Numinous / Thread
The Dark Universe
What is everything made of, if it is not made of anything we can see?
Roughly 95 per cent of the universe neither emits nor absorbs light. We know it is there because of how it pulls, and after ninety years of looking we still cannot say what it is.
How it runs
Fritz Zwicky weighed a cluster of galaxies two ways in 1933, by their light and by their motion, and the answers disagreed by a factor of hundreds. The discrepancy sat mostly ignored for forty years.
Vera Rubin made it impossible to ignore. Galaxy rotation curves should fall off at the edges and do not, in every spiral she measured, which means each one sits inside a halo of something invisible.
The cosmic microwave background supplies an independent count. The pattern of its temperature ripples fixes how much ordinary matter, dark matter and dark energy the universe holds, and it agrees with the galaxies.
Dark energy arrived separately, from supernovae showing the expansion accelerating. It is the larger share of the budget and the less understood half.
Nothing has been detected directly. Decades of underground experiments have ruled out candidates without finding one, which is why the next generation of wide surveys matters.
The entries
In the order they enter the story. Each opens its full entry in the wiki.
Albert Einstein
Albert Einstein rewrote gravity as the shape of spacetime. General relativity, finished in 1915, is still the language every expanding universe and every black hole is described in.
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.
Nancy Grace Roman
NASA's first Chief of Astronomy and its first woman executive, who built the constituency and the political case for putting large telescopes above the atmosphere.
Vera Rubin
Vera Rubin discovered that the universe is mostly invisible. Her rotation curves showed that every spiral galaxy sits inside a halo of matter that pulls on things but does not shine.
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.
Computational Astrophysics
The third mode of inquiry beside observation and theory. When the equations are nonlinear, coupled and spread across ten orders of magnitude, you discretise the problem and run it forward.
European Space Agency
The pooled space programme of 23 European member states, founded in 1975 from ESRO and ELDO, and the source of Gaia, Planck, Rosetta and the Ariane launchers.
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.
Chandra X-ray Observatory
The sharpest X-ray telescope ever flown. It resolves the million-degree gas around black holes, in supernova remnants and between the galaxies of a cluster, at half an arcsecond.
Future Missions
The instruments already being built for the next two decades: three telescopes with mirrors over 24 metres, a survey that photographs the whole southern sky every few nights, and spacecraft aimed at the ocean moons.
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
Four papers from a patent office
In one year Einstein published on the photoelectric effect, Brownian motion, special relativity, and the equivalence of mass and energy. He held no academic post at the time.
The field equations of general relativity
Ten coupled nonlinear equations relating the curvature of spacetime to the energy and momentum it contains. Gravity stopped being a force and became geometry.
Gravitational waves predicted
The equations implied that accelerating masses radiate ripples in spacetime. Einstein later doubted the waves carried energy, and the question stayed open for decades.
A constant added to hold the universe still
Applied to the whole cosmos, general relativity gave a universe that had to expand or contract. Einstein added a term to keep it static. Dark energy later gave that term a job.
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.
Eddington measures the bending of starlight
The eclipse expedition found the deflection general relativity predicted, twice the Newtonian value. Einstein became a public figure outside physics within weeks.
The Nobel Prize, for the photoelectric effect
The prize cited the law of the photoelectric effect and his services to theoretical physics. It did not cite relativity, which the committee still regarded as unsettled.
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.
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.
Starts an astronomy club at eleven
Living in Reno, Nevada, under a dark sky at the edge of town, Roman organized a club with her classmates and worked through a book on constellations and celestial objects. She never lost the interest, despite two decades of adults telling her that science was not for girls.
Vassar, and a door closed at Princeton
Rubin graduated as the only astronomy major in her class and applied to Princeton, which did not admit women to its astronomy graduate program and would not until 1975. She went to Cornell.
Doctorate from the University of Chicago
After a bachelor's degree in astronomy from Swarthmore in 1946, earned at the college's Sproul Observatory, she took her PhD at Chicago, whose graduate astronomy program ran out of Yerkes Observatory in Williams Bay, Wisconsin. She stayed on at Yerkes for six more years.
Links stellar chemistry to stellar motion
Handed the sun-like F5 to G5 stars nobody else wanted, Roman found that those with weaker lines from elements heavier than helium had a far wider spread of velocities and sat further off the plane of the Milky Way. Chemistry records when a star formed; orbit records where. Published in the Astrophysical Journal, received 22 July 1950.
The changed spectrum of AG Draconis
Observing at McDonald Observatory in September 1952, Roman found that the tenth-magnitude object BD+67 922, now known as AG Draconis, had changed almost beyond recognition since the 1940s: an emission continuum swamping all but the heaviest absorption lines. The note raised her profile in the field.
