Download 3D Tour of the Universe Template

3D Tour of the Universe Template
Brendan Mullan
Summer 2008
A/N: This is the “longform” version of the 3D tour, which would probably take ~ 1 hour
to successfully run through. The author recommends picking and choosing certain
targets and topics and assembling your own show.
Visualization Setup – this is automatically done in BLM version
1. Test projector alignment
2. Open extragalactic.sh
3. Set isodensity contours to 0.08
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g10 alpha 0.08
g7 alpha 0.8
g18 alpha 0.8
4. Set 2MASS galaxy label size and luminosities
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g11 lsize 0.023
slum 0.02
5. Set Tully polysize to ~ 0.001 to start
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g9 polysize 0.001
g9 lsize 0.01
alpha 0.99
g9 off
6. Set “Home” and local labels to a reasonable size
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g6 lsize 0.001
alpha 0.99
g8 lsize 0.001
7. Slowly fly into solar system's location in the Galaxy, turn off all extragalactic features and turn
on local objects.
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g6 off
g4 on
g3 on
g2 on
g1 on
gall censize 0
8. Set up 3D perspective
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stereo cross
stereo -0.01 (test this value)
Presentation
A/N: Although this script contains much new content, most of it is a shameless plagiarism of Brian
Abbott's Digital Universe Guide for Partiview and should not be regarded as an independent work.
Additionally, this is far more content than is possible (or appropriate) to present in a typical 30 minute
– 1 hour show. Pare down the material to the perceived interests and intellectual level of the audience.
1. Begin with the night sky
a) Rotate around north pole and point out constellations
We're going to start by looking at the night sky as it's seen from earth. All around us are
many of the stars and constellations you can see from right here in State College, or from
one of the Astronomy department's planetarium shows. Right now we're facing north.
Directly ahead and about 45 degrees up we see Polaris, the North Star, which is at the north
“pole” of the night sky. In front of us is the constellation Ursa Minor, which is usually just
barely visible at the right locations in State College. It's sandwiched by a few other notable
patterns you might have heard of, like Draco over to the left, Ursa Major down below, and
a few others like Bootes and Cetus.
b) Mention the plane of the Galaxy, its dust lanes and its tilt with respect to our planet.
If we keep going in this direction, we notice there's this strange, narrow fuzzy patch
stretching across the sky that makes a 23 degree angle with our equator. Long called the
“Milky Way” for its resemblance to spilled milk across the cosmos, it was only in the early
20th century that Astronomers confirmed that this enormous structure is essentially an
island universe in which we are embedded, i.e. a galaxy. Galaxies are conglomerations gas,
dust, and hundreds of billions of stars; the fundamental building blocks to the large-scale
structure of the universe.
c) Switch to galactocentric coordinates and point out Galaxy center; pan around inside the disk
to ~ 140 degrees galactic longitude
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g4 off
g5 on
Since our home galaxy, the Milky Way, is our starting point to larger distance scales, we
should orient ourselves to it. Now our “equator” coincides with the plane of the Galaxy. At
our galaxy’s center is Sagittarius A* , a compact radio source that astronomers now believe
to be home to a supermasive black hole that weighs as much as a few million suns put
together (in terms of kilograms, that's a one with 36 zeros after it!) .
The north Galactic pole lies in the constellation Coma Berenices, while the
south Galactic pole is in the constellation Sculptor. These points are perpendicular to the
plane of the Galaxy. If you look toward these points in the sky, you are looking directly out
of the Galactic plane. Because there are not as many stars or much gas and dust in this
direction, we can see objects at greater distances when we look toward the Galactic poles.
The same is true for galaxies.
2. Local galaxies
a) Turn on Local Group
b) Turn off stars and constellations
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g2 off
g1 off
g8 on
c) Turn on Tully and scale label sizes as needed (lsize 0.01 is usually good to start)
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g9 on
Besides the stars of our own galaxy, the sky is also home to an immense variety of
other, more extended objects. Once telescopes were put to scanning the skies, many faint
smudges like this “Andromeda” became apparent. Even with a pair of binoculars, many of
these unusual objects can be visible from State College. But are they part of our Milky Way,
or do they lie outside our Galaxy? This was a subject of fervent debate until 1923, when
Edwin Hubble estimated the distance to what was then called the Andromeda Nebula to be
around 1 million light-years (we know now it's closer to 2 million). Astronomers had
determined the dimensions of the Milky Way only a few years prior, and this distance was
well outside the Galaxy. This put to rest the cosmological debate between those who
believed the Milky Way was the entire Universe and those who thought that there was a
Universe outside our Galaxy. Hence the Andromeda Nebula was renamed the Andromeda
Galaxy and extragalactic astronomy was born.
