Are Dark Matter and Dark Energy Actually the Same Thing?

We're getting into some very high-minded science here today. One of the thorniest issues with astrophysics today is dark matter and dark energy. We know they exist, but we have no idea what they are and they seem to make very little sense. Scientists have tried combining the two in models previously, but that involves modifying general relativity, which is a pretty bedrock theory. Modifying it hasn't gone well.

But now, there's a new idea out there, something that combines two things that sound ridiculous but are actually completely compatible with general relativity: negative masses and matter creation. Essentially, this new theory posits that dark matter and dark energy are some sort of exotic fluid that possesses negative mass and also continuously creates new negative mass. Negative mass, if you don't know, is mass, but negative. If you tried to throw a baseball made of negative mass, it would accelerate toward you instead of away. It's weird and completely hypothetical at this point. It was also thrown out as a candidate for dark matter/energy because it would get thinner and less repulsive as the universe expanded, and that's not the result we're seeing. That's where matter creation comes in. If this negative matter constantly creates more negative matter, the density won't thin out and the expansion of the universe works out.

Frankly, this is some EXTREMELY hypothetical stuff, with not a hint of proof behind it. However, it works perfectly within our current laws of physics, and it perfectly predicts the observations we see of dark matter halos. It fits the observations we have, now all we have to do is observe it. No problem, right?

The Good News For General Relativity Keeps On Coming

Not too long ago, I wrote about how alternate theories to general relativity have been having a tough time as of late. Well, things just got even tougher.

The equivalence principle was on trial for this experiment. One of the basic tenets of relativity is that two objects, no matter what their mass or what they're made of, they are affected by gravity in the same way. This has been tested many times on Earth (and famously, on the Moon), but never with really dense objects. Alternative theories to relativity assume that the equivalence principle breaks down at high density, since up to now, there's been room to work.

The test involved  a neutron star-white dwarf pair, and watching the orbit of the neutron star. If there were variations in its orbit, it would have been in violation of the equivalence principle, and the various alternate theories would have some ground to stand on. But there was no variation, and once again, general relativity was proven correct. And not only that, but this test improved the accuracy of the previous best gravity test by a factor of 10. Alternate gravity theories thus have a lot less room to work.

Discovery of Gravitational Waves Announced

It really doesn't sound like very much, does it? The existence of gravitational waves, first theorized by Albert Einstein, is accepted by pretty much everyone. But for the past 100 years, we've never been able to detect them. We've tried, but gravity, as it turns out, is ridiculously weak.

Perfect, now just stuff that in a water bottle

Let's do a little comparison between gravity and the other 3 fundamental forces, just to demonstrate gravity's weakness. Imagine the force of gravity represented as a 1 kilogram object sitting on a table. A big bottle of water that holds 1 liter weighs a kilogram, if you need a visualization. It's not a problem to pick up, right? Everyone can pick up a bottle of water. That bottle represents gravity's comparative force. Now, let's replace gravity with a similarly sized bottle with the comparative weight of the weak nuclear force, the next weakest fundamental force. That bottle, previously 1 kilogram now weighs nearly as much as Earth and Venus combined. It would then turn into a black hole. As it turns out, the weak nuclear force is stronger than gravity by a factor of 1 * 10²⁵. Try picking that up.

The story only gets worse with the other 2 forces. Let's move on to electromagnetism, which along with gravity is the fundamental force we all know. It's stronger than gravity by a factor of 1 * 10³⁶. For reference, the Sun weighs about 2 * 10³⁰ kg. Our bottle would weigh about as much as 500,000 Suns. This is nearly as heavy as Segue 2, a dwarf galaxy and Milky Way satellite which is (according to Wikipedia) the least massive galaxy known; but far, far more than the most massive stars, which weigh in at around 200-250 solar masses. Again, it would then become a black hole. The strong nuclear force is only 100 times stronger than electromagnetism, so our bottle now weighs 50 million solar masses. For comparison, the supermassive black hole at the center of the Milky Way is only about 4.5 million solar masses. At that mass, our bottle actually would not become a black hole...yes it would.

Credit: LIGO

It would clearly be no easy task then to detect gravitational waves. But today, scientists with the Laser Interferometer Gravitational-Wave Observatory announced that they had finally managed to do it. The project used a pair of detectors 2,000 miles apart to detect the waves, which compress a laser in one arm of a detector and stretch the laser of the other arm. 2 detectors are necessary to confirm the result, and to triangulate the location of the gravitational wave. It's an incredibly sensitive experiment, but it has to be. Even the largest gravitational waves, emanating from sources such as supernovas and black hole collisions, cause a change in laser length measured at the subatomic level.

On one hand, this isn't exactly the most exciting news. Gravitational waves have been observed indirectly before, and like I said before, very few doubted that they existed. But on the other hand, direct observation is a lot better than indirect. While we can't rule out an error in the experiment (and verifying this observation will be very difficult, since LIGO is the only detector powerful enough to detect gravitational waves), it seems that this announcement will likely be the real deal. It represents a powerful confirmation of general relativity, and now that we know how to detect gravitational waves, we can take the process further, learning much more about the universe. It really is a bigger deal than it sounds.