Download CosmologyL1

NCPP
Primorsko
June 2007
Topics in Cosmology
Daniela Kirilova
Institute of Astronomy, BAS
Outline
Introduction to Cosmology
Pecularities
Basic Assumptions
Cosmological Principle
Evolution of the cosmological ideas
Our place in the Universe and the scale of the
Universe smoothness
The RW Metric
The Universe Dynamics
The Expanding Universe – observational status
Universe Parameters
H constant
Universe age
The Expansion History of the Universe
Cosmological Parameters
The Early Universe
CMB
BBN
BBN with Oscillating Neutrinos
Baryogenesis and Antimatter in the Universe
DM
Inflation
The subject of Cosmology is the description of the physical
properties and the evolution of the Universe as a whole. The
most widely accepted scenario is the Big Bang Model which is
based on the Einstein's general theory of relativity and supported
by the contemporary observational data.
1.1.Pecularities
Main information source – observations
Research from ground-based and satellite-based telescopes
and other instruments
Deals with enourmous space and time scales
Looks back in time
Cosmic laboratory of bizzare objects
Close connection with Physics, Mathematics,
Astroparticle Physics, Chemistry,…
Fascinating
The whole electromagnetic spectrum
and beyond:
Detection of neutrinos from stars, SN, eventually
relic neutrinos (neutrino telescopes)
Gravitational waves detection
Cosmic Ray searches (electrons, protons, heavier nucleus)
detectors on balloons at the higher part of the atmosphere,
spacecraft searches (AMS, PAMELA , SOHO
collaborations)
Enormous Time Scales
Brief History of the Universe
Inflation
Unified interactions (10-35 sec)
Generation of matter-antimatter asymmetry
Primordial Nucleosynthesis (first 3 minutes).
CMB formation (300 000 years)
Galaxy formation (109 years)
Enourmous space and time scales
Looking back in time
How we can “see” what happened in the past?
The light travels with huge but finite speed:
c  300000 km / sec ond
The light from the Moon reaches us for about a second,
from the Sun - 8 minutes and 23 seconds,
from the nearest other star - over 4 years!
Our star, the Sun belongs to a vast formation of stars called Milky Way
Galaxy. When we receive light or radio waves across our galaxy, it takes
them tens of thousands years to reach us.
Andromeda, one of our nearest neighbour galaxies is 2 million ly from
Earth (the furthest object you can see with your naked eye).
For the nearest galaxies, light has been travelling millions of years.
For the farthest galaxies - the quasars, the light has been travelling to us
for billions of years.
Hence, by observing these distant objects, we in fact, are
observing the distant past of the Universe!
The telescope is a kind of time machine; it lets us see our distant past.
The age of the Universe is about 14 Billion years.
In fact, relic radiation may come to us from epochs not
earlier than CMB formation time. Before that the
Universe was not transparent for radiation.
In neutrino we can reach considerably earlier epoch –
1sec.
Astronomers use special units to measure
huge distances
• Astronomical Unit is defined by the semimajor axis of the
Earth's orbit around the Sun.
A parsec is defined as the distance from the Sun which would
result in a parallax of 1 second of arc as seen from Earth.
Distances of nearby objects can be determined directly using parallax
observations combined with elementary geometry, hence pc was
historically used to express the distances of astronomical objects from
the Earth.
light year – the distance, the light travels per year propagating
in vacuum = 9460 billion km!!
1 pc = 3.26 lys
The most commonly used unit in cosmology is Mpc.
1.2. Basic Assumptions:
1. The universality of physical laws
There is no observation which indicates a departure from the laws of
physics in the accessible Universe!
2. The cosmos is homogeneous.
A belief that the place we occupy is no way special
3. The universe is isotropic
There is no prefered direction (confirmed by recent CMB measurements)
The cosmological principle
Cosmological Principle states that all spatial positions and directions in
the Universe are essentially equivalent or
matter in the Universe is homogeneous and isotropic when averaged
over very large scales.
If viewed from above the disk, our
own Milky Way galaxy would probably
resemble the M100 galaxy, imaged
here by the Hubble Space Telescope.
[Figure courtesy NASA]
\footnote{
Prejudices
It is intriguing that for the bulk of the history
of civilization it was believed that
we occupy the most special location
– the center.
Evolution of the cosmological ideas}
The ancient Greeks believed that the Earth lies at the center of the Cosmos, circled
by the Moon, the Sun, planets and fixed stars. Ptolemy geocentric system
Copernicus heliocentric system
Newtonian static Universe: stars as our Sun are distributed
evenly through infinite space
Stars are located in a disc-shaped assembly (MW)
Hershels identified the disc structure (Sun still at the center) 1700s
• Shapley realized the Sun is 2/3 of radius away from the Galaxy center
(MW still believed to be the center of the Universe) 1900s
•Shapley-Curtis Debate: Are the spiral nebulas within the Milky Way or
extra Galactic objects?
• 1923 - 25 Hubble identified Cepheid variables in “nebulae” NGC
6822, M31, and M33 and proved conclusively that they are outside the
Galaxy, thus demonstrating that our Galaxy is not the Universe.
• Resolution of the Baade demonstrated that MW is a typical galaxy.
Contemporary LSS studies and CMB results: At large scales the
Universe looks the same wherever you are.
In 1917 Einstein invented the cosmological constant as a term in GR
allowing for a static universe.
1923 Friedman proposed the non-static Universe
In 1929, Hubble measured distances to galaxies and with Milton L.
Humason extended Vesto M. Slipher’s measurements of their
redshifts, and in 1929 Hubble published the velocity-distance relation
Galaxies outside the Milky Way are systematically moving away from
us with a speed that was proportional to their distance from us.
