{"id":2490,"date":"2015-07-28T19:11:56","date_gmt":"2015-07-29T02:11:56","guid":{"rendered":"http:\/\/www.deepspace.ucsb.edu\/?page_id=2490"},"modified":"2019-06-10T17:22:31","modified_gmt":"2019-06-11T00:22:31","slug":"greenpol","status":"publish","type":"page","link":"http:\/\/128.111.23.62\/wordpress\/projects\/greenpol","title":{"rendered":"GreenPol"},"content":{"rendered":"

\u00a0 Green<\/strong>land\u00a0Pol<\/strong>arization Experiment<\/p>\n

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Project Overview<\/strong><\/p>\n

GreenPol will use a microwave telescope designed to measure the polarization of emission from our galaxy at frequencies of 10, 15, 20, 30 & 44 GHz at Summit Station in Greenland. The purpose of GreenPol is to understand the galactic emission in order to better understand and remove this structure from deep Cosmic Microwave Background (CMB) anisotropy and polarization maps, such as those generated by the Planck mission. \u00a0By removing the polarized contamination from our galaxy, researchers will be able to generate more sensitive CMB maps, which will help them have a better understanding of the early universe, particularly in the search for primordial gravitational waves, which if found would be strong evidence for the theory of inflation.<\/p>\n

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The Cosmic Microwave Background:<\/strong>
\nThe Cosmic Microwave Background (CMB) is currently the earliest picture of our universe that we can detect. The early universe consisted of very hot dense hydrogen plasma, which was opaque, meaning photons were not able to propagate. As the universe cooled and expanded, the hydrogen plasma became hydrogen atoms making the universe transparent, allowing the photons to propagate. We can still observe these photons today and it is what we call the CMB. Fluctuations or anisotropies observed in the CMB give cosmologists information about density fluctuations in the early universe plasma as well as other conditions at that time. The polarization of the CMB may hold clues of primordial gravitational waves, however contamination from polarized sources in our galaxy must be removed first. In our frequency range the primary sources of polarized microwaves from our galaxy come from synchrotron radiation, which is caused by charged particles moving through the galactic magnetic fields at speeds close to the speed of light, free-free emissions from electron-ion scattering, thermal dust emission from heated interstellar dust grains, and possibly emissions from spinning dust.<\/p>\n

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Why GreenPol?<\/strong><\/p>\n

The millimeter wavelength sky is critical for understanding cosmological foregrounds in order to remove the galactic signature from the cosmological signature. This is especially important in searching for the gravity wave signature from the proposed inflationary era from polarization in the Cosmic Microwave Background (CMB). Our galaxy has two primary and very different emission mechanisms, namely synchrotron emission from high energy Cosmic Rays that dominates below 100 GHz and dust emission from heated interstellar dust grains that dominates above 100 GHz. It is critical that we have a series of very sensitive maps from 10-100 GHz to understand the synchrotron component of our galaxy to combine with the Planck high frequency dust maps from 100-900 GHz. The current WMAP and Planck maps at 23, 30, 40 and 70 GHz are insufficient, especially in polarization which neither satellite was specifically designed to measure and for which there is both insufficient sensitivity and serious systematic concerns. There are two major goals for our longer term effort in Greenland. One is to study the galactic foregrounds by measuring over about 50% of the sky, doing for synchrotron what Planck has done for dust, AND to feed these maps and understanding into the deep cosmological maps we will make from Greenland at 10-44 GHz and eventually 100 GHz to search for evidence of gravitational waves from the early universe.<\/p>\n

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The lines show the sky region covered by a 10 degree (black), 20 degree (blue), 30 degree (white), and 65 degree (gray) opening angle<\/p><\/div>\n

\"\"In order to do the latter (search for gravitational waves) we MUST do the former (characterize the galactic foregrounds) as so poignantly been shown by the recent Planck release. Greenland is one of the best observing sites in the world, particularly with respect to precipitable water vapor, and allows coverage of the northern hemisphere, which is less contaminated by the galaxy and is complimentary to southern hemisphere measurements at higher frequencies that study dust contaminated frequencies.<\/p>\n

A critical factor that is new is that the Planck data has recently shown that the higher frequency (about 100 GHz) polarization measurements are heavily contaminated by a much more complex dust emission than we had anticipated and at a higher level than anticipated. Since the atmosphere is quite absorptive above 100 GHz and essentially opaque above 300 GHz, with only a few observing frequency windows, in order to fully characterize the dust emission space based systems are needed. On the other hand the lower frequencies below 100 GHz are essentially completely open to observation from the ground. This allows us to make much more detailed\u00a0measurements of the galactic contamination from ground based measurements. Additionally the advantage of going into space compared to ground is only about a factor of 2 in system noise for the critical bands while at higher frequencies ground observations are much less sensitive than space both due to the increased opacity of the atmosphere and the decreased flux from the CMB at higher frequencies.\u00a0These two effects give us a strong science case to push to lower frequencies and to use the high altitude site in Greenland (Summit site) as an observing site. Another critical factor that favors observing at lower frequencies is that the technology we will use to make these measurements uses detectors that are extremely linear compared to the bolometers used at higher frequencies. Thus atmospheric perturbations from water vapor fluctuations and pressure waves will be largely cancelled out to a much higher degree than bolometer based measurements, allowing us to get polarization information on larger angular scales that is critical to understanding inflation.<\/p>\n

