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The Planck Satellite and the Cosmic Microwave Background

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The Cosmic Microwave Background, Dark Matter and Dark Energy

Anthony Lasenby, Astrophysics Group,
Cavendish Laboratory and Kavli Institute for
Cosmology, Cambridge

Overview

The Cosmic Microwave

Background — exciting new results from the Planck Satellite Context of the CMB =⇒ addressing key questions about the Big Bang and the Universe,

including Dark Matter and Dark Energy

Planck Satellite and planning for its observations have been a long time in preparation — first meetings in 1993!
UK has been intimately involved

— e.g. Cambridge is the scientific data processing centre for the HFI
Two instruments — the LFI (Low

Frequency Instrument) and the HFI (High Frequency Instrument)

— RAL provided the 4K Cooler

The Cosmic Microwave Background (CMB)

So what is the CMB? Anywhere in empty space at the moment there is radiation present corresponding to what a blackbody would emit at a temperature of ∼ 2.74 K (‘Blackbody’ being a perfect emitter/absorber — furnace with a small opening is a good example - needs perfect thermodynamic equilibrium) CMB spectrum is incredibly accurately black body — best known in nature! COBE result on this showed CMB better than its own reference b.b. within about 9 minutes of data!

Universe History

Radiation was emitted in the early universe (hot, dense conditions) Hot means matter was ionised Therefore photons scattered frequently off the free electrons As universe expands it cools — eventually not enough energy to keep the protons and electrons apart — they ‘recombine’ to form atoms of Hydrogen

History of the Universe: superluminal inflation, particle plasma, atomic plasma, recombination, structure formation

Suddenly the photons are able to free-stream away crossing the entire universe without interruption

Universe History

History of the Universe: superluminal inflation, particle plasma, atomic plasma, recombination, structure formation

Universe History

Can see directly today the imprints present at recombination Good evidence these were created by amplification of quantum-generated irregularities during period of inflation, taking place about 10−35 s after the Big-Bang!

History of the Universe: superluminal inflation, particle plasma, atomic plasma, recombination, structure formation

http://www.sdss3.org/surveys/boss.php (Chris Blake and Sam Moorfield)

Development of these (initially quantum) fluctuations from inflation until recombination, imprints characteristic scales on the universe (basically how far ‘sound’ could have travelled by then) — should see this in both matter and CMB

The Power Spectrum

Look at power in fluctuations as a function of angular scale on the sky Shown is a theoretical curve Series of coherent peaks is crucial — if can observe them, then fluctuations must have been ‘phased up’ Inflation is only known mechanism for achieving this! Details of peak location and height depend on the cosmological parameters such as density and age of the Universe The ‘Omegas’ refer to densities in various components, and H0 is Hubble’s constant (linked to age)

The Power Spectrum

Look at power in fluctuations as a function of angular scale on the sky Shown is a theoretical curve Series of coherent peaks is crucial — if can observe them, then fluctuations must have been ‘phased up’ Inflation is only known mechanism for achieving this! Details of peak location and height depend on the cosmological parameters such as density and age of the Universe The ‘Omegas’ refer to densities in various components, and H0 is Hubble’s constant (linked to age)

The Power Spectrum

Look at power in fluctuations as a function of angular scale on the sky Shown is a theoretical curve Series of coherent peaks is crucial — if can observe them, then fluctuations must have been ‘phased up’ Inflation is only known mechanism for achieving this! Details of peak location and height depend on the cosmological parameters such as density and age of the Universe The ‘Omegas’ refer to densities in various components, and H0 is Hubble’s constant (linked to age)

Dark Matter and Dark Energy

THE TWO FURTHER INGREDIENTS:

We know there are big problems with understanding the dynamics of galaxies and clusters of galaxies There appears to be a large amount of ‘missing mass’ — i.e. inferred dynamically, but not visible Very obvious in the ‘rotation curves’ of galaxies From
Instead rotational velocity is flat or even increasing with distance!

mv2 r
GMm

r2

=

p

expect v ∝ 1/r outside galaxy

Dark Matter (contd.)

For clusters of galaxies, the visible matter is only about 1/10th of that needed to explain the dynamics we see

(First pointed out by Fritz Zwicky in 1933 — so this problem has been round a long time!)

General consensus is that the ‘missing mass’ is provided by a hitherto undetected particle, which only interacts

A cluster showing lensing

gravitationally (Though particularly for the galactic rotation curve problem, many attempts also to explain in terms of modifications to the laws of gravity, e.g. MOND theories.)

Fritz Zwicky

Dark Energy

On the largest scales in the universe we see not extra attraction, but ‘repulsion’ The universe is accelerating, as measured by the brightness of distant supernovae Is this Λ?

Einstein introduced this into his field equations for General Relativity to try to get a static

universe

When he realised the universe was expanding, he discarded this term — we finally knew that it was necessary in about 1998

A source term or geometry?

Schematically, Einstein’s equations are:

G = 8πT

a geometrical object derived from the metric g of spacetime the stress-energy tensor of sources of matter and radiation

Where does the cosmological constant enter?
G − Λg = 8πT Modifies gravity itself or
G = 8πT + Λg A new source of energy

More generally, should we interpret the late-time acceleration of the universe in terms of a modified gravity theory? — or as the action of e.g. a new form of matter, such as a new scalar field (like the Higgs, recently discovered)?