Doctorate at Georgetown under George Gamow
Her thesis showed that galaxies are not scattered at random but clumped into large-scale structures. Galaxy clustering would not be widely accepted for another twenty years.
Leaves Yerkes for the Naval Research Laboratory
Told plainly that a woman had no path to tenure, she took a job in the new field of radio astronomy at NRL, mapped the galaxy at 67 centimeters, and rose to head the microwave spectroscopy section. Four years in an engineering shop taught her how flight projects are actually built.
NASA's first Chief of Astronomy
Approached after a lecture by Harold Urey and asked whether she knew anyone who might build a space astronomy program, she applied herself. She joined six months after the agency opened, becoming its first Chief of Astronomy and the first woman to hold an executive position at NASA. Taking the job ended her own research.
The Orbiting Solar Observatories begin flying
John Lindsay's spinning-wheel design pointed two instrument boxes at the Sun, the easiest target in the sky to lock onto, so the OSOs flew years ahead of the harder pointed telescopes. Eight launched between March 1962 and June 1975, covering a full eleven-year solar cycle.
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.
Orbiting Astronomical Observatories, two of four
Four experiments proposed during the International Geophysical Year were flown on a common spacecraft that had to point accurately anywhere in the sky. Four OAOs launched between 1966 and 1972 and only two worked, which was a poor procurement record and a decisive proof that a pointed telescope in orbit could do real astronomy.
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.
Andromeda does not slow down
With Kent Ford, Rubin measured how fast stars orbit at different radii in M31 and found the curve flat. Stars at the edge move as fast as stars near the middle, which Newtonian gravity forbids.
Colliding galaxies, computed
Toomre and Toomre showed that the bridges and tails of interacting galaxies fall straight out of gravity alone in a restricted N-body calculation. Structures that looked pathological turned out to be tidal.
Ten states sign the ESA Convention
ESA was formed on 30 May 1975 by merging ESRO, which did space research, with ELDO, which built launchers. The ten founding members were Belgium, Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, Switzerland and the United Kingdom.
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.
Retires from NASA
She left during an early-out window, partly to care for her mother. Edward Weiler, her successor as Chief of Astronomy, coined the phrase "mother of Hubble." She then worked half-time for nearly two more decades, ending up effectively running the science side of Goddard's Astronomical Data Center.
The pattern holds, galaxy after galaxy
Flat rotation curves turned up in spiral after spiral, across sizes, types and luminosities. Each one implies a halo outweighing its visible matter by a factor of five to ten.
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.
Elected to the National Academy of Sciences
Only the second woman astronomer so elected. She then spent years pushing for more, and mentoring the women the field had made room for late.
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.
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.
Ariane 5 fails on its first flight
Flight 501 broke up 37 seconds after liftoff on 4 June 1996, destroyed by reused inertial reference software that overflowed on a number the new rocket produced and the old one had not. Ariane 5 went on to fly until 2023.
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.
Columbia carries Chandra to orbit
Deployed from the shuttle payload bay on STS-93 and boosted by an Inertial Upper Stage into a 64-hour elliptical orbit that spends most of each circuit above the radiation belts.
First light on Cassiopeia A
The opening images, released on 26 August, showed the remnant of a 300-year-old supernova and a distant quasar. A point source sat at the centre of Cas A that no earlier X-ray telescope could have separated out.
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 Millennium Simulation maps a virtual universe
Ten billion dark matter particles in a box 500 megaparsecs on a side, from the early universe to the present. Its mock catalogues were compared directly against real galaxy surveys.
Numerical relativity finally holds together
Three groups independently achieved stable simulations of two black holes inspiralling and merging, after decades in which the codes crashed. Einstein's equations became something you could integrate.
The Bullet Cluster separates mass from gas
Two clusters had passed through each other. The hot gas, which carries most of the ordinary matter, was left behind in the middle. The lensing mass went straight on with the galaxies.
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.
Planck launches to map the microwave background
Planck flew on 14 May 2009 alongside Herschel. It measured the cosmic microwave background more precisely than anything before it, fixing the age of the universe at about 13.8 billion years and its composition at 5 per cent ordinary matter.
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.
Cassiopeia A in three dimensions
A deep observation used Doppler shifts in the X-ray lines to give line-of-sight velocities for each ejecta knot, turning a flat image into a map of an explosion that was never spherically symmetric.
Gaia launches to survey the Galaxy
Launched on 19 December 2013, Gaia measures stellar positions, distances and motions at microarcsecond precision. Its catalogues of nearly two billion sources underpin the distance ladder, Milky Way archaeology and asteroid orbit work alike.
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.
Philae lands on a comet
Rosetta released the Philae lander onto Comet 67P/Churyumov-Gerasimenko on 12 November 2014, the first soft landing on a comet nucleus. Rosetta itself found molecular oxygen in the coma, which no model had predicted.