d) Turn on 2MASS, 2dF, and Sloan, turn off Tully
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g11 on
g9 off
g13 on
g14 on
But that’s only the tip of the iceberg. With an estimated 100 billion galaxies in our
observable universe, even a tiny subset (a meager hundred million objects or so) of these
clutters up the screen. You’ll notice there aren’t many galaxies seen in the galactic plane.
They’re still there, but they lie behind the so-called “zone of obscuration,” where the
bright optical band of the Milky Way blocks our view of what’s beyond.
You’ll also notice that our picture of the extragalactic night sky is pretty patchy, with
many galaxies seen in narrow strips to the north and south of the zone of obscuration. As
we pan around the sky, you will see one strip of yellow toward the northern Galactic
latitudes and another strip surrounded by dots in the south Galactic polar region. Again,
this is not a natural distribution of galaxies, but rather a “work in progress” for our
ongoing observations. Like I said, this isn’t a 3D tour of the universe per se, but a 3D tour
of what we know of the universe.
e) Return to galactic south, turn off all but local + Tully galaxies, turn on Galaxy
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g3 off
g14 off
g13 off
g9 on
g5 off
g6 on
Now we're facing directly south, below the plane of the Galaxy. We're going to fly north
and out of the Galaxy, giving us a bird's-eye view of our home. Keep in mind that we're
traveling faster than the speed of light in order to cover this distance in a reasonable
amount of time, which of course in reality is impossible.
f) Shift+f to fly slowly north to look down on the Galaxy. Make sure Galaxy is the target
object
You'll notice that the Galaxy is made up of a few distinct components: the bulge, the
disk, and its prominent spiral arms. The innermost component is the Galactic bulge. This is
the brightest part of the Galaxy and contains a small bar of stars and gas around a central
nucleus. While it contains many older, redder stars, it also contains the active, tumultuous
Galactic center, where stars form and orbit in the disk component, around the giant black
hole I mentioned earlier. Overall this region's stars are also deficient in elements other than
hydrogen and helium, or are what we call metal-poor.
g) Turn on 100 kly scale
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g18 on
From the scale, we see that the surrounding Galactic disk is just less than 100,000 lightyears across and encompasses the main body of the Milky Way, host to a population of
mostly younger, metal-rich stars. The Sun orbits 26,000 light-years (8,000 parsecs) from
the Galactic center and is about 50 light-years above the disk midplane. The most
photogenic features, of course, are the spiral arms that wrap around the Galaxy. These
aren't permanent, solid structures, but are more like traffic jams of dust, gas, and stars that
rotate around the disk and trigger additional star formation and give the arms their distinct
blue colors. With all this in mind, we like to classify our Galaxy as a barred spiral. In a
minute, we'll see different, more exotic galaxy “flavors” that populate the Universe.
h) Turn on the halo, remark
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g18 off
g7 on
The last component of the Milky Way I'll touch upon is the practically invisible halo that
envelops it. The Galactic halo is a large, roughly spherical volume that encompasses the
entire Galactic disk. The halo is filled with old, faint stars and globular clusters. The stars
in the halo and inside globular clusters are metal-poor, older stars that formed close to the
era of our galaxy's formation. But for our purposes, you can think of it as roughly
demarcating a sphere of gravitational influence- anything inside this grid is strongly bound
to the Milky Way and will feel its gravitational effects.
i) Orbit around the Galaxy, point out SMC, LMC, dwarf spheroidals, Galactic Cannibalism
The nearest object to the Milky Way, inside this sphere, was discovered in 1994. The
Sagittarius dwarf spheroidal galaxy is represented here by a point, but this belies the nature
of this mysterious object.
You may have figured out that the object is called Sagittarius because it is seen in that
constellation in the night sky. However, parts of this galaxy have been seen on both sides of
the Galactic disk. So representing it with one point is not really accurate.
The Sagittarius is a dwarf spheroidal galaxy that has been stretched and warped by
our Milky Way. Normally, a dwarf spheroidal (dSph) galaxy appears as a smudge on the
sky in even the largest telescopes. With a low star density, these galaxies often resemble star
clusters rather than galaxies.
