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CHALLENGES OF RELATIVISTIC ASTROPHYSICS Reuven Opher (Univ. of Sao Paulo/Cornell. Univ.)

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WHAT ARE THE BIGGEST PROBLEMS THAT NEED TO BE SOLVED IN RELATIVISTIC ASTROPHYSICS? IF YOU WERE ASKED TO NAME SIX, WHAT WOULD THEY BE?

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HERE ARE MINE!

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SUBJECTS (1) Dark Energy (2) Dark Matter (3) Highest Energy Cosmic Rays (4) Primordial Universe (5) New Physics at > 2 x Nuclear Density (6) Gamma Ray Bursts

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CHALLENGE (1): WHAT IS CAUSING THE RECENT ACCELERATION OF THE UNIVERSE? Gravity always decelerates, so this so-called “Dark Energy” that solves the acceleration problem, acts like “Anti-Gravity”. WHAT IS IT?

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CHALLENGE (2a): WHERE ARE THE PREDICTED ABUNDANT SMALL DARK MATTER HALOS ? The standard theory predicts that there are thousands of small massive non-baryonic “Dark Matter” halos in the Milky Way. WHERE ARE THEY?

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CHALLENGE (2b): WHERE ARE THE PREDICTED SMALL DARK MATTER HALOS WITH CUSPS? The standard theory predicts that in the small non-baryonic “Dark Matter” halos there are cusps. (In a cusp the density goes to infinity as the radius goes to zero.) WHERE ARE THESE CUSPS?

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CHALLENGE (3): WHAT IS THE MOST POWERFUL COSMIC ACCELERATOR? It accelerates particles to energies ten million times higher than the most powerful accelerator on Earth. WHAT IS IT?

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CHALLENGE (4): WHAT IS THE SELF- CONSISTENT THEORY OF PRIMORDIAL INFLATION? There is considerable observational evidence indicating that a primordial inflation period occurred. BUT: (1) AT WHAT ENERGY? (2) WHAT NEW PHYSICS IS INVOLVED? (3) IS IT SUFFICIENTLY CLOSE TO THE PLANCK ENERGY THAT QUANTUM GRAVITY EFFECTS ARE IMPORTANT? (4) ARE THE POTENTIALS USED UNREASONABLE?

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CHALLENGE (5): WHAT IS THE BEST WAY TO DETECT THE NEW PHYSICS At > 2 x NUCLEAR DENSITY IN NEUTRON STARS?

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CHALLENGE (6): WHAT IS THE ENERGY SOURCE OF GAMMA RAY BURSTS (GRBs)? When a GRB explodes it is brighter than the whole universe. It emits in one second what the Sun emits in its lifetime. WHAT IS ITS ENERGY SOURCE?

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(1) DARK ENERGY

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CHALLENGE (1): WHAT IS CAUSING THE RECENT ACCELERATION OF THE UNIVERSE?

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IS VACUUM ENERGY THE SOURCE? All existing data is consistent with the Dark Energy being a small vacuum energy density. The main problem is that theory predicts that the vacuum energy density is 120 orders of magnitude bigger than what is observed.

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ALTERNATE THEORIES INSTEAD OF VACUUM ENERGY Cosmic Axion Tracker Field Exponential Potential Spintessence K-Essence Ghost Condensate Thawing Model Freezing Model Phantom Energy f(R) Theories Scalar-Tensor Theory Palatini Formalism Brane World Gravity Modified Gravity (R.R. Caldwell and M. Kamionkowski, Ann. Rev. Nucl. Sci. 59, 397 (2009)) (J.A. Frieman, M.S. Turner and D. Huterer, Ann. Rev. Astron. Astrophys. 46, 385 (2008)) Because of the large number of theories, we all agree that no final answer has been found.

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MY BEST CANDIDATES FOR DARK ENERGY (1)THERE IS A PHYSICAL REASON WHY THE VACUUM ENERGY IS SMALL. (2) DARK ENERGY IS A MANIFESTATION OF THE GROWTH OF INHOMOGENEITY IN THE UNIVERSE.

