Transcript Lecture22 - Michigan State University

Quasars
• In 1963 Martin Schmidt was trying to understand some
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unidentified lines in the optical spectra from a star that
had a strong radio signal
He realized that the lines were Balmer lines that were
normally in the UV that had been shifted to the visible
by the stars veleocity of recession which was about
15% of the speed of light
This “star” turned out to be very distant and not a star
at all
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Quasi-stellar object - quasar
• Thousands of quasars have been found and they all
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show very large redshifts
The largest shows / = 5
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94% of the speed of light!
ISP 205 - Astronomy Gary D. Westfall
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The Luminosity Puzzle
• The very large redshifts of quasars means that they are
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very far away and because we can see them they must be
very luminous
The quasars also seemed to vary in luminosity over a
period of months
This luminosity variation suggested that the quasars were
large objects
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Distances to Quasars
• To figure out how far away quasars are astronomers
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looked for quasars associated with galaxies that to which
they could measure the distance
This is difficult because quasars outshine entire galaxies
by a lot
Then
astronomers
used previous
techniques to
measure the
distance to the
galaxy and
hence the quasar
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Seyfert Galaxies
• Active galaxies produce abnormal amounts of energy,
mostly in their centers
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Active galactic nuclei, AGN
• One example is Seyfert galaxies
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Seyferts are spiral galaxies
• Seyfert galaxies produce emission lines rather than
absorption lines indicating hot gas clouds
• Seyfert galaxies have
The
central
region
of
Seyfert
galaxy
NGC
1068
luminosity variations on
the scale of months like
quasars and have pointlike bright centers that
are brighter than the sum
of individual stars
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Active Elliptical Galaxies
• Elliptic galaxies are also observed to have active nuclei
• Elliptic galaxy M87 has such an active center
• Jets of ionized gas are visible coming from the center
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The Power Behind Quasars
• Astronomers are now convinced that quasars have
massive black holes at their centers
• We can say a black hole exists if we can
demonstrate that there is a very massive object
such that no star or cluster could account for that
mass
• We can determine the mass of an object using
Kepler’s law as before
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We simply measure the period of an object orbiting the
quasar and calculate the mass
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The Mass of the Quasar in M87
• The period of material orbiting the center of M87 was
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calculated by measuring the redshift of material circling
the center
The rotation speed was
measured to be 550
km/s
Using Kepler’s law we
get a mass of about 2.5
billion Msun
There is evidence for
black in the center of
many galaxies
Once formed, these black holes continue to absorb
material and grow
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Radio Jets
• Material falling into a black hole forms an
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ISP 205 - Astronomy Gary D. Westfall
accretion disk
Models show that these accretion disks can
lead to jets along the axis of the disk
These jets glow with radio, light, and x-rays
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Evolution of Quasars
• When we see a distant object we see it as it was long ago
• If we see more quasars far away, there must have been
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more quasars long ago
The theory is that quasars are black holes with enough
fuel around them to make bright accretion disks
This theory leads to the conclusion that quasars should
have formed early in the history of the universe
• This theory leads to
the conclusion that
quasars should have
formed early in the
history of the universe
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Gravitational Lenses and Quasars
• The quasars brilliance and immense
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distance makes it ideal for the study of
deep space
Gravitational lensing was first
discovered using quasars in 1979 when
identical images of a quasar were
observed
• On the right one can see four identical
optical images of a quasar (top) and an
“Einstein ring” of a quasar made with
radio waves using the VLA
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The Distribution of Galaxies in Space
• Looking at distant galaxies is
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like looking back in time
When we look at
astronomical objects we find
they are seldom alone
The question arises: do
galaxies cluster also?
Hubble used the 100 inch Palomar telescope to sample
the sky in 1283 places
He found that number of galaxies visible is about the
same
He found that the number of galaxies increased with
faintness
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More evidence for a constant density of galaxies
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The Cosmological Principle
• The universe is the same everywhere
• The universe appears to be isotropic and
homogeneous
• Without the cosmological principle, we could not
make progress in understanding the universe
around us
• Hubble had simply counted the number of
galaxies
• Recently astronomers have measure the distances
of thousands of galaxies and have built up a
picture of the distribution of galaxies
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The Local Group
• The Milky
Way is a
member of a
small group of
galaxies called
the Local
Group
containing
more than 40
members
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Neighboring Groups and Clusters
• Galaxies form clusters
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Rich galaxy cluster have thousands of galaxies
• The nearest rich galaxy cluster is called the Virgo
Cluster
• An much larger galaxy cluster is the Coma cluster
with a diameter of 10 million LY
• Large galaxy clusters
such as Coma have few
spirals in the center but
have many ellipticals
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Superclusters and Voids
• Galaxy clusters form
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ISP 205 - Astronomy Gary D. Westfall
superclusters
Among the superclusters are
giant voids
The Milky Way is located in the
Local Supercluster
One conclusion we can draw is
the space is mostly empty
The clusters occupy only about
5% of the space
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Slices of the Universe
• Enormous
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volumes of space
lie beyond the
Local Supercluster
This space has not
been completely
mapped
One striking
structure that has
been found is the
“Great Wall”
There are obvious
sheets and
filaments separated
by huge voids
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When Did Galaxies Form?
