Download Globular Cluster Formation in CDM Cosmologies

Chapter 5: Cosmic foundations
for origins of life
Is life a natural consequence of cosmology?
- Evidence for origin of evolution by Big Bang becoming very
secure…
- expansion of the universe,
- cosmic microwave background radiation,
- synthesis of simple elements - hydrogen, helium, lithium.
- Galaxies are a consequence of small fluctuations produced
during the Big Bang…
- Stars form in galaxies … first stars created first carbon in
universe; life not possible before this?
- Stars produce all the heavier elements (dubbed “metals”)
produced by fusion in stellar interiors (eg. O, C, N, ….)
- Synthesis of carbon “finely tuned”… a slight change in
strength of forces could result in universe with *no*
heavy elements, so no life.
Our Milky Way Galaxy is a spiral galaxy much like Andromeda –
our most massive partner in the Local Group of Galaxies.
Note this
spiral galaxy
has a
galactic
bulge, halo,
and disk.
There are
also 2 dwarf
galaxies
visible that
orbit
Andromeda.
THE EXPANDING UNIVESE
Edwin Hubble:
1889 - 1953
At the 48 inch on Mt. Palomar
Cepheid variable stars in M100 –
galaxy in the Virgo cluster: used
by Hubble to measure the
distance to galaxies. P vs. L
relation:
• Hubble Diagram (1929): all galaxies receed from us.
Speed of separation (v) is measured from the red shift of
some typical well-observed line.
- Hubble founds that v is proportional to the distance to
the galaxy – HUBBLE’S LAW: v  H d
o
Hubble’s
original data
and graph.
Red-shift distance
relation for
galaxies
enormously more
distant than in
Hubble’s original
study. Galaxies
studied here are
members of
galactic clusters
(eg. Virgo, Hydra,
etc)
Galaxies move away from us - the same for
observers in any other galaxy: at some past time,
all the matter must have been at one point – a “Big
Bang must have occurred!
• Best measured value for Hubble’s
constant:
H o  65  70 km / s / Mpc
We will use 65
- Good distance measurements
from 1. HST observations of
Cepheids in other galaxies and
2. “Tully-Fischer” relations
• Hubble constant has units of
1/time – so a good estimate of age
of the universe is:
1/Ho = 15 billion years!
CFA survey of galaxies – out to 200 MPC covering 6
deg. thick slice of sky. 1057 galaxies shown.
Cosmological Model – a homogenous and
isotropic universe
Probe structure in the
universe on ever larger
scales – there is a largest
scale seen: Great Wall of
about 200 Mpc in scale.
• On scales > 300 Mpc –
universe appears to be
homogenous (same
everywhere).
• Universe is same in every
direction on these largest
scales – isotropic.
23,697 galaxies within 1000 Mpc (Las Campanas
survey). Voids and walls no larger than 100 – 200
Mpc.
Picturing the Expansion of the Universe:
• It is spacetime that expands – the distance between galaxies
grows!
• The Big-Bang is not like a bomb going off inside some space –
it is spacetime itself that “explodes” – about 15 billion yrs ago.
• Picture dots on a balloon (galaxies). As balloon expands (ie
spacetime), galaxies carried away from one another. Every
observer on every dot sees all the other dots recede – ie,
Hubble’s law is measured by all observers in the universe
General relativity and cosmology:




Cosmological model: assume homogenous and isotropic
distribution of matter on largest scales
General relativity shows space-time is dynamic! Evolution
of universe depends on its density.
Critical density for universe: at greater than the critical
density, the expansion of the universe will ultimately cease
and it will re-collapse. At densities lower that critical value,
the universe will continue to expand forever.
Critical density in general relativity can be accurately
calculated using Hubble’s law, and Newtonian gravity to
find:
 c ,o
2
3H o
 27
3
3

 8 10 kg / m  5 H atoms / m
8G
General relativity: The fate of the universe
depends on how dense it is.
A low density universe
(open) will continue to
expand forever.
A critical density
universe will coast to a
stop after an infinite
time.
A high density universe
(closed) will expand to
a maximum size in a
finite time – and
collapse into a
singularity again: the
“Big Crunch”
Distance between galaxies as a
function of time: model with critical
density labelled “marginally bound”
Geometry of the universe in
four dimensions:
Closed Universe
(sphere)
Critical
(flat)
Open Universe
(saddle)
• Predicted in 1948
(Alpher, Bethe, and
Gamow)
• Discovered in 1964
by Penzias and
Wilson – Nobel prize
Cosmic Microwave Background
Radiation (CMBR)



