Astronomy & Celestial Mechanics
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Dark energy — the series

8 essays on one idea, from the one that introduces it to the one that assumes the rest.
  1. Distance modulus against redshift, for three universes. The distance modulus μ = 5 log₁₀(D_L/10 pc) against redshift for three universes, all with H₀ = 67.36 km/s/Mpc, with 60 model supernovae drawn from the ΛCDM curve with 0.15 magnitudes of scatter. The point of the figure is how little difference there is: across two decades of redshift the three curves stay within a few tenths of a magnitude, and at z = 0.5 the accelerating and decelerating cases differ by 0.387 mag. A cosmology is not read off this plot. It is read off the residual, which is the next figure.

    An expansion that was supposed to be slowing

    Gravity is attractive, so an expanding universe full of matter must be decelerating, and the only question was by how much. Two teams set out to measure the deceleration and both found a quarter of a magnitude of extra faintness at redshift half — which is the wrong sign.

    part 1 · cosmology
  2. w = −0.9 is 34 millimagnitudes, and one supernova scatters by 120. Above: how much the distance modulus moves when the dark energy is not a constant. Each curve is a universe with the same Ωₘ = 0.315 and a different equation of state w, drawn as a difference from w = −1 in magnitudes. At redshift a half, w = −0.9 is worth 34 millimagnitudes — the shaded band is the 0.12-magnitude intrinsic scatter of a single standardised type Ia supernova, and the signal is a fifth of it. Nothing about one object can see this; the measurement is the mean of 1500, whose error on the mean is 3.1 millimagnitudes, and even that only works because the shape of the curve in redshift is different from every systematic anybody has thought of. Below: why the supernovae are not enough on their own. Each locus is the set of (Ωₘ, w) that a measurement cannot tell apart from the fiducial model — computed, not sketched: the supernova curve is the ridge of the same sum of squares a fit would minimise over 0.02–1 in redshift, and the acoustic-scale curve is the exact set of models with the same comoving distance to last scattering, which is what fixes the angle the microwave background's first peak subtends. They cross at 36 degrees. Neither is a measurement of w and the pair is, which is why the constraint on the equation of state is a picture of two loci crossing rather than a number read off a curve — and why −1.03 ± 0.03 is a statement about how well they cross rather than about how well anything was measured.

    The number that would say whether it is a constant

    Whether dark energy is a cosmological constant is the question of whether w is exactly −1, and w = −0.9 changes a distance modulus by thirty-four millimagnitudes at redshift a half — a fifth of the scatter of a single supernova, along a degeneracy only the acoustic scale can cut across.

    part 2 · cosmology
  3. Residuals of 112 per cent nearby and 2.2 far out. Deviations from a pure Hubble flow, in per cent, against distance. Each galaxy carries a peculiar velocity of a few hundred kilometres a second — part a coherent bulk flow shared with its neighbours and part a random dispersion — and that velocity is added to its recession. Since the recession grows with distance and the peculiar velocity does not, the fractional error falls as one over the distance: it is 112 per cent at 5 megaparsecs and 2.2 at 250. The practical consequence is a lower cut-off on any Hubble-constant measurement: below about 40 megaparsecs the motions dominate, and the coherent part does not average away over a sample because neighbouring galaxies share it. Choosing that cut-off is one of the analysis decisions a local expansion rate depends on.

    A residual that is somebody else's velocity

    A redshift is not a distance until the galaxy's own motion has been removed, and galaxies move at a few hundred kilometres a second. Nearby that is comparable to the expansion itself, so the local Hubble diagram's scatter is motions rather than measurement — and the motions are shared between neighbours, so they do not average away.

    part 3 · cosmology
  4. Three fits of the dark energy's equation of state, and the line they all cross. The equation of state w of the dark energy against redshift, for the three combined fits published with the second year of acoustic-scale data from the largest spectroscopic galaxy survey — the same galaxy and microwave-background data with three different supernova compilations — each written as w = w₀ + wₐ z/(1 + z). A cosmological constant is the horizontal line at −1. All three fits have w above −1 today (between −0.84 and −0.67) and below it in the past, and they cross it at redshift 0.35, 0.44, 0.41. Below −1 lies the phantom region, where a density grows as the universe expands; a single field with ordinary kinetic energy can be on either side of the line but cannot cross it. The preference for these fits over a constant is quoted at 2.8, 3.8 and 4.2 standard deviations, and the only thing that differs between the three is the supernovae.

