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Cosmic Rays: A Century of Mysteries
Angela V. Olinto*
Department of Astronomy & Astrophysics
Kavli Institute for Cosmological Physics
Enrico Fermi Institute, The University of Chicago, Chicago, IL, U.S.A.
If you open your hand parallel to the ground, as if you were catching some raindrops,
your hand will be traversed by a number of elementary particles moving close to the
speed of light. Some of these particles were produced in very energetic events far
away from our Solar System. The most common of these particles have been travelling
throughout our Galaxy, the Milky Way, for tens of millions of years. A rarer more
energetic type of these showering particles travelled from far away galaxies all the
way to Earth taking as little as tens of millions of years to large fractions of the age of
the Universe of 13.7 billion years. These messengers that reach us constantly bringing
mysterious puzzles to Earth are what we call cosmic rays.
Today we know that cosmic rays are particles like the nucleus of common atoms such
as protons (the nucleus of the hydrogen atom), helium nuclei, carbon nuclei, oxygen
nuclei, etc… all the way to iron nuclei and beyond. These nuclei have been accelerated
to relativistic energies, i.e., energies much larger than the particle mass by some yet to
be unveiled cosmic accelerators. The nature and mechanism operating in these cosmic
accelerators is a century old mystery. Recent advances in observations, experiments,
and theoretical models have been pointing the way to an eminent resolution.
1 Early history
The year 2012 marks the centenary of the famous balloon
flights by Victor Hess in 1912 (see fig. 1) when he showed that
the flux of cosmic rays at high altitude was greater than the
flux at lower altitudes. This kind of measurement established
the fact that what was then called “ionizing radiation” had a
cosmic origin, outside of the Earth.
* E-mail: [email protected]
The idea that some form of ionizing radiation was present
throughout space dates back to questions raised by Coulomb
in 1785 [1]. He found that electroscopes would spontaneously
discharge even if very well insulated. The discovery of
radioactivity at the end of the 19th century gave a partial
answer to this discharge phenomena: there are energetic rays
crisscrossing space produced by radioactive materials that
can cause the discharge of electroscopes. The fact that some
ionizing radiation does originate in radioactive materials
in the ground and some come from outer space awaited
another decade of experiments.
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Fig. 1 Victor Hess back from his balloon flight
in August 1912.
Fig. 2 Increase of ionization with altitude
as measured by Hess in 1912 (left) and
by Kolhörster (right). (Source: Alessandro
De Angelis.)
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By 1909 scientists had taken electroscopes into
tunnels and surrounded them with metal shields to try
to understand the origin of the penetrating radiation. Was
it coming from the crust of the Earth, the atmosphere itself,
or did it originate outside of the Earth? The idea to measure
the rate of the ionizing radiation with height begins with
Theodor Wulf, a German Jesuit priest and scientist who
developed a transportable electroscope and took it up the
Eiffel Tower (300 m high) in Paris in 1909. He did not observe
a significant change at that altitude. A number of scientists
followed the quest for reaching higher altitudes by taking
electroscopes in balloon flights with mixed results before
1912.
A different strategy was developed by the Italian physicist,
Domenico Pacini, who made measurements of the ionizing
radiation underwater in 1911 [1]. He found that the variation
of the flux of ionizing radiation underwater (3 meters deep
and 300 meters from land) could be explained exclusively
by water absorption. His results questioned the idea that
the crust of the Earth was responsible for the radiation. The
possibility of the atmosphere itself or a cosmic origin would
still be plausible.
In 1910 and 1911, several scientists attempted to measure
the change of flux of the ionizing radiation with height using
balloons including K. Bergwitz, who reached 1.3 km, J.C.
McLennan, E.N. Macallum, and A. Gockel who reached 3 km.
The results were inconclusive until Hess made a series of
balloon flights 1912.
The Austrian physicist, Victor F. Hess, carried electroscopes
up in balloon flights seven times from April to August 1912.
