And she was blinded by the light. Cut loose like a deuce Another runner in the night. Blinded by the light She got down but she never got tight, but she'll make it alright Bruce Springsteen, Blinded by the Light (He did it first)

Assignments: Read Chapter 37, Section 1 (pp. 620-624) Problem Set #8 due Tuesday 27 April, 5:00 pm Check out the page dscribing the Nobel Proze winning work of Robert Wilso and Arno Penzias. Note: Final Exam is on 6 May at 8am in Olin 268

In Class: consider what the universe muct have been like a long time ago - much smaller - objects have been separating for 10 Byr - they must have originally been closer together --> the universe was DENSER - same stuff in a smaller volume - higher density usually means hotter, too. - the early universe was hotter than today --ASIDE--------- another way to understand this: CO2 fire extinguisher - gas in bottle is at room temp. - when it's released, it expands - cools considerably - result of the expansion (i.e., not a chemical reaction) - CO2 fire extinguishers actually freeze a fire more than they suffocate it - the universe has been expanding for 10 Byr - it's been cooling off - so it must've been hotter before --END-ASIDE-------------------------- So the early universe was hot - dense, hot stuff emits like a blackbody - but we're talking about everything in the universe - if you were there, you'd see BB emission coming from every direction - uniform - the whole universe was hot -- like sitting in the middle of a hot oven - radiation from all sides We'll, we _were_ there, so we can see that radiation from all sides - that was a long time ago - to see light that's that old, we have to look out to the largest distances - but stuff that far away is very highly redshifted because of the expansion of the universe Redshift of about 1000: dlambda/lambda = 1000 means that optical wavelength photons are shifted by a factor of 1000 optical photons lambda = 500nm shifted to 500 nm x 1000 = 500,000 nm = 5 x 10^5 x 10^-9 m = 5 x 10^-4 m = 0.5 mm It's at these wavelengths that we see a uniformly bright sky. --------------------- CBR (or CMB): direct proof that a Big Bang ocurred - light from all directlions - uniform in intensity (or very nearly so) - spectral characteristics of a blackbody (hot stuff redshifted) only reasonable expanation is that it came from an explosion which occurred everywhere at once in the distant past "glowing ember of the Big Bang" measuring the CBR - actually pretty hard to do - most of the measurement we do is differential or relative - one student is farther away than another - distances measured in construction w/tape measure - compare length to standard length (a foot) - same thing for measuring the flux of a star - compare number of photons coming from a star versus number of phtons coming from blank sky - sky might have stray light (from a prison, say) - instrumental "noise" - usually, even then, we compare to a "standard star" - you're making both of these relative measurements in the Observing Lab - can't make a differential, or comparative measurement of something that's everywhere (ie., the CBR) - need to make an absolute measurement - very tough - need to understand all of the sources of noise in your instrument - even the guano discovery by Penzias and Wilson in 1965 - working for Bell Labs (communications research) - could not find an instrumental source for static - serendipitous discovery - sounds like they were just lucky - not so - even lucky people have to be smart enough to realize their good fortune - not everyone would link radio static with the beginnings of the universe ---------------- From those humble beginnings, the only real piece of information regarding the beginning of the universe has been exploited COBE satellite launched in 1989 to study the CBR - careful spectral study - is it really a blackbody? - if it's the glowing ember of the BB, it should be - very sensitive isotropy study - is it really uniformly distributed around the sky? - tells us how lumpy the erly universe might have been both are important diagnostics of what happenned way back then - spectral anisotropies - tells us about the time-evolution of the Big Bang - a perfect blackbody indicates a single time (or redshift) at which the explosion became visible to us - if we see more than a blackbody spectrum weird things must've happened either at different times or at different temperatures COBE's result: 2.728+/-0.004 K - one of the most perfect blackbodies ever measured - spatial anisotropy - is there structure in the CBR? - colors indicate intensity of radiation - all same color would indicate isotropy - Doppler shifts - due to local motion - Earth around Sun (tiny) - Sun around MW (pretty big - MW around local group - local, peculiar, motions just like those we saw for galaxies - although the doppler effects are the largest sources of anisotropy, they're not the interesting ones - another source of local "foreground" anisotropy: The Milky Way galaxy - dust in our galaxy is hotter than the background radiation - maybe 10-30 K - emits strongly like a blackbody - need to subtract that out if you want to see the cosmic background - look in directions away from the galactic plane - "up" and "down" from the galaxy - also need to remove the contribution from nearby galaxies that emit like the Milky Way. - getting all of the uninteresting sources of anisotropy out of the way so that you can look at the "real" background it tough - not only tough, but dangerous - we're looking for tiny fluctuations in the brightness of the background - the parts we're trying to ignore are _way_ brighter than the parts we're trying to detect - need to be really careful about the subraction process - actual structure in the background is interesting - the CBR is an image of the universe at a really young age - if the universe were a really smooth mixture of matter and energy - each part of the universe would emit the same - the CBR would be very smooth - BUT if the universe had structure way back then - some parts would have more mass or energy - might be a bit hotter or cooler than surroundings --> there might be little bright spots "fluctuations" in CBR The existence of fluctuations in the CBR, and their size (if they exist) is really important - look around - there's structure everywhere - had to come from somewhere - if the early univere was completely smooth - how do you get the rich structure we see today? - gravity can't act - particles pulled equally in every direction - the universe must develop structure - theories indicate that some structure must have developed very shortly after the Big Bang --> should show up in the CBR It does - fluctuations in temperature of 1 part in 100,000 - tiny variations - still, they indicate that even back then - there were parts of the universe that were denser than other parts --> that's all you need - gravity can take it from there So somehow, the Big Bang wasn't really smooth or uniform