Numinous / Thread
The Distance Ladder
How do you measure something you can never reach?
Every number in cosmology rests on a chain of methods, each calibrated against the one below it. Parallax gives way to variable stars, variable stars to supernovae, and any error low down travels all the way to the top.
How it runs
Parallax is the only direct rung: watch a nearby star shift against the background as the Earth swings around the Sun, and trigonometry gives you the distance. It runs out quickly, which is why everything above it is inference.
Henrietta Swan Leavitt found the second rung in the Magellanic Clouds. Cepheid variables pulse at a rate set by their true brightness, so their period tells you how luminous they really are, and comparing that with how bright they look gives the distance.
Edwin Hubble put Leavitt's rung to work on Andromeda and proved it lay far outside the Milky Way. The universe acquired other galaxies, and then, from their redshifts, an expansion.
Type Ia supernovae carry the ladder further still, and in 1998 they revealed that the expansion is speeding up. The ladder had found something nobody was looking for.
The rungs now disagree. Measurements built from the ladder and measurements built from the cosmic microwave background give different expansion rates, and the gap has not closed. That is the Hubble tension.
The entries
In the order they enter the story. Each opens its full entry in the wiki.
Henrietta Swan Leavitt
Henrietta Swan Leavitt found that a Cepheid variable's pulse rate gives away its true brightness. That is the first standard candle, and the rung every measured distance beyond our galaxy rests on.
Edwin Hubble
Edwin Hubble proved the spiral nebulae are galaxies of their own, then found that the farther one lies the faster it recedes. The universe acquired billions of galaxies and an expansion.
Cosmology
The universe as a single object with a history. A six-parameter model fits the microwave background, galaxy clustering, supernova distances and light element abundances at once, and still explains nothing about 95 per cent of the contents.
Dark Matter and Dark Energy
Ninety-five per cent of the energy content of the universe has been measured precisely and identified not at all. One holds galaxies together; the other is pushing them apart.
Cosmic Microwave Background
The oldest light there is: the thermal afterglow of the Big Bang, released when the universe cooled enough to turn transparent, and still arriving from every direction at 2.725 kelvin.
Hubble Space Telescope
A 2.4-metre telescope in low Earth orbit that arrived with the wrong mirror shape, was repaired by hand, and became the most scientifically productive instrument ever built.
Gravitational Wave Astronomy
Astronomy without light. Kilometre-scale interferometers measure the stretching of space itself as black holes and neutron stars merge, at strains of about one part in a sextillion.
Observational Techniques
Astronomy cannot touch its subjects, so everything comes out of the signal. Photometry, spectroscopy, astrometry, interferometry and polarimetry are the ways of getting more out of the same photons.
The Hubble Tension
The universe has one expansion rate, and two good ways of measuring it disagree. The distance ladder says about 73.5 km/s/Mpc, the microwave background says 67.4, and neither side can find the error.
Timeline
Hipparchus ranks the stars by brightness
Six magnitude classes, brightest to faintest visible. The scale survived unchanged for two thousand years because it was good enough, and it survives now because it was made logarithmic rather than replaced.
Fraunhofer maps the dark lines
More than 500 absorption features catalogued in the solar spectrum, without knowing what caused them. Identifying them with elements half a century later turned light into chemical analysis.
The magnitude scale gets a definition
Pogson fixed five magnitudes as exactly a factor of 100 in brightness, making an ancient eyeball ranking into a measurable logarithmic quantity.
Hired as a computer at Harvard College Observatory
After years as a volunteer assistant, Leavitt was put on Edward Pickering's staff to measure and catalog stellar brightness from glass plates. The women were paid roughly half what male assistants earned.
A note about the brighter ones
Working through plates of the Magellanic Clouds taken from Harvard's Peru station, Leavitt published a brief observation: among the Cepheids, the brighter stars had the longer periods.
The period-luminosity relation
Twenty-five Cepheids in the Small Magellanic Cloud, all at roughly the same distance, fell on a straight line when period was plotted against magnitude. Four pages. The paper went out under Pickering's name.
