The Big Bang theory is widely confirmed by numerous observations that have led to the cosmological model known as ΛCDM: "Lambda" (Λ) represents the cosmological constant associated with dark energy, which is responsible for the acceleration of the Universe's expansion, and "CDM" stands for "Cold Dark Matter," a component made up of particles that interact only gravitationally and are essential in the formation of cosmic structures.
A new study conducted by cosmologists from SISSA—Sandeep Haridasu, Paolo Salucci, and Gauri Sharma—and recently published in the prestigious journal “Monthly Notices of the Royal Astronomical Society,” has paved the way for understanding one of the theory’s predictions that seems to be in serious disagreement with observations.
Recent investigations have highlighted an anomaly in the current speed of the Universe’s expansion H0, known as the Hubble Tension. Haridasu explains: "The value of H0 appears to be different depending on how it is measured.” Using the properties of Supernovae and Cepheid variables observed in nearby galaxies as distance gauges from us, the resulting value is H0=73 km/s per megaparsec, while properties of the cosmic microwave background that pervades the entire Universe imply H0=67 km/s per megaparsec. These two determinations are extremely precise, continues Haridasu: "the probability that they are statistically in agreement is less than one in a billion." The anomaly of the "Hubble Tension" could be the sign of the presence of unknown physics beyond Einstein's equations.
In their new work, Haridasu et al. have devised and used a new method to investigate the expansion of the Universe based on the properties of spiral galaxies. In these, stars located at a certain radius from their centers maintain a gravitational equilibrium due to their rotation speed: consequently, a galaxy’s luminosity and the rotation speeds of stars at specific radii are tightly correlated. This leads to a new distance gauge for galaxies that has precision similar to those mentioned above but can be applied to a much larger number of objects. Using this technique, explains Salucci, “we have managed to trace the expansion of the Universe up to 150 Megaparsecs, analyzing a sample of 843 spiral galaxies.” The work reports 3650 measurements of the ratio between the Hubble parameter at various distances from us and its value at redshift zero, each with precision up to 15%. These measurements imply that, up to 200 megaparsecs from us, the expansion of the Universe corresponds to the predictions of the standard Lambda CDM model with the value of H0=73.
No decrease in this quantity is observed as one moves away from our galaxy, in contrast to many "local explanations" of the Hubble Tension that have been proposed, among all the presence of a gigantic cosmic void near our galaxy. Sharma concludes: "If the presence of new physics is the origin of the Hubble Tension, it must manifest on scales much larger than 200 Mpc and perhaps even in the first moments of the Universe’s life."
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