Could Quantum Gravity Models Arising from Holography Explain Cosmological Acceleration?https://phys.org/news/2023-06-quantum-gravity-holography-cosmological.htmlResearchers at University of Maryland and University of British Columbia recently carried out a theoretical study exploring the possibility that holography, an approach to quantum gravity that includes some features of conventional holograms, could be used to describe quantum mechanical phenomena. Their paper, published in
Physical Review Letters,
introduces a theoretical argument that could suggests a link between observable cosmological phenomena and the physics that would underpin wormhole spacetimes.... "We now have fully consistent models of quantum gravity via an approach called holography, where the gravitational physics is encoded in a simpler, lower dimensional non-gravitational quantum system. Holographic gravity theories have taught us a lot about the physics of black holes and even about the fundamental nature of spacetime, but so far, there hasn't been much progress in understanding realistic cosmological spacetimes with a Big Bang."
The key objective of the recent work by Van Raamsdonk and his colleagues was to describe the physics of cosmological spacetimes using a holographic approach. Spacetimes essentially entail the combination of the three dimensions of space and the one dimension of time into a single four-dimensional manifold, which underpins phenomena such as the Big Bang and cosmic expansions.
"We argue that these models can also explain the fact that the expansion of our universe is accelerating, but in a different way that what's usually assumed," Van Raamsdonk said. "The most conventional explanation, in what's called the Lambda-CDM (cold dark matter) model, is that we have a positive 'cosmological constant,' a type of dark energy that always has the same density throughout the universe. We argued that the holographic models can also naturally explain cosmic acceleration, but they do it via a dark energy whose density changes with time."
Holographic models suggest that at some point the density of dark energy decreases below zero, reaching a negative value. This could in turn cause a deceleration and the eventual re-collapse of the universe, which is sometimes referred to as a "big crunch." Van Raamsdonk and his colleagues thus propose that these models may thus offer a different perspective on cosmological acceleration.
"We observed that quantum gravity models arising from holography can naturally explain cosmological acceleration in a novel way, with a changing dark energy that eventually becomes negative," Van Raamsdonk said. "We don't know for sure if our universe works this way, but it's something that we can look for in cosmological observations.
"We have compared the predictions of our class of model (the decreasing dark energy) with what we see from direct observations of the expansion of the universe, to see whether these seem consistent," Van Raamsdonk added. "With my student Chris Waddell, we looked at the most recent redshift vs. brightness data for type IA supernovae. These observations can tell us quantitatively what the expansion of the universe looked like over the past six or seven billion years. While the data don't definitively tell us whether or not the dark energy is decreasing, we found that most of the models that fit the data acceptably do have decreasing dark energy at present."
Stefano Antonini et al,
Accelerating Cosmology from a Holographic Wormhole,
Physical Review Letters (2023)
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.221601-------------------------------------------------------------
The Expansion of the Universe Could Be a Mirage, New Theoretical Study Suggestshttps://www.livescience.com/physics-mathematics/dark-energy/the-expansion-of-the-universe-could-be-a-mirage-new-theoretical-study-suggestsThe expansion of the universe could be a mirage, a potentially controversial new study suggests. This rethinking of the cosmos also suggests solutions for the puzzles of dark energy and dark matter, which scientists believe account for around 95% of the universe's total energy and matter but remain shrouded in mystery.
The novel new approach is detailed in a paper published June 2 in the journal
Classical and Quantum Gravity, by University of Geneva professor of theoretical physics Lucas Lombriser.
Scientists know the universe is expanding because of redshift, the stretching of light's wavelength towards the redder end of the spectrum as the object emitting it moves away from us. Distant galaxies have a higher redshift than those nearer to us, suggesting those galaxies are moving ever further from Earth.
More recently, scientists have found evidence that the universe's expansion isn't fixed, but is actually accelerating faster and faster. This accelerating expansion is captured by a term known as the cosmological constant, or lambda.
The cosmological constant has been a headache for cosmologists because predictions of its value made by particle physics differ from actual observations by 120 orders of magnitude. The cosmological constant has therefore been described as "the worst prediction in the history of physics."
Cosmologists often try to resolve the discrepancy between the different values of lambda by proposing new particles or physical forces but Lombriser tackles it by reconceptualizing what's already there.
... In Lombriser's mathematical interpretation, the universe isn't expanding but is flat and static, as Einstein once believed. The effects we observe that point to expansion are instead explained by the evolution of the masses of particles — such as protons and electrons — over time.
In this picture, these particles arise from a field that permeates space-time. The cosmological constant is set by the field's mass and because this field fluctuates, the masses of the particles it gives birth to also fluctuate. The cosmological constant still varies with time, but in this model that variation is due to changing particle mass over time, not the expansion of the universe.
In the model, these field fluctuations result in larger redshifts for distant galaxy clusters than traditional cosmological models predict. And so, the cosmological constant remains true to the model's predictions.
"I was surprised that the cosmological constant problem simply seems to disappear in this new perspective on the cosmos," Lombriser said.
Lombriser's new framework also tackles some of cosmology's other pressing problems, including the nature of dark matter. This invisible material outnumbers ordinary matter particles by a ratio of 5 to 1, but remains mysterious because it doesn't interact with light.
Lombriser suggested that fluctuations in the field could also behave like a so-called axion field, with axions being hypothetical particles that are one of the suggested candidates for dark matter.
These fluctuations could also do away with dark energy, the hypothetical force stretching the fabric of space and thus driving galaxies apart faster and faster. In this model, the effect of dark energy, according to Lombriser, would be explained by particle masses taking a different evolutionary path at later times in the universe.
In this picture "there is, in principle, no need for dark energy," Lombriser added.
Lombriser, Lucas
Cosmology in Minkowski space,
Classical and Quantum Gravity, (2023)
