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22. Supernovae

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    Professor Charles
    Bailyn: We had gotten,
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    at the end of last time,
    to the Big Rip.
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    This is where the
    ever-expanding Cosmological
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    Constant--which is,
    of course, a contradiction in
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    terms.
    If you imagine that dark energy
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    is not a Cosmological Constant,
    that it's actually increasing
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    in time, rips everything to
    shreds.
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    The Universe becomes infinite
    in size in a finite amount of
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    time and everything gets pulled
    apart.
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    What I want to start by doing
    today is to go back and do that
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    again slowly,
    okay?
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    Because I think this is not a
    simple line of reasoning that
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    leads you to this.
    So, let's go all the way back
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    to what we know about the
    Universe from observing
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    galaxies, supernovas,
    things like that.
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    We know that the Universe is
    expanding and that,
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    we know.
    Hubble already figured that out.
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    We know this from the Hubble
    Law, and from other--just,
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    in general, from the study of
    standard candles at relatively
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    low redshift.
    So, distances out to,
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    I don't know,
    Z of less than 0.2,
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    or so.
    You can't tell the difference
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    between an accelerating or a
    decelerating Universe.
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    All you know is that it's
    expanding, and you can figure
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    out how fast it's expanding.
    Now, the next question then
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    becomes the rate of expansion.
    Is the rate of expansion
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    changing?
    Is it accelerating?
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    Is it decelerating?
    What's going on there?
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    And so, you want to compare
    acceleration versus
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    deceleration,
    which is a fancy word,
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    as you know,
    for slowing down.
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    And deceleration:
    this is relatively easy to
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    grasp.
    This is what happens because of
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    matter.
    Matter exists.
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    It's got gravitational force,
    and gravitational force tends
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    to hold things together.
    So, if you've got something
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    that's moving apart,
    and there's some gravitational
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    force, it'll tend to hold it
    together, and thus,
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    slow it down.
    Most of the matter,
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    it turns out,
    is this weird dark matter,
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    which we don't understand,
    but that's what's responsible
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    for the deceleration.
    If you're going to make it go
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    faster, you need something much
    weirder that pushes outward,
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    that has repulsive force.
    This, we believe,
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    actually exists.
    This, we label dark energy.
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    And so, the question of,
    "is it accelerating or is it
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    decelerating," is basically a
    question of how much dark energy
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    is there versus how much matter
    is there,
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    because the more--if you have
    more of this than this,
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    then it will accelerate and
    vice versa.
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    And what we have discovered by
    supernovae observations with
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    redshifts of greater than 0.3,
    and by now, out to about a
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    redshift of 1,
    or so, demonstrate that--well,
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    what exactly does this say?
    Let's go slowly.
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    In the past,
    the Universe was expanding more
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    slowly than it is now,
    and therefore--those three dots
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    are the mathematical symbol for
    therefore.
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    Therefore, the Universe is
    accelerating,
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    and therefore,
    there's more dark energy than
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    there is matter.
    And you'll recall,
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    we did the little pie chart of
    the Universe,
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    and it turns out,
    it's 3/4 dark energy and 1/4
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    matter.
    And those proportions are
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    determined by the rate at which
    the Universe is accelerating.
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    Not the rate at which it's
    expanding, but the rate at which
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    it's accelerating,
    because that's the change in
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    the expansion.
    Fine, so far--or maybe it isn't
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    fine.
    I should pose that as a
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    question.
    Fine?
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    This is the basic premise of
    where we had gotten to about a
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    week ago.
    Yes sir?
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    Student: How does dark
    energy cause acceleration?
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    Professor Charles
    Bailyn: Huh?
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    Student: How dark energy
    [inaudible]
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    Professor Charles
    Bailyn: How dark energy
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    causes acceleration?
    Excellent question.
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    I have no idea,
    because we don't have any idea
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    what it is.
    And it shows up-- the reason we
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    have even a name for it,
    you know, is that it showed up
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    in Einstein's equations as a
    potential contribution to the
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    Universe that you could add,
    but didn't have to.
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    A constant of integration,
    for those of you who like that
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    kind of thing.
    What it is, physically?
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    Very hard to understand.
    The only physical explanation
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    that's been offered--well,
    many have been offered.
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    But the only one that connects
    to anything else we know is the
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    idea that it's the vacuum energy
    predicted by quantum mechanics.
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    And it turns out that if that's
    true, it ought to be 10^(120)
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    times stronger.
    And so, that's not a good
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    prediction.
    And so, exactly how this
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    works--what the mechanism is,
    what the nature of this stuff
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    is?
    Completely unknown.
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    And the only reason we think
    it's there is that we see its
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    effect.
    We see the Universe getting
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    faster.
    And so, it's just a name that's
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    attached to whatever is causing
    that effect.
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    Okay. Yes?
    Student: How do you know
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    the Universe is expanding more
    slowly?
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    Professor Charles
    Bailyn: Was expanding more
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    slowly than it is now,
    right.
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    So, that is this transformation
    between these two different
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    kinds of graphs,
    one of which is the observed
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    graph and one of which is the
    plot of the scale factor versus
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    time.
    Here's now.
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    Here's 1.
    We're at 1 and now.
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    And so, if it were not
    accelerating,
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    if it were just coasting along,
    that would be a straight line.
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    And what happens is,
    we can look back into the past
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    and what we see is this.
    So, at this point,
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    if you think about it--believe
    me, for a sec,
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    that this is true.
    Supposing you were sitting here.
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    The rate of expansion is the
    slope of this line.
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    That's how much you're
    expanding.
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    And so, at this point,
    the slope is shallower than it
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    is up here.
    And so, one interpretation of
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    this line is that in the past,
    it used to be expanding,
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    you know, like this,
    and now, it's expanding like
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    this.
    Student: [Inaudible]
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    Professor Charles
    Bailyn: Ah,
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    how do you know it's doing that
    particular shape?
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    Well, you, of course,
    observe points all along the
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    way, here.
    Now, we're about to get to the
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    question of whether that shape
    actually continues.
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    If you asked,
    how do you know that it would
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    continue this way,
    that, of course,
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    you don't know for sure.
    If you invent some piece of
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    magic whereby everything changes
    right now for some obscure
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    reason, it could go off in some
    other direction.
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    But the assumption is that
    whatever it is that's--that we
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    do not live at a special time,
    and that, therefore,
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    whatever it is that's doing the
    accelerating is going to keep at
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    it.