Astronomers have come to realize that galaxies interact with one another more often
than once thought. The Sagittarius dSph is in fact in the midst of an interaction. As it
orbits our Milky Way, our Galaxy slowly rips the Sagittarius dSph apart, stripping away
streams of stars on each pass near the Milky Way’s disk. It's a sordid display of galactic
cannibalism, highlighting the cruel realities of a kind of natural selection that pervades the
cosmos. In fact, this has also happened to two other nearby companions, the Large and
Small Magellanic Clouds.
The Magellanic Clouds (labeled LMC and SMC for Large and Small), are called
irregular galaxies for their lack of ordered morphologies. These are seen in the southern
hemisphere and appear as two isolated patches of faint light, similar in appearance to the
light in the band of the Milky Way as seen from Earth. They are actually small galaxies that
have their own star formation, globular clusters, and the famous supernova of 1987 (in the
LMC). These two galaxies are likely in the process of colliding with our Milky Way. Because
our Milky Way is far more massive than either of these galaxies, they have been ripped
apart by our more dominant Galaxy.
j) Zoom out until Andromeda is visible. Orbit until the disk is close to screen center
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g19 on
g9 lsize 0.005
Flying further away from our home galaxy, we need to establish a new, larger distance
scale- 1 million light years. This next grid sets the scale for the distances we need to talk
about our local neighborhood of galaxies.
k) Explain similarities and differences between Andromeda and the Milky Way. Also discuss
their future merger
As you can see, the Milky Way is surrounded by a number of diminutive companions. But
if we fly back further, we run across the next large system on our block, the Andromeda
galaxy. Similar to the Milky Way, Andromeda commands attention from its own compliment
of smaller, nearby companions, and has well-defined spiral arms in its disk. At 2.3 million
light-years, Andromeda is the largest galaxy we’ve seen thus far, just slightly larger than the
Milky Way, and is the biggest, most luminous among the so-called “Local Group” galaxies.
It is the farthest object we can see with our unaided eye in a dark sky.
As we orbit Andromeda we notice that, like the Milky Way grouping, most of the galaxies
around Andromeda are smaller dwarf spheroidals or dwarf ellipticals. One exception, M33,
is the other spiral galaxy in the Local Group.
All the galaxies I've indicated are part of a group of galaxies that interact with one
another under the group’s mutual gravity. Many of the nearby galaxies are interacting
with the Milky Way, but Andromeda is also on a collision course. In 3 billion to 5 billion
years, the two galaxies will collide. By then, our Sun will be in its late stages, expanding
its envelope as it evolves into a white dwarf star.
l) Explain local group
There are roughly three dozen galaxies in this Local Group. Most of the galaxies in this
group are small dwarf or irregular galaxies, with the exception of three spirals:
Andromeda, the Milky Way, and M33, a face-on spiral in Triangulum.
If you look at the Andromeda Galaxy tonight (assuming it’s visible), you are looking at a
galaxy that is 2.3 million light-years away. The light that reaches your eye left the galaxy
2.3 million years ago, so again, you are looking at the galaxy when the Universe was 2.3
million years younger. This may sound impressive, but this is still a very small distance
scale for the rest of the Universe.
m) Re-center Milky Way, and turn on 10 Mly scale. Zoom out until 10 Mly grid is about 1/2- ¾
the screen size
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g20 on
g9 lsize 0.23
Flying out to scales of tens of millions of light years away from the Milky Way, we see a
great number of well-studied deep sky objects whose names may sound familiar. These are
all large photogenic galaxies, illustrating an important observational bias we experience in
astronomy. Although low-mass, low-luminosity dwarf galaxies are the largest
subpopulation of galaxies in the universe and actually make up the majority of its visible
mass, they are so dim that we can only see them relatively close to us. As we look to greater
and greater distances, we preferentially pick up brighter and bigger objects. This represents
an increasingly inaccurate census of the types of galaxies that are really present. Luckily
for our purposes it doesn’t really matter- dSphs are pretty boring to look at, anyway.
n) Brief tour of some local galaxies. Swoop around galaxies as you orbit (may need to zoom
out/in to avoid passing through Virgo). Suggested path: M31/M33 --> Centaurus A -->
M104 (Sombrero) --> M51 (Whirlpool).
