Download PowerPoint Presentation - The Origin of the Universe

Dark Energy & overview of Cosmological
parameters
Today:
Review last time
Revisit the accelerating universe
Look at values of cosmological parameters
from different methods
Discuss the final exam and review lecture
Last Time
• We heard about methods used to determine
the ultimate fate of the universe
–
–
–
–
average density of matter
expansion rate in the distant past
large-scale geometrical properties of universe
motions of galaxies/clusters relative to smooth
expansion
– abundances of light elements synthesized
shortly after the Big Bang
Last Time
• Most successful recent technique has been
determining expansion rate as a function of
time by measuring the apparent brightness
of distant supernovae
• We discussed this a bit back in lecture 8
Type 1A Supernovae
SNe 1a
• What produces a SNe1a?
– Start off with a binary star system
– One star comes to end of its life – forms a “white
dwarf” (made of helium, or carbon/oxygen)
– White Dwarf starts to pull matter off other star… this
adds to mass of white dwarf (accretion)
– White dwarfs have a maximum possible mass… the
Chandrasekar Mass (1.4 MSun)
– If accretion pushes White Dwarf over the Chandrasekar
Mass, it starts to collapse.
An accreting White Dwarf
SNe 1a
• White Dwarf starts to collapse…
– Rapidly compresses matter in white dwarf
– Initiated runaway thermonuclear reactions – star turns
to iron/nickel in few seconds
– Liberated energy blows star apart
– Resulting explosion briefly outshines rest of galaxy
containing it… these are the SNe1a events
• SNe1a
– No remnant (neutron star or black hole) left
– Since white dwarf always has same mass when it
exploded, these are “standard candles” (i.e. bombs with
a fixed yield)
Hubble’s constant with SNe1a’s
• The program:
– Search for SNe1a in distant
galaxies
– Compare expected power
with observed power to
determine distance
– Measure velocity using
redshift
• “Low redshift” galaxies
give measurement of H0
• “High redshift” galaxies
allows you to look for
deceleration of universe
The results…
• This program gives
Hubble’s constant
– H=65 km/s/Mpc
• and find acceleration,
not deceleration!
– Very subtle, but really is
there in the data!
– Profound result!
What are we trying to do ?
If expansion rate does not change
Further/
fainter
than
expected
Brighter
/closer
than
expected
Low-z SNe Ia
w/ time, expect all the data to lie
along here
If expansion rate was lower in the past
- distant objects should be closer
than the H=constant case
So data will be here
High-z SNe Ia ?
Filippenko & Riess (2000)
So, where’s the data?
Further/
fainter
than
expected
=1,L=0,
qo=0.5
We see Acceleration !
Filippenko & Riess (2000)
Sne Ia for Cosmology (z > 0.1)
In the mid/late 1990s,
as SNe 1a were detected out to greater distances (z > 0.1)
became clear that the observed fluxes of
SNe 1a at 0.3 < z < 0.8
were systematically lower than expected
eg. for a critical density (matter-dominated) universe,
& even for a constant-velocity (empty of matter) universe.
The SNe must have gotten farther than expected for some epoch of the
universe
The universe appears to have been accelerating
between the epoch equivalent to z ~ 0.5 and now !
i.e. expanding faster and faster !
The cosmic concordance
• What is our universe like?
– Geometry (flat, spherical, hyperbolic)?
– Matter content?
– Anything else strange?
• Remarkable agreement between very
different experimental techniques
• Cosmologists, it seems (amazingly!), really
do know what they’re talking about!
Measurements of the matter content of
the Universe (recap)
• Primordial nucleosynthesis
– Theory predicts elemental abundances as a function of
baryon density
– Observed abundances  B0.036
• Galaxy/galaxy-cluster dynamics
– Look at motions of stars in galaxies, or galaxies in
galaxy-clusters…
– Infer presence of large quantities of “dark” matter
which is gravitationally affected observed objects.
 from DM
• Analysis of galaxy
motions suggests a
dark matter density of
DM0.3
• Same conclusion from
gravitational lensing
by clusters
 estimates
• First stunning conclusion:
– Compare B0.036 and DM0.3
– Normal matter only accounts for about 1/8 of
the matter that’s out there!
