dipartimento di fisica -...

27 ottobre 200827 ottobre 2008 Lezione 1Laboratorio di strumentazione

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The first light in the universe

Aniello Mennella

Università degli Studi di MilanoDipartimento di Fisica


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Photons in the early universe

Early universe is a hot and dense expanding plasma

Photons are coupled to matter via Compton scattering

At ~300000 yrs T falls below 3000 K and matter becomes neutral

At that time photons freely travel carrying the “imprinting” of the “baby” universe

We detect it today as a microwave signal, redshifted by a factor ~1000

14 May 1964, 11:15 am


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“Perfect” black body

Radiation and matter in the primordial plasma are in thermal equilibrium

Expansion provides a redshift in frequency but maintains the shape unaltered

This implies a blackbody spectrum of the CMB today at a temperature 1000 times smaller


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“Perfect” black body

Radiation and matter in the primordial plasma are in thermal equilibrium

Expansion provides a redshift in frequency but maintains the shape unaltered

This implies a blackbody spectrum of the CMB today at a temperature 1000 times smaller

No significant deviations from blackbody distribution over 5 decades in frequency


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Seeds in the universe

COBE

DMR

¢T

T» 10¡5

CMB angular distribution closely tracks matter density distribution at the last

scattering surface


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CMB polarisation

d¾Td

/ j²̂ ¢ ²̂0j2

Thompsonscattered light is polarised. Only polarisation perpendicular to propagation direction is maintained

To produce average polarisation in the CMB photon (and therefore matter) distribution must have a non zero quadrupole.


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CMB polarisation

Scalar perturbations (perturbations of the energy density at last scattering)

Vector perturbations (matter vortical motions)

Tensor perturbation (metric perturbations, i.e. gravitational waves)

Three mechanisms can produce quadrupolar anisotropy

Stokes Q;U ! E;B modes


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COBE

DMR

¢T

T» 10¡5

¢T (µ; Á) =1X

`=1

X̀

m=¡`a`;mY`;m(µ; Á)

Spherical harmonics


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Power spectrum£104

0

1

2

3

4

1 10 100 1000 10000`

`(`+1)C

`(¹K2)

C` = hja`;mj2i

The power spectrum provides a statistical description of the angular distribution of anisotropies in the sky

180± » 20± » 2± » 100 » 10


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Power spectrum£104

0

1

2

3

4

1 10 100 1000 10000`

`(`+1)C

`(¹K2)

180± » 20± » 2± » 100 » 10Large angular scales (> 1°): CMB anisotropies trace scalar perturbations in gravitational potential before inflation

¢T

T= ¡1

3

±Á

Á


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Power spectrum£104

0

1

2

3

4

1 10 100 1000 10000`

`(`+1)C

`(¹K2)

180± » 20± » 2± » 100 » 10Medium to small angular scales (1° < δθ < 5') trace perturbations driven by acoustic oscillations in the expanding plasma before decoupling

Ä£+k2c2

3£ ' 0

£ =±T

T¡ 1

c2±Á

Á


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Power spectrum£104

0

1

2

3

4

1 10 100 1000 10000`

`(`+1)C

`(¹K2)

180± » 20± » 2± » 100 » 10

Below 5' anisotropies are “washed out” by photon diffusion happening during recombination.


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Experimental requirements for high precision anisotropy

experiments

±C`C`

=

s2

fsky(2`+ 1)

"1 +

µpix¾2pixC`W`

#

As a first approximation power spectrum uncertainty depends on surveyed area,

optical and noise performances


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Low multipoles

±C`C`

=

s2

fsky(2`+ 1)

"1 +

µpix¾2pixC`W`

#

Large sky surveys required for accuracy at low multipoles


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High multipoles

±C`C`

=

s2

fsky(2`+ 1)

"1 +

µpix¾2pixC`W`

#Lownoise detectors


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High multipoles

±C`C`

=

s2

fsky(2`+ 1)

"1 +

µpix¾2pixC`W`

#

Subdegree angular resolution


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High multipoles

±C`C`

=

s2

fsky(2̀ + 1)

"1+

µpix¾2pixC`W`

#

Optimal optical response reconstruction


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An example from Planck

±C1500=C1500

Expected Planck performances


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r = 0:1

CTT`

CEE`

CBB`

Requirements for polarisation measurements

1 10 100 1000 1000010¡6

10¡4

10¡2

10

102

104

106

`(`+1)C`(¹K2)

`


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r = 0:1

CTT`

CEE`

CBB`


1 10 100 1000 1000010¡6

10¡4

10¡2

10

102

104

106

`(`+1)C`(¹K2)

`

Sensitive to reionisation


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r = 0:1

CTT`

CEE`

CBB`


1 10 100 1000 1000010¡6

10¡4

10¡2

10

102

104

106

`(`+1)C`(¹K2)

`

Emode polarisation between 12 order of magnitudes smaller than T anisotropies

Bmode even smaller, depending on tensorscalar ratio

Requires new generation of instruments and experiments

Key issues:

Low noise detectors

Clean optics

Modular, scalable instrument architecture


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15 years testing the universe

1992 – COBE DMR First evidence of

temperature anisotropies

First points on the power spectrum

C2 is somewhat surprisingly low


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First evidence of temperature anisotropies

First points on the power spectrum

C2 is somewhat surprisingly low

2000 – waiting for WMAP


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15 years testing the universe First subdegree CMB

measurements

Boomerang and MAXIMA measure first peak – the universe is FLAT!

