Galaxies, Dark Matter and Cosmology
Review for Astronomy 123 Midterm Exam 2

posted May 10, 2017 jeb

Outline

Galaxies and Dark Matter
Cosmology

Galaxies and Dark Matter

Dark Matter in the Universe
More than 80% of the matter in the universe is dark
Dark matter is detected in many ways
Orbital velocity of stars and gas in the Milky Way
Rotation curves of galaxies
Orbital motion of binary galaxy systems
Motions of galaxies within clusters of galaxies

Dark matter is probably mostly composed of undiscovered, exotic subatomic particles
Dark galaxy
Possible galaxy composed almost entirely of dark matter, emitting virtually no visible light
Existence not yet established

Galaxy Collisions
Cartwheel galaxy
Antennae
Collisions have substantial effects on galaxies
Starburst galaxies
Star ejection
Shape distortion
Eventually, galactic cannibalism

Andromeda is approaching the Milky Way, and will collide in a few billion years

Galaxy Formation and Evolution
We know of no simple evolutionary path
Hierarchical merging
Galaxies grew through repeated merging of smaller objects

Galaxies often experience close encounters or collisions
A galaxy left alone will evolve slowly, in a predictable way
Elliptical, lacking interstellar gas, will become fainter and redder as stars burn out
Spiral or Irregular, with much gas, will remain bluish as long as new star formation continues

Galaxy mergers
Major mergers turn spirals into ellipticals
collisions between galaxies of comparable size destroy galactic disks

Minor mergers leave larger galaxy intact
collisions of small galaxies with large ones - smaller galaxy is absorbed

Black Holes in Galaxies
Quasars and active galaxies are powered by supermassive black holes
Central engine of million/billion solar mass black hole
Black hole is a region of space where the pull of gravity is so great nothing can escape

Accretion disk
Jets of high-speed particles
synchrotron radiation

Supermassive black holes are common
Every bright galaxy, whether active or not, may contain a central black hole

Quasar epoch
early universe - quasars appear, and fade away

Evolutionary sequence for galaxies
Highly luminous quasars
Radio galaxies
Seyfert galaxies
Normal spiral and elliptical galaxies

Universe on Large Scales
Superclusters
Local supercluster is Virgo Supercluster
tens of thousands of galaxies

Galaxies distributed on "bubbles" surrounding unpopulated "voids"
Largest structure: 200-300 Mpc in size
Great Wall is one of largest known structures
about 100 Mpc away
about 200 Mpc across

Gravitational lenses
predicted by Einstein's general theory of relativity
Quasar mirages
Binary (or double) quasar
Einstein cross

Blue arcs
Images of young, background galaxies, warped by the gravitational field of a foreground galaxy cluster

Bullet cluster
Collision of two galaxy clusters
Cluster about one billion pc distant
Dark matter passed through collision
located by lensing studies of background galaxies

Intergalactic gas collided, producing separated X-ray emitting region

Cosmology

Cosmology
The study of the structure and evolution of the entire universe
Largest structures of universe
200 - 300 Mpc

Cosmological Principle
On a large scale, universe is homogeneous and isotropic
Universe has no edge
Universe has no "center"

Universality of physical laws
The same physical laws apply everywhere in the universe

The Expanding Universe and the Big Bang
Olbers's Paradox - why is the sky dark at night?
red-shift of starlight
finite age of universe

Big-Bang established by observation
Hubble's Law (1st evidence)
v = H × d
H ≈ 20 km/s/Mly
for d = 100 Mly, v = 2,000 km/s
for d = 15,000 Mly, v = 300,000 km/s
(Note - in these examples, H is set to 20 km/s/Mly for simplicity. It is actually slightly larger, so d never exceeds 13,800 Mly)

existence of quasars (early universe was different)
cosmic microwave background (CMB)
abundances
Helium, deuterium, lithium, beryllium
Helium is about 25% of the mass of normal matter in the universe
this is about 10% of the number density; hydrogen is about 90% of the number density

