1. Survey of the Universe Greg Snow U Nebraska Lincoln CROP

2. 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

3. 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 20 th century led to the conclusion that: The farther an object is from us, the faster it is moving away from us.

4. Survey of the Universe This observation is summarized by the “Hubble Law”. Velocity of recession, v = H  distance, where H is the Hubble constant. Distance in light years Velocity in kilometers/second The slope of this line is the Hubble constant. (See data in figure from text.)

5. 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”.

6. Survey of the Universe 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

7. Survey of the Universe 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: Unstable object with mass 2  10 20 eV Decays to two protons with energy 1  10 20 eV Decays to two protons with energy 1  10 20 eV

8. Survey of the Universe The “Top Down” scenario 2  10 20 eV is an enormous mass for a single particle. Unstable object with mass 2  10 20 eV 2  10 20 eV is equal to 2  10 11 GeV, which is the mass of 2  10 11 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

9. Survey of the Universe 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”.

10. Survey of the Universe 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 10 19 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 10 19 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.

11. 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. 14 Billion LY radius Universe Sphere of radius 150 Million LY, the region where our highest energy cosmic rays originate according to the GZK cutoff.

12. Survey of the Universe 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

13. 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)

14. Survey of the Universe Two possible sources of the highest energy cosmic rays Colliding galaxies 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 Supernovae are not violent enough – “Fermi shock acceleration” can explain cosmic rays up to energies of only about 10 15 eV

15. Survey of the Universe The Pierre Auger Observatory – the World’s Largest Array Southern Hemisphere: Malargüe Province of Mendoza Argentina (being constructed now) Northern Hemisphere: Millard County, USA (to be built) 1600 detectors, 3000 km 2 each site The Southern Hemisphere site is being built first since it has a better view of the center of the Milky Way galaxy where there might be a black hole emitting high energy cosmic rays.

16. 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)