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Observational Cosmology Roger Emery, Space Science & Technology Dept, RAL. (firstname.lastname@example.org) (with acknowledgement to Dr M. Griffin at Queen Mary College for many figures). AIM - Give an overview of observational cosmology - Look at current work - What’s coming in the future
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Roger Emery, Space Science & Technology Dept, RAL. (email@example.com)
(with acknowledgement to Dr M. Griffin at Queen Mary College for many figures)
- Give an overview of observational cosmology
- Look at current work
- What’s coming in the future
A. What sort of measurements have cosmological significance?
B. What kinds of practical problems and technologies are involved?
C. Some key observations.
- Measurements of the distance scale of sources / the Universe.
- Large scale structure
- Source counts versus ‘distance’
- Stellar and galactic evolution.
- Abundance of the elements.
- Cosmic Microwave Background.
- The mass of galaxies / in the Universe
- Particle accelerators and the structure of matter.
* PARSEC(pc) - distance at which a star would have a parallax of 1 arc second
= 3.1x1016metres 3.26 light years.
- Primary observations allowed 1000 stars to be measured, including Centauri (1.3pc)
- Hipparcos (ESA) satellite: Accuracy of parallax measurement 1 milli arcsec so has measured out to 1000pc (1kpc).
Compare Orion region at 1.5 kpc, Galactic centre at 8 kpc
or LMC at 55 kpc, Andromeda at 770 kpc - Local group of galaxies
- New proposals for satellites, e.g. GAIA, aiming for few micro arcsec.
2. Standard Candles.
- Cepheid variable stars
- Takes measurements to limits of the Local Supercluster and beyond (~ 15Mpc)
- Supernovae etc.
- Supernovae as distance indicators, as discussed by Robert Laing yesterday.
Speed of recession (v) (km/second)
Distance (Millions light years)THE HUBBLE PARAMETER, H
How fast is the expansion today?
H = 40 - 80 km/s/Mpc
How long since the
10 - 15 billion years
Red shift (z) = /o , [ 1+z = (1+v/c)/(1-(v/c)2 )1/2 ] therefore z v/c
form in the cloud and emit mainly
UV radiation ( ~ 0.1 - 0.3 m)
The UV radiation is absorbed by dust grains in the cloud
The grains are warmed up to about 40 K and re-emit the energy as far infrared radiation ( ~ 50 - 100 m)
1. Galactic and stellar evolutionSTAR-FORMING CLOUDS SHINE MAINLY IN THE FAR INFRARED
INTENSITY of electromagnetic radiation, as a function of :
FREQUENCY - Photometry, Spectroscopy
POSITIONon the sky - Imaging
TIME - Variability
POLARISATION - Polarimetry
Sensitivity: as high as possible
Angular (spatial) resolution: Good enough to resolve interesting spatial details of the source(s)
Spectral (wavelength) resolution: Good enough to resolve details and features of the spectral distribution of the source intensity
Wavelength coverage: Wide enough to take in all parts of the spectrum that contain useful information
• Very high sensitivity – objects are at immense distances
• Good angular resolution to avoid confusion (overlapping sources)
• Observations at different wavelengths
• Optical/NIR: source positions and redshifts, galaxy type, chemistry, etc. (HST, NGST, large ground-based telescopes)
• FIR-mm: re-processed and redshifted UV (FIRST, SCUBA, Millimetre Arrays)
S = Signal DS = Uncertainty in signalNS = Signal photon rate (photons/second/Hz) NB = Background photon rateDn = Bandwidth of radiation frequencies acceptedt = Exposure (integration) time
Back-to-back horns at 4 K
Filters at 1.6 K
Bolometers, horns and filters at 0.1 K
Mounting flange (20 K)A modern cryogenic focal plane: Planck HFI instrument.
= 550 nm Theoretical Achieved Resolution Resolution ("seeing")
Palomar 5-m: 0.03" ~ 1”Keck 10-m: 0.015" ~ 1"
HST 2.4-m: 0.06" ~0.06"
Plane wavefront at top of atmosphere
Distorted (corrugated) wavefront at the ground
Distortion varies: Timescale of ~ 10 ms (coherence time) Length scale of ~ 30 cm (coherence length)
1st order distortion (wavefront tilt):
Image “dances around” in the focal plane
2nd order distortion (wavefront curvature):
Image comes in and out of focus
FIRASFar Infrared Absolute Spectrometer
Measured CBR spectrum
DMRDifferential Microwave Radiometer
Measured spatial fluctuations in CBR temperature
TCBR = 2.73 K
Hot gas in clusters of galaxies
FOREGROUNDS MUST BE SUBTRACTED
The real signal
q = 30 arcmin.
q = 10 arcmin.
FOREGROUND FLUCTUATION LEVELS
EXTRAGALACTIC POINT SOURCES
GALACTIC (HIGH LATITUDE)
Best frequency for cosmological signal ~ 150 GHz
Depends on , B, H, etc.
PrimordialANGULAR POWER SPECTRUM OF CBR VARIATIONS (INFLATIONARY CDM MODEL with o = 1)
Large angles (1/Angular scale) Small angles
1. Measure to one part in 1,000,000 Use a cold telescopeand put it as far away from the Earth as possible
2. Cover a large amount of sky
3. Observe in the 1 - 2 mm wavelength region
4. Use a big telescope to see fine details (resolution of ~ 0.1o = 6 arcminutes)
5. Look through the fog . . .
Planck at L2
1.5 million kmfrom Earth
SIMULATED PLANCK SKY MAPCDM MODEL = 1 Beam = 1/6 oT/T = 2 x 10-6
1- uncertainty (%)
1/3 sky coverageT/T = 2 x 10-6 per pixel
Angular resolution (degrees)
Multi-element interferometer with ~ 60 12-m
Mountain-top site in Chile (alt. 5000 m)
Variable configuration (100 m – 10 km)
Angular resolution ~ 20 milli-arcseconds
Operating ~ 2008
3.5-metre telescope, cooled to ~ 80 K
Sunshield with solar panels on other side
Star-trackers for pointing
2500-litre tank of liquid helium cools instruments inside to 2 K
Warm electronics and thrusters
Wavelength range50 - 700 m
Major scientific projects:- Study of distant galaxies- Star formation - Interstellar chemistry
Space observations of the CBR will either
Prove that currently trendy ideas in cosmology and
high energy physics are valid and quantify all of the
main cosmological parametersor
show that new ideas are needed
Cosmologists will have to find other questions to argue about