• We can study old, distant galaxies and get
information about times near the beginning of the
universe
• Most galaxies we can see are least a few billion
years old
• We can learn about a galaxy by measuring its color
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Blue means young, hot stars
Yellow or red means old stars
• Another way to learn about a galaxy is to study its
shape
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Spiral galaxies are young
Elliptical galaxies are old
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The Ages and Compositions of Galaxies
• Nearly all galaxies are old
• The Milky Way contains stars that about the age of the
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universe
Distant galaxies show evidence for heavier elements that
were not present at the beginning of the universe
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A least one generation of stars has passed
• Star formation has stopped in elliptical galaxies while it
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continues in spiral galaxies
Elliptical are poor in interstellar gas but galaxy clusters
have a large amount
Galaxies in clusters collide!
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Colliding Galaxies
• Galaxies can collide which stimulates
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star formation
Indivdual stars are not affected much
because of the large distance between
stars
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The Life History of Galaxies
• Elliptical galaxies formed early
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and turned all the gas and dust to
stars in the first 3 billion years
Spirals converted gas and dust at
a much slower rate and are still
producing stars today
The peak of star formation
occurred when the universe was
between 3 and billion years old
• When the universe was 3 - 6 billion years old, the
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galaxies were small
Galaxies have merged to form larger galaxies since that
time
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Cosmology
• The study of the universe as a whole is called
cosmology
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How did the universe come into being?
What will its ultimate fate be?
• What have we observed about the universe?
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All galaxies show a redshift proportional to distance,
implying that the universe is expanding
The distribution of galaxies on the largest scale is
isotropic and homogeneous
The contents of the universe evolve with time:
hydrogen and helium are changed into heavier
elements inside stars
Gravity warps the fabric of space-time
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The Age of the Universe
• The universe cannot be static
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The universe must either be contracting or expanding
• If we had a movie of the expanding universe and ran it
backwards, we would see the galaxies moving together
until they were all in one place
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The big bang
• We can estimate how long the galaxies would take to be
back in the same place
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v = Hd, Hubble’s Law
From physics we know d = vt
t = d/v = d/(Hd) = 1/H
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Hubble time
H = 20  2 km/s per million LY
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t = 15  1.5 billion years
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Estimates of the Age of the Universe
• The estimates of the age of the universe depend
on our knowledge of the distance of galaxies
• An error of a factor of 2 in the distance would
mean a factor of 2 in the age of the universe
• Over the past 20 years debate has raged among
astronomers about the value of H
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It has varied from 15 to 35 km/s per million LY
Recent data from the Hubble Space Telescope have
yielded
20  2km/s per million LY
 70  7 km/s per million parsecs
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Deceleration/Acceleration
• The Hubble time is the correct age of the universe only if
this expansion has been constant throughout the age of
the universe
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Constant H
• Gravity creates attraction and should slow the expansion
of the universe
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Deceleration
The universe would actually be younger than the Hubble time
• New measurements of type Ia supernovae can interpreted
to mean that the universe is accelerating
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The universe is expanding more slowly now than in the past
The universe would be older than the Hubble time
Based on the observation that distant type Ia supernovae are
20% dimmer than they should be if expansion were constant
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Other explanations?
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The Age of the Universe
• Astronomers generally agree that the modifications to the
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Hubble time for acceleration and deceleration make the
age of the universe 15  5 billion years
Another way to estimate the age of the universe is to find
the oldest objects whose age we can measure
Computer models show that the age of globular clusters
is about 13 billion years and assuming that it took a
billion years for stars to form, the oldest stars are younger
than the age of the universe
This agreement has only occurred in recent years
• Previously the Hubble time was shorter
• Previously the age of globular clusters was longer
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The Geometry of Spacetime
• The gravity from the matter of the entire universe
warps spacetime
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We must consider a fourth dimension of space
• Thinking in 4 dimensions is difficult so we will
think in 3 dimensions (2 + 1)
• In the world of 2 dimensions the third dimension
if curvature
• A 2-dimensional observer would observe odd
things in his 2-dimensional curved world
• Let’s use a balloon as an example
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A Balloon Universe
• If you go in one direction, you
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get back to where you started
There is no center, all points on
the balloon are the same
If the balloon grows, all points
move away from each other
The points move away from
other other because the balloon
is growing, not because any
point is doing something special
This example represents a closed
universe
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An open universe is also
possibility but harder to
visualize
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The Expanding Universe
• If the universe is dense enough, it will stop expanding and collapse
• If the universe not dense enough, it continue to expand forever
• At a critical density, the universe will just stop expanding at
infinite time
1.
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3.
4.