In August 7, 1912, he reached 5200 meters. He found that
as one ascends in a balloon, the flux of ionizing radiation
decreases immediately above ground and begins to increase
again around 1 km in height, reaching twice the rate of
the penetrating radiation on the ground between 4 and
5.2 km. He called this radiation höhenstrahlung (radiation
from above). He also showed that höhenstrahlung was not
A. v. olinto: cosmic rays: a century of mysteries
Source: Cushing Memorial Library and
Archives, Texas A&M University.
Fig. 3 Robert A. Millikan,
Arthur H. Compton, and the New
York Times 1932.
AIP Emilio Segre Visual Archive.
coming from the Sun as there was no day-night variation. In
1936, Victor F. Hess received the Nobel Prize for the discovery
of the extra-terrestrial origin of the ionizing radiation, now
called cosmic rays.
The result of Hess were carefully verified by Werner
Kolhörster who reached 9.2 km by 1914 (see fig. 2). These
very courageous scientists had to use oxygen to reach these
altitudes. World War I interrupted the studies of cosmic rays
from 1914 to 1918. After the war, Kolhörster continued his
studies and in 1934, tragedy struck one of his expeditions
where two of his collaborators, Dr. Schrenk and Masuch, died
after reaching 12 km altitude.
After WWI, the focus of research in the field moved to the
United States. Robert A. Millikan, who received a Nobel prize
in 1923 for his measurement of the charge of an electron
and the photoelectric effect, was convinced by 1926 that
the ionizing radiation were gamma-rays (i.e., very energetic
light particles or photons). He proposed that these rays
were produced by hydrogen fusion in intergalactic space and
coined the name, cosmic rays. If cosmic rays were gammarays they would have zero electric charge.
In 1927 the Dutch scientists, Jacob Clay, observed that
the cosmic ray flux varied with latitude as he travelled from
Java, Indonesia, to Genova, Italy. A clear confirmation of
the effect came from a large scale expedition mounted
by Arthur H. Compton who enlisted about 100 scientists
throughout the world to measure the cosmic ray flux in
different latitudes and altitudes. (He published a single
authored paper on this effort in 1933.) The latitude effect can
be explained if cosmic rays are charged particles deflected by
the magnetic field of the Earth. The charged nature of cosmic
rays was further clarified by Compton and Luis W. Alvarez
who discovered the excess of cosmic rays coming from the
West relative to the East using an experiment designed by
the Italian Bruno Rossi. This East-West effect also showed that
cosmic rays were mostly positively charged (fig. 3).
From 1933 to 1953, a large number of new particles were
discovered through the studies of cosmic rays. In 1933, the
positron, the first antimatter particle to be identified (the antielectron, e+), was discovered by Carl. D. Anderson using tracks
left by cosmic ray particles in his cloud chamber. For this
discovery, he received the Nobel Prize in 1936. In 1937, the
muon (µ) was discovered followed by the pion (p), the kaon
(K), and the lambda (L0 ) in 1947. These were the first hints
that the building blocks of nature are complex.
2 From Particle Physics to Astrophysics
By the early 1950s, the study of the fundamental nature of
matter and its interactions moved from the use of cosmic rays
to man-made particle accelerators. In 1955 the antiproton
(the antimatter version of the proton) was discovered at the
Bevatron of the Lawrence Berkeley Laboratory. By the mid
1970s, Sheldon Glashow, Steven Weinberg, and Abdus Salam
formulated the Standard Model of particle physics based
on gauge bosons as force carriers and three generations of
quark and leptons. The subsequent discoveries of the charm
quark (1974 at Brookhaven National Laboratory and Stanford
Linear Accelerator Center), the bottom quark (1977 at Fermi
National Laboratory or Fermilab), the W and Z bosons (1983
at the European Organization for Nuclear Research or CERN),
the top quark (1995 at Fermilab), and the tau neutrino (2000
at Fermilab) established the Standard Model. Most recent
the announcement, on July 4, 2012, of a Higgs-type particle
observed at the Large Hadron Collider (LHC) at CERN has
completed the predictions of the model up to the LHC energy
scale which is about 8,000 times mpc 2, where mp is the mass
of a proton. (mp is 1.673 10−4 grams which corresponds to and
energy unit of mp c 2 = 0.938 GeV, where gigaelectronvolt or
GeV = 109 eV, and 1 eV, or electronvolt, is the kinetic energy
an electron gains when it crosses a 1volt potential).