– We’re made of the minority stuff!
 estimates
• Confirmed by taking
an inventory of a
cluster…
– Find that about 1/8 of a
cluster’s mass is in
baryons
– We believe that
clusters should be
representative samples
of the universe…
– Confirms DM0.3
Geometry
• Distribution of galaxies versus z shows…
– The universe is so close to flat we can’t see any
deviations
The accelerating Universe
• Back to those observations of Type-1a
Supernovae (SN1a)
– Very good “standard candles”
– Can use them to measure relative distances very
accurately
– they show the universe expansion is
accelerating
Dark Energy
• There is an “energy” in the Universe that is
making it accelerate
– Call this “Dark Energy”
– This makes up the rest of the gravitating energy in the
Universe, and causes it to be flat!
– Completely distinct from “Dark Matter”
• Remember Einstein’s cosmological constant…?
– Dark Energy has precisely the same effect as Einstein’s
cosmological constant
– So, he was probably right all along!
Inflationary Cosmology
• Dark Energy, inflationary cosmology,  universe all refer
to this accelerating expansion
• Recall the period of inflation in the early universe
– that had the universe undergoing a brief period of exponential
expansion-perhaps due to vast energy liberated by a symmetry
break
• Inflationary Cosmology has the same thing happening
…galaxy separations increasing with time (not so dramatic
as the initial period of inflation)
Inflation/Dark Energy
Solves 3 problems with the Big
Bang Model:
Horizon Problem
We worried about how long
things were causally-connected
and could equilibrate, need
equilibrium to explain
uniformity of CBR - include 
and things could equilibrate and
then expand suddenly…so OK
Dark Energy
Solves 3 problems with the Big Bang Model:
The Structure problem
Models match observed structure better than simple BB
models
The Relic problem
Theories had predicted production of particles and
other stuff which should be around today, but which are
not observed, e.g. the magnetic monopole. Predictions
from inflationary cosmology match observation better
The magnetic monopole density is v. small under inflation,
so lack of detection expected
Dark Energy
• Read Chapter 15 for some more details….
INVENTORY OF THE
UNIVERSE
• So… our best guess for the inventory of the
Universe is:
–
–
–
–
Small fraction is radiation we are familiar with.
3.6% is baryonic matter
About 30% is dark matter
Rest (about 70%) is dark energy!
• Second stunning fact… most of the Universe isn’t
even made of matter!
– BDML1
Energy/Matter Density
• Results… get good agreement between
theory and observed structure if:
– DM0.3 (mostly “cold” dark matter)
– L0.7
• Remarkable agreement with SNe1a results
(but a completely independent method!).
The Age of the Universe
• Using this cosmological model, we can figure out
the age of the Universe.
– Answer – 13 to 14 Gyr
• Prediction…
– There should be no object in the Universe that is older
than 13 or 14 Gyr.
– This agrees with what’s seen… there’s no object in the
Universe that appears to be older than this!
– This was a big problem with old cosmological models
that didn’t include dark energy.
Again, what is “dark energy”?
• An “energy” that is an inherent component
of space…
• Consider a region of vacuum
–
–
–
–
Take away all of the radiation
Take away all of the matter
What’s left? Dark energy!
But we have little idea what it is…
Final Exam
Thursday May 15 10:30am - 12:30pm
Room 107 Closed Book
Bring a calculator
Final Exam
Excluded topics:
Telescope session
Luminosity functions of planetary nebulae & globular clusters
Far distant fate of the universe (details out to 10100 years)
Included topics:
Everything else, note, ancient cosmology (which was excluded
from the mid-term) is in the final exam
You will have a section of straightforward questions to test
breadth of knowledge plus section of more involved questions
for depth of knowledge (some choice of questions)