Centaurus A looks nothing like the galaxies we've seen so far. This is one of the most
peculiar local galaxies; it exhibits a very odd morphology and is an unusually strong
source of radio emission. Its strange shape results from a projection effect- it is aligned
with its disk edge-on to us. But you can see, the bulge is a good deal thicker than the other
flat, pancake-like spirals so far. It is possible this galaxy is distended from the consumption
of at least one large spiral galaxy in the past billion years. Kind of like a boa constrictor.
M104 was named the Sombrero Galaxy because of its appearance as viewed from just 6
degrees south of its equatorial plane and outlined by a thick dark rim of obscuring dust.
This galaxy shows both a big bright core, and as one can see in shorter exposures, also
well-defined spiral arms. It also has an unusually pronounced bulge with an extended and
richly populated globular cluster system - several hundred can be counted in long
exposures from big telescopes. On a historical note, M104 was one of the fist galaxies
whose extragalactic origin was verified by its recessional velocity. Its redshift corresponds
to a recession velocity of about 1,000 km/sec ( caused by the Hubble effect, i.e. the cosmic
expansion). This was too fast for the Sombrero to be an object in our Milky Way galaxy.
M104 is the dominating member of its own small group of galaxies, the M104 group, or
NGC 4594 group of galaxies.
The Whirlpool galaxy, M51, was the first galaxy where spiral structure was discovered,
in 1845 by Lord Rosse. Thus, M51 is sometimes referenced as Rosse's Galaxy or Lord
Rosse's "Question Mark.” According to our present understanding, the uniquely
pronounced spiral structure is a result of M51's current encounter with its neighbor, NGC
5195. From this interaction, the gas in the galaxy was disturbed and compressed in some
regions, resulting in the formation of new young stars. As is common in these kinds of
encounters, spiral structure was induced in the more massive galaxy. M51 is an easily
found astronomical showpiece if the sky is dark, where suggestions of its spiral arms may
be visible. As is also common with these types of galaxy interactions, the central region of
M51 is home to a compact, energetic birth site of massive and luminous stars, whose
genesis was triggered by interstellar gas forced to the center by the gravitational tug-ofwar between the galaxies.
o) Zoom out, go around past Virgo for Antennae (grid is about 1/5th of the screen size), explain
galaxy mergers. You may need to scale down Tully labels and increase luminosity (A/N:
description may change based on the images uploaded into the software).
Here's a more violent example of galaxy interactions: a head-on collision between two
nearly equally-massed galaxies. In this case, extensive plumes of stars and interstellar
medium are sloughed off into space as tidal forces tear at both systems. The gravitational
stresses from the galaxies compress the gas and dust within them, generating hotspots of
fresh star formation along their tails, as well as in the nucleus like M51. With star
formation kicked into high gear, the galaxies' supply of interstellar fuel is rapidly
extinguished, and this activity is brought a halt in a ten to a hundred million years.
Individual stars within the galaxies rarely collide, but are clearly flung from their tidy disk
environments to more chaotic orbits. Cumulatively, smaller scale interactions like in M51,
along with more dramatic mergers like the Antennae play a big role in disrupting delicate
spiral patterns in these systems and transforming them into a new breed of galaxy
altogether.
3. Galaxy Clusters
a) Zoom out turn on isoDensity contours and 100 Mlyr scale, explain clumpiness, clustering
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g21 on
g10 on
From this perspective, we see that galaxies are not uniformly or randomly distributed in
space on this 100 Mlyr scale; there is some structure to their arrangement. Let’s take a blue
highlighter to the densest parts of the universe. In this way, you can tell that our local
universe contains a variety of galactic environments, from what looks like sparselypopulated “rural” zones to compact, vibrant “cities” of galaxies. If we want to continue
this analogy, we might say that our local group of galaxies is a lot like State College- a
medium-sized, comfortable suburban locale. There are a few things of consequence going
on, but the level of activity is never too frenzied. Not too far from us are a number of galaxy
cities, given by the red points. I'll just remove all but these galaxies to give you a better
picture of these urban environments, or what we call galaxy clusters.
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see abell
g9 lsize 0.04
These clusters can contain about 50 to 1000 gravitationally bound galaxies. Like cities,
clusters are fast-paced, frenetic places, hosts of many different kinds of galaxies and their
sometimes violent interactions (A/N: this is a good place to randomly label clusters with
real city names for fun). The closest one to us, the Virgo cluster (our New York or Philly,
perhaps) is an archetype of this kind of environment, so let's have a closer look.
b) Pan around to Virgo, center. Explore its galaxies.
