Probing fundamental physics and cosmology using Gamma-ray observations Hassan Abdalla & Markus B¨ ottcher CSR, NWU, Potchefstroom November 28, 2019 H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 1 / 23
Outline 1 Introduction: 2 The spectral hardening 3 EBL inhomogeneity 4 Lorentz-Invariance Violation LIV: Cosmic opacity 5 LIV and Void 6 LIV: Compton scattering 7 Summary and Conclusions H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 2 / 23
Introduction: Gamma-ray sources (e.g., AGNs) H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 3 / 23
Introduction: The Detection of Gamma-Rays The atmosphere is opaque to gamma-rays! H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 4 / 23
Introduction: Artist’s view: Fermi LAT satellite detector H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 5 / 23
Introduction: Schematic drawing: Imaging Atmospheric Cherenkov Technique H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 6 / 23
Introduction: High Energy Stereoscopic System (H.E.S.S.) H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 7 / 23
Introduction: High Altitude Water Cherenkov Observatory H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 8 / 23
The spectral hardening Introduction: Very High Energy gamma-rays (VHE; more than 100 GeV) from cosmological gamma-ray Sources such as Blazars can be absorbed by the Extragalactic Background Light (EBL), which leads to a high-energy cut-off at the VHE end of Blazar spectra. The probability of absorption depends on the photon energy and redshift. This process has been intensively studied during the last few decades ( e.g., Stecker 1969 - Dom´ ınguez 2011). Acciari et al. 2010 H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 9 / 23
The spectral hardening The spectral hardening What is the problem? From recent observation, the universe is more transparent to the VHE gamma-rays than was expected! Archambault et al. 2014 These VHE signatures in the spectra of distant blazars are currently the subject of intensive research. Finke et al. 2010 H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 10 / 23
The spectral hardening The spectral hardening What is the solution ?! To explain this VHE gamma-ray imprint there are many suggestions: The existence of exotic Axion Like Particles (ALPs) Dom´ ınguez et al. 2011 Interactions of extragalactic Ultrahigh Energy Cosmic Rays (UHECR) Essey et al. 2010 The existence of cosmic voids between such Blazar and the observer on the earth Furniss et al. 2013 We did detailed calculations about the possibility of a cosmic void along the line of sight to such distant Blazar. We considered the possibility of Lorentz invariance violation and its astrophysical implications. H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 11 / 23
EBL inhomogeneity Void: The void radius represented by R The void center represented by z v The source located at z s We set local star formation rate zero inside the void Abdalla & B¨ ottcher 2017 H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 12 / 23
EBL inhomogeneity Opacity deficit due to the presence of the voids z = 0.3 0.175 z = 0.4 z = 0.5 z = 0.6 0.150 z = 0.7 Relative opacity deficit 0.125 0.100 0.075 0.050 0.025 10 1 10 2 Energy (GeV) H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 13 / 23
Lorentz-Invariance Violation Lorentz-Invariance Violation At quantum gravity scale, VHE photons could be sensitive to the microscopic structure of space-time. Higher energy photons are expected to propagate more slowly than their lower-energy counterparts. Image credits: Colin Gillespie, MGM; timeone.ca H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 14 / 23
Lorentz-Invariance Violation Lorentz-Invariance Violation Quantum-gravity theories predict in general the breakdown of familiar physics when approaching the Planck energy scale, E P ∼ 1 . 2 × 10 19 GeV Currently such extreme energies are unreachable by experiments on Earth, but for photons traveling over cosmological distances the accumulated quantum gravity effect can be measured Studies of time delays in the arrival times of γ − rays of different energies due to LIV can be used to probe fundamental physics (Lorentz & Brun 2016; H.E.S.S. 2019). H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 15 / 23
