Download Cosmology Physics 466E Olbers Paradox Cosmological principle

Cosmology
Physics 466E
Olbers Paradox
Cosmological principle
Expansion of the Universe
Big Bang Theory
Steady State Model
Dark Matter
Dark Energy
Structure Formation
"The evolution of the world can be
compared to a display of fireworks
that has just ended; some few red
wisps, ashes and smoke. Standing
on a cooled cinder, we see the slow
fading of the suns, and we try to
recall the vanishing brilliance of the
origin of the worlds."
Lemaitre.
Cosmological Principle
• On large scales (greater than 100 Mpc) the
Universe is homogenious and isotropic
• The Earth is not at a preferred place
(Copernican Principle)
• Homogenious: Every point is equivalent
• Isotropic: Every direction is equivalent
Homogeneity does not imply
isotropy
Homogeneity does not imply isotropy
Cosmological Principle (cont)
Isotropy and Homogeneity
• Homogeneous -> we see no difference when we
change position; there is no preferred
position in the universe (translational
invariance)
• Isotropic -> no difference when we look at a
different direction
• Examples: Surface of uniform cylinder is
homogeneous but not isotropic- what about
the surface of a sphere – or chessboard ?
• Cosmological Principle (CP)-> universe is
homogeneous and isotropic (at a given
cosmological time)
Cosmological Principle
• Cosmological principle means that physical
laws are assumed to be the same
everywhere, too
• The cosmological principle of isotropy and
homogeneity, like other scientific
hypotheses, is testable by confrontation
with data.
• So far, observations support this
hypothesis.
Tests
Galaxies arranged in
superclusters that appear as long
sheets surrounded by voids
Cosmological Principle Tested
The Perfect Cosmological Principle
Perfect Cosmological Principle
• What about time? Every “time” equivalent?
• The Universe is homogenious and isotropic in
space and time.
• The universe looks the same everywhere (on the
large scale) as it always has and always will.
• The evolution of Galaxies does not confirm this
principle. The universe seems to evolve.
Olbers’ Paradox (1826)
•
•
•
•
Consider a static, infinite universe of stars
Every line of sight would end in a star
Then why isn't the night sky bright?
Mathematically, radiative flux drops by r-2
but the number of stars in a volume
increases with r3.
• So the night sky should be bright if the
Universe is sufficiently large!
http://en.wikipedia.org/wiki/Olbers'_paradox
Olbers’ Paradox
Olbers’ Paradox
• A star filled
spherical shell, of
radius r, and
thickness dr,
centered on the
Earth.
Possible Explanations
•
•
•
•
•
There's too much dust to see the distant stars.
The Universe has only a finite number of stars.
The distribution of stars is not uniform. So, for
example, there could be an infinity of stars,
but they hide behind one another so that only a
finite angular area is subtended by them.
The Universe is expanding, so distant stars are
red-shifted into obscurity.
The Universe is young. Distant light hasn't even
reached us yet.
http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html
Correct Answer(s)
• The Universe is expanding
• The Universe is young
• In fact the sky is ablaze, but the
temperature of the radiation is only 2.7 K
(CMBR)
• All starlight ever emitted amounts only to
a few percent of the CMBR energy density.
The Universe is young
• We live inside a spherical shell of
"Observable Universe" which has radius
equal to the lifetime of the Universe.
• Objects more than about 13.7 thousand
million years old (the latest figure) are too
far away for their light ever to reach us.
• Redshift effect certainly contributes. But
the finite age of the Universe is the most
important effect.
References: Wesson, 1991, ApJ. 367, 399
Other galaxies
• Telescopic images of the
night sky reveal many
other galaxies
– What do they look like?
• are they all like the Milky
Way?
– Where are they?
• spread randomly through
space, or grouped?
– What can we learn about
the Universe?
A long time ago in a galaxy
far, far away...
• How do we know these fuzzy blobs are
distant galaxies?