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WHAT IS NEEDED At a given redshift, due to Dark Energy, objects are more distant, older and contains more volume. We need, then, many more precise observations as a function of redshift of: (1) STANDARD CANDLES (e.g., Supernovae Ia), (2) STANDARD RULERS(e.g. Baryon Acoustic Oscillations), (3) STANDARD DENSITIES (e.g. Clusters of galaxies, galaxies and Dark Matter Halos)(observed with gravitational lensing)).

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(2) DARK MATTER

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CHALLENGE (2a): WHERE ARE THE PREDICTED ABUNDANT SMALL DARK MATTER HALOS ? The standard theory predicts that there are thousands of small massive non- baryonic “Dark Matter” halos in the Milky Way. WHERE ARE THEY?

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DARK MATTER HALOS WITHOUT STARS Primordial Magnetic Fields strongly influence the baryon gas fraction of a Dark Matter halo and can prevent small Dark Matter halos to have visible stars. (R. De Souza, L.F. Rodrigues and R. Opher, MNRAS 410, 2199 (2011))

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CHALLENGE (2b): WHERE ARE THE PREDICTED SMALL DARK MATTER HALOS WITH CUSPS? The standard theory predicts that there are thousands of small non-baryonic “Dark Matter” halos with cusp central densities in the Milky Way. WHERE ARE THE CUSPS?

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DARK MATTER HALOS WITHOUT PREDICTED CENTRAL CUSPS A single supernova explosion in a small Dark Matter halo can transform a cusp into a core (a finite density at the center). (R. De Souza, L.F. Rodrigues, E. Ishida and R. Opher, MNRAS 415, 2969 (2011)) (The supernova expels the baryonic matter which perturbs the gravitational potential sufficiently to transform the cusp into a core.)

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WHAT IS NEEDED (1) Gravitational lensing detection of small Dark Matter Halos that have no stars, to see if they exist. (2) Observing stellar orbits in small galaxies with Dark Matter halos to determine the gravitational potential and the central Dark Matter halo density, to see if it is a cusp or not.

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CHALLENGE (3): WHAT IS THE MOST POWERFUL COSMIC ACCELERATOR?

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THE HIGHEST ENERGY COSMIC RAYS The highest energy cosmic rays have energies eV, ten million times more energetic than the most energetic particles accelerated on earth (e.g., in the LHC in CERN, Switzerland). WHAT IS THE ACCELERATOR?

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GENERALLY ACCEPTED THEORY OF THE ACCELERATION OF COSMIC RAYS IN SHOCKS It is generally accepted that the acceleration of high energy cosmic rays, until at least eV, is First Order Fermi acceleration in a supernova shock. In this process, particles bounce back and forth between the downstream turbulence and the upstream Alfven waves that were produced by a streaming instability of the accelerated particles. Assuming that the shock velocity is higher than the Alfven velocity, the particles gain energy each time they bounce off the Alfven waves streaming into the shock.

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CONDITIONS FOR ACCELERATION The limit ~ eV is due to the radius of curvature of the accelerated particle ( proton) not being greater than the size of the supernova remnant in the ambient magnetic field ~ 5-10 microgauss. We can only reach ~ eV if : a) the Magnetic Field is amplified; and/or b) the acceleration region is bigger; and/or c) the accelerated particle is an iron nucleus and not a proton.

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ACCELERATION IN THE BIG POWERFUL ACTIVE GALACTIC NUCLEI (AGN) SHOCKS The highest energy cosmic rays, ~ eV, need to come from a distance < 75 Mpc. The few AGNs < 75 Mpc are not strongly correlated with the high energy cosmic rays eV. (K-H. Kampert et al., arXiv: )

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CREATION OF LARGE MAGNETIC FIELDS A very large random Magnetic Field, much greater than the ambient Magnetic Field ( microgauss), can be generated by the cosmic ray streaming instability in the precursor of supernova shocks. (A.R. Bell, MNRAS 353, 550 (2004)) THUS THE ACCELERATED PARTICLES NOT ONLY PRODUCE ALFVEN WAVES BUT ALSO CREATE LARGE MAGNETIC FIELDS.