The opportunity to test particle interactions with cosmic
rays is still possible as cosmic rays can reach much higher
energies than current particle accelerators. The work of
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Fig. 4 Extensive Atmospheric Showers.
the Italian Bruno Rossi, led to the discovery of airshowers
by Rossi, K. Schmeiser, W. Bothe, Kolhörster and Pierre
Auger. They established that a single particle in the upper
atmosphere can produce a very large cascade of particles,
now called an extensive airshower (see fig. 4), by placing
particle detectors at different distances and observing the
coincidence in arrival time of particle signals on the ground.
By 1939, Auger estimated that the energy of the primary
cosmic ray (the original particle that generated the particle
cascade) reached 1 million times mp c 2. Now we know
that there are cosmic rays with energies above 100 billion
mp c 2, 7 million times the LHC energy. (For these extremely
energetic particles, the typical interaction energy with
atmospheric atoms is about 100 times that of the LHC.)
The study of cosmic rays became of great interest to
astrophysicists, curious to understand how an astrophysical
source can impart such extreme energies to subatomic
particles. In 1934 the German astronomer Walter Baade and
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the Swiss astronomer Fritz Zwicky suggested that supernova,
the explosive death of stars, are responsible for accelerating
cosmic rays based on how much energy would be necessary
to explain the observations. In 1949 the Italian physicist
Enrico Fermi proposed that cosmic rays are accelerated via
a stochastic process in the interstellar space by collisions
against moving magnetized clouds. His theory explained
the puzzling power law behavior of the spectrum of cosmic
rays and is still the basis for most current explanations for the
acceleration of cosmic rays.
Modern theories of the origin of cosmic rays divide
cosmic rays into a galactic origin from energies of about
mp c 2 to about a billion times mp c 2, and an extragalactic
origin for energies above that. The primary model for the
origin of galactic cosmic rays involves the combination of
Baade and Zwicky’s suggestion with Fermi’s mechanism, i.e.,
stochastic acceleration in the remnants of the supernova
explosion. This type of model can explain the observed
spectrum up to 10 million mp c 2, and may be able to reach
higher energies. Given that cosmic rays are charged, their
distribution of arrival directions is isotropized by magnetic
fields in their path to Earth. Observations using photons
(from radio to gamma-rays) are the best route to try to
identify the acceleration sites in the Galaxy. Recent gammaray observations by the NASA Fermi satellite and the groundbased HESS, MAGIC, and VERITAS observatories are beginning
to resolve possible cosmic ray acceleration sites. One prime
candidate is the Tycho supernova remnant shown in fig. 5.
In the next decade these efforts may lead to the resolution of
the mystery of the origin of Galactic cosmic rays.
After acceleration in sites such as supernovae remnants,
cosmic rays diffusive around our Galaxy for long periods of
time depending on their energy (the lower the energy the
more they diffuse). It would take a neutral relativistic particle
about sixty thousand years to cross our Galaxy (travelling
in a straight light), while cosmic rays take tens of millions of
years to reach Earth. This long delay is due to magnetic fields
in the Galaxy that significantly bend their paths to Earth.
This magnetic diffusion process is studied by measuring
the relative abundances of different elements as a function
of energy. From the 1960s, this study used short- and
long-duration balloon experiments and space missions. These
experiments observed that spallation products of common
nuclei are much more abundant in cosmic rays than in solar
system material; for example, lithium, beryllium, and boron
nuclei which are produced mainly by the spallation of carbon
A. v. olinto: cosmic rays: a century of mysteries
and oxygen are 100,000 times more abundant
in cosmic rays than their solar values. The
overabundance shows that cosmic rays have
traversed about 10 g/cm2 as they propagate
in the Galaxy, corresponding to trajectories of
millions of light years in length, which is much
larger than the thickness of the galactic disk
of only thousands of light years.
