I have good news and bad news.

Bad news first: two years ago we reported on the Crisis in Cosmology.

Since then, it’s only gotten worse.

And actually  the good news is also that the crisis in cosmology   has actually gotten worse, which  means we may be onto something!

The most exciting thing for any scientist is when something they thought they knew turns out to be   wrong.

So it’s no wonder that many cosmologists are starting to get excited by what has become   known as the Hubble tension, or the crisis  in cosmology.

The “crisis” is the fact that   we have two extremely careful, increasingly precise measurements of how fast the universe   is expanding which should agree with each other, and yet they don’t.

We first reported on the   growing hints of this tension 2 years ago.

Back then, the most likely explanation was that new,   refined measurements would bring the numbers into agreement.

So far that’s not been the case.

But just recently, one of these methods  received a massive refinement due to the   Gaia mission and its unprecedented survey of a billion stars in the Milky Way.

And guess what - the tension is now even tenser.

So is it time to rethink all of cosmology?

Before we can even think about that, we should probably do a refresher on what the issue actually   is.

So you may have heard that the universe is expanding.

Space on the largest scales   is stretching, throwing galaxies apart from  each other.

We’ve talked about how astronomers   first figured this out.

Long story short  - when a distant galaxy’s light travels to   us through the expanding universe it gets  stretched out - its wavelength increases.

If we also know how far that light traveled - the distance to the galaxy - then we can figure   out the rate at which space is expanding - at least along the path to that galaxy.

Combine   the redshifts and distances of many, many galaxies and you have the expansion rate of the universe,   typically expressed as Hubble’s constant after Edwin Hubble, the guy who first  properly measured it back in 1929.

By comparison, getting the distances is much, much trickier than getting the redshifts.

It depends   on a long chain of distance measurements that we call the cosmic distance ladder.

First you measure   distances to objects in the solar system - then use those to measure distances to nearby stars,   then more distant stars, then nearby  galaxies, then distant galaxies,  etc.

If one of those distance measures is wrong,   all the subsequent rungs of  the distance ladder are off.

Hubble’s distance measurements were based on a method pioneered by Henrietta Swan Leavitt.

She developed one of the first so-called standard candles.

These are objects whose true brightness   or luminosity can be known.

Knowing the true luminosity of an object means you can figure   out its distance just by observing how its  brightness has been dimmed by that distance.

Swan Leavitt realized that a type  pulsating star known as a Cepheid variable   has a rate of pulsation that  depends on its luminosity.

Measure the pulsation rate and you know its true brightness, and so can find its distance.

And if the Cepheid is in another galaxy,  you have the distance to that galaxy also.

Cepheid variables are great standard candles, but they’re just stars, and are too faint to see   beyond a certain distance.

In the 1990s two teams of astronomers employed a new type of   standard candle - the incredibly bright “type 1a” supernovae that result when a white dwarf   star explodes after cannibalizing its binary  partner.

Using these supernovae to get distances   to galaxies halfway across the universe, they found something totally unexpected - not only is   the universe expanding, but that expansion is accelerating.

And so dark energy was discovered   - a mysterious and ubiquitous energy that grows as the universe grows, speeding up its expansion.

Dark energy very likely holds deep, deep  clues about the fundamentals of reality.

With its discovery it suddenly became VERY important to perfect our measurements of the   expansion rate - both to confirm dark energy’s existence and to learn of its nature.

And this   is where our story splits.

There are, broadly, two approaches to improving that measurement.

One is to double down on the old method - find more type 1a supernova and improve those   distance measures.

The other is to find a totally independent measurement of the expansion rate.

A good reason to do that is that the  supernova method is a pretty high rung on   the cosmic distance ladder - which means if any rung below it is broken, the method fails.

So different teams of astronomers pursued both approaches - and this is where the crisis   emerged.

One alternative method for getting the expansion rate is to study the oldest light in the   universe - the cosmic microwave background.

This  light was released only a few hundred thousand   years after the big bang, and carries with it vast information about the universe’s early state.

I’ll leave you to watch our previous video on the subject to see how our map of the CMB using the   Planck satellite can give us the expansion rate.

The Planck team calculated a Hubble constant of   67.6 km/s/Mpc - let’s not worry about the weird units right now.

But they also claim an uncertainty of   about half a percent, making it the most precise measurement of the expansion rate ever made.

Meanwhile, Adam Reiss, one of the  Nobel-winning discoverers of dark energy   has doubled down on the supernova  method.

A couple of years ago his   team published a new Hubble constant  of 73 and a half.+/- 1.5 km/s/Mpc.

That’s in the same ballpark, but far  enough off to raise many, many eyebrows.

One possible explanation for the difference is that the nature of dark energy has   changed over time.

The Planck team’s Hubble constant assumes that dark energy has had a   constant density for the  entire age of the universe.

That’s what you expect for the simplest models of what dark energy might be, But if dark energy   has HAS changed over time it could explain the discrepancy AND indicate the dark energy   is even weirder than we thought.

It’s hard to overstate how huge a discovery that would be.

So you can see how it might be nice  to find out one way or the other   if the difference between the Planck  and supernova results is real.

Most people still think that there are  unknown errors that are affecting one or both.

For example, the cosmological distance  ladder could have a broken rung.

