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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
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On distance scales of less than 100 Mpc the Universe is not homogeneous and isotropic The Virgo cluster sits at the center of this disc like structure Most galaxies in our vicinity lie in a plane (the supergalactic plane) which is approximately perpendicular to the galactic plane. On larger distance scales the universe appears isotropic
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CMBR 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 What does CMBR imply about the isotropy of the universe?
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T Temperature Fluctuations about the mean Two Point Correlation Function Statistical isotropy implies
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If we assume that T (and a lm ) are Gaussian random variables (with 0 mean) then all the statistical information is contained in the two point correlation function or
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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
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Quadrupole Octopole Cleaned Map Oliveira-Costa et al 2003 All the hot and cold spots of the Quadrupole and Octopole lie in a plane, inclined at approx 30 o to galactic plane
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Extraction of Preferred Axis Imagine T as a wave function Maximize the angular momentum dispersion Oliveira-Costa et al 2003
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Extraction of Preferred Axis k = 1 …3, m = - l … l Preferred frame e k is obtained by Singular Value Decomposition e represent 3 orthogonal axes in space The preferred axes is the one with largest eigenvalue Ralston, Jain 2003 Alternatively Define
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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
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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
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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
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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 = polarization offset angle
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Distribution of RM The cut eliminates the data near the central peak
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The radio dipole axis also points towards Virgo Jain and Ralston, 1999
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Anisotropy in Extragalactic Radio Polarizations beta = polarization offset angle Using the cut |RM - <RM>| > 6 rad/m 2
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Anisotropy in Extragalactic Radio Polarizations Using the cut |RM - <RM>| > 6 rad/m 2 Galactic Coordinates
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Equatorial Coordinates Anisotropy in Extragalactic Radio Polarizations A generalized (RM dependent) statistic indicates that the entire data set shows dipole anisotropy
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Possible Explanation An anisotropically distributed background pseudoscalar field of sufficiently large strength can explain the observations To account for the RM dependence
Rotation in polarization =g (
change in the pseudoscalar field along the path
Pseudoscalar field at source g < 10 -11 GeV -1
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Hutsem é kers Effect Optical Polarizations of QSOs appear to be locally aligned with one another. (Hutsem é kers, 1998) A very strong alignment is seen in the direction of Virgo cluster 1<z<2.3
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Hutsem é kers Effect Equatorial Coordinates 1<z<2.3
Maximizing d i ( ) with respect to gives a measure of alignment D i and the mean angle
k , k=1…n v are the polarizations of the n v nearest neighbours of the source i k i = contribution due to parallel transport Statistic Jain, Narain and Sarala, 2003
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Alignment Results We find a strong signal of redshift dependent alignment in a data sample of 213 quasars
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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
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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
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Preferred Axis Two point correlation Define the correlation tensor Define where is the matrix of sky locations S is a unit matrix for an isotropic uncorrelated sample
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Preferred Axis Optical axis is the eigenvector of S with maximum eigenvalue
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Alignment Statistic Preferred axis points towards (or opposite) to Virgo Degree of Polarization < 2%
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Prob. for pairwise coincidences Ralston and Jain, 2003 0.008 radio 0.026 0.059 octo 0.004 0.023 0.015 quad 0.024 0.042 0.061 0.020 dipole optical radio octo quad dipole
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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. However it is not possible to attribute optical alignment to a local effect Future observations will hopefully clarify the situation Radio anisotropy may also arise due to some local unknown effect
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Anisotropy in Extragalactic Radio Polarizations sin(2 ) < 0 + sin(2 ) > 0 Using the cut |RM - <RM>| > 6 rad/m 2
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Radiation propagating over cosmological distances also probes isotropy of the Universe
CMBR
Radiation from distant AGNs
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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 On Large scale it is assumed that Universe is Isotropic and Homogeneous
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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
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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 CDM model with Dodelson (2003) Maddox et al (1990)
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