\nSUMMARYWhat if gravity repelled instead of attracted? Repulsive gravity may have shaped the entire history of our universe. For centuries gravity was defined by Isaac Newton’s Law of Universal Gravitation which said, “every object in the...\nSUMMARY What if gravity repelled instead of attracted? Repulsive gravity may have shaped the entire history of our universe. For centuries gravity was defined by Isaac Newton’s Law of Universal Gravitation which said, “every object in the universe attracts every other object, with a force proportional to their masses and inversely proportional to the square of their distance”.
But in 1915, Einstein developed a more complete theory of gravity. He unified space and time, previously believed to be universal and independent, into a single 4-dimensional continuum. He followed that up 10 years later with General Relativity, which showed that mass and energy could warp this spacetime. So that’s when we were introduced to the idea of spacetime as a malleable fabric that could compress and expand. Gravity was better modeled by geometry.
But if mass curves space, why couldn’t things curve it the opposite way—pushing rather than pulling? This might be the only way to explain the current state of our cosmos. Astronomers had assumed the expansion of the universe should be slowing down due to the gravitational pull of matter. But in the late 1990s, distant supernovae observations showed that the universe is not just expanding—but the expansion is accelerating. Something is pushing space apart, acting like an anti-gravity force, dark energy.
We don’t know what dark energy is. It could be a new kind of energy field, or it could be an intrinsic property of empty space. It makes up 68% of the total energy in the universe. In contrast, normal matter only accounts for about 5% of the energy, and Dark matter is 27%.
But dark energy isn’t the only time repulsive gravity may have played a role in shaping the universe. The birth of the universe, the Big Bang, might have been driven it too. According to the theory of cosmic inflation, a brief but immense expansion occurred in the first tiny fraction of a second after the Big Bang, causing the universe to grow exponentially. This rapid expansion requires a form of repulsive gravity, possibly caused by a high-energy field known as the inflaton field. Repulsive gravity could have shaped both the beginning of the universe and its long-term fate.
How can gravity repel? Einstein’s equation can be rewritten in the following way known as the Friedmann equation. It shows that there are two ways to get an accelerated expansion, or repulsive gravity: negative energy, or negative pressure.
Normal matter has positive energy and exerts a negligible positive pressure. It attracts, and its energy density decreases as 1/a^3 with expansion. Radiation has positive energy and pressure. It also attracts, and its energy gets diluted faster than that of normal matter, evolving as 1/a^4 due to redshift in addition to volume dilution. Dark energy has an unusual property: negative pressure. A substance with negative pressure will get diluted slower than normal matter, or not at all. #gravity #darkenergy In the latter case, known as “cosmological constant”, the energy density remains unchanged throughout cosmic history. In other words, as the volume of space increases, more energy is continuously created; the cumulative repulsive effect then leads to exponentially accelerated expansion.
But what is this dark energy made of? For its density to remain constant with expansion, this mysterious component must be an intrinsic property of spacetime, present in equal amounts at every point and continuously stretching the fabric of the cosmos. A possible solution could be that the vacuum energy cancels out to 0 through an unknown symmetry, and that dark energy is made of exotic particles. This scenario, known as ‘evolving dark energy’ or ‘quintessence’, relies on a scalar quantum field.
Just like the Higgs field, vacuum energy is a scalar field that has a single value at every point of spacetime. This is unlike, for example the electromagnetic field which is vector field, having both a have a value and direction. The energy of such a field can vary in time and space. If it varies, it would be a form of evolving dark energy called quintessence.\n
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