Dark energy, paradigm shifts, and the role of evidence
Ofer Lahav and Michela Massimi
- Author Affiliations
Dept of Physics & Astronomy, University College London, UK; (o.lahav@ucl.ac.uk).
School of Philosophy, Psychology and Language Sciences, University of Edinburgh, UK; (michela.massimi@ed.ac.uk).
Abstract
Ofer Lahav and Michela Massimi examine the current state of the Standard Model of cosmology in a historical and philosophical context.
Cosmological measurements, recently confirmed and refined by the Planck space mission and other probes, strongly favour a “concordance” model, according to which the universe is flat and contains approximately 5% ordinary matter (baryons), 25% cold dark matter and 70% dark energy (Ade et al. 2013). The concept of dark energy is a variant on Einstein's cosmological constant, lambda (Λ), and the proposition for a Λ-like linear force can even be traced back to Newton (e.g. Calder and Lahav 2008, 2010 for a historical perspective). This “Λ + cold dark matter” (ΛCDM) paradigm and its extensions pose fundamental questions about the origins of the universe. If dark matter and dark energy truly exist, we must understand their nature. Alternatively, general relativity and related assumptions may need radical modifications.
Commonly, dark energy is quantified by an equation of state parameter, w, which is the ratio of pressure to density. The case w = −1 corresponds to Einstein's cosmological constant in general relativity, but in principle w may vary with cosmic epoch, e.g. in the case of scalar fields. Essentially, w affects both the geometry of the universe and the growth rate of structures. These effects can be observed via a range of cosmological probes, including the cosmic microwave background (CMB), galaxy clustering, weak gravitational lensing and Type Ia supernovae. The Hubble diagram of Type Ia supernovae (Perlmutter et al. 1999, Riess et al. 1998), for which the 2011 Nobel Prize in Physics was awarded, revealed that our universe is not only expanding but is also accelerating in its expansion. The main problem is that we still have no clue as to what is causing the acceleration, or what dark matter and dark energy are.
The key point we are addressing in this article is the following: should a discrepancy between data and the existing cosmological theory be resolved by adding new entities such as dark matter and dark energy, or by modifying the underlying theory? The Dark Energy Survey (DES; see box “The Dark Energy Survey” p3.15) and other similar projects aim to address this important question by looking for further experimental evidence for dark energy.
There is still the possibility of another major paradigm shift in our understanding of the cosmos, including the following options:
Violation of the Copernican principle: for example, if we happen to be living in the middle of a large void.
Dark energy being something different than vacuum energy: although vacuum energy is mathematically equivalent to Λ, the value predicted by fundamental theory is as much as 10120 times larger than observations permit.
Modifications to gravity: it may be that general relativity requires revision to a more complete theory of gravity.
Multiverse: if Λ is large and positive, it would have prevented gravity from forming large galaxies, and life would never have emerged. Using this anthropic reasoning to explain the cosmological constant problems suggests an infinite number of universes (the “multiverse”) in which Λ and other cosmological parameters take on all possible values. We happen to live in one of the universes that is “habitable”.
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PERGUNTA DESTE BLOGGER:
Quantas anomalias que uma teoria científica não explica são necessárias para haver uma mudança paradigmática? O exemplo proposto por Lahav e Massimi se fosse aplicado em biologia evolucionária, nós veríamos o maior número de mudanças paradigmáticas do que em quaisquer outras áreas científicas. Explico. Uma teoria que tem n mecanismos evolucionários (de A a Z, vai que um falhe), se veria obrigado a lidar mais frequentemente com as montanhas de anomalias não explicadas pela atual teoria geral da evolução.
Ainda há esperança. Vem aí uma nova teoria geral da evolução - a SÍNTESE EVOLUTIVA AMPLIADA ou ESTENDIDA que será anunciada somente em 2020. Enquanto isso, faz-se biologia evolucionária sem fundamentação teórica... No vazio epistemológico. A ciência não abomina o vácuo epistêmico? Abomina, menos em Darwin...
Ainda há esperança. Vem aí uma nova teoria geral da evolução - a SÍNTESE EVOLUTIVA AMPLIADA ou ESTENDIDA que será anunciada somente em 2020. Enquanto isso, faz-se biologia evolucionária sem fundamentação teórica... No vazio epistemológico. A ciência não abomina o vácuo epistêmico? Abomina, menos em Darwin...