Einstein applies relativity to everything at once
The field equations of 1915 turned out to describe a universe that could not sit still. Einstein added a cosmological constant to hold it static, which was a philosophical preference rather than an observation.
Calibration, and the size of the Milky Way
Hertzsprung and then Shapley tied the relation to an absolute scale using statistical parallax. Shapley used it on globular clusters and found the Sun is nowhere near the center.
Arrival at Mount Wilson
After a Rhodes scholarship in law, a doctorate at Yerkes and infantry service in France, Hubble took a post at the observatory holding the 100-inch Hooker reflector, the largest telescope in the world.
The Great Debate
Shapley and Curtis argued at the National Academy of Sciences over whether the spiral nebulae were clouds inside the Milky Way or galaxies beyond it. Nobody could measure the distances, so nobody could settle it.
Death, and a Nobel nomination that arrived too late
Leavitt died of cancer at 53. Gosta Mittag-Leffler wrote in 1924 intending to nominate her, and was told she was gone. The prize is not awarded posthumously.
Andromeda is another galaxy
Hubble found Cepheid variables in M31 and applied Leavitt's relation. His figure of roughly 900,000 light years was too small, and it was still far outside the Milky Way.
Lemaitre derives the expansion and its consequence
A Belgian priest and physicist worked out the expanding solution independently and took the obvious next step: run it backwards and everything was once compressed into what he called a primeval atom.
Hubble measures the recession
Distances and velocities for a couple of dozen galaxies showed the two proportional. Every galaxy is receding, and the farther away it is, the faster. Space itself is stretching.
Velocity rises with distance
Combining his own distances with redshifts measured by Milton Humason and Vesto Slipher, Hubble published a linear relation between how far a galaxy is and how fast it recedes.
Zwicky weighs the Coma Cluster
The galaxies were moving far too fast for the visible mass to hold them together. He named the missing component dunkle Materie, and the field ignored the result for forty years.
The tuning fork
Hubble sorted galaxies into ellipticals, normal spirals, barred spirals and irregulars. He read it as an evolutionary sequence, which was wrong, and the classification is still in use.
A hiss that would not go away
Penzias and Wilson, mapping radio emission at Bell Labs, found an isotropic signal they could not eliminate. Dicke's group at Princeton, building a detector to look for exactly this, supplied the interpretation in a companion paper.
The afterglow is found
The microwave background confirmed that the universe had been hot and dense, and left the Steady State model without an explanation for a signal it had never predicted.
The CCD is invented
Bell Labs built a device that converts photons to electrons linearly and with better than 90 per cent efficiency. Photographic plates managed one to three per cent and were not linear at all.
Rubin and Ford measure Andromeda's rotation
Stars at the edge of the disk orbited as fast as those near the centre, which Newtonian gravity forbids if the visible mass is all there is. The flat rotation curve became dark matter's signature.
A Nobel Prize for aperture synthesis
Martin Ryle was recognised for the technique of combining antennas so that Earth's rotation traces out the baselines of an aperture nobody could build.
The Nobel Prize for the discovery
Penzias and Wilson shared the physics prize. The background had already finished off the Steady State model, which predicted no such thing, and made the hot Big Bang the working picture.
Inflation solves two problems at once
Alan Guth proposed a brief burst of exponential expansion in the first fraction of a second. It explains why opposite sides of the sky are the same temperature and why space is flat, and it supplies the density seeds as stretched quantum noise.
MOND offers the alternative
Milgrom proposed that gravity departs from Newton at very low accelerations. One universal parameter reproduces galaxy rotation curves well, and it has never handled clusters or the microwave background as cleanly.
COBE launches
Two instruments, two results. FIRAS measured a spectrum indistinguishable from a perfect blackbody, and DMR was built to look for the departures from uniformity that structure needs.
A telescope named for him
The Hubble Space Telescope launched with the measurement of the expansion rate as a founding science goal, and its Key Project used Cepheids exactly as he had.