Several measurements all over the world all speak the same language.

The power spectrum actually looks like what it should

2000 – waiting for WMAP


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First faint detection of Emode polarisation

Bmodes still outofreach

Ground measurements show their potential for the forthcoming polarisation challenges

2002 – DASI detects polarization


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2001 – WMAP: the baby universe smiles again

WMAP W-band feed horn

Instrument Front-end Assembly


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2. Radiometer gain model

If not corrected for, it would lead to ∼mK effect on maps

Gain data

1st yr model

New model

WMAP 3yrs: six Jupiter-crossing data sets

Beam solid angles found 1% larger than assumed in 1st year (1.5% effect on power spectrum).

1. Window function determination

Systematic error control: the key factor

From WMAP 3 years data(Jarosik et al 2006)


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Foregrounds: a complex issueForeground minumum contamination at

~70 GHz;

Polarisation: even at optimal frequencies, signal (on average) is dominated by polarized galactic signal


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2003-2008: 5 years of WMAP


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C2is always low...


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TE correlation clearly shows a signal


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Emodes start outlining


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Bmodes still out of reach...


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Planck, the next big step

ESA space mission, unprecedented combination of sensitivity, angular resolution, sky coverage and systematic error control

Successful coldvacuum testing completed this summer

Launch with Herschel foreseen first half 2009

14 month continuous observations from L2


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Low Frequency Instrument

20 K radiometric array

304470 GHz

All polarisation sensitive

Single receiver


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High Frequency Instrument

0.1 K bolometric array

100857 GHz

Polarisationsensitive between 100 and 353

Spiderweb PSB


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27/10/2008

WMAP PLANCK

Angular resolution 14'-56' 5'-33'

Average ∆T/T per pixel/yr 40 ×10-6 2 ×10-6

Average ∆P/P per pixel/yr 56 ×10-6 4 ×10-6

Mission lifetime 4+ yr >14 monthsSpectral coverage 23-95 GHz 27-900 GHzDetector technology HEMT HEMT+BOLDetector temperature 90 K 20K/4K/0.1KCooling Passive Active


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COBE resolution

Imaging the first lightESA-SCI(2005)1 (“Blue book”)


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Planck resolution



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WMAP 2 yrs



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Imaging the first light

WMAP 8 yrs

ESA-SCI(2005)1 (“Blue book”)


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Imaging the first light

Planck 1 yr

ESA-SCI(2005)1 (“Blue book”)


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T power spectrumESA-SCI(2005)1 (“Blue book”)


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E-mode polarisationESA-SCI(2005)1 (“Blue book”)


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B-mode polarisationESA-SCI(2005)1 (“Blue book”)

r = 0:1


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Cosmological parametersESA-SCI(2005)1 (“Blue book”)

WMAP, 4yr observation

PLANCK, 1yr observation

Marginalized posterior distributions for each parameter


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ForegroundsWMAP

Planck


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What after Planck? Many of the outstanding questions standing out from

current theories will require new measurements What is dark matter?

Does dark energy exist and what is its nature?

Did inflation actually happened and what were the conditions?

Did the primordial gravitational potential present tensor fluctuations (gravitational waves)?

How did structures start forming? When and how the universe reionised on large scales?

Many of these issues can be addressed by precision CMB polarisation measurements


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The quest for polarisation: experimental challenges

SubmicroK sensitivity (need for thousandsized arrays of very low noise detectors)

Micropackaged coherent receivers (QUIET experiment)


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SubmicroK sensitivity (need for thousandsized arrays of very low noise detectors)

Antennacoupled bolometers

(Boch, 2002, POLARBEAR exp.)

~ 1 mm

Antenna

Superconducting Nb microstrip

Resistor


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Clean, modular, scalable optical front ends

Wband 2x2 feed array developed from machined platelets

(UniMi, UniMib, 2007)


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Polarised foregrounds, systematic error control, calibration....

Strong galactic contamination at all latitudes.

Need precise measurements of galactic polarisation

Hinshaw et al, 2008


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Conclusions

The CMB is like the snapshot of a newborn baby from which we want to know about his past and infer his future

Clearly we want the picture to be a GOOD one!

In 50 years the CMB has proved a powerful means to understand the universe. Measurements and theories often wonderfully match

But nature never gets short of surprises. And the future is rich with profound and interesting questions.

The game is not a quest for the ultimate answer, but to establish a dialogue with the surrounding world which is, ultimately, a part of ourselves

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