High-Z Supernovae: The
Evidence for An Accelerating
Universe
R. Mutel
Departmental Astrophysics Seminar
February 19, 2003
Outline
• Synopsis of 20th Century Cosmology (Chronology)
• High-z Supernovae and the Accelerating Universe
– Potential problems: evolution, dust, lensing
• Dark Matter update: role of ancient White dwarfs?
• Latest results from WMAP – all cosmological parameters
determined!
• Heretical view: Burbidge’s QSSC
Brief history of Cosmological Models: pre-1980’s
•
1917 – Einstein’s General Relativity: static cosmological model with
‘cosmological constant’ . Einstein assumed static Universe, needed small
o > 0 to prevent collapse.
•
1930 – Hubble finds Universe is expanding, Einstein drops need for 
(‘biggest blunder of my life’).
•
1948 – QED predicts non-zero vacuum energy. Casimir effect provides
experimental verification. Calculation implies  ~10120 o!
•
1950’s Development of Steady State Model: mean density constant –
expanding vacuum creates matter. Time invariance. Cosmol. constant = 0.
•
1965 – Discovery of microwave cosmic background radiation (CBR),
development of Big Bang cosmology. Provides natural explanation for
expansion.
•
1980’s – CBR too uniform (T/T < 10-4) – solved by inflation (Guth 1980).
This requires a ‘flat’ Universe (tot = 1)
•
Early 1980’s – Observations of ordinary (luminous) matter find LM << crit
(tot << 1), i.e. ‘open Universe’ geometry.
Brief history of Cosmological Models: 1980’s-1990’s
•
1980’s – Rotation curves of galaxies, clusters require most gravitational
mass reside in ‘dark matter’ component
•
1990’s – Refined observations find LM ~ 0.02- 0.04 , CDM ~ 0.2-0.4: If
tot = 1, where is the rest?
•
1992 – COBE First comprehensive measurements of T/T ~3·10-5 on
angular scale 6°
•
1998 – First results for high-Z Type Ia SN show evidence for acceleration:
explained by ‘dark energy’ (‘quintessence’)
•
late-1990’s – MACHO, OGLE, etc. micro-lensing surveys find microlensed
events consistent with compact sub-stellar halo objects, total  ~ 0.2-0.4.
•
1999 – Hubble Key Project for nearby galaxies: Ho = 73 ± 6 km/s/Mpc.
Standard Friedmann cosmo. Model predicts Universe’s age t = 2/3 H0-1 =
10 ± 0.8 Gyr.
•
2000 – Refined measurements of stellar ages in GC (using distances from
HIPPARCOS) find oldest stars are 13.4 ± 2.2 Gyr, in conflict with Ho
measurements: Crisis!
Brief history of Cosmological Models
late-1990’s-present
• 2001 – Discovery of extremely cool (old) WD’s. Tentative
density WD  0.1 CDM.
• 2001-2002 – More high-z Type Ia SN enhance claims for
acceleration at z<1.
• 2002 – Highest redshift SN discovered on Hubble Deep
Field (z~1.7, SN1997ff). Mag. Confused by lensing?
• Feb 2003 – WMAP results – acoustic modes in CBR
confirmed, finds tot = 1.02 ± 0.02, with =0.73 ±0.04!
Standard model: distance scale
factor
Ho = 65 km/s/Mpc
 = 0 (green), 1 (black), 2 (red)
SN1994D, A
prototypical
Type Ia SN
• Host galaxy
NGC4526,
Hubble type
S0, member of
Virgo Cluster
(15 Mpc)
• Vmin =
•
Hubble diagram of Type Ia
Supernovae. The upper panel
shows the classical Hubble
diagram with distance modulus
vs. redshift. All data have been
normalized to the m15 method.
Lines of four cosmological
models are drawn: full line for an
empty universe ( M = 0, = 0),
long-dashes for an Einstein-de
Sitter model ( M = 1, = 0),
dashed line for an universe
dominated by the vacuum ( M =
0, = 1), and the dotted line for a
flat universe ( M = 0.3, = 0.7).
The lower panels are normalized
to an empty universe and show
the data of the High-z SN Search
Team (filled squares; Riess et al.
1998) and the Supernova
Cosmology Project (open
squares; Perlmutter et al. 1999a).
Using distances to fit for , M
1. Observed flux, assumed luminosity
of Type Ia SN are used to determine
the luminosity distance:
2. From GR model, luminosity
distance can be calculated as
a function of , M
Where:
k  1   M   
sinh, k  0
sin n  
 sin, k  0
Resulting , M plot
Adding CBR
results
Potential Problems with interpreting Type Ia SN
Observations
•
Evolutionary effects in SN and/or host galaxy
– Could SN Ia at z=0.5 (5 Gyr look-back time) be intrinsically fainter than at
present?
– Effect would have to change luminosity-light curve half-width relationship
– Models at solar metallicity and 1/3 solar metallicity show only small (1%)
differences in the relevant passbands
Variation in luminosity of nearby Type
Ia SN over a wide range of Hubble
type (and metallicity ) show little
spread after MLCS correction)
Host environment, evolutionary effects continued
Also, little variation in SN
luminosity vs. host galaxy
color:
Or distance from host center
Are there spectral
differences in low,
high-z Type Ia SN?
