Units to cover: 75, 78, 82, 83, 84

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Units to cover 75, 78, 82, 83, 84
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Quasars

• Quasars are small, extremely luminous, extremely
  distant galactic nuclei
• Bright radio sources
• Name comes from Quasi-Stellar Radio Source, as
  they appeared to be stars!
• Can have clouds of gas near them, or jets racing
  from their cores
• Spectra are heavily redshifted, meaning they are
  very far away
• Energy output is equivalent to one supernova
  going off every hour!
• The HST was able to image a quasar, showing it to
  be the active core of a distant galaxy

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Energy Source for Active Galactic Nuclei

• Active galactic nuclei emit a tremendous amount
  of radiation over a broad range of wavelengths
• A black hole can be both very small, and have an
  accretion disk that can emit enough radiation
• Likely that at the centers of these galactic
  nuclei, there are supermassive black holes
• Intense magnetic fields in the accretion disk
  pump superheated gas out into jets that leave the
  nucleus
• There are still many questions to be answered

4
Seyfert Galaxies

• Seyfert galaxies are spiral galaxies with
  extremely luminous central bulges
• Light output of the bulge is equal to the light
  output of the whole Milky Way!
• Radiation from Seyfert galaxies fluctuates
  rapidly in intensity

5
Radio Galaxies

• Radio Galaxies emit large amounts of energy in
  the radio part of the spectrum
• Energy is generated in two regions
• Galactic nucleus
• Radio lobes on either side of the galaxy
• Energy generated by energetic electrons
• Synchrotron radiation
• Electrons are part of the gas shooting out of the
  core in narrow jets

6
The Redshift and Expansion of the Universe

• Early 20th century astronomers noted that the
  spectra from most galaxies was shifted towards
  red wavelengths
• Edwin Hubble (and others) discovered that
  galaxies that were farther away (dimmer) had even
  more pronounced redshifts!
• This redshift was interpreted as a measure of
  radial velocity, and it became clear that the
  more distant a galaxy is, the faster it is
  receding!

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The Hubble Law

• In 1920, Edwin Hubble developed a simple
  expression relating the distance of a galaxy to
  its recessional speed.
• V H ? d
• V is the recessional velocity
• D is the distance to the galaxy
• H is the Hubble Constant (70 km/sec per Mpc)
• This was our first clue that the universe is
  expanding!

8
Which two quantities are shown to be related to
one another in Hubble Law?

• A. distance and brightness
• B. distance and recession velocity
• C. brightness and recession velocity
• D. brightness and dust content

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Large Scale Structure in the Universe

• Using modern technology, astronomers have mapped
  the location of galaxies and clusters of galaxies
  in three dimensions
• Redshift is used to determine distance to these
  galaxies
• Galaxies tend to form long chains or shells in
  space, surrounded by voids containing small or
  dim galaxies
• This is as far as we can see!

10
An Expanding Universe

• The expansion of the Universe is not like the
  explosion of a bomb sending fragments in all
  directions
• Space itself is expanding!
• We can detect photons that appear to have moved
  at different speeds through space

• Rather, the speed of light is constant, and it is
  space that was moving relative to the photon
• If each galaxy is like a button attached to a
  rubber band, an ant walking along the band as it
  is stretched will appear to have a velocity
  slower than it really does. The buttons
  (galaxies) are fixed relative to space, but space
  itself is moving.

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One More Analogy

• The expansion of the universe and the increasing
  distance between galaxies is similar to the
  increase in distance between raisins in a rising
  loaf of raisin bread.
• The raisins are fixed relative to the dough, but
  the dough expands, increasing the space between
  them.
• Problem with these analogies loaves and rubber
  bands have edges!
• We have seen no edge to the Universe there are
  an equal number of galaxies in every direction!
• Also, galaxies can move relative to space, as
  sometimes gravity can accelerate one galaxy
  toward another faster than space expands!

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The Meaning of Redshift

• As light waves travel through space, they are
  stretched by expansion
• This increases the waves wavelength, making it
  appear more red!
• An objects redshift, z, is
• Here, ?? is the change in wavelength, and ? is
  the original wavelength of the photon

• This is equivalent to

13
The Age of the Universe

• Thanks to the Hubble Law, we can estimate the age
  of the universe
• At some point in the distant past, matter in the
  universe must have been densely packed.
• From this point, the universe would have expanded
  at some high speed to become todays universe
• Assuming a constant expansion over time, we find
  that the age of the universe is around 14 billion
  years.

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Static Universe and Big Bang
Alexander Friedmann
Fred Hoyle
Died at the age 27
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Light from the Big Bang

• Every time we look at the night sky, we are
  looking back in time
• Can we see light from the Big Bang?
• Almost!

G. Gamow
Alpher, R. A., H. Bethe and G. Gamow. The Origin
of Chemical Elements, Physical Review, 73
(1948), 803
A. Penzias and R. Wilson
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The Last Scattering Epoch

• Minutes after the Big Bang, the Universe was
  opaque
• High temperatures kept all matter ionized
• Photons could only travel a short distance before
  being absorbed

• After 400,000 years, the Universe cooled enough
  for electrons and ions to recombine, allowing
  light to pass
• Now the Universe was transparent!

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Light from the Early Universe

• So what should light from 400,000 years after the
  Big Bang look like?
• It should have a spectrum that corresponds to the
  temperature of the Universe at that time, 3000 K.
• Expansion of space will stretch this light,
  however
• The Universe has expanded by a factor of 1000
  since this time, so the wavelength will have
  stretched by the same amount
• Spectrum will correspond to a temperature of 3K.
• This light from the early Universe has been
  found, and is called the Cosmic Microwave
  Background

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Clumpiness in the CMB
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A Timeline of the Universe
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The Origin of Helium

• Immediately after the Big Bang, only protons and
  electrons existed
• Shortly after the BB, temperature and density was
  high enough for deuterium to form by fusion
• After 100 seconds or so, temperature cooled
  enough so that deuterium could fuse into helium
  nuclei
• The temperature continued to cool, and fusion
  stopped after a few minutes.
• Big Bang theory predicts that around 24 of the
  matter in the early universe was helium, which
  matches what we see.

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The Epoch of Inflation

• Modern technology allows us to test theories back
  to a time 10-33 seconds after the Universe Birth
  (UB).
• Physics as we know it ceases to function at 10-43
  seconds after the UB, called the Plank Time
• Using particle colliders, scientists have
  uncovered a number of clues about what happened
  in the early universe, after the Plank time
• The early universe underwent a period of very
  rapid expansion

• By 10-33 seconds, the universe expanded from the
  size of a proton to the size of a basketball
• This expansion is called inflation

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Expansion Forever? Or Collapse?

• The fate of the universe is ultimately controlled
  by its total amount of energy
• Energy of expansion (positive)
• Gravitational energy that can slow the expansion
  (negative)
• Binding energy

• If the total energy is positive or zero, the
  expansion continues forever
• If the total energy is negative, the expansion
  will halt, and the universe will contract and
  eventually collapse.

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Dark Energy

• Dark energy may provide the solution to the
  mystery
• Dark energy remains constant everywhere,
  regardless of the universes expansion
• Provides an outward push to accelerate expansion

• Dark energy must make up around 70 of all of the
  energy in the universe
• Much work remains to be done on this frontier