the search for a single number the Hubble constant the rate of expansion of our universe has consumed astronomers for generations finally too 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 reveal 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 it tells us how fast the galaxies appear to be retreating from us dependent on their distance apart but more fundamentally H naught tells us the rate of expansion of the universe in the modern era ever since Hubble's great discovery the search for H naught 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 Qwest to measure the hubble constant until the new millennium the best we could do was to estimate H naught 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 mega parsecs is for its distance with one mega parsec being around 3.3 million light-years if the Hubble constant were say 75 calmness per second per megaparsec then for every one mega parsec distance we expect the galaxies 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 Parton is relatively easy just do a Vesta slifer did and measure redshift this is the lengthening of the wavelengths 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 through in 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 and other galaxies gave Hubble their true brightnesses as though undimmed by distance cepheid's 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 cepheid's 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 surfies 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 with down the errors in h-naught an important advance was the development of new standard candles cepheid's 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 research is behind this discovery is Adam riess riess has continued the quest to refine our measurement of H naught to ever greater precision a big part of his work is to improve the calibration of type 1a supernovae as standard candles Risa's supernovae H nought for the equation of state project shoes uses the Hubble Space Telescope to match old supernova observations with new more reliable separate variables by improving this run on the cosmic distance ladder all past supernovae distances also improve recent team have now narrowed down the Hubble constant to 73.5 plus or minus one point seven kilometers per second per megaparsec that 2% ish uncertainty is a hell of a lot better than the old factor of two uncertainty so where's the crisis well in order to fully believe a measurement like this we prefer 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 the 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 H naught from observation of the early universe it's that that observation I'm referring to is far more reliable than say feeds 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 palace bactrim which basically shows the abundance of speckles of different sizes the location of these Peaks tells us which oscillation modes just happen 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 the current expansion rate from all of this well first you figure out what starting cosmological parameters could give the power spectrum observed by plunk 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 plant power spectrum is so rich with information that the Planck team claimed to have calculated H naught - even better precision than shoes the problem is the results don't agree the Planck H naught is sixty six point nine plus or minus point 6 kilometers per second per megaparsec compared to the supernova result of seventy three point five plus minus one point seven 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 three point seven Signet difference in fact which means a one in seven thousand 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 Reese's new calibration of the supernova derived H naught reveal it to be in real conflict with the plunk 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 paps we don't understand Cepheid variables like we thought or perhaps gravitational lensing alters the plunk 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 to 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 constants 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 H naught in the modern universe that 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 find 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 central on quest to measure the expansion rate of the universe will be concluded or perhaps the discrepancy will persist that that will 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 H naught 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 two episodes first it's our journal club on dr. Jamie ferns paper about negative mass dark fluid there's a unifying explanation of both dark matter and dark energy then we'll get two comments on our CPT symmetry episode so a friend of a friend of dr. fans chimed in Leo staleys friend says that dr. Fonz 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 Paul Freeman 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 then 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 masses 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 Marek zu Berman'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 Tony Foxx Tom asks where the 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 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 fineman's path integral formulation of quantum mechanics but it's not so obvious whether this idea corresponds to anything physical Rashid Bora asks how eighty inversion would affect a black hole well a true time reversal that included the interior of the black hole should transform it into a white hole everything that ever felt in would come rushing out and presumably reassemble itself into the Stars spaceships monkeys that originally fell in as for 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 sin so a rewound black hole shouldn't revert exactly to whenever 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 the dark energy equals dark matter times the speed of dark squared genius on so many levels not only scientific levels but still levels