– some types of star (especially variable stars)
have well-known intrinsic brightness
– by measuring how bright
they appear to be we can
infer their distance
– “standard candles”
Types of galaxies
• Galaxies seem to come in
two basic types
– smooth, featureless
elliptical galaxies
• circular or elongated
• made of old, reddish stars
– spiral galaxies like the
Milky Way
• some with round bulges,
some with bars
Hubble’s tuning fork
old stars; no
recent star
formation
E0
no
spiral
arms
••• E6
Sa
Sb
Sc
Irregular
S0
looser spiral arms —>
smaller bulge
—>
SB0
more elongated
—>
old stars
dwarf
elliptical
(dE/dSph)
lots of
young
stars
SBa
SBb
SBc
old stars in bulge;
younger in disc;
youngest in spiral arms
amorphous
or
disrupted
Where do we fit in?
•
•
•
The Milky Way is clearly not
an elliptical galaxy
– it has a disc, and contains
young stars
It has spiral arms
– so, not S0
– size of bulge and arm
pattern suggest Sbc
• between Sb and Sc
There is evidence for a small
bar
– SBbc, or SABbc
• SAB means
intermediate between
barred and unbarred
The Local Group
•
The Milky Way is not
alone: it is part of a small
group containing
– M31 (the Andromeda
galaxy)
• a large Sb spiral,
bigger than us
– M33
• a small Sc spiral
– the Large Magellanic
Cloud
• an irregular
satellite of the
Milky Way
– at least 30 dwarf
irregular and dwarf
elliptical galaxies
– but no large elliptical
galaxies
Galaxy groups and clusters
• The Local Group is small:
some rich clusters contain
thousands of large galaxies
– elliptical and S0/SB0
galaxies are much more
common in rich clusters
– spiral and irregular
galaxies are much more
common in small groups
and the outskirts of
clusters
Galaxy properties
• Elliptical galaxies
– contain old stars
– have little net rotation
• star orbits are
randomly directed,
as in our halo
– have little internal
structure
– are much more common
in galaxy-rich
environments
– include the most
massive galaxies (but
also some with very low
mass)
• Spiral galaxies
– show recent star
formation (in disc)
– have rotating discs
• stars all orbit in
same direction
– have complex internal
structure
– are more common in
low-density
environments
– have a smaller range of
masses
Lenticular (S0/SB0) galaxies are like spiral galaxies with no gas
Irregular galaxies are mostly like spirals too small to become organised
Galaxy problems
• What makes some galaxies elliptical and others spiral?
– their mass?
– their age?
– their rotation?
– their history?
• How do spiral galaxies avoid “winding
up” their spiral arms?
• How does the evolution of galaxies
relate to the presence of central
supermassive black holes?
– the Milky Way’s is, if anything, less
massive than most!
Galaxies and cosmology
• Almost all galaxies are
moving away from us
– and the greater their
distance, the faster they
recede (Hubble’s law)
• Clusters of galaxies group
to form huge superclusters,
separated by vast voids
– how does this large scale
structure develop?
• What is the dark matter?
From http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/Footprints.html
From http://astron.berkeley.edu/~jcohn/lens.html
From Brian Schmidt’s web site, http://msowww.anu.edu.au/~brian/PUBLIC/public.html
http://www.aip.org/history/einstein
Henrietta Leavitt discovers a correlation between
Cepheids' period and luminosity (1912).
Leavitt discovered a direct correlation between the time it took a star
to go from bright to dim and how bright it actually was.
During her career, Leavitt discovered more than 2,400 variable stars.
http://www.pbs.org/wgbh/aso/databank/entries/baleav.html
The age of the universe as determined from the Hubble constant
is now consistent with that determined from the oldest stars
in globular clusters.
Robert W. Wilson
Cosmology becomes a
science:
Robert W. Wilson
and Arno Penzias
discover the cosmic
background
radiation in 1964.