    And indeed, the whole of the
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    Big Rip, which is where I'm
    going with this,
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    comes about by asking the
    question,
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    supposing the acceleration rate
    is changing in a different way
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    from the way we thought it was
    going to.
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    Just to complete the thought
    here, the way this works out in
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    the observational plane,
    you'll recall,
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    looks like this.
    Here's the empty Universe in
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    this set of units.
    And you observe a bunch of
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    supernovae and they seem to be
    doing that.
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    And that line,
    if you, then,
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    transform redshift and
    distance--this is kind of a
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    weird measure of distance--into
    scale factor and time.
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    Let me make this line solid,
    so that the two graphs
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    correspond.
    This line and that line
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    correspond with each other.
    And so, as you observe many
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    points along here,
    that's essentially observing
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    many points along here.
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    Okay, yes?
    Student: Given the
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    empirical data that we know now,
    if we were to extra--to improve
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    to extrapolate that curve,
    would it actually intersect
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    with the t-axis or would
    we need to have [Inaudible]?
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    Professor Charles
    Bailyn: Ah,
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    let me come back to that.
    Let me come back to that.
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    The answer is,
    yes, it does go to zero.
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    It doesn't keep going up like
    this, but let me come back to
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    that in a second,
    and you'll see why.
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    Okay.
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    So, it's all a balance of dark
    energy versus dark matter,
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    or matter, in general,
    but the dark matter
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    predominates.
    And, at the moment,
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    it is true that the dark energy
    is--there's more of it.
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    It exerts more,
    more of an influence on the
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    Universe;
    therefore, the Universe is not
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    just expanding,
    but also accelerating.
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    We know that because these
    points are above the line,
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    not below the line.
    But this didn't always have to
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    be true.
    This balance changes with time.
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    So, currently,
    DE blows away the DM,
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    but this can change.
    This changes.
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    In fact, this is almost certain
    to change with time.
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    And the reason is that the
    energy density of the dark
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    energy and the matter density of
    the dark matter behave
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    differently as you change the
    size of the Universe.
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    It's all about the density
    here, so, matter density in the
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    past.
    You have the same amount of
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    matter, but the Universe was
    smaller.
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    Density is equal to mass over
    volume.
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    So, if you have the same
    M, but smaller V,
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    this would have to have been
    bigger in the past.
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    And, in fact,
    it goes as the scale factor
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    cubed, or 1 over the scale
    factor cubed.
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    Just because volume goes as a
    linear scale cubed.
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    So, in the past,
    there was--the matter density
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    was much greater than it is
    right now,
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    and therefore,
    the gravitational force trying
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    to hold the Universe together
    was greater than it is right
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    now.
    But the Cosmological Constant
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    is constant, and what that means
    is that its density is constant.
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    And what that means is that,
    in the past,
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    the dark energy density was the
    same as it is now.
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    That's just the way this
    particular piece of Einstein's
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    equations works.
    It's a constant.
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    Now, if you're about to ask the
    question, well,
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    how does anything behave that
    way, I'm going to give you the
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    same answer.
    You know, they have this
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    wonderful thing in the British
    Parliament where they ask the
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    Prime Minister
    questions--obnoxious questions.
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    And when they start getting too
    many obnoxious questions in a
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    row, what he'll do is,
    he'll look at the camera and
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    say, "I refer the Honorable
    Gentleman to the answer I gave
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    some moments ago."
    So, if you're going to ask what
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    this is and why it behaves that
    way, I will refer you to the
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    answer I gave to the Honorable
    Gentleman some moments ago;
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    namely, I don't have a clue.
    Something they don't generally
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    say in political situations.
    All right.
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    So, it's the same as now.
    But look at the implication of
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    this.
    If the matter density gets
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    bigger and bigger and bigger as
    you go into the past,
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    and the dark energy density
    does not, then at some point in
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    the past, it must have been true
    that the matter density
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    overwhelms the dark energy.
    And then, at some point in the
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    past, the dark energy and the
    dark matter exert comparable
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    effects on the Universe.
    And before that,
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    dark matter wins--that is to
    say, the Universe was
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    decelerating.
    Now turn this around and go in
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    chronological order.
    From the start of the Universe
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    until some moment,
    the Universe decelerated.
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    It was expanding,
    but it kept getting slower.
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    Then, after some moment--and we
    know that this moment was in our
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    past, because after this moment,
    it starts to accelerate.
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    And we know that it's
    accelerating now.
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    And so, at some point in the
    past, there was a magic moment
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    where everything balanced,
    and then, it started to
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    accelerate.
    So, now, what does this look
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    like on our graphs?
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    What does this look like on the
    graphs, here?
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    Well, let's do the a
    versus t thing,
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    again.
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    Here we are.
    Here's our reference empty
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    Universe.
    And so, recently,
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    it's been accelerating.
    So, you know,
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    if we look into the past.
    And then, the prediction is,
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    the extrapolation back to zero
    looks kind of like this.
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    And remember,
    if it looks like this,
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    it's decelerating.
    If it looks like this,
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    it's accelerating.
    And so, the dotted lines are
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    extrapolations based on the idea
    that the dark energy is the
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    Cosmological Constant,
    and the dark matter behaves
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    like matter ought to behave.
    And so, if you make those two
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    assumptions, this is the curve
    you get.
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    And so, this is what you expect
    for Ω_matter = .25;
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    Ω_λ = .75 at
    present.
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    And so, you can predict,
    given these two quantities,
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    and a measurement of
    H_0 equals
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    about 70.
    This gives you an age of the
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    Universe.
    That is to say,
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    the time from when a = 0
    to now--of the Universe,
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    of something like 13 point;
    I think it's actually,
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    at the moment,
    13.4 billion years,
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    plus or minus,
    I don't know .4,
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    or something like that.
    Under the assumption that λ
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    equals--that the Cosmological
    Constant really is constant,
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    that you've got these kinds of
    proportions, and this kind of
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    current Hubble Constant.
    And then, you can just
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    extrapolate all these lines in
    whatever direction you like,
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    and figure out where it crosses
    a = 0.
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    Yes?
    Student: Can we see back
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    to [Inaudible]
    see it as deceleration or is it
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    [Inaudible]?
    Professor Charles
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    Bailyn: Excellent question.
    The question is,
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    "Can we see far back in the
    past to see this deceleration?"
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    The answer is,
    just about yes,
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    and I'll show you some data in
    a second.