Sixteen of the Messier galaxies (the ones with an “M” label) are members of the Virgo
Cluster, but this is a mere fraction of Virgo’s 2,000-plus galaxies. While it is about 50
million light-years away, the immense gravity of the cluster affects our Local Group
galaxies and pulls other galaxies toward it. Let's increase the size of the galaxies so we
have a better picture of their shapes and colors.
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g9 polysize 0.006
slum 0.001
At the center of the Virgo Cluster is M87, a massive galaxy extending out more than half
a million light-years, or five times larger than the Milky Way. Its most peculiar feature is
the long, knotty jet of material streaming out from its center. This is extremely different from
our home Galaxy.
You've noticed by now that the galaxies we've visited come in a variety of shapes, sizes,
and colors. Astronomers find that galaxies can range from giant, red ellipticals shaped like
a watermelon to bluer spirals like our Milky Way. Galaxies that do not fit the elliptical or
spiral mold are deemed irregular. These are typically smaller and have bluer stars than
spirals or ellipticals. Some have active, ongoing star formation (these tend to be spirals),
and others, mostly Ellipticals, are relatively inactive, dead galaxies.
Very loosely, we can talk about “early types,” galaxies toward the quiescent elliptical
end of the spectrum, vs. “late type” galaxies more spiral-like in nature. Close to the center
we have the neighboring early type galaxies NGC 4477 and NGC 4429, which as we move
out, we can contrast with the beautiful M100, the archetypal “grand design” spiral.
c) Galaxy Morphology and Environment
Why is there such a diverse assemblage of galaxies in clusters? Let's look at where these
galaxies like to live (A/N: This part requires additional dialog to explain each command
criteria. Also, DO NOT ORBIT AS YOU ENTER COMMANDS! THE PROGRAM WILL
CRASH ON THE DAVEY 538 PC!).
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g10 off
only= type
only= type
only= type
only= type
see all
-5 (E)
-2 (S0)
1 3 (Sa-Sb)
4 8 later S
There is some ambiguity in the trends we see here, but for the most part it looks like
early types like to hang out in the dense centers of clusters, while the delicate spiral systems
prefer the outskirts. Apparently, when an ordinary spiral galaxy starts off on the edge of a
cluster and is pulled inward, a complicated series of galaxy-galaxy interactions ensues,
which effectively puffs out its stars to make an elliptical shape and makes it burn up its starforming fuel like M51 or the Antennae. At more or less the same time, the galaxy travels
through a hot, ionized, and diffuse gas that permeates cluster interiors and strips it of the
remaining gas it needs to form new stars and stay a healthy blue color. Timescales are a bit
tricky here- sometimes morphology is affected first, and the star-formation properties
second, as is the case for M60. This large galaxy exhibits many visual characteristics on an
elliptical, with a large amount of star formation going on in its center, speaking to very
recent interactions and a quicker morphological change. Other times, star formation is
affected first, and the galaxy's shape second. Most of the spirals in Virgo follow this pattern.
Ultimately, galaxies that stay in clusters long enough become ellipticals, growing to larger
and larger sizes as they sit and eat smaller satellite galaxies. This is a continuous, ongoing
process- the nearby M61, for example, is a galaxy we know is rushing headlong into the
Virgo Cluster and may be transformed over millions of years into an early type system like
many of its cluster-bound compatriots are.
d) Galactic Census. Zoom out, switch on isodensity map
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g10 on
g21 on
only= type -5 -2
only= type 1 2
only= type 3 8
Let's look at this phenomenon on a larger scale and see each subset again. What was
true for Virgo also applies to the universe as a whole. And overall, we see that within 500
million light years, we have about 7000 early type E/S0 galaxies, 8000 early type spirals,
and 3000 late type spirals, depending on how you divide the sample. Following our
discussion of galaxy interactions and morphological transformations, you might expect the
numbers for the first two categories to roughly increase with time.
4.
Large scale structure
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g10 off
g9 polysize 0.001
slum 151
Let's shrink the galaxies back to a reasonable size. If we orbit the Milky Way with the
Virgo Cluster in sight, you will notice a stream of light-blue color-coded galaxies extending
up toward the Virgo Cluster. This is the Ursa Major Filament that, in the 2-D sky, traces a
path from the constellation Virgo up to Ursa Major. Beyond these local features, galaxies
reveal the grand structure of the nearby Universe. Let's fly to the outer edge of the data and
orbit. From this view, you will see the major structural elements: clusters, filaments, and
voids. You will hear these three words repeated ad nauseam for the rest of the show.
