Transcript Using Galaxy Clusters to Study Structure Evolution

Visible universe is
an infinitesimal
fraction of the
material from the
big bang
What we know about the universe today
Edwin Hubble at Mt. Wilson
Hubble’s observations at the 100 inch during the
1920’s led him to the conclusion that the universe
is expanding, and that an object’s recession velocity
is proportional to its distance from the observer.
Hubble guiding the Hooker
100 inch telescope in 1923.
Photos courtesy Mt. Wilson:
http://www.mtwilson.edu/History
The Hooker 100 inch telescope atop Mt. Wilson
near Pasadena, CA. It was the largest telescope in
the world from 1917-1947.
The View from Mt. Wilson
Photo courtesy Colleen Gino of Mt. Wilson
The Hubble Space Telescope
Thermal and Non-thermal Emission
 Light
– wavelength l, frequency n , speed c
ln  c
E  hf
hc
E
l
l
Dispersion: Use (prism
or grating) to determine spectrum
 Two kinds of sources

– thermal emission: incandescent heated filament: blackbody
radiation…the Planck Law
– non-thermal emission: eg: excited gas emits light at specific colors or
wavelength. eg laser, radio, line spectra

 Identify
pattern of emission lines provides unique fingerprint for each element
spectral lines in distant galaxies
– universality of spectral lines: laws of physics invariant in space and time.
Using the Doppler Effect to Measure Velocity
 Doppler
Shift- applies to sound and light
» If source is moving with respect to observer, the observer experiences
a shift in wavelength l
» Velocities away from observer shift light to longer l (redshift)
» Velocities toward observer shift light to shorter l (blueshift)
» The higher the velocity the larger the shift- provides velocity measure
f 
Blueshift

f (1 v /c)
f 
1 v 2 /c 2

Redshift


f (1 v /c)
1 v 2 /c 2
l l l
z
 1
l
l
lnow
z 1
lthen
z
No recession: z=0

v=0.5c,
z=0.86
Most distant galaxy:z=6
Cosmic Background: z=1000
z=Dl/l=Da/a where a is
characteristic scale size of
universe
Galaxy Spectroscopy
Spectrum courtesy Bob Kirshner
Sodium
Intensity
Stellar Spectrum
Magnesium
Galaxy Spectrum
Wavelength l
Calcium


Spectra of a nearby star and a distant galaxy
» Star is nearby, approximately at rest
» Galaxy is distant, traveling away from us
at 12,000 km/s
Spectra of nearby and distant galaxies
» Nearby galaxy travels at 261 km/s
» Distant galaxy travels at 6,400 km/s
Astronomical Distance Measurement
»Apparent size of a “standard ruler”
– Standard ruler is an object whose intrinsic size is known
– Apparent (angular) size q provides distance d given intrinsic size r
d
r
tan q
 qr
r
q
d
»Apparent brightness of a “standard candle”
– Standard candle is source whose intrinsic brightness is known
– Apparent brightness b provides distance d given intrinsic
brightness B
b
B
4d 2

d
B
4b
Standard Rulers in Everyday Life
The STOP sign is an everyday “standard
ruler”. If we know STOP signs are all
the same size, the apparent size of a STOP
sign provides us with distance information.
Some Known Standard Candles and Rulers
»Standard rulers
– Elliptical galaxies
– Galaxy clusters
»Standard candles
– Certain types of stars (Cepheid variables)
– Spiral galaxies
– Certain types of supernovae (Type Ia SNe)
Exploding white dwarfs
 Emit as much light as an entire galaxy, so can be detected at great
distances

Type Ia Supernova 1998bu in M96
Observations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha
SNe look like bright stars superimposed on galaxies. They brighten toward maximum
and then fade away over time as the hot material expands and cools.
Type Ia Supernova
Observations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha
Repeated observations yield a light curvemeasured brightness versus time- which
astronomers use to determine the peak
apparent brightness and distance of the
SN and parent galaxy. This is SN 1998aq.
A spectrum of the light emitted by
SN 1998bu. The features in this spectrum identify SN 1998bu as Type Ia.
Testing Expansion with Type Ia Supernovae
 Find
SNe in distant galaxies (rare
objects)
Hubble Diagram
spectrum to confirm they
are Type Ia SNe
 For
each SN Ia
» Measure the recession velocity
of the parent galaxy vr
» Measure the maximum
apparent brightness and
compare that to the intrinsic
brightness to calculate the
distance d
» Place that point on Hubble
diagram
Velocity
 Take
vr
d
Distance
A single supernova
Type Ia Supernovae Measurements
Distance measurements to 19 SNe
Riess, Press & Kirshner ApJ 1996
Blue points: 19 SNe
Red line: Hubble Law with
Ho=19.6 km/s/Mly
=64km/s/Mpc
Type Ia supernovae and every
other distance indicator used
provides results consistent with
the Hubble Law: other galaxies
are receding from us, and their
recession velocities are proportional to their distances, in
other words, the farther away
the galaxy, the faster it travels
away from us.
vr  H o d
Interpreting the Expansion
 Galaxies
are receding from us, and their recession velocities
are proportional to their distances from us
 Two
interpretations
»Bad neighbour hypothesis
– We are at the center of the universe, and the rest of the universe is
trying its best to get away from us.
»Homogeneous expansion hypothesis
– The whole universe is expanding, and observers on any other
planet in any other galaxy would note the same proportionality
between recession velocity and distance- the Hubble Law.
 BNH
violates the Cosmological Principle.
 Only linear homogeneous expansion is universally internally
consistent
Summary
 Observations:
galaxies tend to travel away from us, and their
recession velocities vr are proportional to their distances dthe Hubble Law:
vr  H o d
 Our
universe is expanding homogeneously and is not static.
»The universe was denser in the past
»The universe had a beginning….?
 Expansion
is generic to the Big Bang model.
What we know and what we don’t
Matter
density is dominated by cold dark matter
that we know nothing about.
Perturbations which give rise to structure formation
arise in the inflation era due to ultra-high energy
processes about which we know nothing!
The universe is dominated by a property of space
called dark energy, cosmological constant or
quintessence (also called the cosmo-illogical
constant!). We know nothing about it!
Baryon asymmetry arises in the GUT or
Electroweak era due to CP violation but the details
are unknown