Time is on my side, yes it is. The Rolling Stones, Time is on My Side

Assignments: Read Chapter 36, Section 4 (pp. 612-618) Problem Set #7 due Tuesday 20 April, 5:00 pm Problem Set #8 due Tuesday 27 April, 5:00 pm Note: Final Exam is on 6 May at 8am in Olin 268

In Class: ----------------- review: Cosmological Principle 1) The universe is isotropic 2) We don't live anywhere special --> the universe looks isotropic form every vantage point Shur's Theorem: - if the universe looks isotropic form every vantage point it must be homogeneous - what's out there is pretty much what's right here The great thing about the Cosmo Principle is that it means that the universe is the same over here and it is over there. - that means that when we look far away, we're not only seeing what that part of the universe looked like a long time ago BUT ALSO what THIS part of the universe looked like long ago. -- we're seeing our own history -- if we want to see how our universe evolved, look at abjects of varying distance from us. - really distant objects are really young - less distant objects are older - nearby objects are present-day - opportunity to study evolution of objects (galaxies, mostly) Other implications of distance as a time machine provides a nice solution to Olbers' paradox weird things happen in an infinite universe - if the universe is infinite and populated with an infinite number of stars, then we should see a star in every direction - distant stars are fainter because the appear smaller - but their surfaces have the same intensity - in an infinite universe, every line of sight from here should end at the surface of a star - should look bright ---> sky should be uniformly bright this is Olbers' paradox - actually discussed by many people over history; why Olbers gets credit I dunno; right place, right time, I suspect - Olbers lived in the early 1800's - even Kepler in the 1500's had thought a bit about it. so why isn't the sky uniformly bright? - some possibilities: - universe isn't spatialy infinite - ie, we're seeing "past" the universe??? - but what's past the universe? - modified view: universe is infinite, but number of stars isn't - limitless empty space? - sort 16th century view ok if you think the universe is finite - universe is dusty - we can't see faraway objects because of obscuration - sort of 19th century view - starting to think of an infinite universe - well, it's ok for awhile - but after time, photons absorb enough energy to glow - like putting the dust into an oven --> we should see glowing dust everywhere - distant stars are redshifted too much - faraway objects have high velocities - photons from them are shifted to longer wavelengths - but then we should see these photons in infrared or radio emission - we don't see the emission from faraway stars in infrared and radio (but stay tuned on this one....) - early 20th century view still thinking of infinite universe does some of the job, but doesn't make the sky dark - universe isn't infinite in TIME - we look back in time when we look far away - can we look back to a time when there were no stars? - if so, then there would be nothing to see - darkness though it might seem (and did to people awhile ago) that a spatially finite universe is the best solution to Olbers' paradox, it's resally the fact that the universe is temporally finite that solves it. ---> THE UNIVERSE HAD A BEGINNING - we have already inferred that from the Hubble Law relationship - if stuff is expanding outward and has been doing so for some time - then at some time in the past, the universe was much smaller - in fact, if we run the clock backward long enough, the universe was compressed to a single point, or singularity Olbers' paradox says that the universe is temporally finite - therefore had a beginning Hubble Law basically states the same thing, though it adds in that in the beginning, the universe was very small, and that it's been expanding ever since - explosion called the Big Bang - really less of an explosion, and more of a slinky expanding - its the universe itself that expands - not just the material in it - actually, the material is just along for the ride - still, if everything in the universe were packed into a small place - it would be hot - lots of energy - like sitting inside an oven which is expanding Alright, now let's take a look at the universe when it was born - how can we do that? - looking outward in space is looking backward in time - look for the photons that have been travelling for the lifetime of the universe lifetime of the universe = 1/H = 13 Byr t = d/c = v/c 1/H - so v must be = c - well, that's a problem; things can't really move at c - we really can't see the point of creation - but we can see back nearly to the point of creation - that primeval fireball from the Big Bang What would it look like? - a hot oven (hot blackbody) - should be everywhere - look far enough in every direction - see what was there long ago - the whole universe was packed into this oven -- so we should see the oven everywhere - so we should see bright blackbody emission coming from everywhere -- uh-oh, this is Olbers' paradox coming back to haunt us -- now our Big Bang theory says that the sky should be bright -- the sky isn't, so the Big Bang theory must be wrong, right? Wrong - not wrong about the reasoning - wrong about the sky not being bright - the sky IS uniformly bright, with photons coming from every direction - whatza matter? can't see them? - that's right, cuz they're radio photons - looking far away is looking backward in time - look back to the primeval fireball is looking REALLY far away - Hubble Law says that this stuff should appear to be receding from us at nearly the speed of light - redshift should be enormous - 1000 or so - what does that mean? - redshift is d_l/l - d_l/l = 1000 - d_l = 1000 l - l_obs - l_emit = 1000 l_emit = l_obs = 1001 l_emit - big change in wavelegnth - if this cosmic oven originally had a temp of 3000 K (like a star) - l_emit = 0.003/3000 = 1 e-6 m - l_obs = 1001 x l_emit = 1 e-3 m == 1 mm - mm waves are radio waves so we wouldn't expect optical photons even though the radiation started out as optical - the massive redshift turns them into radio photons. Guess what - the sky is uniformly bright in the radio - the spectrum of the radio wavelegnth radiation is that of a redshifted blackbody - the spectrum peaks at about 1mm >>> COSMIC BACKGROUND RADIATION <<<<<<