The search for a single number,
the Hubble constant,... ...the rate of expansion of our universe,... ...has consumed astronomers for generations. Finally, two powerful
and independent methods... ...have refined its measurement
to unprecedented precision. The only problem...
is that they don't agree,... ...and it's causing to question... ...some of the most basic assumptions
about the universe. In 1929, Edwin Hubble...
discovered the universe. He gave us
our first incontrovertible proof... ...that there are galaxies
outside the Milky Way,... ...by measuring the distances
to the spiral nebulae They were many millions
of light years from us,... ...far outside the Milky Way,
and so must be galaxies in their own right. Combined with the Doppler shift
velocity measurements of Vesto Slipher,... ...Hubble revealed that the galaxies
are not only receding from us,... ...but they are receding at a rate
proportional to their distance. An impossibly vast universe
had been discovered beyond the Milky Way,... ...and at the same time
that universe was revealed to be expanding. The galaxies appear to be racing away from us,
because the intervening space is expanding. We encapsulate the expansion of the universe
with a single number, called the Hubble constant. H-naught (H0) It tells us how fast the galaxies appear to be
retreating from us, dependent on their distance apart But, more fundamentally, H0 tells us
the rate of expansion of the universe... ...in the modern era. Ever since Hubble's great discovery,... ...the search for H0
has been the all-consuming obsession... ...of thousands of astronomers
across the generations. And, understandably,... ...the rate of expansion of the universe,
combined with the gravitational effect... ...of the matter and energy it contains,... ...can be used to determine
its entire expansion history,... ...from the Big Bang to its final fate. And it's fundamental for interpreting
our observations of the distant universe,... ...whose light has traveled billions of years
through this expanding cosmos. You can imagine the alarm
when the two most powerful methods... ...used to measure this fundamental parameter,
the Hubble constant,... ...gave different results! But before we get to that, let's talk about
the great quest to measure the Hubble constant. Until the new millennium, the best we could do
was to estimate H0 within a factor of 2,... ... somewhere between 50 and 100
kilometers per second per megaparsec. These strange units
warrant some explanation. Km/s, that's for
the recession speed of a given galaxy. Megaparsecs is for its distance,... ...with 1 megaparsec being
around 3.3 million light-years. If the Hubble constant were, say,
75 km per second per megaparsec,... ...then for every 1 mega parsec distance,... ...we'd expect the galaxy to be retreating from us
at an additional 75 kilometers per second. Historically,
measurement of the Hubble constant... ...meant measuring the recession velocity and distance
for as many galaxies as possible. The velocity part is relatively easy. Just do as Vesto Slipher did,
and measure redshift. This is the lengthening of the wavelength
of light from that galaxy,... ...which was stretched as it travels to us
through an expanding universe. The distance...
