Chapter 05 · The Dark Universe
The Dark Universe
What is the universe really made of?
95% of the cosmos is unknown. Dark matter binds galaxies but has never been detected directly. Dark energy drives accelerating expansion but has no agreed mechanism. The standard ΛCDM picture treats both as established components, but a growing set of serious challenges questions whether the dark sector is real, or whether the data are being misread.
Families (4)
- ConsensusStandard Cosmological ModelΛCDM. Six numbers, set by data, describe almost everything we see in cosmology. Everything else is positioned against this.3 variants
- FrontierDark Matter CandidatesWhat is dark matter? Five candidates, all serious, none confirmed. The most active frontier in particle astrophysics.5 variants
- FrontierModified Gravity / MONDMaybe there is no dark matter or dark energy, only the wrong theory of gravity at low accelerations and large scales. The galaxy-scale pattern looks real; the cluster-scale and cosmological extensions don't fit yet.4 variants
- FrontierDark Energy SkepticsThe accelerating expansion may be a misreading of the data, not a sign of dark energy filling space.1 variant
Bridged from other chapters
These variants live in their own chapters but also answer this chapter's question.
Ch.06Hawking RadiationStrongly supported
Primordial Black Hole Evaporation
If the early universe produced light enough black holes, Hawking radiation would have evaporated them by now or be evaporating them today. Heavier primordial black holes could be some or all of dark matter. Fifty years of searches and no confirmed detection.
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Ch.03Causal Set TheoryFrontier
Phenomenology, Lorentz Tests and the Cosmological Constant
Two empirical handles, one underlying physics. Lorentz-invariant 'swerves' in particle trajectories test discreteness via gamma-ray-burst and neutrino observations. Sorkin's 1991 quasi-prediction that Λ should be of order 10^-120 in Planck units matched the 1998 dark-energy discovery to order of magnitude.
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Ch.03Emergent Spacetime & GravityFrontier
Verlinde Entropic Gravity
Gravity is an entropic force from information on holographic screens. The 2016 dark-matter extension has been substantially constrained by weak-lensing tests.
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