LIGO detects the waves he predicted
Two black holes merged 1.3 billion light years away and moved the detector arms by less than the width of a proton. The prediction was ninety-nine years old.
The templates earn their keep
LIGO identified GW150914 by matching the data against the waveform bank that numerical relativity had produced. Without the simulated waveforms the signal is indistinguishable from noise.
Death, and no Nobel Prize
Rubin died at eighty-eight. Her evidence for dark matter ranks with the expanding universe and the microwave background, both of which were recognized in Stockholm.
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.
IllustrisTNG reproduces a galaxy population
Cosmological magnetohydrodynamics with star formation, feedback, chemical enrichment and black hole growth returned a stellar mass function, a red and blue colour split and a spread of morphologies that match observation.
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.
Dies at 93
Roman died on 25 December 2018, having spent her last decades teaching high school science teachers, recording astronomy texts for blind readers, and telling young women that astronomy was a field they could enter.
A survey telescope takes her name
The observatory in Chile named for her will image the whole visible sky every few nights, and the resulting time-domain survey is aimed squarely at the problem she opened.
WFIRST is renamed for her
On 20 May 2020 NASA announced that the Wide Field Infrared Survey Telescope would become the Nancy Grace Roman Space Telescope. Release 20-054 quotes Barbara Mikulski, who fought for Hubble in the Senate, calling it well deserved.
Uchuu passes two trillion particles
Dark-matter-only simulations kept scaling with the hardware. Each doubling of resolution costs roughly sixteen times the computation, which is why the field tracks supercomputer generations so closely.
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.
JUICE departs for the Jupiter system
The Jupiter Icy Moons Explorer launched on 14 April 2023 on the second to last Ariane 5. It will arrive in 2031 and eventually orbit Ganymede, which would make it the first spacecraft to orbit a moon other than our own.
JUICE leaves for the Jupiter system
ESA's Jupiter Icy Moons Explorer launched on 14 April 2023. It arrives in 2031 and will eventually enter orbit around Ganymede, which would be the first time any spacecraft has orbited a moon other than our own.
Euclid launches to map the dark universe
Launched on 1 July 2023 on a Falcon 9 and stationed at Sun-Earth L2, Euclid is building a three-dimensional map covering a third of the sky and ten billion light-years, using weak lensing and galaxy clustering.
ESA adopts LISA
The Laser Interferometer Space Antenna was formally approved on 25 January 2024: three spacecraft 2.5 million kilometres apart, six laser links, launching on an Ariane 6 in 2035. Construction began in June 2025.
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.
Ariane 6 flies for the first time
Ariane 6 made its maiden flight on 9 July 2024, restoring European access to orbit after Ariane 5 retired and Vega-C was grounded. The upper stage auxiliary power unit shut down early, leaving two reentry capsules in orbit.
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.
Europa Clipper departs for Jupiter
Launched on 14 October 2024 on a Falcon Heavy for a 2.9 billion kilometre cruise. It reaches Jupiter in April 2030 and will make 49 close flybys of Europa, running all nine instruments on every pass.
Reviewed, and kept flying
Chandra went through NASA's 2025 senior review of operating missions and continues in extended operations, twenty-six years into a five-year design life. No approved successor matches its resolution.
Slovenia becomes the twenty-third member state
Slovenia joined on 1 January 2025, following Estonia and Hungary in 2015. Canada sits on the Council under a cooperation agreement, and Cyprus, Slovakia, Latvia and Lithuania are associate members.
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.
Rubin Observatory releases its first imagery
On 23 June 2025, after just over ten hours of test observations, Rubin published images containing millions of galaxies and stars and thousands of asteroids. The ten-year Legacy Survey of Space and Time follows.
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 Bremen ministerial commits a record budget
At the council meeting on 26 and 27 November 2025 member states committed 22.3 billion euros, the largest subscription in ESA's history and 31 per cent above the 2022 round. The money funds ExoMars Rosalind Franklin for a 2028 launch, among much else.
The ten-year survey formally begins
NOIRLab announced on 30 June 2026 that the Legacy Survey of Space and Time had officially started. For the next decade Rubin photographs the southern sky every few nights and issues roughly ten million alerts a night for anything that moved or changed.
The Roman Space Telescope launches
Roman flew on 30 August 2026 on a Falcon Heavy toward Sun-Earth L2. Its mirror matches Hubble's 2.4 metres but its field of view is about a hundred times wider, which is what makes wide-area dark energy and microlensing surveys possible.
The Roman Space Telescope launches
Liftoff at 7:26 a.m. EDT on 30 August 2026, aboard a SpaceX Falcon Heavy from Launch Complex 39A at the Kennedy Space Center. The observatory separated cleanly, deployed its high-gain antenna and aperture cover, powered on its coronagraph on 1 September, and entered a commissioning period on the way to L2. First science images are expected in early 2027.