Recent announcements that the Cosmic Microwave Background (CMB) - the faint
remnant of the Big Bang - is polarized have caused a stir among cosmologists.
Such polarization has long been predicted but could not be detected, until the
recent DASI and WMAP detections. The polarization is a signature of the motion
of the primordial seeds that led to the galaxies and large-scale structure of
the Universe observed today. Earlier measurements of tiny temperature
fluctuations in the CMB have been used to infer the sizes of the primordial
seeds. The polarization signature should help to determine the fundamental
parameters of cosmology and to explain how large-scale structure arose
[Highwire abstract].Comment: Perspective: Astronomy. See summary at
  http://www.sciencemag.org/cgi/content/summary/299/5611/1333 (need sign up) or
  full text (free) at http://www.iafe.uba.ar/relatividad/gangui

Gangui, Alejandro

Science

English

arXiv

Recent announcements that the cosmic microwave background (CMB)-the faint remnant of the big bang-is polarized have caused a stir among cosmologists. As Gangui explains in his Perspective, such a polarization has long been predicted but could not be detected. The polarization is a signature of the motion of the primordial seeds that led to the galaxies and large-scale structure in the universe observed today. Earlier measurements of tiny temperature fluctuations in the CMB have been used to infer the sizes of the primordial seeds. The polarization signature should help to determine the fundamental parameters of cosmology and to explain how large-scale structure arose.Fil: Gangui, Alejandro. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin

CONICET Digital

arXiv:astro-ph/0303048v1  3 Mar 2003A Preposterous Universe∗Alejandro Gangui†For centuries, astronomers have wondered about the mechanisms behind the formationof galaxies and large-scale structures. In the second half of the 20th century, cosmologistsrealized that the witness of it all was a hot bath of light, now cooled to a few degrees Celsiusabove absolute zero, which is the afterglow of the big bang. The sky-pervading cosmicmicrowave background (CMB) radiation [1] was released just before the epoch when matterbegun getting structured. Some ten years ago, the tiny variations discovered in its effectivetemperature [2] told us about the size of the primordial seeds leading to the nascent galaxies,after eons of gravitational evolution.Now, another piece of evidence fits in the right place and shows us the way these pri-mordial seeds were actually moving some 400,000 years after the bang, when the CMBdecoupled from non-relativistic matter. With a radio telescope at the South Pole, scientistsfrom the DASI collaboration [3,4] have recently measured the minute level of orientation, orpolarization, that these microwaves received when they emerged from the seething plasma– a signal that only the peculiar dynamics of the seeds present at that epoch can generate[5,6].Most light around us is unpolarized. Its many individual waves oscillate in differentplanes as it propagates. But unpolarized light becomes polarized whenever it is scatteredor reflected, as in sunglasses or in the surface of a lake. In these cases, most of the intensityof the scattered light is concentrated in one plane along the line of propagation, resulting in∗Perspective: Astronomy, published in Science 299, 1333-1334 (2003) (28 February 2003).†The author is at IAFE, the Institute for Astronomy and Space Physics (CONICET), and atthe Physics Department, University of Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires,Argentina. E-mail: gangui@df.uba.ar1linearly polarized light.Early on, when the universe was hot enough, matter was ionized and the free electrondensity was so high that photons could not propagate freely without colliding with electrons.But the universe was expanding and the ambient temperature decreasing, and so the ener-getic collisions became less frequent. The relatively low-energy photons that ensued couldnot destroy the increasing number of neutral particles (essentially hydrogen and helium) thatbegan to combine. Cosmologists refer to this period as “recombination”, and soon afterwardthe CMB was released free, making its last scattering upon matter. According to theory, itis at this precise time, nearly 14 billion years ago, that the CMB became polarized.CMB polarization was first discussed thirty-five years ago by Martin Rees [7]. However,there was no evidence of its existence until the DASI detection late last year. Polariza-tion is an important probe for cosmological models and for the more recent history of ournearby universe. It arises from the interaction of the cosmic background radiation with freeelectrons; hence, CMB polarization can only be produced at the time of its last scattering1.Unlike temperature fluctuations, polarization is largely unaffected by inhomogeneities in thegrowing distribution of matter after recombination.To understand how the CMB becomes polarized, two points should be clear. First, theoscillating electric field of the incoming radiation will push the electron to also oscillate;the latter can then be seen as an electric dipole, and dipole radiation emits preferentiallyperpendicularly to the direction of oscillation (see Figure 1). Second, after interaction withthe electron, the resulting radiation field will be polarized with the same orientation as theincident electromagnetic wave. These rules will help us understand why the CMB shouldbe linearly polarized.1With the formation of the first stars and quasars, and the subsequent UV radiation emited bythese primitive sources, the hydrogen can re-ionize. As a consequence, the CMB will scatter againupon ionized matter and will also modify its polarization, albeit on a different angular scale.2FIG. 1. Playing tricks with light. Left panel: An electromagnetic linearly polarized wave (inred) oscillates in a given plane (in pink). Reaching an electron (orange ball), the wave induces theelectron to also oscillate, making it emit radiation (in green). This resulting wave is concentratedessentially in the (green) plane orthogonal to the movement of the electron and is polarized like theincident wave. Right panel: Non-polarized light can be decomposed into the sum of two linearlypolarized waves: one along the line of sight (in pink), the other along a perpendicular direction (ingreen). Scattered radiation due to the first wave is contained in the plane orthogonal to the lineof sight and cannot be detected. Only the second component (in green) reaches the observer andis polarized like the incident wave.Before the recombination epoch, the radiation field was unpolarized. In unpolarizedlight, the transverse electric field can be decomposed into the two orthogonal directions(pink and green arrows in Figure 1) to the line of propagation. The electric field componentalong the vertical direction (green arrow) will make the electron oscillate also vertically.Hence, the dipolar radiation will be maximal over the horizontal plane. Looking from theside, we can easily detect this component. However, the incident component oriented alongour line of sight (pink arrow) will not reach us, just because this component causes theelectron to also oscillate along our line of sight, emitting the scattered dipole radiation onthe orthogonal plane. From our position then we cannot perceive this radiation. Thus, it isas