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    Because this is a very strong
    prediction of the model--that is
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    to say that it should turn
    around.
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    If you're imagining that
    there's something wacky about
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    supernovae, and that,
    as you go into the past,
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    they look fainter or something,
    as you were assuming on the
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    problem set,
    then you wouldn't necessarily
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    predict that that would turn
    around and do the other thing as
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    you go further back in the past.
    So, this is a very strong
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    prediction of what ought to
    happen cosmologically,
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    and so, of course,
    people have been trying to test
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    it.
    Yes?
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    Student: If the density
    of the Universe is changing,
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    then wouldn't it indicate that
    we're in some special time right
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    now,
    given the fact that the current
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    density of the Universe is
    really close to the critical
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    density?
    Professor Charles
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    Bailyn: Yeah.
    So, the question is about,
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    if the density is changing,
    why does the--what's so unusual
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    about now, that we're close to
    the critical density?
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    The thing you have to remember
    is, the critical density also
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    changes with time.
    It's based on--remember the
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    critical density.
    It's 3H^(2) over
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    something or other.
    Well, H changes with
  • 18:21 - 18:24
    time, because the velocity is
    more or less the same,
  • 18:24 - 18:26
    but the distances all change.
    Student: So,
  • 18:26 - 18:27
    it's always been pretty close
    to the critical density?
  • 18:27 - 18:29
    Professor Charles
    Bailyn: Well,
  • 18:29 - 18:31
    what is true is that--the way
    it works out,
  • 18:31 - 18:34
    mathematically,
    what's true is that the sum of
  • 18:34 - 18:37
    these two quantities has always
    been close to 1.
  • 18:37 - 18:43
    The ratio changes,
    but the sum of these two--if
  • 18:43 - 18:48
    the total Ω starts as 1,
    it stays at 1.
  • 18:48 - 18:51
    But, it turns out,
    it's the total Ω that you
  • 18:51 - 18:57
    can--that is conserved.
    And we'll come back to that
  • 18:57 - 19:08
    point a little later.
    Okay, let's see.
  • 19:08 - 19:10
    Right. Okay. So, yes.
    Let's go there.
  • 19:10 - 19:12
    Let's go for the observational
    evidence.
  • 19:12 - 19:15
    What would you expect it to be?
    Here we go.
  • 19:15 - 19:19
    This one, again.
    We keep going back and forth
  • 19:19 - 19:22
    between these two plots.
    That's what it's all about.
  • 19:22 - 19:24
    So, here's empty Universe,
    as always.
  • 19:24 - 19:32
    And then, in this plot,
    acceleration means a positive
  • 19:32 - 19:35
    slope.
    So, if you're moving up in this
  • 19:35 - 19:40
    plot, the Universe has been
    accelerating--is accelerating at
  • 19:40 - 19:44
    that particular redshift,
    at that particular time in the
  • 19:44 - 19:45
    past.
    So, you know,
  • 19:45 - 19:49
    the data we've looked at so far
    kind of look like points along a
  • 19:49 - 19:51
    line like this.
    That's the demonstration that
  • 19:51 - 19:53
    the Universe is now
    accelerating.
  • 19:53 - 19:54
    So, here's the prediction.
  • 19:54 - 19:58
  • 19:58 - 20:02
    And, you know,
    here's a magic moment where,
  • 20:02 - 20:05
    at that point,
    this slope is flat,
  • 20:05 - 20:10
    and so, on this side,
    it's decelerating,
  • 20:10 - 20:14
    and on this side,
    it's accelerating.
  • 20:14 - 20:18
    And the question is,
    is it getting further away
  • 20:18 - 20:22
    from--is it moving up compared
    to the empty Universe,
  • 20:22 - 20:25
    or is it moving down compared
    to the empty Universe?
  • 20:25 - 20:28
    Because the empty Universe
    neither accelerates nor
  • 20:28 - 20:31
    decelerates.
    That's the definition of an
  • 20:31 - 20:33
    empty Universe.
    It doesn't have any mass.
  • 20:33 - 20:37
    It doesn't have any energy.
    The expansion rate is constant
  • 20:37 - 20:40
    forever.
    So, if you're going this way
  • 20:40 - 20:43
    with respect to that line,
    or going this way with respect
  • 20:43 - 20:45
    to that line,
    you're accelerating,
  • 20:45 - 20:48
    and if you're coming back,
    going in the other direction,
  • 20:48 - 20:52
    you're decelerating.
    So, here's a closed Universe,
  • 20:52 - 20:55
    right?
    This is a Big Crunch Universe,
  • 20:55 - 21:00
    which is going to re-collapse,
    because it's decelerating all
  • 21:00 - 21:03
    the time.
    Now, interestingly,
  • 21:03 - 21:07
    this turnaround point,
    where you turn around between
  • 21:07 - 21:12
    accelerating and decelerating,
    is not so far back into the
  • 21:12 - 21:15
    past.
    If you believe this set of
  • 21:15 - 21:19
    parameters, the standard model,
    it's at around 0.8.
  • 21:19 - 21:25
    We can see stuff at 0.8.
    And you could hope to go and
  • 21:25 - 21:30
    see things even further out.
    Turns out, it's kind of--now,
  • 21:30 - 21:34
    right at--if you only go out to
    about 0.8, which is about as far
  • 21:34 - 21:36
    out as they did for the first
    time round,
  • 21:36 - 21:39
    you're not going to see the
    turnaround, because,
  • 21:39 - 21:41
    you know, it's just more or
    less flat.
  • 21:41 - 21:44
    You've got a few points out
    there, and you're going to have
  • 21:44 - 21:47
    a tough time telling the
    difference between that and that
  • 21:47 - 21:50
    at this point here.
    But if you go out just a little
  • 21:50 - 21:54
    bit further, you might really be
    able to see this turnaround
  • 21:54 - 21:56
    happening.
    Unfortunately,
  • 21:56 - 22:01
    that turns out to be hard,
    observationally.
  • 22:01 - 22:08
    It's hard to see supernovae
    with redshifts greater than 1,
  • 22:08 - 22:12
    for a number of different
    reasons.
  • 22:12 - 22:16
    First of all, they're faint.
    The further away they are,
  • 22:16 - 22:19
    the fainter they are.
    Second of all,
  • 22:19 - 22:24
    they appear on top of galaxies.
    Obviously, they live in
  • 22:24 - 22:27
    galaxies.
