Cosmologia G.Sironi - Universita` degli Studi di Milano Bicocca I/1

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Challenges and prospects for better intensity
spectrum measurements
Giorgio Sironi Physics Department University of
Milano Bicocca
CMB_at_50 Princeton June 10th, 2015
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CMB Intensity Spectrum
Immediately after the discovery observations were
mainly directed to confirm the first order
properties of the CMB. Blackbody
spectrum and temperature Uniform
spatial distribution
No polarization
by 1990 all of them were confirmed
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CMB Intensity Spectrum

• Almost simultaneously the search for second order
  effects began
• and gradually results arrived
• Dipole anysotropy
  
• Anisotropies
  
• Residual polarization E-modes
  
• 
  B-modes (?)
• S-Z effect
  
• Spectral distortions
  ?
• ! spectral distortions have still to be detected !

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CMB Intensity Spectrum

• Models and observational results on spectral
  distortions will be presented by other speakers
  at this same conference. I will simply remark
  that
• between 0.5 GHz and 500 GHz
• CMB intensity measurements are consistent with a
  thermodynamic temperature of the blackbody
  distribution which best fit the data
• TthCMB (2.7260 /- 0.0013) K
• (2009 revised value of FIRAS original value)
• No evidence of distortions

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CMB Intensity Spectrum

• Models suggests
• wide features which can be
• characterized by
• YFF(free-free)
• µ (chem. pot.,BE spectrum)
• y (comptonization)
• ? Examples from Chluba and Sunyaev 2012
• and
• narrow features
• lines at various frequencies
• (xh?/KT 1.74 10-2 ?(GHz))

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CMB Intensity Spectrum
Recollection of measurements of CMB thermodynamic
temperature Most recent data in red (Old)
models dotted line
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CMB Intensity Spectrum

• so far we can only say that there are
• upper limits to the amplitude of the distortions,
  ranging from 10-3 K
• at high frequencies to 10-1 K
  at low frequencies
• upper limits to the distortions parameter
• 6 10-6 lt Yff lt 1.3 10-5 µBE lt 6 10-5
  y lt 1.5 10-5
• controversial evidence below 3 GHz
• ! very far from expectations !

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CMB Intensity Spectrum
CMB is almost everywhere buried in a sea of
foregrounds and we can think of three frequency
regions where carry on new observations
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CMB Intensity Spectrum Region 1

• ? 0.3 lt?(GHz)lt30 ? 0.1 lt (1/?) cm-1 lt 1
  ? 0.05 lt x lt 0.6
• past observation method absolute measurements of
  temperature at various frequencies, sometime
  coordinated, from ground sites or balloons
• detectors resonant antennae and hetherodyne
  receivers (partially cooled)
• contamination ground and atmospheric emission
• sky foregrounds galactic diffuse emission
  (partially polarized, dominant below 0.6 GHz)
  unresolved extragalactic sources
• distortion expected ?T few times 10-5 K
• better two subregions
• Region 1/A Region
  1/B
• 0.3 lt ?(GHz) lt 3 3 lt
  ?(GHz) lt 30

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CMB Intensity Spectrum Region 2

• 30 lt?(GHz)lt600 1 lt (1/?) cm-1 lt 20
  0.6 lt x lt 10
• past observation method absolute measurements of
  temperature
• - from space (FIRAS)
  with almost continous frequency
• coverage
• - from ground and
  balloons at selected frequencies
• - excitation of CN
  optical lines
• detectors concentrators plus bolometers or
  resonant antennae and
• hetherodyne receivers
  (cooled)
• contamination ground and atmospheric emission
  (where applicable)
• sky foregrounds residual galactic synchrotron
  and dust emissions
• distortion expected ?T ltlt 10-6 K

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CMB Intensity Spectrum Region 3

• 600 lt ?(GHz)lt 3000 ? 20 lt (1/?) cm-1 lt 100 ?
  10 lt x lt 500 ?
• past observation method absolute measurements of
  temperature at various
• 
  frequencies from balloons and in space
• detectors bolometers and bandpass filters
• contamination vehicle emission (where
  applicable)
• sky foregrounds galactic dust, unresolved
  extragalactic sources
• distortion expected ?T ?

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CMB Intensity Spectrum
How to improve observation ? If we represent
the radiation spectrum by T(?) d? k ?-a(?)
d? we can look for deviation from a flat
distribution (a(?) 1) getting the spectral
index a(?) ? -(?T(?)/??)(?m / Tm) by
differential measurements (usually more accurate
than absolute measurements)
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CMB Intensity Spectrum

It can be done a)observing simultaneously the
same region of sky at two frequencies
(absolute measurement of temperature only at few
selected frequencies) b)with continous frequency
coverage (instead of observations at few,
selected frequencies) combined with c)ADC
signal conversion as soon as possible (to let us
adjust via software the system configuration)
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CMB Intensity Spectrum Toward better results

• d)large (5 ?? 15 ) well shaped beam (no
  narrow beam required)
• e)observation of few, properly selected,
  regions of sky ( 10 ?? x 10 ??)
• f)direct measurements of the foregrounds
  necessary (looking for polarization and position
  dependence of a(?) )
• f)preferably from space in very quiet regions (L2
  ?)

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CMB Intensity Spectrum Toward better results

C radiation collector (concentrator/parabola) Ph
Dis phase discrim. S frequency dispersion
(Michelson interf.) PD power detector
(bolometer/diode) MS mass storage and on board
computer To reference noise source
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CMB Intensity Spectrum

• common layout,
• different technical implementations
• at least three different experiments
• ? 0.3 - 30 GHz ground / space
• (parabola?concentrator, preamps
  diodes?bolometers, R.A. techniques)
• 30 600 GHz balloons / space
• (concentrator, bolometers, optical
  techniques)
• 100 900 ? GHz balloons / rockets / space
• (concentrator?optical mirror, bolometers, IR
  techniques)
• (possible ovrerlap of the last two regions)

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CMB Intensity Spectrum

• BUT
• space is expensive and there are long waiting
  lists newly proposed experiments cannot be
  expected to fly before 2030
• therefore to be practical and keep our feets on
  the ground
• Regions 2 and 3 we can decide to be part of more
  ambitious space and balloon experiments aimed at
  measuring the CMB polarization (proposal already
  in the pipeline)
• Region 1 while planning future space
  experiments, we can begin testing new technical
  solutions and making ground observations from
  special site (Antarctica, Atacama, )

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CMB Intensity Spectrum
Past and present proposed experiments we can
delegate part of the observation or take
advantage of technical studies Region 2 and 3
PIXIE, PRISM, Region 1
LOBO, DIME .
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The End of presentation not of observation
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CMB Intensity Spectrum Risultati TRIS

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CMB Intensity Spectrum Risultati Arcade 2

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CMB Intensity Spectrum
x(cm-1) 3,3 10-2 ?(GHz)
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CMB Intensity Spectrum
Glossario bolometer, coherent detector,
diode spectrum analyzer, Michelson
interferometer . .
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CMB Intensity Spectrum Toward better results

C radiation collector (concentrator/parabola) Ph
Dis phase discrim. S frequency dispersion
(Michelson interf.) PD power detector
(bolometer/diode) MS mass storage and on board
computer To reference noise source
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