Download Survey of the Universe

Survey of the Universe
Survey of the Universe
Greg Snow
U Nebraska Lincoln
CROP
Survey of the Universe
• Powers of Ten – a View of the Universe over distance scales
from the very large to the very small
• We live in an expanding Universe
• What’s within 50 Megaparsecs = 150 Million Light Years from us?
• Reminder why this distance is relevant to
Ultra High Energy Cosmic Rays
• Candidate sources of Ultra High Energy Cosmic Rays
• Ultimate fate of the Universe
Survey of the Universe
The Universe began about 14 billion years ago with the
BIG BANG
and has been expanding ever since.
How do we know the Universe is expanding?
Observations of near and far objects in the Universe by
Edwin Hubble in the first half of the 20th century
led to the conclusion that:
The farther an object is from us, the faster it is moving
away from us.
Survey of the Universe
This observation is summarized by the “Hubble Law”.
Velocity in
kilometers/second
The slope of this line
is the Hubble constant.
Distance in light years
Velocity of recession, v = H  distance, where H is the Hubble
constant.
(See data in figure from text.)
Survey of the Universe
Do these observations mean that we (i.e. the earth and our
galaxy) are at the center of the Universe?
• Balloon example
• 5 Volunteers example
The answer is NO – the Universe looks the same from any
vantage point – this is part of the “Cosmological Principle” .
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What’s within 50 Megaparsecs = 150 Million Light Years from us?
Why is this relevant to the study of Ultra High Energy Cosmic
Rays?
There are two current theories about the origin of the highest
energy cosmic rays
1. “Bottom Up” Scenario
Particles are accelerated by some extreme astrophysical
phenomenon – galaxies colliding, inferno in the center of an
active galaxy (an Active Galactic Nucleus, or “AGN”).
2. “Top Down” scenario
Particles are the decay products of heavy unstable objects
created at the time of the Big Bang
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The “Top Down” scenario
Highest energy cosmic ray particles are the decay
products of heavy unstable particles created at the
time of the Big Bang
Such particles would have to be extremely heavy
Simple-minded example:
Decays to two protons
Decays to two protons
Unstable object
with energy 1 1020 eV with mass 2 1020 eV with energy 1 1020 eV
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The “Top Down” scenario
2  1020 eV is an enormous mass for a single particle.
Unstable object
with mass 2 1020 eV
2  1020 eV is equal to 2 1011 GeV, which is the mass
of 2  1011 protons. (Proton mass is about 1 GeV in energy
units.) Reminder – the heaviest particle discovered to date
is the top quark with mass 170 GeV or 170 protons.
Candidate particles that theoretical astrophysicists
talk about:
• WIMPZILLA’s – WIMPs are “Weakly Interacting
Massive Particles”, a dark matter candidate particle
• “Topological defects” – space-time deformities
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The “Top Down” scenario
In the Top Down scenario, we would expect ultra high
energy cosmic rays to come uniformly from all directions
of the sky.
That is, there would be no particular “point sources” .
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The “Bottom Up” Scenario
Particles are accelerated by some extreme astrophysical
phenomenon – galaxies colliding, inferno in the center of an
active galaxy (an Active Galactic Nucleus, or “AGN”).
Reminder of the GZK cutoff:
• If particles accelerated to energies greater than
1019 eV originate at distances greater than about 150 Million
Light Years, they will lose energy by interacting with the
sea of low energy photons that fill the Universe
The Cosmic Microwave Background Radiation
• Hence they will reach the earth with a reduced energy
• So particles with energy greater than 1019 eV must originate
within 150 Million Light Years from earth
In the Bottom Up scenario, we expect primary cosmic ray
directions to point back to particular sources in the sky.
Survey of the Universe
How the Universe is Organized
• First, how big is the present Universe?
• Since the Big Bang happened 14 billion years ago, the largest
the Universe can be is a sphere of radius 14 billion Light Years,
the “light horizon”.
• The farthest galaxies observed from earth are 10-12 billion
Light Years away.
Universe
Sphere of radius
150 Million LY,
the region where
our highest energy
cosmic rays originate
according to the
GZK cutoff.
14 Billion LY radius
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How the Universe is Organized
• The Universe is clumpy – stars are clumped into Galaxies,
Galaxies into Clusters, Clusters into Superclusters, some
Superclusters into Walls.
• The clumpiness is due to the gravitational attraction among
objects small and large, superposed on the overall
expansion of the Universe.
• Otherwise, there are tremendous voids between these
groupings of matter
Survey of the Universe
Some distance scales
• Diameter of our solar system – 0.0012 LY
• Nearest star, Proxima Centauri – 4 LY
• Diameter of our galaxy – 100,000 LY
• Distance to nearest galaxy – the Sagittarius dwarf galaxy, which
is being “eaten” by the Milky Way – 80,000 LY
• Size of our “Local Group” – a collection of at least 30 galaxies,
including Andromeda – 3 Million LY
• Size of our “Local Supercluster” which contains our Local Group,
the Virgo Cluster, and others – 100 Million LY (see text figure)
(See web animation)
Two possible
of the highest
Surveysources
of the Universe
energy cosmic rays
Colliding galaxies
Supernovae are not violent enough –
“Fermi shock acceleration” can explain cosmic
rays up to energies of only about 1015 eV
Active galactic nucleus
But the true origin and
acceleration mechanism
for the highest energy
cosmic rays is UNKNOWN
-- that’s why we want to
study them
The Pierre
Auger
– the
Survey
of the Observatory
Universe
World’s Largest Array
S
#
The Southern Hemisphere
S
#
site is being built first since
it has a better view of the
center of the Milky
Southern Hemisphere:
Way galaxy where
there might be a
Malargüe
black hole emitting
Province of Mendoza
high energy
Argentina
cosmic rays.
(being constructed now)
1600 detectors, 3000 km2 each site
Northern Hemisphere:
Millard County, USA
(to be built)
Survey of the Universe
The ultimate fate of the Universe
Since the discovery of the Hubble Law, it has been a question
whether we live in:
A closed Universe – gravitational attraction will ultimately
slow the Universe’s expansion and lead to the eventual
coalescing of all matter – the “Big Crunch”
An open Universe – expansion will continue forever in all
directions
It has now been determined that the Universe rate of expansion
is in fact accelerating – recession speeds are increasing –
so we live in an open Universe.
The “engine” that powers the acceleration is unknown and is
referred to as the Dark Energy of the Universe.
(See web animation)