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This article provides an overview of the physical properties and evolution of the Universe, including the Big Bang Model, observational data, and the connections to physics, mathematics, and astroparticle physics.
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NCPP Primorsko June 2007Topics 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 ScalesBrief 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)
Looking back in time How we can “see” what happened in the past? The light travels with huge but finite speed: 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 ishomogeneous. 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 inhomogeneousat 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. AllSb 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.
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 x-rays 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 smoothnessConvincing 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-Walkermetric: 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.