that's tricky. Hubble used Cepheid variables,... ...giant stars,
during the last phases of their lives. They pulsate with a period... ...that's related to their true brightness,
as discovered by Henrietta Leavitt. Measuring Cepheid periods in other galaxies
gave Hubble their true brightnesses,... ...as though undimmed by distance. Cepheids became what we call "standard candles",
objects of known luminosity,... ...whose observed brightness, therefore,
tells us their distance. But this calculation
involves assumptions and uncertainties. For one thing, the Cepheid
period-luminosity relationship... ...first had to be calibrated,... ...based on nearby Cepheids,... ...whose distances can be figured
using stellar parallax Tracking their tiny motions on the sky,
as Earth orbits the Sun. This stepwise determination of astronomical distances
is called the cosmic distance ladder. With each step on the ladder,
uncertainties compound. Add this to our uncertainties in the behavior and observation of Cepheids themselves,... ...and the precise measurement
of the Hubble constant... ...has been a slow laborious process. As larger telescopes
and more expansive surveys were completed,... ...we gradually whittled down
the errors in H0. An important advance was
the development of new standard candles. Cepheids are good, but can only be seen
out to a certain distance. Supernovae can be seen much further,... ...and type 1a supernovae are the key. These result when white dwarfs,
ancient remnants of dead stars,... ...absorb too much material
from a binary partner Runaway fusion
causes them to detonate. The resulting explosion
has highly predictable brightness,... ...making them
excellent standard candles. In the 1990s,... ...astronomers were using these supernovae
to better nail down the Hubble constant. They inadvertently discovered... ...that the expansion of the universe
is actually accelerating,... ...revealing the existence of dark energy. One of the Nobel Prize winning researchers
behind this discovery is Adam Riess. Riess has continued the quest... ...to refine our measurement of H0
to ever greater precision. A big part of his work is to improve the calibration
of type 1a supernovae as standard candles. Riess's Supernovae H0 for the Equation of State
project, - SHOES -,... ...uses the Hubble Space Telescope
to match old supernovae observations... ...with new, more reliable Cepheid variables. By improving this run
on the cosmic distance ladder,... ...all past supernovae distances
also improve. Recent teams have now narrowed down the Hubble constant to 73.5 ± 1,7... ...kilometers per second per megaparsec That 2%-ish uncertainty... ...is a hell of a lot better
than the old factor of 2 uncertainty. So, where's the crisis? Well, in order to fully believe
a measurement like this,... ...we prefer it to be made
through independent methods. The SHOES project measures the recession of galaxies
up to around 2 billion light years away. So it's a more or less direct measurement
of the CURRENT expansion rate. But there's another way to go. What if we could measure the expansion rate
of the universe at the very beginning? Then, we could figure out what its current expansion rate should be, given our best understanding... ...of all the gravitational influences
that affected that expansion since the Big Bang. So, we'd better hope
that it does give the same result,... ...or there is a big problem,
with either our supernova measurements... ...or with our understanding
of how the universe evolved. Spoiler: ...
there IS a problem. There's another reason to try to calculate H0 from observation of the early universe It's that that observation I'm referring to
is far more reliable than Cepheids and supernovae. I'm talking about the
Cosmic Microwave Background radiation, the CMB. This is a topic we've been over,
so, for now, just the TLDR. The Cosmic Microwave Background is the remnant
heat glow of the universe's initial hot dense state. Released around 400,000 years
after the Big Bang,... ...when the universe had finally cooled down enough
to become transparent to light. We still see it today,... ...now stretched by a factor of 1,100
by its near 14 billion year journey... ...through an expanding universe. This is the map of the CMB across the entire sky,... ...created by the Planck satellite. The speckles are
tiny differences in temperature,... ...corresponding to tiny
differences in density. The blue regions are a factor of 100,000
cooler than the red regions,... ...and also slightly more dense These over-densities... ...would go on to collapse into the vast
clusters of galaxies of the modern universe. So,... how can the CMB
tell us the Hubble constant? The key is
the sizes of those speckles. In the era just before the release of the CMB,
matter and light were trapped together. Matter wanted to collapse
under its own gravity,... ...while light generated a powerful pressure
to resist that collapse. These counteractive forces
produced oscillations,... ...really vast sound waves
