means that large
streams of galaxies are moving toward
regions of higher density.
Another thing we've learned is thatmost of the universe is not radiating.
It's not radiating any wavelengththat we can see with our telescopes
(03:40):
undetectable, with all the instrumentsthat we have so far been able to build.
We're really seeing only perhaps
10% of the universe.
I think there are really verymajor features
of the universethat we have not yet detected.
All the observational evidencewe see is consistent
(04:01):
with a low density universethat will expand forever, even counting
the dark matter which we can detectby its gravitational attraction.
We cannot count enough matterto halt the expansion of the universe.
In the next section.
Doctor Rubin explainedhow stars arrange into galaxies.
(04:23):
I presume all of you know thatif you go outside on a very clear night,
all of the starsyou see are members of our own galaxy.
That means they're gravitationally bound.
They're all orbitingin mostly circular orbits,
about a very distant centerat the position of the sun
it takes us about 200 million yearsto go once around the galaxy.
(04:47):
So we've been around 10 or 20 timesin the age of our galaxy.
The stars are not distributed at random,but they're flattened to a plane
so that when we look through the plane,we see vast numbers of stars
along the line of sight,
and we see these in a bright bandthat we call the Milky Way.
(05:08):
It was a puzzle to the ancientswhat the Milky Way was.
It was not until Galileobuilt his telescope
in 16 nineand pointed it at the Milky Way.
Could he say that the Milky Waywas composed of lots of stars?
As you see it, from Chile,the Milky Way goes overhead.
We can't see all the way to the center,because the Milky Way
(05:31):
is composed of more than just stars.
There's gas and dust, very low density,
but long enough path lengthso that it brightens.
But we're not lookingtotally toward the center.
Seen head on, we think our galaxywould look like this a central bulge.
Even in the disk,the stars are not arranged at random,
(05:55):
but tend to clump in these long,stringy arms, which we call spiral arms.
The brightest features are not individualstars,
but clusters in which generallynew stars are being born.
Stars are born from the gas in the dust.
They evolve and they die.
We are located somewhere on an outer arm.
(06:18):
As I said, it takes us 200 million years.
Some stars are born and evolve and die
before 200 million years has gone by.
And so they don't even make onerevolution.
The arms. They're not a fixed pattern.
The orbit of individual starscarries them into the arm where they spin,
(06:39):
due to the extra gravity,due to the higher mass.
Spend more timethere, move out and continue their orbit.
The arms are really the locus of pointsof regions of higher density,
much like a traffic jam on a highway.
If you look from an airplane viewdown in a region where
(07:01):
there was a bridge or a bottleneck,you would almost always see cars there,
but they would be differentcars at the same time.
An important feature of galaxies is thatstars are very,
very far apartrelative to their diameters.
You could put about 1%
of all the starsyou could put a billion stars
(07:24):
in our galaxybetween the average separation of two.
And that means the stars don'tgravitationally interfere.
In contrast to stars in the galaxy,which are very far apart,
galaxies relative to their diametersare very close together.
Virtually no galaxy is isolated.
(07:45):
We have these two small satellite galaxies
visible to the naked eyein the Southern hemisphere.
They are gravitationallybound to our galaxy.
They orbit in pathsthat take these stellar systems
through the disk of our galaxy.
And when the gas in these systems meetsthe gas in our disk, there are shocks.
(08:08):
There are gravitational tidal disruptions.
These small galaxieswill soon cease to exist.
Successive passes through our galaxywill force them to lose energy,
and they will ultimately become partof our galaxy.
And we now understandthat this merger process
is a very active driving forcein the evolution of galaxies.
(08:33):
The galaxies I've been talking abouthave been
principally spiral galaxies,disk galaxies.
When we look at clusters of galaxies,especially centers of clusters,
we see thatmost of the galaxies are spheroidal,
relatively featureless,and we now understand
these as the end productsof the merger of two disk galaxies.
(08:57):
Clusters are importantfor several other reasons.
It was in the early 1930s
that Fritz Zwicky first suggested
that there must exist in clustersmuch matter that was dark.
He called it missing matter.