The round, compact Virgo-like clusters we’ve seen. The other structures one sees in the
nearby Universe are the filaments and voids. Filaments are strings of galaxies that often
connect larger clusters and now we see that Virgo and the Ursa Major Filament are part of
a large network of clusters and connecting filaments.
Follow the Ursa Major Filament up to the Virgo Cluster and beyond, up to another red
cluster of galaxies. This is Abell 3526, located in the constellation Centaurus in our 2-D
sky. Looser than Virgo, it seems to have a yellow line of galaxies extend over to it. These
lead to the Antlia Cluster, at which point the yellow galaxies seem to take a right turn,
where the filament connects up to another group of galaxies.
The yellow strands of galaxies define an edge, inside of which you see few
galaxies until the aqua strand that seems to connect with the Virgo Cluster on the opposite
side of Ursa Major. That large area where few galaxies are found is like the inside of a
bubble. These regions of few large galaxies are called voids. The overall density of these
regions is low relative to that of the galaxy-rich filaments. However, there are likely small
dwarf galaxies that we cannot see in these regions.
The local Universe seems resemble a bubble-like form, where filaments of galaxies
reside on the bubble surface and vast voids exist between them. These filaments connect
large clusters of galaxies and make larger superclusters.
a) 2MASS Galaxies
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g11 on
g9 off
g11 slum 0.045
lsize 0.2
see clusters / see lss / see all
It’s time for some more galaxies. From our current perspective, you should see the Virgo
Cluster and strands of multi-colored points. As before, galaxies in dense clusters are given
the color red, but now, color describes galaxy density. As such, red regions have the highest
density, followed by orange, yellow, green, and aqua for the lowest density regions. Flying
backward, we see the Fornax Cluster, as well as the Centaurus and Hydra Clusters. These
are the nearest galaxy clusters to Earth, very similar to Virgo in their structures and
assortment of galaxies.
An especially interesting cluster is the so-called Great Attractor, a very large
concentration of galaxies that's so gravitationally strong, it's attracting everything else in
you see towards it. The Milky Way, the Local Group, and the Virgo and Hydra-Centaurus
clusters are being pulled in this direction at velocities of around 600 to thousands of
kilometers per second. Based on the observed galactic velocities, the unseen mass
inhabiting the voids between the galaxies and clusters of galaxies is estimated to be 10
times more than the visible matter in this region of the universe (for a total of around 5.4 x
1016 solar masses) and so must be composed of mostly the elusive dark matter that lurks in
the cosmos and accounts for an amazing 90% of all mass. But this isn't the only place
where dark matter is important.
b) Highlight structure
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g11 color const
color prox5
g22 on
Looking out to even larger scales of a billion light years, we again see how galaxies are
organized into clusters, filaments, and voids. The Milky Way is in the Local Group, and the
Local Group is part of what we call the Virgo Supercluster. The “Great Wall,” extending
from Coma cluster around about ¼ of the sky is a particularly lengthy example of this.
Meanwhile, large voids occupy space like the inside of a bubble, where space is relatively
empty.
While galaxy groups and clusters are gravitationally bound systems such that no galaxy
can escape, it is important to note that superclusters have not yet settled into this state. In
our analogy, this would be similar to the major cities in the Northeast United States. From
Washington, D.C., to New York, one can travel from town to town almost without leaving an
urban area. Interstate 95 connects them all as one area. But this urban region has not yet
merged into one large megalopolis, just as a supercluster has not yet become
gravitationally bound.
c) 2DF galaxies
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g23 on
g13 on
gall color const
Flying out to distances on the order of billions of light years, we see this universal
structure on even larger scales. Ultimately, this large-scale structure of the universe is
governed by the distribution of dark matter. Though we have never directly observed these
mysterious particles of noninteracting matter, we know it provides the backbone for
clusters and filaments, preventing these objects from disassembling. It also surrounds every
galaxy in an extensive halo, affecting how galaxies develop and interact. Along with “dark
energy,” it is probably the most fundamental, yet poorly understood aspect of cosmology.
d) Stop, turn on Sloan Galaxies and Quasars
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g14
g13
g14
g15
on
off
slum 0.06
on
There are even more galaxies I could show you, extending out to billions of light years
away. Recall that the bow-tie shape of these data indicates the patches of sky that were
surveyed. Imagine if the entire sky were surveyed, you would see similar data surrounding
the Milky Way in a spherical distribution. Overall, we see that large-scale patterns tend to
disperse as we go past 3 billion light years, or 3 billion years in the past. This is a function
of both the evolution of these structures (i.e. that we're seeing their more nebulous infancy),
and that ordinary galaxies are simply getting too faint to see. In order to observe anything
at still greater distances, we need a new class of even brighter objects.
