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.