He recognized as soon as he hadvelocities of the individual galaxies,
(09:19):
that the galaxies
were individually moving so rapidlythat the cluster should disband.
But it was not disbanding,and therefore he concluded
that there had to be more matterin the cluster than you could see,
and that that matter was in factholding the cluster together.
The gravity is sufficient to have haltedthe expansion of the universe,
(09:42):
so centers of clusters of galaxiesare not expanding.
We are expanding from the Virgo Cluster,
but we're expanding slowerthan if the cluster were not there.
Astronomers often carelessly saywe're falling to Virgo, but that's not so.
We are expanding, but slowly.
(10:03):
If the universe lives long enough,our expansion from Virgo
may ultimately be halted,and we might then truly start to fall in.
Astronomers can point to the regionaround the cluster
in which new galaxies are at present,starting to fall in.
(10:24):
We're living in an erawhen clusters of galaxies are forming.
Doctor Rubin next explainedhow you might measure how much matter
is to be found in the universe.
And I'd now like to turn to the discussion
of how you learn how much matteris involved in all of this,
(10:45):
especially if some of it is dark.
And the answer starts with Newton.
What Newton understood when
the apple hit him on the headwas not that apples fall,
because everybody knewthat apples fall to the ground.
But what he understood wasit was the same force of gravity
(11:09):
that pulled the apple to the ground,
that pulled the earth around the sun,pulled the moon around the Earth,
and we would now say, pulls the sunaround the center of the galaxy.
The forward motion of the earth aroundthe sun is so great
that although we are continuallyfalling to the sun, we can't hit it
(11:30):
because our forward motioncarries us in the forward direction
and the sun is curling under us,if you like.
So if you know the distancesand the velocities,
you can calculatehow much matter there is in the sun.
And so you are using each of the planetsas a test particle
(11:52):
in the gravitational field of the sun.
What Galileo knew when he dropped
the, feather and the lump of coalor something from the Tower of Pisa,
was that it doesn't matter what the testparticle is, it doesn't matter
whether this is a planet here,or a bit of dust,
(12:15):
or an elephant or another sun.
If you just have somethingwhose velocity and distance
you can measure,you can interpret from that
how much matterthere is interior to that position.
And this is what we dowhen we attempt to determine
(12:35):
how much matter there is in a galaxy.
After she explained
how Isaac Newton and Galileo's workwas still relevant to measuring matter.
Doctor Rubin moved on to how observationsshe and her colleagues
made provided evidencefor the existence of dark matter.
So some years ago,
with my colleague Kent Ford,who is here tonight, we embarked on
(12:59):
a long range program to measure the motions of stars in galaxies.
Much of the work was done at Kitt Peak.
Kitt Peak is also run by a consortium
of universitiesfunded by the National Science Foundation.
So what Kent and I did was pick
some galaxies that were moderatelyinclined.
(13:21):
Stars are orbiting in the sensethat the arms are trailing.
So we use this instrumentcalled a spectrograph with a grading,
which dispersesthe light into its constituent
frequencies like a rainbow.
And we use a long slitso that each point on the galaxy,
we just make a skinny cutacross the center.
(13:45):
Reproduces in the spectrum
this line of hydrogen alpha.
And by measuring the location of eachone of these knots on the spectrum,
we can determine how rapidlyeach point in the galaxy is moving.
So even though we go observing on nightsthat we call dark,
the radiation from the night skyis displaced this much from the night sky.
(14:09):
So that's the evidencethat the universe is expanding.
This galaxy is moving awayfrom us, and its hydrogen
alpha line is displaced toward the red.
But you can see on one sidethe galaxy velocities
are low with respectto the center, high on the other.
This is due to the rotation of the stars.
(14:31):
And that'swhat we're attempting to measure.
And what you see and what was unexpectedis that the velocities
as far out as we canmeasure do not, in fact, fall.
The light from this galaxy would predictthat the rotation velocity
should have gone up, turned over,and then fallen way down.
(14:54):
And the fact that they didn't,
we interpret as meaning that there is
more matter in the galaxy that we can see,
and the velocities of these starsare remaining high in response
to the gravitational attractionof this matter that we don't see.
