CHALLENGES OF RELATIVISTIC
ASTROPHYSICS
Reuven Opher
(Univ. of Sao Paulo/Cornell. Univ.)
WHAT ARE THE BIGGEST
PROBLEMS THAT NEED TO BE
SOLVED IN RELATIVISTIC
ASTROPHYSICS?
IF YOU WERE ASKED TO NAME
SIX, WHAT WOULD THEY BE?
HERE ARE MINE!
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
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?
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?
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?
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?
CHALLENGE (4): WHAT IS THE SELFCONSISTENT 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?
CHALLENGE (5): WHAT IS THE BEST
WAY TO DETECT THE NEW PHYSICS
At > 2 x NUCLEAR DENSITY IN
NEUTRON STARS?
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?
(1) DARK ENERGY
CHALLENGE (1): WHAT IS CAUSING
THE RECENT ACCELERATION OF THE
UNIVERSE?
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.
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.
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.
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)).
(2) DARK MATTER
CHALLENGE (2a): WHERE ARE THE
PREDICTED ABUNDANT SMALL DARK
MATTER HALOS ?
The standard theory predicts that there
are thousands of small massive nonbaryonic “Dark Matter” halos in the
Milky Way.
WHERE ARE THEY?
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))
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?
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.)
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.
CHALLENGE (3): WHAT IS THE
MOST POWERFUL COSMIC
ACCELERATOR?
THE HIGHEST ENERGY COSMIC RAYS
The highest energy cosmic rays have energies
1020 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?
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 1015 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.
CONDITIONS FOR ACCELERATION
The limit ~ 1015 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 ~ 1020 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.
ACCELERATION IN THE BIG POWERFUL
ACTIVE GALACTIC NUCLEI (AGN)
SHOCKS
The highest energy cosmic rays, ~ 1020 eV,
need to come from a distance < 75 Mpc.
The few AGNs < 75 Mpc are not strongly
correlated with the high energy cosmic
rays 1020 eV.
(K-H. Kampert et al., arXiv: 1207.4823)
CREATION OF LARGE MAGNETIC
FIELDS
A very large random Magnetic Field, much
greater than the ambient Magnetic Field ( 510 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.
ACCELERATION IN SUPERNOVA
SHOCKS WHERE THE MAGNETIC FIELD
HAS BEEN AMPLIFIED
The Magnetic Fields in young
supernovae remnants are observed to
be amplified to 150-500 microgauss.
(H.J. Volk, E.G. Berezhko and L.T. Ksenfontov,
A&A 433, 229 (2005)
Younger supernovae remnants have stronger
fields.
ACCELERATION OF IRON NUCLEI IN
SUPERNOVA SHOCKS WHERE THE
MAGNETIC FIELD HAS BEEN AMPLIFIED
Iron nuclei can be accelerated to ~ 1019 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
1020 eV.
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.
WHAT IS NEEDED
More data on the highest energy cosmic
rays to localize and identify the sources
and their nature (protons or iron nuclei).
CHALLENGE (4): WHAT IS
THE SELF-CONSISTENT
THEORY OF PRIMORDIAL
INFLATION?
EVOLUTION OF THE UNIVERSE
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……
ENERGY SCALE OF INFLATION
The energy scale of inflation is
predicted to be on the order of 1016
GeV, VERY MUCH HIGHER THAN THE ENERGIES
OF THE STANDARD MODEL OF PARTICLE
PHYSICS ~ 104 GeV.
FROM 104 GeV to 1016 GeV there could
easily be new physics.
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 m2F2/2, the mass needs to
be m ~ 4 x 1012GeV to satisfy observations.
FOR AN EXPECTED INFLATION PERIOD ~ 1016GeV, THE
MASS IS EXTREMELY SMALL AND UNNATURAL.
AMPLITUDE OF DENSITY
FLUCTUATIONS
In the standard model of Inflation, the amplitude
of the density fluctuations at the Inflation Era is
3H3/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
( ~ 10-5).
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 ~
(1016 GeV)4, changes negligibly with a change in
the field, DF ~ Mpl ~ 1019 GeV or
V/(DF)4 < 10-12 .
NO KNOWN PARTICLE HAS SUCH A FLAT
POTENTIAL.
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,
065003, (2012))
(U. Machado and R. Opher, arXiv:1211.6478)
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.
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/Epl might be expected, where E is
the energy of the photon and Epl is the Planck
energy.)
CHALLENGE (5): WHAT IS THE NEW
PHYSICS At > 2 x NUCLEAR DENSITY IN
NEUTRON STARS?
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))
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:1012.0045)
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:1203.4352
E.E. Flanagan and T. Hinderer, Phys. Rev. D77, 021502 (2008)
J.E. Vines and E.E. Flanagan, arXiv:1009.4919,
C. Messenger and J. Read, arXiv:1107.5725)
WHAT IS NEEDED
(1) GRAVITATIONAL WAVE DETECTION
Advanced LIGO is expected to detect ~ 40 ( with
an uncertainty of 0.4-400) binary neutron star
merger events per year and could detect the
Tidal Polarizability.
( J. Abadie et. al., Class. Quant. Grav. 27, 17001 (2010))
(2) DETECTION OF M vs R OF NEUTRON
STARS.
CHALLENGE (6): WHAT IS THE ENERGY
SOURCE OF GAMMA RAY BURSTS
(GRBs)?
ARTIST VISION OF A GRB AND ITS
RELATIVISTIC JETS
OBSERVING RELATIVISTIC JETS OF
GRBs
Evidence of the Relativistic Jets of GRBs is the
observed achromatic break from radio to Xrays.
(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))
ENERGY SOURCE: A SPINNING BLACK
HOLE IN A MAGNETIC FIELD?
A Relativistic Jet is produced with a maximum
power:
P ~ 1051 B2 M2 ergs/s
where B is in units of 1015G 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))
ENERGY SOURCE: A SPINNING BLACK
HOLE IN A MAGNETIC FIELD?
The 1015G could come from the collapse of a
Neutron Star (Magnetar) with a field > 1013G.
The 1015G could also come from a spinning
Neutron Star which amplifies its Magnetic Field
by the dynamo effect during collapse.
ENERGY SOURCE: A SPINNING BLACK
HOLE IN A MAGNETIC FIELD?
FOR M SEVERAL SOLAR MASSES AND B ~ 1015G,
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.
WHAT IS NEEDED
MORE DETAILED SPECTROSCOPIC
DATA OF GRBs AT SHORT TIMES.
CONCLUSION
WE MIGHT NOT ANSWER THESE
CHALLENGES IN THE NEXT TEN YEARS,
BUT
WE CERTAINLY WILL ENJOY
OURSELVES TRYING!!