    The further away you look,
  • 22:27 - 22:31
    the galaxies look smaller,
    and so, the supernova appear to
  • 22:31 - 22:37
    be--the further away you look,
    the more light from the galaxy
  • 22:37 - 22:41
    the supernova is superimposed
    on.
  • 22:41 - 22:44
    And it becomes harder to
    separate the light from the
  • 22:44 - 22:48
    supernova and the light from the
    galaxy that it lives in.
  • 22:48 - 22:54
    And the third part is that the
    light is redshifted.
  • 22:54 - 22:58
    Very redshifted.
    Redshifted by a factor of two.
  • 22:58 - 23:01
    So, supernovae give off most of
    their light in optical light,
  • 23:01 - 23:04
    which we can observe.
    But by the time they're at a
  • 23:04 - 23:06
    redshift of 1,
    all the wavelengths are
  • 23:06 - 23:09
    doubled.
    And so, most of the radiation
  • 23:09 - 23:13
    is in the infrared,
    which is much harder to see
  • 23:13 - 23:18
    from the ground,
    because everything glows in the
  • 23:18 - 23:21
    infrared.
    And, as I said before,
  • 23:21 - 23:23
    it's like looking in the
    daytime.
  • 23:23 - 23:26
    The whole sky is glowing.
    Your telescope is glowing.
  • 23:26 - 23:31
    It's just really hard to make
    those kinds of observations.
  • 23:31 - 23:34
    It is possible,
    however, to do--it's much
  • 23:34 - 23:36
    easier to do this from space.
  • 23:36 - 23:41
  • 23:41 - 23:43
    Now, faint is faint.
    It doesn't matter whether
  • 23:43 - 23:46
    you're in orbit or not.
    But the other two things are
  • 23:46 - 23:48
    greatly aided by looking from
    space.
  • 23:48 - 23:55
    Point two.
    You get much better images from
  • 23:55 - 23:58
    space.
    And you may recall,
  • 23:58 - 24:00
    last time, I showed you a
    picture of a supernova,
  • 24:00 - 24:03
    and from the ground,
    you know, it was a little extra
  • 24:03 - 24:07
    light on top of some galaxy.
    And then, when you looked from
  • 24:07 - 24:10
    space, the galaxy was clearly
    delineated.
  • 24:10 - 24:13
    And then, the supernova was a
    tiny point in the outskirts of
  • 24:13 - 24:15
    that galaxy.
    So, it makes it much,
  • 24:15 - 24:20
    much easier to separate the
    galaxy from the supernova.
  • 24:20 - 24:26
    And it is also true that the
    infrared background light is
  • 24:26 - 24:31
    much fainter,
    because you can keep stuff cold
  • 24:31 - 24:36
    out there.
    The infrared background light
  • 24:36 - 24:42
    is much fainter and there's no
    atmospheric absorption.
  • 24:42 - 24:48
    The atmosphere actually absorbs
    a large fraction of the infrared
  • 24:48 - 24:52
    light that hits us.
    That's actually why the
  • 24:52 - 24:54
    greenhouse effect works,
    because light comes in as
  • 24:54 - 24:57
    optical light and it makes it
    all the way through.
  • 24:57 - 24:59
    Then, it heats up the surface
    of the Earth.
  • 24:59 - 25:02
    The Earth radiates heat,
    which doesn't make it through
  • 25:02 - 25:04
    the atmosphere and it gets
    trapped.
  • 25:04 - 25:08
    So, there's no atmospheric
    absorption out there,
  • 25:08 - 25:12
    and so, you have a much easier
    time doing this from space.
  • 25:12 - 25:16
    So, what they have been doing
    is, recently,
  • 25:16 - 25:20
    people, and in particular,
    a guy named Adam Riess,
  • 25:20 - 25:27
    have been using the space
    telescope, the Hubble Space
  • 25:27 - 25:33
    Telescope, to find high redshift
    supernovae.
  • 25:33 - 25:37
    There is a flaw in the way the
    space telescope is designed from
  • 25:37 - 25:40
    the point of view of trying to
    do this experiment.
  • 25:40 - 25:44
    And the flaw is that it doesn't
    look at very much of the sky at
  • 25:44 - 25:47
    once.
    It only looks at a small field
  • 25:47 - 25:48
    of view.
  • 25:48 - 25:53
  • 25:53 - 25:59
    So, you don't find many,
    because you're just not looking
  • 25:59 - 26:05
    at that big a part of the sky.
    And so, you only look at a tiny
  • 26:05 - 26:08
    piece of the sky at once.
    However, they have
  • 26:08 - 26:11
    spent--because this is of some
    clear importance,
  • 26:11 - 26:13
    they have spent many,
    many, many hundreds of
  • 26:13 - 26:18
    observing hours with the space
    telescope trying to track down a
  • 26:18 - 26:21
    few supernovae at a redshift
    greater than 1.
  • 26:21 - 26:24
    And they have now succeeded in
    doing so.
  • 26:24 - 26:26
    So, here's what the data look
    like.
  • 26:26 - 26:30
  • 26:30 - 26:33
    This is from Adam's recent
    paper, 2004.
  • 26:33 - 26:35
    I guess it's no longer quite so
    recent.
  • 26:35 - 26:42
    And this is the plot.
    Let me see if I can focus this.
  • 26:42 - 26:47
    It's actually the Xerox quality.
    We have a joke in astronomy
  • 26:47 - 26:51
    that many important objects turn
    out to be LSXTS,
  • 26:51 - 26:56
    which stands for Low
    Significant Xerox Transient
  • 26:56 - 26:58
    Source.
    That means you've made a Xerox
  • 26:58 - 27:01
    and a little blip appears on
    your graph and somebody says,
  • 27:01 - 27:03
    what's that point in the upper
    right-hand corner?
  • 27:03 - 27:05
    And you say,
    oh, that's the Xerox machine.
  • 27:05 - 27:07
    All right.
    So, I apologize for that.
  • 27:07 - 27:09
    But look what's happening.
    This is redshift.
  • 27:09 - 27:14
    This is the plot we've been
    looking at before,
  • 27:14 - 27:16
    0, .5,1.0,1.5,
    and 2.
  • 27:16 - 27:21
    And this is the Delta (m
    – M) axis.
  • 27:21 - 27:27
    So, 0 is what you expect the
    empty Universe to have.
  • 27:27 - 27:30
    And he has marked on here a
    different set of lines from the
  • 27:30 - 27:33
    set of lines I've been drawing.
