Is dark matter real?
Dark matter sounds like something straight out of a sci-fi film, and even though it’s not, it is just as mysterious and weird. For me, dark matter is one of the most interesting concepts in astrophysics, for a couple of reasons. The first one being it’s a very recent discovery and what you could call frontier science. Though that means we don’t know much about it, it also means we can watch all the new and exciting discoveries happen right before our eyes. Not to sound too dramatic but it’s like watching history being made, which is pretty cool. The second reason is that while learning about dark matter you can learn a little bit about the process of scientific discovery, how physicists actually come up with a new theory.
The interesting thing about dark matter is that we don’t actually know what it is. The only thing that we do know is that we need it to explain certain astronomical observations.
The need for dark matter
Einstein’s theory of special relativity is quite special (sorry I just had to), partly because it is one of the most unintuitive and confusing scientific theories ever developed (I know this sounds irrelevant but just hold on). It basically says (this is a VERY crude summary) that if you have something heavy enough, it could bend the very space around itself. That doesn’t mean it would just bend things floating around it, but bend empty space itself. Anything passing through this space would be bent for that moment, and come out in its original form. This includes light. So light could bend around extremely heavy objects.
Here’s what that would look like through a telescope:
This image shows blue light from a background galaxy being bent around a foreground yellow galaxy. This phenomenon is called gravitational lensing and such rings are called Einstein rings. Note, the blue galaxy is not actually curved, but it appears that way.
But here’s the problem, physicists have now discovered that most galaxies do not have nearly enough mass to bend space-time so much*. But we can clearly see the light bending, so there has to be some missing mass that we cannot see but which still affects the space around it. This missing mass is what we call dark matter.
More evidence
The movement of galaxies is another piece of evidence for the existence of dark matter. Most galaxies spin, either around themselves or around a larger galaxy, but recent calculations show that most galaxies are spinning so fast that they should tear themselves apart. They don’t have enough mass, and thus enough gravity, for that, and yet they do spin, without breaking apart. There is again the same problem of missing mass, or extra gravity.
What is dark matter?
Dark matter is often described as a form or type of matter that does not interact with light in any way (hence dark), does not interact with normal matter in any way, which is why it’s so difficult to detect, but still has mass. You can see how that’s not so much of a definition but more of a convenient way to explain what we observe. It’s like a placeholder, until we discover what is actually causing these discrepancies, and what that could be is anyone’s guess.
Dark matter could be some exotic fundamental particle that we haven’t discovered yet due to its elusiveness, or it could be a massive celestial body, like the aptly named hypothetical maCHOs. It could be a hypothesized type of black holes called primordial black holes, which may have formed right after the big bang. There are several different theories which you can read more about here, but there is a certain group of them that is particularly interesting. What if dark matter doesn’t exist?
A new theory of gravity?
Basically, we know that anything that has mass has gravity, no exceptions. But what if mass isn’t the only source of gravity? What if gravity doesn’t need mass? This would call for a modified theory of gravity, since there is no such thing as massless gravity in our current model. This is, however, the much less likelier possibility. It is also a sort of last resort that we may turn to if we cannot find proof for any other contenders for dark matter. This is because we cannot simply assume that our current theory of gravity is wrong, because:
1. It has worked pretty well for us so far, and
2. Doing so would require a completely redesigned model of physics since gravity is such a fundamental part of it.
However, we do also know that our current theory of gravity is definitely incomplete, because some more anomalies have been observed in disc galaxies**. In fact, some scientists now believe that we need both dark matter and a modified theory of gravity in order to completely explain these observations, you can learn more about that here.
The scientific method
This is what I was referring to when I said that while learning about dark matter you can learn about the process of scientific discovery. When they first noticed the extra gravity, scientists could have just declared our theory of gravity wrong or incomplete and tried to change it, but that wouldn’t have gotten us much far. So instead they introduced a placeholder, and that allows them to do so many things at once. They can look for a new theory of gravity, while still using the current one wherever it’s applicable. They can look for new science without disturbing ongoing research based on the current theory. I find this quite interesting.
A note on dark energy
A concept often associated with dark matter is dark energy and though they have similar names they are not actually closely related. One might think that dark energy must be the ‘dark’ equivalent of normal energy, which as we know from Einstein’s famous equation E=MC² is often interchangeable with matter. That is not the case with dark energy, but it is similar to dark matter in the sense that it is also a placeholder.
An expanding universe
In the 1930s, astronomer Edwin Hubble, after whom the Hubble Space Telescope is named, discovered that all the light we observe was redshifted, which means it looked redder than the source. This indicated that the source, mostly galaxies, was moving away from us, which causes an apparent increase in wavelength, known as the doppler effect (the light appears red because red light has the longest wavelength in the visible light spectrum). Furthermore, it was discovered that the redshift is not only due to the doppler effect. The empty space between us and the observed galaxies itself was expanding, causing the light to stretch with it. This led to the conclusion that the universe is expanding, an idea that was not as common as it is today.
The need for dark energy
So, we know for sure that the universe is expanding (there have been many experiments after Hubble’s initial discovery and they all agree on this) and that in and of itself isn’t a problem, because it agrees with our theory of the big bang. The universe was hot and dense (♫♩ Our whole universe was in a hot, dense state♫♩….) and eventually the pressure caused it to expand.
This expansion was so powerful that the universe is still expanding today. But here’s the problem: according to our calculations, this expansion should have slowed down or even stopped by now due to gravity, and the universe should have started collapsing in on itself. But not only is the universe still expanding, the rate at which it expands is increasing, which means it is accelerating. And, according to Newton’s laws of motion, acceleration only happens when there is a constant force being exerted. This unknown force is what we call dark energy. We don’t know what it is yet, but we know it exists.
Dark energy is also famously associated with Einstein, who once included a repulsive form of gravity in his theories, called the cosmological constant, but later removed it, calling it his biggest blunder. Now, we know that he was right in including the constant, and indeed his biggest blunder might have been calling it his biggest blunder.
Again, similarly to dark matter, there are several theories in contention for dark energy, a list of which can be found here.
P.S.
Here’s some videos about the topic if you’re more of a visual learner:
Fermilab’s videos on Dark matter and Dark Energy
* The mass of a galaxy is measured by using certain celestial objects called tracers, which could be asteroids, or comets or other noticeable bodies that orbit the galaxy. The speed at which they orbit the galaxy and the size of their orbit can be used to calculate the mass of the galaxy. You can read more about the details of the process here and here, since it’s much more complex than what I’ve mentioned.
** Here’s a great video that talks about the need for a modified theory of gravity by Dr. Sabine Hossenfelder, an expert in the subject. I highly recommend checking out her other videos on astrophysics as well.
