Here are some counterfactuals for named entities that can be independently verified, based on the provided sources: - **ΛCDM model of cosmology (dark matter):** If **dark matter did not exist as a non-baryonic particle**, we would expect to observe different gravitational effects in galaxies and galaxy clusters than what is currently observed. Specifically, galaxy rotation curves should fall off according to the visible matter distribution, and galaxy clusters should not exhibit the mass discrepancies inferred from gravitational lensing and dynamical studies. Alternative theories like Modified Newtonian Dynamics (MOND) predict galaxy dynamics without dark matter, and continued astronomical observations can either strengthen the evidence for dark matter (e.g., through direct detection) or provide more support for these alternative gravitational models on various scales. The persistent null results in direct dark matter detection experiments provide indirect support for this counterfactual. - **ΛCDM model of cosmology (dark energy):** If **dark energy was not the cause of the accelerating expansion of the universe**, we would expect to see a different rate of expansion over cosmic time. Future high-precision cosmological observations, such as those measuring Type Ia supernovae, baryon acoustic oscillations, and the cosmic microwave background anisotropies, could reveal deviations from the ΛCDM model's predicted expansion history [Implied]. If the expansion was not accelerating or was decelerating at a different rate, it would necessitate alternative explanations for the observed redshift-distance relation or challenge the need for dark energy altogether. - **Big Bang theory (Horizon Problem):** If the **uniformity of the cosmic microwave background radiation (CMB) was not due to an early period of rapid expansion called inflation**, we would expect to see significant temperature variations in the CMB across regions that were causally disconnected in the standard Big Bang model. The observed high degree of uniformity in the CMB, with only very small temperature fluctuations, is a key piece of evidence supporting inflation. If future, more precise CMB observations revealed large-scale anisotropies inconsistent with the predictions of inflation (e.g., specific patterns of non-Gaussianity), it would challenge the inflationary solution to the horizon problem and necessitate alternative explanations for the CMB's uniformity. - **Lorentz invariance:** If **Lorentz invariance was violated at a fundamental level**, we would expect to observe phenomena such as the speed of light varying with energy or direction, or different limiting speeds for different particles. Astroparticle observations, such as the arrival times of photons and neutrinos from distant astrophysical events, and high-precision laboratory experiments comparing the properties of different particles in different orientations relative to a preferred frame, are constantly testing Lorentz invariance. Any confirmed detection of such variations would directly contradict Lorentz invariance. - **CPT theorem:** If **CPT symmetry was violated**, we would expect to find measurable differences in fundamental properties (like mass, lifetime, charge magnitude, magnetic moment) between particles and their corresponding antiparticles. High-precision experiments, such as those conducted at CERN's Antiproton Decelerator comparing protons and antiprotons, and spectroscopic studies of antihydrogen compared to hydrogen, are designed to detect such minute differences. Any statistically significant and reproducible difference found would constitute a violation of CPT symmetry and challenge the foundations of the Standard Model and Quantum Field Theory. Current experiments show exquisite agreement with CPT symmetry. - **Newtonian gravity (anomalous precession of Mercury's orbit):** If **Newtonian gravity had accurately predicted the precession of Mercury's orbit**, there would have been no need for a new theory of gravity (General Relativity) to explain this discrepancy. The observed 43 arcseconds per century anomaly that Newtonian mechanics could not account for was a key piece of empirical evidence that Einstein's General Relativity successfully explained by considering the curvature of spacetime due to Mercury's proximity to the Sun. Independent astronomical observations of Mercury's orbit continue to confirm this precession rate, thus verifying the failure of Newtonian gravity at this level of precision and supporting the need for a more accurate theory like GR. - **Naturalness arguments in Beyond the Standard Model (BSM) physics:** If **naturalness arguments correctly predicted the scale of new physics needed to address the hierarchy problem**, experiments at the Large Hadron Collider (LHC) would have likely discovered new particles, such as low-mass supersymmetric particles, within its current energy reach. The absence of such direct evidence for many natural BSM scenarios (like minimal supersymmetry) at the LHC's TeV scale suggests that either these specific models are incorrect, or naturalness might not be a reliable guiding principle for physics beyond the Standard Model at these energy scales. Continued searches at higher energies and more comprehensive analyses of existing data can further test this counterfactual by either discovering the predicted particles or further constraining the parameter space of these theories. These counterfactuals highlight specific predictions or expectations that would be different if certain well-established ideas were incorrect. The fact that experiments and observations can (or have) tested these expectations allows for independent verification and is a crucial aspect of the scientific process.