Nobel Prize, for what has been called
“the greatest scientific discovery ever”.
Robert Woodrow Wilson – Autobiography
[copied and edited from www.nobel.se]
During my pre-college years I went on many trips with my father into the oil
fields to visit their operations. I puttered around the machine, electronics,
and automobile shops while he went about his business.
I used to fix radios and later television sets for fun and spending money. I
built my own hi-fi set and enjoyed helping friends with their amateur radio
transmitters.
I did a senior thesis with C.F. Squire building a regulator for a magnet for
use in low-temperature physics. Following that I had a summer job with
Exxon and obtained my first patent.
Following Rice, I went to Caltech for a Ph.D in physics. David Dewhirst, a
Cambridge astronomer, suggested that I see John Bolton and Gordon
Stanley about radio astronomy. Maarten Schmidt, who had previously done
galactic research and was currently working on quasars, saw me through
the last months of thesis work.
I joined Bell Laboratories at Crawford Hill in 1963 as part of A.B. Crawford's
Radio Research department in R. Kompfner's laboratory. I started working
with the only other radio astronomer, Arno Penzias, who had been there
about two years.
In early 1990’s, COBE sees inhomogeneities in cosmic background radiation
(about one part in 100 000): the seeds of the structure (galaxies, clusters etc.) seen in our
present universe, and evidence for both quantum fluctuations and inflation in the
extremely early universe. In addition, the peak associated with acoustic oscillations
(more later) indicates that the universe is flat.
http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/Footprints.html]
A bonus: COBE image of the Milky Way
(credit to Ned Wright)
http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/Images/Geometry_lg.jpg
From Brian Schmidt’s web site, http://msowww.anu.edu.au/~brian/PUBLIC/public.html
From Brian Schmidt’s web site, http://msowww.anu.edu.au/~brian/PUBLIC/public.html
Credit: Riess et al. 2004 and NASA
Evidence from Type Ia supernovae for a decelerating,
then accelerating universe, and thus for dark energy.
These are images of three of the most distant
supernovae known, discovered using the Hubble
Space Telescope as a supernova search engine.
The stars exploded back when the universe was
approximately half its current age.
The light is just arriving at Earth now. Supernovae
are so bright they can be seen far away and far back
in time. This allows astronomers to trace the
expansion rate of the universe, and to determine
how it is affected by the repulsive push of dark
energy, an unknown form of energy that pervades
space.
Credit to Adam Riess et al. and NASA.
Images from http://www-int.stsci.edu/~ariess and
http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/12/image/
a
Fritz Zwicky (1930’s) and Vera Rubin (see next page) discover dark matter.
http://www.dynamical-systems.org/zwicky/Zwicky-e.html
In the thirties, Zwicky and Smith both examined closely two
relatively nearby clusters, the Coma cluster and the Virgo cluster.
They looked at the individual galaxies making up the clusters
individually, and the velocities of the clusters. What they found
was that the velocities of the galaxies were about a factor of ten
to one hundred larger than they expected. The velocities can
indicate the total mass inside the cluster. The more mass in the
cluster, the greater the forces acting on each galaxy, which
accelerates the galaxies to higher velocities..
http://www.swemorph.com/zwicky.html
From http://www.astro.queensu.ca/~dursi/dm-tutorial/dm1.html
Vera Rubin determined the velocities as a function of distance from
the galactic center of clouds of ionized hydrogen (in astrophysical
terminology, HII regions). This was done by measurement of the
Doppler shift of their H-alpha emission lines. The hydrogen clouds
move with the stars and other visible matter in the galaxies. Their
velocities are more easily and directly measured than other visible
matter.
Rubin found that the velocities of the clouds did not decrease with
increasing distance from the galactic center, and in some cases
even increased a little. This is in striking contrast to the decrease in
velocity with radius predicted by Keplerian motion, which would
occur if all the mass of the galaxy were concentrated in the center of
the galaxy.
