Download 02 Observing the Universe I - Jodrell Bank Centre for Astrophysics

COSMOLOGY – PHYS 30392
OBSERVING THE UNIVERSE Part I
http://www.jb.man.ac.uk/~gp/
[email protected]
Giampaolo Pisano - Jodrell Bank Centre for Astrophysics
The University of Manchester - January 2013
OBSERVING THE UNIVERSE
→ Darkness at Night
Universe at different Wavelengths
Large Scale Structures of the Universe
The Cosmic Microwave Background
Homogeneity and Isotropy
The Expansion of the Universe
Particles in the Universe
References: Harrison, Cosmology – Chap. 24
Ryden, Introduction to Cosmology – Par. 2.1
Harrison
Figs 24.2-3
Darkness at Night (1/6)
Dark Night Sky Paradox
Digges (1576), Kepler (1610), Olbers (1826)
• Why the night sky is dark at visible wavelengths instead of being bright ?
• If the Universe has infinite extent, populated everywhere with bright stars,
the entire sky should be covered by stars with no dark gaps.
A line of sight must eventually
intercept the surface of a distant star.
Forest analogy
Harrison
Figs 24.4-5
Darkness at Night (2/6)
Interpretation A
Sky covered by stars with no gaps
but most stars cannot be seen
Interpretation B
Sky not covered by stars
and dark gaps are real
dr
Darkness at Night (3/6)
Halley’s approach (1720)
r
- Let’s assume an infinite Universe with:
n: average # density of stars
L: average stellar luminosity
- Flux received on Earth from a star at a distance r :
- Brightness from a shell of stars:
L n × 4π r 2
nL
→ dB(r ) =
dr
=
dr
2
4π r
4π
4π
f (r ) =
L
4π r 2