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Greenland Summit Station PWV analysis:\u00a0http:\/\/128.111.23.62\/wordpress\/wp-content\/uploads\/2010\/08\/T-TST-REG-04-2013-00044.R1_final.pdf<\/a><\/p>\n

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B mode Polarization forecasts for GreenPol<\/strong><\/p>\n

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RMS anisotropy of CMB compared to Galactic foreground emission (Rayleigh Jeans Temp). Grey is the predicted range for inflation for simple models. The actual signal level is unknown. The red and blue bands show typical high-latitude emission from polarized synchrotron and dust emission within the Galaxy.<\/p><\/div>\n

Gravitational waves from inflation acting as tensor perturbations can produce B-mode polarization (named for its divergence free nature as is the case with magnetic fields) when interacting with light. If inflation is correct we should then be able to detect B-modes in the CMB. The amplitude of B-modes is parameterized with what is known as the tensor to scalar ratio r, currently measured to be r < 0.07 at 95% confidence by the BICEP2\/Keck collaboration. If there were no gravity waves in the early universe then we would only see scalar perturbations in the CMB and r would be zero. As it stands, the small upper bound on r illustrates the need to very accurately characterize galactic foreground emission.<\/p>\n

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The following data is taken from a soon to be published paper by Unni Fuskeland et al. at the Institute of Theoretical Astrophysics at University of Oslo which presents tensor to scalar ratio forecasts for GreenPol.<\/p>\n

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Summary of GreenPol instrument properties (top section), model parameters (middle section), and experimental setups (bottom section) for our proposed experiment. Temperatures are given in thermodynamic units.<\/p><\/div>\n

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Tensor to scalar ratio r for various experimental set ups at the 95% confidence limit. Each point represents the mean of the 95% confidence limits evaluated from 20 simulations. The error bar indicates the 68% region among the same simulations. The horizontal gray dashed line is the value of the baseline configuration SN10.<\/p><\/div>\n

The experimental setup for each point in the above plot is as follows. Unless stated otherwise, all set ups follow the baseline configuration (SN10) of 10-44GHz GreenPol channels + 100-353 GHz Planck channles at a 10 degree opening angle.<\/p>\n

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  • SN10, SN20, SN30, & SN65 – opening angle of 10, 20, 30, & 65 degrees. Each frequency channel is weighted by its signal to noise ratio<\/li>\n
  • Mask04 – Pixels are masked by thresholding the Planck synchrotron polarization, leaving a full sky fraction of 0.73<\/li>\n
  • Mask05 – Pixels are masked by thresholding the CMB posterior RMS map at 0.4 uK<\/li>\n
  • U10 – Uniform weighting across frequency channels<\/li>\n
  • Wide prior – Gaussian priors on the spectral parameters for synchrotron and dust are tripled<\/li>\n
  • LFI HFI – Planck LFI + HFI channels only<\/li>\n
  • LFI HFI G10 – Planck LFI + HFI + 10GHz GreenPol channel<\/li>\n
  • SN10H – 10-44GHz + 90, 143 GHz GreenPol channels<\/li>\n
  • SN10H P353 – 10-143 GHz GreenPol channels + Planck 353 GHz channel<\/li>\n<\/ul>\n

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    Combing Planck HFI observations on polarized thermal dust emission with the proposed experiment, our collaborators from the Institute of Theoretical Astrophysics at University of Oslo estimate a limit of r < 0.02 at 95% confidence for the baseline configuration (SN10). This limit is derived from ideal and simplified simulations which account for foregrounds (thermal dust and synchrotron) and white noise, but not instrument systematics. Variations in a number of experimental parameters such as sky coverage, detector weighting, and foreground priors, have little effect on this limit, making it very robust. GreenPol therefore has the potential to provide the most sensitive low frequency CMB polarization measurements of the northern galactic hemisphere in the foreseeable future.<\/p>\n

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    Greenland Summer 2016:<\/b><\/p>\n

    A brief RFI and site survey was done in the summer of 2016 in preparation for deployment.<\/p>\n[slideshow_deploy id=’3612′]\n

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    Greenland Deployment Summer 2018:<\/b><\/p>\n

    We successfully deployed and field tested our GreenPol system to Summit Station, Greenland from June 29 to August 13. While weather conditions presented more challenges than anticipated, we were able verify the functionality of our instrument design and shipping methods and are now able to provide a detailed account of observing conditions and infrastructure for the summer season. Our main take-away is that while cosmological or astrophysical studies from Summit Station are not impossible, there are significant obstacles to overcome based on our experience. The telescope was successfully shipped back to Santa Barbara on October 8th 2018 and stored at a location near campus. We are currently set up to run where we are and are using the opportunity of a nearby observation site to improve upon the system and obtain additional data. Eventually we would like to move the instrument to our White Mountain observation site or another California based site\u00a0for improved observing conditions.<\/p>\n

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    White Mountain Research Station site survey:\u00a0http:\/\/128.111.23.62\/wordpress\/wp-content\/uploads\/2010\/08\/2005_wmrs_newastronomy.pdf<\/a><\/p>\n

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