ASTRONOMY
The Universe
Key Terms
• Astronomy: The scientific study of the universe; it includes the observation and interpretation of celestial
bodies and phenomenon
• Big Bang: The theory that proposes that the universe originated as a infinitely small mass which
For unknown reasons began to expand
• Cosmology: The study of the origins of the universe
• Doppler Effect / Red Shift: the wavelength of an object lengthens and shifts towards the red end of the
spectrum as it recedes from our position.
• Galaxy: A collection of stars, gas and dust held together by gravity
• Hubble’s Law: galaxies are receding from our galaxy at a speed that is proportional to their distance
• Universe: All matter and energy, including the earth, the galaxies, and the contents of intergalactic space,
regarded as a whole
Origins
The Big Bang
• The universe formed approximately 12 – 14 billion years ago
• Universe was condensed into an infinitesimally small point  SINGULARITY
• For reasons still unknown, expansion of the singularity began
• All matter and space were created at this instant.
• In the first microseconds of expansion (Planck Era), the universe increased in sized at a faster than light rate 
Inflation theory: increased expansion rate due to an unknown and unstable form of energy
– The universe went from the size of an atom to the size of a grapefruit in 10 -43 sec.
• At 10 -32 sec. electrons, quarks and other subatomic particles formed
• 10 -6 sec. protons and neutrons form
• 300,000 years after the big bang the first atoms of H and He form  Light
• 1 billion years after the big bang  First Stars Form
Evidence for the Big Bang Theory
The Big Bang Model is supported by a number of important observations.
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The expansion of the universe
Edwin Hubble's 1929 observation that galaxies were generally receding from us provided the first clue that the
Big Bang theory might be right.
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The abundance of the light elements H, He, Li
The Big Bang theory predicts that these light elements should have been fused from protons and neutrons in
the first few minutes after the Big Bang.
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The cosmic microwave background (CMB) radiation
The early universe should have been very hot. The cosmic microwave background radiation is the remnant
heat leftover from the Big Bang. (COBE and WMAP)
The COBE satellite was developed by NASA's Goddard Space Flight Center to measure the diffuse infrared and
microwave radiation from the early universe to the limits set by our astrophysical environment.
Cosmic Background Explorer
The COBE satellite carried three instruments
1. Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background radiation (CIB).
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The CIB represents a "core sample" of the Universe; it contains the cumulative emissions of stars and
galaxies dating back to the epoch when these objects first began to form.
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The COBE CIB measurements constrain models of the cosmological history of star formation and the
buildup over time of dust and elements heavier than hydrogen, including those of which living
organisms are composed. Dust has played an important role in star formation throughout much of
cosmic history.
2. Differential Microwave Radiometer (DMR) to map the cosmic radiation sensitively
– The CMB was found to have intrinsic "anisotropy“ or fluctuations for the first time, at a level of a part
in 100,000.
– These tiny variations in the intensity of the CMB over the sky show how matter and energy was
distributed when the Universe was still very young.
– Later, through a process still poorly understood, the early structures seen by DMR developed into
galaxies, galaxy clusters, and the large scale structure that we see in the Universe today.
3. Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave
background radiation with a precise blackbody.
– The cosmic microwave background (CMB) spectrum is that of a nearly perfect blackbody with a
temperature of 2.725 +/- 0.002 K.
– This observation matches the predictions of the hot Big Bang theory extraordinarily well, and indicates
that nearly all of the radiant energy of the Universe was released within the first year after the Big
Bang.
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WMAP has mapped the Cosmic Microwave Background (CMB) radiation (the oldest light in the universe) and
produced the first fine-resolution (0.2 degree) full-sky map of the microwave sky
WMAP determined the age of the universe to be 13.73 billion years old to within 1% (0.12 billion years)
WMAP nailed down the curvature of space to within 1% of "flat" Euclidean, improving on the precision of
previous award-winning measurements by over an order of magnitude
WMAP's precision determination that ordinary atoms (also called baryons) make up only 4.6% of the universe
(to within 0.1%)
WMAP's complete census of the universe finds that dark matter (not made up of atoms) make up 23.3% (to
within 1.3%)
WMAP's accuracy and precision determined that dark energy makes up 72.1% of the universe (to within
1.5%), causing the expansion rate of the universe to speed up.
WMAP discovered that the universe was re-ionized earlier than previously believed. By measuring the
polarization in the CMB it is possible to look at the amplitude of the fluctuations of density in the universe that
produced the first galaxies.
WMAP has started to sort through the possibilities of what transpired in the first trillionth of a trillionth of a
second
Large Scale Structures
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Galactic Filaments
– Also known as Great walls , Supercluster complexes or Sheets
– Not gravitationally bound and therefore take part in the Hubble expansion
– Largest known cosmic structures in the universe
– massive, thread-like structures with a typical length of 50 to 80 megaparsecs that form the
boundaries between large voids in the universe.
