According to the The standard model of particle physicsThe universe is governed by four fundamental forces: electromagnetism, the weak nuclear force, the strong nuclear force, and gravity. While the first three are described by quantum mechanics, gravity is described by Einstein’s theory of general relativity. Surprisingly, it is gravity that presents the greatest challenges for physicists. While the theory accurately describes how gravity works for planets, stars, galaxies, and clusters, it doesn’t exactly apply on all scales.
While general relativity Validate repeatedly over the past century (starting with Eddington Eclipse Experience in 1919), the gaps still appear as scientists try to apply them on the quantum scale and to the universe as a whole. According to a new study led by Simon Fraser University, an international team of researchers has tested general relativity on the largest scales and concluded that it might need a tweak or two. This method could help scientists solve some of the biggest mysteries facing astrophysicists and cosmologists today.
The team included researchers from Simon Fraser, The Institute for Cosmology and Gravity at the University of Portsmouth Center for particle cosmology at the University of Pennsylvania Osservatorio Astronomico di RomaThe UAM-CSIC Institute for Theoretical PhysicsLeiden University Lorentz Instituteand the Chinese Academy of Sciences (cup). Their findings appeared in a paper entitled “Imprints of cosmic tensions in reconstructed gravityRecently published in natural astronomy.
According to Einstein’s field equations for GR, the universe was not static and must be in a state of expansion (otherwise the force of gravity would cause it to contract). While Einstein initially resisted this idea and tried to propose a mysterious force that keeps the universe in equilibrium (the “cosmological constant”), observations by Edwin Hubble in the 1920s showed that the universe was expanding. Quantum theory also predicts that the void of space is filled with energy that goes unnoticed because conventional methods can only measure changes in energy (rather than its total amount).
By the 1990s, new observatories such as Hubble Space Telescope (HST) Pushing the Frontiers of Astronomy and Cosmology. Thanks to surveys like Hubble Deep Fields (HDF), astronomers can see objects as they appeared 13 billion light-years away (or less than a billion years after the Big Bang). To their surprise, they discovered that over the past four billion years, the rate of expansion has been accelerating. This led to what is known as the “old cosmological constant problem,” where gravity is weaker on cosmic scales, or a mysterious force driving cosmic expansion.
Lead author Levon Bogosian (Simon Fraser University Professor of Physics) and co-author Kazuya Koyama (University of Portsmouth Professor of Cosmology) summarized the issue in a recent article via Conversation. As they point out, the problem of the cosmological constant stems from a single question with radical implications:
“[W]Whether the energy of the void actually attracts – causing gravity to force and change the expansion of the universe. If yes, why is its gravity so much weaker than expected? If the vacuum does not attract at all, what is causing the cosmic acceleration? We don’t know what dark energy is, but we need to hypothesize that it exists in order to explain the expansion of the universe. Similarly, we also need to postulate some kind of invisible matter presence, dubbed dark matter, to explain how galaxies and clusters evolved to be the way we observe them today.”
The existence of dark energy is part of the standard cosmological theory known as the Lambda Cold Dark Matter (LCDM) model – where Lambda is the cosmological constant/dark energy. According to this model, the mass and energy density of the universe is made up of 70% dark energy, 25% dark matter, and 5% ordinary (visible or “luminous”) matter. While this model has successfully matched observations cosmologists have collected over the past 20 years, it assumes that most of the universe is made up of undetectable forces.
It is for this reason that some physicists have ventured that GR may need some modification to explain the universe as a whole. Moreover, a few years ago, astronomers noticed that measuring the rate of cosmic expansion in different ways yields different values. This problem, as Bogosian and Koyama explain, is known as the Hubble tension:
“The discord, or tension, between two values of the Hubble constant. The first is the number predicted by the LCDM cosmological model, which has been developed to match The light left over from the Big Bang (cosmic microwave background radiation). The other is the rate of expansion, which is measured by observing exploding stars known as supernovae in distant galaxies.”
Several theoretical ideas have been proposed to modify the LCDM model to explain the Hubble tension. Among them are alternative theories of gravity, such as Modified Newtonian Dynamics (MOND), a modified approach to Newton’s law of universal gravitation that eliminates the existence of dark matter. For more than a century, astronomers have tested GR by observing how the curvature of space-time changes in the presence of gravitational fields. These tests have become particularly extreme in recent decades, which include how supermassive black holes (SMBHs) affect stars in their orbit or how gravitational lensing amplifies and alters the passage of light.
For their study, Bogosian and his colleagues used a statistical model known as Bayesian inference, which is used to calculate the probability of a theory given more data. From there, the team simulated cosmic expansion based on three parameters: CMB data from the European Space Agency; Planck satellitesupernova and galaxy catalogs such as Sloan Digital Sky Survey (SDSS) f Dark Energy Survey (DES) and predictions for the LCDM.
“Together with a team of cosmologists, we tested the fundamental laws of general relativity,” said Bogosian and Koyama. We also explored whether modifying Einstein’s theory could help solve some open problems in cosmology, such as the Hubble tension. To determine whether GR is true on the largest scales, we set out, for the first time, to investigate three aspects of it simultaneously. These were the expansion of the universe, the effects of gravity on light, and the effects of gravity on matter.”
Their results showed some inconsistencies with Einstein’s predictions, although they were of fairly low statistical significance. They also found that solving the problem of Hubble tension was difficult simply by modifying the theory of gravity, suggesting that additional force may be required or that there are errors in the data. If the first is true, Bogosian and Koyama said, it is possible that this force existed during the early universe (about 370,000 years after the Big Bang) when protons and electrons first combined to form hydrogen.
Numerous possibilities have been developed in recent years, starting with a special form of dark matter, an early type of dark energy, or Primordial magnetic fields. In any case, this latest study indicates that there is future research to be done that may lead to a revision of the more widely accepted cosmological model. Pogosian and Koyama said:
“[O]Your study showed that it is possible to test the validity of general relativity at cosmic distances using observational data. While we haven’t solved the Hubble problem yet, we’ll have plenty of data from the new probes in a few years. This means that we will be able to use these statistical methods to further modify general relativity, and to explore the limits of modifications, to pave the way for solving some of the open challenges in cosmology.”