-
Asked by anon-251974 on 29 Apr 2020.
Question: What do we know about dark matter and what would change if we knew more?
- Keywords:
-
Anne Green answered on 29 Apr 2020: last edited 29 Apr 2020 12:46 pm
(Along with many other scientists) I’ve spent the last 20 years trying to find out what dark matter is!
.
If Newton’s laws of gravity (and general relativity) are correct, then galaxies, galaxy clusters and the Universe as a whole contain ~5 times as much matter as the things we can see (stars, clouds of gas).
.
We’ve deduced various other things about dark matter:
.
i) It has to have a lifetime much longer than the age of the Universe (13 billion years), otherwise it would have already decayed and disappeared. (Many fundamental particles aren’t stable. For instance an isolated neutron, outside of the nucleus of an atom, decays in minutes!)
.
ii) It can’t have electrical charge, otherwise it would form strange charged atoms, which we would have already seen.
.
iii) It has to move slowly (we call this ‘cold’). Otherwise the dark matter in small dwarf galaxies wouldn’t stay together.
.
iv) It can’t be one of the normal particles that we already know about (we call this ‘non-baryonic’). We can measure the amount of normal (‘baryonic’) matter in the Universe in various ways (one of them is via the amounts of hydrogen, deuterium, helium and lithium made in the first few minutes of the big bang). And there isn’t enough normal matter.
.
Many potential dark matter candidates have been suggested. Some of the most popular are:
.
i) Weakly Interacting Massive Particles are exactly what their name says. They are heavy (weight similar to atoms) and interact only weakly with each other and normal matter.
.
ii) Axions are extremely light and extremely weakly interacting.
.
iii) Primordial Black Holes are black holes which could be produced just after the Big Bang (rather than via the collapse of stars). They can weigh as much as a mountain up to many times what the Sun weighs.
.
Axions and WIMPs are popular as the come from Particle Physics models which have been suggested to solve other problems, rather than just to conjure up a dark matter candidate.
.
Lots of different lab experiments and astronomical observations are taking place to try and detect the dark matter particles. Hopefully these experiments/observations will tell us what type of particle (or black hole) the dark matter is, how much it weighs and how it was produced. We’ll definitely learn more about what dark matter can’t be and rule out some of the candidates.
.
As I mentioned above all of the observational evidence we have for dark matter so far comes from how it affects visible things via gravity. So in principle its possible that dark matter doesn’t exist and instead we don’t understand the laws of gravity completely. However no-one has yet managed to come up with a theory of modified gravity that explains all of the observations.
-
David Sobral answered on 29 Apr 2020:
Interestingly enough, we know more about how dark matter works, what it does and how it behaves in a way that is even better to “ordinary matter”. Dark matter, as far as we can tell, ignores all forces except gravity, making it much easier to study.
What we don’t know is, of course, quite important: what is it? So we know how it works so so well, but we still have not found what it is made of!
-
Mark Laughton answered on 29 Apr 2020:
Two nuclear researchers where I work have a new theory of what dark matter is. You may like to read this article.
-
Ry Cutter answered on 29 Apr 2020:
Okay, this is a biiiiiiig question!
Let’s start with why we think there is dark matter. When we look at galaxies, we can see the stars that comprise them. When we measure the speeds of the stars in these galaxies, we see the stars are moving too fast! The only force that works on the distance scales of a galaxy is gravity (that we know of). These stars are moving faster than what our current gravity calculations predict. There are two solutions to this problem:
1.) our gravity calculations are wrong: the new maths is called Modified Newtonian Dynamics (MoND).
2.) There is more matter in the galaxies that we can see. This hidden matter is called, you guessed it, Dark matter!
Dark Matter exists to explain why galaxies behave the way they do because of the ‘stuff’ that we can’t see.
Dark Matter was originally thought to be made of ‘dark’ baryonic matter. That is, regular matter that is too small or too heavy for light to interact with. However, even with the maximum amount of allowable black holes in a galaxy, this type of dark matter would only account for about 2% of hidden matter we expect to see in these galaxies.
This led to the development of non-baryonic dark matter. This is dark matter, that has mass, but doesn’t interact with light!
There are really two main schools of thought on this on the nature of dark matter:
1.) MACHOs, massive astrophysical compact halo object, which is a type of dark matter that spans the entire galaxy. (Usually, but not always baryonic)
2.) WIMPs, weakly interacting massive particles. (always non-baryonic)
Dark matter is still a big mystery, but it’s helping us understand the parts of the universe we can’t see!
Fantastic Question,
Ry
-
Marios Kalomenopoulos answered on 29 Apr 2020:
In reality, as David explained, we know a lot about dark matter (DM), but also nothing. What do I mean?