– consist of gravitationally-bound galaxies; parts where a large number of galaxies are very close to
each other are called superclusters.
Voids
– the empty spaces between filaments
– contain very few, or no, galaxies
– have a diameter of 11 to 150 Megaparsecs
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Great Attractor
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a gravity anomaly in intergalactic space within the range of the Centauries Supercluster that reveals
the existence of a localized concentration of mass equivalent to tens of thousands of Milky Ways
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observable by its effect on the motion of galaxies and their associated clusters over a region
hundreds of millions of light years across.
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Galactic cluster
– A system of galaxies containing from several to thousands of member galaxies
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Galaxy
– A collection of stars, gas and dust held together by gravity
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Galactic Filaments of the Local Universe
A Close up View of Home
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It doesn’t add up.
– Data collected from COBE and WMAP indicates that the universe is too large and “lumpy” in structure
to have formed in 13.7 billion years.
– More time was needed
– Inflation theory: microseconds after expansion of the universe began there was an inflated increase
in the expansion rate of the early universe.
– This was due to the interaction of a mysterious form a matter and energy which accounts for the
majority of matter and energy in the universe today
• Dark Matter
• Dark Energy
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Dark Matter
– matter that is inferred to exist from gravitational effects on visible matter and gravitational lensing of
background radiation
– neither emits nor scatters light or other electromagnetic radiation (and so cannot be directly
detected via optical or radio astronomy).
– Its existence was hypothesized to account for discrepancies between calculations of the mass of
galaxies, clusters of galaxies and the entire universe and calculations based on the mass of the visible
"luminous" matter these objects contain
• stars and the gas and dust of the interstellar and intergalactic medium.
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Evidence
– Galactic rotation curves: stars in galaxies move at different rates
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Velocity dispersions of galaxies: galaxies are being dispersed at different velocities from one another
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Galaxy clusters and gravitational lensing: the structure of clusters and the lensing effect of galaxies
are caused by the gravitational effect of dark matter that surrounds galaxies in halo type structures
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Cosmic microwave background: CMB not uniform on a large scale (lumpy); fluctuations in the
radiation caused by gravitational effect of dark matter
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Dark Energy
– a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion
of the universe.
– the most accepted theory to explain recent observations that the universe appears to be expanding at
an accelerating rate.
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Evidence
– Supernovae: rate at which light reaches us is increased
– Cosmic Microwave Background: universe in nearly flat therefore mass/energy = critical density to
have a flat universe; visible energy not enough
– Large Scale Structure: only accounts for 30% of total matter in universe
– Late-time Integrated Sachs-Wolfe Effect: Accelerated cosmic expansion causes gravitational potential
wells and hills to flatten as photons pass through them, producing cold spots and hot spots on the
CMB aligned with vast supervoids and superclusters. This so-called late-time Integrated Sachs-Wolfe
effect (ISW) is a direct signal of dark energy in a flat universe
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Shape of the Universe
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Critical density ~ 6 H atoms/m3.
Density of the Universe is close to this amount, therefore the Universe is infinite.
ACTIVE GALACTIC NUCLEI
Seyfret galaxies, Quasars and Blazars
1. All three are associated with super massive black holes.
2. Many astronomers think they are the same object viewed from different angles.
3. They shine brighter than any other objects in the universe
4. They all emit great amounts of energy in the form of …
 Infrared radiation
 Radio waves
 UV radiation
 X-ray radiation
 Gamma radiation
Seyfert Galaxies
1. Spiral shaped galaxies
2. low-energy gamma-ray sources
3. Rotating super massive black hole releases gamma ray jets from either pole as matter is destroy
BLAZARS
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QUASARS
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A class of active galaxy characterized by strong, compact, flat-spectrum radio emission
Extremely bright
Strong gamma ray emissions
Believed to be active galactic nuclei whose jets are aligned within 10° of our line of sight
Quasi-stellar objects of small angular size and immense power output.
Strong radio sources
Because they are so bright, quasars are some of the most distant objects we can see in the universe.
Huge power output is believed to be fueled by interactions between the central black hole and a
surrounding "accretion disk":
 a disk of matter that gathers around the black hole in the galactic nucleus.
GALAXIES
A galaxy is collection of stars, gas and dust held together by gravity.
GALAXY FORMATION
STEP 1
• About 1 billion years after the “big bang” dark matter and cooling gas condense and collapses under its own
gravity to form a PROTOGALAXY
STEP 2
• Gravity separates out the protogalaxy into a core and a halo.
STEP 3
• The subatomic particles that make up the gas interact to lose energy and fall to the core of the protogalaxy.
STEP 4
• The dark matter which only weakly interacts, remains in the halo.
Galaxy Evolution
Galaxy evolution is process of gravitational interaction between dark and visible matter and collisions between large
galaxies.
1. Irregular galaxies condense to form globular clusters.
2. Globular cluster collide to form spirals which collide to form ellipticals.
Types of Galaxies