Recent direct studies of cosmic ray
abundances include the balloon payload
projects named CREAM (Cosmic Ray
Energetics And Mass), TIGER (Trans-Iron
Galactic Element Recorder), and TRACER
(Transition Radiation Array for Cosmic
Energetic Radiation). Chief among
these efforts is the PAMELA (Payload for
Antimatter Matter Exploration and Lightnuclei Astrophysics) space mission which
discovered a very interesting excess of
positrons (antielectrons) and an unexpected
change in the behavior of cosmic ray protons
and helium. The positron excess generated
a lot of excitement over the possibility that
the source of these positrons are due to the
mysterious dark matter which comprises 85%
of the matter in the Universe. There are more
mundane explanations for these positrons
based on nearby astrophysical accelerators
such as pulsars and supernovae. The proton
and helium flux behavior is also quite new and
may be due to details of the most energetic
accelerators in the Galaxy.
The latest observatory to be deployed in
space, the Alpha Magnetic Spectrometer
(AMS), is now running at the International
Space Station. AMS (fig. 6) will follow up on
the findings of PAMELA and make precise
measurements of the composition and
spectrum of different types of cosmic rays
over a wide energy scales and is sensitive
enough to find rare unknown components in
these mysterious rays. This major international
effort has the sensitivity to clarify the nature
of galactic cosmic rays and to discover some
previously unknown components of the
cosmic radiation.
Fig. 5 Tycho Supernova
Remnant. (Credit: X-ray: NASA/
CXC/SAO; Infrared: NASA/JPLCaltech; Optical: MPIA, Calar
Alto, O. Krause et al.)
Fig. 6 AMS installed at the
International Space Station
(NASA Image S134E007532).
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3 From galactic to extragalactic cosmic rays
Different observational techniques allow the observation of
cosmic rays over 12 orders of magnitude in energy (from 108
to 1020 eV) as shown in fig. 7. Up to 1014 eV, direct detection
is feasible with balloon and space experiments. Above
this energy, the flux is too low for space-based detectors
and cosmic rays are studied by observing their air-shower
development based on the discovery of Pierre Auger in
1939. He showed that very-high-energy cosmic rays trigger
extensive air showers in the Earth’s atmosphere, distributing
the original cosmic ray energy among billions of lowerenergy particles (called secondaries) that arrive together
on the ground. These secondary particles can be detected
with arrays of particle detectors and trough ultravioletsensitive telescopes that observe the fluorescence of nitrogen
molecules in the air.
Direct detection shows that at low energies the cosmic ray
flux is modulated by the solar cycle through the magnetic
field of the Sun, which shields the solar system from charged
particles below about 108 eV. From 108 eV to about 1015 eV,
the cosmic ray spectrum is well described by a power law, i.e.,
the number of cosmic rays arriving on Earth per unit time,
area, solid angle, and kinetic energy, E, is proportional to a
power of the energy as E –2.7. At higher energies, air shower
observatories have shown that the spectrum steepens to E –3
and the transition region is called the “knee.” At about 1018 eV
the spectrum hardens again, giving rise to a feature named
the “ankle.”
Below the knee cosmic rays are dominated by light
nuclei (protons and helium) while at higher energies the
composition becomes heavier. This transition to heavier
elements is expected because galactic cosmic rays propagate
diffusively in the magnetic field of the galaxy with a
probability of escape that depends on the ratio of energy
to the charge (called rigidity). Within this picture, the knee
would represent the transition from confined trajectories
to trajectories that escape the Galaxy and thus produce
the change in the spectrum. This model fits well with
observations by the Karlsruhe Shower Core Array Detector
(KASCADE) experiment and the KASCADE-Grande extension.
These data provide evidence for a transition from light nuclei
to heavier ones, with the indication of nuclei from carbon to
iron becoming dominant just below the ankle. A transition
back to lighter nuclei at the ankle is also observed, which
is a signal that the extragalactic component has become
dominant at these energies.