if only the vertical component of the incoming electric field has caused the radiation weperceive. Recalling now the second rule mentioned above, the resulting radiation reachingthe observer should have the same polarization as the incident vertical component (green3arrow). In conclusion, the observer only receives one part of the incident radiation, and thisfraction is linearly polarized.But so far, we are leaving aside the fact that, in the real case, the target electron will behit by radiation waves coming from all possible directions, independently. Thus, to conveythe total effect, we need to consider all these contributions, and sum them up. Each ofthese waves will be scattered to the observer in the form of linearly polarized radiation, buteach one with a different orientation. If the incoming radiation were completely isotropicin intensity, then radiation coming, say, from the left would provide the polarization statethat is missing from that coming from above the electron, leaving the net outgoing radiationunpolarized: just from symmetry arguments, in a spherically symmetric configuration nodirection is privileged.But the CMB is not perfectly isotropic. It has a tiny “quadrupole anisotropy”, first dis-covered by experiments onboard of the COBE satellite [2]. Hence, from any point of view,the orthogonal contributions to the final polarization will be different, leaving a net linearpolarization in the scattered radiation. Theorists believe that this is how the CMB polar-ization detected by DASI, and recently confirmed with data from the Wilkinson MicrowaveAnisotropy Probe (WMAP) satellite [8], arose.There is one last point to emphasize. Before recombination, ionized matter, electrons andradiation formed a single fluid. In this fluid, inertia was provided by massive nucleons whilstthe pressure was that of radiation. And this fluid supported sound waves: the gravitationalclumping tendency of the effective mass in the perturbations was resisted by the restoringradiation pressure, and therefore gravity-driven acoustic oscillations in both the fluid densityand local velocity appeared. Now, the polarization field responds to the local quadrupolemoment at recombination, and this quadrupole is mainly due to the Doppler shifts inducedby the velocity field of the plasma [9]. That is why we know with certainty that the CMBpolarization shows the uncontaminated dynamics of the primordial seeds at recombination.However, the polarized fraction of the temperature anisotropy is small, since only thosephotons that scattered at the very last instants of the decoupling process possess a suffi-4ciently large quadrupole moment that is not washed out in subsequent interactions. Thereason is that the scattering that generates polarization also suppresses the quadrupolefrom which polarization arises. The result is that the polarized fraction of the temperatureanisotropy is no more than a 10%. Since the temperature anisotropies are at the 10−5 level,the polarization signal was expected to be at the 10−6 level, just a few microkelvin – aphenomenal experimental challenge.The level of polarization detected by the DASI and WMAP collaborations was compared[5,6,8] with predictions of one of the currently most favored theoretical models, the so-called concordance model. This is the model that best fits the bulk of current astrophysicalobservations, and contains 5% of ordinary matter, 22% of dark matter, and the rest in darkenergy in the form of Einstein’s cosmological constant (or something even more bizarre): atruly preposterous universe. Comparing their results with the prediction of the concordancemodel, scientists claim that polarization has been detected with a 95% confidence level.As more independent experiments confirm and extend these findings, the CMB polariza-tion will become the next gold mine of cosmology. A new window on the physics of the earlyuniverse is opening wide in front of us. Through it, we expect to obtain key information onthe fundamental parameters of cosmology – and perhaps even the actual mechanism behindthe formation of large-scale structures in our universe.5FIG. 2. A mesure of motion. Intensity and polarization of the cosmic microwave background(CMB) radiation measured with the DASI telescope. The small temperature variations of the CMBare shown in false color, with yellow indicating hot and red cold. The polarization at each spot inthe image is shown by a black line. The length of the line shows the strength of the polarization;its orientation indicates the direction in which the radiation is polarized. The size of the whitespot (lower left) approximates the angular resolution of the observations.6REFERENCES[1] S. Dodelson and W. Hu, Annu. Rev. Astron. and Astrophys. 40, 171 (2002).[2] G. F. Smoot et al., Astrophys. J. 396 , L1 (1992).[3] DASI (Degree Angular Scale Interferometer) home page: astro.uchicago.edu/dasi/[4] E. Hivon and M. Kamionkowski, Science 298, 1349 (2002)[5] E. M. Leitch et al., Nature 420, 763-771 (2002), astro-ph/0209476.[6] J. Kovac et al., Nature 420, 772-787 (2002), astro-ph/0209478.[7] M. Rees, Astrophys. J. 153, L1 (1968).[8] C. L. Bennett et al., First Year Wilkinson Microwave Anisotropy Probe (WMAP)Observations: Preliminary Maps and Basic Results, see map.gsfc.nasa.gov , astro-ph/0302207.[9] M. Zaldarriaga and D. Harari, Phys. Rev. D52, 3276 (1995).[10] The author is supported by Consejo Nacional de Investigaciones Cient´ıficas y Te´cnicas(CONICET), the University of Buenos Aires and Fundacio´n Antorchas (Argentina).“Basta que sea irracional un solo hombre para que otros lo seany para que lo sea el universo”.La historia universal abunda en confirmaciones de ese temor.–Jorge L. Borges (1944), Pro´logo a Bartleby de Herman Melville.7

