Piero Scaruffi

These are excerpts and elaborations from my book "The Nature of Consciousness"

Dark Matter and Dark Energy The real conundrum for cosmologists, however, is to find the missing matter: most of the matter required for the universe to be the way it is (to account for the gravity that holds together the galaxies the way they are held together) has never been observed. In 1933 Swiss astronomer Fritz Zwicky proved theoretically that the universe should contain more mass than it is visible. He observed that the luminosity of some galaxies implied that their gravitational mass should be up to 400 times smaller, i.e. that most of the matter was not luminous, and coined the term "dark matter". Starting in 1971, the US astronomer Vera Rubin carried out other measurements of galaxies that further implied a much larger mass than observed. Physicists are still searching for this "dark matter" (perhaps 23% of the mass-energy of the universe and five times as abundant as matter) that does not interact with ordinary matter, does not emit and does not reflect light, but whose gravitational effect has been inferred by observing the motion of galaxies. Dark matter cannot be any of the known particles, but it is made of particles. Furthermore, in 1998 the US physicists Saul Perlmutter, Brian Schmidt, and Adam Riess discovered that the expansion of the universe is not constant, as Hubble thought, but is instead accelerating ("Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant", 1998). After all, Einstein was right the first time about the cosmological constant, except that it is positive, although very small. In fact, a universe made only of matter would decelerate because of gravitational attraction. It is true that the energy density due to matter decreases as the universe expands, but this decrease is not enough to account for the observed acceleration: there has to be some kind of energy at work that is not due to matter. What is needed is a fixed amount of energy at every point in the universe. This “dark energy” is not associated with particles and it must be about 73% of the mass-energy of the universe in order to account for the observed acceleration. Dark energy is not made of particles. It is not a new particle, otherwise it would be part of matter. Dark energy must be uniformly distributed throughout space to explain the accelerating universe, whereas “dark matter” must be organized in large chunks surrounding galaxies to explain the existing structures. Dark matter exerts positive pressure (it pulls matter together and creates structures), whereas dark energy exerts negative pressure (it pulls the universe apart). Unlike matter, which dilutes while the universe expands, “dark energy” is a form of energy that is constant no matter what the universe does. An obvious candidate is vacuum energy, the energy that Heisenberg’s principle might create in the vacuum as particles pop up randomly. The anti-gravitational energy that Einstein called “cosmological constant” would be physically equivalent to this vacuum energy. However, this vacuum energy would be many orders of magnitude more than the required amount of dark energy. So then one would have to explain what happens to all the vacuum energy that does not become “dark energy”, i.e. that does not contribute to the acceleration of the universe.  The Canadian physicist Cliff Burgess (“Extra Dimensions and the Cosmological Constant Problem “, 2007) suggested that vacuum energy may be hidden away in other dimensions, dimensions that are curled and only sensitive to gravitation. Matter (the sum of “our” matter plus dark matter) may have dominated at the beginning, thus slowing down the expansion. Energy density decreases as the universe expands. After a while, dark energy became predominant and caused the universe to accelerate. Unless some process reverses this trend, eventually matter will be so diluted that its density will be zero and there will only be dark energy to control a universe spinning out of control. Whatever the reason, more than 90% of the mass-energy of the universe has not been found. Today’s Physics is really a scientific theory about the mere 4% of mass-energy that we can account for. Many other hypotheses for dark energy have been suggested. For example, the Iranian physicist Niayesh Afshordi suggested that dark energy is a field created by black holes ("Stellar black holes and the origin of cosmic acceleration", 2009). Dark Energy as Disappearing Energy Lee Smolin ("Quantization of unimodular gravity and the cosmological constant problems", 2009) rediscovered "unimodular gravity", a variation of general relativity discovered by Einstein in 1919 ("Do gravitational fields play an essential part in the structure of the elementary particles of matter", 1919). Unimodular gravity does not explicitly mandate the conservation of energy and momentum. If one actually decides that energy and momentum don't need to be conserved (i.e. that they can be destroyed and created), unimodular gravity yields a value for the cosmological constant that would explain "dark energy". This is what Thibault Josset, Alejandro Perez and Daniel Sudarsky did ("Dark energy as the weight of violating energy conservation", 2016). In that case dark energy is simply a measure of the energy-momentum that the universe has lost since the big bang. Dark Energy as Mass Gravity A logical alternative to dark matter is that gravity is weaker than everybody assumed. This would be the case if gravitons had a mass. Einstein's theory implicitly assumes that gravitons have no mass, that the gravitational force takes place with no interference. If gravitons have a mass, then gravity itself plays the role of dark energy. Claudia De Rham of Imperial College London and Gregory Gabadadze of New York University showed that this is indeed possible ("Resummation of Massive Gravity", 2011). The Big Bang of Dark Matter The theory of “Cold Dark Matter” (CDM), originally proposed by Canadian astrophysicist James Peebles ("Large-scale background temperature and mass fluctuations due to scale-invariant primeval perturbations", 1982), assumes that dark matter is both “cold” (it moves slowly compared with the speed of light) and “dark” (it interacts very weakly with ordinary matter and electromagnetic radiation) and only interacts gravitationally. Dark matter would be made of massive particles called Weakly Interacting Massive Particles (WIMPs). Dark energy is represented by a cosmological constant called Lambda. A model of the origin and evolution of galaxies that combine both CDM and dark energy was proposed by the US physicists Joel Primack, George Blumenthal and Sandra Moore Faber ("Formation of galaxies and large-scale structure with cold dark matter", 1984). To account for discrepancy between astronomical observations and CDM, the theory of “Self-Interacting Dark Matter” (SIDM), proposed by US astrophysicist David Spergel, ("Observational Evidence for Self-Interacting Cold Dark Matter", 2000), assumes the existence of both dark particles and dark forces (the forces through which these dark particles interact), possibly just one dark particle and one dark force. If dark matter can interact with itself, the problems of CDM are solved. SIDM particles would be much lighter than WIMPs. This theory implies that there should exist “dark stars” and US astrophysicists Katherine Freese and Martin Winkler pointed out some dark-star candidates while at the same time hypothesizing that matter and dark matter were not born together but instead that an independent “ big bang” event created dark matter after the Big Bang had created “normal” matter (“Dark Matter and Gravity Waves from a Dark Big Bang”, 2023). Back to the beginning of the chapter "The New Physics" | Back to the index of all chapters