Chapter 16-17: Cosmology  Hubble’s law Expansion of Universe 1  Galaxy spectra Optical spectra of remote galaxies show redshift (galaxies appear to move away) Redshift increases with distance of galaxy Increase of recessional velocity with distance

A survey of 24,000 galaxies suggests that the universe is roughly homogeneous at the largest scale. Cosmological Principle - Homogeneous universe (the same everywhere) - Isotropic universe (the same in all directions)  Galaxy survey

Hubble’s law recessional velocity = H 0 x distance H 0 : Hubble parameter at the present time Age of the Universe = Time of Big Bang:  Age of the universe

- Receding galaxies are a result of the expanding universe - The Universe has no center and no edge Cosmological principle + Hubble expansion  Expanding pniverse  Receding galaxies

Cosmological redshift - As the universe expands, the radiation is stretched in wavelength  Cosmological redshift

 Models of expansion(cont’d) Analogy between escape velocity and universe expansion: - A rocket with insufficient velocity (< escape velocity) will fall back to the planet and the distance between the rocket and the planet eventually will become zero. - The distance between two two galaxies moving away from each other with insufficient speed will eventually decrease to zero.

 Models of expansion/contraction Low density universe: will expand forever. High density universe: Will collapse

(a) high-density universe(b) oscillating universe(c) low-density universe  Models of expansion/contraction (cont’d)

 Evidence for accelerating universe Use of Type I SN as a standard candle - All the Type I SN’s have similar luminosity and are bright. - Distance to these SN’s can be measured. - If the universe is decelerating, more distant galaxies move faster than what Hubble’s law suggests (Hubble’s law obtained from closer galaxies’ movement). - If the universe is accelerating, more distant galaxies move slower than what Hubble’s law suggests – light from these galaxies emitted when the universe was expanding slower. deccelerating universe accelerating Universe (due to dark energy?)

 Age of the universe II

Nucleosynthesis  History of cosmos Right after the Big Bang the universe was filled with radiation (photons) at high temperature. The temperature was high enough for photons to produce particles that make matter such as protons, electrons, neutrons and possibly particles that make up dark matter. Then in 15 minutes after the Big Bang protons and neutrons formed nuclei of hydrogen (75%) and helium (25%). In tens of thousands years, the amount of energy in form nuclei, electrons, and dark matter dominated over the amount of energy in form of radiation. In next a few hundred thousands years when the temperature went down to 3000 K, atoms were formed. The universe became transparent to radiation (epoch of decoupling between radiation and matter).

10 12 K 3 K 5,000 K Cosmic Microwave Background  Cosmic blackbody radiation Right after Big Bang the universe was filled with radiation. As the universe expands, the wave- length of radiation shifted red (longer wavelength) – in microwave region now. As the universe expands, the temperature of the radiation decreases to about 3 K now.

T = 2.7 K  Cosmic microwave background (CMB) spectrum

COBE map shows indicating tiny ripples of millionth of a kelvin.  Cosmic microwave background (CMB) map Early structure of the universe shows up as structure in CMB at 400, 000 years- redshift~1100, 1100 times hotter and smaller.