EXTREME ENERGY COSMIC RAYS AND THE UNIVERSE General scope: a new universe Cosmic rays: facts and puzzles

In the past 40 years or so our knowledge of the universe has made spectacular progress but the more we learn of it and the more puzzles we need to solve: what are this “cold dark matter” that fills up galaxies? This “dark energy” that accelerates the expansion of the universe? This “inflation” that blew it up in the first tiny fraction of a second? And while we have learnt a lot about stars, their birth, their life and their death, we still do not know precisely how cosmic rays are accelerated.

Our current knowledge of the cosmic microwave background is extremely accurate. The Planckian shape of the spectrum, T=2.725±0.001K, tells us that we are looking at photons that had been emitted when the universe was ~ years old (at a redshift z~1100). Until then it had been made of electrons, photons and nuclei (mostly protons) in thermal equilibrium. At that time electrons and nuclei combined in atoms and the universe became transparent to photons, entering a long era of “dark ages”. It was then remarkably homogeneous and isotropic.

WMAP Fluctuations at the 10 ppm level have been measured with an accuracy of a fraction of a ppm and excellent angular resolution. Their maximal size at recombination time was defined by the horizon at that time and is well known. Measuring their size today tells us how the universe has evolved since then: one finds that it is flat, Ω=1.02±0.02, namely that its density equals the critical density.

Greatly improved accuracy of the Hubble diagram, V (Doppler red shift) = Hr (standard candles) Type Ia supernovae H = 71±3 km/s/Mpc High z supernovae reveal acceleration of the expansion, q (deceleration parameter) =  0.66±0.10

Detailed understanding of the abundance of light elements in the universe. Nucleosynthesis took place three minutes or so after the big bang. “Visible” stars account for only 1% of the total energy density, normal (baryonic) matter for only 4.4 ± 0.4 %. Invisible 3% or so made of brown dwarfs, planets, hot gas, dark clouds, black holes...

Far away from the galaxy bulbs the movement of stars reveals the presence of a halo of dark-matter. Gravitational lensing and clusters of galaxies confirm this finding. Cosmologists claim that dark matter must be cold, i.e. have velocities <<c, for galaxies to have formed. Dark matter (23±4 %) +baryonic matter (4 %) make up only 27% of the energy density of the universe (photons and neutrinos are at most at the percent level). What are the 73±4 % of remaining “dark energy” responsible for the acceleration of the expansion? Dark Energy 73% CDM 23% Baryons 4%  + < 1%

Stars are born from the condensation of molecular clouds under gravity. Particularly frequent in density waves of spiral arms of galaxies (OB associations). Most massive stars burn their fuel fast and explode, triggering the formation of new stars. Less massive stars evolve along the main sequence of the Hertzsprung-Russell diagram.

When stars have burned up their nuclear fuel they blow up their outer envelope into a nebula. Their core condenses into a white dwarf, a neutron star (pulsar) or a black hole depending on their mass. White dwarfs are mostly made of light nuclei and electrons in the form of a Fermi gas (earth size). Neutron stars are made of neutrons at nuclear densities, possible quark gluon plasma in the centre (km size). Black holes look like dense neutron stars from outside but may be enormously more massive, up to billions of solar masses.

Compact objects (white dwarfs, neutron stars, black holes) may accrete matter from a companion star or more generally from the surrounding environment, which ever state it is in. This is known to be the case of many nearby compact objects having stellar masses. Recently it has been understood that it is also the case of very massive black holes having nearly galactic masses, the so called active galactic nuclei (AGN). Depending on their detailed configuration they include quasars, radio-galaxies, Seyfert galaxies, blazars, BL Lac, etc

x = 2.7 x = 3.0 x = 2.7  Cosmic rays are ionized nuclei that travel in space up to extremely high energies, ~10 20 eV=16 Joules!  There are very few of them but they carry as much energy as the CMB (nearly 1 billion photons per proton) or the visible light or the magnetic fields ~1eV/cm 3  Their composition is similar to that of matter in the universe except that the rare nuclei of the latter are more common in cosmic rays: spallation reactions in ~7g/cm 2 of interstellar matter  They have a power law spectrum over 32 decades (12 decades in energy), ~ E The ankle and the knee are not really understood.