Lambda CDM

Putting the two together, we get ΛCDM

This is now the ‘standard model of cosmology’ (in analogy

with the Standard Model of particle physics) Here dark matter particle is ‘cold’ — basically moving slowly and non-relativistically today Suitable candidates could be e.g. large mass WIMPS And what provides the repulsion for the accelerating universe is a simple cosmological constant Λ This has a constant ratio of pressure to energy density

= −1

Other possibilities like scalar fields, this changes with time Key tests come from the CMB power spectrum

The Planck Satellite

Planck has been called ‘the coolest spacecraft ever built’! Certainly payload is one of the most complex scientific mission ever put into space Cost 700M euros, and mass at

launch 1.9 tonnes

It flew out to the Second Lagrangian point (L2) of the Earth/Sun system
Scanning strategy (1 rpm,

plus 1 degree advance per day) leads to 2 × 7 month surveys, each covering entire sky once
Semi-stable — flies in a Lissajous orbit about L2

Planck Science

So what did Planck see, and why is it such a big advance? The key is much improved resolution and sensitivity compared to the previous missions At the higher frequencies, each Planck sky map gives about 50 million pixels at each frequency — compare ∼ 3 million for WMAP Sensitivity about 10 times higher per beam Frequency coverage much improved compared to previously as well — can better discriminate the CMB from Galactic and other foregrounds

Planck Science

So what did Planck see, and why is it such a big advance? The key is much improved resolution and sensitivity compared to the previous missions At the higher frequencies, each Planck sky map gives about 50 million pixels at each frequency — compare ∼ 3 million for WMAP Sensitivity about 10 times higher per beam Frequency coverage much improved compared to previously as well — can better discriminate the CMB from Galactic and other foregrounds

Planck Science

So what did Planck see, and why is it such a big advance? The key is much improved resolution and sensitivity compared to the previous missions At the higher frequencies, each Planck sky map gives about 50 million pixels at each frequency — compare ∼ 3 million for WMAP Sensitivity about 10 times higher per beam Frequency coverage much improved compared to previously as well — can better discriminate the CMB from Galactic and other foregrounds

Planck Cosmology Results

28 papers plus associated data products released Mar 21 Made headlines around the world, including front page of the NY Times Release based on first 15 months of data rest of data (another 15 months) + crucial polarisation data, due in 1 year HFI cryogens ran out in early 2012 — LFI observations finished recently and Planck now ‘de-orbited’

Planck Cosmology Results

Broad overview of results would be: Spectacular overall agreement with ΛCMD cosmology But with some hints of departures in places And some tensions with other results For example rate of universe expansion (H0) from CMB now discrepant with recent optical and IR determinations at about 2.5σ level (Universe has got slightly older Planck about 40 Myr > WMAP9 value.)

Planck Cosmology Results

Angular scale

  • 90◦
  • 18◦

1◦

  • 0.2◦
  • 0.1◦
  • 0.07◦

6000

5000 4000 3000 2000 1000
0

Planck has produced a wonderful power spectrum of the fluctuations in the CMB sky Very big increase in accuracy — can now definitely say Dark

Energy and Dark Matter exist,

just from primordial CMB alone

  • 2
  • 10
  • 50
  • 500
  • 1000
  • 1500
  • 2000
  • 2500

Multipole moment, `

Proportions of DE and DM now slightly different: instead of what’s shown in Pie chart (based on previous space experiment (WMAP) values), Planck now has 69% for DE, 26% for DM and 5% for ordinary matter
WMAP values

Planck Cosmology Results (contd.)

Many other interesting results A key result for inflation is the slope of the primordial power spectrum of perturbations Generic inflation models predict a primordial power spectrum slope of about 0.96 whereas pre-inflation theory expected value was 1 Planck gets 0.9603 ± 0.0073 Incredible that something first predicted about 30 years ago, concerning the first 10−35 seconds of the universe, we are now starting to get confirmation of

Planck Cosmology Results — still to come

Polarisation results will be key over next year – potentially can tell us directly energy scale of inflation (which is currently constrained to 1012 times larger than LHC can probe) Detecting this mode of polarisation (the B-mode) is equivalent to detecting gravitational waves in early universe!
LIGO
This may give first point of contact with String Theory, since this component predicted to be generically small in string-based cosmologies

String Theory

Further space missions

PRISM (Polarized Radiation Imaging and Spectroscopy Mission) is a proposal for an L-class mission to be the ‘ultimate’ mapper of both temperature and polarisation for the CMB Unfortunately has now lost out to Athena (X-ray) and eLISA (gravitatonal waves)

For Dark Matter and Dark Energy, the future is brighter(!) Euclid is an M-Class mission already selected, due for launch 2020 Important for both DE and DM — DM via lensing, and DE via mapping the distribution and redshift of galaxies, and seeing how characteristic scales evolve with time

Euclid

Planck Results — still to come

Returning to Planck, quality of polarisation data on small angular scales already extremely impressive Line shown is not a fit, but predicted from Temperature data Also Planck, with its high resolution and large frequency coverage, is a very impressive instrument for Galactic studies First release, with about 1000 pages total, has just scratched the surface — definitely many mysteries remaining!

Planck image of dust in the Galaxy

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