5. Edge of the observable universe
a) Zoom out, introduce QSOs. Fly through a strip.
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g16 on
A quasar is essentially a galaxy with a supermassive black hole at its core, generating
immense amounts of energy hundreds of times more luminous than the Milky Way. The
energy source responsible for this incredible luminosity is the heating and colliding of gas
as it spirals into a supermassive black hole at the core.
Just as we see a slightly earlier Universe when we look to Andromeda, we see a much
younger Universe when we look at quasars, which lie billions of light-years away. Could
the active quasars represent an era in the earlier Universe when all galaxies were more
active and luminous? Astronomers are answering these questions as we study the origin of
our Universe.
As you can see, they appear as a natural extension to the galaxies. Nearby, there seems
to be a lower density of them, and the number of quasars increases as the distance
increases toward their average distance of 13.4 billion light-years. By about 15 billion to
20 billion light-years, the density of quasars begins to fall off. This fall-off is due to the
limiting magnitude of the telescopes—their cut-off is set by brightness, typically around
20th magnitude—and objects dimmer than this will not be detected. This is very similar to
the Sloan survey's quasar coverage, as seen here.
As we venture further and further away from the Milky Way, we join galaxies and
quasars that seem to be receding faster and faster away from us. This is called the Hubble
expansion, a phenomenon that is akin to ants on an inflating balloon. As the balloon
universe expands, each ant will see the others expand away from it, so there is really no
privileged center to the universe. On large enough scales, everything recedes from
everything else. Even more interesting, it was recently discovered that during the past few
billion years the expansion has increased. This acceleration is attributed to something
astronomers call “dark energy.” Dark energy is the mysterious force that is causing the
expansion of the Universe to speed up, overcoming the force of gravity.
While quasars are the farthest objects we can see, there is light from an earlier epoch
that pervades the Universe, called the cosmic microwave background, or CMB. We will
discuss this next.
b) Zoom out, introduce CMB, WMAP
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g17 on
The CMB pervades the Universe. It is an imprint of the time of recombination in the
Universe, 379,000 years after the big bang, when the universe was cool enough for protons
and electrons combined to form hydrogen, and when light began to travel in space
unimpeded. When we look at that light, we are looking back in time to that event, 379,000
years after the Big Bang. Before that time, the Universe was opaque, so we can never see
the Big Bang event itself. This primordial light from recombination has since expanded with
space, cooling down to an average wavelength of 1 nm, and a temperature of 3 degrees
above absolute zero.
The CMB, then, defines that part of the Universe that we can see, what we call our
observable Universe. While the CMB is everywhere in the Universe, we can also think of it
as our outer limit, that which we cannot see beyond. It also shows the minute variations in
temperature that were present in the Universe at that early era. These temperature
variations are on the order of millionths of degrees around the average temperature of
2.725 Kelvin.
6. Slow zoom back into milky way
Finally, as we return to the Milky Way, let's discuss why these minute variations, or
anisotropies, in the CMB are so important. Essentially, they are directly responsible for the
complex large-scale structure in today’s Universe. You can think of them as density
fluctuations in the early Universe, where these slight differences in density become
amplified by gravity over time. Regions that are more dense, even by tiny amounts, will
gradually collapse because of this excess gravity. This process, called gravitational
instability, worked slowly over billions of years until galaxies formed. Those galaxies are
now hundreds of times denser than the nearly empty space that surrounds them.
In addition to galaxy-sized objects, this process gave rise to clusters and filamentary
strands of galaxies. Again, this is much like our previous galaxy city, suburbs, and rural
environments analogy. We settle in towns of various sizes (galaxy groups), or larger, busier
cities (galaxy clusters). Connecting these places are trails, rivers, and highways, along
which smaller towns or groups of towns are found (galaxy filaments).
7. End Show, solicit questions