Closed universe
Open universe
Universe with critical
density
Universe with less than
critical density and
positive cosmological
constant
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Facing the Future
• If the mass density of the universe is high enough,
the expansion of the universe will reverse and the
universe will collapse
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The Big Crunch
• If the mass density of the universe is low enough,
the universe will expand forever and slowly die
out
• At critical density, the universe can just barely
expand forever
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Flat universe
Zero curvature
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Who’s Winning?
• The observed matter density is too low to close
the universe
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Dark matter may play a role
• The ages of stars suggest that we live in an open
universe
• Type Ia supernovae suggest that the universe if
accelerating
• The lookback time is how long ago the light from
an object was emitted
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Depends on our model of the universe
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The Big Bang
• The big bang theory states that the universe began
as a gigantic explosion
• This idea has entered popular culture
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History of the Idea of the Big Bang
• Georges Lemaitre proposed a big bang-like theory
in the early 1920s involving fission
• In the 1940s, George Gamov proposed the a big
bang model incorporating fusion
• Since that time, many astronomers and physicists
have added their work to what is now known as
the standard model of the big bang
• Three main ideas underlie the big bang model
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The universe cools as it expands
In very early times, the universe was mostly radiation
The more hotter the universe, the more energetic
photons are available to make matter and anti-matter
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The Evolution of the Early Universe
• With the three previous ideas in mind, we can
trace the evolution of the universe back to when it
was 0.01 s old and had a temperature of 100
billion K
• We can go back farther but not all the way to zero
time
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At 10-43 s most of our physical laws become
impractical
• At times before 0.01 s, the universe was filled
with quarks and gluons
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After 0.01 s
• Our picture after 0.01 s is that
the universe was filled with
radiation and with types of
matter that exist today
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Protons and neutrons
Photons and neutrinos
• The temperature was no
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longer hot enough to create
neutrons and protons in
collisions of photons
At about 3 minutes, nuclei
begin to form
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75% hydrogen, 25% helium,
some lithium
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Learning from Deuterium
• All the deuterium in the universe was formed in
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the first 3 minutes
If the universe was very hot and dense when the
deuterium formed, it would have been broken up
If the universe expanded and then out thinned out
rapidly, deuterium would survive
The density extracted from the surviving
deuterium is 5 x 10-31 g/cm3
Suggests a low enough mass that the universe is
open
Dark matter may still play a role
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The Universe Becomes Transparent
• For several hundred thousand years the universe
resembled the interior of a star
• After that time, atoms began to form
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The universe became transparent
Radiation and matter decoupled
• 1 billion years after the big bang, stars and
galaxies began to form
• The radiation from the big bang faded but it left
an indelible fingerprint, the cosmic radiation
background
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The Cosmic Radiation Background
• In the 1940s Adler and Herman realized that just before matter and
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radiation decoupled, the universe must have been radiating like a
blackbody at a temperature of 3000 K
That was 15 billion years ago, and the universe has expanded,
leaving an afterglow of the big bang with a temperature of 3 K
In the 1960s, Penzias and Wilson were using a microwave antenna
to study the sky
They could not make their receiver work without background noise
that seemed to be coming from everywhere in the sky
They thought is was their detector but soon realized that it was real
and was coming from space
Penzias and Wilson got in touch with some cosmologists from
Princeton and who realized that this radiation was the cosmic
background radiation (CBR)
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Properties of the CBR
• The first accurate
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measurements of the
CBR were made by the
COBE satellite
They observed that the
CBR matched perfectly
with a blackbody with a
temperature of 2.73 K
• Astronomers concluded that the universe
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we see today evolved from a hot,
uniform state
We live in an evolving universe
The universe looks uniform in all
directions but not completely uniform
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Problems with the Big Bang Model
• The standard big bang model explains many
things but there are remaining issues
• It does not explain why there is more matter than
antimatter in the universe
• It does not explain the observed uniformity of the
universe
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Parts of the universe could never have been in contact
yet they show the same background temperature
• It does not explain why the density of the universe
is close to the critical density
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Grand Unified Theories
• There are 4 forces
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Gravity, weak,
electromagnetic, nuclear
• At high temperatures,
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these forces become one
force
Theories exist that unify
weak, electromagnetic
and nuclear
Grand unified theories
(GUTs)
No theory yet exists
incorporating gravity
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The Inflationary Hypothesis
• GUTs predict that at 10-35 s, a rapid, early expansion took place
• Prior to this inflation, the universe was small enough to
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communicate with itself
After inflation, parts of the universe were beyond each other’s
horizon
The inflationary model also predicts that the universe is exactly at
critical density
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Lucky Accidents
• The temperature of the radiation emitted when the
universe became transparent varies by about 16 millionth
of a K
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Smaller variations would have led to no galaxies
Larger variations would have led to black holes
• The fine balance between expansion and contraction
• The existence of only matter and not anti-matter
• The production rate of nuclei in the big bang produced
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only hydrogen and helium, and did not go all the way to
iron
Neutrinos have to have just the right interaction
properties with matter to allow supernovae
Anthropic principle
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