Cosmic rays with energies well above the ankle are certainly
extragalactic. At these high energies a galactic component
would give a clear signal in the sky distribution, instead of
the observed isotropic distribution, the image of the galactic
plane should emerge. Cosmic accelerators far away from
the our Galaxy produce these ultrahigh-energy particles.
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The precise energy above which the galactic component is
overtaken by the extragalactic component is still an open
question. More mysteries remain such as what could be the
source of these ultrahigh energy extragalactic particles. Are
they produced in the super-massive black holes in the center
of distant galaxies? Or perhaps in shocks produced by the
largest structures in the Universe? Or were they accelerated
in more energetic explosive deaths of star that create black
holes or neutron stars? Finally, how high an energy do cosmic
rays reach?
In 1962, the Volcano Ranch array led by John Linsley
observed a cosmic ray event with an energy of tens of joules
or around 1020 eV (about 100 billion mp c 2). This kind of
energy is common among a good serve of a tennis ball, but it
is extreme for a subatomic particle to carry. Four years later,
Kenneth Greisen in the United States and Georgiy T. Zatsepin
and Vadim A. Kuzmin in the USSR predicted the abrupt
steepening of the cosmic ray spectrum around 1020 eV as a
result of cosmic ray interactions with the newly discovered
cosmic microwave background (CMB), the relic radiation
from the Big Bang. In his landmark article of 1966, Greisen
announced that the measurement of such a flux steepening
would clarify the origin of ultrahigh energy cosmic rays by
showing their “cosmologically meaningful termination.”
Ultrahigh-energy cosmic rays are detected by two main
techniques: ground arrays (of scintillators or water Cherenkov
tanks) and fluorescence telescopes. Ground arrays sample
the extensive air shower as the secondary particles reach
the ground. Historic arrays built to explore these extremely
energetic events include Haverah Park (1967 to 1987), Sydney
University Giant Air-Shower Recorder (SUGAR) (1968 to
1979), Yakutsk (1991 to present), and the Akeno Giant AirShower Array (AGASA). The 111 surface detectors of AGASA
covered 100 km2 and operated for just over a decade (1990
to 2004). An alternative technique based on the atmospheric
fluorescence of extensive airshowers was pioneered by the
Fly’s Eye detector, which in 1991 observed an event with
energy of 3 × 1020 eV, the current record holder, challenging
the prediction by Greisen, Zatsepin, and Kuzmin (GZK).
The fluorescence technique was further developed by the
High-Resolution Fly’s Eye (HiRes) experiment, which reached
very large exposures accumulating enough ultrahigh energy
cosmic rays to verify that the GZK prediction was correct.
Fluorescence observatories detect the ultraviolet light
produced by the fluorescence of nitrogen molecules in the
atmosphere as the shower develops above the ground.
Mirrors focus the ultraviolet light onto photomultiplier
tubes that record the fast-moving shower pattern in the
atmosphere. These fast and sensitive cameras can record the
light equivalent to a 40 Watt light bulb moving at the speed
of light tens of kilometers away. This technique can observe
the full development of the shower giving the energy and
A. v. olinto: cosmic rays: a century of mysteries
the likely composition of the primary cosmic
ray. However, it has a low duty cycle since it
works best during clear moonless nights while
ground arrays work 24 hours a day.
Since the prediction of the GZK effect in
1966, the existence of the steepening of the
spectrum was a great open question. The
AGASA observatory found a flux that did not
follow the expected shape, suggesting that
new physical may be at play at these extreme
energies. This discrepancy was settled by
HiRes and the Pierre Auger Observatory.
Located in the Mendoza province
of Argentina, the Pierre Auger Observatory is
the largest detector of cosmic rays ever built.
Covering an area of 3,000 km2 with an array
of water Cherenkov detectors and the four
fluorescence telescope overlooking the site, it
began full operations in 2008 (see fig. 8).