None yet, but spectra of
high-z SN are difficult to
measure with high SNR
Problems, continued
• Extinction by dust
– Ordinary dust: Reddening,
measured by E(B-V), is
consistent for low-z high-z SN.
This implies IG dust must be
grey.
– Gray dust: Intergalactic
graphite whiskers (1µ), with
density  ~10-5 could cause
0.25 mag extinction at 5 Gpc,
with only small amt of extinction
(but one SN at z~1.7 has
insufficient reddening, 2.5)
Problems, continued
•
Gravitational Lensing
–
Lenses most often dim
distant objects (although
integral effect is 1.0x).
–
For z=0.5 and M~0.5,
effect is 2%, much too low
to explain observed 25%
dimming
–
This will be more important
for higher-z SN surveys
Dark Matter
• Rotation curve
obs => CDM ~ 102
M/pc3
CDM densities
• Central density
appears to be
independent of scale,
from dwarf galaxies
to clusters
• ctr ~ 10-2 Msun pc-3
• Model Radial profile
CDM  r-1 (cusp at
center)
• CDM ~ 0.2
Dark Matter: Very cool (old) White Dwarfs?
• Indirect evidence for the `dark matter'
being comprised of cool white dwarfs
first came from the MACHO (MAssive
Compact Halo Object) gravitational
microlensing experiment.
• The MACHO project monitored ten
million stars in the Magellanic Clouds in
the hope of detecting the occasional
brightening caused by a dark Halo object
moving across our line of sight to one of
the stars.
• Microlensing experiments suggested the
existence of a large number of dark
objects in the Halo of our Galaxy with
masses about half that of our Sun. The
likely candidates for these invisible
objects are distant, faint, cool white
dwarfs.
• However as such objects had never
actually been seen before there was some
doubt as to their nature. The MACHO
results suggest that these stars are very
numerous, and could contribute
approximately 50% of the total mass of
the Galaxy.
WD0346+246 (V=19.1, µ = 1.3''/yr, T=3,500K)
Survey of nearby cool WD’s (Ibata et al 2001)
• Survey covered of 28° x 28°
• Looked for cool, faint stars with
high proper motion
• Complete to R=19, d = 33pc
• Two detections
 survey  790 2  0.24 sr
3
4 3  survey  survey d
Volsurvey 
d 

3
4
3
NWD  M WD
3
WD 

M WD
3
Volsurvey
 survey d
WD  7  104 M pc 3  0.1CDM
 MACHO 
WD  1.3  105 M pc 3 ( IMF estimates )
Several new cool WD discoveries
(Scholz et al. 2002)
WMAP map of CBR
Canonical cosmological parameters
(from WMAP Feb 2003)
tot  1.02  0.02
   0.73  0.04
CDM  0.27  0.04
baryon  0.0224  0.0009
TCBR  2.724  0.002
tUniverse  13.7  0.2 Gyr
tdecoupling  379  8 kyr
treionization  180( 180, 80) Myr
zdecoupling  1089  1
zreionization  20 ( 10, 9)
Using WMAP angular spectrum to determine
curvature: Result is =1.02 ± 0.02 (flat)
Angular scale for acoustic peaks are determined by density, ratio of
electrons/photons, and distribution of dark matter at 300,000 yr. For flat
Universe, first peak is expected at  ~ 1 deg
If the curvature is k<0 (open), ‘frozen out’ sonic density waves if a given
physical wavelength appear at smaller angular scales w.r.t flat model
Using CBR spectrum to measure Electron/photon ratio at Decoupling
Increasing the ratio of electrons to photons also has the
effect of decreasing the sound speed of the fluid. Since the
fluid moves more slowly, the secondary oscillations occur
at larger angular scales. This effect shifts the location of
the latter peaks in the fluctuation spectrum.
WMAP: Baryon density n = (2.5 ± 0.1)·10-7 cm-3
• The cosmic background radiation propagating
from the surface of last scatter to is also
affected by gravitational fluctuations along its
path. Photons falling into the gravitational
potential wells of clusters gain energy. They
lose this energy when they climb out of the
potential wells. If the universe is flat and
composed entirely of matter, then these two
effects cancel exactly and the matter along the
photon path has no net gravitational effect.
• If there is a cosmological constant, then the
depth of gravitational potential wells decay with
time. Thus, a photon which falls into a deep
potential well gets to climb out of a slightly
shallower well. The net effect leads to a slight
increase in photon energy along this path.
Another photon which travels through a low
density region (which produces a potential
"hill") will lose energy as it gets to descend
down a shallower hill than it climbed up.
• Because of this effect, a model with a
cosmological constant will have additional
fluctuations on large angular scales. Large
angular scale measurements are most sensitive
to variations in the gravitational potential at low
red shift.
Using CBR spectrum to
Measure Cosmological
Constant 