² The universe observed ² Relativistic world models ² Reconstructing the thermal history ² Big bang nucleosynthesis ² Dark matter: astrophysical

Transcription

1 ² The universe observed ² Relativistic world models ² Reconstructing the thermal history ² Big bang nucleosynthesis ² Dark matter: astrophysical observations ² Dark matter: relic particles ² Dark matter: direct detection ² Dark matter: indirect detection ² Cosmic rays in the Galaxy ² Antimatter in cosmic rays ² Ultrahigh energy cosmic rays ² High energy cosmic neutrinos ² The early universe: constraints on new physics ² The early universe: baryo/leptogenesis ² The early universe: inflation & the primordial density perturbation ² Cosmic microwave background, large-scale structure & dark energy

2 Per Carlson (Physics Today, Feb 2012) 1912: Victor Hess discovers cosmic rays (named so in 1927 by Millikan) Nobel Prize 1936 [1928: Paul Dirac predicts the existence of antiparticles Nobel Prize 1933] 1932: Carl Anderson discovers the positron in cosmic rays - Nobel Prize 1936 (cloud chamber invented by C T R Wilson - Nobel Prize 1927) [1935: Hideki Yukawa predicts the existence of mesons Nobel Prize 1949] 1937: Seth Neddermeyer & Carl Anderson discover the muon in cosmic rays 1947: Cecil Powell discovers the pion in cosmic rays Nobel Prize : George Rochester & Clifford Butler discover the kaon (Patrick Blackett awarded Nobel Prize 1948 for development of the Wilson cloud chamber method )

3 Another cosmic ray signal of new physics? As the most precise measurement of the cosmic ray positron flux to date, these results show clearly the power and capabilities of the AMS detector, said AMS spokesperson, Samuel Ting. Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin. Phys. Rev. Lett. 110: (2013)

4 We are constantly bombarded by cosmic rays with energies ranging up to ~10 11 GeV arriving isotropically from all directions in the sky so their sources are unknown knee Courtesey: Simon Swordy Most of them are protons (although heavier elements are also present) and the energy spectrum is a featureless power-law, at least up to the knee at ~1 PeV

5 The composition of cosmic rays (up to the knee ) mirrors that of the interstellar medium with refractory elements enhanced, suggesting acceleration of circumstellar gas & dust grains by supernova shock waves (Meyer, Drury & Ellison, ApJ 487:182/197, 1997)

6 The sources of galactic cosmic rays have long been presumed to be supernova remnants Direct evidence for acceleration of electrons (to > 40 TeV) from observation of synchrotron emission: radio X-rays Energetics: GCR energy density Volume of extended halo Total GCR energy Residence time of CRs in Galaxy Power needed Galactic SN rate Cassiopeia A: Chandra Required output/sn (remnant) This is only ~5-10% of the benchmark kinetic energy of erg produced in a SN explosion Cassiopeia A: VLA

7 1 st -order Fermi acceleration by shock waves (DSA) CR trajectory B 1 B 2 Shock velocity v s : β = v s /c Simple diffusion theory: prob. of CR crossing shock > m times is (1-β) m Average fractional energy gained at each crossing is: Δε/ε = β differential spectrum ε -2 High velocity plasma Low velocity plasma For 2 nd -order Fermi acceleration with Δε/ε β 2, the slope can be harder However if ~10% of the shock wave K.E. is converted into relativistic particles, then backreaction of cosmic ray pressure on shock will make spectrum somewhat harder and slightly concave but the time-integrated spectrum may still be close to the Fermi form (Caprioli, Amato & Blasi, Astropart.Phys.33:160,2010) If cosmic rays diffuse out of Galaxy on a time-scale decreasing 1/ε 0.6, then the observed spectrum ε -2.6 is matched (but why is no anisotropy ε 0.6 observed?)

