As an alternative to the dark matter particles known as WIMPs, astrophysicists considered “Massive Astronomical Compact Halo Objects” (MACHOs). These are basically ways of making “normal” matter invisible, such as very faint stars or planets. We’ve become much better at detecting these objects now, and although they exist, they cannot account for all of the dark matter effects in the Universe.
Month: September 2019
WIMP #4: the circle of light
Gravity pulls on particles of light, changing their trajectories and resulting in the phenomenon of “gravitational lensing”. One of the strongest manifestations of this is called an Einstein ring, which occurs when a bright source of light (such as a galaxy), a big mass (i.e. the “lens”) and the Earth form an almost perfect straight line: then the bright source is seen as a ring. In different gravity theories, this ring can have a different radius, or might not even appear at all.
Figure credit: Wikipedia HERE
WIMP #3: The “recent” importance of dark energy
By the time we noticed it, dark energy had been the most prevalent thing in the Universe for more than 3 billion years. This is around a quarter of the Universe’s current age, hence the description “recent”.
Star trekking for dark matter
Should we send a spacecraft with a fancy clock and an accelerometer (literally a device for measuring acceleration) to the outskirts of the Solar System?
This is the question under consideration in a recent paper (HERE), which contemplates sending a spacecraft 150 times further from the Sun than we are. The answer is a pretty convincing ‘yes’ and, despite only performing “local” measurements (to a cosmologist, the whole Milky Way is “local”, and frankly a nuisance sometimes), this spacecraft could make some really important measurements relating to the dark sector.
Health warning: this is fairly speculative paper, basically showing that the mission concept they discuss is possible and useful. Do not expect to see the launch of this mission on the news over the next few years, or go telling your friends that this is definitely happening just yet!
One of the difficulties with a mission like this is how to get the spacecraft so far from the Sun in a sensible amount of time. The authors posit that the Breakthrough Starshot idea may have reached sufficient maturity by the 2040s for the spacecraft to reach these distances in 2-6 years. This technology involves making small spacecraft with sails and then shooting a laser at them, see HERE and HERE.
The technology is not my expertise, so I will leave it up to you guys to investigate these links if you are interested. I will discuss the two main ways that such a spacecraft could help us to understand the dark sector.
(A) The “desert” of gravitational tests
A few years ago some cosmologists made a plot to show the range of possible gravitational phenomena that could occur, and which of these we have tested so far. The vertical axis of this plot is the “potential”, which essentially means the strength of gravity at that location. In terms of the trampoline analogy (see HERE), it corresponds to how stretched the trampoline is, and therefore how much mass is on the trampoline at that point. The different colours and lines correspond to different types of gravitational tests that we have have done over the years (I won’t go into the details of each of these tests here).
The crucial thing is that there is a band across this plot corresponding to potentials between known astrophysical systems (like the Solar System) in the upper half of the plot, and large scale cosmological observables (such as the expansion of the Universe) at the bottom of the plot. Since we think we understand gravity in the Solar System, but have to introduce dark matter and dark energy on large scales, this previously unexplored ‘desert’ could yield some clues.
Aside: the gravitational regime that this spacecraft probes would approach something called the MOND acceleration scale. Some of you may have already heard or read about this: it is a proposed modification to gravity where things work differently at really small accelerations. I will say more about this in a future blog post.
This spacecraft mission plans to test gravity in this ‘desert’ (and is one of the few ways we might be able to do so). The idea is to put a really good atomic clock, and a really good accelerometer onto the spacecraft. But how would these help us to test gravity?
One of the consequences of Einstein’s gravity (and most subsequent theories that have the same view of space and time), is that clocks in a stronger gravitational field go slower than those in a weaker gravitational field. So, by including a really sensitive atomic clock on the spacecraft, and seeing how fast it goes compared to clocks on the Earth, we can calculate how strong gravity is at the spacecraft’s location. At the same time, the trajectory of the spacecraft is tracked. Finally, the accelerometer is used to measure any extra effects on the trajectory of the spacecraft due to non-gravitational effects, such as gas and dust friction, solar radiation pressure, outgassing from the spacecraft, and lots of other annoyances that would contaminate our measurement.
