Everything in the universe has gravity – and feels it too. Yet this commonality of all fundamental forces is also the one that presents the greatest physical challenges.
Albert Einstein’s theory of general relativity has been remarkably successful in describing the gravity of the stars and planets, but it does not seem to apply perfectly on all scales.
General relativity has been proven by many years of observations, from Eddington’s measurement of the deflection of the Sun in 1919 to the recent detection of gravitational waves in stars.
But gaps in our understanding begin to appear when we try to apply it to very small distances, where the laws of quantum mechanics operate, or when we try to describe the entire universe.
Our new study published in Nature AstronomyEinstein now proved the theory on the largest scales.
We believe that our approach may someday help solve some of the biggest mysteries in cosmology, and the results suggest that a general theory of relativity may be needed in this form.
A bad example?
Quantity predicts empty space, empty, filled with energy. We do not notice its presence, because our thoughts can only measure changes in energy rather than its total amount.
However, according to Einstein, energy has a repulsive force – it pushes vacuum space apart. Interestingly, in 1998, it was discovered that the expansion of the universe is actually accelerating (a discovery awarded with the 2011 Nobel Prize in Physics).
However, the amount of empty energy, or dark energy, as it has been said, to explain the acceleration is several orders of magnitude smaller than what quantum theory predicts.
Hence the big question, called the “old cosmological constant question”, is whether the vacuum of energy actually pulls – exerting the force of gravity and changing the expansion of the universe.
If so, why is its gravity so much weaker than predicted? If the vacuum does not gravitate at all, what causes cosmic acceleration?
We don’t know what dark energy is, but we need to postulate it to explain the expansion of the universe.
Similarly, we need to assume the appearance of an invisible presence of matter, called dark matter, in order to explain how galaxies and clusters have evolved, as we observe them today.
These principles are baked into the standard cosmological theory of scientists, called the lambda cold dark matter (LCDM) model – suggesting that there is 70 percent dark energy, 25 percent dark matter, and 5 percent ordinary matter in the cosmos. And this model fits remarkably well everything that has been collected by cosmologists over the past 20 years.
However, the fact that most of the universe consists of dark forces and substances, with odd values not making sense, has led many physicists to wonder if Einstein’s theory of gravity needs modification to describe the entire universe.
A new twist appeared a few years ago, when it became apparent that different ways of measuring the rate of cosmic expansion, called the Hubble constant, gave different answers – a question known as the Hubble tension.
The difference, or tension, between the two values of the Hubble constant.
There is one number predicted by the LCDM cosmological model, which has been developed as a pair of light left over from the Big Bang (pronounced cosmic rays).
Another is the expansion rate measured by observing stars exploding known as supernovae in distant galaxies.
Many theoretical ideas have been proposed for ways to modify the LCDM to explain the Hubble tension. Among them are alternative theories of gravity.
We can examine the plan to check if the universe obeys the precepts of Einstein’s theory.
General relativity describes gravity as bending or warping space and time, bending the paths along which light and matter travel. Most of all, it predicts that the trajectories of light rays and matter are bent in the same way by gravity.
Together with a team of cosmologists, we are testing the fundamental laws of general relativity. We also explored whether Einstein’s theory could be modified to help solve some open problems in cosmology, such as the Hubble tension.
In order to investigate whether general relativity is correct in large-scale calculations, we first proceed to investigate three aspects of it simultaneously. These were the expansion of the universe, the effect of gravity on light, and the effect of gravity on matter.
Using a statistical method known as Bayesian inference, we reconstruct the gravity of the Universe throughout cosmic history in a computer model based on these three parameters.
We were able to estimate the parameters using cosmic ray color data from the Planck satellite, supernova catalogs as well as observations of the shape and distribution of distant galaxies by the SDSS and DES telescopes.
We then compared our reconstruction with the prediction of the LCDM model (essentially Einstein’s model).
We have found some known researches about a possible match with Einstein’s conjecture, although the statistical significance is rather low.
This means, however, that there is a possibility that gravity works differently in large states, and the theory of general relativity can be abandoned.
Our study also found that it is very difficult to solve the Hubble tension problem by changing the theory of gravity.
A full solution would require a new ingredient in the cosmological model, present before the time when protons and electrons first combined to form hydrogen just after the Big Bang, as a special form of dark matter, the first form of dark energy, or primordial magnetic fields.
Or perhaps there is an as-yet-unknown systematic error in the data.
That said, our study demonstrated that it is possible to test the validity of general relativity over cosmological distances using observational data. While we haven’t solved the Hubble problem yet, we will have a lot more data from new probes in a few years.
This means that we will be able to use these statistical methods to continue general relativistic tweaking, explore the limits of modification, and pave the way to solving some of the open challenges in cosmology.
Kazuya Koyama, Professor of Cosmology, University of Portsmouth and Levon Pogosian, Professor of Physics, Simon Fraser University
This article is republished from The Conversation under the Creative Commons license. Read the original article.
#Scientists #Projected #Einsteins #Relationship #Cosmic #Scale #Odd