In addition to confirming the shape of the
spectrum at the highest observed energies,
the Auger Observatory has found hints of
anisotropies in the distribution of arrival
directions of cosmic rays with energies above
6 ×1019 eV. These can be the first signs of
the mysterious sources from outside our
Galaxy [2]. The number of events at the
highest energies is not enough yet to sharpen
the image of the real source distribution
but one strong candidate for a source of
anisotropies is Centauros A, a nearby galaxy
with a jet produced by a supermassive black
hole at its center (see fig. 9). The quest for
resolving this mystery continues with future
observatories being designed to gather
many more particles of extreme energies and
sharpen up the picture of these mysterious
and very powerful sources.
In addition to hints of the source
distribution in the sky, the Auger Observatory
has found an interesting behavior of the
shower profiles. They are better explained
by heavier nuclei than protons, a complete
surprise to most astrophysicists. This puzzle
may indicate that particle interactions are
different at these extreme energies or that
astrophysical accelerators inject more heavy
nuclei than what is available thorough
intergalactic space. Another ground array
recently complete, named the Telescope Array
(TA), has not yet confirmed this unexpected
behavior at the highest energies. Covering
Fig. 7 Spectrum of Cosmic Rays
(source: Swordy - U. Chicago).
Fig. 8 Pierre Auger Observatory
covering 3,000 km2 near the city
of Malargue in the Mendoza
province in Argentina. Each
red dot is a water tank detector
(separated by 1.5 km each)
and the green lines represent
the fluorescence telescope
field of view. The schematic
picture shows how particles
are observed jointly by the
water tanks and fluorescence
telescopes at night (courtesy of
Pierre Auger Observatory).
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Fig. 9 Centaurus A – a nearby galaxy with
a jet produced by the supermassive black
hole at its center. (Credit: X-ray: NASA/CXC/
CfA/R.Kraft et al; Radio: NSF/VLA/Univ.
Hertfordshire/M. Hardcastle; Optical: ESO/
WFI/M.Rejkuba et al.)
700 km2 in Utah, USA, it observes the
highest-energy events arriving in the
Northern Hemisphere complementing the
Southern Auger Observatory. These two
giant arrays will likely continue to unravel
the mysteries behind these extremely
energetic particles during this decade or
more.
A new generation of observatories
is now being planned with the goal of
accumulating enough particle events to
solve the mystery behind the extragalactic
origin of cosmic rays of ultrahigh energies.
A powerful fluorescence telescope
is being designed by an international
collaboration to be installed in the
International Space Station to look down
on Earth, the JEM-EUSO (Extreme Universe
Space Observatory on the Japanese
Experiment Module) project (fig. 10).
It can accumulate ten times more events
than the current ground arrays and
observe showers from upward-going
particles such as high-energy neutrinos.
This first space mission for the highestenergy particles may pioneer the space
exploration of the Earth’s atmosphere as a
giant particle detector. A first step towards
understanding the nature of the more
than a billion particles of extreme energies
that reach the Earth annually.
References
[1] A. deAngelis, P. Carlson, N. Giglietto,
S. Stramaglia, in the “Proceedings of the
32nd International Cosmic Ray Conference,
ICRC 2011”, Beijng, China, 2011.
[2] K. Kotera and A. V. Olinto, Annu. Rev. Astron.
Astrophys., 49 (2011) 119.
Fig. 10 The Extreme Universe Space Observatory
on the Japanese Experiment Module JEM-EUSO
(source: JEM-EUSO website).
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Angela V. Olinto
Angela V. Olinto is Professor and Chair of the Department of Astronomy
and Astrophysics, and member of the Enrico Fermi Institute and the
Kavli Institute for Cosmological Physics, at the University of Chicago. She
received her Ph.D. in Physics from MIT (1987) for work on the physics of
quark stars. She worked on inflationary theory, cosmic magnetic fields, the
nature of the dark matter, and now leads the effort to understanding the
origin of the highest energy cosmic particles, cosmic rays, gamma-rays
and neutrinos. She is the US PI of JEM-EUSO and a member of the Pierre
Auger Observatory. Olinto is a Fellow of the APS and the Chair-Elect of
the APS DAP. She received the Quantrell Award at Chicago and the Chaire
d’Excellence of the French ANR.