that rippled across the universe. These are
the baryon acoustic oscillations,... ...and they occurred
on all different sized scales,... ...sloshing between high and low density,
over those 400,000 years. Then,... ...the release of the CMB meant that light and matter
were no longer coupled together. And so those oscillations stopped. The state of the oscillations
at the moment of that release... ...is imprinted on the CMB,
in those speckles. We usually show
the distribution of speckle sizes... ...with what we call a power spectrum,... ...which basically shows the abundance
of speckles of different sizes. The location of these peaks... ...tells us which oscillation modes... ...just happened to be at their peaks... ...at the moment the CMB was released. This, in turn, depends
on the density of matter and radiation,... ...as well as the expansion rate
of the universe in that early epoch. So, how do you get the Hubble constant,
i.e., the current expansion rate, from all of this? Well, first you figure out
what starting cosmological parameters... ...could give the power spectrum
observed by Planck. Those parameters include the starting combination
of both dark and light matter, and radiation,... ...as well as the initial expansion rate. And then,... ...you figure out how the universe
described by these parameters... ...should evolve to the present day. This sounds involved,... ...but the Planck power spectrum is so rich
with information, that the Planck team... ...claim to have calculated H0
with even better precision than SHOES. The problem is,
the results don't agree. The Planck H0 is 66.9 ± 0.6
kilometers per second per megaparsec,... ...compared to the supernova result
of 73.5 ± 1.7. Now, they're actually
remarkably close,... ...given we figured them out
from data at the opposite ends of time. But they also seem
irreconcilably different,... ...3,7 sigma different in fact. Which means a 1/7000 chance... ...that that level of difference
could have happened through random errors. This is the crisis in cosmology. This discrepancy first emerged in 2016, when
Riess's new calibration of the supernova-derived H0... ...revealed it to be in real conflict
with the Planck result from a couple of years earlier. Since then, calibrations have been improved,
results have been rechecked,... ...and independent methods have been used
to calibrate the supernovae as standard candles. The difference is real,... ...and, in fact,
the error bars are only getting smaller. Okay, before we declare
all cosmology broken,... ...let's think about the two main possible sources
of this discrepancy. First: there are unknown
systematic sources of uncertainty... ...in either the supernova
or Planck measurements. Biases, that are driving one or the other
to be too high or too low. Perhaps we don't understand
Cepheid variables like we thought,... ...or perhaps gravitational lensing alters the
Planck speckles differently to how we thought. Ongoing efforts are ruling out
systematic errors one by one,... ...but it's possible there's still something
we haven't thought of yet Second: there's some unknown physics... ...that needs to be taken into account
for the CMB calculation. This is the most exciting possibility There are a few options. So let's start a new list. One: A new type
of very fast-moving particle. Insufficient numbers could skew the energy balance
of the early universe, and mess up the calculation. That particle could be
the sterile neutrino,... ...a hypothetical, non-interacting neutrino,
that isn't part of the standard model. Two: Dark matter particles
behave differently to how we thought. Perhaps dark matter interacts more strongly
with matter and radiation,... ...which would shift the sizes
of those CMB speckles. Three: Dark energy isn't constant. The current calculations assume that dark energy
is described by the cosmological constant,... ...which, by definition,
doesn't change. But if dark energy increases,... ...that could explain why we observe
a higher H0 in the modern universe... ...than is predicted by extrapolating
from the early universe. The answer will depend on whether the more
correct measurement of the Hubble constant... ...comes from Planck or SHOES. New observations and new telescopes
will refine these numbers even further. Independent methods, like using
gravitational lensing, or gravitational waves,... ...will weigh in on one side or the other. Perhaps the uncertainties will be refined,
and the two results will converge. That'd be cool. The near centennial quest to measure
the expansion rate of the universe will be concluded. Or perhaps the discrepancy will persist. That would be even cooler We'll have a new tool... ...to investigate the mysterious physics
of dark energy, dark matter,... ...or of unknown particles
beyond the standard model. For now, we continue
our obsessive quest for H0... ...and for what it'll tell us
of the origin and fate of our expanding space-time. In today's comment responses,
we need to catch up on 2 episodes. First, it's our journal club