Based on some observations of the universe (how galaxies rotate, how the universe expands and galaxies form, how light gets deflected on its way to us etc), we have trouble to explain them with the “normal” matter in the universe, i.e. matter that you and I and everything around us is made from.
So what scientists do is to imagine a new type of matter, called dark, because it doesn’t emit light (in most of the models), and put it in our equations. These new equations with DM work better on describing the things we see in the Universe. However, beware, that noone has ever DIRECTLY detected this new type of matter.
What it can be? Again, noone is really sure, but the standard explanation, coming from particle physics, is that it is a new particle! And usually very massive that we haven’t yet observed in accelerators/detectors on Earth.
*If you want to be more radical, you can even say that DM doesn’t exist at all. At the last analysis it’s something that we put into our equation to make them work. And you can think instead that the equations we were using at the first place are wrong and need to find new ones! Of course, this is also difficult, and can have its own problems, so the majority of physicists think that DM is most likely a particle that we just haven’t seen yet.
-
Susan Cartwright answered on 29 Apr 2020:
What we know about dark matter is basically the following:
– A number of different astrophysical and cosmological observations agree that it makes up about 5/6 of the matter density of the universe (the fact that different techniques give the same answer is good evidence that this is real, and not some error in the calculations)
– It is detected because it exerts gravity, but it does not feel the strong nuclear force (if it did, it would have interfered with the formation of helium in the early universe) or the electromagnetic force (if it did, t would emit or absorb light, and we would see it – which we do not). It might or might not feel the weak nuclear force – most experiments aiming to detect it assume that it does, but there is no evidence for this.
– It is stable, or at least has a lifetime which is long compared to the age of the universe (because measurements from the very early universe such as the cosmic microwave background agree with measurements from the local universe such as the masses of clusters of galaxies).
– It is not fast-moving, because we see that it is gravitationally bound to galaxies (so its typical speed must be similar to the Sun’s, a few hundred kn/s, not close to the speed of light like, say, neutrinos).What we do not know is what it actually is. There is nothing in the conventional standard model of particle physics that has these properties, but there are a number of popular extensions to the standard model that offer candidates. The preferred candidates are those where the extension to the standard model has a good rationale: it is not good scientific practice to make arbitrary additions to theories just to explain one anomalous observation. The most popular of these extensions are supersymmetry, which seeks to explain why the Higgs boson has the mass that it does (and which predicts a very massive dark matter particle) and axions, which seek to explain why the strong interaction, unlike the weak interaction, is the same for both matter and antimatter (and which predicts an extremely low-mass dark matter particle). There are experiments looking for both these types, but so far they have not found anything.
If we detected dark matter, it would be a very exciting time for theoretical particle physicists, because it would require a major change to the current theoretical model – the first for nearly half a century (changes had to be made to accommodate massive neutrinos, but they were very minor by comparison). So it could very much change our ideas of the fundamental structure of matter. It would not, however, make much change to our everyday lives, because dark matter interacts so weakly with anything else that it would be very difficult actually to use it for anything.
-
Roan Haggar answered on 29 Apr 2020: last edited 29 Apr 2020 10:38 pm
Although we know quite a lot about the effects of dark matter on the Universe around us, we know very little about what it actually is!
–
There’s plenty of evidence for dark matter: for example, we can look at the speeds of stars moving in galaxies, and work out how much stuff (‘matter’) there would need to be in the galaxy to produce the gravitational forces to cause this motion. However, we can’t see enough matter in these galaxies to explain why the stars move so quickly, which means that there must be more matter that we can’t see, which we call ‘dark matter’.
–
Other evidence includes measurements of ‘gravitational lensing’, which involves large objects in space, such as galaxy clusters, bending the light from other objects by gravity (there’s some amazing photos of this online!). Again, we can work out how much matter is needed to cause this lensing, and once again, it’s much less than what we can actually see!
–
These are both examples of ‘indirect detection’, where we see the effects of dark matter on other objects. There’s a lot of work currently going into ‘direct detection’ experiments to try to detect new particles that could make up dark matter, as this would be the final bit of evidence to prove that dark matter exists, but these haven’t found anything yet. However, some scientists don’t think that dark matter is made of these tiny particles, but is made of larger objects that we can’t see, such as a type of black hole called ‘primordial black holes’. We really don’t know which of these is the case — dark matter could even be a combination of them both!
–
Trying to work out whether dark matter is made up of tiny individual particles or larger objects is one of the big questions for scientists right now. Either way, it would force us to re-think our ideas about the very beginning of the Universe, and come up with ideas to explain how either these dark matter particles or primordial black holes (or something else!) were formed. Knowing more about the dark matter would also help us better understand its effects on visible matter, such as galaxies, and how this might have changed over the history of the Universe. Hope this helps!
Comments