Lorentz-Invariance Violation Lorentz-Invariance Violation At Planck energy scale Lorentz symmetry will breakdown, the deviation from Lorentz symmetry can be described by modification of the dispersion relation as follows: � n � E E 2 = p 2 c 2 + m 2 c 4 + S E 2 (1) E LIV where S = − 1 for a subluminal case , S = + 1 for a superluminal case , and n is the order of the leading correction . The modified pair-production threshold for n = 1 , can be written as: ǫ min = m 2 c 4 E 2 � � − S (2) E γ 4E LIV where E LIV = E P /ξ 1 , ξ 1 is dimensionless parameter. 10 0 10 0 10 -1 10 -1 S = − 1 S = + 1 ǫ ( eV ) ǫ ( eV ) Standard Standard 10 -2 10 -2 E LIV = E P E LIV = E P E LIV = 5 E P E LIV = 5 E P E LIV = 20 E P E LIV = 20 E P 10 -3 E LIV = 100 E P 10 -3 E LIV = 100 E P E LIV = 400 E P E LIV = 400 E P 10 -1 10 0 10 1 10 2 10 -1 10 0 10 1 10 2 E ( TeV ) E ( TeV ) H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 16 / 23
LIV: Cosmic opacity LIV: Cosmic opacity The standard relation for optical depth τ γγ ( E γ , z s ) at the energy E γ and for a source at redshift z s is modified as (Fairbairn et al. 2014) � z s � smax ( z ) � ∞ c dz n ( ǫ, z ) [ s − m 2 γ c 4 ] σ γγ ( s ) ds τ γγ ( E γ , z s ) = 8E 2 H ( z )( 1 + z ) 3 ǫ 2 0 smin ( z ) γ ǫ min (3) γ c 4 and m 2 γ c 4 ≡ S E 3 where smin = 4m 2 e c 4 , smax = 4 ǫ E γ ( 1 + z ) + m 2 E LIV . 10 0 10 0 10 −2 10 -2 S = − 1 exp(− τ γγ ) S = + 1 10 −4 exp( − τ γγ ) 10 -4 z = 0.6 z = 0 . 6 10 −6 10 -6 standard standard E LIV = E P E LIV = E P 10 −8 10 -8 E LIV = 5 E P E LIV = 5 E P E LIV = 10 E P E LIV = 10 E P 10 −10 10 -10 10 0 10 1 10 0 10 1 E ( TeV ) E ( TeV ) H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 17 / 23
LIV: Cosmic opacity LIV and Void LIV and Void: Cosmic opacity Comparison between the impact of 10 typical voids size R = 100h − 1 Mpc and the effect of Lorentz Invariance Violation 10 0 10 0 10 -2 10 -2 10 -4 S = − 1 10 -4 S = + 1 exp( − τ γγ ) exp( − τ γγ ) z = 0 . 6 z = 0 . 6 standard standard 10 -6 10 -6 E LIV = E P E LIV = E P E LIV = 5 E P E LIV = 5 E P 10 -8 10 -8 E LIV = 10 E P E LIV = 10 E P void void 10 -10 10 -10 10 0 10 1 10 0 10 1 E ( TeV ) E ( TeV ) 10 0 E LIV = E P 10 -1 S = − 1 E LIV = 5 E P z = 0 . 6 E LIV = 10 E P Relative Opacity Deficit Relative Opacity deficit void void + ( E LIV = E P ) S = + 1 z = 0 . 6 10 -1 10 -2 E LIV = E P E LIV = 5 E P E LIV = 10 E P void void + ( E LIV = E P ) 10 0 10 1 10 0 10 1 E ( TeV ) E ( TeV ) H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 18 / 23
LIV: Compton scattering LIV: Compton scattering One of the most important fundamental high-energy radiation mechanisms is Compton scattering. In the leptonic Blazar models, the high-energy component is produced by Compton scattering. The question that could arise is, could the influence of the LIV effect on the Compton scattering process explain the spectral hardening of the VHE end of spectra of several Blazars? H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 19 / 23
LIV: Compton scattering LIV: Compton scattering Compton scattering is the process whereby photons gain or lose energy from collisions with electrons E γ i / c , − → E ei / c , − → E γ f / c , − → E ef / c , − → � � � � � � � � P γ i + P ei = P γ f + P ef , (4) Using energy-momentum conservation with the LIV-modified dispersion relation (1) we derive the scattered photon energy E f as a function of incoming photon energy E i and scattering angles θ � E 3 E 3 � 2 E γ i E γ f +2( E γ f − E γ i ) m e c 2 = S γ i γ f + E LIV E LIV (5) � � E γ i E γ f + 2 µ E γ i E γ f 1 − S − S . 2 E LIV 2 E LIV H. Abdalla & M. B¨ ottcher (CSR, NWU) Fundamental physics and cosmology November 28, 2019 20 / 23
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