Detailed observations were first made by Rubin and W.K. Ford of
the Andromeda galaxy and published in "Rotation of the Andromeda
Nebula from a Spectroscopic Survey of Emission Regions,"
Astrophysical Journal 159, 379 (1970). They then made
observations of over 60 other spiral galaxies which apparently
confirmed that the presence of dark matter was a general
phenomenon ["Rotation Velocities of 16 Sa Galaxies and a
Comparison of Sa, Sb, and Sc Rotation Properties," Astrophys. J.
289, 81 (1985), with D. Burstein, W. K. Ford, Jr., and N. Thonnard].
Photo credit: Mark Godfrey
by Benjamin Johnson
http://www.physics.ucla.edu/~cwp/articles/rubindm/rubindm.html
From http://astron.berkeley.edu/~jcohn/lens.html
From http://astron.berkeley.edu/~jcohn/lens.html
http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/Images/CosmicTimeline_gr.jpg
The early
universe must
have been
extremely hot
and dense
3 minutes
T about 109 K
Wien's displacement law
T  const
~R
1 1
T~ ~
 R
Hubble image of spiral galaxy NGC 4414. [http://nssdc.gsfc.nasa.gov/photo_gallery]
Resembling a gigantic hubcap in space, a 3,700-light-year-wide dust disk
encircles a 300-million- solar-mass black hole in the center of the
elliptical galaxy NGC 7052.
The disk, possibly a remnant of an ancient galaxy collision, will be swallowed
up by the black hole in several billion years. The black-and-white image
on the left, taken by a ground-based telescope, shows the complete galaxy.
The Hubble picture on the right is a close-up view of the dust disk surrounding
the black hole.
[http://hubblesite.org/newscenter/archive]
Charles Bennett presenting Wilkinson Microwave Anisotropy Probe (WMAP) results at a press
conference in Feb. 2003. He is the principal investigator for WMAP, which recently determined
the age, content, history, and other key properties of the universe with unprecedented accuracy.
The Wilkinson Microwave Anisotropy Probe (WMAP) is named in honor of David Wilkinson of
Princeton University, a world-renown cosmologist and WMAP team member who died in
September 2002.
WMAP has made the first detailed full-sky map of the oldest light in the universe. It is a "baby
picture" of the universe. Colors indicate "warmer" (red) and "cooler" (blue) spots. (The oval
shape is a projection to display the whole sky.) The microwave light captured in this picture is
from 380,000 years after the Big Bang, over 13 billion years ago. The data brings into sharp focus
the seeds that generated the cosmic structure we see today. These patterns are tiny temperature
differences within an extraordinarily evenly dispersed microwave light bathing the Universe,
which now averages a frigid 2.73 degrees above absolute zero temperature. WMAP resolves slight
temperature fluctuations, which vary by only millionths of a degree. These data support and
strengthen the Big Bang and Inflation Theories.
[http://map.gsfc.nasa.gov]
Power Spectrum (Fingerprint of
the Universe)The "angular
spectrum" of the fluctuations in
the WMAP full-sky map. The
shapes of these two curves
contain a wealth of information
about the age and content of the
universe and about the source of
the fluctuations seen in the picture.
The rise in the bottom curve at
large angles (~90 degrees) is the
indication that the first stars in
the universe formed very quickly.
Looking Back In Time. WMAP, in the present era, looks
back to the first light to break free in the Universe, the
afterglow of the Big Bang that emerged 380,000 years
after the Big Bang. This light, seen today, has taken over
13 billion years to reach us. During that time, giant clouds
of gas in the early Universe condensed under the force
of gravity to form the first stars (200 million years after
the Big Bang). Then, galaxies and galaxy clusters formed
into the vast structure we see today. The temperature
fluctuations seen today correspond to the seeds that
grew to become galaxies.
The First Stars. The first stars in the Universe turn on.
WMAP data reveals that this era occurred 200 million
years after the Big Bang, much earlier than many
scientists had suspected.
