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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

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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 supergalactic plane) which is approximately perpendicular to the galactic plane.

The Virgo cluster sits at the center of this disc like structure

On larger distance scales the universe appears isotropic


What does CMBR imply about the isotropy of the universe?

WMAP released very high resolution data in march 2003

Total number of pixels = 512 x 512 x 12

The data is available at 5 frequencies

There is considerable contamination from foreground emissions which complicate the interpretation of data


WMAP multi-frequency maps

Ka band 33 GHz

K band 23 GHz

Q band 41 GHz

W band 94 GHz

V band 61 GHz


DT(q,f) = Temperature Fluctuations about the mean

Two Point Correlation Function

Statistical isotropy implies


If we assume that DT (and alm) are Gaussian random variables (with 0 mean)then all the statistical information is contained in the two point correlation function



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


Cleaned Map



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
Extraction of Preferred Axis

Imagine dT as a wave function y

Maximize the angular momentum dispersion

Oliveira-Costa et al 2003

extraction of preferred axis1
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

  • Quadrupole


  • Octopole

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
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
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
  • (Berera, Buniy and Kephart, 2003)
  • Sunyaev Zeldovich effect due to local supercluster

(Abramo and Sodre, 2003)

anisotropy in radio polarizations
Anisotropy in Radio Polarizations

Radio Polarizations from distant AGNs show a dipole anisotropy

  • Offset angle b = c - y
  • q(l2 ) = c + (RM) l2
    • RM : Faraday Rotation Measure
    • c = IPA (Polarization at source)

b shows a Dipole ANISOTROPY

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


<RM> = 6 rad/m

distribution of rm
Distribution of RM

The cut eliminates the data near the central peak


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

possible explanation
Possible Explanation

An anisotropically distributed background pseudoscalar field f of sufficiently large strength can explain the observations

Pseudoscalar field at source

To account for the RM dependence

  • Rotation in polarization =gfgg (Df)
  • f = change in the pseudoscalar field along the path

gfgg < 10 -11 GeV-1



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




Equatorial Coordinates

statistical analysis
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


qk, k=1…nv are the polarizations of the nv nearest neighbours of the source i

D ki = contribution due to parallel transport

  • Maximizing di(q) with respect to q gives a measure of alignment Diand the mean angleq


Jain, Narain and Sarala, 2003

alignment results
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 level2
Significance Level

Large redshifts (z > 1) show alignment over the entire sky

alignment results1
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

possible explanation1
Possible Explanation

Optical Alignment can also be explained by a pseudoscalar field.

As the EM wave passes through large scale magnetic field, photons (polarized parallel to transverse magnetic field) decay into pseudoscalars

The wave gets polarized perpendicular to the transverse magnetic field

But we require magnetic field on cosmologically large distance scales

Jain, Panda and Sarala, 2002

preferred axis
Preferred Axis

Two point correlation

Define the correlation tensor



S is a unit matrix for an isotropic uncorrelated sample

is the matrix of sky locations

preferred axis1
Preferred Axis

Optical axis is the eigenvector of S with maximum eigenvalue

alignment statistic
Alignment Statistic

Preferred axis points towards (or opposite) to Virgo

Degree of Polarization < 2%

prob for pairwise coincidences
Prob. for pairwise coincidences

Ralston and Jain, 2003

concluding remarks
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 supercluster.

The physical mechanism responsible for this is not known so far.

Radio anisotropy may also arise due to some local unknown effect

However it is not possible to attribute optical alignment to a local effect

Future observations will hopefully clarify the situation


Anisotropy in Extragalactic Radio Polarizations

sin(2b) < 0 +

sin(2b) > 0 

Using the cut |RM - <RM>| > 6 rad/m2


Radiation propagating over cosmological distances also probes isotropy of the Universe

  • CMBR
  • Radiation from distant AGNs

On Large scale it is assumed that Universe is Isotropic and Homogeneous

The 3-dim space appears the same in all directions and at all locations

One way to test for isotropy and homogeneity is by observing the density of matter (galaxies) in different directions and positions

Angular correlation function

or 3-D correlation function


APM Survey

100 degrees by 50 degrees around the South Galactic Pole

Intensities scaled to the number of galaxies

blue, green and red for bright, medium and faint galaxies


The APM survey has about 5 million galaxies

It gives an accurate measure of the angular two point correlation function to about 10 degrees

The results agree reasonably well with the LCDM model with

WL = 0.7

Dodelson (2003)

Maddox et al (1990)