The question, does it exist?
(15:16):
Is generally answered affirmatively.
The only way we could explainthe observations without dark
matter is to say that Newton's lawsdo not hold over distances this large.
And the scientific communityis really reluctant to say that Newton's
laws don't hold,although there are very capable scientists
(15:39):
attempting to make relativisticcosmologies which are altered.
And if they are successful,we may have to reconsider.
But barring ignoring Newton's laws,then we have evidence that it exists.
Newton's laws,
for those of youthat remember your high school physics,
tell you that the mass interior to any R
(16:01):
can be calculated if you just knowthe distance R and the velocity squared.
So if for any test particle at a distanceR from the center of a mass M,
the mass can be calculatedwith a constant.
We have a rotation curve.
The light in the galaxywould predict that.
And so this is the contributionto the velocity from the dark matter.
(16:25):
And thereforeif you want to study the dark matter
you want to go to the largest radiusthat you can possibly get.
And the other commentI wanted to remember to make
was the clearlythe contribution to the dark matter
gets more importantas you go to larger and larger distances.
(16:45):
We close our visitwith Doctor Rubin's 1994
What's the matter in the universe lectureby hearing how measuring matter
lets astronomers and cosmologistsanswer questions about the universe.
And the question that cosmologists
would like to answer, and I thinkmost people would like to answer is,
do we live in a universethat will expand forever?
(17:06):
Or will the gravitational attractionof all the matter
inside of the universe ultimatelyhalt the expansion
and start it recontracting,or is it just exactly flat?
Do we live in a universe in whichthe density is just the closure density,
so that it will sort of halt the expansionand just kind of sit there?
(17:27):
And the evidence that's been gatheredmost recently, a lot of it
from X-ray observations of clusters,
is, in fact, that it does turn over.
We can observe many of the very
large clusters in X-rays,
and it turns out that the dominant mass inthe clusters
(17:48):
is very hot gas,but they still have densities
relative to the closure densitythat just sit around here.
And it's based on that kind of evidence.
The density in luminous galaxiesfalls off like this.
Dark matter is more dominantthan the galaxies.
(18:09):
The X-ray gas becomes even more dominantas you go farther out
the total density implies a mass
relative to its luminosity of about 100.
And so we are left, I think, with evidence
that clearly indicatesthat at the present time,
there are no observations that implythat we live in a closed universe.
(18:34):
Now, the problem isthat based on the most accepted
cosmologies,theorists would like the amount
of matter in the universeto just match the closure density.
So if we live in a universewhich is closed,
if the density of the universeis high enough to hold the expansion,
(18:55):
then that matter must be unconventional.
It must be unlike the matterwe have so far.
Detected in the universe.
And that's the problem that cosmologistsand particle physicists are attempting
to seewhether they can build particle detectors
(19:17):
to detect non baryonic matter.
That could be the constituentthat closes the universe.
Doctor Rubin and her colleagueat the Carnegie Institute of Washington,
Kent Ford, studied more than 60 galaxies.
The work resulting from these observationseventually
convinced the science communitythat dark matter was real.
(19:37):
Doctor Rubin passed away in 2016,but her legacy lives on.
The NSF DOE Vera C.
Rubin Observatory will provide a vastastronomical dataset
for unprecedented discoveryof the deep and dynamic universe.
As the observatory embarks on its questto capture
the cosmos, we'll have to wait and see ifdark matter is revealed once and for all.
(19:59):
But there are sure to be countlessdiscoveries in the years to come.
What's the matter in the universe?
was the 29th annual Jansky Lecturefrom the National Radio
Astronomy Observatory,recorded in November of 1994.
For the Discovery Files, I'm Nate Pottker.
If you want to see the entire lectureor watch video versions of other podcasts,
head over to NSF's YouTubechannel @NSFscience.
(20:22):
Please subscribe wherever you get podcastsand if you like our program,
share with a friend and considerleaving a review.
Discover how the U.S.
National Science Foundationis advancing research at NSF.gov.
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