8 We are witnessing rapid advances in γ-ray astronomy the sources of low energy cosmic rays may soon be known: SNRs? Ø Do we see γ-rays from hadronic interactions (π 0 decays), or are they from inverse-compton scattering by (radio synchrotron emitting) electrons? Ø Can 1 st -order Fermi acceleration at SNR shocks explain the spectrum (injection, magnetic field amplification, diffusion losses vs anisotropy)? Ø What are the unidentified Milky Way γ-ray sources are there new RXJ (HESS, 2004) source classes (micro-quasars, PWN, binaries ), acceleration mechanisms? Fermi HESS Southern Plane Survey 2005 Much progress has been made but these questions are not fully answered to unambiguously identify the cosmic ray sources, we must detect TeV neutrinos! also the PAMELA and Fermi anomalies have highlighted the limitations of the standard cosmic ray diffusion model

9 Cosmic ray acceleration in RXJ : electrons or protons? B = 10 μg models: F. Aharonian (HESS collabora-on, 2006) γ-ray emission well fitted by IC scattering of ~10 2 TeV electrons on CMB/starlight alternatively γ-rays may be from decays of π 0 s produced by ~10 3 TeV protons There is no definite evidence yet that all SNRs accelerate protons to high energies this will be proven only when the neutrinos from π 0 decay are detected

10 Detection of the Characteristic Pion-Decay Signature in Supernova Remnants Fermi has now identified several SNRs where the γ-rays are definitely from decays of π 0 s Nevertheless we are far from having a standard model of cosmic ray acceleration

11 Efficient diffusive shock acceleration should give a concave spectrum (due to feedback on the shock by cosmic ray pressure) (Voelk et al, A&A396:649,2002) but the synchrotron radio spectrum of this young SNR is a convex power-law well-fitted by log-normal spectrum expected from 2 nd order Fermi acceleration by MHD turbulence behind the shock wave (Cowsik & Sarkar, MNRAS:207:745,1984)

12 The standard model for galactic cosmic rays SNR shock waves accelerate relativistic particles by Fermi mechanism power law spectrum (synchrotron radio/x-ray + γ-ray emission) Diffusion through magnetic fields in Galaxy (disk + halo) Secondary production during propagation: e ± lose energy through synchrotron & inverse Compton scattering Measurables: Energy spectra of individual species, diffuse radiation

13 Diffusion of galactic cosmic rays Transport equation: diffusion energy losses injection Boundary conditions: Green function: describes flux from a discrete, burst-like source integrate over spatial distribution and time-variation of injection GALPROP (Moskalenko & Strong ApJ 493:694,1998, 509:212,1998) solves time-dependent transport equation but yields ~the same answer for equilibrium fluxes as the leaky box model in which cosmic rays have small energy dependent probability of escape from Galaxy exponential distribution of path lengths between cosmic ray sources and Earth

14 The leaky box model (Cowsik, Yash Pal, Tandon & Verma 1966) Transport equation: diffusion energy losses injection Averaging over extended cosmic ray halo steady state solution Escape through diffusion: τ esc ~ E -δ, with δ ~ 0.6 (from secondary/primary ratios) Energy loss through synchrotron radiation/ic scattering: τ cool ~ E -1

15 The production of secondaries provides a diagnostic of the propagation of nuclear cosmic rays in Galaxy interstellar medium ~90% H, ~10% He Acceleration of protons

16 Elements such as Li, Be, B in cosmic rays are not found in the interstellar medium they are secondaries created in the spallation of the high energy primary nuclei in interstellar collisions Cosmic rays Solar system

17 Primary e Production: Energy spectra (from radio spectrum) Propagation: Observed: Primary protons/nuclei Production: presumably same as e Propagation: Observed: Secondary production: propagation: observed:

18 Secondary-to-primary ratios Evoli, Gaggero, Grasso & Maccione, JCAP 10:018,2008 All measured ratios consistent with leaky box model with τ esc ~ E -δ, δ ~ NB: Kolmogorov spectrum for interstellar magnetic field turbulence implies δ = 1/3, while Kraichnan spectrum implies δ = 1/2

19 But we do not see anisotropy increasing monotonically with energy as might be expected for diffusive transport in the Galaxy with τ esc ~ E -0.6 (Ptuskin, 2005)

20 The two zone USINE model Maurin, Taillet, Donato, Salati, Barrau & Boudoul [astro-ph/ ] Semi-analytic formulation provides better insight and estimation of uncertainties