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ACCELERATION IN SUPERNOVA SHOCKS WHERE THE MAGNETIC FIELD HAS BEEN AMPLIFIED The Magnetic Fields in young supernovae remnants are observed to be amplified to microgauss. (H.J. Volk, E.G. Berezhko and L.T. Ksenfontov, A&A 433, 229 (2005) Younger supernovae remnants have stronger fields.

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ACCELERATION OF IRON NUCLEI IN SUPERNOVA SHOCKS WHERE THE MAGNETIC FIELD HAS BEEN AMPLIFIED Iron nuclei can be accelerated to ~ eV in these amplified Magnetic Fields in supernova remnants. (V. Ptuskin, V. Zirakashvili and E-S. Seo, Ap. J. 718, 31 (2010)) A factor of ten, however, is missing to reach eV.

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ELIMINATING THE SHOCK IN FIRST ORDER FERMI ACCELERATION The First-Order Fermi accelerated particles can slow down the incoming matter into the shock, eliminating the shock entirely, and the source might be observed as just a smooth adiabatic compression. (G. Medina-Tanco and R. Opher, Astron. Ap. 240, 178 (1990) THESE COMPRESSIONS COULD BE AROUND GALAXIES AND CLUSTERS OF GALAXIES, WITH NO SHOCK BEING OBSERVED.

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WHAT IS NEEDED More data on the highest energy cosmic rays to localize and identify the sources and their nature (protons or iron nuclei).

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CHALLENGE (4): WHAT IS THE SELF-CONSISTENT THEORY OF PRIMORDIAL INFLATION?

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EVOLUTION OF THE UNIVERSE

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VARIOUS SUCCESSES OF ASSUMING A PRIMORDIAL INFLATION ERA (1) Non-causally connected regions today were causally connected in the past; (2) The universe has little curvature today (Kinetic Energy ~ Potential Energy); (3) The density perturbation spectrum is almost scale invarient ( independent of scale, they have the same amplitude when they enter the horizon); (4) Relics of gauge symmetry breaking are not observed (e.g. monopoles); (5) Almost Gaussian perturbations ( one part in a thousand); and (6) Perturbation modes began with the same phase (could have been random). BUT……

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ENERGY SCALE OF INFLATION The energy scale of inflation is predicted to be on the order of GeV, VERY MUCH HIGHER THAN THE ENERGIES OF THE STANDARD MODEL OF PARTICLE PHYSICS ~ 10 4 GeV. FROM 10 4 GeV to GeV there could easily be new physics.

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THE POPULAR CHAOTIC INFLATION MODEL The popular chaotic inflation model of a massive scalar field F with a mass m and a potential equal to m 2 F 2 /2, the mass needs to be m ~ 4 x GeV to satisfy observations. FOR AN EXPECTED INFLATION PERIOD ~ GeV, THE MASS IS EXTREMELY SMALL AND UNNATURAL.

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AMPLITUDE OF DENSITY FLUCTUATIONS In the standard model of Inflation, the amplitude of the density fluctuations at the Inflation Era is 3H 3 /V’, where H is the Hubble radius in the Inflation Era and V’ is the derivitive of the Inflation potential with respect to the field. THE THEORY DOES NOT GIVE THE VALUE OF V’ TO OBTAIN THE OBSERVED DENSITY FLUCTUATION ( ~ ).

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FLATNESS OF INFLATION POTENTIAL The present popular model of inflation requires a very flat slow-roll potential to obtain the observed density fluctuations, in which the potential, V ~ (10 16 GeV) 4, changes negligibly with a change in the field, DF ~ M pl ~ GeV or V/(DF) 4 < NO KNOWN PARTICLE HAS SUCH A FLAT POTENTIAL.

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INFLATION FROM QUANTUM GRAVITY Non-commutation of space and time and Lorentz Invariance Violation, indicated by quantum-gravity, can produce inflation. (U. Machado and R. Opher, Class. Quant. Grav. 29, , (2012)) (U. Machado and R. Opher, arXiv: )

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WHAT IS NEEDED (1)Detailed observations of Tensor Fluctuations. (2) Detailed observations of Gaussianity Fluctuations. (3) Investigation of Lorentz Invariance Violation. The Tensor Fluctuations give information on the energy and the fields and potentials of the inflation era. The Gaussinaity can give information on, for example, the possible existence of Cosmic Strings, predicted by String Theory.