Content of the Universe. The Universe is much more than
what meets the eye. The contents of the Universe
include 4% atoms. This is ordinary matter, the stuff
from which stars and everything we see and touch is
made. WMAP data reveals that 23% of the Universe is
unseen dark matter, a mysterious form of matter
intrinsically different from atoms. This matter does not
radiate light like ordinary matter, but is detected only
indirectly by its gravity. Most of the Universe, 73%, is a
mysterious form of energy, dubbed dark energy, that
acts as sort of an anti-gravity force and is responsible
for accelerating the expansion of the Universe.
The light that is reaching us has been stretched out as
the universe has stretched, so light that was once beyond
gamma rays is now reaching us in the form of microwaves.
Microwaves are the same kind of electromagnetic
radiation as the light we see with our eyes, but
stretched out to a longer wavelength.
Map showing an asymmetry (light and dark gray tones),
called the "dipole", due to the motion of the spacecraft.
The foreground signal of the Milky Way can be separated
from the cosmic background because they are different
colors.
WMAP Conclusions
The new WMAP data has been combined with other diverse cosmic measurements
(galaxy clustering, Lyman-alpha cloud clustering, supernovae, etc.) to yield a new
unified understanding of the universe:
*Universe is 13.7 billion years old with a margin of error of close to 1%.
*First stars ignited 200 million years after the Big Bang.
*Light in WMAP picture from 379,000 years after the Big Bang.
*Content of the Universe: 4% Atoms, 23% Cold Dark Matter, 73% Dark energy.
*The data places new constraints on the dark energy. It seems more like a
"cosmological constant" than a negative-pressure energy field called
"quintessence". But quintessence is not ruled out.
*Fast moving neutrinos do not play any major role in the evolution of structure in
the universe. They would have prevented the early clumping of gas in the universe,
delaying the emergence of the first stars, in conflict with the new WMAP data.
*Expansion rate (Hubble constant) value: Ho= 71 km/sec/Mpc (with a margin of
error of about 5%)
*New evidence for Inflation (in polarized signal)
*For the theory that fits our data, the Universe will expand forever. (The nature of
the dark energy is still a mystery. If it changes with time, or if other unknown and
unexpected things happen in the universe, this conclusion could change.)
The Sloan Digital Sky Survey is the most ambitious astronomical survey project ever
undertaken. The survey will map in detail one-quarter of the entire sky, determining
the positions and absolute brightnesses of more than 100 million celestial objects. It will
also measure the distances to more than a million galaxies and quasars.
Credit and copyright: Sloan Digital Sky Survey Team, NASA, NSF, DOE
Evidence for a mysterious dark energy in the universe:
Gazing to the far reaches of space and time, NASA's Hubble Space
Telescope identified the farthest stellar explosion ever seen, a supernova
that erupted 10 billion years ago. By examining the glow from this dying
star, astronomers have amassed more evidence that a mysterious,
repulsive force is at work in the cosmos, making galaxies rush ever faster
away from each other.
[http://hubblesite.org/newscenter/archive]
From Brian Schmidt’s web site, http://msowww.anu.edu.au/~brian/PUBLIC/public.html
This and several
following slides from
http://wwwsupernova.lbl.gov
The predicted abundance of elements heavier than hydrogen, as a function of the density of baryons
in the universe (expressed in terms of the fraction of critical and the Hubble parameter).
From http://astron.berkeley.edu/%7emwhite/darkmatter/dm.html
The Four Pillars of the Standard Cosmology
The four key observational successes of the standard Hot Big Bang
model are the following:
Expansion of the Universe
Origin of the cosmic background radiation
Nucleosynthesis of the light elements
Formation of galaxies and large-scale structure
The Big Bang model makes accurate and scientifically testable
hypotheses in each of these areas and there is remarkable agreement
with the observational data.
from http://www.damtp.cam.ac.uk/user/gr/public/bb_pillars.html