Why all these prejudices against a constant? (1002.3966v3)

Abstract: The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.

• The paper challenges prevailing notions about the cosmological constant by demonstrating its consistent integration within general relativity to explain universal expansion.
• It critiques the coincidence problem, arguing that the apparent balance between dark energy and matter is a natural outcome of cosmic evolution rather than a mystery.
• The authors highlight limitations in applying flat-space quantum field theory to curved spacetime, calling for refined approaches in calculating vacuum energy.

An Examination of Prejudices Against the Cosmological Constant

The paper by Eugenio Bianchi and Carlo Rovelli addresses a critical issue in contemporary cosmology: the role and perception of the cosmological constant ($\Lambda$) within the framework of general relativity, particularly in the context of explaining the observed acceleration in the expansion of the universe.

The authors argue against the widespread perception of dark energy as a profound mystery that challenges our understanding of cosmology. They assert that current physical theories, specifically the $\Lambda$CDM model, which incorporates a non-vanishing cosmological constant, offer a straightforward explanation of this phenomenon. The expansion is consistent with general relativity when a cosmological constant is considered, and the observed acceleration merely allows us to measure the non-zero value of this constant.

Historical and Theoretical Perspective

The paper revisits the historical context regarding the cosmological constant's inclusion in Einstein’s equations. Einstein's introduction of $\Lambda$ was initially motivated by the desire to maintain a static universe, leading him to dismiss the term as a "greatest blunder." The authors clarify this misconception by emphasizing that the real oversight was in Einstein's failure to predict an expanding universe, a possibility inherent in his equations regardless of the presence or absence of $\Lambda$. They highlight that the cosmological constant forms a natural component of the Einstein equations, suggesting that it would be more puzzling if $\Lambda$ were zero.

Critique of the Coincidence Argument

The paper challenges the "coincidence problem," which questions why the cosmological constant and matter density are of comparable orders of magnitude today. The authors argue that this perspective emerges from a misapplication of the cosmological principle and faulty probabilistic reasoning. The universe's current state, where $\Omega_b$ and $\Omega_\Lambda$ are of similar magnitudes, aligns with the time constraints necessary for life as we know it, without contravening principles of generality or statistical normality.

Quantum Field Theory and the Cosmological Constant

A significant focus of the paper is the cosmological constant problem in quantum field theory (QFT), where the vacuum energy density calculations vastly exaggerate the observed cosmological constant's value. The authors discuss the radiative corrections in QFT and highlight the failure of traditional methods to deal with vacuum energy on gravitational scales due to inappropriate assumptions in applying flat-space QFT concepts to curved spacetime scenarios. The discrepancy, often cited as a theoretical failure, suggests unresolved aspects in our understanding of QFT rather than a flaw in the cosmological constant's role in cosmology.

Implications for Cosmology and Physics

The authors emphasize that while it is prudent to test the $\Lambda$CDM model and explore alternative theories, sensationalizing the concept of dark energy as unresolved or mystical is unproductive. Instead, acceptance of a small but non-zero cosmological constant within current models should be seen as a resolution to prior observational uncertainties regarding universe expansion. The paper calls for a more grounded approach in addressing the phenomenology of dark energy and the cosmological constant, asserting that misconceptions are often fueled by the quest for dramatic scientific narratives rather than a genuine lack of understanding.

Overall, Bianchi and Rovelli's work advocates for clarity and precision in how cosmology addresses the cosmological constant, urging the scientific community to focus on outstanding theoretical issues in particle physics and quantum gravity that influence cosmological theories without mischaracterize $\Lambda$'s established role in explaining universal expansion.