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WHAT IS NEEDED Lorentz Invariance Violation can be investigated by measuring the velocity of high energy photons. (A deviation from the velocity of light as a function of E/E pl might be expected, where E is the energy of the photon and E pl is the Planck energy.)

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CHALLENGE (5): WHAT IS THE NEW PHYSICS At > 2 x NUCLEAR DENSITY IN NEUTRON STARS?

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NEW PHYSICS AT THE CENTER OF NEUTRON STARS Most models of dense matter predict that at the density > 2 times nuclear density in Neutron Stars, the formation of exotic matter takes place ( e.g., free quarks). (J.M. Lattimir and M. Prakash, Phys. Rep. 442, 109 (2007))

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SUPERFLUIDITY IN NEUTRON STARS The cooling Neutron Star, in the Cassiopeia A supernova remnant, gives evidence for superfluidity in the core. (P.S. Shternin et al., arXiv: )

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DETECTING THE NEW PHYSICS IN NEUTRON STARS Tidal Polarizability, dependent on the new physics at the center of Neutron Stars, is measureable in the Gravitational Wave signal of merging Neutron Star binaries. (T. Damour et. Al., arXiv: E.E. Flanagan and T. Hinderer, Phys. Rev. D77, (2008) J.E. Vines and E.E. Flanagan, arXiv: , C. Messenger and J. Read, arXiv: )

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WHAT IS NEEDED (1) GRAVITATIONAL WAVE DETECTION Advanced LIGO is expected to detect ~ 40 ( with an uncertainty of ) binary neutron star merger events per year and could detect the Tidal Polarizability. ( J. Abadie et. al., Class. Quant. Grav. 27, (2010)) (2) DETECTION OF M vs R OF NEUTRON STARS.

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CHALLENGE (6): WHAT IS THE ENERGY SOURCE OF GAMMA RAY BURSTS (GRBs)?

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ARTIST VISION OF A GRB AND ITS RELATIVISTIC JETS

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OBSERVING RELATIVISTIC JETS OF GRBs Evidence of the Relativistic Jets of GRBs is the observed achromatic break from radio to X- rays. (Predicted to occur when the Lorentz factor becomes equal to one over the opening angle). (S.B. Cenko et al., Ap. J. 711, 641 (2010))

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ENERGY SOURCE: A SPINNING BLACK HOLE IN A MAGNETIC FIELD? A Relativistic Jet is produced with a maximum power: P ~ B 2 M 2 ergs/s where B is in units of G and M is in solar mass units. (J.C.M. McKinney, A. Tchekhovsky and R. D. Blandford, MNRAS 423, 3083 (2012)) (R.D. Blandford and R.L. Znajek, MNRAS 179, 433 (1977))

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ENERGY SOURCE: A SPINNING BLACK HOLE IN A MAGNETIC FIELD? The G could come from the collapse of a Neutron Star (Magnetar) with a field > G. The G could also come from a spinning Neutron Star which amplifies its Magnetic Field by the dynamo effect during collapse.

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ENERGY SOURCE: A SPINNING BLACK HOLE IN A MAGNETIC FIELD? FOR M SEVERAL SOLAR MASSES AND B ~ G, THE POWER OBTAINED IS COMPARABLE TO THAT OF GRBs. THE SPINNING BLACK HOLE IN THE MAGNETIC FIELD COULD COME FROM THE COLLAPSE OF A MASSIVE STAR.

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WHAT IS NEEDED MORE DETAILED SPECTROSCOPIC DATA OF GRBs AT SHORT TIMES.

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CONCLUSION WE MIGHT NOT ANSWER THESE CHALLENGES IN THE NEXT TEN YEARS, BUT WE CERTAINLY WILL ENJOY OURSELVES TRYING!!

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