We live in an expanding Universe What’s within 50 Megaparsecs = 150 Million Light Years from us?

Survey of the Universe

• We live in an expanding Universe

• What’s within 50 Megaparsecs = 150 Million Light Years from us?

• Why is this distance relevant to Ultra High Energy Cosmic Rays we detect on Earth?

• What is the ultimate fate of the Universe?

Learning Objectives


The Universe began about 14 billion years ago with the BIG BANGand 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.


This observation is summarized by the “Hubble Law”.

Velocity of recessionv = H distance

where H is called the Hubble constant

Distance in light years

Velocity in kilometers/second

The slope of this lineis the Hubble constant.


Do these observations mean that we (i.e. the earth and ourgalaxy) are at the center of the Universe?

• Balloon example

• 5 Volunteers example

The answer is NO – the Universe looks the same from anyvantage point – this is part of the “Cosmological Principle”.

Survey of the Universe The Primary Cosmic Ray

Energy Spectrum

Changes in slope mayindicate different

acceleration and/orpropagation processes

The highest energycosmic rays are

studied with “extensive air shower”

experiments

Second “knee”at 1017.8 eVSecond “knee”at 1017.8 eV

Energy (eV)

“Ankle” at 1019 eV“Ankle” at 1019 eV

“Knee” at 1015.5 eV“Knee” at 1015.5 eV

Balloon and satelliteexperiments

Extensive air showerexperiments

Ultra-high meansE > 1019 eV


What’s within 50 Megaparsecs = 150 Million Light Years from us?

Why is this relevant to the study of Ultra High Energy CosmicRays?

There are two current theories about the origin of the highestenergy cosmic rays1. “Bottom Up” ScenarioParticles are accelerated by some extreme astrophysical phenomenon – galaxies colliding, inferno in the center of anactive galaxy (an Active Galactic Nucleus, or “AGN”).

2. “Top Down” scenarioParticles are the decay products of heavy unstable objectscreated at the time of the Big Bang


The “Top Down” scenarioHighest energy cosmic ray particles are the decay products of heavy unstable particles created at the time of the Big Bang, some of which have not yet decayed

Such particles would have to be extremely heavy

Simple-minded example:

Unstable objectwith mass 2 1020 eV

Decays to two protonswith energy 1 1020 eV

Decays to two protonswith energy 1 1020 eV


The “Top Down” scenario2 1020 eV is an enormous mass for a single particle.

Unstable objectwith mass 2 1020 eV

2 1020 eV is equal to 2 1011 GeV, which is the massof 2 1011 protons. (Proton mass is about 1 GeV in energyunits.) Reminder – the heaviest particle discovered to dateis 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


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”.


The “Bottom Up” ScenarioParticles are accelerated by some extreme astrophysical phenomenon – galaxies colliding, inferno in the center of anactive 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 earthIn the Bottom Up scenario, we expect primary cosmic raydirections to point back to particular sources in the sky.

Survey of the UniverseWMAP Satellite Map of Irregularities in the Cosmic Microwave Background Radiation

Survey of the UniverseThe GZK Cutoff

1966 - K. Greisen - G.T.Zatsepin & V.A.Kuz’min

GZK showed the recently discovered Cosmic Microwave Background Radiation (CMBR) effectively makes the universe opaque to sufficientlyhigh energy cosmic particles.

There are 400photons per cm2

with peak energy6 10-4 eV (2.70K)

Survey of the UniverseTwo possible sources of the highest energy cosmic rays

Colliding galaxies

Active galactic nucleus

But the true origin andacceleration mechanismfor 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 1015 eV


There may be super-massive

black holes at the centerof some galaxies whichemit “jets” of extremely

energetic particles


#S

#S

The Pierre Auger Observatory – the World’s Largest Array

Southern Hemisphere:Malargüe

Province of MendozaArgentina

(being constructed now)

Northern Hemisphere:Millard County, USA

(to be built)

1600 detectors, 3000 km2 each site

The Southern Hemispheresite is being built first sinceit has a better view of the center of the MilkyWay galaxy where there might be a black hole emitting high energy cosmic rays.


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

UniverseSphere of radius150 Million LY,the region whereour highest energycosmic rays originateaccording to the GZK cutoff.


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


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 web animation)


The Milky Way Galaxy – a spiral galaxy

100,000 Light Years


Our Local Group

It contains about 30 galaxiesincluding Andromeda

3 million Light Years

Milky Way

Andromeda


Local Supercluster or VirgoSupercluster of galaxies

Block shown is 100 millionlight years across

Galaxies are found in clumpsand small groups, but much of space contains no galaxiesat all – there are great “voids”

Let’s look on a larger distancescale now

100 million Light Years

Local group

Virgo cluster


Slice aboveand below ourgalactic planeshowing galaxydistributionout to 400million lightyears 400 million

light years150 millionlight years

Great NorthernWall of galaxies

Great SouthernWall of galaxies


The ultimate fate of the Universe

Since the discovery of the Hubble Law, it has been a questionwhether we live in:A closed Universe – gravitational attraction will ultimatelyslow the Universe’s expansion and lead to the eventualcoalescing of all matter – the “Big Crunch” An open Universe – expansion will continue forever in alldirections

It has now been determined that the Universe rate of expansionis in fact accelerating – recession speeds are increasing – So it is thought that we live in an open Universe.

The “engine” that powers the acceleration is unknown and isreferred to as the Dark Energy of the Universe.

(See web animation)


What makes up the Universe

Stars mainly,which wecan see

Unknown matter needed to explain the gravitational attraction of matter we can see

Unknown energy whichpowers the acceleratingexpansion of the Universe


Documents

We live in an expanding Universe What’s within 50 Megaparsecs = 150 Million Light Years from us?