Discovery carries Hubble to orbit
Launched on 24 April after more than a decade of delays and near-cancellations. Low Earth orbit was chosen deliberately, so that shuttle crews could reach the telescope and replace its instruments.
The mirror is the wrong shape
A flawed null corrector had been used to test the primary during grinding. The surface was off by roughly a fiftieth of the width of a human hair, enough to spread every star into a halo.
The first map of the fluctuations
DMR found temperature variations of about one part in 100,000 at seven degrees resolution. Crude, but they were the density seeds that everything else in the universe grew from.
Five spacewalks fix it
STS-61 installed COSTAR, a set of corrective mirrors, and a replacement camera with the correction built in. The aberration had been characterised precisely, which is why it could be cancelled precisely.
Ten days on an empty patch of sky
The Hubble Deep Field stacked 342 exposures of a piece of Ursa Major chosen for having nothing in it. It came back with about 3,000 galaxies, most of them billions of light years away.
Hipparcos publishes the first space astrometry catalogue
Positions and parallaxes for over 100,000 stars at milliarcsecond precision, from above the atmosphere. It rebuilt the bottom of the distance scale.
The expansion is found to be speeding up
Two supernova teams working independently reached the same unwelcome result. Something with negative pressure dominates the energy budget, and no mainstream model had predicted it.
Dark energy arrives uninvited
Two supernova teams found distant Type Ia events fainter than a decelerating universe permits. Something with negative pressure makes up most of the energy density, and nobody had asked for it.
The expansion turns out to be accelerating
Two competing teams found distant Type Ia supernovae fainter than a decelerating universe predicts. Hubble supplied the photometry that ground telescopes could not measure accurately enough.
The HST Key Project settles H0 to 10 per cent
Freedman and colleagues published the final Key Project result, H0 near 72 km/s/Mpc with about 10 per cent uncertainty. It ended a decades-long argument in which credible estimates differed by a factor of two, and set the precision everything since has had to beat.
WMAP starts precision cosmology
Forty-five times COBE's resolution resolved the acoustic peaks. The angular scale of the first peak fixed the geometry as flat, and the relative peak heights split the contents into ordinary matter, dark matter and dark energy.
The Ultra Deep Field goes deeper
A longer exposure in Fornax reached galaxies seen when the universe was a few hundred million years old. The deep fields together turned galaxy evolution into a statistical subject.
The Bullet Cluster separates mass from matter
Two clusters passed through each other. The gas, which is most of the ordinary mass, was slowed by ram pressure; the lensing mass carried straight on with the galaxies. Modified gravity has no easy account of that.
Planck launches
Nine frequency bands from 30 to 857 GHz at five arcminutes, which is what it takes to separate the background from galactic dust and synchrotron emission rather than model them away.
The last servicing mission
STS-125 installed Wide Field Camera 3 and the Cosmic Origins Spectrograph, repaired two instruments that had failed, and replaced all six gyroscopes. NASA had cancelled the flight after Columbia and reinstated it under public pressure.
The Nobel Prize for the acceleration
Perlmutter, Schmidt and Riess shared the physics prize. Thirteen years after the result, the cause of the acceleration was, and still is, unidentified.
Planck publishes the parameters
The first cosmological results from the full microwave sky fixed the composition at roughly five per cent ordinary matter, twenty-seven per cent dark matter and sixty-eight per cent dark energy.
A B-mode detection that was dust
BICEP2 announced a primordial polarisation signal in March. A joint analysis with Planck's dust maps concluded by January 2015 that polarised galactic dust could account for it.
GW150914 arrives before the run starts
On 14 September, four days early, both LIGO detectors recorded a chirp lasting about 0.2 seconds. Two black holes of roughly 36 and 29 solar masses had merged 1.3 billion light years away.
The detection is announced
Five months of checking later, the collaboration published. The ringdown matched the predicted modes of a Kerr black hole, so the result confirmed both the waves and the objects at once.
