Tell me Doctor, where are we going this time? Is this the fifties? Or Nineteen ninety-nine? Please don't drive eighty-eight; I don't wanna be late again. So take me away, I don't mind You just better promise me I'll be back in time I gotta be back in time Huey Lewis and the News, Back in Time

Assignments: Read Chapter 36, Sections 1-3 (pp. 602-612) Problem Set #7 due Tuesday 20 April, 5:00 pm Note: Final Exam is on 6 May at 8am in Olin 268

In Class: ------------------ review: ugly redshift formula, due to Einstein's Theory of Special Relativity can't exceed the speed limit of the universe but that doesn't mean that redshift can't be very large redshifts greater than one don't imply superluminal motion -------------------------- the observed large redshifts, coupled with the Hubble Law tells us that some of these galaxies are REALLY far away. distances of 100's of Mpc that is, 100's of millions or even billions of ly looking out in space is looking back in time d = v/H time = d/c = v/(cH) we don't know what distant objects look like today we only know what they looked like a long time ago How much does this matter? - depends on what you're looking at AND how fast the objects changes - look at a star 100 pc away (e.g., in our galaxy) - you're seeing light emitted 326 years ago - does that matter? - probably not - we change a lot over 326 years - stars don't - lifetimes are in billions of years - however, this does mean that if a star 100 pc away explodes as a supernova we don't know about it for 326 years. - what about a galaxy 100 Mpc away? - you're seeing light emitted 326 million years ago - does that matter? - probably still no - most stars' lifetimes are still longer (though O and B stars you're seeing now have certainly blown up already) - galactic processes (sf cycle, spiral arm propagation) probably are stable over these timescales (about 1 revolution for MW) - so even though some of the stars we see now aren't there any more, there are certainly others that have taken their place, and the general structure of the galaxy hasn't changed over the 326 MYR This is why redshift surveys of relatively nearby galaxies tell us something of the structure of the current universe - even though over huge scales, distances are small compared to the size of the universe. - travel time for photons is long, but not nearly as long as the Hubble time Example: some of the gax have v = 50,000 km/s H = 75 km/s/Mpc d = v/H = 666 Mpc t = d/c = 666 x 3.09 x 10^22 m/ 3 x 10^8 m/s = 6.9 x 10^16 sec = 2.2 x 10^9 yr = 2.2 Byr or about 15% of the age of the universe - this really isn't looking back too far in time Ex.: the Sun had been around for 2 BYR or so 2.2 BYR ago. Earth/Moon system had formed all of the planets were probably much as they are today - the big changes in the universe - galaxy formation - structure formation (in general) likely occurred at a much earlier age - models indicate that the universe hasn't changed a lot in the last 2 BYR >>>>Thus the redshift surveys of relatively nearby objects show us the structure of the present-day universe - what the universe looks like today >>>>>>>> -- distance as a time-machine<<<<<< What if we look at more distant objects - we look back in time - can see what the universe looked like a long time ago - if we're lucky, we can look far enough back in time to see its birth - almost like archaeology - dig down to see what happened at earlier times - we look far away to see what happened at earlier times most astronomers assume that the universe is homogeneous in space on really large scales - basically every thing that happened here also happened everywhere in the universe i.e., our locations is nothing special (again!) --> then looking far away is the same as looking at our part of the universe long ago this is a statement of the Cosmological Principle 1) The universe is isotropic on really large scales - looks the same no matter whichh direction you look - obviously, on small scales the universe is very ANisotropic - e.g., on one side of the Earth, there's the sun and blue sky on the other ists dark and there are stars out - that's ANisotropy - things look different in different directions --> but that's just because we happen to be close to a star - if we were just at some random point in space, it would look dark and starry in all directions - likewise, our location in the Milky Way galaxy makes our local view anisotropic - look in the direction of the disk -> see lots of stars - look perp --> see many fewer - however, if we ignore all of this local stuff and look out to really large distances we see more-or-less the same number of galaxies in every direction --> the large-scale universe IS isotropic - there can be structure - lots of structure in our present-day universe - but the structure should look more-or-less the same in every direction. Ex: block of swiss cheese - see more-or-less the same number of holes no matter which way you look 2) We live nowhere special - therefore our view of the universe is the same as the view from anywehere else - we saw in lab that the Hubble Law satisfies this - principle requires that the structure also look the same - e.g., the bubbly, filamentary structure we see must be seen from other locations, too this is the part that makes it a principle, rather than a theorem or observational result - we know that from our vantage point, the universe looks isotropic, but we have NO IDEA what the universe looks like from other vantage points - the "we live nowhere special" statement is a philosophical one - we used to have the opposite opinion i.e., we ARE somewhere special so this is a big shift in attitude - maybe because we've been burned so many times