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Language: en
So, now let's get into the Hubble
tension.
>> The Hubble tension. Now, when I think
about the Hubble tension, you know,
sometimes, man, you know, I'm gonna be
honest. I love me some nerds.
>> Yeah.
>> But I don't always vibe with the nerds.
And why do I not vibe with the nerds?
Cuz sometimes I feel like they're making
too much out of a a topic. And and so
the classic example to me is what
whether or not black holes have hair,
>> right? I'm just like, is it really that
deep? You know, right?
>> So, the Hubble tension, where does that
fall? much bigger deal to me. Much
bigger deal.
>> Well, okay. So, going back to this idea,
>> define it. What is it? Yeah.
>> Yeah. Yeah. Yeah. Well, so what is the
Hubble tension? Okay. So, we reach a
point today where we say we kind of
think we have a pretty good model of the
universe, but you know, has big areas of
ignorance. 95 96% is in the form of dark
matter and dark energy that we have kind
of cartoonike explanations for, but
that's fine. Okay. And then we say, all
right, let's really test this model.
Let's see if it's really right. Right.
So the what I've been calling the best
end to end test of the model, right? You
do an endto-end test when you really
want to know does my thing, you know,
operate as I expect. The best end to end
test of the universe is to look at the
cosmic microwave background, which tells
you the state of the universe shortly
after the big bang. Okay? And it allows
you to predict how fast the universe
should be expanding today. It's like if
you had a kid who was 2 years old, you
could predict what height they will grow
to based on growth charts and your
understanding of human physiology. Okay?
But the end test of that story is to
actually measure today how fast is the
universe expanding. A number called the
Hubble constant. That would be like
measuring that kid's height when they
are fully grown.
>> Right?
>> If you really understand things, the two
will match within the error bars, within
the uncertainties. And so over the last
decade, we've seen this mismatch growing
and growing in significance. First it
was one or two times the error bars
away. Then it was three. Then it was
four.
>> Then it passed five. Now in physics,
five is considered the kind of gold
standard for like going from uh don't
bother me with that uh to like this
doesn't make sense. And now we're
probably up to six or six and a half.
Okay. And the reason that it has grown
is because the data is getting so much
better. We've had the Hubble Space
Telescope. Now we have the James Web
Space Telescope. Previously we had crude
parallaxes. Uh now we have the European
Space Agency Gaia mission measuring
parallaxes. Um you know the cosmic
microwave background data has gotten
better. First it was groundbased, then
it was WMAP, then it was plank, and now
it's also these groundbased experiments
with high resolution like ACT and SPT.
So, and then we have many techniques for
making these measurements. So, when we
measure the Hubble constant locally, we
build what is called a distance ladder.
>> Right. So, speaking of that, I just want
to define a term. You mentioned parallax
which is a geometrical way of
determining distances and it works best
for nearby objects but you can crossc
calibrate more distant objects where
they overlap. Correct. Right. Something
of that nature.
>> Yeah. Yeah. Yeah. And that process we
call the distance ladder. I mean ideally
you would look at some distant galaxy
and you would measure its distance from
us geometrically by looking at parallax.
the the parallax is when the earth goes
around the sun and your perspective on a
nearby object changes with respect to
something distant, you form a triangle
in space and you can measure how far
away it is. The problem is things are
far away. They're really far away. So
they that that shift in position becomes
imperceptibly small. And so like you
said, you can only measure it for stars
in the Milky Way, but you calibrate the
luminosity of a certain kind of star
called a sephiid variable whose period,
the rate at which it pulsates uh
>> correlates tightly to its luminosity.
And so then you see one, you calibrate
one that has a 20-day period with
parallax. Then you look in a supernova
host galaxy and find more 20-day
sephiids and you go, "Ah, it's the same
kind of object just further away." Now I
know how far away it is. Now I've
calibrated the luminosity of the
supernova and this is called a distance
ladder.
>> You build it up from nearby objects that
you get a very precise distance to and
move further and
>> further. So at the turn of the last
millennium, uh, astronomers used this
technique to get the Hubble constant to
about 10%. And that was great. But over
the last 20 years we have been improving
that now approaching about 1%. Wow.
>> And ever since we got to about 5% we
started seeing this tension.
>> So different techniques give you
different answers.
>> Well really that the cosmic microwave
background route starting from the early
universe and using the model gives you a
lower Hubble constant than when you
measure locally around us with many
different techniques. So that you know
more and more it starts to look to
people like the problem isn't in the
cosmic microwave background
measurements. They've been duplicated
and replicated. The problem isn't with
the local measurements. They've been
duplicated and replicated. The problem
might be with the story we tell
ourselves that connects the two. This
lamb to CDM model. Maybe there's
something else going on that we haven't
yet understood. Maybe dark matter and
dark energy are more complicated. Maybe
there's been more episodes of dark
energy than inflation at the beginning
and dark energy at the end. Maybe
there's been a in between dark energy.
And so these are the things that people
are thinking about because otherwise we
don't know how to explain what we're
seeing.
>> So there's a so [clears throat] you so
the cosmic microwave background
radiation comes from 13 12 billion years
ago. Correct. This local measurement
that you're making with the
>> comes from now essentially.
>> Now essentially
>> close close. I mean you know red 005.
No, not even. No, no, I would say maybe
200 million years.
>> Oh, wow.
>> Yeah. So, it's really now
>> it's a big gap.
>> Yeah. Yeah.
>> Oh, boy. Okay. So, what's the solution
to filling in that gap? What's
>> Well, what you would like
>> observational solution.
>> Yeah. What you would like is you would
like to be able to measure something in
between where you had an absolute
>> knowledge of distance. And so this is
always the the big rub in our field is I
want to start out with something that's
absolute that it's like running a tape
measure out and going this many inches,
right? But instead, we get lots of these
standards or standard rods or things
where we go, well, it's it's uniform at
least. So if I see it here and I see it
there, I could tell how much further
away it is. Um and so this clash really
comes down to uh the clash between
parallax which is the the geometric sort
of starting point for distances nearby
and at the other end is physicists
>> theoretical understanding of something
called the sound horizon which is it's
like the it's the distance that uh a
fluctuation in the early universe can
travel from the moment of the big bang
until the universe becomes transparent a
few hundred thousand years after the big
bang. And so this is like a a standard
ring for them. They could calculate it
from first principles. And so each of us
are starting with these absolute
references at opposite ends. It's like,
you know, that those famous uh that
famous uh meter stick in a uh in France
that is kept in a refrigerator. You
know, it's like this is the meter,
right? So we each have our this is the
meter. And then we use tools to try to
bring them closer together, but when we
hold them up next to each other with
these other tools, they're not agreeing.
Oh geez. So
>> that's tough. So
>> no, it's great.
>> Well, it's great from your perspective.
>> Yeah. Because th this is how we learn
things in science. It's the opportunity
that we get. I mean 1998 things didn't
fit either. Um they didn't fit the
conception at the time. So to me, this
is what makes science so much fun.
>> So there's a discovery waiting in
resolving this tension. I think so
potentially. I think so. Yeah. Yeah.