A1 23 The Universe

Matter in the Universe
LACC: §27.3, 27.4, 27.5

• Galaxy Distributions
• Galaxy Cluster Distributions
• Dark Matter

An attempt to answer the “big questions”: What is
out there?

Thursday, May 20, 2010 1

The Known Universe

http://www.haydenplanetarium.org/universe


Galaxy Distribution
One of the candidates for "largest
known structure in the
Universe" is the "Sloan Great
Wall", discovered in 2003 by J.
Richard Gott III and Mario Juric
and their colleagues, using SDSS
data. The wall is nearly 1.5 billion
light years in length and is
located approximately one billion
light years from Earth. The Sloan
Great Wall is almost 3 times
longer than the "Great Wall" of
galaxies discovered in Margaret
Geller and John Huchra's 1989
survey (which is also sometimes
called the CfA2 Great Wall). At
about 500 million light-years long,
300 million light-years wide and
15 million light-years thick, it was
was previously the "record holder"
for largest known structure.

http://heasarc.gsfc.nasa.gov/docs/cosmic/sheets_voids_info.html

Voids, Walls, and Filaments

http://www.absoluteastronomy.com/topics/Large-scale_structure_of_the_cosmos


Voids, Walls, and Filaments
Stars are organized into galaxies, which in turn form clusters and
superclusters that are separated by immense voids, creating a vast foam-like
structure sometimes called the "cosmic web". Prior to 1989, it was commonly assumed that
virialized galaxy clusters were the largest structures in existence, and that they were distributed
more or less uniformly throughout the universe in every direction. However, based on redshift
survey data, in1989 Margaret Geller and John Huchra discovered the "Great Wall," a sheet of
galaxies more than 500 million light-years long and 200 million wide, but only 15 million light-
years thick.... In April 2003, another large-scale structure was discovered, the Sloan Great Wall.
One of the biggest voids in space is the Capricornus void, with an estimated
diameter of 230 million light years. In August 2007, a possible supervoid was detected in
the constellation Eridanus. It coincides with the 'WMAP Cold Spot', a cold region in the
microwave sky that is highly improbable under the currently favored cosmological model. This
supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a
billion light-years across.

In more recent studies the universe appears as a collection of giant bubble-like
voids separated by sheets and ﬁlaments of galaxies, with the superclusters
appearing as occasional relatively dense nodes. This network is clearly visible in the
2dF Galaxy Redshift Survey. In the ﬁgure a 3-D reconstruction of the inner parts of the survey is
shown, revealing an impressive view on the cosmic structures in the nearby universe. Several
superclusters stand out, such as the Sloan Great Wall, the largest structure in the
universe known to date.

http://www.absoluteastronomy.com/topics/Large-scale_structure_of_the_cosmos

A Universe of Dark Matter
This three-
dimensional map
offers a ﬁrst look at
the web-like large-
scale distribution of
dark matter, an
invisible form of
matter that accounts
for most of the
Universe’s mass

http://www.youtube.com/watch? http://www.esa.int/esaSC/
v=gCgTJ6ID6ZA SEMZ6GSVYVE_index_0.html



Latest research

In 2005, astronomers in the United Kingdom
announced the discovery of the ﬁrst dark galaxy,
VIRGOHI21, lying 50 million light years away in the
Virgo Cluster. Radio observations of the rotation of
hydrogen gas in VIRGOHI21 have revealed there must
be about a thousand times as much dark
matter as hydrogen in this galaxy, and that its
total mass is about one-tenth that of the Milky Way.

http://www.daviddarling.info/encyclopedia/D/darkmat.html

Although the nature of dark matter remains uncertain, results published
in February 2006 based on a study of 12 nearby dwarf galaxies have for
the ﬁrst time put numbers to some of its physical properties. This
research shows that dark matter comes in "magic volumes",
or standard packages, about 1,000 light-years (300
parsecs) across and containing about 30 million solar
masses of material. Apparently, dark matter can't be crammed into
spaces that are smaller than this. Another surprise is that the particles
of which dark matter is made are moving on average with a speed of
about 9 km/s, corresponding to a temperature of about 10,000 K
– much higher than expected. This indicates that dark matter is neither
hot nor cold but somewhere in between.

In addition to dark matter, the universe contains large amounts of
another invisible ingredient known as dark energy. Overall, the cosmos
seems to consist of 4% ordinary matter (mostly in the form of
hydrogen and helium), 23% dark matter, and 73% dark
energy.
http://www.daviddarling.info/encyclopedia/D/darkmat.html

Matter in the Universe
LACC: §27.3, 27.4, 27.5
• Galaxy Distributions: clusters and
superclusters (clusters of clusters)
• Galaxy Cluster Distributions: walls, voids and
ﬁlaments
• Dark Matter: seems to come in clumps of
about 30 million solar mass, about 300
parsecs across, with a temperature of about
10,000 K
An attempt to answer the “big questions”: What is
out there?

LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch. 27, pp. 637: 8

Due at the beginning of next week’s ﬁrst class
period (unless there is a test that week, in which
case it’s due the same period as the test).


Properties of Our Universe
LACC: §27.3, 27.4, 27.5

• The Cosmological Principle
• The Shape of the Universe
• The Size of the Universe

An attempt to answer the “big questions”: What is out
there?


The Cosmological Principle
The cosmological principle is a Copernican idea. It means we are not in a
special place.
On size scales of billions of light years, the universe is assumed to be uniform.... The
discovery of the long superclusters may seem to endanger this assumption. On large
enough scales though, the universe has many superclusters in all directions.... The idea
of a uniform universe is called the cosmological principle. There are two aspects
of the cosmological principle:
• The universe is
homogeneous. This
means there is no preferred
observing position in the
universe.
• The universe is also
isotropic. This means you
see no difference in the
structure of the universe as
you look in different
directions.
http://www.astronomynotes.com/cosmolgy/s3.htm


Isotropy means there are no special directions to the Universe,
homogeneous means there are no special places in the Universe.

http://abyss.uoregon.edu/~js/cosmo/lectures/lec05.html



...for 100 Mpc regions the Universe is
smooth to within several percent. Redshift surveys
of very large regions conﬁrm this tendency toward
smoothness on the largest scales, even though nearby
galaxies show large inhomogeneities like the Virgo
Cluster and the supergalactic plane.

http://www.astro.ucla.edu/~wright/cosmo_01.htm


The Shape of the Universe

http://www.youtube.com/watch?v=3WL_vtu4r1w
4:36


Shape of the Universe
Three-dimensional curved space is
not possible to visualize, but
curvature in two dimensions can be
illustrated. A positively curved
universe is like the surface of a
sphere; a negatively curved
universe, like a saddle. A universe
with zero curvature is like a
plane. The geometry of curved
space, two- or three-dimensional, is
different: parallel lines may intersect
(positively curved space) or may
diverge (negatively curved space) and
the sum of angles in a triangle may be
more than 180 degrees (positively
http://www.nap.edu/openbook.php? curved space) or less than 180
record_id=10079&page=82 degrees (negatively curved space).

Shape of the Universe
"Closed," "open," and "flat"
actually refer to the shape, or
curvature, of space-time itself.
Impossible to picture in three spatial
dimensions, this is easy enough in
two:
• Two-dimensional space with positive curvature would resemble
the surface of a sphere (on which parallel lines converge).
• Two-dimensional space with negative curvature would be like
the surface of a saddle or a Pringle's potato chip (on which parallel
lines diverge).
• A flat two-dimensional universe would resemble a sheet of paper
(on which parallel lines stay parallel).
Many independent observations indicate that the universe is in fact flat.
http://www.lbl.gov/Science-Articles/Archive/SNAP-3.html


Size of the Universe
Imagine the universe just a million years after it was born, Cornish
suggests. A batch of light travels for a year, covering one light-year. "At
that time, the universe was about 1,000 times smaller than it is today,"
he said. "Thus, that one light-year has now stretched to become 1,000
light-years."
All the pieces add up to 78 billion-light-
years. The light has not traveled that
far, but "the starting point of a photon
reaching us today after traveling for
13.7 billion years is now 78 billion light-
years away," Cornish said. That would
be the radius of the universe, and twice
that -- 156 billion light-years --
is the diameter. That's based on a
view going 90 percent of the way back
in time, so it might be slightly larger.
http://www.space.com/scienceastronomy/mystery_monday_040524.html


Properties of Our Universe
LACC: §27.3, 27.4, 27.5
• The Cosmological Principle--our universe is
homogeneous (looks the same no matter where you
are) and isotropic (looks the same in all directions)
• The Shape of the Universe: open (negatively curved),
closed (positively curved), or ﬂat (zero curvature); our
universe seems remarkabley ﬂat
• The Size of the Universe: light from an object that
appears 13 billion lightyears away is now probably
about 75 billion lightyears away
An attempt to answer the “big questions”: What is out
there?

LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch. 27, pp. 637: 11 (for #11, I just need yes-or-no answers).

• Ch 28: Tutorial Quizzes accessible from: http://
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of next week’s ﬁrst class
period (unless there is a test that week, in which
case it’s due the same period as the test).


A1 23 The Universe

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