    A crossing that may belong to the fitting form

    Allow the dark energy's equation of state to change and three independent fits say it has: above −1 today, below it before redshift half. The data hold w well at one redshift only, a line crossed from below is one no single field can cross, and a straight line in the scale factor fitted to a curved history crosses it anyway.

    part 4 · cosmology
  5. Where solved fields land in the plane the surveys fit. The w₀–wₐ plane, with the cross at a cosmological constant. Each small point is the best two-parameter description, through the same mock survey as the earlier figures, of a scalar field solved from its own equation: four thawing potentials at seven values of w₀ from −0.97 to −0.62, and freezing trackers on inverse-power potentials with exponents from a quarter to two. The thawing fields all land on one line through the constant, wₐ ≈ −1.49(1 + w₀), with no point more than 0.08 from it; the freezing fields land on the other side, with wₐ positive. The three published fits (larger points, with their quoted errors) lie below the thawing line by −0.38, −0.59, −0.49 in wₐ: steeper than any thawing field with their present value would be. The diagonal is w₀ + wₐ = −1. The thawing line is steeper than it, so every thawing field is fitted as having crossed −1 — but early: these fits cross between redshift 1.7 and 3.3, where a line through the constant with this slope must, near z = 1/(1.49 − 1) = 2.0. The published fits cross near redshift 0.4.

    Every thawing field falls on one line

    A scalar field that is the dark energy sits still while the universe expands quickly, and begins to roll only when the expansion has slowed to the field's own mass. Solve the field instead of fitting a line to it, and four quite different potentials give one history and one line through the plane the surveys fit — a line the published fits do not sit on.

    part 5 · cosmology
  6. How much each change moves the growth rate. The percentage difference in fσ₈ from growth in relativity inside a cosmological constant's expansion, against redshift, for three alternatives started from the same early amplitude. Changing the expansion to the evolving fit with DES-SN5YR and keeping relativity moves it by −1.2 per cent at redshift 0.5, and by at most 2.1 per cent anywhere below redshift 2. Keeping the constant's expansion and changing gravity to γ = 0.68 moves it by −8.6 per cent; doing both, −10.3. The shaded band is ±8 per cent, about the error of one of the better redshift-space measurements so far. The difference between a constant and the evolving fit is several times smaller than any single measurement; the difference between two laws of gravity is comparable to one. Growth is a weak test of whether the dark energy is changing and a strong test of whether it is gravity.

    The growth that tells a substance from a law

    In relativity an expansion history leaves no freedom in how fast structure grows inside it, so every fitted history predicts its own growth. The evolving dark energy the surveys prefer changes that growth by one or two per cent; a change in the law of gravity, with the expansion held fixed, changes it by nine. Growth is a weak test of whether the dark energy is changing and a strong test of whether it is there at all.

    part 6 · cosmology
  7. A ratio relativity predicts from one density. The ratio EG — written E with a subscript G in the literature — of the lensing around a set of galaxies to their clustering and infall, against redshift. In relativity it is Ωₘ₀/f(z) and depends on nothing else: with Ωₘ₀ = 0.3153, 0.598 today, 0.455 at z = 0.32 and 0.360 at z = 1, falling because f rises towards one as matter comes to dominate. The dashed curves keep light's bending as in relativity and change how hard matter pulls, by −10% and +10% today, growing with the dark energy's share: a weaker pull slows the infall and raises EG. The point is the first measurement, 0.39 ± 0.06 at z = 0.32 from galaxy–galaxy lensing in the Sloan survey.

    A ratio in which the galaxies cancel

    Matter falls in one gravitational potential and light is bent by the sum of two, and relativity says the two are equal. A survey that measures both the infall of galaxies and the lensing around them can divide one by the other so that the galaxies' bias and the fluctuations' amplitude both cancel — leaving a ratio that relativity predicts from the matter density alone, and that a gravity which bends light differently from the way it pulls matter cannot fake.

    part 7 · cosmology
  8. The static universe is a saddle in the plane of size and speed. Every history of a closed universe of dust and an exactly balancing cosmological constant, as curves of constant energy in the plane of size against rate of change of size, in units of the static radius and 1/√(4πGρ). The two thick curves cross at the static point: they are the histories that approach it for an infinite time or leave it after an infinite time. Above them a universe expands from a big bang through forever or contracts from forever; to the left it rises from a big bang and falls back; to the right it contracts, bounces, and expands. No history other than the point itself stays near it.

    A balance that is a saddle

    In 1917 Einstein added a constant to his equations to hold the universe still, and the constant does exactly that: there is one size at which the attraction of matter and the repulsion of the constant cancel. Eddington showed in 1930 that the balance is a pencil on its point. A thousandth out and it is gone in seven units of its own free-fall time. The instability is not a flaw of that one model. It is why the real universe could cross the same balance without stopping.

    part 8 · cosmology

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