    But, nevertheless,
  • 27:33 - 27:36
    you can see what's going on.
    Here are--these are all
  • 27:36 - 27:40
    supernovae--all the supernovae
    they know.
  • 27:40 - 27:43
    These kinds of fuzzy gray
    points are the ones they found
  • 27:43 - 27:46
    with the space telescope.
    The ones in here are the
  • 27:46 - 27:49
    ground-based ones known at the
    time.
  • 27:49 - 27:53
    And what they've done is
    they've averaged them together
  • 27:53 - 27:56
    in groups of redshift,
    grouping by redshift.
  • 27:56 - 27:58
    That's the bottom thing.
    So, each one of these points is
  • 27:58 - 28:03
    an average of many supernovae.
    This point is an average of
  • 28:03 - 28:05
    these two.
    And then, as you get down to
  • 28:05 - 28:08
    lower redshifts where they have
    lots and lots of them,
  • 28:08 - 28:10
    the precision gets much better.
    But you can definitely see
  • 28:10 - 28:13
    what's happening.
    Here, out to about,
  • 28:13 - 28:15
    you know, .5,
    .8, it's going up.
  • 28:15 - 28:19
    And then, it does definitely
    seem like it's going down.
  • 28:19 - 28:24
    And so, there really is,
    now, some evidence for this
  • 28:24 - 28:27
    turnaround.
    Now, it kind of all depends on
  • 28:27 - 28:31
    these two points.
    I have a rule of thumb.
  • 28:31 - 28:34
    My rule of thumb is you can put
    your thumb over any one point,
  • 28:34 - 28:38
    because you never know what
    screw up might have happened in
  • 28:38 - 28:41
    any one measurement.
    So, I can't really do that on
  • 28:41 - 28:44
    the top.
    But if I do this,
  • 28:44 - 28:50
    and I make that point go away,
    you could still kind of draw a
  • 28:50 - 28:55
    line that keeps going up.
    It would miss this point by a
  • 28:55 - 28:59
    little, but not by a lot.
    It's really this that does it.
  • 28:59 - 29:03
    It's these last two individual
    supernovae down there,
  • 29:03 - 29:07
    which really make the case that
    the thing is turning over and
  • 29:07 - 29:10
    coming back down.
    Now, you have to worry a little
  • 29:10 - 29:14
    bit--and there's actually a good
    reason to worry about the
  • 29:14 - 29:18
    brightness of high redshift
    supernovae.
  • 29:18 - 29:28
    And let me give you one
    particular reason to worry,
  • 29:28 - 29:37
    namely, gravitational lensing.
    Remember gravitational lensing?
  • 29:37 - 29:40
    You stick some mass in the
    middle, between you and the
  • 29:40 - 29:43
    object you're looking at.
    It focuses the light.
  • 29:43 - 29:49
    Gravitational lensing makes
    things look brighter.
  • 29:49 - 29:52
  • 29:52 - 29:55
    The further away you look at
    something, the more likely it is
  • 29:55 - 29:57
    that there's something in
    between that will do the
  • 29:57 - 30:01
    lensing.
    So, first of all,
  • 30:01 - 30:09
    point one: more distant
    objects, more likely to be
  • 30:09 - 30:11
    lensed.
  • 30:11 - 30:15
  • 30:15 - 30:19
    Point two: if you're out
    looking for very distant,
  • 30:19 - 30:22
    very faint objects,
    which are the ones you're going
  • 30:22 - 30:24
    to see first?
    Student: The bright ones.
  • 30:24 - 30:27
    Professor Charles
    Bailyn: The bright ones,
  • 30:27 - 30:30
    yes.
    At the, sort of,
  • 30:30 - 30:36
    faint limit,
    you see abnormally bright
  • 30:36 - 30:41
    things first.
    I can't even spell
  • 30:41 - 30:48
    that--abnormally bright things,
    preferentially.
  • 30:48 - 30:51
  • 30:51 - 30:54
    So, you could imagine,
    if you're basing your entire
  • 30:54 - 30:58
    cosmology on one or two high
    redshift supernovae,
  • 30:58 - 31:01
    you got to ask yourself the
    question: supposing those happen
  • 31:01 - 31:05
    to be lensed?
    So, let me now go back to this
  • 31:05 - 31:08
    plot.
    Here's the plot.
  • 31:08 - 31:12
    So, if they're lensed,
    then they're brighter--then
  • 31:12 - 31:15
    they seem to be brighter than
    they actually are.
  • 31:15 - 31:17
    They're actually fainter than
    they look.
  • 31:17 - 31:22
    That would have the effect that
    this one actually ought to be up
  • 31:22 - 31:25
    here, and this one actually
    ought to be up here,
  • 31:25 - 31:28
    if they were lensed by some
    amount.
  • 31:28 - 31:30
    So, if these things are lenses,
    then the true,
  • 31:30 - 31:33
    correct position of these
    points would be higher up in the
  • 31:33 - 31:37
    graph than it actually is.
    So, this is the effect of
  • 31:37 - 31:38
    lensing.
  • 31:38 - 31:43
  • 31:43 - 31:46
    Now, when it was only one of
    these points,
  • 31:46 - 31:48
    people were very concerned
    about this.
  • 31:48 - 31:51
    Now that there's two,
    people are a lot less
  • 31:51 - 31:54
    concerned, because,
    you know, it would be pretty
  • 31:54 - 31:58
    bad luck to have both of the
    supernovae that you know about
  • 31:58 - 32:01
    at high redshift turn out to be
    lensed.
  • 32:01 - 32:04
    If they had twenty,
    this whole problem would go
  • 32:04 - 32:08
    away, because there would be no
    possible way that you could be
  • 32:08 - 32:11
    so unlucky--well,
    you could calculate the
  • 32:11 - 32:14
    probability that you would be so
    unlucky as to have the first
  • 32:14 - 32:18
    twenty high redshift supernovae
    you know happen to line up right
  • 32:18 - 32:22
    behind some massive object,
    and it would be some incredibly
  • 32:22 - 32:24
    small probability.
    So, each time you observe
  • 32:24 - 32:27
    another one of these things out
    there and it's low,
  • 32:27 - 32:29
    you get around this lensing
    problem.
  • 32:29 - 32:31
    I should say this is only one
    of a number of problems.
  • 32:31 - 32:33
    These things are very hard to
    observe.
  • 32:33 - 32:37
    They're very faint objects.
