Cosmology

1. Cosmology • Structure in the Universe has seemed to exist on the largest scales we can measure • distributions of superclusters and voids of galaxies get as large as 200 Mpc • no measurements done out to average quasar distances • use “pencil-beam” surveys to look at structure beyond • only count galaxies as function of redshift in a vary small region of the sky • see structures no larger than 200 Mpc extending out to about 3 Gpc

2. Cosmology (cont.) • if sample is taken over a range larger than 300 Mpc, the universe will appear homogeneous • ther is no structure on scales larger than 200 Mpc, so the Universe looks smooth for scales of that size • The Cosmological Principle has two basic tenets • the Universe is homogeneous as mentioned above • the Universe is isotropic • it looks the same in any direction • consequences of the cosmological principal include • the Universe has no edge so that homogeneity will not be violated • isotropy means the Universe has no center • The age of the Universe can be estimated from the Hubble constant • since the current value of 65 km/s/Mpc is actually a frequency, just invert to estimate the age • current value gives ~ 15 billion years • not really too accurate since the Hubble constant is likely to have varied as a function of time • simply demonstrates the finite age of the Universe • Hubble’s law implies that ~ 15 billion years ago, all matter and radiation in the Universe existed in a single point

3. Cosmology (cont.) • there was an explosion where all of space and time were thrusted outward -- the Big Bang • Hubble’s Law seems to violate the cosmo. principle • individual redshifts of galaxies are different when taken from different host galaxies • but when the redshift vs. distance relation is graphed again, a line with the same slope, Hubble’s constant, appears • The expansion of the Universe has no central point of origin • the Big Bang was not an ordinary explosion • the key point is that space exploded along with the matter and radiation • imagine the two-dimensional analog, a surface, on an inflating balloon, marked with fiducial points • as the balloon inflates, the points ALL separate from each other, as the spacial surface of the balloon expands

4. Cosmology (cont.) • The same analogy of expanding space can be extended to radiation • radiation (photons) are tied to space from theory of general relativity • as space stretches due to expansion, photons must stretch • wavelengths get longer • known as the cosmological redshift • not related to velocity like the doppler shift • Some things may be said about what might have been prior to the Big Bang • not even general relativity is sufficient to describe the workings of a singularity • not even time existed prior to the Big Bang • there was no “before” • current known physics can only describe what occurred in the Big Bang at times greater than 10-43 sec after Bang

5. Cosmology (cont.) • The fate of the Universe depends on the amount of matter in the Universe • there are three possibilities • the Universe is unbound • it will keep expanding forever • the Universe is bound • at some point the Universe will stop expanding and begin contracting back towards a spacetime singularity • the Universe is not expanding and has always been the way we currently see things • this is called the steady state model of the Universe • there is absolutely no evidence to support this • the key parameter is known as the critical density of the Universe • for H = 65, this density is 5 H atoms / cubic meter ! • if the density of matter in the Universe is greater, then the total gravity from all the matter causes acontraction

6. Cosmology (cont.) • if the density of the Universe is less than the critical density, the Universe will expand forever • usually astronomers talk about the ratio of the actual density to the critical density • call it 0 • = 1 , the Universe is marginally bounded • > 1 , the Universe is bounded • < 1 , the Universe is unbounded • we can try to measure the mass content of the Universe • mass in luminous matter (galaxies) => L = 0.01 • dark matter from cluster motions and galaxy light curves => D = 0.1 - 0.3 • possible very large scale distributions of dark matter from 100-150 Mpc scales => D2 ~ 0.5 - 1.0 ? • another way to determine 0 is through the deceleration of the Universe • the Universe expanded faster in the past • the greater the density, the greater the deceleration • look for changes in the Hubble law diagram where, at the most distant galaxies, Hubble’s Law breaks down

7. Cosmology (cont.) • galaxies show some signs of deceleration but the errors are large • recent measurements of Type Ia SNe standard candles shows the Universe is accelerating, not decelerating • if true, would mean a total rewrite of cosmology theory • don’t know if all these Type Ia SNe are of the proper behavior, because some are known to be very bright or very faint • The best hint that the Big Bang happened at all is the Cosmic Microwave Background • discovered in 1964, by Robert WIlson and Arno Penzias while searching for sources of radio noise that might interfere with phone transmissions

8. Cosmology (cont.) • they noticed that this background noise appeared in every direction they looked at the same intensity • realized it was the remnant signature of the Big Bang • the CMB is the redshifted radiation of the Big Bang • started out as gamma ray, but became cosmologically redshifted to the microwave region • in 1989 the COBE satellite determined the CMB is a perfect blackbody curve with a characteristic temperature of 2.735 K • showed an extreme degree of isotropy (to 1 in 100,000) in all directions

9. Cosmology (cont.) • also showed net redshift/blueshift caused by the Earth’s velocity through space => 400 km/s