Power
 Brightness =

Unit
area
×
Solid
Angle


(it doesn' t depend on r )
∞
- Total brightness from all the stars in the Universe:
∞
nL
B = ∫ dB =
dr = ∞
∫
4
π
0
0
The night sky should be infinitely bright Absurd
Darkness at Night (4/6)
- We have implicitely made different assumptions:
Assumption 1
Unobstructed line of sight (not true)
• Nearby stars, with their finite angular size, hide distant stars:
Finite sky brightness equal to that of a typical star
• Absorption by Inter-Stellar Medium (ISM) might hide stars (Olbers):
ISM would heat up and emit at the same temperature (Kelvin)
Assumption 2 n L constant throughout the Universe (might not be true)
• Distant stars might be less numerous and less luminous than nearby stars:
Hierarchical clustering Universe would be anisotropic
• Cosmic island (Stoic):
Not supported by observations
Darkness at Night (5/6)
Assumption 3 Universe infinitely large (might not be true)
• Finite Universe with rmax Aristotelian Cosmic edge:
Finite brightness:
B≈
nL
rmax
4π
Assumption 4 Light flux distant source follows the inverse square law
(might not be true)
• Universe might not be Euclidean:
Einstein: the Universe can have different geometries
• Sources non stationary:
Light can be blue- or red-shifted
Assumption 5 Universe infinitely old (might not be true)
• Finite speed of light: We look back in time
• Universe with finite age with t0 : Finite brightness: B ≈
nL
ct0
4π
Darkness at Night (6/6)
Paradox solutions
Edge of observable
Universe
Poe (1848), Kelvin (1901), et al..
• The Universe has a finite age
• Speed of light is finite
• Stars beyond the horizon distance are invisible to us
• Number of visible stars too few to cover the entire sky
However:
• Stars have a finite luminous age (~1010 y)
• Even in an infinitely old Universe, the stars would not contain enough
energy to shine and fill the space with starlight radiation
Note:
• The sky is not dark at mm-wavelengths CMB...
OBSERVING THE UNIVERSE
Darkness at Night
→ Universe at different Wavelengths
Large Scale Structures of the Universe
The Cosmic Microwave Background
Homogeneity and Isotropy
The Expansion of the Universe
Particles in the Universe
References: Liddle, Introduction to Modern Cosmology – Par. 2.1, 2.2
The sky at different wavelengths
ν
Initially, the Universe was observed only at optical wavelengths
The sky at different wavelengths and beyond..
Beyond visible light
• Radio waves: high resolution maps of very distant galaxies,
many furthest galaxies detected this way
• Microwaves: CMB as BB at 2.725 K, astonishingly uniform,
best evidence of cosmological principle, tiny anisotropies at 10-5 level
• Infrared: surveys spotting different galaxy population: ‘young’ galaxies;
useful to look through dust close to galactic plane
• X-ray: cluster of galaxies hot gas emission at tens of millions K
• γ-ray: gamma-ray bursts
Beyond e.m. spectrum
• Neutrinos (supernova explosions)
• High energy cosmic rays
• Gravitational waves (colliding stars)
OBSERVING THE UNIVERSE
Darkness at Night
Universe at different Wavelengths
→ Large Scale Structures of the Universe
The Cosmic Microwave Background
Homogeneity and Isotropy
The Expansion of the Universe
Particles in the Universe
References: Liddle, Introduction to Modern Cosmology – Par. 2.1, 2.2
Ryden, Introduction to Cosmology – Par. 2.2
Serjeant, Observational Cosmology – Par. 3.11
A.Z. Colvin - Wikipedia
Solar System
~ 0.25 x 10-3 pc Ø
(~50 AU)
1 AU = 150 x 106 km
1pc = 3.261 l.y.
= 3.09×1013 km
- Astronomical Unit: Average distance Earth-Sun
- Parsec (pc): Distance from the Sun corresponding to a parallax of 1 arcsec
(1 AU at 1 pc distance looks with an angular separation of 1 arcsec)
A.Z. Colvin - Wikipedia
Solar Interstellar Neighborhood
~ 40 pc Ø
- Nearest star:
Proxima Centauri
4.2 l.y.
- Main source of light:
Nuclear fusion within stars
- Different type of stars:
Less or more massive than Sun
A.Z. Colvin - Wikipedia
Milky Way Galaxy
~ 30 kpc Ø
- Milky Way:
~100 x109 stars
(0.1-10 Mʘ)
- Solar system:
8 kpc off-centre
- Bulge + disc:
r = 12.5 kpc
t = 300 pc
Rotation period
~200 x106 yr
Solar Interstellar Neighborhood
- Disc: slow
differential rotation
- Globular clusters:
Symmetric distribution: 5-30 kpc
~106 stars per cluster
ESO
- Galactic Halo:
Spherical, larger than disc
Spiral Galaxy M100
A.Z. Colvin - Wikipedia
Local Galactic Group
~ 2 Mpc Ø
- Local Group:
~40 galaxies
- MW and M31:
largest galaxies
- MW accreting
LMC & SMC
- M31 at 770 kpc
- LMC at 50 kpc
HST
- Canis Major dwarf galaxy:
At 7.6 kpc Closer to us than MW centre!
- Sagittarius dwarf galaxy at 25 kpc
- MW falling toward M31:
Future merger
Andromeda Galaxy M31
Local Group: Predicted Merger between Milky Way and Andromeda Galaxies
Today
2 x109 yr
3.75 x109 yr
M31 bigger
angular size
3.85 x109 yr
New star formation
3.9 x109 yr
Star formation
4 x109 yr
MW warped
M31 tydally stretched
5.1 x109 yr
Galaxy cores as
pair of bright lobes
7 x109 yr
Core of huge
elliptical galaxy
A.Z. Colvin - Wikipedia
Virgo Supercluster
~ 50 Mpc Ø
- Galaxy clusters:
M > 1014 Mʘ
- Galaxy groups:
~ few Mpc3
Galaxies
separation
~ 1 Mpc
Largest bounded
structures
Universe
- Nearest cluster:
Virgo cluster
Local group will
be accreted by it
- Local group:
Interacts with Maffei I,
Sculptor, M81, M83 Groups
- Local (Virgo) supercluster:
Contains all these structures
CfA Galaxy Redshift Survey
( 1977-1982-1995 )
~ 220 Mpc deep
‘Stick man’ & ‘Great Wall’
- Survey:
- 1100 spectra / redshifts
- Narrow slice: 6 x 130 deg
- Discovery:
- Galaxy distribution not random Filamentary structure
- Great Wall: Cluster of galaxies surrounded by voids
2dF Galaxy Redshift Survey
( 2 degree field )
( 1997-2002 )
Z < 0.3
Robert
Smith
Survey regions
Milky Way
- Survey:
- 221000 spectra / redshifts
- Range 0 < z < 0.3
- First large scale map:
- Superclusters
- Walls
- Giant Voids
- Very large scales:
Begin of Universe homogeneity
A.Z. Colvin - Wikipedia
Local Superclusters
~ 600 Mpc Ø
COMA
Supercluster:
at 100 Mpc
~10000 galaxies
- Superclusters:
joined by filaments
and walls of galaxies
Longest dimension
~100 Mpc
- Foam-like structures:
with large voids
(up to 50-100 Mpc)
- Superclusters:
Largest structures
in the Universe
2dF Quasar Redshift Survey
Z<3
- Survey:
- 23000 redshifts
- Range: z < 3
Galaxy survey
Billions of light years
Milky Way
Redshift
Quasars
- Quasars:
due to supermassive
black hole in the centre
of galaxies AGNs
Extremely luminous,
visible at large distances
- Distribution of quasars
almost homogeneous
at these scales
A.Z. Colvin - Wikipedia
Observable Universe
Z<3
- 2dF survey &
Sloan Digital Sky:
Hundred thousands galaxies
No structures greater
than Superclusters and Voids
- Large Scale Smoothness
At hundreds of Mpc the
Universe begins to appear smooth
Cosmological
Principle
Observational
demonstration
OBSERVING THE UNIVERSE
Darkness at Night
Universe at different Wavelengths
Large Scale Structures of the Universe
→ The Cosmic Microwave Background
Homogeneity and Isotropy
The Expansion of the Universe
Particles in the Universe