I’m not an astronomer, by any stretch. But there’s this thought about the universe I can’t get out of my head, so I’m sharing it with you. And it has to do with gravity, the fabric of space-time, and dark matter.
So, first, imagine you put a bowling ball on your bed. No matter how tightly you tuck your covers, if you put a bowling ball in the middle of your bed, it’s going to create a divot. And if you roll the bowling ball around in your bed, that divot is going to travel with it…and it’s also going to leave a trail of slightly messed up, looser covers behind it. The reason it leaves a trail is because your covers aren’t perfectly elastic: when they bend due to applying force (in the form of setting a bowling ball on them) then they snap back partially, but not completely, to the shape they were before.
Now imagine that you set the bowling ball in the middle of a steel beam that’s spanning a gap. You can’t really see it with your naked eye, but the steel beam deforms juuuuuuuust a liiiiiitle bit from the weight of the bowling ball. And, when you take the bowling ball back off the beam, it snaps back to its original shape. Well, pretty much. It, also, isn’t 100.00000….% elastic, so it snaps back to its original shape, minus a miniscule, tiny, teeny, itty-bitty bit of deformity. If you take the bowling ball on and off the steel beam, like, a literal billion times, then you might deform it enough to see with your naked eye. Or, maybe, measure with sensitive instruments. Basically no material in the universe is perfectly elastic: if you deform it a little with force, then remove that force, it doesn’t snap back to 100.0000000…% of its original shape. Maybe the most elastic material in the universe snaps back to 99.9999…..% of its original shape. Close enough to be imperceptibly different from 100% elastic, but not exactly 100% elastic.
So, we have this metaphor of space-time being a “fabric”, like your bed sheets. And we say that gravity is a force created by matter deforming that fabric, much like a bowling ball deforms your bedsheets. Well, matter moves around in space, just like that bowling ball moved around on your bedsheets. There is one critical difference between space-time and your sheets, though. When we talk about the fabric of space-time, we talk about it as if, when matter moves from one place to another, then the “divot” that it had created in its previous position moves with it, and the fabric of space-time that it leaves behind snaps back to a state of perfect zero gravity. As if it’s perfectly elastic and retains not a single trace of its previous deformity.
But, what makes whatever that “fabric” of space-time is made of so special? What if it isn’t perfectly elastic? When you disturb anything else in the universe, it does not snap back to exactly its original position. It retains some echo of its deformity. Why would whatever space-time is made of be different than everything else in the universe?
So, imagine space-time is at least a little less than perfectly elastic, like everything else in the universe. Let’s say, it’s 1 minus 10 to the negative eleventy-gazillion-trillionth away from being perfectly elastic. Well, after 13 billion years of the universe existing, with stars and planets and black holes moving around in it, and stars converting matter into energy in their nuclear furnaces, what if all that stretching and snapping back of space-time over 13 billion years left its fabric….floppier? 13 billion years is a very long time for even the tiniest deviation from zero to accumulate into something more interesting.
Well, if that happened, then you’d see something that looks like the effect of gravity without seeing…matter. What you’d be seeing is the “tracks” left behind in space-time from matter moving around and/or being converted into energy. Wouldn’t you? And isn’t that exactly what we call “dark matter”? Dark matter is simply seeing the evidence of there being pockets of gravity without those pockets interacting with matter in any other way. And those pockets are mostly, but not completely, evenly distributed across the universe, just like matter was, especially in the beginning of the universe.
This doesn’t feel as crazy as it sounds. I mean, we just recently actually measured gravity waves. What are gravity waves other than a more dynamic version of what I’m talking about? Something huge like two neutron starts collided, they stretched space-time, it tried to snap back, it overshot zero in the opposite direction, tried to snap back again, overshot zero again by just the tiniest bit less, etc, until the resulting “wave” hit Earth. Well, if gravity can exist in wave form, without there being matter directly attached to it, then why can’t it also exist in “divot” form? Or waves that are harmonically locked somewhere? Or everything in between?
Ok, I got that off my chest. What do you think? Any actual astronomers out there?