Talk - IIT Kanpur

download report

Transcript Talk - IIT Kanpur

Is there a preferred direction in the Universe
P. Jain, IIT Kanpur
There appear to be several indications of the
existence of a preferred direction in the Universe
(or a breakdown of isotropy)
Optical polarizations from distant AGNs
Radio polarizations from distant AGNs
Low order multipoles of CMBR
On distance scales of less than 100 Mpc the Universe is not
homogeneous and isotropic
Most galaxies in our vicinity lie in a plane (the
supercluster plane) which is approximately
perpendicular to the galactic plane.
The Virgo cluster sits at the center of this disc like
On larger distance scales the universe appears isotropic
CMBR is isotropic to a very good approximation
What does CMBR fluctuations imply about the
isotropy of the universe?
T ( ,  )    almYlm ( ,  )
l 2 ml
Cl  l (l  1) alm a
TT Cross Power Spectrum
The power is low at small l (quadrupole l=2)
The probability for such a low quadrupole to occur by a
random fluctuation is 5%
Oliveira-Costa et al 2003
The Octopole is not small but very planar
Surprisingly the Octopole and Quadrupole appear
to be aligned with one another with the chance
probability =1/62
All the hot and cold spots of the Quadrupole and Octopole lie
in a plane, inclined at approx 30o to galactic plane
Oliveira-Costa et al 2003
Extraction of Preferred Axis
Imagine dT as a wave function y
Maximize the angular momentum dispersion
Oliveira-Costa et al 2003
Extraction of Preferred Axis
Alternatively Define
k = 1 …3, m = -l … l
Preferred frame eka is obtained by Singular Value Decomposition
ea represent 3 orthogonal axes in space
The preferred axes is the one with largest eigenvalue La
Ralston, Jain 2003
The preferred axis for both
points roughly in the direction
(l,b)  (-110o,60o) in Virgo Constellation
Hence WMAP data suggests the existence of a preferred
direction (pointing towards Virgo)
We (Ralston and Jain, 2003) show that there is considerable
more evidence for this preferred direction
CMBR dipole
Anisotropy in radio polarizations from distant AGNs
Two point correlations in optical polarizations from AGNs
Also point in this direction
CMBR Dipole
The dipole is assumed to arise due to the local (peculiar)
motion of the milky way, arising due to local in-homogeneities
The observed dipole also points in the direction of Virgo
Physical Explanations
Many explanations have been proposed for the
anomalous behavior of the low order harmonics
Non trivial topology
(Luminet, Weeks, Riazuelo, Leboucq
and Uzan, 2003)
Anisotropic Universe due to background magnetic field
(Berera, Buniy and Kephart, 2003)
Sunyaev Zeldovich effect due to local supercluster
(Abramo and Sodre, 2003)
A satisfactory explanation of the observations is still lacking
Anisotropy in Radio Polarizations
Radio Polarizations from distant AGNs show
a dipole anisotropy
 Offset angle b  c  y
 (l2 )  c  (RM) l2
 RM : Faraday Rotation
 c = IPA (Polarization at
b shows a Dipole
Birch 1982
Jain, Ralston, 1999
Jain, Sarala, 2003
b = polarization
offset angle
Likelihood Analysis  The Anisotropy
is significant at 1% in full (332 sources) data set and
0.06% after making a cut in RM (265 sources)
|RM - <RM>| > 6 rad/m 2
<RM> = 6 rad/m 2
Distribution of RM
The cut eliminates the data near the central peak
The radio dipole axis also points towards Virgo
Jain and Ralston, 1999
Anisotropy in Extragalactic Radio Polarizations
beta = polarization offset angle
Using the cut |RM - <RM>| > 6 rad/m2
Anisotropy in Extragalactic Radio Polarizations
Using the cut |RM - <RM>| > 6 rad/m2
Galactic Coordinates
Anisotropy in Extragalactic Radio Polarizations
A generalized (RM dependent) statistic indicates that the
entire data set shows dipole anisotropy
Equatorial Coordinates
Hutsemékers Effect
Optical Polarizations of QSOs appear to be locally
aligned with one another. (Hutsemékers, 1998)
A very strong alignment is seen in the direction of Virgo cluster
Hutsemékers Effect
Equatorial Coordinates
Statistical Analysis
 A measure of alignment is obtained by comparing
polarization angles in a local neighborhood
The polarizations at different
angular positions are
compared by making a parallel
transport along the great circle
joining the two points
k, k=1…nv are the
polarizations of the nv
nearest neighbours of the
source i
 ki = contribution
due to parallel
•Maximizing di() with respect to  gives a measure
of alignment Di and the mean angle 
Jain, Narain and
Sarala, 2003
Alignment Results
We find a strong signal of redshift dependent
alignment in a data sample of 213 quasars
The strongest signal is seen in
Low polarization sample (p < 2%)
High redshift sample (z > 1)
Significance Level
Significance Level
Significance Level
Large redshifts (z > 1) show alignment
over the entire sky
Alignment Statistic (z > 1)
Alignment Results
Strongest correlation is seen at low polarizations ( p < 2%)
at distance scales of order Gpc
Large redshifts z > 1 show alignment over the entire sky
Jain, Narain and Sarala, 2003
Preferred Axis
Two point correlation
Define the correlation tensor
is the matrix of sky locations
S is a unit matrix for
an isotropic
uncorrelated sample
Preferred Axis
Optical axis is the eigenvector of S with maximum
Alignment Statistic
Preferred axis points towards (or opposite) to Virgo
Degree of Polarization < 2%
Prob. for pairwise coincidences
dipole quad
Ralston and Jain, 2003
Physical Explanation
A satisfactory explanation of the observations is so far
not available
It is possible that the universe may not be isotropic
even at cosmological scales. One should then explore
generalization of the FRW metric