Combining these, we know the non-gravitational forces on the spacecraft, and can subtract them off the trajectory. We then compare the remaining trajectory to the strength of gravity measured by the onboard clock. If they agree, then Einstein’s gravity passes another test. If they disagree, then things get very interesting…
(B) Measure the local dark matter environment.
What if we send the spacecraft out and find gravity works exactly as Einstein predicted and there are no deviations? Whilst this would be an important result by itself, it isn’t very exciting. However, this mission can do other tests too, making it a win-win experiment: the same technology that allows the tests of gravity can also be used to measure the local dark matter density.
Whilst we know quite precisely what the density of dark matter should be on average across the whole Universe, the same can’t be said for the local dark matter density. Dark matter clumps, so it will have different densities in different places in the Universe, and our measurements of this in the Solar System have quite large error bars.
The measurement of the local dark matter density basically works by the same process as the tests of gravity: Measuring the gas and dust friction using the accelerometer will tell us how much normal stuff there is around, and therefore how much of the gravitational strength is coming from this stuff. Therefore, the rest of the gravitational effect must be due to the local dark matter density. We can thus turn this around and work out how much dark matter must be there in order to give the measured strength of gravity.
A measurement of the local dark matter density is important because we use this quantity to draw conclusions from some of our experiments. One of the methods that physicists use to look for dark matter is “direct detection experiments”. These use the fact that dark matter isn’t expected to be perfectly dark, and every so often will interact with an atom they are passing through. For the archetypal dark matter candidate WIMPs (see HERE for an explanation of this acronym), the idea is to build very big and sensitive targets and hope that enough dark matter particles pass through to get a few signals. We haven’t yet detected any signals yet, but we use the lack of a detection to say something about the dark matter properties.
The problem is that saying something about the dark matter properties from this lack of detection depends on knowing the local dark matter density, so if we don’t know this very well then our statements about the properties of dark matter are similarly imprecise. However, if this spacecraft could deliver a good measurement of the local dark matter density, then we would learn a lot more from experiments that we’ve already done, and also that we will do in the future.
Sci-fi pie-in-the-sky, or crucial experiment of the next 20-30 years?
I have focussed on the use of the mission for the dark sector, but some of the details about the technology are quite interesting. The paper is more readable than most because it is a “white paper” rather than a peer reviewed publication, so I encourage people to have a look at the paper itself if they are interested.
As I said above, it is a little speculative. Nonetheless, I am personally quite excited by the prospect of new tests of gravity. These are the sorts of experiments that might deliver what we expect… and might change everything that we thought we knew…
Figure credits
Breakthrough starshot image, see HERE
Gravitational ‘desert’ plot, see the original paper HERE
WIMP #2: Einstein’s biggest error?
Einstein ostensibly described adding the cosmological constant to his equations as his biggest blunder. But, if dark energy turns out to be a cosmological constant as some cosmologists expect, then maybe it will be removing it from his equations that is his biggest error…
WIMP #1: Fritz Zwicky
An astronomer named Fritz Zwicky was one of the first to suggest the existence of dark matter. He noticed that the orbital speed of galaxies in the Coma cluster needed much more mass than could be seen. This was back in 1933! I wonder if he thought we would have found it by now…
Weekly Informative Mini Posts (WIMPs)
Like all good acronyms, this one was arrived at by torturously trying vaguely appropriate words until I got to the answer I already knew I wanted: WIMPs.
One of the most popular class of candidates for dark matter has the acronym WIMPs, which stands for “Weakly Interacting Massive Particles”. These are particles that occur in many extensions to the standard model of particle physics, and which interact directly with other particles only (or primarily) through the weak nuclear force.
One of the reasons that WIMPs are so popular as a possible dark matter candidate is something called the “WIMP miracle”. This refers to the fact that a particle with a mass and interaction strength that are associated with the weak nuclear force, naturally results in dark matter with the correct abundance in the Universe to be the cosmological dark matter. This abundance could have turned out to be many trillions of times too big or too small, so it is one hell of a coincidence if it is by accident!
My plan is to make a small regular post each week with an interesting factoid, or “soundbite” style piece of information: think something that can fit into a single tweet (ish). I will call these “WIMPs” (Weekly Informative Mini Posts) in homage to the popular dark matter candidate. Interspersed with these mini posts will be the regular (longer) blog posts.
Enjoy!
Dark Matters 