on Dr. Jamie Farnes' paper... ...about negative mass dark fluid... ...as a unifying explanation
of both dark matter and dark energy. Then we'll get to comments
on our CPT symmetry episode. So, a friend of a friend
of Dr. Farnes' chimed in. Leo Staley's friend says that Dr. Farnes
doesn't necessarily believe the claims of his paper,... ...but rather its purpose
was to spark interesting ideas among physicists. Well, okay. I totally respect that motivation
to publish even quite fringe ideas,... ...and he certainly
sparked a conversation. I mean, look,
I'm still talking about it. Andrew Paulfreyman points out that... ...the gravitational lensing measurements
of dark matter... ...will give the exact opposite results
if dark matter is due to this negative mass fluid... ...than if it's actual,
positive mass matter. And my intuition tells me
that this is right. Gravitational lensing
is the bending of light by a gravitational field. We see it in the warping
of images of distant objects,... ...due to the gravitational fields
of more nearby galaxies. We can use that warping
to measure masses. And yeah, those measures tell us
that dark matter has positive mass I'd need to do the simulations,
but I have a feeling... ...that we wouldn't even see
this sort of strong gravitational lensing... ...if the effect of dark matter
was due to this dark fluid. Marik Zilberman's
distaste for negative masses... ...is that they produce perpetual motion machines
and paradoxes left and right. Exactly what I thought. When a theory leads to these - sort of -
pathological predictions, it's a big red flag. And we're actually going to do
a challenge question episode,... ...to explore these paradoxes. Stay tuned. Okay, let's move on to our episode
on the ultimate symmetry of nature,... ...the simultaneous reversal
of charge, parity, and time. First up: a few of you asked questions
about time reversal, so I want to clarify. The T in CPT symmetry
isn't a literal rewinding of the clock. It's best thought of
as a reversal of all motion,... ...both linear and angular momentum. Everything reverses direction. If the universe has
this sort of T symmetry,... ...then, if you reverse all motion,... ...the universe will evolve exactly backwards,
to its initial state. Turns out that's not the case,... ...as demonstrated by the different forward/backward reaction rates in certain quantum interactions. But the universe IS symmetric
under full CPT inversion. Now, a CPT inverted universe
is not the same as this universe,... ...but the laws of Physics are the same. The point is that you can't tell
which of the two you're in. TinyFox Tom asks whether mass
would be inverted under CPT symmetry. And I guess you're referring to the idea
that time-reversed energy has its sign flipped. So, the simple answer is no,
because the T in CPT isn't a true time reversal. But in the case of a true time reversal,
the answer is, essentially, yes. And a negative mass particle,
moving backwards in time,... ...is mathematically the same as a positive mass particle moving forward in time That notion makes sense in the math,... ...and is used in, for example, Feynman's
path integral formulation of quantum mechanics. But it's not so obvious whether this idea
corresponds to anything physical. Rishit Vora asks how a T inversion
would affect a black hole? Well, a true time reversal
that included the interior of a black hole... ...should transform it into a white hole. Everything that ever fell in
would come rushing out,... ...and presumably reassemble itself into
the stars, spaceships, monkeys,... that originally fell in. As to the motion reversal symmetry
of the T in CPT,... ...frankly, I'm not sure, because we don't know
the state of matter in the black hole. But, at any rate, remember
that T symmetry is broken. Both the T of CPT
and true time reversal symmetry. So a rewound black hole shouldn't revert exactly
to whatever it formed from. That doesn't mean
information is lost. Just that it ends up
in a different form. And back to dark fluid for a sec. Mr. Nation [?] has his own unified theory
of the Dark Sector. He reveals to us
that dark energy equals dark matter,... ...times the speed of dark... ...squared.
[ DE = dm•(cd^2) ] Genius on so many levels. Not only scientific levels,
but still, levels.
"...a white hole. Everything that ever fell in would come rushing out, and presumably reassemble itself into the stars, spaceships, monkeys that originally fell in"
The idea of every monkey and spaceship ever used in analogies and thought experiments about black holes exploding back out of one is deeply amusing to me
Sup! Great shirt Dr!
In retrospect, I wish that Matt would have gone over the following:
The Milky Way is situated within the KBC void, the largest-known supervoid in the obersvable universe. This means that our region of the universe is underdense... which means less gravity to counteract cosmic expansion... which means that we should measure a larger Hubble constant (compared to the CMB) out to at least several billion light years.