Gravity in the early universe may have been considerably stronger than it is now. This decrease in gravity’s strength could have contributed to the universe’s accelerated expansion. The concept of “dark energy” was introduced about three decades ago to explain this acceleration. However, recent insights suggest that dark energy might not be necessary, implying that the theory of relativity requires revision. This topic was discussed in an article in Nature Communications.
The instigators were the scientists who collaborated on the Dark Energy Survey project. The core of the project involves powerful telescopes creating a three-dimensional map of the distant universe. These telescopes capture images of very distant—and therefore ancient—galaxies.
In mapping, it’s crucial to identify instances where the light from a distant galaxy is warped by the gravitational field of a nearer one. Common knowledge from physics tells us that light, which typically travels in a straight line, bends around massive objects, creating a gravitational lens. The extent of light bending allows scientists to measure the strength of the gravitational field and compare it with the galaxy’s mass.
This process is quite fascinating. We understand how a kilogram of any material exerts gravitational pull, whether it’s a kettlebell or a kilogram of feathers. Basic equations (Newtonian physics suffices, without the need for Einstein’s theories) predict the gravitational force at a given distance. These predictions hinge on the gravitational constant. But the question remains: Is this constant truly unchanging? Has it always been constant throughout history?
Through meticulous observations, scientists have gathered sufficient gravitational lenses to map gravity’s behavior across various epochs, from billions of years ago to the present day.
Here’s an intriguing finding: around 6-7 billion years ago, gravity conformed neatly to Einstein’s equations, acting as a sterile, highly predictable force. However, between 3-5 billion years ago, it appears that identical masses generated significantly weaker fields, indicating that gravity weakened independently.
Coinciding with this period is the enigmatic acceleration of the Universe’s expansion. It seems as though gravity’s “slackening” has caused the Universe to spread out more vigorously. In this context, the concept of “dark energy” becomes unnecessary (and should not be confused with “dark matter,” which is distinct).
Crucially, the research methodology employed is founded on criteria independent of any initial hypotheses, ensuring objectivity. Yet, given the extreme delicacy of these observations, errors are a possibility. Hence, scientists report the discovery of this effect with “high probability,” though not absolute certainty.
Theorists have speculated that the laws of physics, as currently understood, are not immutable but variable. For instance, a recent hypothesis suggests that gravity, after diminishing over distance, may abruptly intensify. The observed phenomenon, however, varies over time rather than distance. It appears the gravitational constant may not be so constant after all.
What might be the reason? It is too early to discuss it
Firstly, scientists measure gravity by observing so-called gravity wells, which are dips in space-time created by massive bodies, including us. Around 3 billion years ago, these wells became less deep. This raises questions: Is gravity weakening, or is the property of space changing, becoming less pliable or sagging more?
Secondly, the distance to a galaxy, and thus the epoch in which it is observed, is generally estimated using redshift. This is because the universe’s expansion makes distant objects appear redder than closer ones. However, this raises the question: What if photons lose energy en route and appear red, altering our understanding?
If we did observe a change in gravity, the question of why remains unanswered because the nature of gravity is still not fully understood. Einstein’s theory describes gravity as a depression in spacetime caused by mass, addressing the “how” but not the “why.” Quantum mechanics suggests that forces are carried by particles, and in gravity’s case, by gravitons, which have not yet been discovered, leaving gravity as a profound mystery.
We find ourselves amidst uncertainty, and only a new, comprehensive theory could resolve it. The theory of relativity is not finished; it predicts phenomena like gravitational lenses and wells. The discussion is about refining some of the theory’s equations, acknowledging that the laws of nature may be subject to change.
