Reunited and it feels so good Reunited 'cause we understood There's one perfect fit And, sugar, this one is it We both are so excited 'cause we're reunited, hey, hey Peaches and Herb, Reunited

Assignments: Read Chapter 37, Section 2 (pp. 624-628) Problem Set #8 due Tuesday 27 April, 5:00 pm Note: Final Exam is on 6 May at 8am in Olin 268

In Class: -------------------------- CBR tells us what happened near the time of creation we see the highly redshifted photons from that very explosion the universe was remarkably uniform - to one part in 100,000 but not completely so - COBE saw small scale variations in the CBR, and revealed some large scale structure - can this structure "seed" the universe to produce the incredibly structured universe we see today? Answer: It's hard gravity can only act when there's inhomogeneity - consider particle of mass M in a big box - if material is evenly distributed in the box - pull due to gravity is the same in every direction - net pull is zero - nothing moves - no structure formation - if instead, there's one part of the box that has a little more mass than the rest of the box - i.e., if the density is higher than average in one place - pull in that direction will be slightly greater - particle will move that way - how hard will the pull be? - depends on how over-dense the region is - strength of pull tells us how long it will take to make the universe structured. - These little over-dense regions are then the "seeds" for galaxies and other structures in the universe COBE says that the early universe was really smooth - very few over-dense regions - the ones that did exist were only slightly over-dense - indicates that formation of strucutre will take a very long time. - if you want to create the structure we see in the luminous mass today - it's hard in 13 Byr Clearly, it happened -- i.e., we're here so how? Escape routes: - give yourself more time - smaller H --> longer Hubble time - not supported by the data (so far) - cheat - say that the universe isn't as structured as it looks - "dark matter" between the galaxies - we've see DM before - to explain gax rotation curves - also needed to explain the motions in clusters of gax - this time, though, we want it for a different reason - to "fill in" the space between the galaxies - make the universe less structured that it looks Is this cheat justified? - well, maybe - what we know is that the luminous matter is highly clumped - but what if this matter were luminous precisely because it's clumped - i.e., low density matter just doesn't emit or doesn't emit much - it would be hard to find - physicists are used to this "well it must be there" arguments - provides reasons to go out and look - has anyone found the DM? - not yet There's reason to believe that there's DM out there, but we're still "in the dark" about what it is or even whether it's there. -- we'll come back to the DM question on Monday when we talk about the fate of the universe Third possibility: - we don't really know how structure forms in the universe - in particular, galaxy formation - hot piece of new research Sum up: COBE tells us that the early universe was very smooth, with only very small variations for place to place Looking around today, we see tons of structure in the universe It's hard to take so little structure then and turn it into so much structure now. --> therefore, we need to choose one of the following statements: - our theory for structure formation is incomplete - the universe has been around longer than we think - the universe isn't really as structured as it looks today ----------------------------------------------------------------------- We've talked about what has gone on since the time of the CBR to the present, but what about the earliest times? - The CBR we see doesn't come from the first instant of the universe - actually, it probably comes form a time when the universe was 300,000 to 1 million years old. -what happened before, and why can't we see it? what goes on in the early universe - energy density (a.k.a. temperature) rules At the earliest times (i.e., in the first second or so) - so much energy packed into one place - energy density was absurdly high - so high that conversions from energy to mass and back were commonplace - when there's enough energy around, you can do almost anything - 1 proton = 1.67 x 10^-27 kg == 1.5 x 10^-7 J - recall that an optical photon has E = 10^-19 J - making a proton is equivalent to putting 10^12 photons (a trillion) all in one tiny place - need a high photon (or equivalanetly energy) density As the universe expanded, it cooled - as the universe cooled, it became harder to switch between energy and mass - once things got cool enough, the universe was basically divided into mass and energy. - still some conversion possible, but not the wholesale back and forth as before - at around 5 seconds T= few billion K - protons neutron, electrons, and a few other particles with mass were stable - however, most of the universe was still energy in the form of photons - at about 90 seconds (a minute and a half) - T = 1 billion - ok temperature for fusion - at higher temps, fusion can take place but everytime you fuse two protons together they'd get blasted apart by a photon. - once it's a little cooler, there are fewer of those high powered photons - the age of cosmic nucleosynthesis - hot enough for fusion to take place - cool enough for fusion products to be stable - tricky balance - only lasts for awhile - timing is important because it determined how much nucleosynthesis goes on - determines element abundances - enough time for about 25% of the H --> He - why not anything else? - stability - He 2He4 is really stable against photons - built tightly; - takes a very powerful photon to break it apart - not too many of those around - D 1H2 is more susceptible - it's easy to make - but it's not as well-made - wimpier photons can blast it apart - accessibility - easiest way to build a nucleus is by aggregation - eg crash 2 H together; get 1H2 - crash another H into it get 2He3 - crash another H into it get 2He4 - problem comes in the next step - crash H into 2He4 - try to make mass=5 particle - eg 2He5 or 3Li5 - very unstable -falls apart - don't get m=5 particles - therefore, don't get m>5 particles either - chain is broken - a few heavier nuclei can be made by other means - crash 2He4 + 1H2 - make 3Li6 - add H; make 3Li7 - crash two 2He4's together - make 4B8 - BUT collisions between two heavier nuclei are much more rare than collisions between anything and H -cuz there's so much H around - bunch of cars and few trucks on the highway - car-car collisions all the time - some car-truck collisions - truck-track collisions rare Once universe cools below 100 million K - cosmic nucleosynthesis stops - mostly H - 25% He - 0.01% D - even wimpier traces of Li, Be, B - how much you get of each type depends critically on how long cosmic nucleosynthesis period lasted - ie., 5 minutes, 10? how dense the universe was at that time - how many collisions took place during that time how fast it cooled ----ASIDE on Observational Cosmology--------- - can turn this problem around and look at the abundances of these elements today as a probe of the early universe conditions - astronomers try to measure light element abundances to constrain theories of the Big Bang - problem: fusion in the cores of stars make these elements too - tough to figure out how much of what you see comes from the cosmic nuke period and how much comes from stellar processing ---End ASIDE-------------------------- Still not to the point of the CBR emission yet - It's still hot in the universe for another million years - cosmic nuking has stopped - but atoms have yet to form - too make high-pwered photons flying around - try to capture an electron into an atomic orbit - photon comes along and ionizes atom; frees e- - As a result, photons can't travel far without being absorbed - by a free electron (just changes speed) - by a temporarily captured e- in an "atom" - any photon can be captured - no quantization of energy levels, because e- aren't trapped in atoms - Once the universe cools enough (to about 3000 K) - there aren't as many high-pwered photons - atoms can form (or "combine") - won't be destroyed by photons - once the e- are all confined in atoms - they can't absorb just any photons - now they only can absorb photons whose energies are equal to the difference in atomic energy levels - only magic photons - THEREFORE, the photons that don't have the right energy are free to travel off into space - they're _not_ absorbed by the atoms - these photons are finally free to start their 13 BYr journey to us The photons that make up the CBR were "released" from their surroundings just as the universe become atomic and therefore transparent to most photons. - the CBR tells us about the universe when it was at a temperature of 3000 K - a time about 300,000 to 1 million years after the Big Bang ------------------------------ We can divide the universe into epochs, based on what the universe's dominant consituents are at each time Energy Epoch first five sseconds Particle Epoch 5-100 seconds Nuclear Epoch 100 seconds - 1 million years Atomic Epoch 1 million years -- now The Future ?????