GW170817 is seen in every channel
Two neutron stars merged on 17 August. Fermi caught a gamma-ray burst 1.7 seconds later, optical telescopes found the source within eleven hours, and the kilonova spectrum showed freshly made heavy elements.
A neutron star merger gives an independent H0
GW170817 was seen in gravitational waves and in light, so the waveform gave a distance and the host galaxy gave a redshift. The result was consistent with both camps and far too imprecise to choose between them, but its systematics share nothing with either.
The Nobel Prize follows in eighteen months
Rainer Weiss, Barry Barish and Kip Thorne shared the physics prize for LIGO. The gap between first detection and the award was unusually short, which is a measure of how little doubt there was.
Planck publishes its final parameters
The full-mission analysis put the age at 13.80 billion years and the expansion rate at 67.4 km/s/Mpc, with sub-percent errors. The fit is excellent and it disagrees with the local measurement.
Planck fixes the early-universe value
The final full-mission Planck analysis of the microwave background gives H0 = 67.4 plus or minus 0.5 km/s/Mpc under flat Lambda-CDM. The fit is internally excellent, and ACT and SPT have since reproduced it with different instruments.
Interferometry resolves a black hole shadow
The Event Horizon Telescope correlated millimetre stations from Hawaii to the South Pole for roughly 20 microarcseconds of resolution, which is a newspaper in New York read from Paris.
SH0ES reaches 1 km/s/Mpc and the gap holds
Riess and colleagues calibrated Cepheids in the hosts of 42 Type Ia supernovae using Gaia parallaxes, the NGC 4258 maser and LMC eclipsing binaries, and got H0 = 73.0 plus or minus 1.0 km/s/Mpc. Roughly 70 analysis variants failed to move it. The discrepancy with Planck is about 5 sigma.
Pulsar timing finds a nanohertz background
NANOGrav and three other timing arrays announced correlated timing deviations across millisecond pulsars, consistent with the combined hum of supermassive black hole binaries across the sky.
ESA adopts LISA
Three spacecraft 2.5 million kilometres apart, sensitive in the millihertz band where supermassive black hole mergers and Milky Way white dwarf binaries live. Launch is planned for 2035.
Hubble goes to one-gyro mode
Three of the six gyroscopes installed in 2009 have now had a flex lead fail. Science resumed pointing on a single gyro, with one held in reserve, at the cost of slower slews and less sky available at any moment.
The Carnegie-Chicago programme lands in between
Using JWST, the CCHP calibrated the tip of the red giant branch and JAGB stars instead of Cepheids and reported H0 = 70.4, with about 1.2 statistical and 1.3 systematic. TRGB and JAGB distances agree to better than 1 per cent, and the number sits between the two camps.
DESI pushes on dark energy
Baryon acoustic oscillations from over 14 million galaxies and quasars fit a constant dark energy less well than one that evolves with time. Combined with the microwave background the preference runs to about 3 sigma.
DESI sees a hint that dark energy changes
Baryon acoustic oscillations across more than 14 million galaxies and quasars prefer an equation of state that evolves over a fixed cosmological constant, at about 3 sigma when combined with the microwave background.
DESI DR2 pushes on the dark energy side instead
Baryon acoustic oscillations from over 14 million galaxies and quasars fit flat Lambda-CDM only in 2.3 sigma tension with the CMB. A dark energy equation of state that evolves with time is preferred over Lambda-CDM at 3.1 sigma. Its implied H0 is 68.17, so DESI does not relieve the tension.
SH0ES widens the gap to about six sigma
Riess and colleagues combined JWST and HST Cepheids across 19 and 37 hosts for H0 = 73.49 plus or minus 0.93, or 73.18 plus or minus 0.88 when 35 tip-of-the-red-giant-branch calibrations are folded in. Both sit roughly six sigma above the CMB value.
The catalogue passes 390 events
GWTC-5.0, released on 26 May, added 161 events from the fourth observing run. Detection has gone from a singular result to a population study in about a decade.