the large scale anisotropies could arise due to :
• propagation in a large scale anisotropic medium
• The active galactic nuclei may be intrinsically
correlated on very large distance scales. Similarly the
CMBR quadrupole and octopole may be aligned at the
Physical Explanation
Alternatively the anisotropies could arise due to the
local inhomogeneous distribution of matter
This possibility cannot be ruled out for the CMBR and
radio anisotropies but is unlikely to account for the
large scale optical correlations, which is a redshift
dependent effect
Physical Explanation
The observations may also represent a fundamental
violation of Lorentz invariance
Lorentz invariance has been observed to be a very
good symmetry of nature.
Theoretically we expect that it is violated due to
quantum gravity effects.
We expect violations of order
(Jain, Ralston 2005)
Light Scalars
We have been exploring the possibility that
the effects may be explained by a light scalar
(or pseudoscalar)
Very light mass pseudoscalars (or scalars) are
predicted by many theories beyond the Standard Model
 Axion (Peccei-Quinn)
 Supergravity
 String theory
A very light scalar or pseudoscalar may also be required to
explain dark energy
A common model for dark energy is a scalar field slowly rolling
towards its true vacuum
Coupling to Photons
• Such a scalar field will have an effective coupling to
• It does not matter whether  is a scalar or a
• If  is a scalar then this interaction breaks parity but
parity is not a symmetry of nature.
We are basically interested in electromagnetic waves
propagating over astrophysical or cosmological
distances in the presence of a background magnetic
As the EM wave passes through large scale background
magnetic field, photons (polarized parallel to transverse
magnetic field) mix with pseudoscalars
This leads to reduced intensity of wave if the
incident pseudoscalar flux is assumed negligible
The reduction in intensity due to pseudoscalar photon
mixing in the local supercluster magnetic field may
explain the anomalous CMBR quadrupole and
(Jain and Saha, work in progress)
This may also be partially responsible for dimming of
distant supernovae
(Csaki, Kaloper and Terning, 2002)
The wave gets polarized perpendicular to the
transverse magnetic field since
only the component parallel to the background
magnetic field mixes with pseudoscalars
This may explain the optical alignment
However we require magnetic field coherent
on cosmologically large distance scales
Limit on the coupling
For the invisible axion the current limit on the PecceiQuinn symmetry breaking scale is 109 GeV,
Mass < 0.01 eV
This particle gives very little contribution to mixing
for galactic or intergalactic propagation.
It may contribute in regions of strong magnetic fields
and plasma density.
We are interested in a pseudoscalar whose mass
may be much smaller
g < 6 x 10-11 /GeV
if we assume that the mass is negligible
We will assume that its mass is smaller or
comparable to the plasma density of the medium
Typical scales
Background magnetic field for the case of Virgo
supercluster is roughly 0.1 G,
distance 1-10 Mpc
Plasma density  10-6 cm-3
For intergalactic propagation it may be reasonable to
assume many domains of size 1 Mpc and
B ≈ 0.005 G
Plasma density  10- 8 cm-3
We are interested in the frequency regime from radio to
 = 10- 5 – 1 eV
Pseudoscalar Photon mixing
We have considered this mixing in great detail so that
it can be tested in future observations
Uniform background
Turbulent background
(Jain, Panda, Sarala, 2002)
Slowly varying background (background magnetic
field direction fixed)
(Das, Jain, Ralston, Saha, 2004)
Slowly varying background with the direction of
magnetic field varying with distance.
(Das, Jain, Ralston, Saha, 2004)
Degree of Polarization as a function of l (or )
Uniform Background
Stokes Parameters as a function of  (we set I = 1)
Uniform Background
At source Q=0, U=0.4, V = 0.1
Degree of Polarization as a function of the distance of
The wave is unpolarized at source
Stokes parameter V as a function of Q for several
different parameters
(varying background magnetic field direction)
Background pseudoscalar field
A background pseudoscalar (scalar) field
also leads to a rotation of the
polarization of the wave
Rotation in polarization =ggg ( )
  change in the pseudoscalar field along the path
Possible Explanation of Radio
An anisotropically distributed background pseudoscalar field
 of sufficiently large strength can explain the observations
Pseudoscalar field at
To account for the RM dependence
Concluding Remarks
There appears to be considerable evidence that there is a
preferred direction in the Universe pointing towards Virgo
However the CMBR observations may also be explained in
terms of some local distortion of microwave photons due to
The physical mechanism responsible for this is not known so far.
We are considering the possibility that it may be explained due
to conversion of photons into pseudoscalars due to propagation
through local supercluster magnetic field.
Concluding Remarks
Radio anisotropy may also arise due to some local unknown
However it is difficult to find a physical mechanism which
can accomplish this.
An anisotropically distributed background pseudoscalar
field may explain this effect.
It is not possible to attribute optical alignment to a local effect
since it is intrisically redshift dependent.
We can explain this in terms of pseudoscalar photon mixing
provided there exist magnetic fields coherent on cosmological
distance scales
Future observations will hopefully clarify the situation