Chapter 14 The Expansion of Space 14 1 The Cosmological Principle Shapes Our View of the Universe galaxy a gravitationally bound grouping of stars dust and gas the cosmological principle the realization that the physical laws that apply to one part of our universe apply to all parts cosmology is the study of the universe homogeneous same in all positions isotropic same in all directions 14 2 We Live in an Expanding Universe Edwin Hubble found variable stars in Andromeda similar to Cepheid variables though fainter found they were at great distances Vesto Slipher Arizona obtained spectra of these galaxies the galaxy looked like the spectra of collections of stars with a bit of glowing interstellar gas mixed in all Doppler velocities were redder longer the galaxies were moving away from us the further they were the faster they moved rest wavelength the wavelength in the laboratory upside down y redshift written as z the higher this number the faster it is moving away Hubble s Law Hubble combined Slipher s redshifts with galaxy recession veloci ties and found that the velocity at which a galaxy is moving away from us is pro portional to the distance of that galaxy Ho H naught is the constant of proportion ality between the distance and the speed the Hubble constant Hubble s law helps us test the prediction that the universe is isotropic galaxies in one direction obey the law just as those in other directions and homogenous like pa perclip ant we are sitting in a uniformly expanding universe and expansion looks the same regardless of location therefore we are not the center of the universe exception to the rule in the case of galaxies that are gravitationally bound to gether gravitational attraction dominates over the expansion of space therefore gravi tational or electromagnetic forces can overwhelm the expansion of space SS isn t ex panding the distance ladder relates distances on a variety of scales ways to measure distance for close distances radar near stars parallax Milky Way spectroscopic parallax nearby galaxies Cepheids distant galaxies Ia super novae Type Ia supernova occur when gas glows over from an evolved star onto its white dwarf companion pushing the white dwarf over the Chandresekhar limit for the mass of an electron degenerate object the overburdened white dwarf then begins to collapse and explodes all Ia supernova occur in white dwarfs of the same mass so about the same lu minosity good standard candles because of this and extreme brightness billion times brighter than the sun it has been decided that the Hubble constant is around 72 km s Mpc because velocity and distance are proportional to each other the points on a graph have a slope equal to the Hubble constant once we know the value of the Hubble constant we can use a straightforward measurement of the redshift of a galaxy to find its distance we use redshifts and dis tances to find H0 and then use H0 to find the distances Hubble s law places galaxies in space as well as in time in general when looking at a distant object we speak of its look back time 14 3 The Universe Began in the Big Bang assuming that galaxies have been moving apart at the same speed that we see today than 13 7 billion years ago there was no space between all the bits of matter and energy that make up today s universe all the matter was unimaginably dense and hot Hubble time the value of 1 divided by the Hubble constant it represent uni verse s age the Big Bang theory has grown stronger with time the Big Bang took place everywhere wherever you are in the universe today you are sitting at the site of the Big Bang galaxies aren t flying apart through space space itself is expanding carrying the stars and galaxies with it this is different from stars and galaxies moving apart through space Hubble s law demonstrates this there is no limit to this expansion the fabric of space can go on expanding for ever so if its continually stretching how would we measure it if are units are also stretching this is scale factor it tells us the size of the fabric relative to its size at the time when we drew our ruler and how much the distance between points has changed however the laws of physics are unchanged by the changing scale factor as we get closer to the Big Bang the scale factor gets smaller and smaller ap proaching zero started smaller than an atom at a 50th of a second was Earth sized So where is the center of the Big Bang It didn t occur at a specific point in space space came into existence with the Big Bang the Big Bang is homogenous iso tropic although the distance between galaxies is increasing as a result of the expansion of the universe and we can use the equation for Doppler shifts to measure the red shifts the redshifts are NOT due to Doppler shifts as light comes toward us the scale factor of space is constantly increasing as well as the distance between wave crests so light is stretched out as space expands therefore the redshift from distant galaxies is a direct measure of how much the uni verse has expanded since the time when the radiation left its source 14 4 Major Predictions of the Big Bang Theory Are Resoundingly Confirmed Gamow and Alpher thought about the implications of Hubble expansion in early 1900s they reasoned that since a compressed gas cools as it expands the universe should be cooling and when it was very young and small it must have been hot and dense which would have made it awash in blackbody radiation they reasoned that this radiation would have redshifted to longer wavelengths as the universe expanded 1948 Alpher Bethe and Gamow published a paper asserting that the radiation from the early universe should still be visible today and should have a blackbody spec trum with a temp of 5 10 K 1960s Penzias and Wilson needed a very sensitive microwave telescope to prove it worked tirelessly to eliminate all sources of interference from the instrument bird droppings etc no matter how hard they tried to eliminate extraneous noise they could still detect a faint microwave signal when pointing the telescope at the sky 1965 Robert Dicke of Princeton interpreted it as the radiation left behind by the hot early universe a temp of about 3K In the ionized early universe light was trapped by free electrons radiation had a blackbody spectrum When the universe recombined it became transparent and the blackbody radiation went its own way like a fog clearing cosmic microwave background radiation the radiation left over from the early