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Asked by anon-257942 on 25 Jun 2020.
Question: Can you have different densities of dark matter, and does it affect how things like light and other matter travel through space? Since it's supposedly a matter, it's supposed to have particles, so it should have different densities right? I understand not much is known about it but if you have at least an idea or theory I'd be interested.
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Richard Fielder answered on 25 Jun 2020:
Yes to both questions. Dark matter is thought to consist of some sort of currently unknown particle that doesn’t interact with the electromagnetic force, which means you can’t see it using light. But it still has mass and energy, which means it does interact with gravity. That’s how we know it’s out there – we can’t see it directly, but we can how the gravity it produces affects the things we can see. So the fact that it affects other matter is the only way we know it exists at all. And even though it doesn’t interact directly with light, its gravity still affects the path light travels through space (because of general relativity).
As for density, that’s actually one of the most interesting things about it. Some of the first observations that led to the discovery of dark matter came from looking at the rotation of galaxies. You can calculate how fast things should orbit inside a galaxy by knowing how much mass there is (from the same maths that allows us to calculate things like the orbits of planets). When we checked, it seems almost all galaxies seem to have a lot more mass than we can see. But it’s not just that there’s too much mass, but it seems to be in very different places from where we’d expect – visible matter is more dense towards the centre of galaxies, but dark matter is more dense towards the edges. It’s these variations in density that allow us to figure out some of its properties and how it can’t interact much with ordinary matter, even without knowing exactly what it is or being able to study it in a lab.
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Anne Green answered on 25 Jun 2020:
Yes, the dark matter (DM) density varies lots. In the early Universe, just after the Big Bang, there are small variations in the density (which lead to the temperature fluctuations which we’ve measured in the Cosmic Microwave Background (CMB) radiation https://map.gsfc.nasa.gov/universe/bb_cosmo_fluct.html). Gravity pulls matter together, so regions which were initially overdense get more overdense with time and eventually clumps of DM, called halos, form. The DM density is highest in the middle of DM halos and decreases rapidly with increasing distance from the centre. Later on gas condenses in the middle of these halos and stars and galaxies form. You can see supercomputer simulations of this process here: https://wwwmpa.mpa-garching.mpg.de/galform/data_vis/
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DM does affect light and matter and this is where the observational evidence for DM comes from. Einstein’s general theory of relativity tells us that matter bends space, so that the path of light gets curved when it passes through a region with high density. This phenomena is known as gravitational lensing (the high density regions act like a lens) and it allows us to map out the distribution of DM with galaxy clusters (groups of tens or thousands of galaxies which are held together by DM). You can learn more about this here: https://www.thoughtco.com/introduction-to-gravitational-lensing-4153504
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DM affects the motion of stars via its gravitational force. In spiral galaxies stars move in circular orbits due to the force of gravity (like the planets in the Solar system). By measuring how fast the stars move, and how this varies with distance from the centre of gravity, we can measure how much matter there is in the galaxy and how it’s distributed. The stars in the outer regions of galaxies are moving much faster than they would if galaxies only contained the stars and gas we can see. This tells us that (provided Newton’s laws of gravity are correct) galaxies are surrounded by large, roughly spherical DM halos.
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We know, from the speed at which the Sun is moving around the centre of the Milky Way (roughly 220 km/s), that the DM density in the Solar neighbourhood is equivalent to a bit less than 1 proton per cubic metre. Using the CMB (and also the amounts of hydrogen, deuterium, helium and lithium made in the first few minutes of the Big Bang) we’ve found that the average density of normal matter is roughly 5 times smaller than the average DM density. And the average density of matter in the entire Universe (which we’ve measured using the CMB) is equivalent to roughly 1 hundred thousandth of a proton per cubic meter. Most of the Universe is essentially empty!
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The DM can’t be one of the normal particles we already know about. The most popular DM candidates are new exotic particles (for instance WIMPs and axions). Another possibility is Primordial Black Holes- black holes which could be produced just after the Big Bang (rather than via the collapse of stars). There are lots of experiments underway trying to detect WIMPs and axions. WIMPs can be detected either directly in the lab, when they interact with normal matter, or indirectly via the particles produced when 2 WIMPs come together. The signals expected in these experiments depend on how the DM is distributed in our Milky Way galaxy.
Comments
anon-257942 commented on :
Cool, thanks for replying. If I understood, we detect dark matter by detecting gravity that normal matter is not present to create. You say Its’ impact on gravity affects the path of light, so instead of light travelling in a straight line through space, should it be slightly bent? You also say that it has energy, so does that mean its’ energy is extremely low (as space is apparently 2.73 Kelvin), or is there such thing as negative energy (below 0 Kelvin)? I’d be really interested to know.
Richard commented on :
“You say Its’ impact on gravity affects the path of light, so instead of light travelling in a straight line through space, should it be slightly bent?”
Yes. This was actually an important part of verifying general relativity, since the bending of light near large masses has important effects in astronomy and leads to stars and other objects appearing to be in the wrong place if their light passes close to something large like a star (including the Sun). Since dark matter has mass, it has the same effect on travelling light and this is one of the ways we can detect it’s there.
One very interesting effect is called gravitational lensing, where one object passes in front of another one, and the light from the one behind gets bent as if by a lens and can produce multiple images or a ring surrounding the one in front. Since the focussing depends on the mass of the front object, this provides another piece of evidence for dark matter, because we can sometimes see gravitational lensing happening much more than the visible mass would create. Unfortunately, we haven’t found any clumps consisting only of dark matter, so we haven’t been able to measure it on its own.
https://en.wikipedia.org/wiki/Gravitational_lensing
“You also say that it has energy, so does that mean its’ energy is extremely low (as space is apparently 2.73 Kelvin)”
When we say space has a temperature of 2.73 K, that’s only a measure of mostly empty space and comes from the background radiation left over from the big bang. In places where matter exists, the energy contained in a volume of space can be much higher – obviously somewhere like the centre of a star is quite hot. Since matter and energy are equivalent (according to the famous E=mc^2), any part of space that contains matter must have higher energy than the empty parts.
However, how much energy dark matter actually has is one of the questions we’d like to be able to answer. Early on, the most popular theory was what is called “cold dark matter”, which suggested that because it doesn’t interact with normal matter, dark matter would end up very cold and close the temperature of empty space. More recently, we think it might instead be “warm dark matter”, since observations of its distribution suggest it must be moving around a lot more and actually be quite a bit hotter than the surrounding space. Most of our ways to measure temperature involve production and absorption of light, and since dark matter doesn’t interact with it directly this makes it very difficult to measure.
Anne commented on :
Actually cold dark matter is still the favoured theory. There are some discrepancies between predictions and observations of galaxies on small scales. However the consensus of people who work in this area is that this is due to the complicated physics of the stars and gas. It’s been shown that warm dark matter can’t simultaneously solve all of these issues.
anon-257942 commented on :
Great, thanks for the information. I had no idea that stars and galaxies formed in the center of dark matter halos, or that the stars we see in the sky are not actually in the position we see them in!