Frontiers and Controversies in Astrophysics: Lecture 21 Transcript
April 17, 2007
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Professor Charles Bailyn:
Welcome to prefrosh [newly admitted students visiting during Yale's "Bulldog
Days" program]. I'm talking about black holes at 8 o'clock this evening. Come listen to that, too, and in the
meantime, we'll talk about the future of the Universe. Okay. Logistical questions? Anything?
All right. Where were we? We were here. This is the data that demonstrate that the Universe is filled with
dark energy, a kind of anti-gravity that is pushing the Universe apart. And what this is--this is the same data
on both of these plots, just plotted slightly differently. On the top plot, what they've done is they've plotted
the apparent magnitude of these supernovae versus their redshift. And so, apparent magnitude--because the
absolute magnitude is always the same. These are standard candles. So, from the apparent magnitude, you
can figure out what the distance modulus is. That gives you a measure of the distance. So, this is just distance
versus velocity.
Now, we've plotted distance versus velocity of bunch of times. That's the Hubble Diagram. Most of the times
we've plotted it, it's come out on a straight line. Do you understand why this plot isn't on a straight line? The
top one, I'm talking about, now--why those lines are curved? What's the
y
-axis? The
y
-axis is--yes?
Student:
It's logarithmic.
Professor Charles Bailyn:
It's logarithmic. Thank you very much. Yes, it's magnitude. Magnitudes are
upside down and logarithmic, right? That's why faint stars have high numbers up at the top. And because one
axis is logarithmic and the other axis isn't, of course, it curves. You could--the way--if you want a nice
straight line, you just make this axis logarithmic, as well.
What we've done down on the bottom is just subtracted off the empty Universe--a Universe with no matter
and no energy in it--so that you can see more clearly the way these three lines diverge. And these three lines
are three different models of the Universe. And they're denoted by these Î© factors. Î©
matter
, that's the
density of matter divided by the critical density. Î©
Î»
, that's the energy density of the dark energy divided by
that same critical density.
And down here is an Î©
matter
of 1. That's the dividing line between a Universe that re-collapses and a
Universe that expands forever. So, if the Universe were--if the points that we studied were down here, the
Universe would re-collapse. If they're up here, it wouldn't. This dotted line here is 1/4 and 0.
1/4 in Î©
matter
is the amount of matter that we actually observe in dark matter and galaxies. If you go out and
count up all the dark matter in the galaxies, add it all up, you get to about 1/4 of the critical density. So, what
people were kind of expecting to see was something between here and here.