Cosmological Principle is not exact at small scales
Obviously:
Sitting in the lecture room is not the same as sitting at the
beach…
Conditions on the Earth are much more preferable for us that
those of the outerspace …
Sun’s interior is quite different from the interstellar regions
The conditions within a galaxy differ from those of IGM, etc.
…
The Universe is inhomogeneous
at the scales of planetary systems
The Earth and its moon
The Sun
Our nearest star the Sun is at 0.0001 ly away
from the Earth ( 8 min 23 sec)
1 AU = 149 600 000 km
A giant flare, many times larger than Earth, leaps from the
surface of the Sun.
Inhomogeneous at galaxy scales
Our galaxy contains billions of stars
with mass range b/n 0.1-20 solar
masses. It is a tyical Sb galaxy. All Sb
galaxies have a bulge, disc and halo
The Sun is at 8 kpc from the center.
In cosmology the detail structure of
galaxies is usually irrelevant.
Galaxies are considered as a
point-like objects emitting light.
Structure of the Milky Way:
The Local Group
MW resides within a small
concentrated group of
galaxies known as the Local
group
The nereast to MW is LMC
Galaxy groups occupy a
typical volume of a few cubic
Mpc.
inhomogenious at the scale of
galaxy clusters
On a scale of 100 Mpc variety of
large scale structures exist: clusters
of galaxies, superclusters and voids
Clusters of Galaxies
The cluster of galaxies, Abell 1689, 2 billion ly from Earth in the
constellation Virgo.
Clusters of galaxies are the largest gravitationally-collapsed objects.
Clusters are grouped into superclusters of galaxies, joined by filaments
and walls of galaxies. In b/n lie large voids, deprived of galaxies,
almost 50 Mpc across.
Structure in the Universe
A map of galaxy positions in a
narrow slice of the Universe, as
identified by the CfA (Center for
Astrophysics) redshift survey. Our
galaxy is located at the apex, and the
radius is around 200 Mpc. The galaxy
positions were obtained by
measurement of the shift of spectral
lines. While more modern and
extensive galaxy redshift surveys exist,
this survey still gives one of the best
impressions of the Universe structure.
[Figure courtesy Lars Christensen]
Deep Field
The Hubble "Ultra Deep Field" shows a tiny patch of sky – as narrow as a
grain of sand held at arm's length – in the constellation Fornax, just below Orion.
The light from the closest of these galaxies has taken about 6
billion years to reach us - and the furthest more than twice that long.
So we are seeing this part of the universe not as it looks now, but as
it looked as many as 12 billion years ago.
The Dark Ages
The image, taken with NASA's Chandra X-ray Observatory in space, shows
the most distant (and ancient) galaxies we can see. The dots are thought to be xrays emitted by enormously powerful black holes at the centers of galaxies that
are just beginning to form. In fact, the galaxies may not yet contain stars that have
begun to shine — or they may be so distant that their starlight has been absorbed
by dust.
Large-scale smoothness
Convincing observations about the smoothness of
matter distribution on large scales exist :
Recent extremely large galaxy surveys, 2dF and Sloan
Digital Sky Survey, have surveyed large volumes of few
Gps.
Superclusters and voids are likely to be the biggest structures
At scales 100-200 Mpc the Universe begin to appear smooth.
This key assumption of cosmology for the previous decades
is also confirmed now observationally by CMB.
Sloan Digital Sky Survey
SDSS is the most ambitious astronomical survey ever undertaken. It will
provide detailed optical images covering more than a quarter of the
sky, and a 3-dimensional map of about a million galaxies and quasars.
SDSS uses 2.5-meter telescope on Apache Point, NM, equipped with
two powerful special-purpose instruments. The 120-megapixel camera
can image 1.5 square degrees of sky at a time, about eight times the
area of the full moon. A pair of spectrographs fed by optical fibers can
measure spectra of (and hence distances to) more than 600 galaxies
and quasars in a single observation.
The SDSS completed its first phase of operations in 2005. SDSS-I
imaged more than 8,000 square degrees of the sky in five bandpasses,
detecting nearly 200 million celestial objects, and it measured spectra
of more than 675,000 galaxies, 90,000 quasars, and 185,000 stars.
The furthest we can see…14 billion ly
Very wide-angle view of almost the entire night sky, by NASA's WMAP
satellite, shows the furthest light we can see. It is also the oldest: The light was
emitted shortly after the Big Bang, and has been traveling through space for 13.7
billion years to us.
In this "baby picture" of the universe, the red and yellow patches are regions that are
just a few millionths of a degree hotter than the blue and black areas. This tiny difference
helped seed the formation of galaxies out of the shapeless gas that filled the early universe.
CMB, the remnant heat from the Big Bang, has a temperature which is highly uniform over
the entire sky. This fact strongly supports the notion that the gas which emitted this radiation
long ago was very uniformly distributed.
Cosmological Principle is exact at large scales
> 200 Mpc (containing mlns of galaxies)
It is a property of the global Universe.
The RW Metric
In case CP holds the most general expression for a space-time metric
which has a (3D) maximally symmetric subspace of a 4D space-time is
the Robertson-Walker metric:
c = 1 assumed. By rescaling the radial coordinate the curvature
constant k may have only the discrete values +1, −1, or 0
corresponding to closed, open, or spatially flat geometries.
The observed homogeneity and isotropy enable us to describe the
overall geometry and evolution of the Universe in terms of two
cosmological parameters accounting for the spatial curvature and the
overall expansion (or contraction) of the Universe.