Magnetic fields prevent pointing back to the sources except, hopefully, at the highest energies.However gamma rays (from π → γγ decays) and neutrinos (from π → μν decays) are expected to be good tracers of their interactions with matter (in particular in the vicinity of the source). Indeed high energy gamma rays have been observed from the Crab and from Cygnus X3, but also from distant AGN's

HESS 2 Galactic Centre - 11  RX J  Linton, WatsonFest, Leeds July 2004

There is now strong evidence from x-ray and gamma-ray astronomy that galactic supernova remnants are sources of cosmic rays and that acceleration takes place on the shock front. ROSAT XMM CANGAROO RXJ RA

Diffusive shock acceleration. A cloud moves into the ISM, both are extremely diluted, with high relative velocity V. No collisions but only interactions via trapped magnetic fields. Relativistic particles gain an energy ΔE=E cosθ V/c each time they traverse the shock at incidence θ. Multitraversals produce the required acceleration as long as some mechanism exists that keeps bringing the particles back toward the shock. This mechanism is not well understood. It is described as a random walk (magnetic field inhomogeneities). The maximal attainable energy is proportional to the product size L × field B (Hillas). V V Cloud ISM θ 1km 1Gm1pc 1Mpc 1G1G 1G 1MG 1TG eV protons RG LOBES SNR SUN SPOTS AGN WD NS L B

Above eV or so one expects the spectrum to be cut off (the Greisen-Zatsepin-Kuzmin, “GZK” cutoff, photoproduction of pions on the CMB photons) unless sources are close to us (ie are not AGN's). This issue is currently controversial.

The Auger Observatory combines the strengths of Surface Detector Array and Air Fluorescence Detectors Independent measurement techniques allow control of systematics More reliable energy and angle measurement Primary mass measured in complementary ways The highest energy cosmic rays are observed from the extensive air showers that they produce when entering the atmosphere. One method consists in sampling the particle density on ground, another method consists in detecting the fluorescence light produced on nitrogen molecules along the shower axis. In both cases timing gives the direction and intensity gives the energy but both methods suffer of very different systematic sources of errors.

Giant detector arrays are made of scintillators or water Cherenkov counters. That of the Pierre Auger Observatory covers 3000 km 2 with a triangular grid having a 1.5 km mesh size (1500 detectors of 10m 2 area each). One >10 20 eV shower detected every week, involving 15 to 20 detectors. Typical angular and energy resolutions are 1.5° and 20%.

Four stations of six eyes each, each eye covering a field of view of 30°×28° with a mirror focusing on an array of 22×20 pixels (photomultiplier tubes), each having 1.5° aperture. They measure the induced fluorescence of nitrogen molecules (near UV).

Typical (nice) Event The Pierre Auger Observatory, being the first large scale hybrid detector ever constructed, will open a new window on the extreme energy region and hopefully answer many of the current puzzles: is there a GZK cutoff? can one identify specific sources? galactic or extra-galactic? which is their composition (protons or heavier nuclei?). Surface array view Lateral distribution function fit

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For now 40 years astrophysics has made fascinating and spectacular progress The whole of physics is invited to the banquet: particle, nuclear, atomic, molecular, plasma, solid state The whole of physics and also the whole world Only a few privileged countries can afford to launch space missions or to build giant telescopes But any country can, in principle, access the data This is an opportunity that developing countries should not miss The sky belongs to all of us We are all made of the same star dust

Coma cluster MANY THANKS To the organizers for the very nice Conference To E. Parizot, P. Sommers and A. Olinto for allowing me to use some of their slides To Pham Thi Tuyet Nhung and Pham Ngoc Diep for help in preparing the talk