A Preposterous Universe

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

arXiv:astro-ph/0303048v1  3 Mar 2003A Preposterous Universe∗Alejandro Gangui†For centuries, astronomers have wondered about the mechanisms behind the formationof galaxies and large-scale structures. In the second half of the 20th century, cosmologistsrealized that the witness of it all was a hot bath of light, now cooled to a few degrees Celsiusabove absolute zero, which is the afterglow of the big bang. The sky-pervading cosmicmicrowave background (CMB) radiation [1] was released just before the epoch when matterbegun getting structured. Some ten years ago, the tiny variations discovered in its effectivetemperature [2] told us about the size of the primordial seeds leading to the nascent galaxies,after eons of gravitational evolution.Now, another piece of evidence fits in the right place and shows us the way these pri-mordial seeds were actually moving some 400,000 years after the bang, when the CMBdecoupled from non-relativistic matter. With a radio telescope at the South Pole, scientistsfrom the DASI collaboration [3,4] have recently measured the minute level of orientation, orpolarization, that these microwaves received when they emerged from the seething plasma– a signal that only the peculiar dynamics of the seeds present at that epoch can generate[5,6].Most light around us is unpolarized. Its many individual waves oscillate in differentplanes as it propagates. But unpolarized light becomes polarized whenever it is scatteredor reflected, as in sunglasses or in the surface of a lake. In these cases, most of the intensityof the scattered light is concentrated in one plane along the line of propagation, resulting in∗Perspective: Astronomy, published in Science 299, 1333-1334 (2003) (28 February 2003).†The author is at IAFE, the Institute for Astronomy and Space Physics (CONICET), and atthe Physics Department, University of Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires,Argentina. E-mail: gangui@df.uba.ar1linearly polarized light.Early on, when the universe was hot enough, matter was ionized and the free electrondensity was so high that photons could not propagate freely without colliding with electrons.But the universe was expanding and the ambient temperature decreasing, and so the ener-getic collisions became less frequent. The relatively low-energy photons that ensued couldnot destroy the increasing number of neutral particles (essentially hydrogen and helium) thatbegan to combine. Cosmologists refer to this period as “recombination”, and soon afterwardthe CMB was released free, making its last scattering upon matter. According to theory, itis at this precise time, nearly 14 billion years ago, that the CMB became polarized.CMB polarization was first discussed thirty-five years ago by Martin Rees [7]. However,there was no evidence of its existence until the DASI detection late last year. Polariza-tion is an important probe for cosmological models and for the more recent history of ournearby universe. It arises from the interaction of the cosmic background radiation with freeelectrons; hence, CMB polarization can only be produced at the time of its last scattering1.Unlike temperature fluctuations, polarization is largely unaffected by inhomogeneities in thegrowing distribution of matter after recombination.To understand how the CMB becomes polarized, two points should be clear. First, theoscillating electric field of the incoming radiation will push the electron to also oscillate;the latter can then be seen as an electric dipole, and dipole radiation emits preferentiallyperpendicularly to the direction of oscillation (see Figure 1). Second, after interaction withthe electron, the resulting radiation field will be polarized with the same orientation as theincident electromagnetic wave. These rules will help us understand why the CMB shouldbe linearly polarized.1With the formation of the first stars and quasars, and the subsequent UV radiation emited bythese primitive sources, the hydrogen can re-ionize. As a consequence, the CMB will scatter againupon ionized matter and will also modify its polarization, albeit on a different angular scale.2FIG. 1. Playing tricks with light. Left panel: An electromagnetic linearly polarized wave (inred) oscillates in a given plane (in pink). Reaching an electron (orange ball), the wave induces theelectron to also oscillate, making it emit radiation (in green). This resulting wave is concentratedessentially in the (green) plane orthogonal to the movement of the electron and is polarized like theincident wave. Right panel: Non-polarized light can be decomposed into the sum of two linearlypolarized waves: one along the line of sight (in pink), the other along a perpendicular direction (ingreen). Scattered radiation due to the first wave is contained in the plane orthogonal to the lineof sight and cannot be detected. Only the second component (in green) reaches the observer andis polarized like the incident wave.Before the recombination epoch, the radiation field was unpolarized. In unpolarizedlight, the transverse electric field can be decomposed into the two orthogonal directions(pink and green arrows in Figure 1) to the line of propagation. The electric field componentalong the vertical direction (green arrow) will make the electron oscillate also vertically.Hence, the dipolar radiation will be maximal over the horizontal plane. Looking from theside, we can easily detect this component. However, the incident component oriented alongour line of sight (pink arrow) will not reach us, just because this component causes theelectron to also oscillate along our line of sight, emitting the scattered dipole radiation onthe orthogonal plane. From our position then we cannot perceive this radiation. Thus, it isas if only the vertical component