Major prediction of the Big Bang is that the
universe started very dense and hot, and then
cooled with time.
CMBR pervades all space, and should be blackbody.
Explanation: Wien’s law for black body radiation:
increasing wavelength implies that temperature
decreases:
T (t )  1 / max (t )
Expansion of universe stretches out the wavelength of a photon –
like wave drawn on a balloon that is stretched with time:
 (t )  R(t )
•This is why galaxies appear to be redshifted! NOT that galaxies
move relative to one another – but that universe expands during
the time it takes light to travel from another galaxy to us.
• Temperature varies inversely as scale factor of universe:
T (t )  1 / R(t )
COBE measurement of CMBR
spectrum: T=2.735 K

Experimental
errors are
smaller than
size of dots in
figure. Blue
line is best fit
black-body
spectrum
COBE map (1989): – fluctuations in
temperature; one part in a hundred thousand
WMAP (2003): 45 times more sensitive, and 33
times the spatial resolution of COBE
Data from WMAP gives age of universe of 13.7 billion yrs; to
accuracy of 1%! *Fluctuations are the seeds of galaxy formation*
At a time when the age of the
universe was;
1035  1033 sec
-Universe underwent a brief(!)
epoch during which
exponentially fast expansion
occurred - increasing its size
by 50 orders of magnitude!
(before and after this episode,
the size of the universe grows
as a power-law function of
time – much slower!).
- Universe grew out of a
single “fluctuation” of this new
phase – solving problems
Inflation and CMBR
(Alan Guth, 1981)
Big Bang Nucleosynthesis
• During first 3 minutes; in time
that universe cools from 10
billion to 1 billion deg. K;
nucleosynthesis is possible.
• Elements fabricated are
Helium-3, Helium-4,
Deuterium, and a trace of
Lithium.
• Deuterium and Helium
isotopes produced using
neutrons – unlike protonproton chain.
1. 1H  n 2H  energy
2. 2 H 1H 3He  energy
3
He  n 4He  energy
3. 2 H  2H  4He  energy
Producing simplest elements in the Big Bang:
Observations of Helium 3
& 4, + deuterium, ->
specific density and
temperature conditions
-> strongly constrain the
cosmological model –
fraction of matter in
baryons.
• Deuterium is not
produced significantly in
stars – so are seeing
primordial abundance.
• Best fit: baryon density
that is a few % of critical.
Supernovae Type Ia’s; mapping the cosmos to
furthest distances… discovery of dark energy
Composition of universe

Measure density in units of the critical density – define density
parameter:
o  o / c,o : ( 1 for critical )

Contribution of baryonic (ordinary) matter:

Contribution of all matter (baryonic + dark):
lum  0.04
 m  0.27

Contribution of “dark energy” (cosmological constant) from
CMBR and supernova measurements:
  0.73

Key result:
o   m     1
History of our universe
Text
Density fluctuations arise at
earliest moments when
universe is still a quantummechanical object. This is the
“Planck scale”:
Origin of
Galaxies
t Planck  1043 sec; lPlanck  1035 m
Quantum fluctuations in Planck
era are ultimate density
fluctuations that grow to be
galaxies!
This spectrum of fluctuations is
preserved in the CMBR
fluctuations - predicts many
aspects of galaxies such as
their size distribution, evolution Computer simulation – courtesy Hugh
with time (“hierarchical”), etc. Couchman (McMaster University)
Formation of the first star




Physics is much simpler – no magnetic
fields, no dust or complex molecules,
primordial gas contains hydrogen and
helium.
Start with a small, cold, dark matter halo –
about a million times mass of the Sun.
Gas within it cools down to 200 K,
(molecular hydrogen is the coolant)
A 100 solar mass core forms inside a
filamentary “molecular cloud”
The very first star… 100 times the mass of our Sun
- a single star forms
Abel et al
(2002),
Science
First stars
turned on
perhaps 200
million years
after the Big
Bang… they
started to
make the
elements out
of which
planets, and
living things,
are made.
Stellar evolution – forming the elements
for biolmolecules and planets….