    But, nevertheless,
  • 32:37 - 32:42
    I think, at the moment,
    the evidence for this
  • 32:42 - 32:50
    turnaround is highly suggestive,
    but not yet wholly conclusive.
  • 32:50 - 32:53
    All right.
    But now, suppose you actually
  • 32:53 - 32:58
    want to get higher accuracy than
    just seeing the thing
  • 32:58 - 33:06
    turnaround.
    Because, suppose dark energy
  • 33:06 - 33:11
    isn't constant.
    Suppose the Big Rip really is
  • 33:11 - 33:20
    going to happen.
    Then, DE density increases with
  • 33:20 - 33:24
    time.
    This leads to the Big Rip,
  • 33:24 - 33:28
    as we discussed last time.
    And therefore,
  • 33:28 - 33:35
    the dark energy density was
    less in the past than in now.
  • 33:35 - 33:38
  • 33:38 - 33:44
    And therefore,
    the deceleration,
  • 33:44 - 33:54
    the moment of balance between
    deceleration and acceleration
  • 33:54 - 34:00
    was more recent.
    Because, as you go back in the
  • 34:00 - 34:04
    past under this new scenario
    where the density of dark energy
  • 34:04 - 34:07
    is increasing with time,
    it's therefore decreasing as
  • 34:07 - 34:10
    you go into the past.
    So, as you go into the past,
  • 34:10 - 34:13
    two things happen – the
    density of matter gets bigger,
  • 34:13 - 34:16
    and the density of dark energy
    gets less.
  • 34:16 - 34:19
    So, there's an additional
    effect that will make that
  • 34:19 - 34:25
    crossover happen earlier.
    And so, to summarize on these,
  • 34:25 - 34:31
    by now, almost boring and
    ubiquitous plots,
  • 34:31 - 34:34
    here.
    All right.
  • 34:34 - 34:39
    So, here's the,
    kind of, standard cosmological
  • 34:39 - 34:44
    model, and if it's going to be
    a--oh,
  • 34:44 - 34:46
    and then, this predicts in the
    future that it's going to go
  • 34:46 - 34:49
    something like,
    if it's a Big Rip,
  • 34:49 - 34:52
    then what happens is,
    it's doing this,
  • 34:52 - 34:58
    and it's going to do that.
    Because the dark matter takes
  • 34:58 - 35:05
    over from the dark energy more
    recently in the past.
  • 35:05 - 35:08
    Because you've also lost oomph
    in your dark energy,
  • 35:08 - 35:12
    but the dark energy's getting
    bigger and bigger with time.
  • 35:12 - 35:19
    This, then, translates onto
    this plot.
  • 35:19 - 35:23
    And here is what we expect,
    and here's the kind of
  • 35:23 - 35:29
    prediction into the future.
    Future of observations.
  • 35:29 - 35:33
    Further into the past in time.
    And what you would expect is
  • 35:33 - 35:35
    that this would sort of keel
    over earlier.
  • 35:35 - 35:46
    So, this is a Big Rip cosmology.
    Big Rip cosmology has the
  • 35:46 - 35:55
    same--Big Rip would have the
    same expansion rate now,
  • 35:55 - 36:02
    the same acceleration now,
    but more acceleration in the
  • 36:02 - 36:12
    future than you'd expect,
    and more deceleration in the
  • 36:12 - 36:13
    past.
  • 36:13 - 36:18
  • 36:18 - 36:22
    And so, it's important,
    not just to see this
  • 36:22 - 36:26
    turnaround, but to actually
    plot, in detail,
  • 36:26 - 36:30
    where that line goes.
    Because, if it turns out that
  • 36:30 - 36:33
    your supernovae are kind of
    lined up like this,
  • 36:33 - 36:36
    or they're kind of lined up
    like this--you already know that
  • 36:36 - 36:39
    there are a bunch of them like
    this--or that they're kind of
  • 36:39 - 36:42
    lined up like this,
    makes a huge difference in how
  • 36:42 - 36:45
    you understand the dark energy,
    and tells you something new
  • 36:45 - 36:48
    about the dark energy.
    In fact, it tells you the first
  • 36:48 - 36:51
    thing you know about the dark
    energy other than that it
  • 36:51 - 36:54
    exists--namely,
    that it would be getting bigger
  • 36:54 - 36:57
    or less big, or maybe staying
    constant as a function of time.
  • 36:57 - 36:59
    Yes?
    Student: Well if the
  • 36:59 - 37:03
    deceleration was earlier than
    expected following the Big Bip
  • 37:03 - 37:05
    cosmology,
    wouldn't we have seen it
  • 37:05 - 37:09
    already since we [Inaudible]?
    Professor Charles
  • 37:09 - 37:11
    Bailyn: So,
    these three lines,
  • 37:11 - 37:13
    at about, you know,
    0.8, are very,
  • 37:13 - 37:17
    very close together.
    They don't diverge all that
  • 37:17 - 37:21
    much until a redshift--until the
    turnaround, at about a redshift
  • 37:21 - 37:23
    of .8 to 1.
    Now, in principle,
  • 37:23 - 37:27
    you could go out and make a
    whole bunch of--you know,
  • 37:27 - 37:31
    measure 10,000 of these things
    at a redshift of .5,
  • 37:31 - 37:33
    and try and distinguish that
    way.
  • 37:33 - 37:36
    Okay.
    So, how are we going to figure
  • 37:36 - 37:37
    this out?
    The space telescope,
  • 37:37 - 37:39
    kind of, finds these things one
    at a time.
  • 37:39 - 37:41
    It's going to be awfully tough
    to make this distinction.
  • 37:41 - 37:46
    But let me introduce you to two
    projects currently underway,
  • 37:46 - 37:49
    the goal of which is to figure
    this out.
  • 37:49 - 37:52
    They do the exact two things
    that we've just been talking
  • 37:52 - 37:54
    about.
    One is designed to go
  • 37:54 - 37:58
    deep-to-high redshift,
    and the other is designed to do
  • 37:58 - 38:01
    many, many, many,
    at intermediate redshift.
  • 38:01 - 38:08
    So, there is something called
    the--so, there's a space
  • 38:08 - 38:15
    mission, space telescope.
    This is called JDEM,
  • 38:15 - 38:23
    stands for Joint Dark Energy
    Mission.
  • 38:23 - 38:27
    And the joint means joint
    between NASA and the Department
  • 38:27 - 38:30
    of Energy.
