What goes up must come down. Because of Gravity. Maybe.
Excerpted from Bigravity: A Hidden ‘Gear’ for Gravity? By Yuen Yiu in Inside Science:
Scientists have known since at least the 1990s that our universe is expanding at an accelerating rate. But this doesn’t make sense, because gravity — an attractive force like a rubber band — is supposed to cause our universe to contract or at least slow down the expansion.
“It’s like if you took a ball and threw it up in the air, but instead of falling back down, it just kept going up faster and faster,” said Andrew Sullivan, a physicist from Montana State University and author of the paper.
Scientists theorized that some other force must be responsible for ripping the universe apart, and “dark energy” became the placeholder term for the mysterious force.
One way to solve the dark energy riddle is to modify the theory of gravity itself, such that gravity can actually begin to repel over extremely long distances. In other words, the same gravity that is keeping us from falling off the face of the Earth could be ripping apart our universe at the same time.
As early as the 1930s, long before scientists knew about the expansion rate of our universe, Swiss physicists Wolfgang Pauli and Markus Fierz had already speculated that gravity may have a bipolar personality. However, their theory at the time was haunted by what’s known as “ghost modes,” which are parts of a theory that produce nonsensical solutions.
“If a theory has a ghost mode, it’s typically dead in the water,” said Sullivan.
In 2009, physicists Claudia de Rham, Gregory Gabadadze and Andrew Tolley together found a “ghost-free” solution to the bigravity theory, revitalizing the theory as a possible answer to dark energy. But just because the theory is no longer nonsensical doesn’t mean that it is correct. It still needs to stand the test of the scientific method.
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Honestly tho, does anyone else get a bit freaked out by cosmology?Report
It’s real analysis that always gets to me.
There’s just that unfathomably vast set of numbers that we really can’t say anything about at all.Report
This is why I’m a finitist.Report
I can see that. Bigger your perspective on the universe the smaller and more fragile we are. I enjoy it. BTW when doing academic papers, always spell check cosmology, cause take it from me and experience you do not want to submit a rant on cosmetology…still haven’t lived that one down yet.Report
Ha!Report
Maybe you were subconsciously influenced by this?Report
It was a rough day for sure Report
But your hair and makeup were fabulous, right?Report
Yep..that’s pretty much indicative of how the rest of that course went.Report
Big and weirdness doesn’t bother me. Small, OTOH… Quantum mechanics is what chased me out of physics. Electron-slit experiments in particular. Electrons being waves and going through both slits at once if you don’t watch them especially. (Electrons jumping from place to place without crossing the intervening space doesn’t bother me the same way, so I’m fine with flash memory.) I believe what I said to my physics advisor was “If this is how the universe works, I don’t want to know any more.”Report
Just assume the universe is a simulation, granular to the planck length and time, and quantum mechanics is just when the universe is forced to calculate a specific answer, instead of just using some computationally cheaper algorithms. (You know, you don’t need to simulate every drop of water in a stream to know it’ll flow downhill, unless some jerk with a really good set of sensors is measuring the crap out of the flow…)Report
But as far as we can tell nature does compute the entire Hamiltonian in all it’s exponential glory. That’s the model. The model works. Simplifications of the model all fail to match experiment in some important ways.
In some of his video lectures, Susskind describes it this way (paraphrase): a theory like general relativity is durable. You can change all kinds of variables within the theory, and it remains self consistent. By contrast, quantum mechanics is precise and fragile. It works, but each bit is logically connected to every other bit. Any tweaks, the whole system collapses.Report
Yeah. In some ways this makes QM (and especially QFT) the more powerful theory, because it lets you rule out a whhole lot more.
In other ways it can leave everything feeling like a Rube Goldberg machine or a Jenga tower with half the pieces missing.
I always remember my first encounter with the Spin Statistics Theorem, which says that if particles with integral spins are bosons, and particles with half-integral spins are fermions, the correlations between quantum fields at different points in space time cancel out in just the right way to preserve causality.
I still can’t decide if it’s incredibly elegant or a ridiculous hack.Report
It’s a hack, just like inertial mass equalling gravitational mass so that gravity accelerates everything at the same rate. Some day we’ll find a more elegant explanation, like General Relativity.Report
Really religious people? They can always say its’ the will of the divine.Report
For years I’ve been saying we need to re-write the law of gravity… it’d be nice to live long enough to see that.
I have mixed feelings about the potential re-write not leading to a star drive.Report