of the incoming electric field has caused the radiation weperceive. Recalling now the second rule mentioned above, the resulting radiation reachingthe observer should have the same polarization as the incident vertical component (green3arrow). In conclusion, the observer only receives one part of the incident radiation, and thisfraction is linearly polarized.But so far, we are leaving aside the fact that, in the real case, the target electron will behit by radiation waves coming from all possible directions, independently. Thus, to conveythe total effect, we need to consider all these contributions, and sum them up. Each ofthese waves will be scattered to the observer in the form of linearly polarized radiation, buteach one with a different orientation. If the incoming radiation were completely isotropicin intensity, then radiation coming, say, from the left would provide the polarization statethat is missing from that coming from above the electron, leaving the net outgoing radiationunpolarized: just from symmetry arguments, in a spherically symmetric configuration nodirection is privileged.But the CMB is not perfectly isotropic. It has a tiny “quadrupole anisotropy”, first dis-covered by experiments onboard of the COBE satellite [2]. Hence, from any point of view,the orthogonal contributions to the final polarization will be different, leaving a net linearpolarization in the scattered radiation. Theorists believe that this is how the CMB polar-ization detected by DASI, and recently confirmed with data from the Wilkinson MicrowaveAnisotropy Probe (WMAP) satellite [8], arose.There is one last point to emphasize. Before recombination, ionized matter, electrons andradiation formed a single fluid. In this fluid, inertia was provided by massive nucleons whilstthe pressure was that of radiation. And this fluid supported sound waves: the gravitationalclumping tendency of the effective mass in the perturbations was resisted by the restoringradiation pressure, and therefore gravity-driven acoustic oscillations in both the fluid densityand local velocity appeared. Now, the polarization field responds to the local quadrupolemoment at recombination, and this quadrupole is mainly due to the Doppler shifts inducedby the velocity field of the plasma [9]. That is why we know with certainty that the CMBpolarization shows the uncontaminated dynamics of the primordial seeds at recombination.However, the polarized fraction of the temperature anisotropy is small, since only thosephotons that scattered at the very last instants of the decoupling process possess a suffi-4ciently large quadrupole moment that is not washed out in subsequent interactions. Thereason is that the scattering that generates polarization also suppresses the quadrupolefrom which polarization arises. The result is that the polarized fraction of the temperatureanisotropy is no more than a 10%. Since the temperature anisotropies are at the 10−5 level,the polarization signal was expected to be at the 10−6 level, just a few microkelvin – aphenomenal experimental challenge.The level of polarization detected by the DASI and WMAP collaborations was compared[5,6,8] with predictions of one of the currently most favored theoretical models, the so-called concordance model. This is the model that best fits the bulk of current astrophysicalobservations, and contains 5% of ordinary matter, 22% of dark matter, and the rest in darkenergy in the form of Einstein’s cosmological constant (or something even more bizarre): atruly preposterous universe. Comparing their results with the prediction of the concordancemodel, scientists claim that polarization has been detected with a 95% confidence level.As more independent experiments confirm and extend these findings, the CMB polariza-tion will become the next gold mine of cosmology. A new window on the physics of the earlyuniverse is opening wide in front of us. Through it, we expect to obtain key information onthe fundamental parameters of cosmology – and perhaps even the actual mechanism behindthe formation of large-scale structures in our universe.5FIG. 2. A mesure of motion. Intensity and polarization of the cosmic microwave background(CMB) radiation measured with the DASI telescope. The small temperature variations of the CMBare shown in false color, with yellow indicating hot and red cold. The polarization at each spot inthe image is shown by a black line. The length of the line shows the strength of the polarization;its orientation indicates the direction in which the radiation is polarized. The size of the whitespot (lower left) approximates the angular resolution of the observations.6REFERENCES[1] S. Dodelson and W. Hu, Annu. Rev. Astron. and Astrophys. 40, 171 (2002).[2] G. F. Smoot et al., Astrophys. J. 396 , L1 (1992).[3] DASI (Degree Angular Scale Interferometer) home page: astro.uchicago.edu/dasi/[4] E. Hivon and M. Kamionkowski, Science 298, 1349 (2002)[5] E. M. Leitch et al., Nature 420, 763-771 (2002), astro-ph/0209476.[6] J. Kovac et al., Nature 420, 772-787 (2002), astro-ph/0209478.[7] M. Rees, Astrophys. J. 153, L1 (1968).[8] C. L. Bennett et al., First Year Wilkinson Microwave Anisotropy Probe (WMAP)Observations: Preliminary Maps and Basic Results, see map.gsfc.nasa.gov , astro-ph/0302207.[9] M. Zaldarriaga and D. Harari, Phys. Rev. D52, 3276 (1995).[10] The author is supported by Consejo Nacional de Investigaciones Cient́ıficas y Técnicas(CONICET), the University of Buenos Aires and Fundación Antorchas (Argentina).“Basta que sea irracional un solo hombre para que otros lo seany para que lo sea el universo”.La historia universal abunda en confirmaciones de ese temor.–Jorge L. Borges (1944), Prólogo a Bartleby de Herman Melville.7

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A Preposterous Universe

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