    Whoever named this stuff dark
  • 38:30 - 38:32
    energy gets a little prize,
    because now we get money from
  • 38:32 - 38:34
    the Department of Energy to
    study it.
  • 38:34 - 38:39
    Let's see.
    And the idea--of which an
  • 38:39 - 38:43
    example is something called Snap
    1 Proposal.
  • 38:43 - 38:52
    That's the supernova
    acceleration probe.
  • 38:52 - 38:54
    Student: This is part of
    the JDEM?
  • 38:54 - 38:56
    Professor Charles
    Bailyn: This is an example
  • 38:56 - 38:58
    of one mission proposed to be
    JDEM.
  • 38:58 - 39:04
    This is an example.
    And it's an example I'm
  • 39:04 - 39:05
    familiar with,
    because we have people here at
  • 39:05 - 39:09
    Yale who are working on it.
    And there are competitors.
  • 39:09 - 39:12
    They haven't actually selected
    the thing yet.
  • 39:12 - 39:16
    And what this is going to be
    is, it's a space telescope that
  • 39:16 - 39:20
    differs from the current space
    telescope in two major ways.
  • 39:20 - 39:27
    One is, it has a wide field of
    view, dozens of times bigger
  • 39:27 - 39:31
    than the current space
    telescope.
  • 39:31 - 39:34
    So, it can do twenty--let me
    see if I can remember.
  • 39:34 - 39:38
    It's like twenty--it's over 100
    times bigger field of view than
  • 39:38 - 39:41
    the current space telescope.
    So, the discovery rate of
  • 39:41 - 39:44
    supernovae will be much,
    much greater.
  • 39:44 - 39:46
    And it's optimized for the
    infrared.
  • 39:46 - 39:50
  • 39:50 - 39:57
    And so, its goal is to find
    many supernovae with redshift
  • 39:57 - 40:03
    greater than 1,
    perhaps out to a redshift of 2.
  • 40:03 - 40:08
    That's its purpose in life.
    And proposals are currently
  • 40:08 - 40:12
    being evaluated for this.
    Launch time is supposed to
  • 40:12 - 40:15
    be–well, the optimists say
    2013, but that means
  • 40:15 - 40:19
    congressional funding this year,
    which is actually unlikely.
  • 40:19 - 40:23
    More likely 2016 or 2017,
    and then, we'll know.
  • 40:23 - 40:25
    Yes?
    Student: Is optimization
  • 40:25 - 40:26
    for infrared or are there any
    other predicted,
  • 40:26 - 40:28
    sort of, benefits of having
    [Inaudible]
  • 40:28 - 40:29
    Professor Charles
    Bailyn: Oh,
  • 40:29 - 40:33
    it's very nice to look at the
    infrared, because anything at
  • 40:33 - 40:35
    high redshift shines in the
    infrared.
  • 40:35 - 40:38
    And so, if you're looking at
    high redshift galaxies,
  • 40:38 - 40:40
    how galaxies evolve,
    and so forth,
  • 40:40 - 40:41
    this is useful,
    too.
  • 40:41 - 40:44
    There's actually a big debate
    over whether you totally set the
  • 40:44 - 40:46
    thing up to do nothing but
    supernovae,
  • 40:46 - 40:49
    or whether you sort of
    generalize it and spend half
  • 40:49 - 40:53
    your time doing other things.
    And there's one other project,
  • 40:53 - 40:56
    at least, which I'll describe
    next week, that these guys are
  • 40:56 - 40:58
    interested in doing,
    but there's a real tradeoff
  • 40:58 - 41:01
    between making it a general
    purpose telescope like the
  • 41:01 - 41:04
    Hubble,
    or doing this one thing
  • 41:04 - 41:06
    especially well.
    And that's, you know,
  • 41:06 - 41:09
    the kind of thing that's
    currently under discussion.
  • 41:09 - 41:14
    So, SNAP is one example of
    where people might go in the
  • 41:14 - 41:19
    future.
    Another is a ground-based
  • 41:19 - 41:29
    project called the Large
    Synoptic Survey Telescope.
  • 41:29 - 41:33
    This is otherwise known as LSST.
    LSST.
  • 41:33 - 41:36
    The plan is this.
    And this has a lot of other
  • 41:36 - 41:42
    science besides this.
    It's going to do a full survey
  • 41:42 - 41:48
    of the sky every three days.
    So, it's going to--every three
  • 41:48 - 41:51
    nights, it marches across the
    entire sky, takes a really deep
  • 41:51 - 41:56
    image of the sky.
    And so, the consequence of this
  • 41:56 - 42:03
    for cosmology is that you find
    every low and intermediate
  • 42:03 - 42:09
    redshift supernova.
    That's tens to hundreds of
  • 42:09 - 42:18
    thousands--tens of--maybe 10,000
    a year, approximately.
  • 42:18 - 42:20
    So, you find lots of these
    things.
  • 42:20 - 42:22
    So, first of all,
    you build up incredible
  • 42:22 - 42:25
    statistics out to a redshift of
    about 1/2.
  • 42:25 - 42:32
    So, huge statistics to Z
    of about 1/2.
  • 42:32 - 42:34
    That's kind of where its limit
    is going to kick in.
  • 42:34 - 42:44
    And you also can study the
    details of sub-classes of
  • 42:44 - 42:47
    supernovae.
    So, this is important,
  • 42:47 - 42:50
    because you know if you're
    fooling yourself,
  • 42:50 - 42:52
    because the distant supernovae
    are different,
  • 42:52 - 42:54
    somehow, from the local
    supernovae.
  • 42:54 - 42:57
    They're fainter for some reason.
    But in the local sample,
  • 42:57 - 43:00
    only one in 100 is of the faint
    kind.
  • 43:00 - 43:03
    If you've got 10,000,
    that means you've got,
  • 43:03 - 43:06
    you know, a sample of 100 of
    the weird subclass that's
  • 43:06 - 43:09
    causing you trouble.
    So, this will be hugely
  • 43:09 - 43:12
    helpful, not only in beating
    down the statistics so that you
  • 43:12 - 43:15
    can tell the difference between
    lines that are really quite
  • 43:15 - 43:18
    close to each other,
    theoretical predictions that
  • 43:18 - 43:20
    are close to each other,
    but it also will test the
  • 43:20 - 43:23
    systematic problems that you
    might have different kinds of
  • 43:23 - 43:27
    supernovae and generally enhance
    your overall confidence that you
  • 43:27 - 43:31
    know what you're talking about.