Early on the whole universe was compressed to very high density
Since then the entire universe has been expanding
Since the universe is expanding everywhere, distant galaxies appear to be moving away from us in every direction

Discovery of Cosmic Microwave Background
Predicted by Big Bang
Universe's distant past was very dense and hot

First detected by Penzias and Wilson with radio telescope
Very sensitive measurement achieved after eliminating sources of extraneous noise
Fixed loose joints
Repaired faulty connections
Removed pigeon droppings that created noise in detector

Found 2.7 K background
Uniformly coming from all directions
Uniform throughout the day
Uniform through the seasons

Peak wavelength about 1 mm
Matches blackbody spectrum
Emerged from big bang fireball when the universe was 400,000 years old and had a temperature of 3000 K
Following big bang, as universe expands, background radiation gets cooler
from background radiation we can determine motion of earth through universe

COBE
small asymmetry due to the Earth's motion through space

Steady State Theory
Alternative to Big Bang
Consistent with Hubble expansion
Could not explain CMB - therefore rejected

Prediction of Big Bang
Expansion of the universe shifted the initially high energy radiation in the universe to longer and longer wavelengths
Cosmological redshift

The Big Bang predicted that the universe should be "hissing" (static)
Coming from all directions
Follows "black body" curve
Temperature about 3 K (we now know it is 2.7 K)

Age since Big Bang
13.8 billion years measured from CMB fluctuations
Time of expansion depends on the density of matter
and the cosmological constant (or "dark energy")

Space-Time Geometry
3 possibilities in Einstein's General Theory of Relativity
open
flat
closed

Open, flat, or closed depends on density of matter (and "dark energy")
Astronomers define a ratio of densities, Ω₀("Omega nought") = actual density / critical density
the amount of mass needed to close the universe is call the critical density
critical density = 9 × 10 ^-27 kg/m ³
Earth's atmosphere contains about 1.2 kg/m ³

We know Ω₀ is close to 1
Luminous matter explains Ω₀ = 0.02-0.05 ("a few percent" of critical density)
Dark matter is much more, bringing Ω₀ to about 0.3
Dark energy completes closure of the universe, bringing Ω₀ close to 1.00

Einstein found the General Theory of Relativity required the universe to be expanding of contracting, not static
Einstein invented "cosmological constant", a kind of "fudge factor" to add to his theory
Allowed the universe to be static, neither expanding nor contracting
Or it could still be expanding or contracting, depending on the densities of matter, and the cosmological constant
When Hubble discovered the universe was expanding, and it didn't need to be static, Einstein considered the "cosmological constant" his "greatest blunder"
Cosmological constant is similar to "dark energy" or the energy of the vacuum

density of matter nature of universe geometry of universe future

less matter than critical OPEN infinite and unbounded expand forever

critical density FLAT infinite and unbounded expand forever

more matter than critical CLOSED finite and unbounded collapses in BIG CRUNCH

Future of the universe
Future of expansion also depends on the matter density and the dark energy density ("the cosmological constant")
The expansion of the universe is accelerating
Supernovae at large redshift are fainter than they would be if the universe were expanding at a constant speed

This acceleration must be caused by an unknown cosmic force field
Called "dark energy"
Appears like "cosmological constant"
Perhaps Einstein did not "blunder"

Could come from "vacuum energy"

Planck satellite
A European Space Agency Explorer mission that launched May 2009
satellite which measured the tiny fluctuations in the CMB in different directions
CMB fluctuations tell us many things
Age of the universe
Total energy-mass density (Ω₀)
Fractions of ordinary matter, dark matter, and dark energy

Planck results as of 2015
Age of the universe = 13.8 billion years (with less than 0.1% uncertainty)
Ω₀ = 1.00 (less than 1% uncertainty)
in other words, the universe is flat

Ordinary matter makes up about 4.9% of the universe
Dark matter makes up about 27%
Dark energy makes up about 68%

Note- dark matter represents more than 80% of the matter
dark matter / all matter = 27%/(27%+4.9%) = 85%