ASTR 113 – 003 Spring 2006 Lecture 12 April 19, 2006 Review (Ch4-5): the Foundation Galaxy (Ch 25-27) Cosmology (Ch28-29) Introduction To Modern Astronomy II Star (Ch18-24) 1.Sun, Our star (Ch18) 2.Nature of Stars (Ch19) 3.Birth of Stars (Ch20) 4.After Main Sequence (Ch21) 5.Death of Stars (Ch22) 6.Neutron Stars (Ch23) 7.Black Holes (Ch24) Extraterrestrial Life (Ch30) 1.Our Galaxy (Ch25) 2.Galaxies (Ch26) 3.Active Galaxies (Ch27) 1.Evolution of Universe (Ch28) (EXAM 3 on April 26) 2. Early Universe (Ch29 )

Cosmology: The Origin and Evolution of the Universe Chapter Twenty-Eight ASTR 113 – 003 Spring 2006 Lecture 12 April 19, 2006

Guiding Questions 1.What does the darkness of the night sky tell us about the nature of the universe? 2.As the universe expands, what, if anything, is it expanding into? 3.Where did the Big Bang take place? 4.How do we know that the Big Bang was hot? 5.What was the universe like during its first 380,000 years? 6.What is “dark energy”? How does the curvature of the universe reveal its presence? 7.Has the universe always expanded at the same rate? 8.How reliable is our current understanding of the universe?

Olbers’s Paradox: Why is the night sky dark? If the universe is infinite in space and time, and static (the Newtonian universe), Olbers expected that the night sky is bright, since one would always see a star in any direction The resolution: the universe we can see is finite

The universe is expanding The Hubble law: the more distance a galaxy, the greater its redshift and the more rapidly it is receding from us. v = H0 d V: velocity in unit of (km/s) D: distance in init of Mpc H0, Hubble constant, ~ 71 km/s/Mpc, but not certain

The universe is expanding The receding motion of galaxies is due to the expansion of the universe; the galaxies are locked into the space fabric

The universe is expanding Expansion is not an explosion; there is no center, and there is no surrounding space Expansion involves all space

Cosmologic redshift Cosmologic redshift is caused by the expansion of space itself Normal Doppler redshift is caused by the motion of objects through space

Cosmologic redshit For example, observed redshift Z=1 z = (λ – λ0)/λ0 λ = 2 λ0 the wavelength is doubled It means that the universe has expanded by a factor of 2 during the period that takes light to travel from the emitting galaxy to the Earth The larger the redshift, the greater the distance, and the greater the travel time Lookback time: cosmological redshift is also an indicator of the lookback time e.g., Z=1, distance=5 billion light-year, lookback 5 billion years

Big Bang The universe is created by the Big Bang Big Bang is a cataclysmic event originated from a cosmic singularity with infinite density The universe started its expansion after the Big Bang The Big Bang marks the beginning of time Before the Big Bang, the law of physics we know can not apply After the Big Bang, space and time began to behave in the laws of physics we know

The age of the Universe The age of the Universe is measured from the Big Bang It is approximately the time taking all galaxies back to the singularity point at the expansion velocity, that is –T = d / v –Hubble law says v = H0 d –T = d / H0 d or simply –T = 1 / H0 T = 1 / 71 (km/s/Mpc) = 3.09 X / 71 = 4.4 X s and 1 year = 3.16 X 10 7 s The age of our universe is 13.7 (+- 0.2) billion years

Cosmic Light Horizon We cannot see objects whose distance is beyond 13.7 light- years because light from these objects has not had enough time to reach us Cosmic light horizon is the surface of the sphere with a radius of 13.7 light-years It defines the size of the observable universe, even the universe itself may be even bigger, or infinite

Cosmic Microwave Background Radiation There is a background radiation in microwave in all directions from the sky; the peak of radiation is about 1 mm Cosmic background radiation is virtually a blackbody with a temperature of 2.7 K (or more accurately, K)

Mass Densities of Matter and Radiation Mass density of matter (including dark matter) in our universe (from observations of galaxy clusters) at present day is about ρ m = 2.6 X kg/m 3 or about 1 single hydrogen atom per m 3 Mass density of radiation (2.7 K microwave) in our universe at present day is about ρ rad = 4.6 X kg/m 3 Because of E=MC2, radiation is equivalent to mass in terms of energy At present day, the universe is dominated by matter

Evolution of Densities As the universe expands, the volume gets bigger. As a result, both density of matter and density of radiation decrease with time At 2500 years after the Big Bang (redshift Z=25000), the two densities are equal Before 2500 years, the universe is dominated by radiation After the 2500 years, the universe is dominated by matter

Epoch years: era of recombination At 380,000 years after the Big Bang (Z=1100), the temperature of the universe dropped to 3000 K Because photon ionization is sufficient weak at this temperature, electrons start to combine with protons to form neutral hydrogen atoms The time when the first atoms formed is called the era of recomnination

Epoch years: era of recombination

Cosmic background radiation is the afterglow of the Big Bang The cosmic microwave background radiation, corresponding to a temperature of 2.7 K at the present day, is the greatly redshifted remnant of the hot universe as it existed about 380,000 years after the Big Bang During the first 380,000 years of the universe, radiation and matter formed an opaque plasma called the primordial fireball The microwave background radiation we see today are the same photons of the Universe at 380,000 years when the photons started to move freely in the transparent universe.

Three possible types of the Universe Universe could be closed, flat and open What type the Universe could be depends on the density parameter, the combined mass density ρ 0 (including all mass and energy) compares to a critical density ρ c Critical density ρ c = 9.5 X kg/m 3 Density parameter Ω 0 = ρ 0 / ρ c

Flat Universe Flat universe: parallel light beams remains parallel forever converge Flat geometry Curvature of space is zero The sum of the three angles is equal 180 degree The combined density equals the critical density Density parameter equals 1

Closed Universe Closed universe: parallel light beams converge; if travel in a straight line, you will return to your starting point Spherical geometry Curvature of space is positive The sum of the three angles is more than 180 degree Combined density larger than the critical density Density parameter is more than 1

Open Universe Open universe: parallel light beams diverge Hyperbolic geometry Curvature of space is negative The sum of the three angles is less than 180 degree Combined density less than the critical density Density parameter is less than 1

Cosmic background radiation reveals the type of our universe Cosmic microwave background radiation shows very small (1 over 10000) variations in the temperature across the entire sky It has hot spots and cooler temperature regions

Our Universe is flat The theoretically predicted hot spot size (about 1 degree) is very close to what is observed Therefore, our universe is flat, or density parameter is 1.0

Dark Energy The combined mass density should be equal to critical density ρ c = 9.5 X kg/m 3 The observed mass density of matter in our universe is ρ m = 2.6 X kg/m 3 The matter density parameter is 0.27 The energy density parameter should be 0.73 in order to keep the universe flat. However, the mass density of radiation is almost negligible Therefore, 73% of the universe is made of dark energy

State-of-the-Art of the Universe Age: 13.7 billion years Composition: 73% dark energy, 23% dark matter, 4% ordinary matter

Key Words average density of matter Big Bang closed universe combined average mass density compression cosmic background radiation cosmic microwave background cosmic light horizon cosmic singularity cosmological constant cosmological principle cosmological redshift cosmology critical density dark energy dark energy density parameter dark-energy-dominated universe density parameter era of recombination flat space homogeneous hyperbolic space isotropic lookback time mass density of radiation matter density parameter matter-dominated universe negative curvature observable universe Olbers’s paradox open universe Planck time plasma positive curvature primordial fireball radiation-dominated universe rarefaction relativistic cosmology spherical space zero curvature