Astronomy & Celestial Mechanics
the sky, computed
Concept

Intergalactic medium — where it appears

The diffuse gas between galaxies, which holds most of the ordinary matter in the universe. It is detected almost entirely in absorption against background quasars, so its temperature, density and ionisation are read from line widths and depths rather than from any emission.

Named by 5 essays across one field — each of them below, with the objects they name alongside it.

A neutral fraction of 2.4·10⁻⁶ is already opaque. The Gunn–Peterson optical depth against the neutral fraction of the intergalactic medium, at z = 3, 5, 6.3, for Ω_b = 0.0493 and h = 0.674. Note the range of the vertical axis. A fully neutral medium at z = 6.3 gives τ = 4.1·10⁵, which is not absorption but extinction of everything; the medium reaches τ = 1 — the point at which it stops transmitting most of the light — at neutral fractions of 6.1·10⁻⁶ at z = 3, 3.3·10⁻⁶ at z = 5, 2.4·10⁻⁶ at z = 6.3. That is why the argument runs from the flux that survives rather than from the flux that does not. A spectrum showing any transmission at all between Lyman α and Lyman β is a measurement that the medium is ionised to better than one part in 164,727, and no fit to any absorption line is needed to establish it.

A trough that proves the forest survived

A uniform neutral medium at redshift six would absorb Lyman alpha with an optical depth of four hundred thousand. So the existence of any transmitted light in a quasar's spectrum is a measurement — of a neutral fraction below one part in ten thousand — made from a detection rather than from an absorption.

cosmology · Reionisation Bubbles that meet at z = 5.3, and a scattering depth of 0.047. The fraction of the volume of the universe filled by ionised bubbles, integrated from redshift 20 down to 4.5. The equation has two terms and no others: photons escaping from young galaxies open new volume, at a rate taken from the measured cosmic star formation history with an escape fraction of 0.2; recombinations inside the bubbles close it again, on a timescale that is one over the density times the recombination coefficient times a clumping factor of 3. Early on the density is high and recombination wins almost everything; the curve is nearly flat. As the universe expands the recombination time lengthens as the cube of one plus the redshift while the star formation rate is still rising, the balance tips, and the filling factor runs to one in under half a billion years. It reaches unity at redshift 5.31, which is overlap — the moment the bubbles meet and the last neutral walls between them disappear. The same integration gives an electron-scattering optical depth of 0.0465 for the microwave background, against the 0.054 that is measured, and that agreement is the check: the two observations constrain the same history from opposite ends, one fixing when it finished and the other how long it took.

It ends when the walls meet

Reionisation is usually described as the universe becoming transparent, which makes it sound like a change of state. It is not. Each young galaxy opens a bubble of ionised gas around itself and recombination closes it again, and what ends the epoch is geometric — the bubbles meet, and the last neutral walls between them disappear.

cosmology · Reionisation Fine along one axis, coarse across the other two. The smallest scales a forest survey can measure, against how many quasars a square degree it has sightlines to, at redshift 2.33. Along a sightline the sampling is set by the spectrograph: a 69 km/s pixel is 0.29 comoving megaparsecs at this redshift, so the radial wavenumber reaches 10.8 Mpc⁻¹ and it does not depend on the survey's size at all. Across the sky it is set by how densely the quasars sit: at 30 a square degree the mean separation is 18 megaparsecs and the transverse wavenumber reaches 0.17. The ratio is 63, and improving it means finding more quasars, which are a finite population on the sky. That anisotropy is the inverse of a galaxy survey's, where the transverse direction is sampled by a wide sky and the radial one is limited by a redshift shell and contaminated by peculiar velocities — so the forest measures the expansion rate at its redshift better than it measures the distance to it, which is the reverse of every galaxy survey.

A ruler read in absorption

Beyond the reach of any galaxy survey the only tracer left is the hydrogen between here and a quasar, sampled finely along each sightline and coarsely across the sky. That inverts the balance every galaxy measurement is built on — the forest measures an expansion rate better than a distance, which is the harder of the two to get any other way.

cosmology · Baryon acoustic oscillations The intergalactic temperature against density, with and without blazar heating. The temperature of the intergalactic gas against its density relative to the cosmic mean, both logarithmic, near a redshift of two. Photoheating by the ultraviolet background alone (dashed) gives a power law with denser gas hotter, T = 12,000 K at mean density rising as the density to the 0.6. Heating by the pair beams of distant blazars, if they lose their energy to the plasma rather than to scattering, is deposited per unit volume, so each particle in a void gets more of it: adding increments of 3,000, 10,000, 30,000 K at mean density turns the relation over below densities of 0.58, 1.23, 2.44 times the mean, and the emptiest gas becomes the hottest. That inversion is what a void heated from the inside looks like, and it would happen whether or not the void holds a magnetic field.

A void heated from the inside

The missing gamma-ray halos around distant blazars are read as a magnetic field in the voids — unless the electron–positron pairs that should make the halos lose their energy to the plasma first. Then the voids are being heated, the heat goes preferentially to the emptiest gas, and the intergalactic medium's temperature–density relation turns over, with the voids hotter than the filaments. The field and the heating predict different skies, and the Lyman-α forest is where they part.

cosmology · Primordial fields A sightline through the web, and where its field would be. The density of matter along a 300-megaparsec sightline through a model cosmic web, relative to the mean, on a logarithmic scale: a lognormal field whose logarithm has a width of 1.2. Galaxies sit at the 8 density peaks above the mean, and each has blown a magnetised bubble 1.4 Mpc in radius (shaded columns). Together they cover 7% of the sightline. The dots along the bottom are where 60 teraelectronvolt photons, stopped at random along the path, made their pairs: a photon stops where it stops, regardless of what is there, so the pairs fall on the empty stretches as often as on the crowded ones. Four of the 60 are made inside a bubble and are turned by its field. The other 56 are made in unmagnetised space, and their light forms a halo at full strength whatever the bubbles hold.

A field the galaxies cannot spread far enough

The missing gamma-ray halos are read as a field older than the galaxies because the galaxies seem unable to have put one there. The reason is not strength. A galaxy's wind carries a field that, spread over a megaparsec, is still ten million times stronger than the halos require. What it cannot do is reach the empty volume where most of the pairs are made — and a halo is dimmed only by the share of space the field fills.

cosmology · Primordial fields

Named alongside it

The objects these essays reach for when they reach for this one.

The Lyman-α forestOptical depthBlazarThe Gunn–Peterson troughPair cascadePrimordial fieldsRecombinationReionisationThomson scatteringVoidsAnisotropic samplingBaryon acoustic oscillations

All concepts