In recent years, a large and persistent discrepancy has emerged between independent measurements of the Hubble constant (H0) – the parameter that describes the rate of cosmic expansion. Resolving this conflict, known as the Hubble tension, is one of the most pressing challenges in contemporary cosmology. It has prompted serious reflection on the assumptions underpinning ΛCDM.
There are two primary and independent methods used to determine the value of the constant, and they yield results that differ well beyond the range of mutual error bar. The first method infers H0 by analysing temperature fluctuations in the CMB. When interpreted within the ΛCDM model, this method yields a value of 67.4±0.5 km/s/Mpc. This approach is model-dependent. It depends on assumptions made within ΛCDM (especially inflation) which do not necessarily apply to other cosmological models.
The second method derives the constant from observations of astronomical objects in the local universe, using the so-called cosmic distance ladder. This process involves calibrating the intrinsic brightness of Cepheid variables. A Cepheid variable is a type of massive star that pulsates in a regular cycle, changing in brightness with a well-defined period. The crucial characteristic of Cepheids is the direct relationship between their pulsation period and their intrinsic brightness (luminosity), a relationship known as the period-luminosity law, discovered by Henrietta Swan Leavitt. This law makes them powerful "standard candles" for measuring vast cosmic distances: by observing a Cepheid's pulsation period, astronomers can determine its true luminosity and then calculate its distance by comparing it to its observed apparent brightness] and Type Ia supernovae1. The SH0ES (Supernovae, H0 for the Equation of State) collaboration, among others, has consistently obtained higher values of 73.0±1.0 km/s/Mpc. This method is relatively model-independent.
The discrepancy between these two values now exceeds 5 standard deviations, which makes it highly unlikely to be attributable to statistical error. While it has been suggested that unrecognised systematic errors may be responsible, extensive reanalyses and cross-checks using different methods and observatories have failed to eliminate the discrepancy. Very recently, the James Webb Space Telescope has essentially eliminated the possibility that the Hubble Tension is just a measurement error in the distance ladder. JWST's high-resolution infrared data has confirmed the Cepheid distances to an unprecedented degree. The tension is now a "Crisis of Physics," not a "Crisis of Data."
The Hubble Tension suggests there is a deep flaw in our understanding of the universe’s early conditions, the nature of Dark Energy, or the validity of the ΛCDM model itself. Possibilities under investigation include modifications to the physics of the early universe (such as early dark energy or extra relativistic species), revised models of Dark Matter, and even exotic proposals involving varying fundamental constants or departures from GR.
Local measurements of the expansion rate of the universe give a value near 73 km/s/Mpc. The figure extracted from the CMB only gives about 67 km/s/Mpc, but that lower number is not something we ever measured in the world. It only appears when the CMB data are interpreted through ΛCDM, which builds in a seamless physical history stretching from the Big Bang to the present. Most people take this continuity for granted, so the two numbers look like they should match. The tension rests on that expectation. Once you slow down and look at what each number is actually telling you, the mismatch ceases to be a mystery. The local value comes straight from observations, and it reflects the behaviour of the cosmos to which we physically belong. The CMB value is produced by running a simulation that includes inflation, a fixed early energy budget and a fully continuous past. Under 2PC that past is not a classical history at all. It is only the structure visible when our present observational surface is pushed backward through the old model’s rules. If the early universe is not an actual past, then the CMB value is not describing the same thing that the local value is describing. Both procedures assume an accelerating expansion (positive cosmological constant Λ), but they diverge in how they interpret this parameter.
The Hubble tension arises because two different procedures extract a parameter called “H₀” under different ontological assumptions. Local distance-ladder measurements yield H₀ ≈ 73 km/s/Mpc directly from observations within the present cosmic state. The lower value, H₀ ≈ 67 km/s/Mpc, is not measured in the same sense. It is inferred by embedding CMB observables into a ΛCDM model that presupposes a single continuous FLRW spacetime extending from recombination to the present.
Under 2PC, that continuity is not physically real. The early universe is not an actual past evolving forward into the present, but a projection of present Phase 2 structure onto the timeless Phase 1 ensemble, interpreted as if it were a historical past. Consequently, the CMB-inferred H₀ does not describe the same geometric quantity as the locally measured value, even though both are labeled “the Hubble constant.”
Type Ia supernova data independently establish that the present cosmic geometry exhibits accelerating expansion, corresponding to a positive Λ. This result does not depend on inflation or CMB reconstruction. What is disputed is not the existence of acceleration, but the ontological status of Λ. In standard cosmology, Λ is treated as vacuum energy permeating spacetime. In 2PC, Λ is the intrinsic curvature of the Phase 2 manifold – an emergent structural feature of instantiated reality, not a physical substance or force.
The locally measured H₀ therefore directly characterises the actual curvature of the Phase 2 geometry we inhabit. The CMB-derived value characterises the Hubble parameter that would be required for consistency if that same curvature were assumed to belong to a single, continuous classical history extending into a physically real early universe. The numerical mismatch reflects this category difference. It is not a discrepancy between two measurements of the same quantity, but a mismatch between a direct geometric measurement and a model-dependent reconstruction tied to a denied ontology. The tension dissolves once we recognise that one number measures the geometry of our actualised world, while the other measures consistency with a fictional history.
1 Type Ia supernovae are the thermonuclear explosions of carbon-oxygen white dwarfs in binary systems, occurring when the white dwarf accretes mass from a companion star and reaches a critical limit, the Chandrasekhar limit, triggering an explosion. These explosions are known as "standard candles" due to their consistent intrinsic brightness, allowing astronomers to measure vast cosmic distances and the accelerated expansion of the universe to estimate distances and recession velocities of galaxies.