The Biggest Things in the Universe

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The Biggest Things in the Universe

• Thursday, February 21

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Large scale structure of the universe how
galaxies and clusters of galaxies are distributed
through space.
To visualize large scale structure, we need a map
of the universe.
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Making maps isnt easy.
Early maps of the Earth were faulty because of
lack of information.
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Making maps isnt easy.
You must select what information goes on your map.
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Making maps isnt easy.
If the information you want isnt available, you
must use an imperfect substitute.
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Ideally, we would like to know how all the energy
mass is distributed through space.
This is difficult (and perhaps useless).
the Cartographers Guild drew a Map of the
Empire whose size was that of the Empire,
coinciding point for point with it. The following
Generations, who were not so fond of the Study of
Cartography, saw the vast Map to be Useless and
permitted it to decay and fray under the Sun and
winters. Jorge
Luis Borges
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Although dark energy dark matter make up most
of the universe, theyre invisible.
Thus, astronomers map the large scale structure
of the universe by mapping the positions of
galaxies.
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Although stars in galaxies provide only 0.4 of
the universe, theyre good markers for large
scale structure.
Dark energy smoothly distributed. Dark matter
clumped in halos around starry galaxies.
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An advantage of mapping the positions of galaxies
Galaxies are easy to identify.
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Sometimes galaxies are caught in the act of
merging (is it one galaxy, or two?), but thats
fairly rare.
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A disadvantage of mapping the positions of
galaxies
When we simply map their position on
the sky, information is lost.
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Galaxies are spread through 3
dimensions.
When we make a 2-d map of the sky, information
about the 3rd dimension (distance from us) is
suppressed.
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To make a full 3-d map of the positions of
galaxies, we need to know the distance to each
galaxy.
We can mass produce measurements of galaxy
distances.
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Flashback slide!
Why its useful to know H0
Measure redshift of galaxy (?-?0)/?0
Compute radial velocity v c (?-?0)/?0
Compute distance d v / H0
Cheap, fast way to find distance!
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Astronomers have done large redshift surveys of
galaxies.
One such survey was the 2dF galaxy survey, with
gt200,000 galaxy redshifts.
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The 2dF slice of the universe shows a bubbly or
spongy structure.
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One way to visualize the 3-d structure of the
universe is to do a simulated fly-through.
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To map the whole universe, you need a redshift
survey that covers the whole sky.
Big Dipper
This is hard to do.
1) From the northern hemisphere, you cant see
the southern sky.
Southern Cross
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2) Dust in the Milky Way blocks our view of
distant galaxies.
Dust blocks our view in these directions -
but not these directions.
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Best attempt (IMHO) at an all-sky redshift survey
(full view of the universe)
Sloan Digital Sky Survey (SDSS)
Telescope in New Mexico has measured redshifts
for 800,000 galaxies.
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The Sloan Digital Sky Survey has covered about ¼
of the sky.
?Polaris
Green portion of sky surveyed.
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Galaxies 0.1 Mpc across. Clusters of
galaxies 1 Mpc across. The biggest things we
can see are superclusters (clusters of clusters)
elongated filaments 30 Mpc long.
Superclusters are separated by voids, 100 Mpc
across.
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We live in the Virgo Supercluster.
10 Mpc
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Why arent there superduperclusters?
Why are the largest structures lt 100 Mpc across?
If there were superduperclusters as long as the
Hubble distance (4300 Mpc), we could see them.
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Superclusters grew from slightly overdense
regions. Voids grew from slightly underdense
regions.
These density irregularities began as quantum
fluctuations that were then expanded by inflation.
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Inflation must have produced density fluctuations
on all scales
including the superdupercluster scale!
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Superclusters are the largest structures in the
universe because
larger objects have not yet had time to collapse.
A dense region takes time to collapse. For the
largest dense regions, collapse time is longer
than Hubble time.
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Its possible (with a big computer) to simulate
the formation of large scale structure.
- Make a large (imaginary) box. -
Fill it with (simulated) massive objects. - Make
sure the density fluctuations are those predicted
by inflation. -Let er rip.
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then
now
redshift 29
redshift 0
(The size of the box grows from 1.5 Mpc to 43
Mpc.)
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What do we see? Formation of
structure is bottom-up - smallest structure
forms first.
First dwarf galaxies Next
ordinary galaxies Next clusters
of galaxies Now superclusters
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The Cosmic Web
cluster
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Rate at which structures grow depends on 2
competing factors
1) Gravity compresses dense regions.
2) Expansion of the universe pulls them apart.
If the expansion of the universe continues to
speed up, expansion will win! No
superduperclusters
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Tuesdays Lecture
Formation and Evolution of Galaxies
Reading
Chapter 12