The   supernova standard candles are calibrated based on distances from our good-ole Cepheid variables   in galaxies where both are observed.

But those distant Cepheids are in turn calibrated based on   Cepheids in our own galaxy, for which we can get distances by a method that’s much more reliable.

That method is stellar parallax - and it’s  about as direct a method as you can get,   short of building a giant space ruler.

Ultimately, refining the supernova distance measurements comes   down to refining parallax measurements,  and that’s what we’ve finally achieved.

You’re already familiar with parallax.

Place a finger in front of your eyes   and blink left and right.

Your finger moves  relative to the background, which I guess is   me in this case.

Move your finger away and the displacement is less.

Closer and it increases.

We can use this same trick to measure the distance to stars.

As the earth orbits the   sun over the course of the year, nearby stars appear to move relative to more distant stars.

That’s stellar parallax, and our quest to measure it has been central to understanding our universe   for hundreds of years.

Prior to the invention  of the telescope, the fact that we didn’t see   obvious stellar parallax was taken as evidence that the Earth is NOT orbiting the Sun.

It turns out that the stars are just so  far away that you need careful observations   with quite a good telescope to see  parallax in even the nearest stars.

And so it was that in 1912 Henrietta Swan Leavitt used parallax measurements of Cepheids in the   Milky Way to turn these stars into standard candles and so founded our distance ladder,   which ultimately led to the discovery of dark energy.

But this feels like a bit of a house of   cards - the ladder was entirely dependent on the relatively few Cepheids that are close enough for   parallax measurements.

Things started to get better when we put telescopes in space - above   the blurring effect of Earth’s atmosphere it’s possible to make better position measurements.

The Hubble Space Telescope has done great work here, and so has the European Space Agency’s   HIPPARCOS satellite, which tracked the motion of 100,000 stars in our local patch of the galaxy But to really nail down the lowest  rung of the distance ladder,   we need a lot more Cepheid  parallaxes to much greater distances.

And that’s what ESA’s Gaia mission has given us.

Parked in an orbit just past the moon,   Gaia scans the sky year after year, mapping the structure and motion of a good faction   of the Milky Way galaxy.

Gaia is making the most accurate catalogue yet of parallax measurements,   for the nearest brightest stars, it’s 200 times more accurate than any previous measurement.

Gaia has allowed us to recalibrate Cepheid variables as standard candles, which in turned   enabled a recalibration of type 1a supernovae - which in turn gave Adam Reiss and team a refined   measure of the Hubble constant.

So what do you think - do the supernova and Planck results agree?

Not in the least.

The Gaia-based Hubble constant of 73.2 km/s/mpc seems to confirm the previous   type-1a supernova result, now with more surety about the distance ladder it’s based on.

Before we start jumping up and  down and yelling about new physics,   remember that we’re level-headed scientists.

Two independent methods aren’t enough.

We need more - and we have some great  options that will either to break the   tie between Planck and supernova,  or to confirm that the difference is real.

We’ve already talked about one of these options.

It’s to look for vast ring-like patterns in the   way galaxies are scattered across the universe and use those rings as a sort of standard ruler.

These “baryon acoustic oscillations”  are the fossils of ancient sound waves   that reverberated through the hot, dense plasma of the early universe.

Now those ripples are frozen   into the distribution of galaxies that formed from that matter.

The Baryon acoustic oscillations seem   to be coming on the side of the Planck  result - a Hubble constant in the high 60s.

Another extremely promising method is  gravitational lensing - the bending of   light around massive objects due  to their warping of spacetime.

One manifestation of this is when a distant quasar - a giant, gas-guzzling black hole - happens to be   closely aligned behind a more nearby galaxy.

Then, that quasar’s light travels multiple paths through   this gravitational lens, resulting in multiple  images of the quasar from our point of view.

Quasars are violent beasts - the maelstrom of gas fluctuates in brightness as it spirals into   the black hole.

And so we see lensed quasar images flicker - but they flicker out of sync.

There’s a time offset due to the fact that  these different paths through the universe   have slightly different lengths.

By measuring the time delay in these flickering lenses, we can   get a measurement of cosmic distances, and with that a measurement of the expansion rate that’s   independent of the cosmic distance ladder.

So far we’ve only done this with a small number of lenses and so the uncertainty is large - but published results give a Hubble constant in the low 70s - so   in agreement with the supernova guys.

But this game is about to take off, with upcoming giant   surveys set to discover thousands of new lenses that should massively improve this measurement.

And before too long we may even be able to use gravitational waves from merging black holes   to measure the Hubble constant.

These waves get stretched by the expanding universe, just like   light does.

But unlike light, they also encode information about the distance they’ve traveled,   and so can be used to measure the expansion rate without the cosmic distance ladder.

We’re calling these black hole  mergers 'standard sirens’,   and while the error bars they give are still  large, they’ll only get smaller over time.

That is where the crisis stands - it’s  increasingly clear that there’s a hole in   our understanding of the universe - whether it’s a crack in the rung of the cosmic distance ladder or   something more fundamental about how the universe expands.

Scientists love being wrong - because   when you find the source of that wrongness, it can only lead to greater understanding - in this case,   of the strange forces driving  our ever-expanding spacetime.