    Also, these kinds of sky
  • 43:31 - 43:36
    surveys are of benefit for many,
    many other fields of astronomy.
  • 43:36 - 43:40
    The issue with this is the
    following statistic.
  • 43:40 - 43:47
    Thirty terabytes of data per
    night, right?
  • 43:47 - 43:53
    That's 30,000 gig per night.
    Thirty million megabytes every
  • 43:53 - 43:56
    night.
    And so, this is not something
  • 43:56 - 43:59
    that us normal astronomers can
    handle, and so,
  • 43:59 - 44:03
    the recent news from the LSST
    project is, Google has joined
  • 44:03 - 44:06
    the project.
    And so, we're bringing in
  • 44:06 - 44:09
    the--yeah, right.
    We're bringing in the big boys
  • 44:09 - 44:11
    for this.
    And I think this is kind of
  • 44:11 - 44:13
    Google Universe,
    right?
  • 44:13 - 44:17
    Because it's a--you know,
    you take all--you do this for a
  • 44:17 - 44:20
    few years, and you've done 100
    sky surveys.
  • 44:20 - 44:23
    You can then add them all up
    and get incredibly deep data.
  • 44:23 - 44:27
    And so, I guess Google figured,
    you know, if somebody's going
  • 44:27 - 44:30
    to be piling up a catalog of the
    entire known Universe,
  • 44:30 - 44:34
    they'd better be a part of it.
    And it's fortunate for us,
  • 44:34 - 44:37
    because they're probably the
    only people in the world who can
  • 44:37 - 44:41
    create a database of data coming
    in at this rate.
  • 44:41 - 44:43
    And, I should say,
    keep in mind that you have to
  • 44:43 - 44:47
    actually, not just acquire this
    data, you have to actually look
  • 44:47 - 44:50
    at it each night.
    Because you've got to actually
  • 44:50 - 44:53
    discover these supernovae in
    real time, because they're going
  • 44:53 - 44:56
    to be gone three weeks from now,
    or three months now.
  • 44:56 - 44:59
    And so, you not only have to
    pile up 30 terabytes of data
  • 44:59 - 45:01
    every night, you've got to
    actually look at it and find all
  • 45:01 - 45:04
    the interesting objects.
    So - Student:
  • 45:04 - 45:05
    [Inaudible]
    can they do it for?
  • 45:05 - 45:07
    Professor Charles
    Bailyn: The plan,
  • 45:07 - 45:09
    currently, is to operate for at
    least five and probably ten
  • 45:09 - 45:11
    years.
    They have a site picked out.
  • 45:11 - 45:14
    They're going to put it down in
    Chile, actually,
  • 45:14 - 45:16
    on the mountain where our tiny
    telescope is.
  • 45:16 - 45:20
    And so, we actually like this,
    because they'll pay for some of
  • 45:20 - 45:22
    the infrastructure costs down
    there--hopefully,
  • 45:22 - 45:25
    for all of the infrastructure
    costs.
  • 45:25 - 45:27
    But negotiation's still in
    progress.
  • 45:27 - 45:32
    And they have a whole design
    planned for this.
  • 45:32 - 45:34
    It's all ready to go,
    but it's going to cost a fair
  • 45:34 - 45:37
    amount of money to build.
    And what's even more important:
  • 45:37 - 45:40
    it's going to cost an enormous
    amount of money to run,
  • 45:40 - 45:43
    just on a night-to-night basis.
    So, actually,
  • 45:43 - 45:46
    the construction costs of the
    telescope are not the dominant
  • 45:46 - 45:48
    cost.
    The dominant costs are building
  • 45:48 - 45:50
    the software and running the
    program.
  • 45:50 - 45:54
    And it turns out,
    that's harder to get some rich
  • 45:54 - 45:57
    guy to give you a $100,000,000
    for.
  • 45:57 - 46:01
    And it's ground-based,
    so it doesn't fall into the
  • 46:01 - 46:03
    NASA category.
    And so, it isn't clear where
  • 46:03 - 46:06
    the money is going to come from.
    They actually have a fair
  • 46:06 - 46:08
    amount of private money already
    piled up.
  • 46:08 - 46:10
    And, as I say,
    they have detailed,
  • 46:10 - 46:13
    elaborate plans for this thing.
    You can look at their website.
  • 46:13 - 46:15
    In fact, you will have to look
    at their website,
  • 46:15 - 46:17
    because that's going to be part
    of the problem set.
  • 46:17 - 46:23
    And so, these are two of the
    projects, which are currently
  • 46:23 - 46:28
    being designed and hopefully
    soon will be built,
  • 46:28 - 46:32
    that are going to try and push
    this a little further and try
  • 46:32 - 46:37
    and find out more about the dark
    energy than simply the fact that
  • 46:37 - 46:43
    it exists.
    Okay, let me put back up this
  • 46:43 - 46:50
    piece of paper with details.
    Remember, no sections on Monday.
  • 46:50 -
    No sections on Monday.
Titre:
22. Supernovae
Description:

Frontiers/Controversies in Astrophysics (ASTR 160)

Professor Bailyn offers a review of what is known so far about the expansion of the universe from observing galaxies, supernovae, and other celestial phenomena. The rate of the expansion of the universe is discussed along with the Big Rip theory and the balance of dark energy and dark matter in the universe over time. The point at which the universe shifts from accelerating to decelerating is examined. Worries related to the brightness of high redshift supernovae and the effects of gravitational lensing are explained. The lecture also describes current project designs for detecting supernovae at high or intermediate redshift, such as the Joint Dark Energy Mission (JDEM) and Large Synoptic Survey Telescope (LSST).

00:00 - Chapter 1. From Acceleration to Deceleration of Universe Expansion
10:20 - Chapter 2. The Balance between Dark Energy and Dark Matter
18:59 - Chapter 3. Complications from Supernovae Brightness and Gravitational Lensing Effects
37:33 - Chapter 4. The Joint Dark Energy Mission (JDEM) and Large Synoptic Survey Telescope (LSST)

Complete course materials are available at the Open Yale Courses website: http://open.yale.edu/courses

This course was recorded in Spring 2007.

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Équipe:
Veduca
Projet :
Fronteiras/Controvérsias na Astrofísica - Yale
Durée:
46:54
Amara Bot edited Anglais subtitles for 22. Supernovae
Amara Bot added a translation

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